Introduction

            The desired result with all FT-IR sampling techniques is to obtain an absorbance spectrum of the sample as quickly and easily as possible. In many cases, however, direct analysis of “as received” samples by transmission or reflection methods is not practical because the sample either transmits inadequate light to measure or it lacks suitable surface or particle size conditions for reflectance spectroscopies. In other cases, reflectance spectroscopies may not probe deeply enough into the sample to yield the desired information.
            Photoacoustic spectroscopy (PAS)^1-3 is unique as a sampling technique, because it does not require that the sample be transmitting, has low sensitivity to surface condition, and can probe over a range of selectable sampling depths from several micrometers to more than 100 mm. PAS has these capabilities because it directly measures infrared (IR) absorption by sensing absorption-induced heating of the sample within an experimentally controllable sampling depth below the sample’s surface. Heat deposited within this depth transfers to the surrounding gas at the sample surface, producing a thermal-expansion-driven pressurization in the gas, known as the PAS signal, which is detected by a microphone. The magnitude of the PAS signal varies linearly with increasing absorptivity, concentration or sampling depth until at high values of their product a gradual roll off in sensitivity (saturation) occurs. The phase of the PAS signal corresponds to the time delay associated with heat transfer within the sample. These signal components are described in detail in the next section.
            PAS signal generation is initiated when the FT-IR beam, which oscillates in intensity, is absorbed by the sample resulting in the absorption-induced heating in the sample and oscillation of the sample temperature. The temperature oscillations occurring in each light-absorbing layer within the sample launch propagating temperature waves called thermal-waves, which decay strongly as they propagate through the sample. It is this thermal-wave decay process that defines the layer thickness, or sampling depth, from which spectral information is obtained in an FT-IR PAS analysis. The sampling depth can be increased by decreasing, via FT-IR computer control, the IR beam modulation frequency imposed by the interferometer. The lower modulation frequency allows a longer time for thermal-waves to propagate from deeper within the sample into the gas. As the sampling depth increases, the saturation of strong bands in PAS spectra increases just as it does in absorbance spectra measured by transmission as sample thickness increases.
            The discovery of the photoacoustic effect by Alexander Graham Bell in 1880 marked the beginning of the development of the technique as a useful spectroscopic method.⁴ Development was hampered, however, by the weak acoustic signals that must be measured due to the very high thermal-wave reflection coefficient at the sample-to-gas interface. A high fraction of the thermal-wave amplitude is reflected back into the sample and is not detected, leading to signal-to-noise problems. Signal saturation also was a problem in the initial efforts to apply the technique in the ultraviolet and visible spectral regions. Operation in the near- and mid-infrared spectral regions, made practical with the multiplexing capability of FT-IR systems and the higher sensitivity of