Fourier-transformed Infrared Photoacoustic Spectroscopy of Solids

MARK G. ROCKL E Y Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74074

A wide selection of infrared absorption spectra of opaque s a m - p les is presented to show the versatility of Fourier-transformed i n f r a r e d photoacoustic spectroscopy. A method for correcting these spectra for incident radiant power variations with respect to wavelength is also demonstrated. Index Headings: In f ra red ; Fourier-transformed photoacoustic spectroscopy.

INTRODUCTION

Fourier-transformed infrared photoacoustic spectros- copy ~ (FT-IR PAS) offers a real advantage over other methods for obtaining the infrared absorption spectra of solid samples and surface adsorbed species. Specifically, this technique requires no sample preparation and is suitable for studying insoluble materials, catalysts, un- ground samples and opaque specimens. It is the purpose of this paper to describe the method used for measuring FT-IR PAS spectra and for correcting the spectra for incident power variations and to illustrate the versatility of the technique by describing several example spectra.

I. E X P E R I M E N T A L

The FT-IR PAS spectrometer used here was simple in principle and design, except that a commercial FT-IR spectrometer (a Digilab FTS-20 evacuated spectrometer} was used. The PAS detector was constructed from a small cell (volume ~_ 1 cm 3) and a 1/2 GR-1962 foil electret microphone as transducer. The microphone signal was amplified with an Ithaco 143L preamplifier. The output of this detector was then connected directly to the am- plifier section of the FT-IR spectrometer. The tetragly- cine sulfate detector amplifier was used. Conversion from normal IR operation to PAS operation involved a change of cables only. Because the FT-IR spectrometer used here was an evacuated unit, this cable exchange required the venting of the interferometer chamber and subse- quent reevacuation, a 20 min operation.

The PAS sample chamber was constructed from alu- minum with internal dimensions 10 mm by 6 mm by 6 mm. The infrared exciting light from the interferometer entered the sample chamber through a 3 mm thick NaC1 plate. This plate was sealed to the chamber with a small amount of silicon stopcock grease. Because the diameter of the active area of the sample chamber was small, the exciting light was focussed into the chamber with a 4X reflective beam condenser. Subsequent experiments with a lens condensing arrangement provided similarly satis- factory signal intensities.

Received 15 February 1980.

Volume 34, Number 4, 1980

II. RESULTS

It was quicldy realized that single beam spectra were drastic distortions of the true spectra because the inci- dent power was not a uniform function of infrared fre- quency. Fig. 1 shows a typical single beam spectrum of ground activated charcoal. On this spectrum can be seen negative going features associated with atmospheric H20 and CO2 in the optical path between the evacuated interferometer chamber and the PAS sample cell. These are negative going features since the atmospheric H20 and CO2 attenuate the infrared beam resulting in a lower PAS signal at those frequencies. Because it was not known whether or not activated charcoal had a fiat absorption spectrum, a companion spectrum of lead power was run. The ratio of these two spectra is shown in Fig. 2. This spectrum is essentially flat with the posi- tive going features attributable to differing amounts of atmospheric H20 and CO2 present in the two samples. The small absorption at 2950 c m -1 and in the vicinity of 1000 c m -1 is due to a small amount of silicon grease on the periphery of the cell entrance window. It is apparent therefore that either finely divided lead metal powder, powdered C-black or activated charcoal will serve as an appropriate reference sample. Furthermore, a reference spectrum such as that shown in Fig. 2 may be used to subtract out from normal sample spectra the contribu- tions made by atmospheric H20 and CO2 vapors. This

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Fro. 1. Spectrum of ground activated charcoal at 8 cm-; resolution; 160 scans are coadded.

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FIG. 2. Ratio of ground activated charcoal to lead powder at 8 cm -1 resolution; 1600 scans of the lead powder are coadded.

APPLIED SPECTROSCOPY 405

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FIG. 3. CO2 and H20 vapors at atmospheric concentration levels; 160 scans at 8 cm -1 resolution are coadded.

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FIG. 4. Commercial polystyrene beads used for catalyst supports; 800 scans at 8 cm -1 resolution are coadded.

subtraction has been done by software control on all subsequent spectra described in this paper, with the exception of Fig. 3. All figures are unretouched, un- smoothed spectra.

Because the sample chamber used had an entrance window but no exit window, a background spectrum of the sample chamber with a small amount of atmospheric H20 and CO2 was measured. The resulting spectrum shown in Fig. 3 indicates that the PAS signal drops to zero in the absence of H20 or CO2 vapor absorption. That is, there is very little residual absorption by the aluminum sample chamber.

However, single beam spectra I do indicate residual sample absorption in regions of the spectrum, e.g., 2100 to 2800 cm -1 which cannot be attributed to any particular molecular vibration. This residual absorption is broad- band and flat as indicated by a flat baseline in corrected spectra. This absorption is probably due to total absorp- tion by diffuse scattering within the powdered sample. Since there is no particular vibrational mode absorbing at these frequencies, the individual particles of the sam- ple are relatively transparent so that the incident light is fairly evenly absorbed throughout the sample particle. This contrasts with absorption of the incident light nearer the surface when the frequency corresponds to resonant absorption by the sample. Since the absorption near the surface of the particle is more likely to be in the thermally active layer 2 this absorption contributes more

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FIG. 5. A typical aspirin tablet; 1280 scans at 8 cm- ' resolution are coadded.

strongly to the overall PAS signal strength than the absorption by the bulk. Thus, the PAS spectra shown here represent the absorption oscillator strength modi- fied by the thermal diffusivity of the sample at that particular modulation frequency (as the light arrives from the interferometer) and is a complicated function of the fraction of incident light absorbed in the thermally active layer.

As a further test of the versatility of this method several solid samples of varied relevance were analyzed. Fig. 4 shows the spectrum of a commercial sample of polystyrene beads used to support catalysts. The sample is fairly similar to that of polystyrene film except for the presence of liquid phase water shown by absorption in the 3200 to 3600 cm -1 region and the presence of hydro- gen-bonded amine (absorption at 3250 cm-l).

The spectrum of an aspirin tablet is shown in Fig. 5. Commercial aspirin consists of 50% by weight acetylsal- icylic acid with a starch binder. The presence of the acid can be verified by the absorptions near 1700 cm -1 and at 1655 cm -1. It is in the examination of such samples that the minimal sample preparation required is of value. For the spectrum shown here, a whole tablet was placed in the sample chamber. In short, this is a nondestructive testing procedure.

FT-IR PAS spectra have also been taken of cigarette tobacco, insoluble sugar proteins, potassium nitrate salt powder, coal samples, 3 and conducting polymers. The ease with which these spectra can be obtained show the realized potential of FT-IR PAS for examining opaque, insoluble solids and hint at bright prospects for examin- ing surface adsorbed species.

ACKNOWLEDGMENTS

The author gratefully acknowledges the partial support of this research by the National Institutes of Health Grant No. IRO1 GM25353 and by the National Science Foundation for purchase of the spectrometer with Grant No. CHE78- 01764. The author is most grateful for many helpful suggestions made by Professor J. Paul Devlin.

1. M. G. Rockley, Chem. Phys. Lett. 68, 455 (1979). 2. J. F. McClelland, and R. N. Kniseley, Appl. Phys. Lett. 28, 467 (1976). 3. M. G. Rockley, and J. Paul Devlin, Appl. Spectrosc. 34, 407 (1980).

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