BRUKE TENSORTM

Series

Perkin ElmerTM

Spectrum One

Instrumentation

• Dispersive instruments: with a monochromator to be used in the mid-IR region for spectral scanning and quantitative analysis.

• Fourier transform IR (FTIR) systems: widely applied and quite popular in the far-IR and mid-IR spectrometry.

• Nondispersive instruments: use filters for wavelength selection or an infrared-absorbing gas in the detection system for the analysis of gas at specific wavelength.

Dispersive IR spectrophotometers

Simplified diagram of a double beam infrared spectrometer

Modern dispersive IR spectrophotometers are invariably double-beam instruments, but many allow single-beam operation via a front-panel switch.

Double-beam operation compensates for atmospheric absorption, for the wavelength dependence of the source spectra radiance, the optical efficiency of the mirrors and grating, and the detector instability, which are serious in the IR region.single-beam instruments not practical.

Double-beam operation allows a stable 100% T baseline in the spectra.

Dispersive spectrophotometers Designs

Null type instrument

Components of dispersive spectrophotometers

Nernst Glower heated rare earth oxide rod (~1500 K)

1-50 µm(mid- to far-IR)

Globar heated SiC rod (~1500 K) 1-50 µm(mid- to far-IR)

W filament lamp 1100 K 0.78-2.5 µm(Near-IR)

Hg arc lamp plasma 50 - 300 µm(far-IR)

CO2 laser stimulated emission lines 9-11 µm

1. IR source

Thermocouple thermoelectric effect -dissimilar metal junction

cheap, slow, insensitive

Bolometer Ni, Pt resistance thermometer (thermistor)

Highly sensitive <400 cm-1

Pyroelectric Tri glycine sulfate piezoelectric material

fast and sensitive (mid IR)

Photoconducting PbS, CdS, Pb Se light sensitive cells

fast and sensitive (near IR)

2. Detector / transducer

3. Optical system

• Reflection gratings ( made from various plastics): the groove spacing is greater (e.g. 120 grooves mm-1). To reduce the effect of overlapping orders and stray radiation, filters or a preceding prism are usually employed. Two or more gratings are often used with several filters to scan a wide region.

• Mirrors but not lenses are used to focus and collimate the IR radiation. Generally made from Pyrex or another material with low coefficient of thermal expansion. Front surfaces coated with a vacuum-deposited thin metal film of Al, Ag, or Au.

•Windows are used for sample cells and to permit various compartment to be isolated from the environment.

transparent to IR over the wavelength region inert to the various chemicals analyzed capable of being shaped, ground, and polished to the desired optical quality

The Fourier transform method provides an alternatives to the use of monochromators based on dispersion.

In conversional dispersive spectroscopy, frequencies are separated and only a small portion is detected at any particular instant, while the remainder is discarded. The immediate result is a frequency-domain spectrum.

Fourier transform infrared spectroscopy generates time-domain spectra as the immediately available data, in which the intensity is obtained as a function of time.

Direct observation of a time-domain spectrum is not immediately useful because it is not possible to deduce, by inspection, frequency-domain spectra from the corresponding time-domain waveform (Fourier transform is thus introduced).

Fourier Transform Infrared Spectrometer (FTIR)

In one arm of the interferometer, the IR source radiation travels through the beam splitter to the fixed mirror back to the beam splitter through the sample and to the detector. In the other arm, the IR source radiation travels to the beam splitter to the movable mirror, back through the beam splitter to the sample and to the detector. The difference in pathlengths of the two beams is the retardation . An He-NE laser is used as a monochromatic reference source. The laser beam is sent through the interferometer in the opposite direction to that of the IR beam.

Single-beam FTIR Spectrometer

Double-beam FTIR Spectrometer

Interferometer

Michelson interferometer

If moving mirror moves 1/4 l (1/2 l round-trip) waves are out of phase at beam-splitting mirror - no signal

If moving mirror moves 1/2 l (1 l round-trip) waves are in phase at beam-splitting mirror – signal

...

Interferograms

Difference in pathlength called retardation

Plot vs. signal - cosine wave with frequency proportional to light frequency but signal varies at much lower frequency

One full cycle when mirror moves distance l/2 (round-trip = l)

Frequency of signal:

Substituting l=c/n

If mirror velocity is 1.5 cm/s

Bolometer, pyroelectric, photoconducting IR detectors can "see“ changes on 10-4 s time scale!

llMMMM VVf 2

2/

nc

Vf MM2

VMM velocity of moving mirror

nn 1010 10

/103/3

scmscmf

Computer needed to turn complex interferograms into spectra.

Measuring processes

• very high resolution (< 0.1 cm –1 )Two closely spaced lines only separated if one complete "beat" is recorded. As lines get closer together, must increase.Dn(cm1) 1/Mirror motion is 1/2 Resolution governed by distance movable mirror travels

• very high sensitivity (nanogram quantity)can be coupled with GC analysis (–> measure IR spectra in gas-phase)

• High S/N ratios - high throughputFew optics, no slits mean high intensity of light

• Rapid (<10 s)

• Reproducible and • Inexpensive

Advantages of FTIR

Usually to improve resolution decrease slit width but less light makes spectrum "noisier" - signal to noise ratio (S/N)

n # scans

S/N improves with more scans (noise is random, signal is not!)

nNS

SS

SnNS

i

2)(

To improve S/N ratio

For routine instrument calibration, run the spectrum of polystyrene film (or indene) at resolution 2 cm-1. Band positions are available in the literature.

Higher resolution calibrations may be made from gas-phase spectra (e.g. HCl gas).

Spectrum calibration

Sample preparation techniques

The preparation of samples for infrared spectrometry is often the most challenging task in obtaining an IR spectrum. Since almost all substances absorb IR radiation at some wave length, and solvents must be carefully chosen for the wavelength region and the sample of interest.

Infrared spectra may be obtained for gases, liquids or solids (neat or in solution)

• A gas sample cell consists of a cylinder of glass or sometimes a metal. The cell is closed at both ends with an appropriate window materials (NaCl/KBr) and equipped with valves or stopcocks for introduction of the sample.• Long pathlength (10 cm) cells – used to study dilute (few molecules) or weakly absorbing samples. • Multipass cells – more compact and efficient instead of long-pathlength cells. Mirrors are used so that the beam makes several passes through the sample before exiting the cell. (Effective pathlength 10 m).• To resolve the rotational structure of the sample, the cells must be capable of being evacuated to measure the spectrum at reduced pressure. • For quantitative determinations with light molecules, the cell is sometimes pressurized in order to broaden the rotational structure and all simpler measurement.

Gas samples

• Pure or soluted in transparent solvent – not water (attacks windows)•The sample is most often in the form of liquid films (“sandwiched” between two NaCl plates)• Adjustable pathlength (0.015 to 1 mm) – by Teflon spacer

Liquid samples

Regions of transparency for common infrared solvents.

The horizontal lines indicate regions where solvent transmits at least 25% of the incident radiation in a 1-mm cell.

Solid samples• Spectra of solids are obtained as alkali halide discs (KBr), mulls (e.g. Nujol, a highly refined mixture of saturated hydrocarbons) and films (solvent or melt casting)

Alkali halide discs:

1. A milligram or less of the fine ground sample mixed with about 100 mg of dry KBr powder in a mortar or ball mill.

2. The mixture compressed in a die to form transparent disc.

Mulls

3. Grinding a few milligrams of the powdered sample with a mortar or with pulverizing equipment. A few drops of the mineral oil added (grinding continued to form a smooth paste).

4. The IR of the paste can be obtained as the liquid sample.

1. Fundamental chemistryDetermination of molecular structure/geometry.e.g.  Determination of bond lengths, bond angles of gaseous molecules

2.  Qualitative analysis – simple, fast, nondestructiveMonitoring trace gases: NDIR.Rapid, simultaneous analysis of GC, moisture, N in soil. Analysis of fragments left at the scene of a crimeQuantitative determination of hydrocarbons on filters, in air, or in water

Main uses of IR spectroscopy:

Near-infrared and Far-infrared absorption

The techniques and applications of near-infrared (NIR) and far-infrared (FIR) spectrometry are quite different from those discussed above for conventional, mid-IR spectrometry.

Near-infrared: 0.8 -2.5 m, 12500 - 4000 cm-1

Mid-infrared: 2.5 - 50 m, 4000 - 200 cm-1

Far-infrared: 50 - 1000 m, 200 - 10 cm-1

Near-infrared spectrometry

NIR shows some similarities to UV-visible spectrophotometry and some to mid-IR spectrometry. Indeed the spectrophotometers used in this region are often combined UV-visible-NIR ones.

The majority of the absorption bands observed are due to overtones (or combination) of fundamental bands that occur in the region 3 to 6 m, usually hydrogen-stretching vibrations.

NIR is most widely used for quantitative organic functional-group analysis. The NIR region has also been used for qualitative analyses and studies of hydrogen bonding, solute-solvent interactions, organometallic compounds, and inorganic compounds.

Far-infrared spectrometry

Almost all FIR studies are now carried out with FTIR spectrometers.

The far-IR region can provide unique information.

i) The fundamental vibrations of many organometallic and inorganic molecules fall in this region due to the heavy atoms and weak bonds in these molecules.

ii) Lattice vibrations of crystalline materials occur in this region,

iii) Electron valence/conduction band transition in semiconductors often correspond to far-IR wavelengths.

References:J. Workman, A.W. Springsteen, “Applied Spectroscopy”, Academic Press, 1998.

J.M. Hollas, “Modern Spectroscopy”, John Wiley&Sons, 1996.

B. Stuart, W.O. George, D.J. Ando, “Modern Infrared Spectroscopy”, John Wiley&Sons, 1997.

N.N. Colthup, L.H. Daly, S.E. Wiberly, S.E. Wiberly, “Introduction to Infrared and Raman Spectroscopy”, Academic Press, 1997.

B. Schrader, D. Bougeard, “Infrared and Raman Spectroscopy: Methods and Applications”, John Wiley&Sons, 1995.

Infrared Spectrum of CCl4