Elimination of Interferences in Flame Spectrometry Using Spectral Stripping K. W. Busch, N. G. Howell, and G.
H. Morrison'
Department of Chemistry, Cornell University, Ithaca, N. Y. 14853
A simple and rapid spectral stripping technique Is described for the elimination of practically all spectral interferences in flame emission spectrometry. The technique utilizes a vidicon flame spectrometer, which permits the simultaneous monitoring of a 20-nm spectral range. Exa:nples are described for the removal of either molecular band or undesired concomitant interferences from analytical lines of interest, with resultant high accuracy of determination. In contrast to compensation methods, spectral stripping allows samples of diverse matrices to be determined using a simple calibratlon curve obtained with synthetic standards containing only the buffered analyte. The time required to perform spectral stripping is less than one minute per sample.
It is frequently stated that atomic absorption spectrometry is superior to flame emission spectrometry because atomic absorption spectrometry is free of spectral interferences ( I , 2 ) . In atomic absorption spectrometry, the high effective spectral resolution obtained is due t o the effective bandwidth of the line source used, the monochromator being necessary only to isolate the desired resonance line from the other source radiation. Even with the high effective spectral resolution obtained in atomic absorption spectrometry, several reports have appeared which demonstrate that spectral overlap can occur in certain instances, resulting in a spectral interference (3-5). In comparison, the monochromator used in flame emission spectrometry functions to isolate the desired analytical line from all other emitted radiation. The ability t o separate closely spaced radiation in flame emission spectrometry, therefore, depends on the resolution of the monochromator, and spectral interference occurs when the complexity of the emitted spectrum exceeds this limit. Indeed, many of the observed signal enhancements produced by the addition of large excesses of other metals may be due to the presence of an unsuspected minor line within the spectral bandpass of the monochromator. With the recent advent of multichannel detectors in flame spectrometry (6), these limitations can now be overcome. The effective spectral resolution of a flame emission system may be increased instrumentally by use of electronic spectrum unfolding or spectral stripping methods analogous to those used in gamma-ray spectrometry (7-9). T o perform spectral stripping in flame emission analysis, a spatia1 multichannel detector (6) and a multichannel ana-
' Reprint requests should be sent to G. H. Morrison at the above address. (1) W. Slavin, Appl. Spectrosc.,20, 281 (1966). (2) H. L. Kahn, J. Cbem. Educ., 43, A7 (1966). (3) V. A. Fassel, J. 0. Rasmuson, and T. G. Cowley, Spectrocbim. Acta, Part 6,23, 579 (1968). (4) J. E. Allan, Spectrocbim. Acta, Part& 24, 13 (1969). (5) D. C. Manning and F. Fernandez, At. Absorption Newsleft., 7, 24 (1968). (6) K. W. Busch and G. H. Morrison, Anal. Cbem., 45, 712A (1973). (7) K . Siegbahn, "Alpha, Beta- and Gamma-Ray Spectroscopy," North Holland Publishing Co., Amsterdam, 1965. (8) 0. V. Anders and W. H. Beamer, Anal. Cbem., 33, 226 (1961). (9) W. Lee, Anal. Cbem., 31, 800 (1959).
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9
lyzer are employed. The multichannel analyzer required t o perform spectral stripping must have two separate memories and a n arithmetic unit t o make it possible to algebraically combine spectra stored in each memory. This allows a complex spectrum to be accumulated in one memory. An equivalent amount of the interfering spectrum is accumulated in the second memory, and subtracted from the pulse-height distribution of the complex spectrum, t o yield the net resolved pulse-height distribution of the component of interest. Spectral stripping is a universally applicable technique capable of resolving direct spectral overlap. It differs from compensation techniques commonly used in flame analysis in that it requires no matching of solutions, Le., the concentration of the interferent causing spectral overlap need not be known. This paper describes a simple experimental procedure for the elimination of interferences caused by: a) the spectral overlap of an analytical line with either a flame band or a concomitant band; and b) the spectral overlap of line emission, including complete overlap. The technique uses a recently described vidicon flame spectrometer (IO) which utilizes a dual memory multichannel analyzer and a sensitive television tube which is arranged t o monitor the radiation dispersed across the tube face in 500 electronic channels.
EXPERIMENTAL System. The vidicon flame spectrometer used in this study consists of a nitrous oxide-acetylene flame source, a 0.5-m Ebert monochromator, a UV sensitive image vidicon tube, and an optical multichannel analyzer as described in Table I. The principle of operation of the vidicon tube and the characteristics of the vidicon flame spectrometer have already been described (6, 10). An 1180groove mm-' grating causes a 20-nm spectrum t o be dispersed across the 0.5-inch width of the tube target. Using this dispersing system with 25+m slits, lines 1.4 %, apart are resolvable. The spectral lines dispersed by the monochromator are focused on the tube target as vertical images which are aligned with the 500 channels permitting the 20-nm spectral window t o be monitored simultaneously. The radiation striking the vidicon tube target results in a charge pattern being produced on the target, which is read off the target in 500 electronic channels by a scanning electron beam. This charge pattern is stored by the target until read by the electron beam and continues to accumulate as long as light strikes the target. The charge pattern in any given channel is read once every frame scan (32 msec), and the summation of any desired number of frame scans may be accumulated into either of two memories. By selecting the A minus B mode, an arithmetic unit provides the channel-by-channel difference of memory B from memory A, which is displayed on the oscilloscope monitor. The peak height of a spectral line may be obtained by moving the cursor or channel marker to the line peak as displayed on the oscilloscope and reading the contents of that channel on the digital display panel of the optical multichannel analyzer. By selecting the summation mode, the summation or integral of the counts in any number of adjacent channels may be obtained. This mode is used to obtain the average background adjacent to a spectral line. Procedure for Spectral Stripping. Case A . Spectral Interference of a Flame Band with an Analytical Line This is the simplest case and is taken care of almost automatically by the vidicon (10) K. W. Busch, N . G. Howell, and G. H. Morrison, Anal. Cbem., 46, 575 (1974).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
B
A-8
I
>
2
e
z
w
c
z
H W
? 4
W J
OI
304
324 304
324 304
WAVELENGTH, nrn (a)
WAVELENGTH.nrn (b)
324
WAVELENGTH, nrn (C)
Figure 1. Interference of OH flame bands with Bi 306.8-nm line
( a ) Spectrum obtained with 60 ppm bismuth solution in nitrous oxide-acetylene flame, and stored in memory A. ( b ) Spectrum of OH flame bands under identical experimental conditions and stored in memory B. ( c ) Net spectrum of Bi obtained using A minus B mode, where the intensity scale has been divided by ten to show t h e signal more clearly
flame spectrometer. In this case, the sample is aspirated into the flame and the signal accumulated in memory A for a sufficient number of frame scans to provide a signal of adequate intensity. A blank solution of distilled water is then aspirated for the same number of frame scans under identical flame conditions and accumulated in memory B. Since the time integrated intensity of the flame band should be the same in both cases, selection of the A minus B mode displays only the analyte signal without any signal from the flame band. Case B. Spectral Interference of Molecular Rands Emitted by Sample Concomitants on a n Analytical Line. In this case the sample is aspirated into the flame and the signal accumulated in memory A for a sufficient number of frame scans to provide a signal of adequate intensity. This results in a pulse-height distribution in which the analytical line of interest may be obscured. While monitoring the A minus B mode, the analyst aspirates a solution of the concomitant which emits the band, and accumulates this signal in memory B. The accumulation is terminated when the band has been eliminated as determined from the oscilloscope display. For the accumulation time to be the same for both memories, the stripping solution should contain more concomitant than that present in the sample. Nevertheless, this concentration need not be known by the analyst. In this way. the band may be stripped in less frame scans than that used to accumulate the sample in memory A. The remaining difference in frame scans between memory A and memory B is made up by aspirating distilled water and further accumulating this signal in memory B. In this way, both memories have viewed the flame with solution being aspirated for the same total number of frame scans, and a flat base line results. Case C. Spectral Interference of an Analytical Line with a Line Emitted by a Sample Conconzitant. In this case, the sample is aspirated into the flame and the signal accumulated in memory A for a sufficient number of frame scans to provide a signal of adequate intensity. The resulting pulse-height distribution of the overlapping lines cannot be used to monitor spectral stripping as in the case of band overlap; thus a spectrally resolved monitor line of the interfering element a t a different wavelength must be selected for spectral stripping. This monitor line may occur in the same spectral window or a different window. If the monitor line occurs in the same window, a more concentrated stripping solution is accumulated in memory B until the monitor line is erased from the A minus B mode display. The remaining difference in frame scans between memory A and memory B is made up by aspirating distilled water and accumulating the signal in memory B. If the monitor line occurs in another window, the grating is rotated to observe this region, the sample solution is aspirated into the flame, and the monitor line is accumulated in memory A for a selected number of frame scans. This monitor line is then stripped from the resulting spectrum in the manner described above. The number of frame scans required to accomplish this is determined by selecting the accumulation cycles mode on the optical multichannel analyzer and reading the number of frame scans accumu-
lated. The grating is then rotated back to the original window, and the sample is aspirated for the selected number of frame scans in memory A. The stripping solution is then aspirated for the number of frame scans determined using the monitor line. The remaining difference in frame scans between memory A and memory B is made up by aspirating distilled water and accumulating in memory B. The peak height of the resolved analytical line is read digitally on the display panel by moving the cursor to the line peak. The average adjacent background in ten channels is obtained with the summation mode and subtracted from the peak height to give the background corrected peak height used in analyses. In some cases, it may be necessary to use a stripping solution whose concentration is not significantly greater than the concentration of interferent present in the sample. This solution can be prepared by adding a concentrated stock solution to an arbitrary amount of distilled water until the intensity obtained with this solution in real-time with the monitor line is slightly higher than that produced by the sample. This can be conveniently accomplished by delivering the concentrated stock solution from a buret into a beaker containing distilled water. A magnetic stirrer is used to mix the solution.
RESULTS AND DISCUSSION General Considerations. The signal obtained for any channel with the vidicon detector is the sum of the leakage current for the diodes included in the channel and the sum of whatever flame radiation and sample radiation strikes the given channel. The leakage current is primarily dependent on temperature and is constant under ordinary experimental conditions. The leakage current does vary from channel to channel, however, because of target irregularities, and for this reason it is necessary to accumulate the same total number of frame scans into each memory when performing spectral stripping. The portions of the signal due to flame emission, analyte emission, and concomitant emission, all contribute independently to the signal, enabling the contribution of the flame emission signal and concomitant emission signal to be removed from the analyte emission signal by simple algebraic subtraction. Reciprocity. A major disadvantage of compensation methods currently used in flame emission is that the standards and sample must be matched. This means that the analyst must know the concentration of the interferent in the sample, and this requires an additional determination. The reciprocity property of the vidicon flame spectrometer overcomes this limitation. Since the target gain and the slope of the current us. luminous flux curve are both unity, the analog displacement
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 1 4 , DECEMBER 1974
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Table 11. Signal Reciprocity with Barium 553.5-nm Line
Table I. Experimental Facilities External optics
Burner Monochromator
Entrance slit Detector Optical multichannel analyzer Readout Flow meters
5-cm diameter Supracil lens with 12.5-cm focal length. Lens stopped down to 2.6-cm diameter. Varian Techtron 5-cm slot burner for nitrous oxideacetylene. Jarrell-Ash, Model 82000,O. 5m Ebert mounting scanning monochromator with 1180 grooves/mm grating blazed for 3000 A. Reciprocal linear dispersion 16 A mm-' in the first order. Jarrell-Ash, Model 12-080 variable 0-2000 pm straight edged slit. UV sensitive silicon diode vidicon, Model 1205F, SSR Instruments Co. Model 1205A, SSR Instruments
co. Techtronix oscilloscope, 604 Monitor. Brooks Full-view Rotameters calibrated for nitrous oxide and acetylene, Brooks Instrument Division.
Barium concn,a ppm
Frame scans
16.3 32.6 48.5 81.4 114 163
1000 500 333 200 143 100
Line intensity, c o n t i
Av
31142 29835 32035 30608 31612 30121
+
30892
i
331b 628 1322 i. 2045 i 1824 i 1060
+ +
858
1000 ppm of Cs was added as an ionization buffer to all solutions. b Standard deviation of four measurements. Q
where n is the number of counts produced for the given channel, and y is the number of counts per ampere produced by the optical multichannel analyzer. With a source of constant radiance, the total number of counts S for a given channel for f frame scans is given by
(5) Letting k = a(X)WHQtrlT, Equation 5 may be written as
S = kpCf
(6 )
where h is an instrumental constant. Thus, as long as @ remains constant over the concentration range covered, there exists a reciprocity relationship between concentration and the number of frame scans, given by
C,fi = C2f2 current produced with an image vidicon when the scanning electron beam recharges a given channel may be expressed by s =
QeXWHEt hc T
(11
where s = signal in amperes; Q = quantum efficiency of the target; e = charge of electron, coulombs; X = wavelength of the incident radiation, cm; W = width of a channel, cm; H = height of a channel, cm; E = radiant energy striking the target, ergs sec-' cm-2; t = time for which charge is allowed to build up on the target before being erased by the scanning electron beam ( t is the period of a single frame scan), sec; h = Planck's constant, erg sec; c = speed of light, cm sec-'; T = time for the scanning electron beam to erase a given channel, sec. Disregarding the transmission of the optics, E = I O , where I is the radiance of the given spectral line emitted by the flame in ergs sec-l cm-2 ster-' and O is the solid angle of the collimator of the monochromator in steradians. Equation 1may be rewritten, s =
CY
(x)WH8 t l T
(2 1
where a(X) = QeX/hc. The radiance emitted by the flame for a given solution concentration of emitting species is given by I = PC, where (3 is the slope of the growth curve in radiance per unit concentration and C is the solution concentration of the emitting species. Substituting this in Equation 2 gives
s =
CY (A) WHS2tpC
T
(3 )
The number of counts accumulated in memory for a single frame scan for a given channel is given by
n = ys 2076
(4 )
This means that the same signal can be obtained for solutions of different concentrations by adjusting the number of frame scans used. For this reason, the analyst does not need to know the concentration of interfercnt in the sample when performing spectral stripping, and can match signals using another solution by adjusting the number of frame scans used. T o match leakage currents in both memories, the same total number of frame scans must be accumulated into each memory and, for this reason, the interferent should be present in greater concentration in the stripping solution than in the sample solution. This allows the spectral stripping to be completed in less frame scans than used to accumulate the sample in memory A. The remaining difference can then be made up by aspirating distilled water, to give the same total number of frame scans in each memory. Table I1 shows the reciprocity between solution concentration and the number of frame scans for a series of barium solutions. Table I1 shows that strict reciprocity holds for a tenfold range of concentration with the barium 553.5-nm line. Case A. Figure 1 shows an example of the use of spectral stripping to remove the interference of the OH band emitted by the nitrous oxide-acetylene flame from the bismuth resonance line a t 306.7 nm. Figure l a shows the unstripped spectrum obtained by aspirating a bismuth solution into the flame and storing the result in memory A. The bismuth 306.7-nm line is completely obscured by the OH band. Figure l b shows the spectrum of the flame obtained by aspirating distilled water into the flame for the same number of frame scans under identical flame conditions and storing the result in memory B. Figure IC shows the stripped spectrum obtained using the A minus B mode. Case B. The best known example of spectral interference between molecular bands emitted by a sample concomitant and an analytical line, is the case of the determination of barium in the presence of an excess of calcium. In the case of atomic emission spectrometry, several instru-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
A- 0
n a
L 564
WAVELENGTH, nm
(4 Figure 2. Interference of
(C)
CaOH band with Ba 553.5-nm line
( a ) Spectrum obtained with solution containing 0.35 ppm Ba and 165 ppm Ca and stored in memory A. ( b ) Spectrum in memory B after spectral stripping has been accomplished using a Ca stripping solution. ( c ) Net spectrum of Ba obtained using A minus 6,where the intensity scale has been divided by ten to show the signal more clearly
Table IV. Determination of Ba Using 553.3-11,
Table 111. Spectral Stripping of CaOH B a n d Emission from B a 553.5-nm Line for Samples Containing 0.32 ppm Ba a n d Various C a Concentrations C a concn, p p m a
0 275 687 961 1370
Sample
Ra intensity after s p e c e a l stripping, counts
C a content,
USGS W-1
7.82"
NBS Orchard
2 . Ogb
Ea concn detd by spectral stripping, PPm
159 44.3
Line
Actual Ea concn, pprn
16OQ
45.6'
Re1 error, 56
0.62 2.85
Leaves
*
2202 122* 2467 i 112 2485 & 127 2573 + 235 2549 i 172
(SRM 1571) a
Reference 23
NBS certified value. Neutron activation anal-
ysis.
Av 2455 148 1000 ppm Cs was added to all solutions as an ionization buffer. Standard deviation of five measurements. mental techniques have been proposed for the elimination of the interference of the CaOH bands with the barium resonance line a t 553.5 nm, including ac scanning spectrometry (11-13) and sample modulation (14-21). An interference is also present with this determination using atomic absorption spectrometry. In this case, molecular absorption of the barium 553.5-nm line by the CaOH molecule produces an erroneous absorption which must be corrected using background correction techniques ( 2 2 ) . In addition, the signal-to-noise ratio is degraded by the presence of the intense unmodulated CaOH radiation a t the photocathode of the photomultiplier. Figure 2a shows the unstripped spectrum obtained by ( 11) W. Snelleman, Spectrochim. Acta, Part 6, 23, 403 (1968). (12) W. Sneileman, T. C. Rains, K. W. Yee, H. E. Cooke, and 0. Menis, Anal. Chem., 42, 394 (1970). (13) V. Svoboda, Anal. Chem., 40, 1385 (1968). (14) R. Herman, W. Lang, and K. Rudiger, Z. Anal. Chem., 206, 241 (1964). (15) W. Lang, 2. Anal. Chem., 219, 321 (1966). (16) W. Lang, Spectrochim. Acta, PartA, 23, 471 (1967). (17) W. Trampisch and R. Herrmann, Spectrochim. Acta, Part 8, 24, 215 (1969). (18) K. Rudiger. 8. Gutsche. H. Kirchhof, and R . Herrmann, Analyst (London), 94, 204 (1969). (19) M. Marinkovic and T. J. Vickers, Anal. Chem., 42, 1613 (1970). (20) A. AntiCJovanoviC, V. Bojovic. and M. Marinkovic, Spectrochim. Acta, Part 8, 25, 405 (1970). (21) V. Bojovic and A. Antic-Jovanovic, Spectrochim. Acta, Part 8, 27, 385 (1972). (22) S. R. Koirtyohann and E. E. Pickett, Anal. Chem., 38, 585 (1966).
aspirating a digested sample of NBS Orchard Leaves (SRM 1571) into the nitrous oxide-acetylene flame and storing the result in memory A. The spectrum shown in Figure 2b is the barium-free CaOH band which is stored in memory B after the spectral stripping has been completed. The barium line cannot be identified due to the presence of the strong CaOH band emission. A stripping solution was made by dissolving an arbitrary amount of barium-free calcium carbonate in distilled water to give a more concentrated calcium solution than that present in the sample. By selecting the A minus B mode, the progress of the spectral stripping can be monitored visually on the oscilloscope display as the stripping solution is accumulated into memory B. Since the band is much wider than the line, it is easy to determine when the band has been stripped from the spectrum. When the CaOH band has been completely stripped, the accumulation is terminated by unlatching the B accumulate button. At this point, an irregular base line is observed on the screen because both memories do not contain the same total number of frame scans. A flat base line is obtained as shown in Figure 212, by accumulating in memory B the signal from the flame while aspirating distilled water, until both memories contain the same total number of frame scans. Table I11 shows that spectral stripping does not alter the remaining barium intensity when varying amounts of calcium are present as an interferent. Table IV shows the results of the analysis of the U.S. Geological Survey standard rock sample W-1 and NBS standard Orchard Leaves (SRM 1571) for barium using spectral stripping to remove the calcium interference. Both (23) F. J. Flanagan. Geochim. Cosrnochim.Acta, 37, 1189 (1973).
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 14, DECEMBER 1974
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30001
8
1
concentration C,. From Equation 9, f s ( A l ) is given by nl
m
Since C, is not known by the analyst, this quantity must be obtained by spectrally stripping the monitor line. When the monitor line has been completely stripped, Si(&) = S,(X2), where S i ( X 2 ) is the signal produced by the interferent a t A 2 and S s ( h ~is) the signal produced with the stripping solution a t X2. Thus one can write,
Figure 3. A = Strontium signal profile of 5534.81-A line for 1036ppm solution. ppm solution
= Barium signal profile of 5535.48-A line for 0.48-
samples were dissolved using a Teflon bomb technique ( 2 4 ) and Cs was added as an ionization buffer. Both samples were analyzed using the same calibration curve prepared from standard solutions containing only barium and 1000 ppm Cs as an ionization buffer. The standard solutions used to prepare the analytical curve covered a range of concentrations from 0.3 to 1.2 ppm of barium. The results indicate that high analytical accuracy can be obtained in the determination of trace amounts of barium in the presence of a 500-fold excess of calcium, using spectral stripping. Furthermore, the results indicate, that once a conventional calibration curve has been established using standards containing only barium and an ionization buffer, samples of diverse origin may be determined using spectral stripping with the same calibration curve. The analysis time per sample using this simple spectral stripping procedure to remove the CaOH band interference, is less than one minute per sample, once the calibration curve has been established. Case C. The removal of a spectral interference due to the overlap of two spectral lines is somewhat more complicated than the two previous cases, where it was possible to determine when stripping was complete by visually observing the base line adjacent to the line. Since this is not possible in the case of line overlap, another spectral line of the interfering element must be selected as a monitor line. To understand the factors involved in the selection of an appropriate monitor line, it is instructive to consider the factors involved in spectral stripping in more detail. The total signal in a given channel resulting from sample emission is given by
where St(Xl) is the total signal resulting from analyte emission and interferent emission at XI, S,(Xl) is the signal arising from analyte emission a t XI, and S i ( X 1 ) is the signal arising from the interferent a t XI. T o perform spectral stripping, the signal component due to the interferent, Si(X,), must be matched with an equal signal S,(Xl)which is accumulated in memory B. S,(X1) is the signal obtained by aspirating the stripping solution and accumulating the result in memory B. Since S i ( X l ) = S,(Xl) when spectra1 stripping is complete, then from Equation 6,
k p i (A 1)C i f t ~ f ) kp s (A 1)Cs-f s (A 1 )
(9)
where f,(Xl) is the number of frame scans of stripping s o h tion whose concentration is C, required to strip f(X1) frame scans of interferent whose concentration is C1. The quantit y of interest to the analyst is fs(Xl), the number of frame scans required to strip Si(hl) using the stripping solution of (24) B. Bemas, Anal. Chem. 40, 1682 (1968).
2078
Solving Equation 11 for C i and substituting the result in Equation 10 gives
Equation 12 says when [ P i ( X l ) ] l [ P s ( X ~ ) l and [ P ~ ( X Z ) I / [ P ~ ( A ~ ) I are both unity, f s ( X l ) = f s ( X z ) , and the same number of frame scans used to strip the monitor line a t Xz can also be used to strip the interfering line a t XI. Therefore, we must inquire under what conditions [Pi(Xl)]/[j.js(X~)] and [PS(A2)]/[Pi(X2)] will be equal to unity. It should be recalled that P is the slope of the intensity us. concentration curve, i.e., the growth curve, for the concentration in question. In general, the slope of this curve becomes less a t higher concentrations. Thus [@i(Al)]/[Ps(A1)] 2 1 and [ B s ( X ~ ) l l [ @ i ( X ~ ) l I 1, since the concentration of the stripping solution is greater than the concentration of interferent in the sample. Several cases are possible depending on the characteristics of A 1 and XZ. If hl and A 2 are both non-resonance lines, will be linear over a wide the growth curves for A 1 and concentration range, and [Pi(Xl)]l[@s(Xl)] = 1 and [ p s ( X 2 ) ] I [pi(X,)] = 1. In this case, the stripping solution may have any concentration over the linear regions of the growth curves as long as the solution does not saturate the detector. If one or the other line is a resonance line or if both lines are resonance lines, the linear regions of both growth curves will be reduced by self-absorption. Nevertheless, as C s approaches Ci, both [Pi(X1)]/[Ps(X1)I and [ P s ( X d ] / [ P i ( X d I approach unity. Therefore in these last two cases, the stripping solution concentration should not deviate greatly from the concentration of the interferent in the sample. Thus, if either or both lines are resonance lines, the stripping solution should be prepared by adding concentrated stock solution of interferent to distilled water, until the stripping solution gives a monitor line intensity only slightly greater than that observed in the sample solution. These considerations will be illustrated using the overlap of the strontium 5534.81-A nonresonance line with the barium 5535.48-A line. Using a conventional system employing a photomultiplier detector and a 0.5-m monochromator with an 1180 groove mm-’ grating, the reciprocal linear dispersion is 16 A mm-l. With 40-wm matched entrance and exit slits, two lines must be 1.28 8, apart to be completely resolved. In the present example, the lines are only 0.67 A apart, and therefore these lines would not be completely resolved. Figure 3 shows the signal profile, i.e., a plot of pulse height us. channel number, obtained when a solution of strontium (1036 ppm) is aspirated into the flame. A signal profile for a solution of barium (0.48 ppm) obtained under identical conditions is also shown. The degree of overlap of the two signal profiles is readily apparent. Figure 4 shows the signal profile obtained with a mixture of 1036 ppm strontium and 0.48 ppm barium. It is not apparent from the observed profile that a spectral line interference is present. Figure 4 also shows the net signal profile obtained for the barium line after spectrally stripping the interfering strontium line. The profile of the
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
4500
Table V. Spectral Stripping of the Sr 5534.83-A Line Using the Sr 5521.83-A Line as a Monitor Line Sr stripping solution, pprn
1040 2070 2900 4140 6210
-
4000-
Barium Intensit). after spectral stripping, countsQ
+ vr
1257 1270 1289 1279 1252 1271
3500-
z
3
8 3000>
c 9 2500c W
Av The test solution gave an intensity of 2484 counts without spectral stripping while a pure 0.65-ppm solution of Ba gave an intenity of 1277 counts
z
H W
2000-
2
c
a _I cc w
1500-
IO00-
strontium line shown in Figure 4 was obtained by subtracting the barium profile from the mixture profile. These figures demonstrate that spectral stripping may be used t o remove the spectral interference of line overlap in atomic emission spectrometry. Although the two lines selected in the present example are 0.67 A apart, the technique of spectral stripping is applicable even in the case of complete overlap. Furthermore, the use of spectral stripping does not alter the net profile of the analytical line, allowing peak height to be used as a measure of intensity. To demonstrate the efficacy of spectral stripping in the removal of interferences caused by line overlap, a synthetic “unknown” composed of 1036 ppm strontium and 0.48 ppm barium was analyzed for barium using barium standards containing only an ionization buffer. Without spectral stripping, the signal profile obtained with the unknown resulted in a barium value which was 59% higher than the true value. Analyzing the same ‘(unknown” mixture with the same calibration curve and performing spectral stripping using the 5521.83-A strontium line as a monitor line, gave a barium result which differed from the true value by 3%. Obviously, the determination of the amount of barium impurity in a strontium salt would be seriously in error if the barium were determined a t 553.5 nm by atomic emission without spectral stripping. The effect of using a resonance or nonresonance line as a monitor line for spectral stripping was evaluated using the S r 5521.83-A nonresonance line and the Sr 4607.33-A resonance line as monitor lines to strip the Sr 5534.83-A line from the Ba 5535.48-A line. A solution containing 1036 ppm Sr and 0.65 ppm Ba with Cs added as an ionization buffer was prepared. A similar solution containing 0.65 ppm Ba and the same amount of Cs was prepared to determine the true barium intensity under the given experimental conditions. Stripping solutions up to six times more concentrated than the strontium concentration in the test solution gave correct. results when the 5521.83-A nonresonance strontium line was used as a monitor line as shown in Table V. Thus, in agreement with the theory developed for spectral stripping, good results may be obtained without regard for the concentration of the stripping solution when both the interfering line and the monitor line are both nonresonance lines. When the same experiment was performed using the Sr 4607.33-A resonance line as a monitor line, the stripping solution used could be no more than 20% greater than the concentration of interferent present (1036 ppm) if the net barium intensity was to be within 5% of the true value. Stripping solutions more concentrated than this resulted in a reduced barium intensity when the Sr 4607.33-A line was used as a monitor line. Thus, in agreement with the theory for spectral stripping, more care must be used in preparing the stripping solution if either the interfering
CHANNEL NUMBER
Figure 4. 0 = Signal profile obtained with a mixture containing 1036 = Signal profile of Ba 5535.48-A line ppm Sr and 0.48 ppm Ba. after spectral stripping. A = Strontium signal profile obtained by subtracting Ba signal profile from the mixture profile
line or the monitor line is a resonance line. If either line is a resonance line, the stripping solution used should be only slightly more concentrated than the interferent concentration in the sample. Nevertheless, the analyst does not need to know this concentration, and can make the stripping solution by adding a concentrated stock solution of interferent to a portion of distilled water, until the stripping solution gives a monitor line intensity slightly greater than that produced with the sample. CONCLUSIONS Spectral interference caused by the presence of a molecular band or an undesired concomitant line is a potential interference in flame emission spectrometry whenever complex samples are analyzed on systems with table-top monochromators. Spectral stripping, made possible with the advent of the vidicon flame spectrometer, is a rapid and simple procedure for the elimination of practically all spectral interferences in flame emission spectrometry. Spectral stripping does not, of course, remove interferences caused by chemical and physical phenomena which affect both atomic emission and atomic absorption. Spectral stripping is universally applicable in the removal of molecular band interferences. In the case of line overlap, spectral stripping requires that a monitor line be selected. If both the monitor line and the interfering line are nonresonance lines, a stripping solution of arbitrary concentration may be used. If either line is a resonance line, the concentration of the stripping solution should be only slightly greater than the interferent concentration in the sample. The same spectral interference present in a variety of samples may be removed with spectral stripping, allowing each sample to be determined using the same calibration curve produced with simple standards. The time required to perform spectral stripping is less than one minute per sample.
RECEIVED for review May 20, 1974. Accepted August 1, 1974. This research was supported in part by the National Institutes of Health under Grant No. GM-19905-02, and in part by the National Science Foundation through the Corne11 Materials Science Center.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 14, DECEMBER 1974
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