ANALYTICAL CHEMISTRY, VOL. 50, NO. 1‘1, SEPTEMBER 1978 (11) R. Moss and E. V. Browett, Analyst (London),9, 428 (1966). (12) L. J. Purdue, R. E. Enrione, R. J. Thompson, and B. A. Bonfield, Anal. Cbem., 45, 527 (1973). (13) L. J. Snyder and S. R. Henderson, Anal. Chem., 33, 1175 (196‘). (14) L. J. Snyder, Anal. Chem., 39,591 (1967). (15) V. Cantuti and G. P. Cartoni, J . Chromalogr.. 32, 641 (1968). (16) A. J. McCormack. S. C. Tong, and W. D. Cooke, Anal. Chern., 37, 1470 (1965). (17) C. A. Bache and D. J. Lisk, Anal. Chem., 43, 951 (1971) (18) W. E. L. Grossman, J. Eng, and Y. C. Tong, Anal. Chirn. Acta, 60, 447 (1972). (19) H. Kawaguchi, T. Sakamoto, and T. Mizuike, Talanfa. 20, 321 (1973). (20) F. A. Serravallo and T. H. Risby, J . Chromatogr. Sci., 12, 585 (1974). (21) Y. Talmi and D. T. Bostick, Anal. Chem., 47, 2145 (1975). (22) Y. Talmi and V. E. Norwell, Anal. Chem., 47, 1510 (1975). (23) Y. Talrni and A. W. Andren, Anal. Chem., 46, 2122 (1974). (24) M. S.Epstein and T. C. O’Haver, Spectrochim. Acta, Part 6. 30, 135 11975). - -,. (25) E. D. Pellizzari, J. E. Bunch, B. H. Carpenter, and E. Sawicki, f n v m n . Sci. Technol., 9, 552 (1975).
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(26) E. D. Pellizzari, J. E. Bunch, R. E. Berkley, and J. McRae. Anal. Lett., 9. 45 (1976). (27) J. W. Robinson, L. E. Vidaurreta, and D. K. Wolcott, Spectrosc. Lett., 8 (7), 491 (1975). (28) A . Laveskog. Proc. 2nd Int. Clean Air Congr., 549-557 (1970). (29) R. M. Harrison and D. P. H. Laxen, Atmos. fnviron., 11, 201 (1977). (30) S. Hancock and A. Slater, Analyst (London), 100, 422 (1975). (31) R. E. Lee, S. S. Goranson, R. E Enrione, and G. B. Morgan, Envlron. Sci. Technol., 12, 1025 (1972). (32) J. M. Ondov, PhD. Thesis. University of Maryland, College Park, Md., 1974. (33) R. K.Skogerboe, Department of Chemistry, Colorado State University, Fort Collins, Colo., private communication.
RECEIVED for review April 27, 1978 Accepted June 7, 1978. This work wab supported by NSF/RANN Grant No. AEN7C5-02667.
\
Pulsed Radio-Frequency Electrodeless Discharge Lamps for Atomic Absorption and Atomic Fluorescence Spectrometry John W. Novak, Jr., and Richard F. Browner” School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
broadening, due primarily to self-reversal effects ( 4 , s ) . RF EDLs operated in a pulsed mode are shown in the present study to be remarkably free from this problem.
Pulsed radio-frequency excited electrodeless discharge lamps are examined as spectral line sources for atomic absorption (AAS) and atomic fluorescence (AFS) spectrometry. Their properties satisfy all the essential requirements for these techniques, i.e., they are narrow line sources with high radiant output. Comparisons are made to existing CW radio-frequency and microwave systems and show the pulsed lamps generally provide higher sensitivity in both absorption and fluorescence measurements.
EXPERIMENTAL Optics and Electronics. The optical setup and much of the associated instrumentation has been previously described ( I ) . For
the fluorescence studies, a GCA/McPherson Model EU700 Czerny-Turner monochromator with a 1180 grove/mm grating blazed at 1900 A was used. This was fitted with a Model 9783B photomultiplier (EM1 Gencom, Inc., Plainview, N.Y.) with a supply voltage of 700 V from a GCAlhlcPherson Model EU42A power supply. All atomic fluorescence data were taken using 100-km slit widths The remainder of the detection system included a current preamplifier (Model 164A, Ithaco Inc., Ithaca N.Y.), a lock-in amplifier (Ithaco Model 391A) and recorder (Model SR6,Sargent-Welch Scientific. Co., Ill.). Microaave EDLs for Hg, Cd, and Zn were prepared from the pure elements in a manner similar to that described by Dagnall and West (6). Exact constructional details may be obtained by writing to the authors. A Microtron 200 Mark I11 microwave power generator (Electromedical Supplies, England) was used with a 3/4-pL Broida cavity. Operational Parameters. A standard solution was prepared by dissolving pure Hg, Cd, or Zn metal, as appropriate, in 1:l nitric acid. Aliquots were then diluted to the desired concentration with distilled water. All AF measurements were made using an air/hydrogen flame. Fluorescence intensities were measured at an average height of 4 mm above the burner head. The bLrner was of the circular capillary design (10-mm diameter) descr (bed by Aldous, Browner, fitted to a Perkin-Elmer burner chamber. Dagnall. and West
Criteria for proper evaluation of any new radiation source for atomic spectrometry should include lamp lifetime, stability, radiant output a t t h e resonance lines, and line profile characteristics. A recent paper from this laboratory ( I ) has evaluated pulsed radio-frequency electrodeless discharge lamps (prf EDLs) operated a t high power, and proved them to adequately satisfy the previously mentioned criteria for possible application to atomic spectrometry. One of the most desirable properties of these new sources is their high radiant output a t the resonance lines of their fill metal. If used for atomic fluorescence spectrometry (AFS), this would be anticipated to result in good AFS detection limits. Plots of radiant output as a function of power ( I ) for CW and pulsed rf EDLs have indicated a gain of intensity in the pulsed mode. Also, the slopes of the pulsed EDL plots were generally steeper, Le., more radiant output per incident watt. Signal-to-noise gains through the use of a variety of pulsed sources have already been investigated (2-4). Human ( 4 ) used pulsed hollow cathode lamps (HCLs) and a boxcar detection system, and found approximately a fivefold gain in signalto-noise ratio compared to his conventional atomic absorption detection system. Weide and Parsons (3)have also shown an increased signal-to-noise ratio which they attribute to the increased peak photon flux available through pulsing their hollow cathode lamps. However, the major shortcoming of pulsed HCLs has been the sensitivity loss as a result of line 0003-2700/78/0350-1453$0 1 O O / O
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RESULTS AND DISCUSSION The three modes of operation of rf EDLs as a function of incident power are as follows: (1) Low power diffuse glow along the axis of the lamp. (2) Intermediate power “fireball emission”, concentrated in a small ceni,ral volume of the lamp. (3) High power “fireball emission”, filling the entire EDL. This last mode is designated as the High Intensity Emission mode of operation. A detailed description of these modes has been given previously ( I ) .
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1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1 1 , SEPTEMBER 1978
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Figure 2. Influence of incident power on absorbance for Cd rf EDL (0)Pulsed EDL. (A)CW EDL
Influence of incident power on absorbance for Hg rf EDL.
(0)Pulsed EDL. (A)CW EDL
T h e performance of the EDL in atomic absorption and atomic fluorescence studies is closely related to the physical characteristics of the various emission modes, as a result of the combined effect of changes in source emission intensity and source line profile. Therefore, in the remainder of this text, correlations will be drawn between the source emission mode and either the analytical sensitivity (in AAS) or the ratio of atomic fluorescence to source emission signal (in AFS). Both of these parameters are sensitive indicators of changes which may occur in source emission line profiles. [N.B. The term “sensitivity” is used throughout this paper in the sense recommended by IUPAC (8),Le., to refer to the slope of the analytical curve (signal vs. concentration), and not to the “sensitivity for 1%absorption” as commonly used in AAS. This is to ensure uniformity in discussion of both AAS and AFS analytical curves.] Absorption Studies. In AAS it is desirable to operate with a high photon flux a t the photomultiplier in order to improve signal-to-noise, by minimizing the percentage of photon shot noise on the signal. With pulsed HCLs (41, it has been observed that the gain in emission intensity is often accompanied by a loss in sensitivity due to line broadening. Absorption measurements were therefore made with all sources primarily to observe if a loss of sensitivity resulted from pulsing the EDLs. Hg EDLs. Figure 1 shows comparative AAS analytical curves for CW and pulsed rf EDLs. Although the pulsed Hg EDLs were more intense ( I ) , they still provided a greater absorption sensitivity a t all incident powers above ca. 200 R. At lower powers (CW < 100 W, pulsed < 200 W), the absorption signals showed a marked decrease, associated with Mode 1 of lamp operation (diffuse glow along lamp’s central axis). This effect is shown for both CW and pulsed lamps, and is almost certainly a consequence of severe line selfreversal. The emission intensity similarly showed a marked decrease in the low power region, again probably as a result of severe self-reversal, but also because of a reduction in the emitting pathlength. It was also observed that the absorption sensitivity was constant for a much greater power span for the pulsed EDLs than for CW operated lamps. This would make for greater convenience in AAS operation due to the need for less stringent power regulation requirements, and is also encouraging from a fundamental viewpoint in that high power pulsed operation in itself does not appear to lead to appreciable line profile broadening. Cd EDLs. Figure 2 shows a comparison between pulsed and CW operated Cd EDLs. Cd EDLs gave maximum absorption sensitivity near the power setting where the high intensity emission mode began. At higher powers there was
Figure 3. Influence of incident power on absorbance for Zn rf EDL. ( 0 )Pulsed EDL. ( 0 ) CW EDL
a loss of sensitivity a t a fairly constant rate, regardless of the mode of operation, CW or pulsed. As with Hg EDLs, the sensitivity drop as power was increased beyond the optimum setting was less with the pulsed than with the CW operated lamps. Zn EDLs. With Zn EDLs the transition from low intensity to high intensity modes was more gradual than for either Hg or Cd EDLs ( 1 ) . Figure 3 shows that a similar trend is reflected in the AAS analytical curves, i.e., there was a much more gradual absorption sensitiwty rise to the optimum power than for either Hg or Cd EDLs. At powers above the optimum, any further increase of power caused a gradual decrease in absorption sensitivity. A noticeable difference between Zn EDLs and either Hg or Cd EDLs is that the power for maximum sensitivity is quite different for the pulsed and CW lamps. This is probably best explained as a heating effect, resulting from the lower volatility of Zn compared to either Hg or Cd. The average power to the lamps is much less when pulsed, therefore less rf induction heating will occur and a higher power is necessary to produce an adequate Zn vapor pressure for excitation purposes. Atomic Fluorescence Studies. Atomic fluorescence studies were made in two parts: (1) A comparison of fluorescence intensities attained from both pulsed and CW operated EDLs as a function of rf incident power (at constant analyte concentration). ( 2 ) A comparison of fluorescence intensities vs. analyte concentration, again for both pulsed and CW lamp operation. In this study the lamps were run a t the optimum power for maximum fluorescence intensity. The capabilities of microwave excited EDLs have been well characterized in many fluorescence studies (9-1 I ) . Therefore, for reference purposes, results from fluorescence studies using a microwave source are also presented for each element. Experimentally, the rf generator available limited the maximum peak rf power produced in the pulsed mode to the same values as the maximum CW power output in the
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
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WATTS
Figure 4. Hg atomic fluorescence signal as a function of incaent power. (0)Pulsed rf EDL. ( 0 ) CW rf EDL. (A)Microwave EDL. Wattage scale shown is for rf lamps. For microwave lamps, the wattage scale is divided by 10
continuous mode. Consequently, the acerage incident power reaching the lamps in the pulsed mode was always less than the average incident power reaching the lamps in the CW mode, for t h e same incident wattage. In t h e plots of fluorescence intensity vs. power, the wattage scale refers to peak values for pulsed lamps and t o average values for CW operated lamps. For CW signal detection, a piroammeter was used. For pulsed signal detection, a lock-in amplifier was used, set to the pulse repetition frequency. With pulsed source operation and lock-in amplifier detection, a duty cycle less than 0.5 results in reduced signal, because the “push pull” nature of the signal processing subtracts one part of the signal from the other. A correction factor was generated by holding the radiant output constant and plotting signal vs. duty cycle. A calibration factor was then applied to the measured signal to correct for response. This procedure produced a true estimate of peak intensity values for pulsed source operation, using the lock-in. Clearly, the use of a boxcar integrator would providcl greatly enhanced S/Nratios with the pulsed source, and would be essential for producing optimum detection limits. Hg EDLs. With all fluorescence studies, a preliminary plot of fluorescence intensity as a function of incident power to the EDL was necessary to determine the optimum power for further fluorescence experiments. As seen, in Figure 4, the CW operated lamps reached the optimum fluorescence intensity condition a t a lower power than the pulsed operated Hg EDLs. However, it is clear t h a t once the CW lamps reached an optimum condition, any further increase in powei caused a decrease in fluorescence intensity. Within the power range available in this study, a fluorescence intensity maximum was not reached with the pulsed sources and the slope of the plot remained quite steep even a t the higher powers. (Note that the power scale for the microwave plot is l/lo that for the rf lamps). Microwave EDLs produced slightly less maximum fluorescence intensity than the rf CW operated EDLs, but were definitely inferior to the pulsed rf ED1,s. Fluorescence intensity vs. concentration data were obtained using pulsed and CW rf lamps and microwave lamps run a t optimum power for fluorescence intensity. These produced linear plots with relative slopes in the order predicted from t h e fluorescence vs. power plots. Pulsed rf EDLs produced the greatest fluorescence intensities, giving an analytical curve with slope 2x that produced by the microwave lamps. CW rf lamps operated at their optimum power gave an analytical curve with slope 1.5X that of the microwave lamps. Cd EDLs. Figure 5 shows the characteristic Cd atomic fluorescence curves as a function of power. At the incident power corresponding to maximum signal from the CW lamps, the slope of the fluorescence intensity vs. power plot for the pulsed EDLs had its greatest value. Additional comparisons
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Figure 5. Cd atomic fluorescence signal as a function of incident power. (0)Pulsed rf EDL. ( 0 ) CW rf EDL i~
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were made from fluorescence intensii;y vs. concentration data, with all EDLs run a t the rf plower giving maximum fluorescence intensity. The CW rf and microwave operated lamps produced very similar fluorescence intensities. Taking the slope for the microwave lamp as 1.0, the CU’ rf EDL gave a relative slope of 1.4. The pulsed rf Cd lamp produced much more fluorescence intensity than either the CW rf or microwave operated lamps, the relative slope for the pulsed lamp being 5.3. Zn EDLs. The results for Zn EDLs were quite different from either the Hg or Cd EDLs. Figure 6 shows that CW operated rf Zn EDLs produced a higher fluorescence intensity than the pulsed Zn EDLs at all powers available with our generator. The slope of the plot for pulsed lamps was slightly steeper a t peak power than for CW lamps, but the continuation of their plots of higher power could not be followed because of power generator output limitations. A comparison of the fluorescence intensities of microwave, CW, and pulsed rf EDLs, operated a t optimum power (for fluorescence intensity), was also made. The reverse situation from the Hg and Cd lamps was found with the Zn lamps, i.e., the microwave lamps were superior in producing fluorescence intensity. Also the CW lamps were superior to the pulsed rf lamps, producing about three times greater fluorescence intensity. Fluorescence as a Function of Pulse Width. Fluorescence intensity as a function of pulse width was investigated with the pulsed rf EDLs. In a previous study of pulsed EDLs (I), it was found t h a t for maximum source emission intensity there was an associated optimum pulse width. A similar relationship was found between pulse width and maximum fluorescence intensity. For pulsed rf Hg EDLs, the maximum fluorescence intensity resulted from a pulse
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
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600
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Flgure 9. Atomic fluorescence to atomic emission ratio as a function of rf power, CW RF EDL ( 0 )Hg EDL (A) Cd EDL
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Flgure 7. Cd atomic fluorescence signal as a function of peak rf power, variation with pulse width. (x) 0.3 ms. (A) 0.5 ms. ( 0 ) 1 ms. (M) 2 ms. (0)3 ms 7 ,
300
600
900
1200
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Flgure 10. Cd atomic fluorescence/atomic emission ratio as a function of rf power, variation with pulse width. ( 0 ) 1.0-ms pulse width. (A) 2.0-ms pulse width. (0)3.0-ms pulse width
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Flgure 8 . Zn atomic fluorescence and emission signals as a function of duty cycle. (0)Emission, pulsed rf EDL. (0)Fluorescence, pulsed rf EDL
width of 0.5 ms. As the pulse width was increased stepwise t o 0.8 and 2.0 ms, the fluorescence intensity dropped progressively. This followed the pattern of the emission studies. where the 0.5-ms pulse produced greatest emission intensity, and the 2.0-ms pulse produced least emission intensity. The plots of fluorescence intensity vs. concentration had the following relative slopes: 2.0-ms pulse width, slope = 1.0; 0.8-ms pulse width, slope = 3; 0.5-ms pulse width, slope = 7 . 5 . For the Cd EDLs, fluorescence intensity as a function of pulse width is shown in Figure 7 . Maximum fluorescence intensity occurred at a pulse width of 1ms. The shorter width pulses all produced characteristic plots with steeper slopes than did the longer width pulses. However, the final profile of the shorter pulses could not be adequately determined because of power coupling limitations. Zn fluorescence as a function of pulse width was also investigated. Figure 8 shows a maximum was reached a t short pulse widths and as the pulse width was increased, the fluorescence intensity decreased. However, at a duty cycle of about 0.5, the fluorescence intensity again began to rise with increasing pulse width as the lamps approached a condition close to CW operation. This was not surprising since the CW operated rf Zn EDLs produced a higher fluorescence maximum than t h e pulsed lamps. It was also noted that the
fluorescence results followed closely the source emission results. Comparison between F l u o r e s c e n c e and Emission Intensities. Fluorescence intensity (IF1) to emission intensity ( I E ~ratios ) as a function of power are useful for observing indirectly changes in the line profile from the emission source. Hg EDLs operated in the pulsed mode showed very little change in the IFI/IE,,, ratio between 200 and 1000 W (Le., approximately zero slope for a plot of IF1/I~m vs. power) when operated with short duration pulses (51ms). It would appear that the conversion efficiency of emitted radiation to fluorescence radiation was still high even a t maximum power, indicating good spectral overlap and little source emission line profile broadening. Ideally, interferometric measurements would be made to confirm this. From these data there is no indication of any spectral properties displayed by the source u n d e r the power conditcons available which would be anticipated to cause a leveling off of the analytical curves (Figure 4). Higher incident powers than those available with the present generator would be necessary to investigate this interesting region. Figure 9 shows the plots of fluorescence to CW emission intensity ratios as a function of power for Hg (bottom) and Cd (top) lamps. The conversion of emitted radiation to fluorescence became less efficient with increasing power, indicating that undesirable changes were occurring in the atomic line profile a t high continuous powers. These effects are shown to be minimal in the pulsed operation mode. Figure 10 illustrates how the line profile changed with pulse duration using Cd lamps. A marked change occurred when going from 1- to 3-ms pulse duration. The curve fci 1-ms
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
pulses shows that the conversion efficiency of emitted source radiation to fluorescence radiation remained essentially unchanged throughout the observed power interval. In contrast, the plot for 3-ms pulses showed a definite loss in conversion efficiency. T h e initial increase in the fluorescence to emission ratio with increasing power is worthy of note. This corresponds well to our original assumption that a t lower powers the sources are extremely self-reversed because of the physical distribution of the discharge. Detection Limits. No particular attempt was made to optimize experimental conditions for detection limit measurements. Certain unalterable aspects of the experimental setup were far from ideal, e.g., small aperature optics, lock-in rather than boxcar amplifier for pulsed source operation, etc. An unseparated air/Hz flame was used for all elements. The detection limits from a concentration giving a (signal): (standard deviation of baseline) = 2 are as follow: Hg, 0.5 ppm; Zn, 0.2 ppm; Cd, 0.09 ppm. By proper experimental optimization, particularly the use of a boxcar amplifier, these values could undoubtedly be improved very considerably.
CONCLUSIONS Currently, atomic fluorescence spectrometry is used relatively little for practical analytical work, especially when compared to the better established atomic absorption spectrometry. Potentially AFS offers the following advantages over AAS: (1)Improved detection limits. (2) Longer linear analytical range. (3) Better multielement capability. However, a major limitation in AFS development has been the availability of suitably intense and stable excitation sources. T h e following conclusions may be drawn regarding the characteristics of pulsed rf EDLs as sources for AFS. There is a general similarity in the shapes of the curves of fluorescence intensity vs. power for all the EDLs studied (e.g., Hg, Cd, and Zn). The shapes of the analytical curves can be considered as two distinct regions. In the first region, the fluorescence intensity changes little with increasing power (plots have very small slopes). In the second region, the fluorescence intensity rises rapidly with a small power increase (steep slope). When the lamps are pulsed using long (>1.2 ms) pulse widths, the lamps typically reach a “steady state” with respect t o fluorescence intensity, Le., there is little or no change a t higher powers. However, when short pulses are used (
