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corresponding gravimetric values, providing additional evidence that the N as NH4Cl was quantitatively recovered. The NH&l may therefore be used not only for isotopic N analysis b u t for total N determinations. More sensitive N-detection methods, such as indophenol blue (10) and halide pulse polarography (111, might also be used with the resulting NH4C1, especially if this semimicro system were used for microgram quantities of N.
LITERATURE CITED (1) G. E. Schuman, M. A. Stanley, and D. Knudsen, SoilSci. SOC.Am. Roc., 37, 480 (1973). (2) J. M. Bremner in "Methods of Soil Analysis", C. A. Black, Ed., American Society of Agronomy, Madison, Wis., 1965, pp 1149-1178. (3) J. M. Bremner in "Methods of Soil Analysis", C. A. Black, Ed., American Society of Agronomy, Madison, Wis., 1965, pp 1256-1286. (4) T. Yoneyama and K. Kumazawa, Plant Ce/i Physiol., 15. 655 (1974). (5) E. Conway, "Microdiffusion Analysis and Volumetric Error", Crosby Lockwood, London, 1957, pp 139-153. (6) L. K. Porter and W. A. O'Deen, Anal. Chem., 49, 514 (1977). (7) R . D. Hauck and J. M. Bremner, Adv. Agron.. 28, 219 (1976).
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(8) A. E. Martin and P. J. Ross, Trans. I n f . Congr. Soil Sci., 9fh, 3, 521 (1968). (9) R. N. Galiaher, C. 0. Weidon, and F. C. Boswell, SoilSci. SOC. Am. J., 40, 887 (1976). (10) J. K. Fawcett and J. E. Scott, J . Clin. Pathol., 13, 156 (1960). (11) W. A. O'Deen and R. A. Osteryoung, Anal. Chem., 43, 1879 (1971).
RECEIVED for review August 23, 1978. Accepted December 26, 1978. T h e work reported here was presented in part a t the Third International Conference on Stable Isotopes, Oak Brook, Ill., 1978, and at the 20th Annual Rocky Mountain Conference on Analytical Chemistry, Denver, Colorado, 1978. Contribution of USDA, Science and Education Administration, Agricultural Research, Fort Collins, Colorado, in cooperation with Colorado State University Experiment Station, Fort Collins, Colorado. Trade names used in text are included for the reader's convenience, and such inclusion does not constitute any preferential endorsement of products named over similar products available on the market.
General Method for Overcoming Photoacoustic Saturation in Highly Colored Organic and Inorganic Solids William H. Fuchsman" Chemistry Department, Oberlin College, Oberlin, Ohio 44074
Ann J. Silversmith Gilford Instrument Laboratories, Inc., 132 Artino Street, Oberlin, Ohio 44074
Recent interest in the application of photoacoustic spectroscopy ( 1 , 2 )t o samples which absorb visible light has led not only to the commercial production of photoacoustic spectrometers b u t also to careful examination (2) of the advantages and limitations of the technique. A general problem in the application of photoacoustic spectroscopy to highly colored solids is saturation, which results in featureless spectra. Saturation occurs when light is so effectively absorbed that the spectrum ceases t o be dependent on the sample absorptivity. In photoacoustic spectroscopy, a layer of material with thickness approximately equal to the thermal diffusion length p of the material can contribute to the signal; p = f-' *, where f is the frequency of modulation. The relative sizes of p and the optical penetration depth 1/13 determine whether saturation occurs. If the material is so highly absorbing that all of the light is absorbed in the first thermal diffusion length (1/8
