Photoacoustic spectrometry

Fourier Transform Infrared Spectrometry, Second Edition, by Peter R. Griffiths and James A. de Haseth Copyright 2007 John Wiley Sons, Inc. 

Note that the PA spectrum has significant intensity only at those wavelengths where the sample absorbs and the single-beam background is completely suppressed. The same concept was applied commercially by Briihl and Kjaer in Denmark, which developed a portable instrument for monitoring industrial atmospheres, the sensitivity of which was in the low parts-per-milfion range. However, this instrument no longer appears to be marketed. 

PHOTOACOUSTIC SPECTROMETRY OF SOLIDS WITH A RAPID-SCANNING INTERFEROMETER 

The PA signal is generated by the thermal expansion of the gas caused by the sum of all the ATg contributions caused by each absorption band. Contributions originate from each of the sample layers in which the relevant wavelengths are absorbed and which are close enough to the surface that the thermal-wave amplitude has not decayed to a vanishingly small level after crossing the sample-gas interface. 

The infrared absorption coefficient and thermal wave decay coefficients, a(v) and flj, respectively, determine the magnitude of the photoacoustic signal. The term ot( exp —[a( +fls]x in the expression for temperature oscillation leads to a linear PA signal dependence on infrared absorption when a( C a. The reciprocal of as is correctly known as the thermal wave decay length, L, although it is sometimes referred to as the sampling depth, penetration depth, or thermal diflusion depth. The sample layer extending a distance L beneath the surface contributes 

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Photoacoustic spectrometry (pas) differs from the other methods in that the detector is a microphone. This makes pas wavelength independent. 

For the determination of individual additives several spectroscopic techniques are normally employed ultraviolet-visible spectrophotometry, spec-trofluorimetry, liuninescence, and photoacoustic spectrometry. Sometimes, to increase method selectivity, a combination of spectroscopic techniques is used. Another alternative to determine individual additives are electrochemical techniques voltammetry, polarography, amperometry, and potentiometry. 

In this article, the key principles of photoacoustic spectrometry (PAS) will be described together with a simple discussion of the instrumentation required to perform PAS. Some important applications will be discussed and future scope of the technique s application outlined. The introduction brings together some historical generalities of the technique followed by sections on its theoretical aspects, instrumentation, and analytical applications. The emphasis is on chemical analysis throughout. 

Rosencwaig, A. Hall, S.S. Thin-layer chromatography and photoacoustic spectrometry. Anal. Chem. 1975, 47, 548-549. 

Hata M, Tokura Y, Takigawa M, Sato M, Shioya Y, Fujikura Y, Imokawa G. Assessment of epidermal barrier function by photoacoustic spectrometry in relation to its importance in the pathogenesis of atopic dermatitis. Lab Invest. 2002 82 1451-61. 

FTIR photoacoustic spectrometry has been used to analyze TLC spots after removal from the plate (White, 1985). It has also been applied to depth profiling of compound distribution on the sorbent (Vovk et al., 1997). 

Analysis of TLC spots by photoacoustic spectrometry (TLC7PAS) has been preferred over DRIFT analysis of strongly IR-absorbing samples (79). TThe spot containing 1-50 pg of the sample must be physically removed from the chromatoplate. After some preparation, it is placed in the photoacoustic cell for measurement. 

In Chapters 2 to 8 we describe the theory and instrumentation needed for an appreciation of the way that Fourier transform infrared and Raman spectra are measured today. The sampling techniques for and applications of FT-Raman spectrometry are described in Chapter 18. The remaining chapters cover the techniques and applications of absorption, reflection, emission, and photoacoustic spectrometry in the mid- and near-infrared spectral regions. 

There are experiments for which rapid-scan interferometers are not well suited. These experiments include depth profiling by photoacoustic spectrometry (Section 20.3), hyperspectral imaging (Section 14.5), fast time-resolved spectrometry... 

One great advantage with digitized spectra is the capabihty to perform comparisons between the spectra of unknowns and reference spectra in a library. Infrared spectra are largely, but not completely, immune to collection conditions, that is, most spectral collection techniques (e.g., transmission, attenuated total reflection, diffuse reflection, photoacoustic spectrometry) will all produce approximately equivalent (but not identical) spectra if appropriate care is taken. Of course, some methods will be more successful than others for some samples or sample matrices, and those effects must be taken into account. Sample conditions are also important Spectra can change with temperature, solvent, or crystallinity, for example, and as the samples change their physical states, the infrared spectra will reflect those changes. 

TIRES and TIRTS have many of the same properties as photoacoustic spectrometry (see Chapter 20), in that they are largely insensitive to sample morphology and to the optical properties of the sample, such as scattering coefficient and front-surface reflection. Similarly, TIRES and TIRTS spectra are also largely unaffected by the sample backing (if any) and the surrounding atmosphere, although emission from hot water vapor and carbon dioxide must sometimes be subtracted from the spectrum after the measurement. 

Figure 17.6. (a) Emission spectra of a 3-mm-thick polycarbonate sheet made by the TIRES technique and by uniformly heating the sample. A blackbody emission spectrum is shown below for comparison, (fo) Emittance spectra of polycarbonate derived from the upper panel spectra compared to an absorption spectrum of polycarbonate recorded by photoacoustic spectrometry. (Reproduced from [7], by permission of the American Chemical Society copyright 1990.)... 

In summary, although infrared emission spectrometry is by no means as widely used as absorption, reflection, or even photoacoustic spectrometry, the capability of emission spectroscopy for remote, noncontact analysis of samples should not be overlooked. 

Figure 20.2. Interferograms of methanol vapor measured by (a) transmission spectrometry and (b) photoacoustic spectrometry (c, d) corresponding spectra. (Reproduced from [2], by permission of Elsevier Publishing Co. copyright 1978.)...

Figure 20.9. Typical electronics for photoacoustic spectrometry with a step-scan interferometer and a two-phase lock-in amplifier.

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