Anal. Chem. 1987, 59, 2378-2382
2378
Effects of Particle Size on Quantitative Photoacoustic Spectroscopy Using a Gas-Microphone Cell Peter S. Belton* and Reginald H. Wilson AF.*C Institute of Food Research (Norwich Laboratory), Norwich NR4 7UA, United Kingdom
Alfred M. Saffa School of Chemical Sciences, University of E a s t Anglia, Norwich NR4 7TJ, United Kingdom
The effects of particle size on the Intensity of disperslve and Fourier transform Infrared photoacoustic spectra of sucrose, carbon black, and sucrose/potassium bromide mixtures in gas-microphone cells are reported. I t Is concluded that the principal factor affectlng signal tntensltles Is the efflclency of energy transfer from solld to gas. The effects of the concentration of chromophore In sucrose/potassium bromide nrixtures may be quantlfied by using a simple reciprocal relationship between signal intensity and chromophore concentratlon whlch was derived by conslderlng only the thermal term of energy transfer.
Photoacoustic spectroscopy (PAS) is by now well recognized as a valuable spectroscopic technique; however its potential as a quantitative analytical method has not been fully exploited. This is due, in part, t o a n uncertainty about the effects of particle size on signal strength in gas-microphone cells. Such effects have been recognized since the beginnings of the application of PAS t o particulate materials (1-6) and have continued to attract attention (7-10). The main conclusions of these observations is that increasing particle size results in decreasing signal intensity. Three recent publications dealing with the theory of the photoacoustic effect in powders ( 11-13) have indicated that quantitative photoacoustic spectroscopy in particulate systems is difficult. If this is the case in practice, then the value of photoacoustic spectroscopy as an analytical tool is severely limited. Very recently Yang and Fateley (14, 15) have shown that the relative intensities of peaks at different wavelengths vary with particle size in Fourier transform PAS. Once again these results are indicative of potential problems in quantitative analytical applications. Particle size may affect both the absolute intensity of the signal and t h e way in which it changes on dilution of the chromophore. In order to examine these effects, we have chosen to use a model system consisting of sucrose and potassium bromide. Sucrose was chosen because its infrared spectrum is representative of carbohydrates which are of considerable interest for practical analytical applications and it may be obtained in a high degree of purity; further its crystals may be readily ground to give a uniform particle size without risk of chemical change or change in crystal habit. Potassium bromide was chosen because of similar crystalline properties and the fact that it was transparent in the spectral region of interest. Comparative experiments were also carried out with carbon black. Both dispersive and Fourier transform experiments were carried out both to give a range of modulation frequencies and wavelengths and to compare the various spectroscopic techniques.
EXPERIMENTAL SECTION Sucrose and potassium bromide were AnalaR reagents. The carbon black was a commercial lumpwood charcoal made from
Table I. Particle Size Ranges, Bulk Densities, and Porosities of Sucrose (s) and Carbon Black (c) Powders
range of particle mean density density diameters/ diameter/ (s)/g (c)/g porosity porosity
w
,elm
