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construction of the amplifier, and Edwin M. Bryant for many helpful suggestions in preparing the manuscript. Registry No. Disodium protoporphyrin, 50865-01-5; pyrene, 129-00-0.
LITERATURE CITED (1) Rosencwaig, A.; Hlndley, T. W. Appl. Opt. 1981, 20, 606-609. (2) Patel, C. K. N.; Tam, A. C. Rev. Mod. f h y s . 1981, 53, 517-550. (3) hi,E. P. C.; Voigtman, E.; Wlnefordner, J. D. Appl. Opt. 1982, 21, 3126-3128.
(6) ial; E. P. C. Ph.D. Dissertation, Universlty of Florida, Gainesvllle. FL, 1982, pp 97-99.
(7) Farrow, M. M.;Burnham, R. K.;Auzanneau, M.;Olsen, S. L., Purdie, N.; Eyring, E. M. Appl. Opt. 1978, 17, 1093-1098. (8) Blitz, J. ”Fundamentals of Ultrasonics”; Plenum: New York, 1967; p 25. (9) Weast, R. C., Ed. “CRC Handbook of Chemistry and Physics”, 55th ed.; CRC Press: Cleveland, OH, 1974; p E-47. (IO) Major, R. W.; Hutton, S. L. Appl. Opt. 1982, 21, 1159-1161. (11) Lal, P. C., unpubllshed work, Bowling Green State University, Bowling Green, OH, 1983.
RECEIVED for review July 7,1983. Accepted September 14, 1983. This work was supported by Grant No. 8318 from the Biomedical Research Support Program and in part by a part-time associateship from the Faculty Research Committee, Bowling Green State University.
Thermal Gradient Lamps for Dispersive Flame Atomic Fluorescence Spectrometry M. D. Seltzer and R. G . Michel* Department of Chemistry, University of Connecticut, Storrs, Connecticut 06268 Many of the characteristics of thermal gradient lamps (TGLs) have been described by Gough and Sullivan (1-3). Their work demonstrated that for a variety of volatile metals (arsenic, cadmium, selenium, zinc) TGLs are useful as sources for nondispersive atomic fluorescence spectrometry (AFS)and for atomic absorption spectrometry (AAS). This is because of their narrow line widths and high intensities. No results have yet been reported for the use of TGLs as the excitation source for AFS with conventional dispersive detection. Our experience with rigorously optimized microwave excited electrodeless discharge lamps (EDLs) (4-7)for dispersive AFS allows a meaningful quantitative comparison of the intensity of TGLs when compared to microwave excited EDLs, in order to confirm the potential of TGLs for use in atomic spectrometry.
EXPERIMENTAL SECTION The instrumentation used was that described in ref 7 and involved the use of a nitrogen separated air-acetylene flame, an f/4.2 double monochromator with a 0.5-nm spectral bandwidth, a UV-sensitive photomultiplier tube, and photon counting detection. For selenium and arsenic lamps, detection limits were measured with the monochromator flushed with nitrogen to reduce absorption in the UV below 200 nm. Three thermal gradient lamps, selenium, cadmium,and arsenic, and their associated power supply were supplied by Scientific Glass Engineering, Inc. (SGE), of Austin, TX. Atomic fluorescence signals were measured and detection limits were calculated for the TGLs in the same way as for EDLs in all previous papers (4-7). Cadmium metal, selenium oxide, and arsenious oxide were used to prepare 2000 Fg/mL aqueous stock solutions,0.04 M in hydrochloric acid. The arsenious oxide was first dissolved in a minimum volume of 20% NaOH. Chemicals were analytical reagent grade. RESULTS AND DISCUSSION The detection limits obtained here for cadmium and selenium TGLs are compared in Table I with EDL detection limits obtained by us. Cadmium and selenium TGL detection limits were measured 5 OC above the recommended operating temperatures which were 160 ‘C and 170 OC, respectively. The cadmium TGL detection limit was a factor of 7 worse than the EDL, and the selenium TGL detection limit was a factor of 3 worse. The difference in detection limits obtained with TGLs and EDLs is a result of the difference in the relative intensities of the two types of lamps because background signals and noise magnitudes were the same for both TGL and EDL measurements. 0003-2700/83/0355-2444$01.50/0
Table I. Comparison of Atomic Fluorescence Detection Limitsa (rg/L) EDL wavelength, (microwave metal nm TGL excited) 0.07 228.8 0.5 cadmium selenium 100 30 196.0 Cadmium detection a Signal-to-noise ratio of two, limits were measured by using a 5 Mg/L solution and selenium limits with a 1 0 Wg/mL solution. The cadmium EDL detection limit was reported in ref 5 and has been reproduced consistently since then on the instrumentation used here. The selenium EDL detection limit was reported in ref 6 using the same instrumentation as ref 5 . Hence the EDL detection limits are probably very consistent although selenium EDLs were not directly compared in this work. A stability check of the change in cadmium TGL atomic fluorescence signal over an hour revealed a drift of about 1% per hour. The lamp was operated at a temperature of 160 ‘C which is the manufacturer’s recommended operating temperature. This stability is somewhat better than that observed for microwave excited EDLs which have typically a downward drift of 3-5% per hour. This c o n f i i s the TGL drift observed by Gough and Sullivan by direct measurement of TGL output light intensity. Short-term fluctuations in TGL intensity were insignificant since detection limits in the work reported here were limited by flame background noise. At higher concentrations, short term lamp fluctuations did not increase the noise on the fluorescence signal. The power supply provided for the TGLs allows control of the operating temperature of the lamps with much the same idea in mind as the thermostating of EDLs suggested by Browner and Winefordner (8) and used by us ( 7 ) . Temperature control of the TGLs is by heated filament rather than heated air. However, the effect is much the same as shown in Figure 1, where a rapid increase in atomic fluorescence signal with temperature is observed. At operating temperatures higher than those recommended by the manufacturer, lamp stability was the same as a t the recommended temperature but warmup periods of more than half an hour were required in order to attain stable operation. We did not attempt to go beyond the highest temperatures shown in Figure 1because the manufacturer claims reduced lifetime of the TGLs at high temperatures. Hence, it was not 0 1983 American Chemical Society
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Anal, Chem. 1983, 55,2445-2448
"1
3
LL
nm, 193.7 nm, and 197.2 nm. However, detection limits for arsenic were poor, about 100 pg/mL. Hence calibration curves and temperature effects were not investigated for arsenic. TGLs appear to be easier to use than microwave excited EDLs because the current method of wing hot air temperature stabilization for EDLs is more cumbersome than the heated filament used in TGLs. The high stability and excellent sensitivity obtained for AAS (2) and the good detection limits obtained here for dispersive AFS and elsewhere for nondispersive AFS (3) indicate that TGLs are a promising light source for atomic spectrometry. We plan to take a closer look a t the relative intensities of TGLs and EDLs by calibration of our monochromator with a calibrated deuterium arc in order to find out why we could not get better detection limits for arsenic.
P
j 0 ,
100
I
I
I
I
I
I
I
110 120 130 1LO 150 160 170 TGL OPERATING TEMPERATURE P C )
Figure 1. Effect of variation of operating temperature of two TGLs. The selenium concentration used was 100 pglmL. The cadmium concentration was 50 pg/L.
possible to explore the existence of the plateau normally seen for variation in operating temperature of EDLs (9). A plateau is generally thought to improve stability of EDLs (9) and probably improves warmup times of EDLs by making attainment of a particular temperature less critical. This could explain the longer warmup times noted here for TGLs operated at higher than the recommended temperature but it may be that the TGL power supply is slow to drive the temperature to the higher level. Calibration curves for cadmium and selenium were linear from the detection limit to about 4 orders of magnitude above the detection limit. This is as expected for AFS with intense line sources. Attempts were also made to obtain atomic fluorescence signals for three different arsenic TGLs at three lines, 235.0
ACKNOWLEDGMENT We acknowledge with thanks SGE, Inc., for providing the TGLs and their power supply. Registry No. Cadmium, 7440-43-9; selenium, 7782-49-2. LITERATURE CITED (1) Gough, D. S.; Sullivan, J. V. Anal. Chlm. Acta 1979, 108, 347-350. (2) Gough, D. S.; Sullivan, J. V. Anal. Chlm. Acta 1981, 124, 259-266. (3) Norrls, T.; Sullivan, J. V. Am. Lab. (Falrfldd, Conn.) 1982, 14 (12), 67-7 1. (4) Mlchel, R. G.: Coleman, Julia; Wlnefordner, J. D. Spectrochim. Acta, Parl B 1978, 338, 195-215. ( 5 ) Mlchel R. G.; Ottaway, J. M.; Sneddon, J; Fell, G. S. Analyst (London) 1878. - - - - ,103. . _ _ .1204-1209. . (6) Mlchel, R. G.; Ottaway, J. M.; Sneddon, J; Fell, G. S. Analyst (London) 1879. ... - , 104. . - , 687-691. . .. . (7) Seltzer, M. D.; Mlchel, R. G. Anal. Chem. 1983, 55, 1817-1819. (8) Browner, R. F.; Patel, B. M.; Glenn, T. H.; Rletta, M. E.; Wlnefordner, J. D., Spectrosc. Left. 1972, 5 , 311-318. (9) Browner, R. F.; Wlnefordner, J. D. Spectrochlm. Acta., Part B 1973, 288, 283-288.
RECEIVED for review July 18, 1983.
Accepted September 1, 1983. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research.
Sample Deoxygenation for Fluorescence Spectrometry by Chemical Scavenging M. E. Rollie, C.-N. Ho, and I. M. Warner* Department of Chemistry, Emory University, Atlanta, Georgia 30322 Many organic compounds fluoresce and this property is widely used for analysis (1-3). The ground state of most organic molecules is a singlet state (i.e., all of the electrons in the molecule are spin paired). To produce measurable fluorescence emission, molecules must initially be promoted from the ground singlet state (designated So) to an excited singlet state. (The first and second excited singlet states are designated S1and S2,respectively.) This is commonly done by absorption of ultraviolet radiation. If the molecule is promoted to the second excited singlet state (SJ, it will rapidly undergo nonradiative deexcitation to the lowest vibrational level of the lowest excited singlet state (SI) by a combination of internal conversion and vibrational relaxation. Once the molecule reaches the lowest vibrational level of SI,it can return to So by emission of a photon. Thus, fluorescence is essentially the emission of a photon by an excited molecule that is returning to the ground state via a singlet-singlet transition (S, So). Generally, fluorescence emission occurs very rapidly
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after excitation with lifetimes on the order of 1 X to 1 x 10-7 9. Various nonradiative deexcitation processes compete with and often greatly reduce fluorescence emission. Of these processes, quenching has the most pronounced effect. Quenching of fluorescence is defined as any process that results in a decrease in the true fluorescence efficiency of a molecule (4). Quenching processes divert the absorbed energy of the molecule into channels other than fluorescence (5). The presence of molecular oxygen contributes significantly to fluorescence quenching because most organic molecules in an excited state will nonradiatively deactivate after one or two collisions with molecular oxygen (6). Effects of oxygen quenching are most serious for solutions of aromatic hydrocarbons (7,81, but the fluorescence of virtually all organic compounds is quenched, at least slightly, by oxygen (9-12). Therefore, it is often important that samples be deoxygenated prior to fluorometric analysis. 0 1983 American Chemical Soclety
