The real power of the technique appears to lie in the feature that it allows a multielemental analysis directlyi. e., without sample preparation-of many different types of samples. Other samples analyzed but not discussed include human hair, cloth, paper, other filter media, and even a section of a butterfly wing which showed the lead and bromine normally associated with automobile exhaust emissions. Computer analysis of the spectra is fast, permitting the determination of 20 elements in as little as 2 minutes on our off-line computer. Therefore, it does appear that proton excited X-ray emission shows real potential as a rapid, mass-screening or survey tool. This could apply to whole-body tissue analyses as well as analyses of pl samples of blood or urine from large populations being surveyed in environmental pollution studies. Note Added in Proof: Very recently a review article by Goulding and Jaklevic has appeared in Annual Reviews of Nuclear Science (1973) in which comparison is made between detectability limits for X-ray fluorescence and proton-induced X-ray emission systems. The calculations upon which they base their conclusions, i. e., bremsstrahlung flux and characteristic X-ray yields are in reasonable agreement with values that can be derived from our data shown here, e.g., in Figure 5. However, the 3 u detectability limits for Zn that can be determined from Figures 14 and 15 (where Zn is present a t a level of around 40 ppm dry weight) would be around 0.5 ppm (dry weight) for a run of about 5-min duration. These results do not contradict the stated value of a detectability limit of 14 ppm designated by
Goulding and Jakelevic for different beam and target conditions, but it certainly tempers their explicit comparisons and the implicit conclusions presented.
ACKNOWLEDGMENT We would like to acknowledge the generous assistance of Janette Stanford, Alan Larkin, and Mike Corcoran in preparing targets and in data accumulation and analysis. The botanical studies were made possible through contributions of J . Antonovics, and the ion-exchange studies through the efforts of C. H. Lochmuller and J. Galbraith. The cooperation of E. G. Bilpuch and H . W. Newson of TUNL is sincerely appreciated. Contributions from G. K. Klintworth, D. Paulson, and L. Goldwater of the Duke Medical Center, K. V. Rajagopalan of the Duke Biochemistry Department, and B. A . Fowler of the National Institute for Environmental Health Studies is gratefully recognized. Special gratitude is owed to S. M. Shafroth from the University of North Carolina for the use of his X-ray chamber and Si(Li) detector in the first running period and to A. B. Baskin for his efforts to provide us with a suitable, semi-automatic peak fitting code. Received for review July 27, 1973. Accepted February 4, 1974. This work was supported in part by the U S . Atomic Energy Commission and the U S , Environmental Protection Agency. Additional support for one of us (J.M.J.) from the North Carolina Board of Science and Technology is also gratefully acknowledged.
Cavity-Dumped Argon-Ion Laser as an Excitation Source in Time-Resolved Fluorimetry F. E. Lytle and M. S. Kelsey Department of Chemistry, Purdue University, Lafayette, Ind. 47907
Both the operational principles and the experimental aspects of cavity-dumping the continuous wave (CW) argon-ion laser are discussed. The technique is used to construct an excitation source for time-resolved fluorimetry having the advantages of a variable repetition rate (-10 MHz to single-shot), variable pulse width ("9 nsec to CW) and moderate peak power per unit band width (-12 W in the blue-green and 600 mW in the UV). Two classes of sample data are shown for molecules at trace (ppb) levels. The first type is intensity-vs-time at a fixed wavelength. This allows the calculation of the fluorescence lifetime. The second type is in tensity-vs.-wavelength at an arbitrary but constant time with respect to the excitation pulse peak. This allows the display of the fluorescence spectrum without interference from scatter and Raman lines. Finally, future instrumental advances are discussed that should allow much shorter pulse widths and greater wavelength selection.
A brief survey of the review literature (1-3) in the field of time-resolved fluorimetry indicates the tremendous advances made during the past decade in detector and am-
plifier technology. Concomitantly it has become apparent that the excitation source is now usually the restricting feature with respect to the lower achievable limits on concentration and lifetime. Therefore, continued enhancements in performance will require improved methods of generating short duration light pulses. The instrumentation reported in this paper is the first step in an overall program designed to explore solutions to this problem. Before designing an excitation source, it is necessary to decide whether the overall device should yield data based on phase delays or pulse decays. Although both techniques should yield identical results when pushed to their ultimate, some experiments are easier to perform with one arrangement or the other. As an example, phase delays can be used to determine extremely short relaxation rates (1) G. E. Peterson, "Fluorescent Lifetimes of Trivalent Rare Earths," in "Transition Metal Chemistry," Vol. I l l , R. L. Carlin, Ed., Marcel Dekker, New York, N.Y., 1966, pp 202-302. (2) J. B. Birks and I . H. Munro, "The Fluorescence Lifetimes of Aromatic Molecules." in "Progress in Reaction Kinetics," Vol. I V , G. Porter, Ed., Pergamon Press, New York, N.Y., 1967, pp 239-32. (3) W . R. Ware, "Transient Luminescence Measurements," in "Creation and Detection of the Excited State." A . A . Lamola, Ed., Marcel Dekker. New York. N.Y., 1971, pp 213-32.
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with high degrees of accuracy and precision ( 4 ) . On the contrary, time-resolved spectra and complicated decay schemes are obtained more readily from a pulse decay (5, 6). Because of the nature of our interests, we have restricted attention to this latter technique. In pulse decay fluorimetry, the source has three major characteristics: time-intensity profile, peak power, and repetition rate. First, the full width a t half maximum (FWHM) of the excitation should be a t least comparable to the measured relaxation. However, the information is easiest to interpret when the fluctuation appears as a delta function on the time scale of interest. Second, since the magnitude of the signal is proportional to the peak power, it is desirable to have this value as large as practical. Realistically, though, one is probably restricted to pulses
