Background Fluorescence Spectra Observed in Atomic Fluorescence Spectrometry with a Continuum Source W. K. Fowler’ and J. D. Winefordner” Department of Chemistry, University of Florida, Gainesville, Florida 326 1 1
The background spectrum observed in atomic fluorescence spectrometry with a continuum source, consisting of source radiation scatter and molecular fluorescence of flame gas species, has been evaluated for the argon separated air/ acetylene flame and an EIMAC xenon arc continuum source in both the pulsed and cw modes of operation. Molecular fluorescence bands were measured and assigned to PO, NH, CN, and OH. Peaks resuitlng from the grating second order PO fluorescence bands were observed. The spectral distrlbution of the source radlatlQn was measured in both the pulsed and cw modes. Background fluorescence spectra were recorded for fuel-lean, stoichiometric, and fuel-rich conditions and spectra obtained in both the pulsed and cw modes are compared. A commercially-available filter designed to remove PH3 from acetylene was evaluated with respect to its ability to reduce the fluorescence intensity of PO, a species formed in the flame from PH3, present in the acetylene as an impurlty. A general assessment of molecular fluorescence in flames as a source of interference in atomic fluorescence spectrometry is presented.
During a recent study (1) involving the evaluation of a pulsed continuum source of excitation for atomic fluorescence spectrometry (AFS), it was noted that the background spectrum (Le., t h e fluorescence signal obtained a t each wavelength when only distilled water was aspirated into the flame) contained numerous “anomalies” which resulted in sharply elevated background levels at certain wavelengths of interest. At these wavelengths, significant interferences with atomic fluorescence measurements were observed. I t was recognized that these background spectral irregularities could be attributed to either of two phenomena: (a) molecular fluorescence of molecules within the flame, and/or (b) prominent peaks in the source spectral output, transferred to the detector by scattering of source radiation from the flame. Some of the peaks in the fluorescence background were known to have resulted from the molecular fluorescence of PO (2);however, many others had never been noted previously and thus could not be characterized. Although molecular emission (3-6) and absorption (6-8) of flame gas constituents have been fairly well documented, and their effects upon analytical multielement atomic fluorescence measurements recognized (9-1 11, molecular fluorescence in flames has received but little attention in the literature. The fluorescence of the CH radical at 431.5 nm has been excited by a dye-laser in an oxyacetylene flame (12), and several observations of OH flame fluorescence have been reported (13-15). Molecular fluorescence bands were seen, but not identified, by Johnson, Plankey, and Winefordner (16). The fluorescences of several alkaline earth compounds (CaOH, SrOH, and BaC1) have been observed when concentrated solutions of these compounds (or their precursors) were aspirated into flames irradiated by high intensity continuum sources (17, 18). ‘Present address, 302 North Hubert Ave., Apt. 201, Tampa, Florida 33609. 944
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
Reported incidences of observable scatter signals in atomic fluorescence studies have been linked primarily to the use of cool flames and sample matrices with high solids content (19-21). Measurable scatter signals have, in general, not been noted or at least stressed, when pure aqueous solutions were used with the airlacetylene flame. For these reasons, a study of the background anomalies prevalent in multielement AFS, reported by Johnson, Fowler, and Winefordner (I), has been conducted and is described here. An attempt was made to identify each of the observed peaks and bands. Spectra were recorded under three different analytically-useful sets of flame conditions in the flame used for the previous study (1) (argon sheathed airlacetylene flame). Results obtained with the EIMAC lamp operated in the pulsed and cw (continuous wave) modes, respectively, were compared, and the spectral distribution of the lamp radiation was measured for both modes of operation. A commercially-available filter designed to remove phosphine (PHJ from acetylene (22)was tested with respect to its ability to reduce the fluorescence signal of phosphorus monoxide (PO). As reported previously (1,21, PO is formed in the flame from PH3, present in the acetylene as an impurity. When possible, the equipment and experimental conditions employed in the previous pulsed EIMAC study (1)have been retained in the present work in order to preserve the validity of any comparisons which have been drawn.
EXPERIMENTAL Apparatus. Block diagrams of the experimental systems used in the present work are given Figures 1-3. The nebulizer, burner, monochromator, photomultiplier detector (and its shielded housing), mirror, light trap, and entrance optics have been described previously (23). For the pulsed source fluorescence measurements, the experimental arrangement of Figure 1 was adopted. The excitation source and its associated power supply network, including the trigger signal source, are as previously used ( I ) . For the measurement of fluorescence in the cw mode (see Figure 2), the same excitation source was employed but with a different power supply, and the excitation optics, including the chopper are similar to those used previously (19). The lamp dc power supply employed for all cw measurements was a seriescontrolled, constant current power supply designed specifically for use with the EIMAC lamp. To record the spectral distribution of the excitation source radiation in both the pulsed and cw modes, the experimental system was modified as shown in Figure 3. An aluminum block coated with Eastman White Reflectance Coating No. 6080 (Eastman Kodak Company, Rochester, N.Y. 14650) was placed on top of the burner assembly and oriented so as to scatter the source radiation into the monochromator. The coating contained barium sulfate, and when applied according to the manufacturer’s directions, it formed a diffuse reflector whose absolute reflectance was 90% or better at all UV and visible wavelengths above 220 nm (24). To reduce the radiant flux impinging on the photomultiplier, a 2.0 absorbance neutral density filter was placed in front of the monochromatorentrance port, and a diaphragm with a 0.5-cm aperture was positioned between the source and the reflector at a distance of 6 cm from the reflector. The distance from source to reflector was approximately 30 cm. For all measurements except that of fluorescence in the cw mode, signal processing was accomplished with a dual-channel boxcar integrator equipped with two separate integrator modules
dirror 0
1
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OLens Eimac Lamp
v s'
II
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C
r l -
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Figure 1. Schematic diagram of the system used for all flame background molecular fluorescence measurements In the pulsed mode. Switches and diodes are as described previously ( 7 )
Table I. Experimental Components and Manufacturersa
a
Source Item Type Boxcar integrator/integrator 162/164 Princeton Applied Research, Princeton, N.J. 08540 modules (A & B) Ithaco, Inc., Ithaca, N.Y. 14850 Lock-in amplifier 391 Current preamplifier 164 Ithaco, Inc., Ithaca, N.Y. 14850 Sargent-Welsh Scientific, Birmingham, Ala. 3 5 20 2 Strip chart recorder SRG PS 300-1 Eimac, Div. of Varian, San Carlos, Calif. 94070 Lamp power supply (dc) Acetylene (PH,) filter ... L'Oxhydrique Francaise, Malakoff, Paris, France (Laboratory constructed) Mechanical chopper ... Note. All other components used in this study are listed in Reference 1.
labeled A and B, respectively, a variable aperture or sampling interval applicable simultaneously to both integrators, and seaarate aDerture delav circuit for each inteerator module. The aperture delays were referenced to a single trigger signal supplied externally; hence the instrument was capable of being synchronized to the excitation pulse in the pulsed mode by simultaneously triggering both the boxcar and the pulsed power supply from the same trigger source. The outputs of the two integrators could be read individually or could be subtracted electronically to yield the difference signal, A - B. With the aid of an oscilloscope, the aperture of the A integrator was set (using the aperture delay control) to coincide with the lamp output pulse (for pulsed mode measurements) and the aperture of the B integrator was positioned at a point in time approximately half-way between successive A integrator apertures. Therefore, the total signal fed into the A integrator consisted of flame emission signal plus photomultiplier dark current plus any signal (fluorescence and/or scatter) generated by the source output pulse, while the total signal processed by the B integrator consisted of flame emission signal plus photomultiplier dark current plus any residual fluorescence and/or scatter signal produced by the residual lamp output between pulses. Hence, the difference signal, A B, was composed entirely of fluorescence and scatter signals produced by the lamp output pulse, and it was this net signal which was recorded in all spectra obtained in the pulsed mode. As in the previous work ( I ) , the aperture was chosen to be 20 ps, and the excitation source pulse width was fixed at approximately 15 ps. A time constant of 0.8 s was employed for all work with the boxcar integrator. For the measurement of fluorescence in the cw mode, the signal was processed with a lock-in amplifier, together with a current preamplifier whose function wm to amplify the signal current and convert it to a signal voltage. The reference channel of the lock-in I
Table 11.
summary of Experimental
Monochromator Slit width Slit height Ban dpass Spectral scan rate Lamp power supply (dc) (used in pulsed power circuit) Current Lamp power supply (dc) (used in cw studies) Current Pulsed power supply Amplitude setting Pulse width Repetition rate Flame conditions Fuel-lean Fuel-rich Stoichiometric Strip chart recorder chart speed Photomultiplier voltage Boxcar integrator aperature Observation height in flame Chopper speed
Conditions 75 pm 1cm 0.4 nm 5 nm/min
6.5 A 20 A of full scale -15 s 300 Hz
Air 10.5 L/min C2H2 1.3 L/min Ar 10 L/min 9 . 0 L/min Air C,H, 1 . 3 L/min Ar 10 L/min 9.7 L/min Air C,H, 1.3 L/min Ar 10 L/min 1in/min -1400 V 20 ps
2.5 cm above burner top 50 Hz
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Lens C,,
Monoc hromator
I
\
P
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I
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I
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Lock- in
1
Preamplifier
Arnplif ier Flgure 2. Schematic diagram of the system used for the measurement of flame background molecular fluorescence in the cw mode (19, 23
was supplied with a reference output triggering signal from the chopper. The sensitivity was adjusted t o achieve the desired response, and the time constant was set at 0.4 s. The output of the lock-in was fed to the same strip-chart recorder as employed with the boxcar integrator. Procedure and Experimental Conditions. A summary of the experimental conditions applicable to this work is given in Table 11. All equipment was operated as described in the manufacturers’ manuals except as previously noted. The monochromator slit width and the observation height in the flame were chosen to be identical to those used in the previous study (1).
When required, the acetylene filter (for removal of PHJ was spliced into the acetylene supply line between the pressure regulator on the acetylene tank and the flow control valve. In all fluorescence studies, distilled water (blank) was aspirated into the flame. For recording the lamp spectral distribution using the experimental arrangement of Figure 3, all instrumental settings were maintained identical to those used for the fluorescence studies, except that the signal recorded in the cw mode was the output signal from the A integrator only, rather than the difference signal, A - B. Thus, the boxcar integrator was gated with a repetition frequency of 300 Hz and an aperture width of 20 ks in both the pulsed and cw modes; this was done to permit a more direct comparison of the lamp outputs in the two modes.
RESULTS AND DISCUSSION The spectra recorded in this study are shown in Figures 4-10. In each case, the baseline represents the photomultiplier dark signal, Le., the signal observed with the monochromator slits closed. The series of bands lying in the 220-275 nm region of the fluorescence spectra (Figures 4-8) constitute the y system of the PO molecule, arising from the electronic transition A2C-X211. The weak band in the 280-295 nm portion of the spectrum of Figure 4, as well as the much stronger band in the 305-320 nm region, are both due to the OH radical. This system, known only as the 306.4 nm system of OH, is due to the transition A2C-X211. The separate bands are produced by transitions involving different vibrational energy levels of the same electronic energy levels. The band in the 323-330 nm region comprises the p system of PO and corresponds to the transition B2x-X211. The fluorescence of this band was sought in a previous study (2),but it was seen only in emission. The peak or sharp band a t 336 nm is probably the band head of the 336.0 nm system ( A 3 n - X 3 C ) of the NH radical. Interference of this band with atomic fluorescence measurements is unlikely because of its narrowness. The band whose maximum is at 388.3 nm is believed to be due to the CN violet system, i.e., the B2C--A211transition of the CN radical. Another possibility for this assignment, considered less likely by the authors, is the 390.0 nm system of CH. However, this band is described (6)as being degraded to the red and to have maxima at 387.1 and 388.9 nm. Of these characteristics, only a “secondary” maximum at 387.1 nm is evident, which suggests only that the observed fluorescence Neutral Density Filter
I
Monochromator
I
Diaphragm
Figure 3. Schematic diagram of the experimental arrangement employed for the measurements of the lamp spectral output. Note that either the cw or the pulsed power supply circuit can be attached to the lamp input terminals ( 7, 79, 23) 946
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WAVELENGTH, nm
WAVELENGTH, nm
Flame background molecular fluorescencespectrum obtained in the pulsed mode from a fuel-lean CzHz/airflame. The portions of the 190-600 nm spectral region which are not shown here contained no detectable molecular fluorescence Figure 4.
Flame background molecular fluorescencespectrum obtained in the pulsed mode from a CzH,/air stoichiometricflame. The barren portions of the 190-600 nm spectral region have been excluded
Figure 6.
I 320
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~
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in the pulsed mode from a fuel-rich CzH,/air flame. Only selected portions of the spectrum are shown
Selected portions of the flame background molecular fluorescence spectrum obtained in the cw mode with a stoichiometric C,H,/aIr flame
in this spectral region may contain a contribution from both species. The series of bands observable beyond 450 nm are the grating second-order dispersion of the PO y-system bands. Although observed in second-order, these features are nonetheless capable of adverse influence upon atomic fluorescencemeasurements in this spectral vicinity. The above band assignments are summarized in Table 111. At any wavelength in the fluorescence spectra which is
well-removed from the observed bands, the offset between the baseline and the spectral trace represents the scatter signal at that wavelength. There are no “anomalies” in any of these spectra which can be shown to correspond to peaks in the spectral distribution of the lamp radiation, as had been suspected previously. However, small scatter signals are present at all wavelengths and in atomic fluorescence spectrometry with a continuum source ultimately limit the sig-
Figure 5.
Flame background molecular fluorescence spectrum obtained
Figure 7.
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spectral output in the 190-350 nm wavelength region of the EIMAC lamp operated in the pulsed (solid line) and cw (dotted line) modes Figure 9. Lamp
460
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WAVELENGTH, nm Flgure 8. Selected portions of the flame background molecular fluorescencespectrum obtained in the pulsed mode from a stoichomeiric C2H,/air flame while employing the acetylene filter (for removal of PH,) as described in the text
Table 111. Summary of Molecular Fluorescence Band Assignments Spectral region, nm Species Transition Spectroscopic system 220-275 PO AZc-X2n y system 280-295 OH A2zC-X2n 306.4 nm system 305-320 OH A2P-X2n 306.4 nm system 323-330 PO B 2 z - X 2 n p system 336 (max) NH A3n-X3c 336.0 nm system 380-390 CN BZr-Azn cyanogen violet system (PO y-system grating second order) 450-550 nal-to-noise ratios for many analyses, as it did in the previous study ( I ) , particularly for measurements near the limit of detection. An important objective of this study was to effect a comparison of the results obtained with the EIMAC source operated in the pulsed and cw modes, respectively. In Figure 6, the flame background fluorescence spectrum of a stoichiometric flame obtained in the pulsed mode is given; in Figure 7 , the corresponding flame background spectrum obtained in the cw mode is given. The fluorescence of the y system of PO is clearly more intense (relative to the fluorescence signals of OH, NH, and CN) in the pulsed mode than in the cw mode; an apparent reason for this can be discerned from Figures 9 and 10. Not only is the spectral output of the pulsed lamp generally shifted toward the UV as compared to that of the cw source, but also many of the large peaks in the pulsed lamp spectrum coincide with the bands of the PO y system. Hence, the fluorescence excitation of this molecular band system is more efficient in the pulsed mode. Comparing Parts A and B of Figures 9 and 10, it can be seen that the lamp output in the pulsed mode during the 2 0 9 ~ s sampling period is only a little (approximately three to five times) larger than the lamp output in the cw mode during the same sampling period. Considering that practical analytical atomic fluorescence measurements employing a continuum 948
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
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nm wavelength region of the EIMAC lamp operated in the pulsed (solid line) and cw (dotted line) modes source in the cw mode utilize a much larger duty factor (fractional “on-time”) and more efficient excitation optics, atomic fluorescence detection limits obtained in the pulsed mode would not be expected to be improved over those obtained in the cw mode. The reasons for the inefficient conversion of electrical power to light output in the pulsed mode are not known. I t is interesting to compare Figures 4-6 to ascertain the effect of varying the fuel/air ratio upon the observed fluorescence bands. The absence, in the fuel-lean flame, of the bands assigned to CN and NH is striking. Also, the OH band at -285 nm is present to a significant extent only in the fuel-lean flame, and even the band at -310 nm appears to undergo a marked increase in intensity as the fuel/air ratio is decreased. Further scrutiny of the latter band reveals that,
at the wavelengths corresponding to fluorescence maxima in the stoichiometric and fuel-rich spectra (Figures 5-8 a t 306.8 nm and 309.2 nm), there are sharp “dips” indicating virtually no fluorescence at all in the fuel-lean flame. These particular observations reveal that molecular fluorescence in flames may be, in many instances, strongly dependent upon flame conditions, a fact which should be of considerable importance in optimizing experimental conditions for analysis by atomic fluorescence spectrometry. The spectrum of Figure 8 was recorded under the same set of experimental conditions as adopted for the spectrum of Figure 6, except that the acetylene filter for removal of PH3 was inserted into the acetylene line immediately prior to recording the spectrum of Figure 8. The fluorescence of the y system of PO is barely discernible in Figure 8, indicating the excellent filtering capability of the filter. Moreover, the grating second-order bands above 450 nm disappeared entirely. These spectra suggest that the “attenuation factor” for this filter may be as much as 30 or more. At this point, it is appropriate to consider in general terms what effects these findings may have on multielement analysis by atomic fluorescence spectrometry with a continuum source. In applications where it is desired to obtain a qualitative scan of the entire fluorescence spectrum in order to establish the presence or absence of various metals within a sample, molecular fluorescence bands could obscure the presence of an atomic line. However, a detailed knowledge of the molecular fluorescence background associated with the pqrticular analytical conditions (sample matrix, flame gases, flame conditions, impurities, etc.) relevant to the analysis should be effective in accounting for any such spectral interferences in those cases where filtering of the impurity is impossible or incomplete. T h e effect of molecular fluorescence upon quantitative atomic fluorescence multielement analysis is a more complex matter. Because a molecular fluorescence band which overlaps an atomic fluorescence line constitutes an additional component of light viewed by the detector a t that wavelength, it is reasonable to expect an increase in the shot noise amplitude at the output of the photodetector as a result of any spectral interference from molecular fluorescence. Moreover, molecular fluorescence is capable of transmitting source flicker to the detector, should source flicker noise prove to be significant in relation to the total observed noise level in the system. In these ways, interference from flame molecular fluorescence is, in principle, similar to that associated with source radiation scattering from flame components. Because the fluorescence of these species may display a strong dependence upon the location in the flame of the focused source radiation, any general instability or turbulence associated with the flame may lead to measurable fluctuations in the intensity of the molecular fluorescence of these species. I t should be noted that the total effect on atomic fluorescence measurements (signal and signal-to-noise ratio) of the presence (or absence) in the flame of any fluorescing molecular species cannot be established solely from a knowledge of the concentration and fluorescence characteristics of that species, because the absorption and emission properties of that species also affect the signal-to-noise ratio (10, 11, 25) for atomic fluorescence measurements a t any
wavelengths susceptible to interference from molecular band(s). In summary, flame background molecular fluorescence interferences arising from the flame gases or sample matrices can seriously limit signal-to-noise ratios and detection limits and even the accuracy in the case of sample matrix produced interferents in atomic fluorescence spectrometry with a continuum source (as well as with line sources). However, this work has demonstrated the feasibility of optimizing experimental conditions so as to minimize such interferences, and to maximize atomic fluorescence precision, detectability, and accuracy; namely, molecular fluorescence of flame gas species and blank impurities can be minimized with respect to the atomic fluorescence by removal of the impurity (such as PHJ, change of flame conditions, and wavelength modulation (correction for broad band fluorescence and scatter and minimization of source related flicker noise).
ACKNOWLEDGMENT The authors are indebted to Radu Mavrodineanu of NBS for the loan of a PH3 filter which greatly reduced the PO fluorescence.
LITERATURE CITED D. J. Johnson, W. K. Fowler, and J. D. Winefordner, Talanta, in press. H. Haraguchi, W. K. Fowler, D. J. Johnson, and J. D. Winefordner, Spectrocbim. Acta, 32, 1539 (1976). , F. Burriel-Marti and J. Rarnirez-Munonz, “Flame Photometry”, 3rd ed., Eisevier, New York, 1964, p 40. (4) T. J. Vickers and J. D. Winefordner, “Analytical Emission Spectroscopy”, Part 11, in AnaMical Spectroscopy Series, Vol. 1, E. L. Grove, Ed., Marcel Dekker, New York, i972, pp 3iO-312. (5) E. E. Pickett and S. R. Koirtyohann, Anal. Cbem., 41 (14), 28A (1969). (6) R. W. B. Pearse and A. G. Gaydon, “The Identification of Molecular Spectra”, 3rd ed., Chapman and Hall, London, 1965, pp 51-320. 17) H. Haraouchi. N. Furuta. E. Yoshimura. and K. Fuwa. Anal. Cbem.. 48. ‘ 2066 (1376). (6) A. G. Gaydon, G. N. Spokes, arid J. van Suchtelen, Roc. R. SOC. (London), Ser. A, 315, 129 (1970). (9) J. D. Winefordner and T. J. Vickers, Ana/. Chem., 38, 161 (1964). (IO) J. D. Winefordner and R! C. Elser, Anal. Cbem., 43 (4), 24A (1971). (11) C. D. West, Anal. Cbem., 48, 797 (1974). 112) . . R. H. Barnes. C. E. Moeller. J. F. Kircher. and C. M. Verber, Am/. . . ODt.. . 12, 2531 (1973). (13) C. T. J. Alkernade in “Proceedings of the Xth Colloquium Spec!roscopicum Internationale”, E. R. Lippincott and M. Margoshes, Ed., Spartan Books, Washington, D.C., 1963: (14) T. Carrington, Paper presented at the 6th Symposium on Combustion, Pasedena. Calif.. 1960. (15) C Veiion,’J: M. Mansfield, M. L. Parsons, and J. D. Winefordner, Anal. Chem., 38, 204 (1966). (16) D. J. Johnson, F. W. Phnkey and J. D. Winefordner, Can. J. Spectrosc., 19, 151 (1974). (17) D.R. Jenkins, Paper presented at the 2nd International Atomic Absorption and Fluorescence Conference, Sheffield, England, 1969. (18) H. G. C. Human and P. J. Th. Zeegers, Spectrochim. Acta, Part 5,30, 203 (1975). (19) D. J. Johnson, F. W. Plankey, and J. D. Winefordner, Anal. Chem., 47, 1739 (1975). (20) J. 1. Dinnin, Anal. Cbem., 40, 1825 (1968). (21) J. P. S. Harrsma, J. Vlogtman, and J. Agterdenbos, Spectrochim. Acta, Part €I 31, , 129 (1976). (22) R. Mavrodineanuand H. Boiteux, “Flame Spectroscopy”, John Wiley and Sons, New York, 1965, pp 60-63. (23) D. J. Johnson, F. W. Plankey, and J. D. Winefordner, Anal. Chem., 46, 1897 (1974). (24) F. Grurn and G. W. Luckey, Appl. Opt., 7, 2289 (1968). (25) D. R. Jenkins, Spectrochim. Acta, Part 5 ,23, 167 (1967). ~
RECEIVED for review February 9,1977. Accepted March 17, 1977. Work supported by AF-AFOSR-F44620-76-C-0005.
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