Anal. Chem. 1985, 57,871-875
fuel samples. In principle the NMR analysis could be applied to a total saturates fraction. However, if n-alkanes are a major component, it is probable that the accuracy of parameters reflecting the presence and nature of branched and cyclic saturates would be reduced. It is for this reason that in the present work the major emphasis has been placed on a separate branched plus cyclic saturates fraction. Saturates differ in terms of branching and ring structures. It is believed that these characteristics can be sufficiently well-defined using the GASPE 13C NMR procedures to delineate clear differences in the composition of samples obtained from different sources.
(10) Cookson, D. J.; Smith, B. E. Fuel 1983, 62,986. (11) Weiss, F. T. "Chemical Analysis"; "Determination of Organic Compounds: Methods and Procedures" Ewing, P. J., Kolthoff, I.M., Eds.; Wiley-Interscience: New York, 1970; Vol. 32 Chapter 1, p 14 (12) Cookson, D. J.; Smith, B. E. J . Magn. Reson. 1984, 5 7 , 355. (13) Fetterly, L. C. "Non Stoichiometric Compounds"; Mandelcorn, L., Ed.; Academic Press: New York, 1964; Chapter 8. (14) Bendall, M. R.; Pegg, D. T. J . Magn. Reson. 1983, 5 3 , 272. (15) Biidsoe, H.; Donstrup, S.;Jakobsen, H. J. J . Magn. Reson. 1983, 53, 154. (16) "API Technical Data Book-Petroleum Refining"; American Petroleum on of Refining: Washington, DC, 1970; Vol. 1, Chapter 1. (17) Henze, H. R.; Blair, C. M. J . Am. Chem. SOC. 1931, 53, 3077. (18) Lindeman, L. P.; Adams, J. Q. Anal. Chem. 1971, 4 3 , 1245. (19) Grant, D. M.; Paul, E. G. J . Am. Chem. SOC. 1964, 86,2984. (20) Williams, R. B. ASTM Spec. Tech. Pub/. 1958, No. 224, 168. (21) Brown, J. K.; Ladner, W. R. Fuel 1980, 3 9 , 87. (22) Clutter, D. R.; Petrakis, L.; Stenger, R. L.; Jensen, R. K. Anal. Chem. 1972, 44, 1395. (23) Retcofsky, H. L. Appl. Spectrosc. 1977, 3 7 , 116. (24) Bartle, K. D.; Jones, D. W. "Analytical Methods for Coal and Coal Products"; Karr, C., Ed.; Academic Press: New York, 1978; Vol. 2, p 104. (25) Bartie, K. D.; Ladner, W. R.; Martin, T. G.; Snape, C. E.; Williams, D. F. Fuel 1979, 58, 413. (26) Yokoyama, S.;Uchino, H.; Katoh, T.; Sanada, Y.; Yoshida, T Fuel 1981, 60,254. (27) Netzel, D. A.; Miknis, F. P. Fuel 1982, 61,1101. (28) Petrakis, L.; Allen, D. T.; Gavalas, G. R ; Gates, B. C. Anal. Chem. 1983, 55, 1557. (29) Awadalla, A. A.; Cookson, D. J.; Smith, B. E. Fuel, in press.
ACKNOWLEDGMENT We thank C. J. Rix of the Royal Melbourne Institute of Technology and personnel of the BHP Petroleum Laboratory for assistance. LITERATURE CITED (1) Bartle, K. D.; Jones, D. W.; Pakdel, H. "Molecular Spectroscopy"; West, A. R., Ed.; Heydon: London, 1976; Chapter 9. (2) Netzel, D. A. "Synthetic Fuels from Oil Shale Symposium", Atlanta, GA, Dec 1979; p 271. (3) O'Donnell, D. J.; Sigle, S. 0.; Berlin, K. D.; Sturm, G. P.; Vogh, J. W. Fuel 1980, 5 9 , 166. (4) Takegami, Y.; Watanabe, Y.; Suzuki, T.; Mltsudo, T.; Itoh, M. Fuel 1980, 59, 253. (5) Suzuki, T.; Itoh. M.: Watanabe, Y.: Mitsudo. T.: Takeaami. Y. J . JDn. Pet. Inst. 1981, 2 4 , 151. (6) Gillet, S.;Rubini, P.; Delpeuch, J.J.; Escalier, J.-C.; Valentln, P. Fuel 1981, 60,226. (7) Netzel, D. A.; McKay, D. R.; Heppner, R. A.; Guffey, F. D.; Cooke, S. D.; Varle, D. L.; Linn, D. E. Fuel 1981, 60,307. (8) Cookson, D. J.; Smith, B. E. Org. Magn. Reson. 1981, 16, 111. (9) Cookson, D. J.; Smith, B. E. Fuel 1983, 6.2,34.
871
RECEIVED for review August 21, 1984. Accepted December 10, 1984. Support for this work was provided under the National Energy Research Development and Demonstration Program, administered by the Australian Commonwealth Department of Resources and Energy.
Comparison of Photoacoustic and Attentuated Total Reflectance Sampling Depths in the Infrared Region Daniel A. Saucy, Steven J. Simko, and Richard W. Linton*
Kenan Laboratories of Chemistry, Department of Chemistry, University of North Carolina, Chapel Hill,North Carolina 27514
Attentuated total reflectance (ATR) and photoacoustic spectroscopy (PAS) are surface-sensitlve I R techniques. The maximum analytical depths for both techniques were compared for organlc thln fllms by preparing polymers conslstlng of known thicknesses of polystyrene (PS) on poly(methy1 methacrylate) (PMMA). Relative contrlbutlons of PS and PMMA to the spectra (1601 cm-', 1732 cm-', 3028 cm-' bands) suggest that ATR samples to a depth of p. The first part of the table demonstrates the effect of thermal diffusivity on sampling depth. The latter portions examine the effect of band position and mirror velocity, respectively, over a typical range of values. The table shows that, indeed, PS is a representative compound in terms of its thermal properties. The range of predicted maximum sampling depths for frequently encountered experimental conditions is approximately 10-30 pm. Values at the lowest wavenumbers (approaching 400 cm-') are somewhat higher. Table I11 is a similar compilation of sampling depths for ATR. Again, the effects of various experimental parameters are examined. Polystyrene has a slightly higher than average index of refraction and thus the ATR sampling depths calculated for it are slightly larger than those for the averages in the polymer or organic compound categories. The sampling depth range estimated for the typical parameter values in an ATR experiment is approximately 0.6-3 pm. Values a t the lowest wavenumbers (approaching 400 cm-l) are somewhat higher. Comparing the ATR and PAS sampling depth ranges suggests that ATR consistently samples less deeply than does PAS, a conclusion supported by our experimental results. In addition, the variation in sampling depth over a given wavenumber range is greater for ATR than for PAS because of different proportionalities to the band wavenumber. Thus, PA and ATR spectra can exhibit differences in relative peak intensities even in a thick film sample. The assumption of optical transparency made for the PAS calculations deserves some further discussion. The optical
absorption coefficient, p, for the aromatic C-H stretch band in PS (1601 cm-') is approximately 450 cm-'. For a very strongly absorbing band, such as the C=O stretch in PMMA, can be on the order of 2000 cm-' ( 4 ) , giving an optical absorption length of 5 pm. Thus, it is possible to have samples where a weakly absorbing band is classified as being optically transparent while a strong band is opaque. The effect that this situation will have on the spectrum depends on the relation between 1/p and p. As long as p < l/p, regardless of the sample thickness, the PA signal will be proportional to p and the band will be observed. If, on the other hand, 1/p C p C sample thickness, one has photoacoustic saturation and the PA signal becomes independent of 0. As Krishnan ( 4 ) has noted, saturation can indeed occur in mid-IR/PA spectra and affects relative peak heights. Thus, the value of p must be considered in analyzing PA spectra and sampling depths.
ACKNOWLEDGMENT The help of Ann Richard in understanding photoacoustic theory is gratefully acknowledged. Bruce Gay of U S . EPA is thanked for the use of a globar IR source, and Royce Murray is thanked for access to the spin coater and profilometer. LITERATURE CITED (1) Rosencwaig, A.; Gersho, A. Science 1975, 790, 556. (2) Rosencwaig, A,; Gersho, A. J. Appl. Phys. 1978, 4 7 , 64. (3) Rosencwaig, A. "Photoacoustics and Photoacoustic Spectroscopy"; Wiiey-Interscience: New York, 1980. (4) Krishnan, K. Appl. Spectrosc. 1981, 35, 549. (5) Harrick, N. J. "Internal Reflection Spectroscopy"; Wiley-Interscience: New York, 1967. (6)Gardella, J. A., Jr.; Grobe, G. L., 111; Hopson, W. L.; Eyring, E. M. Anal. Chem. 1984, 56, 1169. (7) Woodard, F. E.; Woodward, W. S.; Reilley, C. N. Anal. Chem. 1981, 53,1251A-1266A. ( 8 ) Teng, Y. R.; Royce, B. S. H. Appi. Opt. 1982, 21, 77-80. (9) "Thermophysical Properties of Matter"; Toulouklan, Y. S., Ed.; IFI/Plenum: New York, 1973; Vol. 10. (10) McCleiland, J. F.; Kniseley, R. N. Appl. Opt. 1978, 15, 2658-2663. (11) Vidrine, D. W. I n "Fourier Transform Infrared Spectroscopy: Techniques in Fourier Transform Interferometry"; Ferraro, J. R., Baslle, L., Eds.; Academic Press: New York, 1982; Vol. 3, Chapter 4. (12) Thomas, H. R.; O'Maliey, J. J. I n "Photon, Electron and Ion Probes of Polymer Structure and Properties"; Dwight, Fabish, Thomas, Eds.; American Chemical Society: Washington, DC; 1981; pp 319-338.
RECEIVED for review August 24, 1984. Accepted December 26, 1984.
