Studies of the analytical utility of low-temperature photoacoustic spectroscopy

The present study investigated the analytical utility of laser-excited, low-temperature photoacoustic (PA) spectroscopy, particularly for the detection of weakly fluorescing and non-fluorescing polycyclic aromatic compounds (PACs) isolated in low-temperature (5-20 K) vapor deposited matrices. Photoacoustic spectroscopy of other low-temperature samples was also studied.

In order to accomplish this objective, piezoelectric (PE) transducers were evaluated as low-temperature PA detectors. Two PE materials were studied, X-cut crystalline quartz and lead-zirconate-titanate (PZT). PZT was found to exhibit ~10 times higher sensitivity. PA spectra of Nd(OH)₃ powder at 6-8 K were taken to demonstrate the ability to acquire piezoelectrically-detected PA spectra at low-temperatures.

Laser-excited PA spectra were recorded for a number of matrix-isolated PACs. The detection limits were 300 ng for quinoline and 50 ng for pyrene. Both of these mass detection limits corresponded to a limiting detectable thermal energy liberated of ~175 nJ. Singlet-triplet intersystem crossing (i.e., S₁ T₁) was determined to be the primary source of the liberated thermal energy which generates the PA signal. The S₁-T₁ energy gap is ~30-40 % of the total S_n-S_o energy gap (i.e., the absorbed photon energy) for most PACs. Attempts to significantly enhance the PA signal by using a heavy-atom matrix (Xe) to increase the S₁ T₁ process were not successful.

The main limitation to the analytical utility of the technique was that most of the non- and weakly-fluorescing PACs studied did not yield highly resolved spectra when matrix isolated. This fact adversely affected the ability to identify individual compounds in multi-component samples. A detector was developed for obtaining PA spectra of samples in frozen solutions at 77 K. Laser-excited PA spectra of Nd(OH)₃ powder dispersed in low-temperature glasses (isopropanol and glycerine/H₂O) were obtained. However, attempts to obtain spectra of PACs in frozen n-alkane solutions were not successful because of the highly scattering nature of the samples.