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Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2001 Electronic ReleaseLASER ANALYTICAL SPECTROSCOPY - Laser Analytical Spectroscopy (Michael A. Bolshov, Yuri A. Kuritsyn)

Laser Analytical SpectroscopyREVISED JANUARY 2001

MICHAEL A. BOLSHOV, Institute of Spectroscopy, Russian Academy of Sciences, Troitzk, Russia; Institute of Spectrochemistry and Applied Spectroscopy, Dortmund, Germany

YURI A. KURITSYN, Institute of Spectroscopy, Russian Academy of Sciences, Troitzk, Russia

1. Introduction

Modern technologies in electronics, in high-purity materials and chemical reagents, in toxicology, biology, and environmental control and protection need extremely sensitive analytical techniques. Impurity concentrations at and below pg/mL (pg/g) levels are state-of-the-art for many analyses. In many cases the sample volume is very small; if an impurity concentration of the order of ng/g has to be detected in a sample of mass 1 µg, the analytical technique should have an absolute detection limit of ca. 1 fg (10–15 g). Even techniques such as atomic absorption spectrometry (AAS), optical emission spectrometry with inductively coupled plasma (ICP-OES), neutron activation spectroscopy (NAS), and X-ray fluorescence spectroscopy (XRFS) can not detect directly trace elements at such low levels of concentration and/or absolute mass. In most cases, these techniques need elaborate chemical pretreatment of samples. Contamination or loss of analyte are very probable during any chemical procedure at such low concentration.During the last 30 years the development of laser technology has opened up new possibilities in analytical spectroscopy, both by improvements in traditional analytical techniques, such as absorption and fluorescence or optoacoustic spectroscopy, and by the introduction of new techniques such as multistep photoionization spectroscopy, thermal lens spectroscopy, and site-selection spectroscopy.

© 2001 Wiley-VCH Verlag GmbH, Weinheim, GermanyPage: 1


1.1. Properties of Laser Light

1.1.1. Tunability

Practically all laser analytical techniques are based on systems with tunable wavelength. By using different types of lasers and the methods of nonlinear optics, one can obtain intense tunable laser radiation over the spectral range of 0.2 – 40 µm and in some regions of the vacuum ultraviolet and far infrared. Almost any strong atomic or molecular transition is thus accessible to laser excitation.

1.1.2. Intensity

The intensities of lasers exceed by many orders of magnitude those of conventional light sources. At high intensity levels, nonlinear interaction of light with atomic and molecular systems becomes pronounced. New analytical techniques involving multistep and multiphoton excitation and ionization, optical saturation, and excitation of forbidden transitions become possible. High radiation intensity also increases the sensitivity of laser analytical techniques.

1.1.3. Time Resolution

Pulsed lasers with extremely short pulse durations (down to 10–15 s) are now available, enabling investigations of chemical reaction kinetics, energy and electron transfer in complex excited molecules, etc. The efficiency of excitation of higher states increases because the duration of the laser pulse is often shorter than the decay time of the intermediate state of atoms or molecules.

1.1.4. Temporal Coherence

The spectral linewidth of modern lasers is often less than the homogeneous linewidth of a given atom or molecule. This results in very high spectral intensity and thus very efficient excitation. It also allows highly selective excitation of a particular atom or molecule in a complex mixture. High selectivity is an important advantage of laser analytical techniques.



1.1.5. Spatial Coherence

The ability to focus a laser beam to a very small spot (a few µm diameter) provides high spatial resolution in sample analysis. High spatial coherence also enables collimated laser beams to be used for remote sensing.A review of the physical fundamentals and main characteristics of lasers is given in → Lasers.

1.2. Types of Laser Analytical Techniques

Laser analytical techniques can be classified on the basis of the physical process caused by the interaction of laser radiation with matter (Fig. (1)).The primary process is absorption of photons by atoms or molecules. If the absorption lines are strong and the number density of the absorbing species is sufficient, the difference between incident and transmitted energies may be measurable for a single pass of the laser beam through the sample. In this case the signal can be measured by a simple light detector, such as a photodiode. For weaker absorption lines and lower number densities of the absorbing species multipass schemes are used.Another set of analytical techniques (optocalorimetric methods) is based on indirect measurement of the energy absorbed by the sample. Absorbed energy is transformed into kinetic energy of the sample particles followed by an increase in the local temperature of the sample. The resultant changes in pressure and refractive index can be measured by optoacoustic methods or by laser scattering techniques.The absorption of laser photons leads to excitation of atoms or molecules. In many cases, radiative decay of the excited state is very efficient, i.e., quantum yields of luminescence or fluorescence are very high. Measurement of the intensity of emitted photons as a function of analyte concentration is a widespread analytical method with conventional light sources. The use of intense laser sources greatly increased the sensitivity and selectivity of traditional luminescence methods.Excitation of atoms or molecules may be followed by collisional or multistep (multiphoton) ionization of the excited species. The charged particles so formed may be recorded by conventional methods. Several techniques involve a combination of optical excitation of a neutral particle and its subsequent ionization.



1.3. Peculiarities of Resonance Laser Radiation Absorption

The interaction of intense laser radiation with matter differs from that of conventional radiation sources. The high incident irradiances available with lasers can lead to a significant depletion of the initial level population. As a result, the optical absorption coefficient decreases as a function of the excitation energy and the absorbed energy tends towards a constant value. This so called optical saturation effect leads to a nonlinear dependence of the absorption signal on the light intensity. Because the amount of energy released by radiative and nonradiative relaxation processes depends on the amount of energy absorbed, nonlinear intensity-dependent signals are also obtained in other spectrometric techniques, such as fluorescence, optoacoustic, thermal lens, etc.In the simplest case for the interaction of a two-level system with cw (continuous wave) laser radiation (steady-state mode), the number of transitions per unit time from level 1 to level 2 (absorption) is equal to the number of transitions from level 2 to level 1 (stimulated and spontaneous emission):

where σ0 is the cross-section of the radiation (stimulated) transition, N1 and N2 are the number densities of particles at the levels 1 and 2, respectively, I is the intensity of laser radiation, hv is the energy of the resonance photon, τ is the radiative decay time of upper state 2 (assuming that non-radiative transitions due to collisions are negligible). Both N1 and N2 are dependent on laser intensity, so that

where N is the total number density of particles, is the saturation intensity, defined as the intensity for which the number density of the excited particles N2 reaches one half of its maximum value 1/2 N.If the intensity of laser radiation is much lower than Isat, the absorption energy and the number density of the excited particles N2 are directly proportional to the light intensity, and hence the analytical signal for different techniques (absorption, optocalorometric, fluorescence, etc.) has a linear dependence on the intensity of the radiation. This linear regime is typical for



conventional radiation sources used in elemental analysis: hollow cathode lamps (HCLs), electrodeless discharge lamps (EDLs) and for low intensity lasers.In the opposite case, when the intensity of the resonance laser radiation is high enough (I > Isat), the absorbed laser energy reaches its maximal value and the population of the upper level 2 approaches 1/2 N. The analytical signal for the absorption technique

(where l is the absorption pathlength) tends towards a constant value. Because the shot noise of radiation is directly proportional to √I, then optimum detection of the absorbing particles is achieved at I = Isat.If deep saturation is achieved (I > Isat), two very important advantages of different laser analytical techniques (fluorescence, optocalorimetric, ionization) are gained: Maximum analytical signal and minimal influence of intensity fluctuations of the excitation source on the precision of the analysis. The ultimate sensitivity down to single atom/molecule detection has been demonstrated under saturation conditions in many analytical applications with lasers.In real analytes the scheme of the energy levels is rather complicated. Thus, there may be many channels of radiative and collisional transfer of the absorbed laser energy. The complex structure of the energy levels scheme offers different combinations of excitation and detection of a particular species, but nevertheless the simplest scheme described above gives correct qualitative presentation of the analytical signal behavior.Details on the optimization of laser parameters for different analytical techniques can be found in [1], [2], and [3].

2. Tunable Lasers

In most analytical applications, tunable lasers are used. Tunability allows selective excitation of a particular analyte in a complex mixture and relatively simple "switching" from one analyte to another. For conventional sources with fixed spectral lines it is often necessary to use different lamps for different analytes.For a few laser analytical techniques (molecular fluorimetry of liquids, spontaneous Raman scattering), fixed wavelength lasers are used. The following discussion is limited to the two most widely used types of tunable laser.



2.1. Dye Lasers [4]

For a long time the dye lasers were the most popular type for atomic and molecular analysis. The active medium of these lasers is an organic dye in an appropriate solvent. About 20 different dyes cover the spectral range from ca. 320 to ca. 900 nm.Typically, dye lasers are pumped by an intense fixed-frequency laser, the most often used being nitrogen (337 nm), argon ion (488.0, 514.5 nm), XeCl excimer (308 nm), and frequency-doubled and frequency-tripled Nd-YAG (530, 353.3 nm).Dye molecules have broad emission bands. The optical resonator of a dye laser includes a spectrally selective element (grating, prism, interferometer, Lyot filter, etc.) which narrows the dye laser spectral line. By appropriate adjustment (e.g., rotation of the grating) one can smoothly tune the laser wavelength within the luminescence band of a particular dye. The tuning range may be 30 – 40 nm. At the extremes of the tuning range the energy of a dye laser falls off.The organic dyes used for lasers can be classified on the basis of molecular structure: terphenyls (spectral range: 312 – 400 nm); coumarines (420 – 580 nm); rhodamines (570 –720 nm), oxazines (680 – 860 nm). Each class consists of several dyes with slightly modified chemical structure and, hence, slightly shifted absorption and luminescence bands. There are special dyes for UV and near IR.The most utilized solvents are ethanol, methanol, cyclohexane, ethylene glycol, and dioxane. For different solvents, the tuning curves of the same dye may be shifted by 10 – 15 nm.The dyes for the UV spectral range (below 380 – 400 nm) are not photochemically stable. In most analytical applications, intense UV radiation below 360 nm is obtained by nonlinear optics, i.e., the generation of harmonics and sum frequencies of the visible dye laser radiation in nonlinear crystals. The most widely employed crystals are KDP, ADP, LiF, LiIO3 , and BBO. For example, the second harmonic of a rhodamine dye laser (fundamental band near 640 nm) is a more practical way to obtain tunable UV radiation near 320 nm rather than direct lasing of an unstable UV dye laser.The efficiency of energy conversion of the pumping laser radiation in commercially available lasers varies from a few percent to ca. 20 %, depending on the type of dye and the spectral range.


kuritsyn


2.2. Semiconductor Diode Lasers [5]

In the mid-1960s, tunable semiconductor diode lasers (DLs) were introduced into spectroscopic and analytical research. In conventional DLs the laser action can occur when a forward bias current is applied to a p – n junction. Photons are generated in the process of electron – hole recombination. The wavelength of laser radiation is determined by the energy gap between conduction and valence bands, which, in turn, is determined by the chemical composition of the semiconductor. Wavelength tuning is possible by varying the injection current and by the temperature of the laser.At first, DLs based on the IV – VI (Pb-salt) materials were used for analytical applications. These lasers operate in the 3 – 30 µm region, where the fundamental absorption bands of most molecules lie. Linewidths as small as 2 Ч 10–6 cm–1 were observed for the DLs, but in the commercial systems used for analytical applications the DL linewidth is determined by the quality of temperature stabilization. Usually the temperature is stabilized at a level of 10–2 – 10–3 K, which corresponds to a linewidth of ~ 10–3 cm–1. This width is much smaller than the widths of molecular absorption lines at atmospheric pressure and even smaller than Doppler broadening for light molecules. The IV – VI DLs operate at cryogenic temperatures. In laboratory high-resolution spectrometers operating temperatures of 10 – 120 K are usually provided by closed-cycle Stirling microrefrigerators. Now lasers operating above 77 K are available at most wavelengths, and in analytical instruments temperature-controlled liquid-N2 Dewars are the cooling method of choice. Typical emitting powers are 100 – 300 µW, and cw lasers with powers of 1 mW can also be fabricated.Recently, the characteristics of DLs operating in the visible/near-IR (NIR) region have been greatly improved. High quality lasers, made from the III-V group of semiconductor materials, are now commercially available in a number of discrete spectral ranges between 630 nm and 2.0 µm. In principle, lasers can be made at any wavelength, but because of the market demand the DL are commercialized mainly for specific spectral ranges. These lasers can work in cw mode at temperatures accessible with thermoelectric coolers, generally – 40 to 60 °C. The typical linewidth of a free-running DL is 20 – 100 MHz. Commercially available edge-emitting Fabry – Perot (FP) DLs yield singe-mode radiation up to 150 mW in the 670 – 870 nm spectral range and up to 30 mW in the 630 – 670 nm range. DLs, working in the 630 – 780 nm region, have been shown to be very attractive for atomic absorption analyzers.Compared with other lasers, the output radiation from DLs is highly divergent. The emission angles are typically 10° and 30° parallel and perpendicular to the laser junction, respectively, and may vary from sample to sample. These beam features can be corrected with appropriate collimating and anamorphic beam-conditioning optics.The drawbacks of the FP-DLs are the frequency drift and mode jumping, therefore other types of lasers such as distributed feedback (DFB), distributed Bragg reflector (DBR), external cavity (ECDL), and vertical cavity surface emitting lasers (VCSELs) have been developed. In a typical DFB laser, an index grating is fabricated directly on a chip alongside the active layer, providing



distributed feedback only for a selected wavelength specified by the grating. A DBR laser uses the same principle, but the grating is placed beyond the active region. DFB/DBR lasers provide single-mode operation, can be continuously tunable by current and temperature, and exhibit no mode hops. The drawbacks are limited tuning range and higher cost compared with simple FP lasers.Wavelengths longer than 1 µm are attractive for molecular analysis. Commercially available DLs for this region include: InGaAsP/InP (1.3, 1.55 µm FP structures, 1.2 – 1.8 µm DFB and EC lasers). For wavelengths longer than 1.8 µm, DLs are made from compounds containing antimonides such as AlGaAsSb, InGaAsSb, and InAsSb. Lasing in the 2 – 2.4 µm region at room temperature is available, and the spectral range up to 3.7 µm at liquid N2 is achieved.Vertical cavity surface emitting lasers (VCSELs), in contrast to conventional FP lasers, emit from their top surface. A conventional VCSEL consists of a multi-quantum-well (MQW) active region placed in a cavity between two DBR mirrors. These mirrors control the wavelength and beam divergence. For typical VCSELs, the far field beam characteristic will be a near Gaussian shape, with a beam divergence less than 12°. The narrow beam divergence, circular symmetry, and lack of astigmatism greatly simplify the optical design with these type of DLs. Currently, VCSELs operating in cw mode at room temperature in the visible/NIR range up to 2.3 µm can be produced.An alternative approach for tunable single-mode operation is to build an external cavity. In this case, a FP laser is placed into an optical cavity, and the wavelength is tuned by using a dispersive element, such as a grating. The wavelength can be tuned continuously over many wavenumbers (20 – 50 cm–1) by rotating the grating. The linewidth depends on the construction of the external cavity and its stability and is usually less than 1 MHz.To obtain generations in the regions of mode-hops for conventional FP lasers a simple design with a small external cavity was proposed. A glass plate mounted in front of the diode chip acts as the low-reflectivity mirror of an extended resonator and helps to select particular longitudinal modes within gain profile, even where laser action is not possible without the feedback.Recently developed multi-electrode lasers are very promising devices for multielement sensors. These lasers separate the wavelength selection, gain, and phase control regions of the laser cavity and are available in a number of different configurations. A tuning range over 100 nm around 1.5 µm has been demonstrated. These tuning ranges are equivalent to EC devices, thereby simplifying present multi-laser sensor concepts by allowing comparable wavelength coverage with a single laser.Frequently conversion is used to extend the available spectral range of DLs. Second harmonic generation (SHG) in bulk non-linear crystals, such as LiIO3 and KNbO3, can provide up to 0.1 µW of radiation in the 335 – 410 nm range and up to 1 – 3 µW in the 410 – 430 nm range. The generation of sum frequencies (SFG) in non-linear crystals can open the way to tunable radiation in the UV/visible part of the spectrum. Over the past several years, progress in the development of quasi-phase matching structures (QPMS) and waveguides, based on non-linear crystals, has been achieved. Conversion efficiencies in both types of such non-linear converters are much higher than in bulk crystals, thus single-mode powers up to some mW in the UV/blue range can be produced. Mid-IR radiation can be also produced by difference-frequency mixing of two NIR lasers. Difference-frequency generation in periodically poled LiNbO3 and bulk AgGaSe2 or AgGaS2 has been demonstrated for generating tunable radiation between 3.2 and 8.7 µm.In the mid-IR, very promising substitutes to lead-salt lasers are quantum-cascade lasers (QCLs),



made using III-V compounds. In QCL the active region consists of several gain quantum well (QW) layers separated by the digitally graded electron-injecting regions. Electrons are injected into the high energy level of the first gain layer, fall down to the lower level with photon emission, then through the digitally graded region are injected into the high level of the next gain layer and the process repeats. Quantum efficiencies higher than 100 % are possible in such structures. It is important that such cascades can be organized between quantized states in the conduction band only. The close spacing of these quantized states allows light to be emitted in the mid-IR range. Currently, room temperature lasing up to 8.2 µm in structures fabricated from AlInAs/InGaAs can be obtained. QCLs with wavelength as long as 11 µm at cryogenic temperatures have been demonstrated.In 1999 blue DLs became commercially available. InGaN MQW DLs were demonstrated to have a lifetime of more than 10 000 h under conditions of room temperature cw operation. Single-mode emissions were produced in the region of 390 – 450 nm. Maximum output power is about 30 mW. With an external cavity the narrowband 1 MHz laser linewidth can be coarsely adjusted over a tuning range of 2.5 nm and then fine-tuned by a piezo stack over a more than 20 GHz range without mode hops.

3. Laser Techniques for Elemental Analysis

In this chapter we shall discuss laser analytical techniques for elemental analysis, i.e., for detection of impurity atoms of a particular element in a sample.

3.1. Laser-Excited Atomic Fluorescence Spectrometry [2], [3], [6]

3.1.1. Physical Principles



The principles of the laser-excited atomic fluorescence (LEAF) technique are very simple. A liquid or solid sample is atomized in an appropriate device. The atomic vapor is illuminated by laser radiation tuned to a strong resonance transition of an analyte atom. The excited analyte atoms spontaneously radiate fluorescence photons and a recording system registers the intensity of fluorescence (or total number of fluorescent photons). The extremely high spectral brightness of lasers makes it possible to saturate a resonance transition of an analyte atom. Therefore, the maximum fluorescence intensity of the free analyte atoms can be achieved while the effect of intensity fluctuations of the excitation source are minimized. Both factors provide the main advantage of LEAF — extremely high sensitivity. The best absolute detection limits achieved in direct analysis by LEAF lie in the femtogram-range, which corresponds to concentrations down to pg/mL.Several different excitation – recording schemes used in LEAF are shown in Figure (2). The choice of a particular scheme is dictated by the structure of the atomic energy levels, the sources of background and noise, and spectral interference with the major sample components.The resonance scheme (Fig. (2) A) has the great advantage of a possible cyclic interaction of analyte atoms with the laser radiation and, hence, a high yield of fluorescence photons. The highest sensitivity of LEAF, the detection of a single free atom, has been demonstrated with Na atomic beams and cw laser excitation. This scheme has a serious disadvantage — a high level of scattered laser light and, hence, high background. For a few elements (Na, Cd, etc.) the resonance scheme is the only possible one. For the most elements this scheme is not optimal in real analytical conditions when pulsed dye lasers are used for excitation.The Stokes direct-line or collisionally assisted Stokes direct-line schemes (Fig. (2) B, C) are often employed (Tl, Pb, Au, Ag, Co, etc.). Their main advantage is that different wavelengths are used for excitation and fluorescence. Because of this, the laser scattered light can be easily separated from the weak fluorescence radiation by cut-off optical filters or monochromators. The analytical potential of collisionally assisted detection schemes depends strongly on the efficiency of collisional mixing between the neighboring levels. Typically this efficiency is high enough for energy gaps between the levels of ca. 1000 cm–1. For some elements with low-lying excited states (Ga-type) the anti-Stokes direct-line scheme (Fig. (2) D) is very useful. An advantage of this scheme is a blue shift of the fluorescence and an appropriate reduction in background, caused by scattered laser light. In some cases, double-resonance excitation by two lasers with different wavelengths is used.The possibility to select the optimal excitation – detection scheme often simplifies the problem of spectral interference with the major components, providing the second advantage of LEAF — high selectivity.

3.1.2. Atomizers

Graphite electrothermal atomizers (GETA), flames, glow discharges, and laser plumes are the most widely used types of atomizers in modern LEAF analytical practice.GETA. Graphite electrothermal atomizers are of two types: open and closed. The main unit is a graphite crucible (cup, profiled rod, boat, furnace) held between massive water- cooled electrodes. Typical sample volumes are 2 – 100 µL. The maximum temperature of a crucible



depends on its mass, the electrical power supply, and the type of graphite used. In most applications, it is 2700 – 2900 °C. In open and closed atomizers an inert gas atmosphere is used to minimize quenching of the excited analyte atoms and sublimation of the hot graphite crucible. Graphite furnaces have proved preferable because of the high temperature of the analytical zone and, hence, less chemical interference. Low-pressure noble gas in a closed GETA has been used to minimize matrix interference.Flames. The flames used in LEAF are very similar to those used in AAS. In most applications, cylindrically symmetric burners are used since the dimensions of the analytical zone in LEAF do not exceed 10 mm. Quenching of the excited atoms by components of the flame is a serious problem. As oxygen is one of the most effective quenching agents, flames with minimum oxygen concentration should be used (e.g., an oxygen – argon – hydrogen flame). The high temperature of the analytical zone increases not only the fluorescence signal, but also the background, caused by flame emission. The optimal position of the analytical zone on the flame axis maximizes the signal-to-noise ratio. The background radiaton is usually much higher in flames as compared with GETA because of fluctuations in the optical density of the analytical zone (Rayleigh scattering of the excited laser radiation), luminescence of organic compounds in the flame, Mie scattering by dust particles, etc. As a result detection limits are typically much worse for flame-LEAF than for GETA-LEAF. On the other hand, the relative simplicity of design and the stationary analytical regime are definite advantages of flames.Glow Discharges. Glow discharges (GD) have proved very useful atomizers for direct analysis of solid samples by LEAF. In GD the surface of a solid sample is bombarded by ions of a noble gas, which is continuously pumped through the atomizer at low pressure. Depending on the type of GD, the gas pressure ranges from 10–1 Pa to 10 kPa. At such pressures collisions between analyte or matrix atoms and atoms in the gas phase are relatively scarce, which substantially reduces the probability of gas phase reactions and fluorescence quenching. The sample to be analyzed is used as the cathode. The anode – cathode voltage is typically several hundred volts, the energy range of the bombarding ions is 10–17 – 10–15 J (0.1 – 10 keV). Both direct current and pulsed discharges are used. In the latter case the discharge is synchronized with the laser pulse, with an appropriate time delay, depending on the distance between the sample surface and the analytical zone and the mean energy of the sputtered atoms.The process has substantial advantages over thermal atomization. The sputtering yields of different elements differ by no more than one order of magnitude, whereas their vapor pressures may differ by several orders of magnitude. Sputtering yields are much less sensitive to the chemical composition of the matrix as compared to thermal atomization efficiencies. Ion sputtering avoids the drawbacks of GETA (e.g., chemical interference, recombination of free atoms into molecules and clusters, carbide formation). The problem of surface contamination can be easily solved by preliminary cleaning in a discharge. After the surface layer is removed, sample sputtering stabilizes and the bulk composition of a homogeneous sample or the layer-by-layer composition of an inhomogeneous one can be analyzed.

3.1.3. Limits of Detection

Like most spectroscopic analytical techniques, LEAF needs a calibration procedure. In common with other modern analytical techniques, determination of limits of detection (LOD) poses a new problem. Commonly, LOD are defined as the concentration (or absolute amount) of analyte



that gives an analytical signal three times the standard deviation of the blank. The analyte concentration in the purest solvent used for blank measurements can often be detected by modern analytical techniques. One should differentiate the LOD determined by the limited purity of the solvent from the limit determined by the intrinsic characteristics of the analytical technique (nature of background, electronic noise, recording sensitivity, etc.). The first LOD is measured with the laser tuned to the resonance line of the analyte (on-line LOD). The second is measured with the laser detuned (off-line LOD). The detection power of most laser analytical techniques is very high and most published LOD are off-line values.The best LEAF LODs currently available are listed in Table (1) [7].

3.1.4. Real Sample Analysis

Trace amounts of Pb, Cd, and Bi at femtogram levels have been directly determined in ancient ice and recent snows in the Antarctic by LEAF – GETA. Toxic metal concentrations in deep Antarctic ice cores give unique information about the preindustrial tropospheric cycles of these metals and about recent anthropogenic sources of heavy metals. Concentrations of Pb and Cd in the samples analyzed were 0.3 – 30 pg/mL and 0.08 – 2 pg/mL, respectively. The values measured by LEAF – GETA were in good agreement with those measured by isotope dilution mass spectrometry (Pb) and graphite furnace AAS (Cd). Both traditional techniques required elaborate chemical pretreatment and large sample volumes (50 – 200 mL). The sample volume in LEAF experiments was 20 – 50 µL.The most successful analysis of real samples by LEAF was done with sample atomization in a graphite furnace. The high detection power (absolute LODs at and below femtogram levels) and wide linear range (up to 5 – 7 orders of magnitude) of LEAF were combined with modern furnace technology:Fast heating,platform atomization, matrix modification, and direct solid sampling. As a result low LODs, reasonable accuracy and precision, less matrix interference, and simpler sample pretreatment (if necessary) were achieved (Table (2)). The potential of LEAF with glow discharge atomizers (LEAF-GD) have been demonstrated. The LODs for Pb and Ir as low as 0.1 and 6 ng/mL, respectively, were achieved with hollow cathode GD and aqueous standards. With a similar atomizer 40 ng/g LOD was obtained for direct analysis of Pb traces in Cu-based alloys. With the more efficient magnetron GD even lower LODs were achieved: 3 ng/g for Pb in Cu alloys, 0.8 ng/g and 2 ng/g for Si in In and Ga, respectively.Analysis of pure gases by LEAF is much less developed. Nevertheless, some very impressive results have been demonstrated. Traces of Pb and Tl in the air of a normal laboratory and a class-100 clean room were detected by LEAF. Air from the room was pumped through a tantalum jet, fixed in a hole inside a graphite tube. The heavy particles impacted onto the tube wall opposite the jet. After pumping a certain volume of ambient air in a particular time interval the impacted metal traces were detected by graphite furnace LEAF. LODs of 0.1 pg/m3 and 0.01 pg/m3 for Pb and Tl, respectively, were achieved. The Pb LOD is two orders of magnitude better than the LOD for graphite furnace AAS. The measured concentrations of Pb and Tl in laboratory air were 1.25 ng/m3 and 4.3 pg/m3, respectively. The same values in clean-room air were about 0.3 –0.6 ng/m3 and 0.9 – 1.0 pg/m3. The measured concentrations of the same metals inside a clean bench, in the clean room were 6.5 pg/m3 and 0.035 pg/m3.Detection of atoms and molecules which have transitions in the vacuum UV region is a serious



challenge for any spectroscopic technique. One way to solve the problem is excitation of a species, not from the ground state, but from a higher metastable state which can be efficiently populated by electron impact in a gas discharge. The population of a metastable level by electron impact followed by LEAF detection was successfully applied for the detection of Ne and N2 traces in He, and for detection of NO and NO2 traces in He, Ar, N2 , and air. LODs of ca. 10–7 – 10–8 vol % were obtained. These LODs are 100 – 1000 times better as compared with any traditional analytical technique.

3.2. Laser-Enhanced Ionization [3]


Laser-enhanced ionization (LEI) spectroscopy is based on the difference in collisional ionization rates of atoms in their ground and excited states. If a d.c. voltage is applied across the atom reservoir and analyte atoms are optically excited by laser radiation, the enhanced probability of collisional ionization can be detected as a change of current (or a change of voltage across the load resistor). The LEI signal is linearly proportional to the ionization rate of the excited analyte atom, which in turn depends on its ionization potential, the nature of its collision partners, and the temperature of the analytical zone. The maximum sensitivity of LEI occurs when both the probabilities of ionization and ion collection are maximal (close to unity). The ionization efficiency depends exponentially on the energy difference between the upper excited state and ionization potential of the analyte atom. The main advantage of LEI as compared with LEAF is the very high probability of ion collection (close to unity in the optimal scheme); the collection efficiency of spontaneously emitted fluorescence photons is much lower.Different excitation schemes for LEI are shown in Figure (3). The simplest and most widely used is the one-step excitation scheme (Fig. (3) A). Elements with low ionization potential (mainly alkali metals) can be efficiently ionized by the one-step scheme with dye lasers in the visible range. For a number of elements (Cd, Mg, Pb, etc.) the one-step scheme can be efficient with excitation in the UV (harmonics of visible dye lasers). Excitation by UV photons below 300 nm may increase the background caused by nonselective one- or two-step photoionization of the thermally excited species in the analytical zone. The two-step excitation scheme (Fig. (3) B) is more complicated — thus more expensive — as it utilizes two tunable lasers, but it has significant advantages. Ionization efficiency increases up to three orders of magnitude compared with one-step excitation. The background does not change significantly if both lasers operate in the visible range. Selectivity increases dramatically as the selectivities of the single



steps are multiplied in the two-step process. LEI is an efficient technique only if collision rates are high. Thus, the excitation – ionization scheme (Fig. (3) C), with noncoinciding levels of the first and second steps, might be efficient owing to effective collisional coupling of both levels.

3.2.2. Atomizers

The most commonly used atomizers for LEI are atmospheric flames. Flames have relatively high temperatures in the analytical zone and, hence, high efficiency of analyte atomization. Thermally excited molecular compounds in flames provide efficient ionization of the optically excited analyte atoms. All types of flames used in atomic absorption and emission spectrometry have been investigated in LEI. Relatively hot hydrocarbon flames such as C2H2/air or C2H2/N2O are preferable for LEI because of the high degree of atomization of the elements. A large number of elements can be detected by LEI with C2H2/air flames; the hotter C2H2/N2O flame is more suitable for the detection of refractory elements.Few numerical values can be given for LEI in flames. In a flame at 2500 K, the rate of ionization of an element is enhanced ca. 100-fold for each electron volt (1.6Ч10–19 J) of laser excitation energy. If the energy gap between the excited level and the ionization continuum is less than 1 eV, the probability of ionization within 10 ns is close to unity. If cw laser excitation is used 100 % ionization can be achieved for an energy gap of ca. 3 eV (and less).Other atomizers such as graphite furnaces, ICP, and GD have also been investigated for LEI. Argon buffer gas, typically used in graphite furnaces, is a much poorer collisional partner than the molecular constituents of flames, so collisional ionization is less efficient. On the other hand, good analytical results have been obtained with a combined flame – rod atomizer. A small volume of sample (positioned in a hole in a graphite rod) combined with the high temperature of the analytical zone (located above the rod in the surrounding flame) provided a low absolute detection limit in the picogram – femtogram range. ICP and GD proved to be electrically noisy for LEI.

3.2.3. Limits of Detection

The best LODs for one- and two-step LEI in flames are listed in Table (3).

3.2.4. Real Sample Analysis

The potential of LEI has been investigated by analysis of Standard Reference Materials (SRMs) at the National Institute of Standards and Technology (NIST). Traces of Co, Cu, Cd, Mn, Ni, and Pb were measured in different SRMs of water; Mn and Ni traces in bovine serum; Mn, Ni, and Fe traces in apple and peach leaves; Tl and Ni traces in river sediments; and Na, K, Ca, and Mg traces in rainwater.The concentrations of the alkali metals Cs, Li, and Rb were detected by LEI in different types of reference rock samples. Rock samples were dissolved by standard methods and analyses



performed without preconcentration, with pure aqueous standards for calibration. Concentrations were determined by both calibration curve and standard additions methods. The results obtained by the two techniques coincided within experimental errors. Concentrations varied in the range 0.4 – 17.0 ppm and were in a good agreement with the certified values for the reference samples.The LEI technique has been applied to the analysis of pure materials for the semiconductor industry. The concentrations of Cr, Fe, and Ni in Cr-doped GaAs disks were measured with the simplest pretreatment (dissolution in high-purity nitric acid and dilution with pure water). After dilution, a concentration of 0.1 ng/g in solution corresponded to 1 µg/g in the solid. Impurity concentrations were 4, 8, and 2.3 µg/g, respectively.The rod – flame atomizer avoids the general disadvantage of flame-based analytical techniques — the necessity to dissolve the solid sample. The potential of two-step LEI with the rod –flame atomizer was investigated by analysis of Cu traces in Ge and In traces in CdHgTe. The LODs as low as 1 pg/g for In and 0.5 ng/g for Cu were achieved in direct analysis of 10-mg solid samples. Measured values in real samples varied in the range 30 – 70 ng/g for Cu and 2.6 – 3.4 ng/g for In.Laser-enhanced ionization is a powerful, versatile and relatively simple technique which provides LODs in the ng/g – pg/g range with large linear dynamic range. A serious drawback of the technique is its susceptibility to interference: Thermal ionization of easily ionized matrix components (mainly alkali metals); photoionization of the thermally excited major components (especially if UV laser light is used for excitation); thermal ionization from levels excited by the spectral wing of an intense laser line; and ionization of the molecular components of the flame (especially NO). Interference may be greatly reduced or avoided by the appropriate choice of excitation wavelength, by the two-step excitation scheme, by specially constructed collecting electrodes; by optimal location of the analytical zone within the flame. To reduce interference one should investigate precisely the origins of noise and background for a particular analyte and matrix.

3.3. Resonance Ionization Spectroscopy [2]


Resonance ionization spectroscopy (RIS) is based on resonance multistep excitation of high-lying levels of free analyte atoms and their subsequent ionization. Ionization may be caused by the photons of the final laser step (photoionization) or by an external electric field (field



ionization). Sometimes the latter technique is referred to as field ionization laser spectroscopy (FILS). Multistep excitation of atoms may be effective if the rates of stimulated transitions to each intermediate level are much higher than the total decay rates (radiative and collisional) of any intermediate level. In contrast to LEI, RIS is efficient only if negligible collisions occur in the analytical zone. As in the case of LEI, the high sensitivity of RIS is based on the high efficiency of ion collection (close to unity). Thus, the overall efficiency of neutral atom detection by RIS depends on the efficiency of stepwise excitation and final ionization. Maximum sensitivity is achieved if all the intermediate transitions are saturated and the ionization efficiency is close to unity. In optimal conditions, a single free atom may be detected with a probability close to unity.The three most widely used schemes for RIS are shown in Figure (4). They differ from each other in the final ionization step. Nonresonant photoionization to a continuum (Fig. (4) A) can be efficient if the energy flux of the ionizing laser is high enough (0.01 – 1 J/cm2), as the nonresonant ionization cross sections are relatively low (10–17 – 10–19 cm2). For comparison, typical cross-sections of resonance optical transitions are 105 – 106 times higher.The photoionization scheme via the so-called autoionization state (AIS) is shown in Figure (4) B. The AISs are discrete states of the inner excited electrons with energies above the ionization potential of the atom. On the one hand, the cross section of a transition to AIS may be several orders of magnitude higher than direct photoionization to the continuum (Fig. (4) A); on the other hand, the AIS may decay to the continuum (i.e., the atom is ionized) with a typical lifetime of a few nanoseconds. Thus, in the field of an intense laser pulse with a duration of ca. 10–8 s, the probability of excited AIS depletion and ionization may be close to unity. Ionization via AIS is highly efficient for some elements (Gd, Lu).Ionization through high-lying Rydberg states (Fig. (4) C) has been found to be very effective.Rydberg states close to the ionization limit can be efficiently ionized by an electric field pulse of moderate amplitude. The dependence of ionization yield of a particular Rydberg state on the electric field amplitude has a characteristic Z-profile with a pronounced threshold Ecrit. For electric fields above Ecrit the ionization yield is nearly constant and close to unity. Both the excitation cross section and Ecrit depend strongly on the principal quantum number of the Rydberg state (n*). For higher n*, Ecrit is lower (an advantage), but the excitation cross section is much lower (a disadvantage). Therefore, compromise values of n* are used, which for most elements lie between 15 and 30. For such n*, excitation cross sections are of the order 10–14 – 10–15 cm2 and Ecrit values are 1 – 10 kV/cm. For analytical applications, ionization via Rydberg states has proved to be an ideal scheme for RIS. Because of the relatively high excitation cross sections, maximum sensitivity can be realized with tunable lasers of moderate pulse duration and average power, and with reasonable electric fields. The effectiveness of AIS is limited by the absence of AIS near the ionization potential of many elements and the lack of appropriate spectroscopic information.The fact that RIS measures charged particles is an important advantage. Highly efficient signal ion collection is possible by appropriate electromagnetic focusing. Second, discrimination of signal ions from background ions can be achieved by mass separation methods. These two factors improve both the sensitivity of RIS and its accuracy.In most analytical RIS applications two- or three-step excitation is used. This provides extremely high selectivity (up to 1010 – 1012) as the overall selectivity of a multistep process is a product of one-step selectivities.



3.3.2. Atomizers

Vacuum or low-pressure atomizers are used in RIS to obtain collisionless interaction of the free analyte atoms with resonance laser radiation. Different types of GD and vacuum electrothermal atomizers (VETA) are most commonly used in RIS. In some specific analytical applications (detection of traces of rare radionuclides) RIS is used on-line with fast atomic beams.In many applications of RIS – VETA the generated ions are detected by a secondary-emission multiplier incorporated in the atomizer. The vacuum should be better than 10–3 Pa throughout the analytical volume and the path of free ions from the interaction zone to the multiplier. This limits the speed of sample evaporation and dictates the efficiency of the pumping system. Both unified vacuum systems and systems with differential pumping of the analytical volume and free flight zones are used. In these respects the strategy of electrothermal atomization of a sample in RIS differs significantly from the traditional one used in graphite furnace AAS. In the latter technique fast atomization (1 – 10 s) gives the best analytical results. In RIS, very slow atomization (up to several minutes) is typical.In some applications, a proportional counter operating near atmospheric pressure is used. In this case, the electrons created in the process of atomic resonance ionization are detected. By appropriate choice of gas mixture and accelerating voltage, an electron cascade with a magnification factor of about 104 can be achieved. This gain is high enough for single photoelectron detection.For maximum sensitivity and isotope selectivity RIS is often combined with time-of-flight (RIS-TOF) or quadrupole mass spectrometry (RIMS).

3.3.3. Limits of Detection: Real Samples

The high sensitivity of RIS has been demonstrated in the analysis of pure semiconductor materials, biological samples, seawater, and environmental samples. In some cases, direct analysis was possible; for some matrices, sample pretreatment was used.Traces of Na, Al, and B in high purity Ge were detected by RIS with direct electrothermal atomization of solid Ge chips in vacuum. For all these elements, two-step excitation of Rydberg states and electric field ionization were used. The measured concentrations of Na, Al, and B in real samples were 0.2, 1.0, and 0.2 ng/g while the LODs were 0.05, 0.01, and 0.05 ng/g, respectively. In the case of B, the strong ion background seriously limited the sensitivity and selectivity of traditional RIS. This background was caused by efficient two-step nonresonance photoionization of Ge atoms (the energy difference between the excited 2s23s level of B and the 4s24p5s level of Ge is only 20 cm–1). Both analytical characteristics were greatly improved by a special technique, with two sequential electrical pulses of different duration, polarity, and voltage.The distribution of ruthenium and rhodium traces in the ocean has been investigated by RIS –VETA. Both elements are rare and their distribution is of great geological and geochemical importance. The abundance of Ru and Rh in the Earth's crust is estimated to be 1 – 100 pg/g and can not be directly measured by current analytical techniques. The fire assay method and concentration by polymer sorbents were used prior to RIS analysis. Concentrations of Ru varied



from 1.3 pg/mL in ocean water to 7.6 ng/g in Red Sea sediments. Concentrations of Rh in ocean water were 3 – 10 pg/mL.The detection of 81Kr, produced by cosmic rays, is of great importance for dating samples from the Earth's crust, groundwater, and polar ice cores. Measurement of 81Kr in groundwater is a unique analytical problem as its concentration is ca. 1400 atoms per liter. The problem is further complicated by isotopic interference of stable krypton isotopes (82Kr/81Kr is about 1012). Prior to final analysis by RIS – TOF [8] several consecutive steps of field sampling and isotope concentration were necessary. All steps required extreme care to avoid sample contamination. Water samples of 50 – 100 L were necessary to detect 81Kr above the background. This value is more than 100 times less than for the routine technique of 39Ar dating.The environmentally important radionuclides 90Sr and 210Pb have been detected by RIMS – VETA. Two-step excitation of Rydberg states by two cw dye lasers, followed by nonresonant ionization by a cw infrared CO2 laser, was used. The solid samples were dissolved by standard methods. The LODs were 60 ag (6Ч10–17 g) for 210Pb and 30 ag for 90Sr. Absolute 210Pb concentrations of 0.12 – 0.14 fg were measured in brain tissue. Isotope selectivity of 109 for 90Sr/89Sr was achieved.Recently the diode lasers were successfully used in a multiphoton RIMS scheme for isotopically selective detection of strontium. The two-step resonance excitation of 90Sr was performed with two single-mode cw DLs (λ1 = 689.45 nm, λ2 = 688.02 nm). The excited 90Sr atoms were photoionized by a powerful Ar+ laser (λ = 488.0 nm). The ions were detected in a commercial quadrupole MS. Using graphite crucible atomization the absolute LOD of 0.8 fg and overall (optical + mass spectrometer) isotopic selectivity of > 1010 in the presence of stable 88Sr was achieved for 90Sr. No other non-radiochemical technique can realize such sensitivity and selectivity of 90Sr detection.A similar scheme was used for the spectroscopic studies on hyperfine components and isotope shifts of rare stable isotopes of calcium and the radionuclide 41Ca. Calcium atoms in an atomic beam were excited with single-frequency cw dye and titanium sapphire lasers and then photoionized with the 363.8 nm or 514.5 nm line of an Ar+ laser.The use of RIMS in chemical vapor deposition diagnostics has been investigated.

3.4. Diode Lasers in Elemental Analysis [6], [8]

3.4.1. Physical Principles of Absorption Spectrometry

Quantitative absorption measurements are based on the Lambert – Beer law:



where Pt is the laser power at wavelength λ, transmitted through a layer with absorption coefficient k(λ) and absorption pathlength l. The absorbed power ∆P is

. For small values of the absorption

k(λ)l < 1, this is approximated with .Theoretically, the minimum detectable absorption (MDA) is determined by the shot noise of detected radiation. In single pass measurements (l = L, where L is a sample absorption pathlength)

where η is the quantum efficiency of the photodetector, and B is a detection bandwidth. To obtain noise equivalent relative power variations, ∆Pmin/P0, smaller than 2 Ч 10–8 with a measurement time of 1 s, the laser photon flux should be at least 5 Ч 1015 photons/s. This corresponds to laser powers of 1 mW at 1 µm and 100 µW at 10 µm. In practice there are additional noise factors affecting the detection sensitivity, such as intensity fluctuations of the laser (laser excess noise) and technical fluctuations. Thus, for typical DLs, the value of ∆Pmin/P0 = 1 Ч 10–8 can be considered as the lowest, fundamentally limiting one.Usually, absorption measurements are not considered to be very sensitive since the signal P0k(λ)l is a subtle variation of a large baseline P0. In order to have zero baseline, sample or wavelength/frequency modulation (WM/FM) techniques can be used. In the case when λ is modulated with frequency f, a signal at the n-th harmonics of f is simply proportional to P0knf(λ)l, where knf is the amplitude of the corresponding harmonics of k. As

, where σ(λ) is the absorption cross section,

(see Section 1.3. Peculiarities of Resonance Laser Radiation Absorption), then the signal after the phase-sensitive detector is directly proportional to the concentration of absorbing particles N. Another important advantage of WM/FM is the detection of a signal at high frequency (> 10 kHz), where the low-frequency flicker noise is greatly reduced.Besides reducing noise, improvements in the MDA can be achieved by enhancing the signal. This can be done by increasing the absorption pathlength using multipass cells. By an appropriate mirror arrangement a DL beam can be directed many times through the absorbing layer, increasing the overall absorption with an appropriate decrease in MDA. The number of passes is limited by the quality of the mirrors, which determines the cell transmission, or by interference effects.

3.4.2. Atomic Absorption Spectrometry with Diode Lasers [6], [8]



UV/visible/NIR semiconductor diode lasers (DLs) as sources of tunable narrowband resonance radiation have been discussed in Section 2.2. Semiconductor Diode Lasers . The practical and technical advantages of visible/NIR DLs are ease of operation, small size, room-temperature operation, and low price. In comparison with hollow cathode lamps (HCLs), they have such advantages as high spectral power, spatial coherence, and tunability. Owing to these features, detection limits, dynamic range, and selectivity of analysis are all significantly improved.The presently available powers of DLs are several orders of magnitude higher than the powers of the best commercial HCLs. In contrast to an HCL, a DL emits a prominent single narrow line. This feature dramatically simplifies the construction of an analytical device because one does not need a monochromator, which is used in commercial AAS instruments for isolation of the analytical line from the spectral lines of the HCL buffer gas and unwanted lines of the cathode material. In the case when optimal experimental conditions are realized, more than 1 mW of laser radiation power can be obtained at the photodetector. It has been shown that a near fundamental shot-noise limit can be achieved and relative power variations ∆Pmin/P0, which can be measured with DLs, can be as low as 10–8.The LODs of diode laser atomic absorption spectrometry (DLAAS) have been greatly improved using a WM technique. In conventional atomic absorption spectrometry with HCLs, low-frequency (10 – 60 Hz) amplitude modulation (mainly by mechanical choppers) is used. In contrast, the wavelength of the DL can be easily modulated with much higher frequencies, in DLAAS usually up to 10 – 100 kHz. By recording the analytical signal at the harmonics of the modulation frequency, usually the second one (2f-mode), one can dramatically suppress the 1/f noise, thus decreasing the LOD. Even in the first simple measurements with a DL power of ca. 100 µW, a modulation frequency of 5 kHz, and with 2f-mode of recording, absolute LODs in the femtogram range have been achieved experimentally. These LODs correspond to measured values of ∆Pmin/P0 ≈ 10–4 – 10–5 (compared with 10–2 – 10–3 for conventional AAS).Further improvement of WM-DLAAS has been demonstrated for the case when both the radiation source and the absorption are modulated. Such a situation was realized with a microwave induced plasma (MIP) or a direct current plasma (DCP) as the atom reservoir. The double-beam double-mudulation approach with logarithmic signal processing (LSP) and 2f-mode recording permitted a complete elimination of the background, caused by non-selective absorption, low-frequency fluctuations, and drift of the baseline and laser intensity. A shot-noise limited MDA as low as 3 Ч 10–8 for a 1 s time constant was experimentally realized. The linear dynamic range can be also substantially extended with this technique, in the best cases up to six – seven orders of magnitude as compared with less than two orders of magnitude for conventional AAS.Applications of DLs in analytical atomic spectrometry are collected in the following list (numbers in parentheses are analytical wavelengths in nanometers):

Diode laser atomic absorption spectrometry (DLAAS):Graphite furnace

Al (396.15), Ba (791.13), Cs (894.35), K (769.90), La (657.85), Li (670.78), Rb (780.03), Sr (689.26)

FlameCs (425.44, 852.11), Cr (425.44), K (766.49), Li (670.78), Rb (780.03), Ti (399.97, 842.65)

Discharge (direct current plasma (DCP) or microwave induced plasma (MIP))Cl (837.60), F (739.87), O (777.18)



Laser induced fluorescence (LIF):Li (670.78), Rb (780.03)

Isotope selective detection:Li, Pb, wavelength modulation diode laser atomic absorption spectrometry (WMDLAAS)235U/238U in glow discharge, optogalvanic detection235U/238U in solid samples, laser ablation + LIF with DL excitation

Physical vapor deposition process control:Y (668), external cavity diode laser (ECDL), high frequency modulation spectroscopyAl (394), second harmonic generation (SHG) of ECDL

Control of semiconductor films growth:Al (394), Ga (403), In (410), SHG in bulk crystals

Specific detection of haloform components:Cl (837.60), Br (827.24), MIP or DCP, GC-WMDLAAS, double-modulation

Environmental analysis:CrIII/CrVI speciation in deionized and drinking water

Cr (427.48), SHG in LiIo3, flame, HPLC-WMDLAASCrVI in tap water

Cr (427.48), SHG in KNbO3, flame, HPLC-WMDLAAS, double-beam, LSPChlorinated hydrocarbons in oil from plastic material recycling

Cl (837.60), MIP, GC-WMDLAASChlorophenols in plant extracts

Cl (837.60), MIP, GC-WMDLAAS, double-beam, logarithmic signal processing (LSP)

Speciation of methylcyclopentadienyl manganese carbonyl (MMT) in gasoline, human urine and tap water

Mn (403.1), flame, SHG in LiIO3, HPLC-WMDLAAS, 4f, LSPAnalysis of chlorine in polymers

Cl (837.60), MIP, laser ablation + WMDLAAS, double-modulation, LSPResonance ionization mass spectrometry (RIMS):

La (657.85), three-step RIMS, DL – first stepSr (689.45, 688.02), double-resonance excitation by two DLs

The detection capabilities for various species differ considerably depending on the type of atomizer, oscillator strength of a transition, detection technique used. Multielement analysis with DLs has also been investigated. Several laser beams may be collimated and directed simultaneously through an absorbing layer in a conventional atomizer, either by a system of mirrors, or by optical fibers. The monochromator – photomultiplier system can be replaced by a simple semiconductor photodiode. Appropriate modulation of a wavelength of a particular DL, followed by Fourier transform or lock-in amplification analysis of the photodiode signal, makes it possible to process analytical signals of individual analytes. A six-element analysis with DLs and a commercial graphite tube atomizer has been demonstrated.



3.4.3. Element Specific Detection by DLAAS

Diode laser based analytical devices can be used as element selective detectors for gas/liquid chromatography (GC/LC). The atomic constituents of different molecular species in a sample can be measured in an atomizer, such as a flame or low pressure plasmas, e.g., an MIP or DCP. The GC-MIP/DCP in combination with DLAAS enables the detection of non-metals, such as H, O, S, noble gases, and halogens. Most of these elements have metastable levels, which are efficiently populated in a plasma, and which have strong absorption transitions in the red – NIR spectral region.Recently it has been possible to observe a definite trend in elemental analysis, from detection of the total content of a heavy metal in a sample to the detection of its different chemical compounds or oxidation states (speciation). This is because the essentiality or toxicity of the metals should not be attributed to an element itself but to element compounds with distinct biological, physical, and chemical properties. A compact and relatively cheap DL based instrument could be a reasonable alternative to the powerful but expensive ICP-MS, presently used for speciation. The potential of DLAAS as an element selective detector for speciation has been demonstrated by its coupling with high performance liquid chromatography (HPLC). This system has been used for speciation of chromium (III)/(VI) and organo-manganese compounds. An LOD of 30 pg/mL for CrIII and CrVI was obtained by use of 3 µW power frequency-doubled laser light, a double-beam configuration, and logarithmic processing of the signals. In the case of chromium, the LOD of HPLC-DLAAS is several times lower, than the LOD for HPLC-ICP-MS.

3.4.4. Isotope Selective Detection with Diode Lasers

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroscopy – DLAAS or laser induced fluorescence (LIF). For example, 235U and 238U ratios have been measured by Doppler-limited optogalvanic spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes.

3.4.5. Commercial System

The DLAAS devices are now commercially available from Atomica Instruments (Munich, Germany). The first system was prepared for the measurement of ultra-trace levels of Al in the semiconductor industry. The commercial laser diode module includes the DL with a heat sink for temperature tuning, the microoptics, and the non-linear crystal for SHG. The module has the size of a HCL. Since delivery, the module has successfully worked routinely under the conditions of an industrial analytical laboratory without any repair or maintenance. The semiconductor industry in particular is interested in the measurements of light elements, such as



Al, K, Ca, Na, in ultra pure water or chemicals. For example, the module designed for the detection of K provides an LOD of 0.5 pg/mL (10 µL aliquot). Similar compact and sensitive devices with electrothermal atomizers, flames or micro-plasma are currently available for a number of elements, such as Mn, Ni, Pt, Pd, Ca, Cu, Fe, Ga, the alkali elements, the halogens, C, O, and H. In forthcoming years, one can also expect the commercialization of specific element selective DLAAS detectors for GC or LC.

4. Laser Techniques for Molecular Analysis [6], [8]

4.1. Molecular Absorption Spectroscopy with Diode Lasers

4.1.1. Detection of Molecular Gas Impurities with Lead-Salt Diode Lasers [2], [9]

The Pb-salt-based DLs (Section 2.2. Semiconductor Diode Lasers ) efficiently generate tunable laser radiation in the region 3 – 30 µm, where the great majority of molecules have intense vibrational rotational absorption lines. As the Doppler-broadened absorption linewidths of molecules are of the order of (1 – 3) Ч 10–3 cm–1, DLs provide the best selectivity for linear absorption techniques.Both cw and pulsed (sweep) modes of DL generation have been widely tested in analytical applications. Many well-developed modulation techniques have been used with consequent improvement of analytical sensitivity. The best results were achieved with a high FM technique (f ~ 100 – 1000 MHz). The minimal shot-noise-limited relative power variations ∆Pmin/P0 which can be experimentally detected with FM DLs are about 10–7. Typical values of ∆Pmin/P0 ≈ 10–5 (1 Hz detection bandwidth) are mainly determined by instrumental factors (in most cases by interference effects). For this minimal detectable value of ∆Pmin/P0 and for molecular species at atmospheric pressure the corresponding concentration LODs are ca. 0.1 – 10 ppb m (0.1 – 10 parts per billion by volume oner an optical path of 1 m). Evidently, if 10 m absorption length could be realized, the minimal detectable concentration of absorbing molecules would be 10 times lower.The most important analytical applications of mulecular absorption spectroscopy with mid-IR DLs are environmental monitoring and atmospheric studies [9]. Two different approaches are used: (1) measurements in sample cells at reduced pressure, when the absorption linewidths are of ca. (1 – 3) Ч 10–3 cm–1, and (2) open-path measurements at atmospheric pressure, when the



pressure broadening leads to ca. 10–1 cm–1 linewidths.In a cell, optimal gas pressure and appropriate dilution of primer gas by a buffer can be used. A number of mulecules have been measured with lead-salt DLs: CO, NO, NO2, N2O, HNO3, NH3, SO2, O3, HCl, H2CO, CH4, C2H4, C2H6, CF2Cl2, etc. The effective absorption pathlength of 20 – 300 m was realized with multipass cells with an appropriate decrease of the LOD down to a record value of 10 ppt (5 min integration time). Several Pb-salt DL-based gas monitors have been used to perform in situ trace gas measurements in the troposphere and lower stratosphere from airborne or balloon-borne platforms. The systems were designed to measure CH4, CO, N2O, NO2, N2O, HCl and H2O with LODs at sub-ppb concentration levels (3 – 30 s integration time). A typical system includes a liquid-N2 Dewar for diode lasers and detectors, a multipass cell, and can simultaneously measure up to four gases.Owing to pressure broadening, ground-based remote detection of trace gases with DLs was not as effective as measurements in sample cells. Long-path atmospheric gas analyzers have been exploited mainly to monitor carbon monoxide concentrations. Mirror retroreflectors have been used to reflect the DL probing beam back to the detector providing a round-trip absorption pathlength of up to 1 km. Sensitivities achieved are in the range from 0.06 to 40 ppm · m.Besides atmospheric trace measurements, Pb-salt DLs have successfully been used for detection of impurities in the process control of semiconductor materials, analysis of high purity gases, sensing of toxic gases, explosive detection, isotope analysis of gases, and medical diagnosis of various diseases by analysis of expired air. The use of a DL provides high time resolution (0.1 – 1 µs), facilitating sensitive control of transient processes (e.g., intermediates in chemical reactions, discharges, molecular beams). So far the high price, complexity, and limited reliability of the Pb-salt DL spectrometers limit their applicability to the research laboratories when utmost sensitivity and selectivity have to be realized.

4.1.2. Near IR Gas Sensors

Recent advances in room-temperature NIR lasers has led to a new generation of gas sensors. A number of DL-based instruments have been developed and are now commercially available.These sensors have rather simple constructions, may be very compact, have low cost, are reliable, and allow completely autonomous operation for a long time.Peculiarities of developing NIR-DL sensors are defined by characteristics of molecular absorption in this spectral region. There exists a large quantity of absorption lines that arise from overtones and combination bands. Typical linestrengths of these transistions are two to three orders weaker than those for the fundamental bands in the mid-IR. The strongest absorptions in the 1.3 – 1.6 µm region are provided by the H2O, HF, HCl, and C2H2 molecules. The LODs which are achieved for these molecules in pure gas can be at the ppt level. However, if the absorption of the matrix gas is larger than that of the detected impurity, the LOD will be determined by the interfering absorption of the matrix gas molecules.There are a number of quite successful applications of NIR-DL sensors for monitoring of gas molecules, especially H2O. A few types of hygrometers based on a 1.4 µm InGaAsP-laser are commercially available. A very small LOD of 65 ppt for water in air was achieved in laboratory test measurements with a hygrometer, designed to measure moisture in semiconductor feed gas. Another DL sensor, capable of measuring low levels of water vapor in the presence of methane,



is designed to be attached to a sampling station where pipeline gas is extracted at near-ambient pressures (1 – 2 atm).Flight versions of NIR-DL instruments for trace water vapor measurements have been demonstrated on research aircraft with a sensitivity of the order of 10 ppm. Examples of such systems flown on NASA aircraft include both open-path and closed-path configurations designed for fast-response measurements of H2O from the Earth's surface to an altitude of 20 km.Similar instruments developed for monitoring O2, NH3, HCl, HF, H2S, CO, CO2 are suited to industrial stack monitoring, process and environmental control, where high temperatures, continuous vibration, thermal cycling, or high-speed air flow are present. The automated sensors provide fast and accurate ppm-level measurements and are used for optimizing incineration, sewage and waste treatment, power generation, and so on. Fiber optics are often preferred for their simplicity and ability to place the instrument at distances hundreds of meters from the hazardous zone.

4.1.3. Advanced Developments

Rapid improvements of the DL characteristics make these devices very attractive for absorption spectrometry not only of gas species (atomic or molecular) but also of liquid and condensed-phase samples. Much attention is now focused on using NIR-DLs for molecular fluorescence spectrometry, Raman spectroscopy, thermal lens spectrometry, medical diagnostics, development of fiber sensors, and so on.It is unlikely that the NIR-DLs will permit measurement of all molecular trace constituents in the atmosphere with sensitivities sufficient for environmental control. New, room temperature operated lasers have been tested for trace measurements in mid-IR region. Potentially the most significant technology is quantum-cascade lasers (QCLs, see Section 2.2. Semiconductor Diode Lasers ). Spectroscopic characteristics and noise properties of these structures enable measurements of trace impurities with LODs achieved using cryogenic Pb-salt lasers.New absorption methods, like intracavity spectroscopy, cavity-ring-down and cavity-enhanced spectroscopy, have demonstrated very high sensitivities in laboratory measurements with DLs. An ultrasensitive technique that combines external cavity enhancement and FM spectroscopy has been developed recently. This method, which has been called NICE-OHMS, or noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, is based on frequency modulation of the laser at the cavity free-spectral-range frequency or its multiple. The MDA of 5 Ч 10–13 (kmin = 1 Ч 10–14 cm–1) in the detection of narrow saturated absorption spectra of C2H2 and C2HD has been reported, with a cavity finesse of 105. This result is one of the most sensitive absorption measurement ever reported.



4.2. Laser Optoacoustic Spectroscopy [1], [2], [10]

The optoacoustic effect, based on partial nonradiative dissipation of the absorbed light energy, followed by generation of an acoustic wave, was discovered about a hundred years ago. The effect is very weak and, before the advent of lasers, it was applied mainly for spectroscopic investigations of gases. Lasers and improvements in acoustic vibration detection have led to the development of laser optoacoustic spectroscopy (LOAS). Several physical processes are involved in LOAS: (1) absorption of laser radiation by one or more compounds in a medium; (2) radiationless conversion of a part of the absorbed energy into local heating of the sample; (3) formation of an acoustic wave due to a temperature gradient; and (4) detection of the acoustic vibrations.The main advantages of LOAS are high sensitivity, applicability to samples of any state of aggregation, and relatively simple instrumentation. Its drawback is poor selectivity, due to spectral interference of the molecular compounds in a complex mixture. To improve selectivity, LOAS is often combined with a preselecting technique such as gas or liquid chromatography. This discussion is limited to the applications of LOAS for sensitive analysis of simple mixtures.The LOAS instrument consists of the following main units: A tunable laser, a sample cell with an acoustic transducer (sometimes called a spectrophone or SP), an amplifier, and a recording system. The acoustic response of the medium can be stimulated by amplitude or frequency modulation of the laser light or by Stark or Zeeman modulation of the absorption line of the analyte. Both pulsed and cw lasers with mechanical or electro-optical amplitude modulation are often employed in LOAS analytical applications.The key component of an LOAS instrument is the spectrophone. Its construction depends critically on the state of aggregation of the sample. The SP for gas samples consists of a gas cell with input and output windows for laser radiation and a microphone, fixed on an inside or outside wall of the cell. As in the case of MAS with SDLs, the sensitivity can be improved by the use of multipass SPs. Plane capacitor or electronic microphones are the most widely used. The sensitive element of these microphones is a thin (1 – 10 µm) elastic membrane made of Mylar, Teflon, or metallic foil. The membrane serves as one electrode of a dielectric capacitor (membranes made of dielectric materials have metallic coatings). Thus, the acoustic vibrations of a gas mixture in the cell can be directly converted to an electrical signal. Commercial microphones with sensitivities of 5 – 50 mV/Pa are utilized in routine applications of LOAS; specially constructed and optimized ones are used for ultrasensitive analyses.Resonant and nonresonant SPs can be used in LOAS. In the former type, the acoustic resonances are formed inside the cell. Resonant SPs provide higher sensitivity, but they are less flexible, as the acoustic resonances depend on the physical dimensions of the cell, the type of gas mixture, its temperature, and pressure. The modulation frequency of cw laser radiation should be matched to the acoustic resonance frequency (typically of the order of several kilohertz). The dimensions of nonresonant cells are commonly 5 – 10 mm diameter and 5 – 20 cm length, while some resonant cells have a diameter up to 10 cm.For the analysis of liquid and solid samples, more complicated SPs are used. For liquid samples, the simplest and most sensitive detection of acoustic waves is by direct contact of the microphone and a liquid. The acoustic vibrations of irradiated solid samples can be transduced



to a microphone via a buffer gas or a "pure" liquid. Piezoelectric microphones are the most widely used type for solids and liquids.Some important molecular impurities in pure gases have been successfully detected by LOAS. Typical gas pressures range from 1 – 100 kPa; sample volumes 0.5 – 3 cm3; routine accuracy 5 –15 %; linear dynamic range up to 105. The most impressive LODs are: 0.1 ppb for SO2 (multipass, resonant SP); 10 ppb for NO2 (resonant SP); 1 ppm for HF (nonresonant SP, pulse laser, time resolution); 100 ppb for CH4 (nonresonant SP); about 100 ppb for CO (resonant SP); 0.1 ppb for NO (nonresonant cell, six microphones); 10 ppb for SF6 (resonant SP).The best sensitivity of LOAS for liquid samples (up to 10–6 cm–1) was achieved with pulsed lasers of ca. 1 mJ pulse energy. With modulated cw lasers of ca. 1 W average power, a minimum absorption of 10–5 – 10–6 cm–1 was detected. A few examples of real analyses include the LODs for β- carotene, Se, and Cd in chloroform of 0.08, 15, and 0.02 ng/mL, respectively; the LODs for bacteriochlorophyll, chlorophyll b, and hematoporphyrin in ethanol of 1, 0.3, and 0.3 ng/mL, respectively; the LODs for cytochrome c, vitamin B12 , uranium(IV), and uranium(VI) in water of 30 ng/mL, 4 ng/mL, 8Ч10–7 mol/L and 10–6 mol/L, respectively. Typically, the LODs for direct analysis of liquids are worse as compared with "pure" gases because of higher background absorption of the solvent. To reduce this background, frequency modulation of lasers and differential two-beam detection schemes are used.LOAS has been successfully applied for the detection of thin films and layers deposited on a solid substrate. Both direct measurement by a piezoelectric microphone connected to the substrate and detection of acoustic waves via an intermediate gas or liquid were used. For example, the absorption of thin Al2O3 layers (up to monolayer) on an aluminum substrate was measured with a cw CO2 laser, modulated by a mechanical chopper. Measurements of thin layers of liquids and powders as well as aerosols, suspensions, colloids, and small dispersed particles are important applications of LOAS.

4.3. Thermal Lens Spectroscopy [1], [3]

Thermal lens spectroscopy (TLS) is another laser-based technique, associated with dissipation of absorbed laser energy followed by the formation of temperature gradients in a medium. It was observed for the first time in 1964 when a 1- cm Brewster cell containing benzene was placed inside the resonator of a cw He – Ne laser. Contrary to the expected behavior, the laser intensity began to fluctuate and in a few seconds, when the steady state was reached, the beam diameter at the resonator mirrors had increased. The benzene cell acted as a spherical negative lens with a focal length of ca. 1 m.The physical principles of TLS are rather simple. If a laser beam with a Gaussian-like intensity



distribution propagates in a weakly absorbing medium the local temperature will increase and the transverse temperature profile will match the intensity profile of the laser beam. Since most liquids have a negative temperature coefficient of their refractive index (dn/dT < 0) the induced transverse profile of the refractive index acts as a negative (diverging) lens. The focal length of the lens depends on the parameters of the laser beam (power, diameter) and the characteristics of the medium (absorption coefficient, thermal conductivity, thermal diffusivity, dn/dT, cell length). Calculations show that in benzene a 0.01-W laser beam of 0.1 cm radius induces a negative thermal lens with focal length – 1.14Ч104 cm. If a beam of the same power is focused to radius 0.01 cm, the focal length is – 2.27Ч102 cm [2].The absorption coefficient of a medium and the concentration of the absorbing species can be evaluated from the experimentally measured focal length value of an induced thermal lens. The simplest way to determine focal length is to measure the "far field" diameter of a laser beam with and without the sample. Since the optical strength of a thermal lens is not very high, the distance between the sample and the plane of measurement should be several meters. The maximum TLS signal can be detected optimally when the laser beam is focused just before the sample cell. This distance (the confocal length) is defined by the parameters of the laser beam and the focal length of the lens.An optimal TLS scheme detects the fraction of laser beam power P passed through a pinhole in a screen, located in the farfield, being detected by a photomultiplier (or vacuum photodiode) and lock-in amplifier (Fig. (5)). The TLS signal can be expressed as a normalized fractional change in the detected laser power:

where P (t = 0) is the detected part of a laser beam not affected by the thermal lens effect, and P (t = ∞) is the equilibrium value of P in the presence of the thermal lens. The absorption coefficient of a sample can be directly evaluated from this ratio and the parameters of the sample and laser beam. Both single- and two-beam versions of TLS are widely used in analytical applications. In the latter version, one beam of appropriate power is used as the heating beam while the second, low intensity, stable probe beam is used for signal detection.From the introduction of TLS, cw lasers with discrete wavelengths (e.g., He – Ne, He – Cd, Ar+, Kr+, CO2) are the most widely used. These lasers have the best power stability and beam profile. In most cases, the absorption bands of the trace species of interest do not coincide with a discrete wavelength of the heating laser. Chemical pretreatment is necessary for formation of an analyte derivative with an appropriate absorption band. As in the case of LOAS, TLS has poor selectivity because of spectral interference of sample components. Selectivity depends mainly on the appropriate choice of reagent for analyte derivative formation.The signal also depends on the solvent. Carbon tetrachloride, benzene, and acetone are preferred. For example, the TLS signal of a given analyte in water is 45 times smaller than in carbon tetrachloride. Suspended dust and air bubbles in a sample may perturb the laser beam, affecting the signal. To minimize these perturbations, liquids should be filtered and allowed to equilibrate in the cell before beginning the experiment. Absorption by the cell windows followed by a heating of the sample should also be minimized, as well as the formation of interference fringes due to the reflections of the laser beam by the optical components of the apparatus.Originally, TLS was expected to be an absolute (standardless) technique because of the direct relationship between absorption coefficient and the experimental parameters. It was later



recognized that the accuracy of the experimental and molecular parameters is not sufficient for absolute quantitative evaluation of the trace concentration of an analyte; calibration by standard reference solutions is necessary for quantitative analysis.The minimum absorption coefficients measured by TLS are of the order of10–6 – 10–7 cm–1, which corresponds, for strong absorbers (ε ≥ 105 M –1 cm–1), to minimal concentrations of ca. 10–11 – 10–12 M. Some recent data from real sample analysis by TLS are listed in Table (4).

4.4. Fluorescence Analysis of Organic Molecules in Solid Solutions [2]

Fluorescence is one of the most sensitive techniques for the detection of complex organic molecules such as aromatic hydrocarbons, heterocyclic compounds, porphyrins (including chlorophyll and its derivatives), organic dyes, etc. Its high sensitivity depends on the relatively high fluorescence quantum yields for these classes of molecules and well-developed methods of photon detection. If a pure, single-component solution is analyzed, extremely low LODs can be achieved, e.g., highly luminescent dyes in aqueous solution can be easily detected with LODs down to 10–13 M. For many organic compounds in liquid or solid solution form, single molecules can be detected by modern experimental techniques. In many cases, the main analytical problem is not sensitivity, but selectivity. At room temperature, the absorption and luminescence spectra of organic molecules consist typically of one or more bands whose spectral width is hundreds or thousands of wavenumbers; severe spectral interference is a key problem in the analysis of multicomponent mixtures.The Shpol'skii method is a powerful improvement. The spectra of many organic molecules embedded in a crystalline matrix of short-chain n-paraffins consist, at helium temperature, of well-defined quasi-lines with linewidth 2 –20 cm–1. In spite of many successful analytical applications the Shpol'skii technique is not universally applicable because many organic molecules have low solubility in n-paraffins and, moreover, many compounds in n-paraffin matrices have broad-band spectra even at helium temperature.The discovery that in many cases the spectra of organic compounds at low temperature are inhomogeneously broadened was the next and very important step in the understanding of the spectroscopy of complex molecules. In most cases, the broad spectra have an intrinsic line structure, which can be revealed by selective laser excitation. This new technique, laser fine-structure selective spectroscopy of complex molecules in frozen solutions is often called site-selection spectroscopy (SSS). A physically more realistic term is energy selection spectroscopy. By now the SSS technique is so far developed that inhomogeneous broadening of organic molecules at low temperature can be eliminated by selective laser excitation [2].



The fundamentals of SSS are based on the theory of impurity centers in a crystal. The optical spectrum of an organic molecule embedded in a matrix is defined by electron – vibrational interaction with intramolecular vibrations (vibronic coupling) and interaction with vibrations of the solvent (electron – phonon coupling). Each vibronic band consists of a narrow zero-phonon line (ZPL) and a relatively broad phonon wing (PW). ZPL corresponds to a molecular transition with no change in the number of phonons in the matrix (an optical analogy of the resonance γ-line in the Mцssbauer effect). PW is determined by a transition which is accompanied by creation or annihilation of matrix phonons. The relative distribution of the integrated intensity of a band between ZPL and PW is characterized by the Debye – Waller factor:

and depends critically on the temperature and the strength of electron – phonon coupling. At liquid helium temperature, if this coupling is not too strong, the vibronic bands of an isolated impurity molecule consist of a strong, narrow ZPL and a broad, weak PW. The observed broad band of an assembly of organic molecules in solution at low temperature is an envelope of a great number of narrow ZPLs (accompanied by PWs) of individual molecules which are surrounded by different local fields (mainly because of small variations of the relative orientation of an analyte molecule and the solvent molecules). Statistical variations cause shifts in ZPL positions and, hence, inhomogeneous broadening of a spectral band. A narrow-band laser excites only a small fraction of the molecules with an appropriate ZPL position. Thus, well-separated, narrow ZPLs, accompanied by PWs, are observed in the fluorescence spectra of a multicomponent cryogenic solid mixture. This effect is often called fluorescence line narrowing (FLN). Informative spectra may be obtained for many types of molecule (polar and nonpolar), in different media (glasses, polymers, and crystals). The efficiency of FLN for a particular compound is well defined by the factor α. The higher the value of α, the better is excitation selectivity. For most systems, α varies within the interval 0.2 – 0.8. For compounds with α < 1, selective excitation is inefficient. Examples of such molecules are ionic dyes (proflavines and rhodamines in ethanol).Fluorescence line narrowing depends strongly on temperature. For most organic molecules, ZPL disappears completely at temperatures above 40 – 50 K. This is the main disadvantage of FLN for analytical applications; the sample temperature should be below 15 – 30 K.The main components of FLN instruments for analysis are a cryogenic system, a tunable laser, a medium resolution spectrometer, and a recording system. The cryogenic system consists of a liquid He optical cryostat with a He circulation system. In some cases, commercially available closed-cycle cryostats can be used. The optical system consists of a sample holder, optics for the exciting beam input, and a high aperture collector of the fluorescent radiation.The most intense 0 – 0 ZPLs of organic molecules of interest occupy the spectral range 650 – 250 nm. Thus, lasers for FLN should be tunable within this range. The laser linewidth should match the FLN vibronic linewidth of the analytes, typically of the order of 1 – 3 cm–1. To avoid sample heating, saturation effects, and/or stimulated two-photon photoreactions, the average laser power should not exceed 1 – 10 mW. The cw dye lasers, pumped by Ar+ or Kr+ ion lasers are relatively cheap and reliable sources for the visible spectrum. For UV, frequency-doubled dye lasers pumped by copper or quasi- continuous Nd-YAG lasers (typical repetition rates 1 –10 kHz) may be preferable.As the distance between the wavelengths of excitation and fluorescence in FLN is ca. 100 –200 cm–1, background scattering of the excitinglaser radiation may be a serious problem. Therefore,



high-quality double- or even triple-grating spectrometers are used in FLN analysis. New multichannel recording systems (photodiode arrays, image intensifiers with linear CCDs) are potentially very useful detectors for FLN.Polycyclic aromatic hydrocarbons (PAH) and heterocyclic aromatic compounds are the most widely analyzed organic compounds by FLN techniques. Both types of compound are widespread in nature. Many have strong carcinogenic and mutagenic properties and these properties depend on the isomeric structure of the compound. Analysis of PAH in a glassy matrix by FLN has many advantages compared with other techniques; simple sample preparation, high optical quality of the frozen matrices, etc., provide higher sensitivity and selectivity. LODs down to 10–12 – 10–13 g/cm3 have been achieved for single-component solutions. For example, a concentration of perylene in ethanol of 10–12 g/mL (ca. 105 molecules in the sample) was easily detected in a frozen solution by FLN. The calibration curve was linear over the concentration range 10–6 – 10–9 g/cm3. Well- characterized fine-structure FLN spectra were detected and identified in an artificial mixture of 14 PAHs in a water – glycerol solution. In another experiment, a mixture of 7 PAHs with concentrations of individual components ranging from 10–7 to 10–9 g/cm3 was quantitatively analyzed and all compounds were correctly identified.In some cases, direct analysis of raw products (without sample pretreatment or dissolution) is possible. To avoid self-absorption and energy transfer effects, low analyte concentrations and low optical density of the sample are necessary. Several PAH were successfully detected in gasoline when a frozen sample (T = 4.2 K) was selectively excited by laser radiation of appropriate wavelength. The same sample excited by a conventional Hg-lamp exhibited a smooth, unresolved spectrum with no possibility of identifying different compounds. As an example, the active carcinogen 3,4-benzpyrene was easily detected in gasoline by FLN. The concentration of 10–7 g/mL was measured by the standard additions method. The estimated LOD depended on the background luminescence of a gasoline sample which is mainly determined by its composition. The LODs varied within 10–10 –10–11 g/mL for different gasoline samples. By the same technique, perylene at concentration 5Ч10–8 g/mL was detected in solid paraffin.

5. Laser Ablation [3], [6], [11]

Laser ablation (LA) is a technique in which focused laser radiation is used to release material from a sample. The advantages of LA are now well known: reduced or no sample preparation is needed; conducting or non-conducting samples may be analyzed; spatial resolution up to a few micrometers can be obtained.



Different analytical techniques are used for detection of the elemental composition of the ablated material. The simplest one is direct detection of the emission of a plasma plume formed above a sample surface. This technique is generally referred to as LIBS (laser induced breakdown spectroscopy). Strong continuous background radiation of the hot plasma plume does not allow detection of atomic and ionic lines of specific elements during the first few hundred nanoseconds of plasma evolution. One can achieve reasonable signal-to-noise ratio for the measurement of atomic and ionic spectral line intensities by optimization of experimental parameters: type and pressure of a buffer gas, time delay, and distance from the sample surface for detection of line intensities. The LODs in (mg – µg)/g range have been realized for direct analysis of metals, glasses, and ceramics by LIBS techniques for a variety of trace elements. Better LODs were demonstrated when LA was followed by detection of the ablated atoms or ions by laser atomic absorption or fluorescence spectroscopy (LAAS, LIF). Even though matrix-effects are significant and detection limits are not as good as for LA-ICP-MS, LIBS has one major advantage. It allows fast and on-line analysis in industrial environment, including the nuclear industry. When fibers are used both for excitation and emission collection, a very robust system can be built that allows on-line quality control.The intrinsic drawback of LIBS is a short duration (less than a few hundred microseconds) and significant non-stationary conditions of a laser plume. Much higher sensitivity was realized by transport of the ablated material into a secondary atomic reservoir such as an MIP. Owing to the much longer residence time of ablated atoms and ions in a stationary MIP (typically several ms as compared with at most a hundred microseconds in a laser plume) and due to additional excitation of the radiating upper levels in the low pressure plasma, the line intensities of atoms and ions are greatly enhanced. Because of both factors the LODs of the LA-MIP technique have been improved by 1 – 2 orders of magnitude as compared with LIBS.Recently, analytical capabilities of LIBS and LA-MIP-OES were noticeably improved by use of an advanced detection scheme based on an Echelle spectrometer combined with a high sensitivity ICCDs (intensified charge-coupled devices) detector. This detection scheme enables simultaneous detection of a large spectral range from the UV to NIR in a single laser shot. It allows estimation of temperature of a laser plume by constructing Boltzman plots and correction for plasma temperature variations. The advantages of this technique are: complete sample analysis in a single laser shot, improved accuracy and precision, the possibility to detect sample inhomogeneities.The sensitivity, accuracy, and precision of solid sample analysis were greatly improved by coupling of LA with ICP-OES/MS. The ablated species are transported with a carrier gas (usually argon) into the plasma torch. Additional atomization, excitation and ionization of the ablated species in a stationary hot plasma provide a dramatic increase in the sensitivity of emission detection (LA-ICP-OES) or detection of ions (LA-ICP-MS). The efficiency of the transport of ablated species into an ICP strongly depends on the size of the particles. The optimal conditions for ablation in the case of LA-ICP differ significantly from the optimal conditions for LIBS because the efficient transport of the ablated matter to an ICP requires a fine aerosol (with solid particle diameters less than a few micrometers), whereas direct optical emission spectroscopy of the laser plume needs excited atoms and ions.The LODs in the sub-µg/g – ng/g range were realized for different materials (metals, glasses, polymers) by LA-ICP-OES. Even higher sensitivity was achieved for LA-ICP-MS due to the high efficiency of ion collection and detection. Under optimal conditions of ablation (choice of gas, diameter of a crater, flow rate of a carrier gas) the LODs in the ng/g – pg/g range are now routinely realized for most elements of the periodic table. Today the LA-ICP-MS technique is



accepted as a most powerful technique for direct analysis of solid samples.As a result of its attractive characteristics, LA-ICP-MS is currently being used for a large variety of applications, such as the in situ trace element analysis of glasses, geological samples, metals, ceramic materials, polymers, and atmospheric particulates. Fingerprinting (characterization of the trace element composition) of diamonds and gold and glass and steel using LA-ICP-MS has been reported to be very useful for provenance determination and for forensic purposes, respectively.So far the pulsed lasers with a pulse duration of some nanoseconds have been most widely used for LA – Nd: YAG (1064, 532, 354.7, 266 nm), excimer lasers XeCl (308 nm), KrF (248 nm). For these lasers the mechanisms responsible for the material removal are thermal melting and evaporation or some kind of explosive evaporation. As a result, so called fractional evaporation could occur — the elements with different melting and boiling temperatures evaporate from a melt at different rates. Because of this composition of a laser plume does not match the bulk composition. This was a serious problem as inadequate probing of a sample could not be corrected for by any sensitive detection scheme. The optimal conditions for laser ablation and advanced methods of data processing were found to avoid the problem of fractionation for different classes of samples in the case of nanosecond pulses.The use of the lasers with femtosecond duration opens the way to overcome the problem of fractionation in the most radical way. The first generation of all-solid-state femtosecond lasers is now commercially available, although expensive. For such a fast energy deposition the non-thermal mechanisms of material removal (e.g., ion repulsions due to Coulomb forces) can be significant. The non-thermal character of material removal with fs pulses enables the depth profiling of a solid sample with a few nm resolution, which was impossible with ns pulses because of melting and mixing of different layers.

6. References

ï [1] D. S. Kliger (ed.): Ultrasensitive Laser Spectroscopy, Academic Press, New York 1983.ï [2] V. S. Letokhov (ed.) : Laser Analytical Spectrochemistry, Adam Hilger, Bristol 1986.ï [3] J. Sneddon, T. L. Thiem, Y. Lee (eds.): Lasers in Analytical Atomic Spectrometry, John

Wiley & Sons, New York 1997.ï [4] F. P. Schдfer (ed.): Dye Lasers, Springer Verlag, Heidelberg 1973.ï [5] M. Ch. Amann, J. Buus: Tunable Laser Diodes, Artech House, Boston 1998.ï [6] B. Smith: "Lasers in Analytical Chemistry," Trends Anal. Chem. 17 (1998) nos. 8 + 9.ï [7] S. Sjцstrцm, Spectrochimica Acta Rev. 13 (1990) 407 – 465.ï [8] K. Niemax: "Diode Laser Spectroscopy," Spectrochim. Acta Rev. 15 (1993) no. 5.



ï [9] H. I. Schiff, G. I. Mackay, J. Bechara, in M. W. Sigrist (ed.): Air Monitoring by Spectroscopic Techniques, John Wiley & Sons, New York 1994.

ï [10] S. Martellucci, A. N. Chester (eds.): Optoelectronics for Environmental Science, Plenum Press, London 1991.

ï [11] R. Russo: "Laser Ablation," Appl. Spectr. 49 (1995) no. 9, 14 A – 28 A.

ï first occurrence in article

Laser Analytical Spectroscopy - Table 1

Best detection limits for LEAF-GETA [7]

Element λ11 , nm λ12*, nm λfl , nm Limit of detection

fg pg/mL**

Ag 328.1 338.3 8.0 0.4

Al 308.2 394 100 5.0

Au 242.8 312.3 10 0.5

Bi 306.8 472.2 800 40

Cd 228.8 643.8 361 18 0.9

228.8 228.8 3.5 0.175

Co 308.4 345.4 4.0 0.2

Cu 324.7 510.5 60 3.0

Eu 287.9 536.1 3.6Ч105 1.8Ч104

Fe 296.7 373.5 70 3.5

Ga 403.3 641.4 250.0 1.0 0.05

In 410 451 20 1.0

Ir 295.1 322.1 6Ч103 300

Li 670.8 670.8 10 0.5

Mn 279.5 279.5 90 4.5

Mo 313.3 317.0 105 5Ч103

Na 589.6 589.6 30 1.5

Ni 322.2 361.9 103 50

Pb 283.3 405.7 0.2 0.01

Pt 265.9 270.2 103 50

Rb 780 780 20 1.0

Sb 212.7 259.8 10 0.5

Sn 286.3 317.5 30 1.5

Te 214.3 238 20 1.0

Tl 276.8 353 0.1 0.005

V 264.8 354.4 2.2Ч106 1.1Ч105

Yb 398.8 666.7 246.4 220 11

* λ12 is the wavelength of the second laser in a two-step excitation scheme.

** The pg/mL values are calculated assuming a sample volume of 20 µL.





Real sample analysis by LEAF

Analyte Matrix a Certified value Measured value LEAF LOD (aqueous standard) Comments

Co ASRMs ng/g ng/g fg digested, diluted samples

wheat grains (SBMP) 60 ± 20 45 ± 2 60

dried potato (SBMK) 100 ± 30 90 ± 5

dried grass (SBMT) 60 ± 20 48 ± 2

Sn alloys pg vacuum atomizer

OSCh 4.4 ± 1.5 4

OVCh 2.5 ± 0.8

Mn ASRMs µg/g µg/g fg slurried samples

citrus leaves (SRM 1572) 23.0 ± 2 23.4 ± 0.7 100

bovine liver (SRM 1577 a) 9.9 ± 0.8 9.4 ± 0.5

milk powder (SRM 1549) 0.26 ± 0.06 0.29 ± 0.01

Pb Ni alloys µg/g µg/g fg direct solid sampling

SRM 897 11.7 ± 0.8 12.6 ± 1.2 1

SRM 898 2.5 ± 0.6 2.2 ± 0.4

SRM 899 3.9 ± 0.1 3.7 ± 0.7

ASRMs slurried samples

citrus leaves (SRM 1572) 13.3 ± 2.4 14.1 ± 0.5

Tl ASRMs ng/g b ng/g fg dissolved samples

pine needles (SRM 1575) 50 48 ± 2 3

tomato leaves (SRM 1573) 50 51 ± 4

bovine liver (SRM 1577 a) 3 ± 0.3 3.2 ± 0.2

Ni alloys µg/g µg/g fg direct solid sampling

SRM 897 0.51 ± 0.03 0.56 ± 0.07 10

SRM 898 2.75 ± 0.02 2.5 ± 0.5

SRM 899 0.252 ± 0.003 0.24 ± 0.03

Te Ni alloys µg/g µg/g fg direct solid sampling

SRM 897 1.05 ± 0.07 0.97 ± 0.15 20

SRM 898 0.54 ± 0.02 0.52 ± 0.05

SRM 899 5.9 ± 0.6 5.4 ± 0.7

Sb Ni alloys µg/g c µg/g fg dissolved samples

PW 1 A 5 4.8 ± 0.5 10

PW 2 A 9 8.3 ± 0.5

PW 3 A 17 16.5 ± 0.8

P Ni alloys µg/g µg/g pg digested, diluted samples



SRM 349 30 ± 10 23 ± 3 8

SRM 865 120 ± 30 100 ± 20

SRM 126 c 40 c 37 ± 5

ASRMs mg/g mg/g

SRM 1577 a 11.1 ± 0.4 11.6 ± 0.9

SRM 1549 10.6 ± 0.2 10.9 ± 0.9

SRM 1575 1.2 ± 0.2 1.1 ± 0.2a ASRM = agricultural standard reference materials (Russian State Reference Materials). b Noncertified value. c Reference value.

© 20



Best detection limits for one- and two-step flame LEI of aqueous solutions

Element λ1 , nm λ2 , nm Flame* Detection limit, ng/mL

Ag 328.1 AA 1

328.1 421.1 AA 0.05

Al 309.3 AN 0.2

As 278.0 AA 3000

Au 267.7 AA 1.2

242.8 479.3 AA 1

Ba 307.2 AA 0.2

Bi 227.7 AA 0.2

Ca 227.6 AA 0.006

422.6 585.7 HA 0.03

Cd 228.8 AA 0.2

Co 240.8 AA 0.06

252.1 591.7 AA 0.08

Cr 240.9 AA 0.2

298.6 483.6 AA 0.3

Cs 455.5 AA 0.002

Cu 324.8 AA 3

324.8 453.1 AA 0.07

Fe 302.1 AA 0.08

271.9 468.1 AA 0.1

Ga 294.4 AA 0.01

In 271.0 AA 0.001

451.1 571.0 AA 0.0009

K 766.5 AA 0.1

Li 670.8 AA 0.001

670.8 610.4 AA 0.0002

Mg 285.2 AA 0.003

01 Wiley-VCH Verlag GmbH, Weinheim, GermanyPage: 36


285.2 470.3 AA 0.001

Mn 279.5 AA 0.04

279.5 521.5 AA 0.02

Mo 319.4 AN 10

Na 285.3 AA 0.001

589.0 568.8 AA 0.0006

Ni 229.0 AA 0.02

282.1 501.4 AA 0.04

Pb 283.3 AA 0.2

283.3 600.2 AA 0.09

Rb 780.0 HA 0.09

Sb 287.8 AA 50

Si 288.2 AN 40

Sn 284.0 AN 0.4

284.0 597.0 HA 0.3

Sr 230.7 AA 0.003

460.7 554.3 AA 0.2

Ti 320.0 AN 1

Tl 276.8 AA 0.006

291.8 377.6 AA 0.008

V 318.5 AN 0.9

W 283.1 AA 300

Yb 267.2 AA 1.7

555.6 581.2 AA 0.1

Zn 213.9 396.6 AA 1

* AA = acetylene – air; AN = acetylene – nitrous oxide; HA = hydrogen – air.



Best detection limits for pure solutions and some examples of real sample analysis by TLS

Analyte Solvent Sample* Laser Detection limit, wt %

Co2+ CHCl3 PS He – Ne 1.2Ч10–6

Cu2+ water PS He – Ne 3.8Ч10–6

Fe2+ CHCl3 PS Ar+ 1.7Ч10–10

Nd3+ D2O PS dye 3.0Ч10–5

Pr3+ water PS dye 7.2Ч10–3

Tb3+ butanol PS Ar+ 7.0Ч10–6

UO22+ water NW dye 4.7Ч10–7

U6+ HClO4 NW dye 7.0Ч10–6

Am3+ HClO4 PS dye 7.0Ч10–7

Pu4+ HCO3– PS dye 2.5Ч10–6



Po43– water – acetone PS dye 5.0Ч10–10

AsO33– 2-butanol NW He – Ne 5.0Ч10–8

SO32– water PS Ar+ 1.3Ч10–7

S2– 2-butanol water, gas He – Ne 5.0Ч10–8

I2 CCl4 PS Ar+ 1.0Ч10–16**

Formaldehyde water PS dye 4.5Ч10–8

Nitroaniline methanol – water PS He – Cd 1.2Ч10–9**

Trimethylpentane CCl4 PS He – Ne 8.0Ч10–9

Azulene liquid CO2 PS Kr+ 3.0Ч10–10

NO2 air air Ar+ 5.0Ч10–7

Methanol N2 PS CO2 1.2Ч10–6

CCl2Fe2 Ar PS CO2 1.0Ч10–7

CS2 Ar PS CO2 1.0Ч10–7

* PS = pure solution; NW = natural water.

** Absolute detection limit, g.


Laser Analytical Spectroscopy - Figure 1



The basic laser analytical techniques



Classification of LEAF schemes



A) Resonance fluorescence; B) Direct-line fluorescence; C) Collisionally assisted direct-line fluorescence; D) Anti-Stokes direct-line fluorescence



Classification of LEI schemesA) One-step excitation; B) Two-step excitation; C, D) Collisionally assisted two-step excitation





Classification of RIS schemesA) Nonresonance photoionization to a continuum; B) Photoionization via an autoionization state; C) Electric field ionization of a Rydberg state



Schematic diagram of a single-beam thermal lens spectrometera) Laser; b) Lens; c) Sample; d) Screen; e) Pinhole; f ) DetectorThe dashed lines show the beam divergence due to the thermal lens effect



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[2]V. S. Letokhov (ed.) : Laser Analytical Spectrochemistry, Adam Hilger, Bristol 1986.

[3]J. Sneddon, T. L. Thiem, Y. Lee (eds.): Lasers in Analytical Atomic Spectrometry, John Wiley & Sons, New York 1997.

[4]F. P. Schдfer (ed.): Dye Lasers, Springer Verlag, Heidelberg 1973.

[5]M. Ch. Amann, J. Buus: Tunable Laser Diodes, Artech House, Boston 1998.

[6]B. Smith: "Lasers in Analytical Chemistry," Trends Anal. Chem. 17 (1998) nos. 8 + 9.

[7]S. Sjцstrцm, Spectrochimica Acta Rev. 13 (1990) 407 – 465.

[8]K. Niemax: "Diode Laser Spectroscopy," Spectrochim. Acta Rev. 15 (1993) no. 5.

[9]H. I. Schiff, G. I. Mackay, J. Bechara, in M. W. Sigrist (ed.): Air Monitoring by Spectroscopic Techniques, John Wiley & Sons, New York 1994.

[10]S. Martellucci, A. N. Chester (eds.): Optoelectronics for Environmental Science, Plenum Press, London 1991.

[11]R. Russo: "Laser Ablation," Appl. Spectr. 49 (1995) no. 9, 14 A – 28 A.


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