Microplasmas for analytical spectrometry

Joachim Franzke,* Kerstin Kunze, Manuela Miclea and Kay Niemax

Institute of Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11,44139 Dortmund, Germany. E-mail: franzke@isas-dortmund.de

Received 7th January 2003, Accepted 8th May 2003

First published as an Advance Article on the web 13th June 2003

Recent developments of miniaturized powerful and robust plasmas for analytical applications are reviewed. The

plasmas described and discussed may be used for analyte detection in chromatographic or electrophoretic ‘‘lab-

on-a-chip’’ systems.

Introduction

Recently there has been a strong interest in the development ofminiaturized analytical measurement systems. In particular,separation systems have been downscaled and produced in achip-based flat format. Designs and implementations can befound, e.g., for gas chromatography (GC),1–3 liquid chroma-tography,4,5 and capillary electro-separation methods.6 A veryimportant point is the integration of a powerful detectionsystem on the chip.7,8 Therefore, it is the aim of several researchlaboratories to develop powerful and robust micro-plasmaswhich can be integrated on a chip because plasmas incombination with separation techniques are widely used inanalytical science.

The advantage of a miniaturized plasma in lab-on-a-chipsystems would be a further improvement in the compactness ofthe total analytical system, which allows its application outsideof the laboratory. Furthermore, parallel and high-throughputmeasurements can be performed if many systems are usedsimultaneously.

Plasmas in analytical spectrometry and theirminiaturization

Plasmas coupled with optical or mass spectrometry (OES, MS)are important tools for the detection of molecular fragmentsand even for elemental analysis. In application fields, such asquality control in industry, environmental control and thebooming field of bio-technology and -medicine they are used tofragment and excite or even atomize molecular species afterseparation (e.g., by chromatography or electrophoresis). Severalplasma sources have been used. These include the microwaveinduced plasma (MIP),9–11 the capacitively coupled plasma(CCP),12,13 the inductively coupled plasma (ICP),14 the radio-frequency (rf) glow discharge (GD),15 and the direct current(dc) glow discharge.16,17

The first work on the use of spectral emission-type detectorsin gas chromatography was carried out by McCormack et al.9

A MIP was used as an energy source for molecular frag-mentation and excitation. Low minimal detectable quantities(1027–10216 g s21) were obtained by observing molecularemission bands (e.g., from C2, CH, and CN). Detection limitsas low as 10216 g s21 were obtained for hexane. Although theauthors of the paper mentioned also the use of a direct currentdischarge as an emission detector no analytical data of thisdevice were presented. At present, the MIP is the most popularplasma for GC-OES. The commercially available apparatusapplies slightly modified Beenakker-type cavities to operateatmospheric plasmas at low power levels.11

Nevertheless, the direct current discharge was considered by

Braman et al. to have several distinct advantages as a plasmaemission detector.16 They suggested that the apparatus couldbe made more compact in size than the MIP detector becausethe relatively large MIP power supply can be replaced by asmaller one. The authors employed a quartz capillary(1–2 mm id) with inserted platinum wires. Plasmas were createdat 10–15 W of power at a helium flow rate of 140 ml min21. Theyobtained detection limits of 10212–10214 g s21 of hexane andselected halocarbon compounds with a dc plasma by observingthe CH diatomic bands and the atomic lines of F, Cl, Br and I,respectively. Direct current,16 alternating current (ac),18 andhigh-voltage pulsed plasma detectors19 have been developed.These detectors are simple to build and consume less powerthan other plasma sources in analytical spectroscopy.

In order to miniaturize plasma sources different laws have tobe regarded. Townsend20 showed that Paschen’s law was aspecial case of a more general similarity theorem for directcurrent discharges. This theorem was extended by Holm21 todescribe the current conditions applying geometrically similarelectrodes. The fundamental processes have been discussed byvon Engel and Steenbeck.22 Later Margenau showed theore-tically how the theorem can be extended to plasmas with highfrequency alternating fields.23 Assuming that the discharge iscompletely governed by linear laws, a set of reduced quantitiescan be defined. Linear processes are, for example, Maxwell’slaw, ionization by electron impact, secondary electron emis-sion, drift of charged particles in an electric field, and diffusion.Two discharge devices of similar shape should behaveidentically, if all the reduced quantities are the same for bothdevices. The most significant reduced discharge parameterswith dependence on the pressure (p) are the linear dimensionsof the electrodes and the vessel (D?p), the electric field (E/p), thevoltage (U) and the current density (j/p2). Proper scaling of adischarge device should keep the expressions given in bracketsconstant. For example, the operating pressure has to beincreased when the size of the device is reduced.

Recently, the development of microplasmas for spectro-chemical analysis was described by Broeckaert.24 The presentpaper gives a review of different miniaturized plasmas whichhave been developed and applied in analytical molecular oratomic spectrometry.

Miniaturized direct current plasmas

A molecular emission detector on a glass chip employing aminiaturized direct current helium plasma for molecular frag-mentation and excitation has been presented by Eijkel et al.25

Fig. 1 shows a typical schematic chip layout consisting of a topand a bottom plate. The channels and the plasma chamber areproduced by HF-etching. The electrodes are formed by

802 J. Anal. At. Spectrom., 2003, 18, 802–807 DOI: 10.1039/b300193h

This journal is # The Royal Society of Chemistry 2003

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deposition of 50 nm of chromium and 250 nm of gold. Gas in-and outlet holes of 400 mm diameter were drilled by ultrasonicabrasion. The plasma was generated in chambers of differentgeometrical dimensions, varying the chamber volume, theelectrode distance, the in- and outlet channel as well as theequivalent radius of the plasma chamber. In- and outletchannels were chosen to obtain a plasma chamber pressure ofabout 100 hPa. A chamber with a volume of 50 nl at a typicaloperating pressure of 170 hPa was used as an excitationsource. Methane could be detected with a detection limit of 3 610212 g s21 (600 ppm v/v) by observing the molecular emissionof the CH radical. The lifetime of the device was limited to 2 hdue to cathode sputtering.

In a further paper Eijkel et al. described an atmosphericpressure dc glow discharge on a microchip to be used as amolecular emission detector.26 The scaling theory for directcurrent glow discharges predicts that normal discharges canexist at atmospheric pressure in microscale discharge tubes.The validity of this theory was demonstrated by the creation ofan atmospheric helium plasma in a nanoliter-size dischargechamber on a microchip. Three chambers of volumes of52.5 nl, 180 nl, 50 nl, with flow rates of 5.75 nl s21, 500 nl s21 and50 nl s21, respectively, were used in order to determine the limitof detection of methane in a helium gas flow by measuringmolecular bands of CH. The limits of detection were 7 ppm,3 ppm and 400 ppb, which correspond to 10214 g s21, 10212 g s21

and 10213 g s21, respectively. In the case of low flow rates, theanalytical signal was linear over two decades. It was shown thatthe microchip plasma could be successfully applied formolecular emission detection. The authors are claiming thatthe simple instrumentation, the small detector size and thegood sensitivity make the device highly suitable for integrationin microanalysis systems.

In a third paper the same group coupled the plasma chip to aconventional gas chromatograph in order to investigate itsperformance as an optical emission detector.27 Although thelowest detection limit of 10214 g s21 was obtained earlier whenthe plasma volume was 52.5 nl and the flow rate 5.75 nl s21, a180 nl volume plasma with a higher flow rate of 320 nl s21 wasapplied for the purpose of preventing broadening and tailing ofthe chromatographic peaks. The plasma was generated inhelium and the applied power was 9 mW (770 V, 12 mA). Anumber of carbon-compounds were detected in the columneffluent recording the CO-emission at 519 nm. For hexane, thedetector showed a linear dynamic range over two decadesand a detection limit of 10212 g s21 (800 ppb). However, allcomponents of the chromatogram showed considerable broad-ening and tailing. The authors proved that these effects werenot related to the performance of the plasma but to deadvolumes in the connection tubes and the channels of the glasschip. It can be expected that the integration of the chromato-graphic column and the plasma detector on a single chipreduces the dead volumes and improves the signals. The device

was operated for more than 24 h without a significant change inperformance. The operation is stable and instrumental require-ments are simple.

Recently, atomic detection of bromine and chlorine wasreported by Bessoth et al.28 A micro-plasma chip of 2 6 0.07 60.07 mm3 size and 125 mW power was coupled to aconventional, large size gas chromatograph. Specific detectionof elements and molecular fragments in the eluted peaks wasdemonstrated. Using the 479.5 nm emission line a detectionlimit of 800 pg s21 for chlorine was found, taking into accountthe injected amount of chlorinated compounds.

The use of solution as one of the electrodes

A method named electrolyte as a cathode discharge (ELCAD)has been developed by Cserfalvi et al. to perform continuousmonitoring of trace metals dissolved in water via glowdischarge-atomic emission spectrometry (GD-AES).29 Anopen, continuously flowing fountain of the sample solution isused as the cathode in the discharge. More recently, a verysmall glow discharge for optical emission spectroscopy (GD-OES) has been described by Marcus et al.30,31 This device,which is called liquid-sampling atmospheric-pressure glowdischarge (LS-APGD), has been developed for the analysis ofmetals in electrolyte solutions. An abnormal glow discharge isformed between the electrolyte solution in a capillary and a Cucounter electrode. The liquid in the capillary acts either as thecathode or the anode of the discharge. Stable discharges couldbe realized with flow rates of 0.5–1.5 ml min21 using hydrogen,sodium or lithium as the electrolyte species. Discharge currentsof 25–60 mA and voltages of 300–1000 V are typical. Analyticalresponse curves were generated for the elements Na, Fe, andPb, with absolute limits of detection in the order of 60 ngobtained for 5 mL sample injections. Jenkins et al. presented thefeasibility of performing AES with liquid samples on a glassmicrochip using the ELCAD technique and a thousand-foldsmaller sample flow rate has been achieved.32

Miniaturized pulsed plasma detectors

A low power plasma detector for molecular emission spectro-metry has been described by Jin et al.33 This detector consists oftwo platinum plate electrodes of 0.04 mm thickness and 3 mmwidth placed face to face in a 40 mm long Teflon tube (outerdiameter 7 mm). The distance between the electrodes is 1.5 mm.The plasma was generated in He with a home-built high-voltage pulsed power supply at atmospheric pressure. Theaverage operational power of the detector was smaller than0.2 W. Because of the low power requirement, the detectorcould be operated with two 1.5 V alkaline batteries for morethan 10 h. The influence of plasma gases, flow rates anddischarge voltages on the performance of the detector, as wellas the reproducibility and sensitivity of the detector to organicvapours, were studied using dimethyl sulfoxide.

Capacitively coupled microplasmas at 13.56 MHz

A miniaturized, parallel-plate capacitively coupled plasma (PP-CCP) at atmospheric pressure, first described by Liang andBlades,34 has been investigated for analytical atomic spectro-metry applications.35 The PP-CCP is sustained by applicationof radiofrequency power to a pair of electrodes which areseparated by a quartz discharge tube, forming a capacitive,transverse discharge. The plasma is normally operated at13.56 MHz using He as a plasma gas. However, other gases andfrequencies can also be used.

Bass et al. describe the implementation of the PP-CCP on a0.25 6 0.25 6 5 mm3 micro-machined fused silica chip.36

The He plasma, schematically shown in Fig. 2, operates at

Fig. 1 Schematic chip layout of the direct current discharge used byEijkel et al. (2 6 0.45 6 0.2 mm3).27

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atmospheric pressure. The power is 5–25 W and the gas flow isbetween 17 and 150 ml min21. The plasma is self-igniting. Theauthors claim that parallel-plate capacitive power coupling isnearly ideal for generating and sustaining a plasma dischargeon a chip since it can be implemented using a very simpleelectrode structure and does not require tuned or resonantstructures. The quartz torch is mounted between two copperelectrodes. The bottom electrode is connected to a groundedcopper block. It also serves as a cooling element for the plasmatorch. The upper electrode is 1 mm wide. The electrodes canalso be made by vapour deposition to build a more compactsource and, if necessary, by electrode patterning to generateseveral plasma discharges on a single wafer. The emission linesof OH, NH, N2, N2

1 and He were measured by applying a 20 WHe plasma and a gas flow of 70 ml min21. Although the PP-CCP has been investigated for application in analytical atomicspectrometry,33 no element selective measurements have beencarried out by Bass et al. so far with the miniaturized version.36

A similar capacitively coupled microplasma, named CCMPsource, operating at atmospheric pressure is described byYoshiki et al.37 It is assembled on a quartz chip of 20 6 20 mm2

that consists of two glass plates with a thickness of 500 mm anda spacer between the plates of 65–500 mm. The thickness of thespacer determines the depth of the capillary. Parallel-plateelectrodes of 5 6 5 mm2 are externally attached to the quartzchip so that the capillary is sandwiched between the electrodes.The He-plasma is generated in channels with cross sections of65–500 6 500–5000 mm. The length of the plasma inside thecapillary is as long as the length of the electrodes, which areexternally attached to the quartz plates. The plasma is excitedby a conventional operational frequency of 13.56 MHz and isignited by an incident power between 1–3 W in the range of80 hPa to atmospheric pressure. Various He emission lines andthe lines of O and OH were obtained when the capillary with across section of 150 6 5000 mm was applied. The He atomicexcitation temperatures are estimated to be about 2000 K.The incident power and the gas flow rate were 5 W and475 ml min21, respectively. The CCMP is supposed to have apotential use as an on chip plasma device but no analyticalmeasurements have been performed yet.

Miniaturized inductively coupled plasmas

A very interesting miniaturized plasma is the down-scaled ICP.Whereas large-size ICPs are typically operated at a frequencyof 13.56 MHz, Hopwood et al. showed that the optimumfrequency for plasma generation increases to 460 MHz whenthe coil diameter is reduced to 5 mm.38 It was found that theelectron density is about an order of magnitude lower than witha large scale ICP. However, it was also shown that the electrondensity increases with the frequency.

The miniature ICP (mICP) was fabricated by etchingplanar spiral inductors in a copper clad epoxy board. Anotherpaper reports on the microfabrication and testing of amonolithic mICP fabricated on glass wafers using surface

micromachining.39 The plasma is sustained by coupling a450 MHz current into a low pressure gas. Ar as well as airplasmas have been generated in the range 0.1 hPa–13 hPa. Theoperation power is 350 mW, although 1.5 W is required toinitiate the discharge. A new single-loop mICP source which isthree times more efficient than the former one has beenfabricated.40 In this case the coil was situated closer to theplasma and was operated with even higher frequencies, up to818 MHz. Ion densities of 1011 cm23 in Ar at 0.5 hPa wereobtained with only 1 W power. Recently, first calibrationcurves for SO2 were measured in an Ar mICP by opticalemission spectrometry of the sulfur atomic 469.5 nm line. Theplasma chamber consisted of a cylindrical hole of 6 mmdiameter and 6 mm in length. The pressure was 7.4 hPa and theplasma power 3.5 W. The limit of detection of SO2 was about190 ppb v/v.41

Microwave induced plasmas based on microstriptechnology

Bilgic et al. described a new low-power, small-scale 2.45 GHzmicrowave plasma source based on microstrip technology,which can be operated at atmospheric pressure and used foratomic emission spectrometry.42,43 This MIP is named MicroStrip Plasma (MSP). The MSP is integrated in a quartz waferand designed as an element-selective detector for miniaturizedanalytical applications. The quartz wafer is a sandwich of twoquartz plates shown in Fig. 3. The dimensions are 1 6 33 690 mm3. Both plates have a 0.45 6 1 6 90 mm3 groove and areglued together by water-glass. The wafers lie on a copper plate,which works as the ground electrode and as a cooling block. Onthe upper plate is a copper electrode of 30 mm thickness. Thiselectrode consists of a small strip placed over the gas channel, asmall matching device and an electric contact to a microwaveconnector (not shown in Fig. 3). The MSP operates at amicrowave input power of 1–40 W and Ar gas flows of 50–1000 ml min21. No homogeneous plasma is formed in the MSPwhen the plasma gas flow is very low. Therefore, allmeasurements were performed with a plasma gas flow of300 ml min21 and a forward power of 30 W. Rotational (OH)and excitation (Fe) temperatures were found to be 650 and8000 K, respectively. Hg could be determined by applying theflow injection cold vapour (FI-CV) technique, and could bemeasured with a detection limit of 50 pg ml21.

Miniaturized radiofrequency plasma at 350 kHz

In 1988 Skelton et al. developed a 350 kHz radiofrequencyplasma (RFP) which was used for sulfur selective gas chro-matographic analysis by GC-OES.44–46 This detector wasfound to have low limits of detection of sulfur (0.5 pg s21) and agood linear response (4 decades). The detector consists of a

Fig. 2 Exploded schematic view of the capacitively coupled micro-plasma discharge chamber used by Bass et al. (0.25 6 0.25 6 5 mm3).33

Fig. 3 Exploded schematic view of the microstrip microwave inducedplasma chamber used by Engel et al. (0.9 6 1 6 20 mm3).40

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helium radiofrequency plasma sustained inside of a 1 mmquartz tube. The gas flow was about 70 ml min21 with anadditional oxygen flow about 0.03 ml min21. A macor ceramiccell body was used to provide thermal and electrical insulationas well as to support the stainless steel electrode. The powerconsumption was about 50–80 W.

The RFP was further miniaturized by Pedersen-Bjergaardet al.47,48 A 5 cm long piece of polyimide coating and stationaryphase was carefully burned off at the end of a fused silica GCcolumn. The last 2 cm of this uncoated GC capillary served asthe plasma tube placed inside a piece of silica tube forprotection. A steel wire placed at the column outlet served asthe top electrode, as shown in Fig. 4. The plasma was generatedinside the end of the fused silica GC column between the topelectrode and the grounded reducing union by a radio-frequency power supply. Atomic emission was measuredside-on through the wall of the fused silica column and theprotecting silica tube. The detection limits for bromine andchlorine were 0.9 and 1.1 pg s21, respectively.

A further paper reports on the application of this plasmadevice for mass spectrometric detection in capillary gaschromatography.49 The plasma was sustained at low pressurein the last 35 mm of a capillary GC column (0.32 mm id) whichwas put inside the ion source housing of a quadrupole massspectrometer. This allowed direct introduction of ions from theplasma into the mass analyzer using only a repeller andelectrostatic lenses to focus the ions. The plasma was sustainedonly with 25 ml min21 of helium, which was accepted by themass spectrometer vacuum system. This low gas flow alsoserved to enhance the energy density of the discharge and toproduce a narrow spray of ions toward the mass analyzer. Dueto the miniaturized nature of the plasma, it was operated at alow power level (2.0 W), and traces of oxygen were added toavoid deposition of carbon on the capillary wall. Chlorine wassuccessfully monitored down to the 2.2 pg s21 level withoutinterference from elements such as C, S, P, O, F, and N.

Dielectric barrier discharge

The dielectric barrier discharge (DBD) or silent discharge isgenerally characterized by the presence of at least one dielectriclayer (barrier) in the discharge volume between two metallicelectrodes. An alternating rectangular high voltage waveformin the range from a few Hz up to MHz is applied to sustain theplasma. This type of discharge is widely used, e.g., in flat-panel

plasma displays, for industrial production of ozone or insurface treatment.50

Miclea et al. have developed a dielectric barrier dischargesuitable for analytical applications,51 which is shown in Fig. 5.The discharge chamber consists of two glass plates coveredwith aluminium electrodes (50 mm length, 1 mm width). Theelectrodes are covered with a 20 mm thick glass type dielectriclayer. The glass spacers in between the glass plates define thedistance of 1 mm between the electrodes and form the plasmachannel of 1 6 1 6 50 mm3. The discharge works at reducedpressures of 10–100 hPa in argon as well as helium with a gasflow of 10–1000 ml min21. The applied voltage has arectangular shape with a frequency of 5–20 kHz and anamplitude of 750 Vpp. The discharge displays a transientbehavior with a half width of the current peak of about 10 ms. Itis ignited over the whole length of the electrodes.

Plasma diagnostics reveals that the hot region of the plasmais constricted for a short time of each discharge cycle close tothe temporary cathode. In this thin layer the electron densityreaches 1015 cm23 and a gas temperature of about 1000 K,while the rest of the discharge remains cold.52

The mean power consumption of the discharge is muchsmaller than 1 W. The slopes of the applied rectangular voltage(e.g., 10 kHz) induces a short current pulse and therefore thehalf width of the current pulse is only a tenth of the wholeplasma cycle. Furthermore, the highest atom density islocalized in about a tenth of the discharge volume. Conse-quently, the peak power density of 1 kW cm23 is of the sameorder of magnitude as the mean power density of an inductivelycoupled plasma. An ICP typically has an input power of about1 kW, a conical plasma torch length of about 25 mm and adiameter of 25 mm, resulting in an average power density ofabout 0.5 kW cm23.

Measurements of halogenated hydrocarbons by diode laseratomic absorption spectrometry (DLAAS) in the dielectricbarrier discharge were performed by absorption of excitedatoms using a laser beam of 1 mm diameter along the dischargechannel. The absorption signal was measured by a phasesensitive detection using twice the modulation frequency of theDBD. The different concentrations of halogenated hydro-carbons were obtained by diluting an initial mixture of 17 ppmdown to concentrations near the detection limits. The limitsof detection of CCl2F2, CClF3 or CHClF2 in the Ar DBDwere about 5 ppb v/v using the 837.824 nm Cl absorption line,which starts from a metastable level of Cl. In He the detectionlimits for CCl2F2 were 400 ppt v/v and 2 ppb v/v using the Cl837.824 nm and the F 685.792 nm line, respectively.

The authors proved that the absorption signals of Cl andF correspond to the stoichiometric ratios of these elements in theanalyte molecules. This can be regarded as an indication ofcomplete dissociation of the molecules in the probed volume.

Fig. 4 Schematic layout of the radiofrequency plasma used byPedersen-Bjergaard et al. (inner diameter of the capillary: 1 mm,length: 20 mm).47

Fig. 5 Exploded schematic view of the dielectric barrier dischargechamber used by Miclea et al. (1 6 1 6 50 mm3).51

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Microstructured electrode discharge

The microstructured electrode (MSE) discharge is a multilayersystem consisting of two metallic foils of Cu, Ni, Pt or Wseparated by an insulator (e.g., Kapton, mica or ceramic). Thethickness of the layers is typically 30–150 mm. A bore with adiameter of 10–500 mm is drilled through the structure.53 Aplasma is produced between the electrodes in noble gases, gasmixtures or air using ac or dc voltages. A very high currentdensity is generated in the discharge due to the hollow cathodegeometry. The reduced dimensions of the electrodes enablehigh pressure operation according to the laws of similarity.23

The discharge can be operated in the pressure range from10 mbar up to 2 bar.

Recently, the plasma parameters of a particular Ar MSEdischarge operated with a dc current voltage were measured byDLAAS.54 This MSE structure was made of two Cu electrodes(120 mm thickness) separated by a 50 mm Kapton insulator witha bore diameter of 300 mm. It could be shown that the gastemperature increases linearly from near room temperature at50 mbar to about 1200 K at 400 mbar. The electron density alsoincreases with the pressure up to 5 6 1015 cm23 at 400 mbarand a constant gas flow of 100 ml min21. Scaling these values toatmospheric pressure, it is expected that the gas temperatureand the electron density are approximately 2000 K and1016 cm23, respectively. This was proved using a structurewith 30 mm thick Pt electrodes and Al2O3 as an insulator madefor atmospheric pressure operation (Fig. 6). The diameter ofthe bore and the distance between the electrodes were about100 mm and 200 mm, respectively. Measurements of the gastemperature and the electron density revealed the extrapolateddata given above.55

The MSE discharge operates in a stable mode for currentsbetween 4 and 8 mA in He as well as in Ar. In this range the gasvoltage is constant and does not exceed 250 V. For the givengeometric parameter and a current of 6 mA the current density andthe input power density are about 60 A cm22 and 1 MW cm23,respectively. One can see that the power density of a MSE dis-charge is 3 orders of magnitude higher than that of an ICP.56

Due to the high gas temperature and high energy ions thelifetime of these structures is only a few days. However, thelifetime can be improved using ceramic as an insulator andmetal electrodes with low sputtering rates.

Atomic emission spectroscopy of the MSE discharge wasapplied for the detection of chlorine and fluorine resulting from thedecomposition of the halogenated molecules (CCl2F2, CHClF2)introduced into the He plasma gas.56 The intensities are linear over3 orders of magnitude. The detection limits for CCl2F2 are20 ppb v/v using either Cl 912.114 nm or the F 739.868 nm.

Concluding remarks

The experimental data such as the dimensions of the discharge,the resulting volume, the pressure, the input power, the flowrate, the detected analyte and the limit of detection are given inTable 1 for the different plasmas discussed above. The mean

Fig. 6 Schematic view of a microstructured electrode discharge used byMiclea et al. (inner diameter of the bore: 0.1 mm, length of the bore0.2 mm).56

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806 J. Anal. At. Spectrom., 2003, 18, 802–807

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power density and the exchange rate are calculated from theexperimental data. An important parameter for the develop-ment of a miniaturized plasma is the mean power density,which is calculated from the input power and the volume ofeach discharge. The mean power density must be on the onehand high enough in order to dissociate and to excitemolecules, and on the other hand not too high to causedamages. A comparison of the calculated mean power densityof the dc discharge shows that for molecular detection a meanpower density of 0.05 kW cm23 is sufficient to obtain a lowlimit of detection for hexane, whereas a mean power density of12.5 kW cm23 is affordable for atomic detection. Theapplication of the RFP and the DBD shows excellent resultsaccording to the limit of detection, although the mean powerdensity is less than 1 kW cm23 in each case. In the case of theDBD, the excellent dissociation capability is due to therelatively high peak power of about 1 kW cm23. Therefore,a mean power density of 1 kW cm23 should be sufficient forelement spectrometry. All other plasmas show mean powerdensities, which are even higher than 10 kW cm23. In order toprevent damage to the plasma device, it should be cooled asmentioned by Bass et al.36 (PP-CCmP), who used a groundedcopper block as the bottom electrode, or Bilgic et al.42 (MS-MIP), who connected the ground electrode to a copper socketwith an active cooler. An alternative or additional possibility toprevent damage to the plasma device is to operate the plasmawith a high flow rate. A measure of the quality of cooling by thegas flow can be the ratio of the flow rate to the plasma volume,called the exchange rate. It is obvious that plasmas operatedwith a high mean power density have also a high exchange rate.However, to reduce the sample and gas consumption it isdesirable to develop plasmas, which can be operated with lowflow rates.

Acknowledgements

The authors gratefully acknowledge financial support by theDeutsche Forschungsgemeinschaft.

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