experimental value of 1.02 mg/ml compared t o a theoretical value of 1.OO mg/ml for the determination. The large error for the determination of 1-124 in the 1-10,11,12,14-s mixture is attributed to the inclusion of a portion of 1-11-s in the gel sample strip. This was observed by applying pinacryptol yellow to the entire sample area after removal of the sample strip. As previously mentioned, the samples in the middle of the cell were behind the outsides samples by as much as 4 mm in these determinations. The results obtained for the 1-10,12,14-s mixture show, as in the case of the p 1-10,3-12,14-s mixture, that if the sample strip is accurately selected, this method of analysis is accurate and dependable.
ported system (4). The basic features in the design of this cell which have eliminated these problems are direct contact between the gel and buffer solution and direct contact between gel and the cooling surface. The slightly uneven migration patterns that were observed are due to failues in construction and materials and can be corrected. There is no reason why the length of the cell cannot be extended t o accommodate runs of longer duration. The results of this limited study demonstrate the method has potential for the separation and quantitative analysis of alkylbenzenesulfonates and similar ionic species.
CONCLUSION
The electrophoretic cell presented in this paper was developed to overcome problems inherent in a previously re-
RECEIVED for review Juiy 22, 1971. Accepted November 24, 1971.
Factors Affecting the Use of a Nondispersive System for Atomic Fluorescence Flame Spectrometry T. J. Vickers, P. J. Slevin, V. I. Muscat, and L. T.Farias Department of Chemistry, Florida State University, Tallahassee, Fla. 32306
A nondispersive atomic fluorescence system responding to radiation of wavelengths less than 2800 A is described, and the effect of the high energy throughput and broad spectral response of the system on the signal-to-noise ratio of atomic fluorescence measurements with line excitation sources is discussed. Dispersive and nondispersive measurements for Hg and As are compared to support conclusions on the utility of the system with flame atom reservoirs of low spectral radiance. THEBENEFITS TO BE DERIVED from a nondispersive system for atomic fluorescence measurements with atomic line excitation sources were recognized early in the application of atomic fluorescence to chemical analysis (1, 2). The principal benefits t o be expected have been listed as: (i) greater energy throughput (3-6), (ii) simultaneous collection of multiple lines for elements with a complex fluorescence spectrum (3), (iii) convenience of simultaneous multi-element analysis (3, 7-9), (iv) simplicity and ruggedness of instrumentation ( 4 ) . Attainment of the latter two of these would seem to be incontrovertible, but the real gain to be attained through (i) and (1) D. R. Jenkins, Spectrochim. Acta, 23B,167 (1967). (2) T. S. West and X. K. Williams, ANAL.CHEM., 40, 335 (1968). (3) P. L. Larkins, R. M. Lowe, J. V. Sullivan, and A. Walsh, Spectrochim. Acta, 24B, 187 (1969). (4) T. J. Vickers and R. M. Vaught, ANAL.CHEM.,41, 1476 (1969). (5) P. D. Warr, Talanta, 17, 543 (1970). (6) R. C. Elser and J. D. Winefordner, Appl. Spectrosc., 25, 345 (1971). (7) D. G. Mitchell and A. Johansson, Spectrochim. Acta, 25B,175 (1970). (8) D. G. Mitchell, “Advances in Automated Analysis,” Vol. 11, Thurman Associates, Miami, Fla., 1971, pp 503-6. (9) D. R. Demers and D. F. Mitchell, “Advances in Automated Analysis,” Vol. 11, Thurman Associates, Miami, Fla. 1971, pp 507--11.
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
(ii) is not so clearly established because it depends in part on the characteristics of the atom reservoir used. The ideal nondispersive atomic fluorescence system would make use of an atom reservoir which did not emit background radiation within the spectral response of the detector. Such ideal behavior has been approached in the use of a flameless system for mercury (IO), and presumably other flameless systems will have similar advantages for other elements. In a conventional dispersive flame atomic fluorescence system, the monochromator serves chiefly to limit the amount of flame background emission which falls on the detector. In nondispersive systems, several steps have been taken to circumvent the difficulties due to background emission: (i) Flames of low spectral radiance have been employed (4-6). (ii) Solar blind multiplier phototubes have been used in place of detectors of wider spectral response (3-6). The most widely used of these (HTV R166) responds only to radiation of wavelengths less than 3200 A. (iii) Bandpass filters have been used with both solar blind and broadband response detectors t o limit the amount of background emission reaching the detector (5-7). Filters with half-bandwidths of 4 to 50 nm have typically been employed. (iv) In work published since completion of the work described in this report, Larkins (11) and Larkins and Willis (12) have described a very useful nondispersive atomic fluorescence system in which the flame background emission was greatly reduced by separation of the flame by a sheath of inert gas. In this report we describe a nondispersive atpmic fluorescence system for the spectral region below 2800 A and discuss the benefits derived from the greater energy throughput and multiple line collection of such a nondispersive system. (10) V. I. Muscat, T. J. Vickers, and A. Andren, ANAL.CHEM., 44, 218 (1972). (11) P. L. Larkins, Spectrochim. Acta, 26B,477 (1971). (12) P. L. L,arkins and J. B. Willis, ibid., p 491.
EXPERIMENTAL The measurement system is shown in schematic fashion in Figure 1 . A premix burner with provision for flame sheathing (but not flame separation) was employed. The properties of this burner as an atom reservoir for atomic fluorescence have been previously described (13). The optical properties of the filter are described in a subsequent section. For dispersive measurements, the entrance slit of the monochromator was placed, as shown, 7.5 cm from the flame center, and the detector module was placed behind the exit slit. For nondispersive measurements, the monochromator was replaced by the detector module such that the distance from the flame center to the entrance aperture (a rectangular opening 1 . 5 X 2 cm) was 7.5 cm. The distance from the entrance aperture to the detector photocathode was approximately 4 cm, and the photocathode dimensions were approximately 10 mm X 20 mm. The entire width of the flame was illuminated by source radiation. The detector was a solar blind multiplier phototube (HTV R166). With the filter in place the measurement systep responded only to radiation at wavelengths less than 2800 A . The remaining components of the measurement system are described in Table I.
t.
7.5cm
5.2cm
3-
RESULTS AND DISCUSSION
Effect of Spectral Bandpass. With a flame atom reservoir, the full benefits predicted for a nondispersive system cannot be attained because the flame background emission adversely affects the signal-to-noise ratio of the fluorescence measurements (14). This is easily demonstrated by considering an idealized experiment in which a single fluorescence line is observed on a continuum of uniform intensity at all wavelengths by a monochromator-detector system of uniform spectral respnnse. The signal due to the fluorescence emission is, in the simplest case, given by (14). iF
=
k y WIp
where iF is the fluorescence signal, kM is the entrance optics factor, W is the slit width, and Ip is the fluorescence intensity. The noise due to flame background is given by (14)
where [ is the relative flame flicker factor, 4fis the frequency response bandwidth of the amplifier readout system, IB is the intensity of flame background emission, and s is the monochromator spectral bandwidth. When a line source of excitation is used, only the noise due to flame background is directly proportional to s, and, if ZB is not zero, at some value of s the fluctuation in the flame background emission becomes the principal source of noise, and the signal-to-noise ratio for the fluorescence measurement is given by
(3) It can be seen from Equation 3 that under the conditions proposed if the spectral bandpass of the measurement system is increased the S/Nratio decreases. Thus, for this special set of circumstances, a nondispersive system, which can be considered as the limiting case of increasing the monochromator spectral bandpass, would not be advantageous. The actual situation is much more complex since more than one fluorescence line may be observed, the flame background emission is not usually a uniform continuum but has features such as the (13) P. J. Slevin, V. I. Muscat, and T. J. Vickers, Appl. Spectrosc., in press. (14) J. D. Winefordner, M. L. Parson, J. M. Mansfield, and W. J. McCarthy, ANAL.CHEM.,39,436 (1967).
Figure 1. Measurement system used in this study 1, Excitation source; 2, mechanical chopper; 3, fused silica lens; 4, sheath burner; 5, spherical front surface mirror; 6 , chlorine
filter; 7,monochromator or detector module
Table I. Experimental Facilities As electrodeless discharge tube operated in wave cavity. Power supplied at 2450 MHz from 100 W continuous wave oscillator (Scintillonics, Inc.). Hg pen-lamp (Ultraviolet Products) Chopper Rotating circular chopper with chopping frequency of 160 Hz Lens 30-mm diameter, 50-mm focal length, fused silica Flame Premix with provision for flame sheathing Mirror 50-mm diameter, 75-mm focal length, front surface spherical mirror Filter 25-mm diameter X 52-mm length quartz tube with fused quartz end windows containing approximately 1 atm chlorine (Ophthos Instrument Co.) Monochromator 350-mm focal length, Czerny-Turner mount, 1180 gropveslmm grating blazed for 2500 A, f/7, 20 A/mm reciprocal linear dispersion at exit slit in 1st order (Model EU-700, Heath Company) Detector Heath EU-701 with Hamamatsu T V R166 module multiplier phototube Amplifier Lock-in Amplifier (Model HR-8, Princeton Applied Research Corp.) Sources
OH bands, and the spectral response of the monochromatordetector system is strongly wavelength dependent, but the major conclusion is the same: as the spectral bandpass is increased, the effect of flame background emission becomes more perceptible. From the preceding discussion, it should be apparent that the background spectral radiance of the atom reservoir is an important consideration in the application of nondispersive ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
931
90
t
30
-
%T
20
-
10 -
Wavelength, nm
Figure 2. Transmission characteristics of chlorine filter
atomic fluorescence. The use of flames of low spectral radiance usually requires some sacrifice in sensitivity due to reduced atomization efficiency, since relatively cool flames are employed, and the region viewed is determined primarily by its background spectral radiance rather than the atomic concentration of the element of interest. For example, in the nondispersive A F measurements of Vickers and Vaught ( 4 ) the region of observation was above the visible portion of a n Hl/air flame (approximately 9.3 cm above the tip of a Beckman 4020 atomizer-burner). The use of solar blind multiplier phototubes reduces the effect of flame background emission but restricts the spectral bandpass of the measurement system
and thus reduces the number of lines which can be observed in fluorescence. The R166 solar blind multiplier phototube is reported (3, 5 ) to have a lower sensitivity a t wavelengths above approximately 2000 A than some broad-band response detectors ($5, S-19), but in our experience the noise of the R166 is correspondingly lower and the decrease in photocathode sensitivity can be more than compensated by a n increase in amplifier gain. A more serious limitation of the R166 and similar phototubes is that their spectral response includes the strong OH band emission of flames in the 2800 t o 3200 A region. Thus, if flame regions of other than very low OH background radiance are to be viewed, some additional steps must be taken t o restrict the spectral bandpass of the measurement system. When bandpass filters are used, the spectral bandpass is further restricted, with a consequent further reduction in the ability of the nondispersive system to accept multiple lines of the element of interest; and the energy throughput of the system is reduced, since peak transmittance of such filters is typically in the range of 5 to 25 %. The nondispersive measurement system of Figure 1 offers a novel approach in which the spectral bandpass is tailored to eliminate most of the OH flame background emission but a relatively broadband response at wavelengths less than 2800 A is retained. This compromise is achieved by use of a chlorine cutoff filter with a solar blind multiplier phototube. The transmission characteristics of the filter are illustrated in Figure 2. The filter is transparent in the region beyond 3900 A and thus is not suitable for use with a broad-band response detector. The effect of the filter on the flame background emission signal observed for various flames with the without filter
4 1850
2150
2450
2750
3050
Figure 3. Background emission spectrum of H,/air flame recorded in 1850 to 3200 A region with and without chlorine filter Flame conditions: Hz-20 psig, 10.0 l./min; air-45 psig, 5.5 I./min.
1850
2150
2450
2750
3050
Figure 4. Background emission spectrum of H2/02/Arflame recorded in 1850 to 3200 A region with and without chlorine filter Flame conditions: H2-20 psig, 2.10 I./min; 02-20 psig, 3.5 I./min; Ar-30 psig, 6.0 l./min. 932
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Figure 5. Background emission spectrum of C2H2/airflame recorded in 1850 to 3200 A region with and without chlorine filter Flame conditions: C2H2-10psig, 1.3 l./min; air-30 psig, 5.5 l./min.
Figure 6. Background emission spectrum of H2/NS0flame recorded in 1850 to 3200 A region with and without chlorine filter Spectra are shifted for clarity. Flame conditions: H2-20 psig, 18.0 I./min; H20-40psig, 5.5 I./min. R166 detector is shown in Figures 3-6. All the spectra were recorded a t the same sensitivity setting. For the H$air and Hy/02/Arflames shown in Figures 3 and 4, use of the filter reduces the background emission signal t o a very low level-the only features remaining in the spectra are weak OH bands with heads a t 2608.5 and 2677.3 A (15) and, for H2/02/Ar,a weak continuum. This is a gratifying result since the H2/02/Arflame is the flame of choice for many A F measurements (16). From Figures 5 and 6, it can be seen that a less satisfactory result is obtained with C2Hz/airand H2/N20 flames. In the C2H2/airflame a relatively strong continuum emission and barely discernible features due to the 2608.5 and 2677.3 A OH bands remain when the filter is used. In the HZ/N20flame the continuum remains, and the two OH bands are stronger than in the C2Hz/airflame; in addition, a band progression due to NO (15) appears. It may be possible by proper choice of flame conditions and region of observation t o further reduce the background emission signals observed in (15) J. B. Willis, V. A. Fassel, and J. A. Fiorino, Spectrochim. Acta, 24B, 157 (1969). (16) J. D. Winefordner, V. Svoboda, and L. J. Cline, Crit. Reo. Anal. Chem., 1, 233 (1970).
Table 11. Optimum Flame Conditions for Hg and As Mercury Inner flame Hydrogen 20 psig 2 . 5 I/min 20 psig 1 . 0 l/min Oxygen 30 psig 5 . 5 l/min Argon Region of observation: 1.5 cm above burner top. Arsenic Inner flame Hydrogen 20 psig 3 . 5 l/min Argon 40 psig 6 . 7 l/min Outer flame Hydrogen 20 psig 3.5 l/min Argon 40 psig 3 . 2 l/min Region of observation: 2.0 cm above burner top
the latter two flames, but it seems likely that flame separation (1I,12)is required for effective use of either of these flames in a nondispersive system. Mercury Measurements. The fluores!ence spectrum of mercury consists of a single line at 2537 A ; and, if either an H2/air or H2/02/Arflame serves ,as the atom reservoir and only the spectral region below 2800 A is considered, the conditions ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
933
Figure 7. Effect of monochromator slit width on mercury fluorescence signal Mercury concentration10 pglml. Numbers across lower edge are slit widths in pm. 1, shutter closed; 2, shutter open; 3, solution aspirated
v
4
I
0
2350H
0 Nondispersive I
I I I , 1 t 1
IO
I
I
I
I IIIII
I
I
100 Concentration, pg/ml
I
I ' 1 L
IC
Figure 8. Analytical curves for arsenic All intensity values are to same scale, but the nondispersive values have been multiplied by
of measurement correspond rather closely to the conditions proposed in connection with Equation 1. Thus we may expect the A F signal to increase as the energy throughput of the system increases, but there can be no gain from a broader spectral bandpass since there are no additional lines to be collected. Optimum flame conditions for mercury mrasurements were investigated and found to be as shown in Table 11. With these conditions, measurements were made for mercury concentrations (HgCI2aqueous solutions) over the range 1 to 1000 pg/ml in the nondispersive mode and 6 to 1000 pg/ml in the dispersive mode (spectral bandpass of approximately 4 A). Analytical curves prepared from these data were linear from the lowest concentration measured to in excess of 100 pg/ml but showed some bending toward the concentration axis at 1000 pg/ml. Limits of detection were not extensively investigated but were found to be approximately 1 pg/ml in the nondispersive mode and 6 pg/ml in the dispersive mode for the conditions used. 934
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
In order to assess the effect of greater energy throughput and spectral bandpass on the measurement signal-to-noise ratio, measurements were made as a function of monochromator slit width with the dispersive system for a Hg concentration of 10 pg/ml. The results are shown in Figure 7: the fluorescence signal and background reading increase linearly with slit width, as predicted by Equation 1, but the noise increases only slightly over the range investigated, implying that flame background is not the principal source of noise. (The background signal is due to stray source radiation.) The near constant magnitude of the noise to quite large slit widths suggests that in the dispersive mode the noise is principally due to detector dark current shot noise. With the same Hg concentration, the nondispersive system gave a signal reading 350-fold greater than the dispersive system with a 500-pm slit width, and the signal-to-noise ratio was improved by approximately a factor of 6. In the nondispersive mode, noise due to flame background emission (flicker and shot noise) is significant, but it is clear that with a flame of low spectral radiance, the greater energy throughput of the nondispersive system results in an improvement in signal-to-noise ratio despite the greater intensity of flame background radiation incident on the detector. Arsenic Measurements. The fluorescence spectrum of arsenic consisted of four main lines, 1890, 1937, 1972, and 2350 A, with signals in the ratio 0.25:1.0:0.97:0.44 when the filter and R166 detector were used, Thus we may expect the A F determination of As to benefit from both the increased energy throughput and wider spectral bandpass of the nondispersive system. Optimum flame conditions for As were investigated and found to be as shown in Table 11. With these conditions, analytical curve measurements were made with the dispersive system for each of the four lines and with the nondispersive system. The results are shown in Figure 8. The limits of detection were Fstimated to be 6 pg/ml for the dispersive system at 1937 A and 0.3 pg/ml for the nondispersive system. At a concentration of 50 pg/ml, the signal increased by a factor of approximatFly 2200 in going from the dispersive measurement at 1937 A with a 500-1m slit width to the nondispersive measurement. The size of the gain is somewhat surprising since one would anticipate from the measurements with mercury and the relative intensities of the As lines a gain of approximately 350 (0.25 + 1.0 0.97 0.44) = 931. This discrepancy appears to reflect the rapid decrease in the transmission efficiency of the monochromator at shorter wavelengths and points out another advantage of a nondispersive
+
+
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) (11) (12) (13) (14) (15)
W. Slavin, At. Absorption Newsleft.,6 , 9 (1967). D. R. Demers and D. W. Ellis, ANAL.CHEM., 40, 860 (1968). H. L. Kahn, Adoan. Chem. Ser., 73, (1968). P. L. Larkins, Spectrochim. Acta., 26B, 477 (1971). H. L. Kahn, Amer. Lab., Aug., 52 (1969). R. Browner and D. C. Manning, ANAL.CHEM., 44,843 (1972).
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