Anal. Chem. 1982, 5 4 , 2191-2194
2191
Effects of Concentration and Sample Preparation in Photoacoustic Spectroscopy of Powdered Samples Doreen King and
R. Stephen
Davldson"
Chemistry Department, The City University, Northampton Square, St. John Street, London EC1 V 4PB, United Kingdom
M. Phiillps Physics Department, The City University, Northampton Square, St. John Street, London EC 1 V 4PB, United Kingdom
The technique of dilution of powdered samples with inert supports to overclome signal saturation has; been Investigated. Dilution OX potassium dichromate was accomplished by grlnding with inert supports and also by evaporation of aqueous solutions of dichromate containing the support. The evaporation procedure was found to have a profound infiuence on the phaiioacoustic signal and it is suggested that unless the effects of the evaporation process on the crystal form and distribution of the sample on the support are known, this method of dilution should be avoided. Satisfactory spectra were obtained by grinding samples in the presence of either magnesium oxide or silica, and a Kubeika-Munk treatment resulted in a linear relationship between sample concentratlon anti photoacoustic signal amplitude over a wide concentration range for these and the other supports. The other supports studied were alumina, lithium fluoride, and barium sulfate.
A theory for the photoacoustic effect was proposed by Rosencwaig and Gersho ( I , 2) and assumes that the primary source of the acoustic signal results from a periodic heat flow from the solid t o the surrounding gas, as tlhe solid is cyclically heated by the absorption of the chopped light. This theory is now widely accepted and provides the groundwork mathematical treatment of the photoacoustic effect of solids. The theoretical analysis shows that the phosoacoustic signal is, to a large extent, gioverned by the magnitude of the thermal diffusion length of the solid. When the sample is optically opaque, if wuo(optical path length) is less than the value of w8 (thermal diffusion length), the photoacoustic signal obtained will depend upon the source power spectrum a t the wavelength of inherest, and the photoacoustic signal will not be proportional to the value of the absorption coefficient. This is because all the incident radiation is absorbed. T o avoid saturation of the photoacoustic signal in opaque samples the signal modulation frequency may be increased, or alternatively the sample may be diluted with a substrate. Potassium dichromate can be considered to be an extremely strong absorber. The photoacoustic spectrum of potassium dichromate has been investigated as a function of particle size distribution, and it has been found that the undiluted sample behaves as a typical strong absorber with an increase in the apparent absorption with a decrease in particle size (3). Particle size has a similar affect upon diffuse reflectance (3). Furthermore, a number of different colorless inert substrates have been used to dilute various compounds (3-5). Dilution may be achieved by grinding the sample and substrate togetheir or by evaporating a solution of the sample with the substrate. It has been reported that a full and careful Kubelka-Munk type analysis of photoacoustic spectroscopy data could yield an optical spectrum closely fitting the true absorption spectrum of the solid sample over a wide range of
sample absorbance values (3),and for undiluted arsenic sulfide it has been reported that for a small value of l othe photoacoustic signal could be explained in terms of Melamed's analysis of diffuse absorption and reflection (6). However, concentration effects have not been systematically studied when compounds are diluted with various substrates, and a comparison of the effects of dilution by grinding and by evaporation with several substrates is now reported.
EXPERIMENTAL SECTION Dilutions of crystalline potassium dichromate (BDH) were made by hand-grinding a 1:l mixture with the diluent using a ceramic mortar and pestle. Further dilutions were then made from this 1:l mixture by manually tumbling the sample vial containing the two powders. Dilutions 01' crystalline potassium dichromate were also made by dissolving the dichromate in distilled water containing the inert substrate?,followed by evaporation with a rotary evaporator. The sampleti were then allowed t o dry at 60 "C for 16 h. The diluents used were silica (Whatman), magnesium oxide (Fisons), lithium fluoridc (BDH), alumina (BDH), and barium sulfate (Fluka). Photoacoustic spectra were obtained from the EDT Model OAS 400 spectrometer. The light source was a 300-W high-pressure, short arc xenon continuous source with an integral parabolic reflector and a 25 mm diameter sapphire window mounted within a fan-cooledhousing. Afkr the monochromator exit slit a fraction of the dispersed irradiation is reflected onto a pyroelectric detector to provide a reference signal for correction of the photoacoustic signal for variation in source intensity with wavelength and for fluctuations in source ini,ensity with time. An aluminum nonresonant cell assembly was used with a transparent fused silica window. The sample tray was aluminum and measured 1 mm deep, 4 mm wide, and 1Ei mm long. The tray was loaded with the powders to a uniform level without compaction. Diffuse reflectance measurements were carried out on a Perkin-Elmer recording spectrophotometer (EPS-3T) using magnesium carbonate as a reference. Scanning electron microscopy was performed on a Jem 100-B transmission microscope with a scanning attachment. The samples were coated with approximately 5 nm thick sputtered gold (as opposed to the evaporation technique) and examined
