Anal. Chem. 1982, 5 4 , 1043-1048
1043
Optimization of Slit Parameters for Continuum Source Atomic Absorption Spect rometry James M. Harnly Nutrient Composltlcm Laboratoty, Beltsville Human Nutrition Research Center, S&E, ARS, USDA, Beltsvllle, Maryland 20705
The absorbance rrlgnal and the absorbance noise were measured as a functlon of the sllt wldth and helght and the modulation frequency for high dlspersion wavelength modulated, continuum sourco atomlc absorption spectroscopy (WMCSAAS). The absorbance signal asymptotically approaches a maxlmum as the sllt wldth Is decreased. The absorbance nolse asymptotlcally approaches a constant as the sllt wldth and helght are Increased and the source flicker noise becomes domlnant. The signal-to-nolse ratlo reflects both trends. The sllt parameters for optimum signal-to-nolse ratlo vary from element to element. The h e a r ranges of the callbratlon curverr are relatlvely unaffected by the choice of sllt parameters. The appllcatlon of these data to slrnultaneous multielement WMCEAAS Is llmlted by a common entrance slit whlch prevents optlmlzatlon of the sllt parameters for each element.
Wavelength modulated, continuum source atomic absorption spectroscopy (WMCS-AAS) was first described by Snelleman (1). Assuming a medium resolution monochromator and ideally behaved low frequency, or “flicker” noise, Snelleman demonstrated that above a critical frequency only statistical noise, or photomultiplier tube shot noise, is significant, As a result, if the continuum source is sufficiently intense, the signal-to-noiseratio of WMCS-AAS is comparable to that of conventional, line source AAS. In addition, as long as the critical frequency is exceeded, the signal-to-noise ratio of WMCS-AAS is only dependent on the reciprocal of the square root of the angular dispersion of the monochromator. Restated, Snelleman’s model predicts that the signal-tonoise ratio of WMCS-AAS is independent of the spectral band-pass and independent of the width of the entrance and exit slits. For CS-AAS, the source intensity is proportional to the square of the slit width. Consequently, photomultiplier tube shot noise, which is proportional to the square root of the source intensity, is proportional to the slit width. The absorption signal, a convolution of the slit function and the absorption profile, also varies linearly with the slit width for a medium resolutiion monochromator (the spectral band-pass > the absorption profiie half-width). Thus, the signal-to-noise ratio is constant. Jn terms of absorbance, the signal is inversely proportional to the slit width. The absorbance noise, which is inversely proportional to the square soot of the source intensity, is also inversely proportional to the slit width producing a constant signal-to-noise ratio. The signal-to-noise ratio can be improved by increasing the slit height. The absorbance signal is independent of the slit height, but the abfiorbance noise is inversely proportional with the square root of the source intensity and the slit height since the intensity and slit height vary linearly. As a result, the signal-to-noise ratio is inversely proportional to the slit height. In practice, WMCS-AAS can be expected to deviate from the ideal model presented by Snelleman in two ways. First, with high resolution, the absorption signal no longer increases linearly with the idit width. High-resolution spectrometers
have historically been used with CS-AAS (2-5) and now with WMCS-AAS (6, 7). The high dispersion of the echellle spectrometer satisfies Snelleman’s criteria for maximizing the signal-to-noise ratio, while the high resolving power provides sensitivities and calibration curve linearities comparable to those of conventional AAS. All figures of merit which have been reported for WMCS-AAS using an echelle spectrometer (6-9) have employed the smallest slit width in order to obtain maximum resolution. The resolution of the echelle is approximately an order of magnitude better than that of a medium resolution monochromator assumed by Snelleman’s model. As the spectral band-pass of the spectrometer approaches and then becomes less than the width of the absorption profile, the computed absorbance will asymptotically approach a maximum (10) rather than increasing inversely with the slit width. Second, Snelleman has assumed a well-behaved flicker noise component which can be eliminated by modulation at frequencies greater than a critical frequency to produce a shot noise limited instrument. However, as the slit width and height are increased, the increased photon flux is accompanied by a shift in contributions of the various noise componentla. At higher fluxes, the flicker noise component becomes more dominant and the critical frequency increases. If modulation above the critical frequency cannot be achieved, then the absorbance noise will asympototically approach a minimum instead of decreasing inversely with the square root of the source intensity. The deviation of the absorbance signal and absorbance noise from ideality at small and large slit widths, respectively, predicts a maximum signal-to-noise ratio at an intermediate setting. In addition, it has recently been shown that maximurn resolution is no longer necessary to optimize the linear range of calibration curves for WMCS-AAS. WMCS-AAS can be designed to make absorption measurements in the wings of the absorption profile (9). As a result, a set of calibration standards will determine a family of calibration curves which can cover 4-6 orders of magnitude in concentration. The overlap of these curves is controlled by the shape of the modulation wave form and the sampling frequency. The linearity of each curve is now far less critical. Whereas maximum resolution had been previously required to optimize linearity, the slit parameters can now be adjusted to maximize the signal-to-noise ratio. A continuum source spectrometer employing an echelle polychromator and wavelength modulation (SIMAAC) (7,9, 11),was used to examine the absorbance signal and absorbance noise as a function of the slit width and height and the modulation frequency. This was done in the single element mode for four elements between 280 and 590 nm. The implications of these results for multielement WMCS-AAS are considered.
EXPERIMENTAL SECTION Equipment. The SIMAAC system has been previously described (7). For these experiments the single element exit cassette was used instead of the multielement cassette. Thus, both thle
This article not subject to US. Copyright. Published 1982 by the American Chemical Society
1044
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
Table I. Relative Absorbance Signals, Base Line Absorbance Noises, and Signal-to-Noise Ratios for Mn vs. Slit Parameters slit height, Pm 100
200 300
slit width (pm) component a
25
50
100
200
100 100
71
OB
48
20 24
A/oB
1.0
A
100
1.5 71
40 33 1.2 40
OB
64 1.6
33 2.2
26 1.5
100
71
55
32 2.2
40 24
A
A l o ~ A OB
500
AIOB A
1.8
OB
44 2.3
A/oB
100
71 26 2.7
1.7
40 21 1.9
0.8 20 21 0.9 20
20 0.9 20 19 1.0
a A = absorbance signal, UB = base line absorbance noise (56 Hz), A / U B = signal-to-noise ratio. intensity saturated the PMT.
entrance and exit slits could be set as desired. All the exit slits for the multielement cassette are fixed at 25 pm wide and 300 pm high. A conventionalburner assembly (Perkin-Elmer,Corp., Norwalk, CT) with a 10-cm slot burner head (Varian Associates, Palo Alto, CA) was employed as an atom source. Reagents. All analyte solutions were prepared from commercially availableAA standards (Fisher Scientific Co., Fair Lawn, NJ). Methods. Absorbance Noise us. Slit Parameters. All noise levels were computed in absorbance units. The data acquisition and processing programs have been described in detail elsewhere (11). Data were acquired for 30 s with a modulation frequency of 28 Hz with detection in the 2f mode (a detection frequency of 56 Hz). A total of 1120 intensity points were acquired each second for each channel resulting in 20 intensity measurements per 1/2 cycle, or pass across the absorption profile. Each set of 20 measurements can be used to compute an absorbance. In this case, 10 passes were ensemble averaged before the absorbances were computed. As a result, for each element 167 absorbances were computed over the 30-9 data acquisition period. Absorbance noise waa defined, as the standard deviation of the mean of the 167 absorbances. These measurements were repeated for each set of slit parameters. As the slit parameters were increased, the transmitted light increased by several orders of magnitude. To keep the signal from the photomultiplier tube (PMT) within the working range of the computer's analog-to-digitalconverter, it was necessary to decrease the minimum amplification of the interface circuit. This was achieved by connecting a suitable resistor in parallel with the feedback resistor of the current-to-voltage amplifer (7). Thus, all the PMT signals were maintained within the working range of the computer without any change in the optical alignment. Absorbance Signal us. Slit Parameters. The analyte absorbance as a function of the slit parameters was determined without wavelength modulation. After the desired entrance and exit slit widths and heights were selected, the wavelength setting was optimized (set at the center of the absorption profile) and a standard was atomized. Absorbance ( A = log [Io/Z'J was computed by using the source intensity before sample atomization as Io and the intensity during atomization as I. These measurements were repeated for each set of parameters using the same standard solution. Care was taken to ensure the standard absorbance fell well above the detection limit (at least 15 times the base line absorbance standard deviation, uB) and within the linear response range. Absorbance Noise us. Frequency. The absorbance noise was determined at each modulation frequency as described previously. In this case, no ensemble averaging was used. A variable data acquisition time was used to ensure that approximately 1000 absorbances were used at each frequency to compute the standard deviation, The absorbance noise was defined as the standard deviation of the mean of the individual absorbances. The maximum modulation frequency that could be obtained was 36 Hz
500 8 23 0.3
8 N.D. N.D. 8 N.D. N.D. 8 N.D. N.D. not determined, source
with detection in the 2f mode, a maximum detection frequency of 12 Hz. For higher frequencies, the data acquisition program was modified to double the modulation frequency and to make only 10 intensity measurements per cycle. This permitted a maximum modulation frequency of 72 Hz, a maximum detection frequency of 144 Hz. Order Scans. The order, or vertical, axis of the echelle spectrometer was scanned by employing a motor making an external friction contact with the fine order control wheel of the echelle spectrometer. The PMT signal was used to drive the pen of a strip chart recorder. All order scans were made with constant strip chart recorder and scanning motor speeds. RESULTS AND DISCUSSION Absorbance Signal and Noise Measurements for Mn. Table I shows the relative absorbance signals, the relative base line absorbance noises, and the signal-to-noise ratios for Mn as measured a t all possible slit widths and heights. Unless otherwise stated, symmetrical slit parameters, i.e., identical width and height settings for the entrance and exit slits, will be assumed. The slit parameters given in Tables 1-111 correspond to the settings of both the entrance and the exit slits. In addition, a set of slit parameters will always be expressed as the width by the height with all units in micrometers (pm). The absorbance signals shown in Table I vary according to the medium resolution model of Snelleman only at slit widths of 100 ym or larger. Below 100 pm, the absorbance signals asymptotically approach a maximum value as the resolution of the echelle approaches the half-width of the Mn absorption profile. The theoretical half-width of the Mn absorption profile has been determined to be 0.014-0.018 nm (12) while the theoretical resolution of the echelle monochromator is 0.026, 0.048,0.092, and 0.178 nm a t 25, 50, 100, and 200 pm slit widths, respectively. There is no significant change in the absorbance signals as a function of the slit height. Shot noise is the dominant noise component only for the smallest slit widths and heights. At higher settings the base line absorbance noise approaches a constant value as the flicker noise component becomes dominant. This change in the dominant noise is reflected by the deviation of the measured base line absorbance noises from the noise levels predicted for the shot noise limited case. The change in the character of the dominant noise is easily seen in Figure 1. The log of the base line absorbance noise has been plotted as a function of the log of the reciprocal of the detection frequency for four different sets of slit settings. These settings represent a diagonal sampling through the slit width by height matrix of Table I. On this plot, a shot noise dominated system shows no frequency dependence and ap-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1045
Table 11. Relative Absorbance Signals, Base Line Absorbance Noises, and Signal-to-Noise Ratios for Several Elements vs. Slit Parameters re1 source intens slit parameters (pm) wavelength, (25 pm X element (mm) 100 pm) componenta 25 X 100 50 X 200 100 X 300 200 X 500 40 20 0.05 100 71 Mn 279.5 33 19 24 100 1.7 1.0 2.2 1.0 23 81 46 324.7 0.24 CU 109 40 28 20 100 Ca
422.6
1.00
Na
589.6
0.18
1.0 100 100 1.0 100 100 1.0
2.0 62 76 1.1
1.6 34 43
1.2 17 N.D.
0.8
N.D.
76
54
32 2.5
18
31 12
3.4
A = absorbance signal, UB = base line absorbance noise (56 Hz), A/oB = signal.to-noise ratio.
2.8 Not determined, source
intensity saturated the PMT. Mn, 279.5nrn -4
I
0
z a
E
/
zm ,005
-
a b
optimum signal-to-noiseratios for a medium resolution system would be expected a t narrower slit widths. Despite the deviations from Snelleman's model a resistance to dramatic changes in the signal-to-noise ratio is observed. The absorbance signal and flicker noise deviations affect opposite ends of the width settings. As a result, for Mn, a 20-fold variation in the slit width produced a 475-fold variation in the source intensity with only a 2.5- to &fold (depending on which slit height is examined) variation in the signal-tonoise ratio. Absorbance Signal and Noise Measurements for Other Elements. The optical characteristics of the SIMAAC system
,002
,001 1/100
1/50
1/20
1/10
I/FREOUENCY (HI)
Flgure 1. Base line absorbance noise for Mn (279.5 nm) as a function of the detection frequency for slits of (0) 25 p m X 100 pm, (0)50 pm X 200 pm, (A)100 pm X 300 pm, and (0) 200 pm X 500 pm.
pears as a horizontal line. Systems dominated by flicker noise exhibit finite slopes as a function of the reciprocal of the frequency. The I / f noise has a slope of unity while higher powers of flicker noise, l/f"(13),have lower slopes. In Figure 1, the 25 pm X 100 pm slits show a shot noise limited system. For the 50 pm X 200 pm slits, the shot noise limited case exists only above 50 Hz,.The larger slit settings are flicker noise dominated up to 120 Hz, the highest frequency tested. The slopes of the flicker components become steeper as the slit parameters are increased. The signal-to-noise ratio matrix in Table I reflects the combined variations of the absorbance signal and base line absorbance noise. For every slit height, a 50-pm slit width provides the maximum signal-to-noise ratio while for every slit width the signal-to-noise ratios increase steadily with slit height. The change in the signal-to-noise ratio as a function of slit width (at a constant height) is brought about by the expected deviations from the medium resolution model of Snelleman. The absorbance signal does not continue to increase linearly with decreasing slit width as the spectral band-pass approaches the half-width of the absorption profile. At large slit widths, the absorbance noise does not decrease inversely with the dit width as the source flicker noise becomes dominant. This last factor would affect medium resolution monochromators as well as the echelle. This suggests that
result in distinctive absorbance signals and noise responsw for each element as a function of the slit width and height. The different responses are caused by the wavelength dopendence of the intensity of the xenon arc source, the s e ~ ~ sitivities of the PMTs, and the resolution of the echelle spectrometer. The intensity of the xenon arc source is relatively constant between 450 and 600 nm. Below 450, the intensity decreaseas gradually at first and then becomes increasingly less intense in the far-UV. At 200 nm the source intensity is a factor of 25 worse than a t 450 nm. The source intensity variation is also affected by the spectral response characteristics of the PMTs (types R-268, R-292, R-374, and R-431 are used). The combination of the source spectra and the PMTs spectral response produces a maximum signal response between 450 and 500 nm with the response at 200 nm and 600 nm a factor of 100 times and 5 times less, respectively. Thus for a givein set of slit parameters, the P M T cathodic signal, and conscquently the shot noise, is highly wavelength dependent. A similar wavelength dependence exists for the resolutioin of the spectrometer. The wavelength range of the echelle spectrometer is composed of short, overlapping wavelengtlh sections located on successive orders (28-112), with the highest wavelengths located at the lowest orders. Consequently, the resolution is poorest at the longer wavelengths and improves in discrete steps as one moves to shorter wavelengths. The measured absorbance signals, base line absorbancte noises, and signal-to-noise ratios, for four elements at four slit settings are shown in Table 11. The four slit settings are the same diagonal sampling through the width by height matrix used for Figure 1. In general, the variations of the absorbance signals as a function of the slit width and height are relatively unaffected by the wavelength while the deviation of the base line absorbance noise, from that predicted for the shot noise limited case, increases with wavelength, or the relative source intensity. This can be easily seen by comparing the data for
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1046
.01
L
Ca,
A -
422.7 nm
Hg
2 5 3 . 6 nm I
W
a
B ORDER CO NTAl N IN0 253.6 nm
,005
D:
w
m a
Y
,002
,001 1/100
1/50
1/20
1/10
I/FREPUENCY ( H z )
Flgure 2. Base line absorbance noise for Ca (422.6 nm) as a function of the detection frequency for slit settings of (0)25 pm X 100 pm, (0)50 pm X 200 pm, and (A) 100 pm X 300 pm. ox r 1.0
01
-
100
Figure 4. Order scans over the Hg (253.6 nm) line using (A) a Hg pen lamp and (6) a xenon arc source.
-
A Ha 404.6 nm
.01
0.I
I
1
I
I
10
10
100
1000
0 ORDER CONTAINING 404.6 nm
CONCENTRATION (ug/ml)
Figure 3. Mn (279.5 nm) calibration curve at slit widths of (0)25 pm, (A)50 pm, (0)100 pm, and (A) 200 Mm. Silt heights were constant at 100 pm.
Mn and Ca at slit settings of 100 pm by 300 pm. The relative absorbance signal for Ca is only 15% less than that for Mn. However, the relative base line absorbance noise for Ca is 43% of that of the smallest slit setting compared to 24% for Mn. For a shot noise limited system, the absorbance noise with 100 pm X 300 pm slits should be 12% of that using 25 p X 100 pm slits. The net result is that the signal-to-noise ratio for Ca, a t slits 100 pm X 300 pm, is half the value for Mn. The log of the base line absorbance noise for Ca as a function of the log of the reciprocal of the modulation frequency is shown in Figure 2. The flicker noise component is even more dominant for Ca than it is for Mn (Figure 1). Only above 60 Hz, using the smallest slit settings, is Ca shot noise limited. This is because the xenon arc source is 20 times more intense at 422.6 nm than at 279.5 nm (Table 11). The photon flux for Ca with 25 pm X 100 pm slits correspond roughly to that for Mn with slits between 50 pm X 200 pm and 100 pm X 300 pm. The data presented in Table I1 indicate that most elements exhibit a signal-to-noise vs. slit parameter matrix similar to that of Mn in Table I. As the source intensity increases, however, the signal-to-noise ratio advantage obtained at intermediate slit widths (50 pm and 100 pm) decreases as the flicker noise becomes dominant at lower slit settings. For this same reason, the improvement in the signal-to-noise ratios obtained at higher slit heights also becomes less significant. Analytical Calibration Curves. So far only the signalto-noise ratios as a function of the slit widths and heights have been considered. However, for SIMAAC, the linearity of the analytical calibration curves is also a function of these slit
Flgure 5. Order scans over the Hg (404.6 nm) line using (A) a Hg pen lamp and (6) a xenon arc source.
parameters. Increasing both slit widths and heights results in increased nonlinearity of the analytical curve. That is, a deviation from Beer's law, curvature toward the concentration axis, is observed at increasingly lower absorbances as the slit width and height are increased. For a continuum sowce instrument, increasing the slit width increases the spectrometer's spectral band-pass and produces larger deviations from the ideal, monochromatic source which is assumed by Beer's law. This results in reduced sensitivities and the onset of nonlinearity at lower absorbances. Figure 3 shows the effect of 25-, 50-, loo-, and 200-pm slit widths on the most sensitive calibration curve of Mn (279.5 nm). Nonlinearity occurs at increasingly lower absorbances as the slit widths increase, but the concentration at which nonlinearity occurs remains approximately constant. A similar phenomena occurs at the lower end of the calibration curve. The signal-to-noise ratio at a given absorbance increases with increasing slit width (Tables I and 11). However, because the sensitivity is decreasing with the increasing slit width, the concentration, at which a given signal-to-noiseratio results, remains approximately constant. Consequently, the dynamic linear range is unaffected by the slit width. Although the sensitivities of the curves are reduced at a higher slit width, the detection limits and the upper ends of the linear range
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
104,7
B -
--A
ORDER CONTAINING 546.1 nm
H 18 546.1 nm
1 ,001
I
I
0.01
0.1
I
I
I
I
1.0 10 100 CONCENTRATION (!Jg/ml)
1000
Figure 8. Family of calibration curves for Na (589.6 nm) using 50 pm X 500 pm slit settings.
Table 111. Relative Signal-to-Noise Ratios for Asymmetrical Slit Widths (Slit Height = 300 bm) Figure 6. Order scans over the Hg (546.1 nm) line using (A) a Hg pen lamp and (B)a xenon arc source.
entrance slit width(bm) 25 50 100 200
500
I
0.1
1
1
10 10 100 CONCENTRATION (!Jg/nl)
I
,
1000
Figure 7. Family of calibration curves for Na (589.6 nm) using 50 pm X i 0 0 pm slit settings.
do not change in terms of concentration. The slit height determines the vertical segment of any order which is examined by the echelle spectrometer. If the slit height is too large, light from the orders immediately above or below the order of the wavelength of interest will also reach the PMT. This light is defined as “order overlap” stray light. The problem of “order overlap” stray light is compounded by the fact that the resolution of the orders by the echelle’s prism is not constant. Increased order overlap is observed as the order number decreases (wavelength increases). Figures 4-6 show vertical scan^^ through three adjacent orders as a function of the slit height and the wavelength. The scans were made with the Eimac lamp and a Hg pen lamp, to allow the width of a single order to be determined. At 253.6 nm (Figure 4) base line separation of the orders is achieved at all slit heights. The offset of the base line from zero is far stray light and increases with slit height. Figures 5 and 6 reflect the poorer resolution of the orders at higher wavelengths. At 404.6 nm (Figure 5), base line separation of the orders is achieved only with slit heights of 100 or 200 pm while at 546.1 nm (Figure 6) base line separation occurs only for the smallest slit height. At 546.1 nm, the individual orders cannot be distinguished using the largest slit height. Thus, severe stray light effects can occur over most of the visible spectrum if any but the smallest slit height is used. Figures 7 and 8 show the family of calibration curves for Na (589.6 nm) detjermined at slit heights of 100 and 500 pm, respectively, using SIMAAC. In each figure, the curve on the left is for the center of the absorption line and the other curves are for wavelength8 on the wings. Each curve is plotted from the quantitation limit (the concentration whose absorbance is equivalent to the arbitrarily chosen value of 15aB)to the highest standard (ZOO0 ppm) or the reversal point, the max-
exit slit width (hm) 25
50
86 87 63 39 22
93 100 82 45 24
100
200
78 84 65 42 25
54 58 50 49 28
imum absorbance before reversal of the calibration curvle occurs. The nature of the absorbance computation for SIMAAC results in reversal of each calibration curve (9). Appropriate computer algorithms are used to detect absorbances occurring on the reverse side of a curve and sample concentrations are computed only for absorbances falling on the low concentration side of the curve. Three things are readily noticeable upon examining the curves in Figures 7 and 8:(1)the quantitation limits are three times lower for each curve using the higher slit height, (2) although the sensitivities of the corresponding curves are equivalent a t lower concentrations, absorbances at higher concentrations are significantly reduced using the higher slit height, and (3) reversal of the calibration curves occurs at lower concentrations with the higher slit height. The overall effect is to shift the curves obtained at the higher slit height to loweir concentrations. For the higher slit height, the signal-to-noise ratios are improved for each curve providing quantitation limits three times lower, but the increased stray light shortenri the upper limit of each curve by approximately the same factor. The dynamic range of the combined family of curves is approximately the same in each case. Optimum Multielement Operating Conditions. Prior to these studies, SIMAAC had been operated in the multielement mode at a detection frequency of 56 Hz, using 25 pm X 300 pm slit parameters for each element. It is obviouc; from Tables I and I1 that improved signal-to-noise ratios can be obtained by using larger slit widths and heights. This improvement in the signal-to-noise ratio is accompanied by a negligible effect on the linearity of the calibration curves (Figures 3, 7, and 8). Lower base line absorbance noise isl obtainable at frequencies higher than 56 Hz. However the reduction in noise is highly dependent on the element and thc slit settings. Three major factors influence the applicability of these results to routine, multielement operation. First, the optimum conditions for each element cannot be used simultaneously since the entrance slit is common to all wavelengths. Second, the exit slits are finely positioned and then epoxied to the multielement cassette. This prevents an easy change of the exit slit parameters. And third, in all but a few cases, the improvement in the signal-to-noise ratios over those of the current operating parameters are less than a factor of 3.
1048
Anal. Chem. 1982, 5 4 , 1048-1052
Of these three factors, the first is the most significant. Since the entrance slit is common to all 16 elements, asymmetric slit settings, unequal slit widths and/or heights, would be necessary. However, asymmetric slit settings provide rather complex compromises for the signal-to-nosie ratios. This is shown in Table I11 where the signal-to-noise ratios were determined for Mn (279.5 nm) for all the possible entrance and exit slit width combinations at a constant slit height. The slit height matrix at a constant slit width was similar. I t is apparent that the determination of the optimum compromise slit parameters for each element is a formidable task. The gain in the signal-to-noiseratios does not justify the cost, time, and effort required to change the existing exit slits. With the purchase of a new multielement cassette, larger exit slit parameters would allow improved signal-to-noise ratios. Data for more elements are necessary, however, before the optimum compromise asymmetric entrance and exit slits can be selected. LITERATURE CITED (1) Snellernan, W. Specfrochim . Acta, Part B 1968, 2338, 403. (2) Nltls, G. J.; Svoboda, V.; Wlnefordner, J. D. Spectrochim. Acta, Part B 1972, 2 7 8 , 345. (3) Velllon, C.; Merchant, P. Appl. Spectrosc. 1973, 2 7 , 361. (4) Kellher, P. N.; Wohlers, C. C. Anal. Chem. 1974, 4 6 , 682.
(5) Keliher, P. N.; Wohlers, C. C. Anal. Chem. 1976, 48, 140. (6) Zander, A. T.; O'Haver, T. C.: Kellher, P. N. Anal. Chem. 1978, 4 8 , 1166. (7) Harnly, J. M.; O'Haver, T. C.; Golden, 9.;Wolf, W. R. Anal. Chem. 1979, 5 1 , 2007. (8) O'Haver, T. C.; Harnly, J. M.; Zander, A. T. Anal. Chem. 1977, 4 9 ,
666.
(9) Harnly, J. M.; O'Haver, T. C. Anal. Chem. 1981, 5 3 , 1291. (IO) Dealan, L.; Winefordner, J. D. Spectrochim. Acta, Part B 1968, 2 3 8 , 277. (11) Harnly, J. M. Mlller-Ihll, N. J.; O'Haver, T. C. J . Autom. Chem., in Dress. (12) barsons, M. L.; McCarthy, W. J.; Wlnefordner, J. D. Appl. Spectrosc. 1968. 20. 223. (13) O'Haver, T. C. "Trace Analysis: Spectroscopic Methods for Elements"; Wlnefordner, J. D., Ed.; Wiley: New York, 1976;Chapter 2.
RECEIVED for review November 19, 1981. Accepted March 8, 1982. Presented in part at the 7th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, 1980, and the 32nd Pittburgh Conference, Atlantic City, NJ, 1981. Mention of trademark or proprietary products do not constitute a guarantee or warranty of the product by the US. Department of Agriculture and does not imply their approval to the exclusion of other products that may also be suitable.
Hydride GenerationKondensation System with an Inductively Coupled Argon Plasma Polychromator for Determination of Arsenic, Bismuth, Germanium, Antimony, Selenium, and Tin in Foods M. H. Hahn,' K. A. Wolnik," and Fred L. Fricke Elemental Analysis Research Center, USFDA, 114 1 Central Parkway, Cincinnati, Ohio 45202
J. A. Caruso Depatfment of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
A hydride generation/condensation system is interfaced to an inductively coupled argon plasma poiychromator for the simultaneous determinatlon of As, Bi, Ge, Sb, Se, and Sn in foods. Detection llmlts range from 0.02 ng/mL for As to 0.80 ng/mL for Sn, and preclsion values at 10 ng/mL are less than 6% relatlve standard devlatlon. Results of analyses of NBS standard reference materlals (wheat flour, rlce flour, spinach, and orchard leaves) demonstrate the appllcation of the method to food matrices.
The analytical potential of the inductively coupled argon plasma (ICAP) polychromator for the analysis of many trace elements in food products is unmatched in terms of speed, cost, and relative simplicity. However, several elements that are of current interest to scientists in food-related disciplines are often present at concentrations that are near or below the direct quantitative capabilities of the ICAP. 'Present address: B e l l Laboratories, 2525 Shadeland Ave., anapolis, IN.
Indi-
Because of the nutritional and/or toxicological significance of elements such as arsenic, antimony, bismuth, germanium, selenium, and tin, accurate and precise quantification is required for proper assessment of their metabolic roles. A combination of factors, most significantly the poor sensitivity of the ICAP polychromator for the emission lines of these elements, necessitates the use of one or more preconcentration steps prior to their determination by ICAP ( 1 ) . The formation of volatile hydrides with this group of elements has been frequently used to improve sensitivity and detection limits for spectrometric determinations and is well documented in the literature (2). Coupling of the hydride generation reaction to an ICAP system, on the other hand, is fairly recent. In 1978, Thompson et al. ( 3 ) introduced a continuous hydride generation/ICAP system for the simultaneous determination of As, Bi, Sb, Se, and Te. Since that time, several modifications of their basic design have appeared in the literature ( 4 , 5 ) . These continuous generation systems provide improved detection limits by at least an order of magnitude over conventional nebulization and include studies on the plasma determination of hydrides of Ge and Sn (6) and P b (5). Wolnik et al. (7) expanded the technique with the
Thls article not subJect to U.S. Copyrlght. Published 1982 by the American Chernlcal Society
