A
system. The Hg line nearly coincides with the 2500 first order blaze wavelength of the grating and thus is transmitted with high relative efficiency. The relative efficiency of a diffraction grating falls off rather rapidly on the short wavelength side of the blaze wavelength (Z7),and at the wavelength of the As lines reflectivity losses lead to a yet more rapid decline in
the overall monochromator transmission efficiency. The 20fold improvement in the limit of detection with the nondispersive system indicates a comparable gain in signal-to-noise ratio and justifies our earlier assumption that a broad bandpass system was advantageous for elements, such as As, which exhibit a complex fluorescence spectrum.
(17) J. F. James and R. S . Sternberg, “The Design of Optical Spectrometers,” Chapman and Hall, London, 1969, p 56.
RECEIVEDfor review September 29, 1971. Accepted February 8, 1972.
Comparison of Atomic Fluorescence with Atomic Absorption as an Analytical Technique William B. Barnett and Herbert L. Kahn The Perkin-Elmer Corporation, Norwalk, Conn. 06856 The virtues of atomic fluorescence were compared with those of atomic absorption for a number of elements. Employing both a modification of a doublebeam atomic absorption instrument and a specially built fluorescence test unit, detection limits, linearities, and analytical interferences were measured. While highly useful results were obtained for atomic fluorescence, it was not superior to atomic absorption in any of the tests which were made.
ATOMICFLUORESCENCE SPECTROMETRY has been described for the past seven years as the wave of the future. Ever since the 1964 publications of Winefordner and coworkers (Z-31,many workers have investigated the technique and found various reasons to prefer it over atomic absorption, including better detection limits for many elements (4-7), a higher dynamic range, and simpler instrumentation (8, 9). Others have indicated doubt as to whether atomic fluorescence could be made applicable to the entire range of elements which is commonly determined. As instrument manufacturers, the authors were interested in investigating atomic fluorescence in sufficient depth to determine whether it would be useful to produce analytical equipment for it. This could take several forms. In its simplest embodiment, it could be an atomic fluorescence accessory to presently existing atomic absorption spectrophotometers. If tests showed fluorescence to possess substantial advantages for a number of frequently determined elements, it would be worthwhile to develop an optimized single-channel instrument. If, on the other hand, fluorescence proved itself substantially equal to absorption for many or most elements, it might be possible to take advantage of the greater optical freedom offered by fluorescence to construct a multi-element spectrophotometer, with which a number of (1) J. D. Winefordner and T. J. Vickers, ANAL.CHEM.,36, 161 (1964). (2) J. D. Winefordner and R. A. Staab, ibid., p 165. (3) Ibid., p 1367. (4) T. S . West and X. K. Williams, ibid., 40, 335 (1968). (5) T. S. West and X. K. Williams, Anal. Chim. Acta, 42,29 (1968). (6) R. M. Dagnall, M. R. G. Taylor, and T. S . West, Spectros. Lett., 1, 397 (1968). (7) J. D. Winefordner and R. C. Elser, ANAL.CHEM., 43, (14) 25A (1971). (8) P. L. Larkins, R. M. Lowe, J. V. Sullivan, and A. Walsh, Spectrochim. Acta, 24B, 187 (1969). (9) D. G. Mitchell and A. Johansson, ibid., 25B, 175 (1970).
elements could be determined simultaneously or in rapid sequence. Before beginning the investigation, certain ground rules were defined, based on the fact that atomic absorption has useful detection limits, relatively few chemical interferences, and almost no spectral interferences. Atomic fluorescence, if it is to be useful, must retain most or all of the advantages of atomic absorption. Therefore, it was decided at the outset to investigate only premixed, acetylene-fueled flames, since it is well known that total consumption burners (IO), and cool flames involving hydrogen or propane (1Z,12)have considerably greater chemical interferences. All experiments were performed with the standard Perkin-Elmer premix burner, with a circular head capable of being employed with a nitrogen or argon sheath gas. The detection limits, linearity, and interferences for various elements obtainable with different instrumental configurations were examined. While this manuscript was being prepared, we obtained a paper by Larkins (13) which also noted that the use of an air-acetylene flame would be necessary for many practical analyses by atomic fluorescence. EXPERIMENTAL
Modified Atomic Absorption Instruments. The first apparatus to be used was a Perkin-Elmer Model 403 Atomic Absorption Spectrophotometer, modified to operate in an atomic fluorescence mode. The Model 403, a double-beam instrument, has been described elsewhere (14). Figure 1 shows a schematic of the accessory optical system added to the Model 403 for these studies. Two small flat mirrors guide the light from the hollow cathode or other lamp into and out of the flame. The unit is also provided with a spherical “backing” mirror to increase the intensity of exciting radiation, in a manner which has by now become conventional. A light trap is also shown, designed to prevent the fluorescence results from becoming confused by possible scattering of room light. This design differs somewhat from the modification employed by Browner and Manning (15) to study atomic fluorescence.
(10) W. Slavin, At. Absorption Newsleft.,6 , 9 (1967). (11) D. R. Demers and D. W. Ellis, ANAL.CHEM.,40, 860 (1968). (12) H. L. Kahn, Adoan. Chem. Ser., 73, (1968). (13) P. L. Larkins, Spectrochim. Acta., 26B, 477 (1971). (14) H. L. Kahn, Amer. Lab., Aug., 52 (1969). 44,843 (1972). (15) R. Browner and D. C. Manning, ANAL.CHEM., ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
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Table I. Atomic Fluorescence Detection Limits on Modified Atomic Absorption Instrument AF limit, AA limit,& Element m/ml dml Notes Cd 0.005 0.001 ED lamp, 12 Watts Ca 0,05* 0.001 Cr cu
Fe Ag
Figure 1. Schematic of accessory optical system designed to convert Model 403 for fluorescence studies
Comparative tests indicated, however, that the two configurations give roughly equivalent analytical performance. A Versatile Test Instrument. The next step was to build a test instrument optimized for atomic fluorescence. It is well established that the requirements for spectral isolation for atomic fluorescence are not as stringent as those of atomic absorption. Therefore, as has been suggested by others ( 4 , 16, 17), interference filters instead of a monochromator can readily be used for isolation. In the spectral region below 3000 A, a solar-blind photomultiplier can be employed without further spectral isolation (8, 18). The optical arrangement of the test instrument is shown schematically in Figure 2. A fast toroidal mirror focuses a reduced image of the lamp (approximately 3 :1 reduction) onto the flame. This optical system, with an effective speed of f l . 4 , provides a high flux density in the sampling region of the flame. The collection optics, which “see” both fluorescence and flame emission, are stopped so that the size of the collection beam matches the size of the illuminated area in the flame. The strength of the fluorescence signal depends directly on the intensity of the source, whereas the noise or fluctuations depend, among other things, on the emission and instability of the flame. It is possible to increase the intensity of a hollow cathode lamp for short periods using pulse excitation techniques (9). In the top part of Figure 3, it is seen that a short (16) D. R. Jenkins, Spectrochim. Acta., 23B, 167 (1967). (17) P. D. Warr, Talanta, 17, 543 (1970). (18) T. Vickers and R. Vaught, ANAL.CHEM., 41, 1476 (1969).
0.56 0.4 0.15b 0,026 0.005
0.003 0.002 0.01 0,002
Zn 0.002 ED lamp, 20 Watts a Atomic absorption detection limits from Reference 19. * These data are from Browner and Manning (15) and were ob. tained with a separated air-acetylene flame. pulse of high current through the lamp is followed by a longer period of no current at all. By this means, a lamp rated at a steady-state current of forty milliamperes can be run with peak currents of, say, four hundred milliamperes. Pulsing the lamp gives an advantage only when the photomultiplier is gated, and synchronous demodulation (the “lock-in” system) is employed. The gating of the amplifier is shown in the lower half of Figure 3. During periods when the lamp is not on, the amplifier is also switched off, in order not to observe the noise from the flame. However, to distinguish flame emission from fluorescence, there must be a period when the system looks at the flame itself. Therefore, the amplifier is gated at twice the rate of the lamp pulses. The desired output is the difference between Voltage A and Voltage B. A series of tests indicated that, within wide limits, the repetition rate of the pulses made little or no difference. A frequency of about 100 Hz and a duty cycle of approximately 5 % were selected for the experimental work with hollow cathodes. For the experiments with electrodeless discharge lamps, the light output was electronically modulated a t 1 KHz with a sine wave and detected with the amplifier set for a 50% duty cycle. RESULTS
Detection Limits. Table I presents the detection limits obtained on a modified atomic absorption instrument with a (19) “Analytical Methods for Atomic Absorption,” Perkin-Elmer Corp., Norwalk, Conn., March 1971, p 32.
Figure 2. Optical schematic of atomic fluorescence test instrument 936
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Table 11. Atomic Fluorescence Detection Limits with the Test Instrument Detection limits (pgiml) Fluorescence" Element HCL EDL AAb As ND 0.8 0.5 0.007
0.1
Cd
>
t AMPLIFIER GATES
7
0.001
cu 0.01 ... 0.002 Fe 0.08 ... 0.01 Zn 0.01 0.002 0.002 Unseparated air-acetylene flame employed for all measurements. * Reference 19.
TIME
OUTPUT
Cd
cu
Fe Zn
...
0.9 0.02 Large difference 0.1% c o 0.1 0.03 Large difference a Test instrument with Zn EDL and R166 solar blind PM. * Model 403.
cence, undoubtedly due to the same sodium effect. Again, when the blank value was subtracted from the fluorescence value, the result for fluorescence was the same as that for absorption. It can be concluded that fluorescence and absorption have very nearly the same interferences with the determination of iron. The sodium problem by fluorescence is undoubtedly severe, but it could possibly be minimized by the development of a brighter source for iron (which would reduce the relative emission intensity of sodium), or by the use of a double monochromator, which would give lower levels of stray light. A different type of interference was examined with the test instrument equipped with a solar blind photomultiplier. When a solar-blind system is used, without filters or other spectral isolation, a potential problem arises. If the excitation lamp (in this case zinc) is not very pure, but contains traces of other elements, and if those elements are also present in the sample, fluorescence of these other elements can occur and be mistaken for that of the element of interest. It is also conceivable that a false reading could be obtained by a scattering effect of the light from the excitation lamp, caused by particles of the sample in the flame. This interference is analogous to the background absorption that sometimes occurs in atomic absorption. Comparative results for the determination of zinc in various off-the-shelf atomic absorption standards are shown in Table V. The standards used are nominally free from zinc, although it is entirely possible that trace levels are in fact present. Since the intent of the investigation was to compare fluorescence and absorption, rather than specifically to determine the trace zinc concentration in our various standard solutions, no effort was made to test whether the atomic
absorption results for zinc were actually correct, or could themselves have been influenced by background absorption. If the two techniques agree, it indicates that their performance is equal; if one method shows a substantially higher zinc concentration than the other, it is presumed that method is incorrect. As is seen in Table V, there is close agreement for all but three materials. Substantially higher values for nickel and cobalt were obtained by the fluorescence technique, indicating that these materials may be present in trace quantities in the lamp. Also, there is an elevated value for silicon, which is due either to scatter or to the sodium problem noted earlier. DISCUSSION
The extensive series of tests conducted on two different instrumental systems has shown beyond doubt that atomic fluorescence is a useful analytical technique. However, we have not been able to show that it has any significant advantages over atomic absorption. The linearities of the two methods are comparable. Analytical interferences are also comparable, although the sodium problem in fluorescence is still unsolved, and fluorescence appears to present greater risks of spectral interferences. There is also no counterpart in absorption of the double-value problem found in fluorescence. Some of the detection limits we obtained in fluorescence were similar to those of absorption, but others were poorer. Two of the “good” elements, cadmium and zinc, appear very often in the atomic fluorescence literature because they are determined much more favorably than many other elements. It is well established that many other elements, particularly those requiring a nitrous oxide-acetylene flame, present considerably greater difficulties in fluorescence, though many of them have been successfully determined. Before we draw conclusions from the above work, it is necessary to set up a framework for these conclusions. In a technical endeavor, it is almost never possible to show that a particular thing cannot or can never be done. All that is possible is to indicate that, with the equipment and skill available, success was not achieved. Thus the discouraging atomic fluorescence results obtained in this work may well be improved in the future, especially if brighter emission sources for more elements are developed, or if an important advance occurs in the atomization system. In a paper received by us after the completion of our work, Larkins (13) obtained detection limits 5 to 7 times lower than atomic absorption for Hg, Sb, and Zn, and comparable limits for eight other elements. As we did, he used an air-acetylene flame and an R-166 solar-blind photomultiplier. For mercury, he used a Philips “germicidal” lamp, and for antimony and zinc, he employed experimental high-intensity hollow cathode lamps developed in his laboratory, which are not yet generally available. Though he did not make a very exhaustive interference study, he called attention to the potential for some of the difficulties that we have found. He also used an atomic fluorescence accessory to an instrument with a monochromator and, as we did, found no advantages over absorption. CONCLUSIONS
With the light sources that are currently known, there appears to be little use for atomic fluorescence equipment employing monochromators, except possibly for theoretical studies.
A single-channel instrument with filters or a solar-blind detector can give slightly better detection limits than atomic absorption for a few elements under some circumstances. However, it will have the disadvantages of lack of general applicability, the double value problem, and increased spectral interferences. Moreover, it will be more expensive and more difficult to use than a medium-cost atomic absorption spectrophotometer,even though it does not require a monochromator. Normal hollow-cathode lamps are easy to use, moderate in cost, and require a relatively simple power supply. By contrast, many laboratories have found electrodeless lamps difficult to control, hard to make stable, and subject to a variety of mystifying problems. The high-intensity hollow cathode lamps used by Larkins (13) offer new hope. However, if they become commercially available, they will undoubtedly be much more expensive than conventional hollow cathodes. Solar-blind photomultipliers cost moreq than conventional units, while filters usable below 3500 A are also far from negligible in price. Finally, the electronics employed by our breadboard were fairly elaborate, though they could perhaps be simplified. On balance, therefore, there seems little attraction at the moment in any kind of single-channel atomic fluorescence instrument. The elimination of the monochromator, and the greater optical freedom enjoyed by atomic fluorescence, makes it potentially attractive as a base for a multi-element instrument. However, it must be remembered that the chief justification for the expense of a multi-element instrument is the potential time saving, and increase in sample throughput, compared to a single-element unit. Therefore, it must be possible to determine all the elements in a given sample at the same dilution. Interferences should be at a minimum, because any requirement for extra sample treatment would quickly throw away the time advantage. Because of the expense of the instrument, it should be able to determine most elements with a high degree of competence. In our hands, the sodium problem, the double-value difficulty, the somewhat limited dynamic range, the poor sensitivity achievable for many elements, and the inability to do certain others, tend to dampen our enthusiasm for multielement instrumentation along these lines. However, it is always possible that a breakthrough could occur. It could take the form of improved devices to atomize samples, the taming of electrodeless discharge lamps, the design of a hybrid instrument to use absorption and emission as well as fluorescence, or some optical or electronic arrangement which did not occur to us. The present situation for analytical atomic fluorescence seems dim, but the future may yet be bright. ACKNOWLEDGMENT We are extremely grateful to E. Horace Siegler, who was responsible for the optical design of the test equipment described in this paper; to Walter Bohler, who designed the electronics with almost eerie perfection; and to David C. Manning who was, as usual, on hand with valuable comments and advice.
RECEIVED for review October 28,1972. Accepted Feburary 8, 1972. Part of this material was presented at the 3rd Conference on Atomic Absorption and Atomic Fluorescence, Paris, September 1971, and part at the XXII Pittsburgh Conference on Analytical Chemistry and Spectroscopy, Cleveland, Ohio, March 1972.
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