Anal. Chem. 1985, 57, 420-424
420
application of this latter spectral interference was reported by Fulton et al. (27), who measured cadmium atomic absorption a t the 228.802-nm line with an arsenic electrodeless discharge lamp. Solutions of Cu, Fe, and As in the concentration range of 2-10% were aspirated into the DCP for the appropriate spectral overlap applications. However, in none of the cases could the nonresonance lines be broadened sufficiently to provide suitable pseudocontinuum radiation over the narrow wavelength modulation interval centered around the analyte resonance absorption line. Solutions containing both the analyte and the appropriate interfering element in various concentration ratios were also aspirated into the DCP in an attempt to achieve pseudocontinuum radiation. This experimental procedure also proved to be unsuccessful because the asymmetrical self-reversal dip in the analyte profile could not be completely removed by the overlapping nonanalyte profile to achieve an emission intensity distribution with a desirable flat plateau region over the entire modulation interval.
CONCLUSIONS While the stray radiation characteristics of the DCP source used for WM-AAC are superior to those of the XAL, the intensity of absorbable radiation from the DCP is less than that from the XAL, and SNR using the DCP is significantly degraded by plasma emission "self-reversal" flicker noise. Thus, while in theory the DCP may be a practical alternative to the XAL as a primary radiation source for future analytical WM-AAC studies, successful application can only be realized if self-reversal can be eliminated by redesign of the gas-flow system of the DCP. ACKNOWLEDGMENT J.D.M. gratefully acknowledges the Inorganic Analytical Research Division of the National Bureau of Standards for a guest worker appointment during which this research was conducted in collaboration with the University of Maryland. Special acknowledgment is also made to Theodore C. Rains for his encouragement and support during this period.
Registry No. Ar, 7440-37-1;Zn, 7440-66-6;Mg, 7439-95-4;Cd, 7440-43-9; Fe, 7439-89-6;Cu, 7440-50-8;Ca, 7440-70-2.
LITERATURE CITED Messrnan, J. D.; Epstein, M. S.;Rains, T. C.; O'Haver, T. C. Anal. Chem. 1983, 5 5 , io%-1058. Greenfield, S . ; Smith, P. B.; Breeze, A. E.; Chiiton, N. M. D. Anal. Chim. Acta 1988, 4 1 , 385-387. Uchida. H.: Tanabe. K.: Noiiri. Y.: Haraouchi. H.: Fuwa.. K. Soectro, chim. Acta, Part B'1980, 35B, 881-883. Uchida, H.; Tanabe, K.; Nojiri, Y.; Haraguchi, H.; Fuwa, K. Spectrochim. Acta, P a r t 8 1981, 3 6 8 , 711-718. Nojiri, Y.; Tanabe, K.; Uchida, H.; Haraguchi, H.; Fuwa, K.; Winefordner, J. D. Spectrochim. Acta, Part 8 1983, 388,61-74. Montaser, A. Spectrosc. Lett. 1979, 72, 725-732. Ornenetto, N.; Nikdei. S.; Bradshaw, J. D.; Epstein, M. S.; Reeves, R. D.; Winefordner, J. D. Anal. Chem. 1979, 5 1 , 1521-1525. Epstein, M. S.; Nikdel, S.; Omenetto, N.; Reeves, R.; Bradshaw, J.; Winefordner, J. D. Anal. Chem. 1979, 5 7 , 2071-2077. Epstein, M. S.; Omenetto, N.; Nikdei, S.; Bradshaw, J.; Winefordner, J. D. Anal. Chem. 1980, 5 2 , 284-287. Cavalli, P.; Ornenetto, N.; Rossi, G. A t . Spectrosc. 1982, 3 , 1-4. Cavalli. P.; Rossi, G.; Ornenetto, N. Analyst (London) 1983, 108, 297-304. Ornenetto, N.; Crabi, G.; Nesti, A.; Cavalli, P.; Rossi, G. Spectrochim. Acta, Part 8 1983, 388, 549-555. Kosinski, M. A.; Uchida. H.; Winefordner, J. D. Anal. Chem. 1983, 55, 688-692. Long, G. L.; Winefordner, J. D. Appl. Spectrosc. 1984, 3 8 , 563-567. Hendrick, M. S.;Goliber, P. A.; Michel, R . G. paper presented at the 13th ACS Northeast Regional Meeting, West Hartford, CT, June 1983, No. 23. Yasuda, K. Anal. Chem. 1968, 3 8 , 592-599. De Jong, G. J.; Piepmeier, E. H. Spectrochim. Acta, Part 8 1974, 298, 159-177. Pieprneier, E. H.; de Galan, L. Spectrochim. Acta, Part 8 1975, 308, 263-279. Wagenaar, H. C.; de Galan, L. Spectrochim. Acta. Part 8 1975. 3 0 8 , 361-381. Human, H. G. C.; Scott, R. H. Spectrochim. Acta, P a r t 8 1976, 3 1 8 , 459-473. Kawaguchi, H.; Oshio, Y.; Mizuike, A. Snectrochim. Acta, Part 8 1982,-378, 809-816 Epstein. M S I Rains, T C , Brady, T J , Moody, J R , Barnes, 1 L Anal .. -. . Chnm -. .-. .. . 1978 ... . , 50 - ., 874-880 . . . Bower, N. W.; Ingle, J. D., Jr. Appl. Spectrosc. 1981, 3 5 , 317-324. Inole. J. D.. Jr. Anal. Chem. 1974. 4 6 . 2161-2171. K&.'W. R.';Moore, C. B. Anal. Chem: 1973, 4 5 , 1274-1275. Larkins, P. L.; Willis, J. B. Spectrochim. Acta. Part 8 1974, 298, 3 19-337. Fuiton, A.; Thompson, K. C.; West, T. S. Anal. Chim. Acta 1970, 57, 373-380.
RECEIVED for review June 25, 1984. Accepted October 11, 1984. In no instance does identification of commercial products by manufacturer's name or label imply endorsement by the National Bureau of Standards nor does it imply that the particular products or equipment identified are necessarily the best available for that purpose.
Sample Introduction by Solvent Extraction/Flow Injection To Eliminate Interferences in Atomic Absorption Spectroscopy Jamal A. Sweileh and Frederick F. Cantwell* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Aqueous samples are injected into a solvent extraction/flow injection analysis system which employs a porous Teflon membrane phase separator. The organic extract phase containing the anaiyte as a metal-ligand complex is introduced continuously into the nebulizer of the atomic absorption (AA) spectrophotometer. As an example, trace zinc is determined in an iron matrix by extracting Zn(SCN),.
Several types of interference plague flame atomic absorption spectroscopy (AAS). "Matrix effects" include vaporization and ionization effects as well as differences in aspiration rate and nebulization efficiency due to differences in viscosity and surface tension between samples and standards (1-3). Spectral 0003-2700/85/0357-0420$0 1S O / O
interferences include light scattering and spectral band or line overlap. Spectral line overlap, though uncommon in AAS, occurs in some cases ( 4 , 5 ) . A variety of strategies are routinely used to minimize various types of interference: the addition of releasing agents, ionization suppressors, and other matrix modifiers (6, 7),spectral background correction (8,9),standard addition (IO), and separation of the element of interest from sources of inteference in the sample prior to AAS (11-13). Solvent extraction is one of the most commonly used separation techniques prior to AAS. The vast body of literature on the solvent extraction of metals serves as a readily available source of extraction systems to separate almost any metallic element from other metals or nonmetals (11,14-18). However, manual solvent extraction procedures are operator intensive C 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985 SarnDle
I Waste
Waste
421
Table I. Instrument Parameters Used with the PE 290B and the P E 4000 Spectrophotometers for Direct Aspiration and for FIA solvent
-L I 1 cm
Figure 1. Diagram of the extraction/FIA/AASapparatus with enlarged
instrument lamp current, mA wavelength, nm spectral slit width, A aspiration rate, mL/min acetylene pressure, psig air pressure, psig recorder full scale, mV recorder speed, cmimin
PE 4000 10
213.9 7
PE 4000 PE 290B 8 10 213.9 213.9 7
12
7 2.7 12
23 50 1.25
23 50 1.25
55
5.5
2.5 8
50
2.50
view of the solvent segmentor (see text for details).
and relatively time-consuming so that alternative approaches t o minimizing interferences in AAS, which do not involve separation, are generally preferred. In the past few years devices have been reported for performing automated solvent extraction in the flow injection analysis (FIA) mode (19-23). Such devices can be flow-coupled t o the nebulizer of an AA spectrophotometer t o provide a rapid and convenient m e w s of performing solvent extraction separations prior to AAS (24, 25). T h e solvent extraction/FIA system used in the presently reported study is based on one previously described (19) which employs constant pressure pumping and a porous Teflon membrane phase separator. The organic extractant is methyl isobutyl ketone (MIBK) which, after it extracts t h e analyte element, is introduced directly and continuously into t h e nebulizer. Pure MIBK is used t o compensate the disparity between the flow rate of t h e organic extract stream and the aspiration rate of the nebulizer. As an example, zinc is determined in the presence of a large excess of iron. This determination is known t o suffer interference from spectral line overlap of the zinc analytical line at 213.856 n m by a weakly absorbing iron line a t 213.859 n m (26-28). A second zinc line at 307.6 nm is 7 X lo3times less sensitive than the 213.856-nm line and is not suitable for trace zinc determination. Chemically, the extraction/FIA determination of zinc involves the prereduction of Fe(II1) t o Fe(I1) by ascorbic acid followed by extraction of Zn(SCN), into MIBK. T h e Fe(I1) remains unextracted. Quantification is based on peak height of the AA signal for the extracted zinc. T h e method is analogous t o a manual procedure used by Headridge for the determination of tin (29). EXPERIMENTAL SECTION Apparatus. The instrument is shown diagrammatically in Figure 1. The components prior to the segmentor (T2)are similar to those previously described (19,20). The rinse solvent, acetone, is used only when it is desired to flush out the system. Flow rates of carrier, reagent, and MIBK were adjusted by inserting suitable lengths of 0.3 mm i.d. tubing immediately downstream of valves VI. Sample solutions were injected manually into the carrier stream (0.1 M HCl) via valve Vs. The aqueous reagent stream which joins the carrier a t T, is 0.5 M KSCN in 0.1 M HC1. The combined aqueous stream joins the MIBK stream a t the segmentor, T2, and the resulting segmented flow stream passes through the extraction coil, C, in which ZII(SCN)~is extracted into the MIBK segments. As previously (19,20),the coil is made of 0.8 mm i.d. Teflon tubing. In the membrane phase separator, M, which has already been described (19), a fraction of the MIBK phase passes through the porous Teflon membrane and is directed to the nebulizer of the AA spectrophotometer (either a Model 290B or a Model 4000, Perkin-Elmer). Because the aspiration rate of the nebulizer is usually higher than the flow rate of MIBK through the membrane ( F M )a, flow compensator tee (T3)provides for the difference to be made up by a compensating flow (F,) of
MIBK from the flask. Additional flow rates shown in Figure 1 are Fcarrier, Freagent, and F,, the flow rate of MIBK. The flow rate plus Freagent, and the of the aqueous phase, Fa,is equal to FCarrLer total flow rate through the extraction coil, FT,is equal to Faplus F,. The strip chart recorder, R, is a Model 7127 A (HewlettPackard). The phase segmentor, T2,is shown in enlarged detail in Figure 1. It was made by drilling out the small bore cylindrical chamber of a commercially available Kel-F tee (Part CJ-3031, Laboratory Data Control) to 1/16-in.i.d. and inserting 2 mm long flared pieces of 0.8 mm i.d. X in. 0.d. Teflon tubing into the three branches of the tee. Glassware used in all experiments was soaked overnight in 30% nitric acid and rinsed with deionized water prior to use. Reagents and Chemicals. All chemicals were reagent grade except for spectroscopically pure iron (Specpure, Spex Industries Inc.). Distilled deionized water was used throughout. Zinc stock solution (1000 wg/mL) was prepared in 0.1 M HC1. MIBK and acetone were reagent grade and used without further purification. All solutions and solvents to be pumped in the extraction/FIA system were filtered through lG20-pm sintered glass before use. The 2% iron solution containing negligible zinc was prepared in 0.1 M HC1 by the procedure described by Headridge (29). Zinc Determination. Following is the general procedure for the determination of trace zinc in the presence of a large excess of iron. If the sample is a solid it is dissolved by the procedure described by Headridge (29). To 25 mL, of the 0.1 M HC1 solution containing up to 1 g of iron is added 4 g of solid L(+)-ascorbic acid. The solution is diluted to 50 mL with 0.1 M HC1, mixed, and allowed to stand for 10 min. A volume of 300-400 wL, precisely controlled, is injected via valve V, shown in Figure 1. Attempts to incorporate the required high concentration of ascorbic acid in the acid/thiocyanate reagent led to occasional breakthrough of the aqueous phase through the porous Teflon membrane. Assays of synthetic samples containing various trace concentrations of Zn in the presence of a large excess of Fe were performed both by direct aspiration AAS and by solvent extraction/FIA/AAS, using the PE 4000 spectrophotometer. Instrument parameters for the P E 4000 were optimized in both cases a t the values shown in Table I. Extraction/FIA conditions employed were as follows: carrier, 0.1 M HCl; reagent, 0.5 M KSCN in 0.1 M HC1; N2pump pressure = 25 psig; extraction coil length, 300 cm; F, = 3.2 mL/min; Fa = 6.0 mL/min; F M = 2.1 mL/min; sample volume injected, 335 wL; sample injection rate l/min. The injector loop was filled for = l o s with the valve in the load position, the valve was switched to the inject position and held there for =40 s, until the entire peak had been recorded, and it was then switched back to the load position. Synthetic samples for this study were prepared by first spiking 0.1 M HCl solutions containing 1 g of Specpure iron per 25 mL with accurately known quantities of Zn. Samples to be assayed by direct aspiration were then diluted to 50.0 mL with 0.1 M HC1. Samples to be assayed by FIA/AAS were treated as described above starting with the addition of ascorbic acid. Zinc standard solutions in 0.1 M HC1 were prepared to contain the same concentrations of Zn as the synthetic samples, without iron present, and were treated in the same manner as the corresponding syn-
422
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
thetic samples for use in direct aspiration or in FIAIAAS.
RESULTS AND DISCUSSION In other studies the design of the “tee” segmentor shown in Figure 1and the physicochemical nature of the MIBK/H20 segmentation process (30) have been discussed, and the detector performance (31) has been characterized in solvent extraction/FIA systems. In this report the particulars of the use of AAS detection in such a system are reported. PIA/AAS Interface. Important constraints on the present instrument design arise from the following features: (i) the AA spectrophotometer, disconnected from the phase separator, has its own aspiration rate which can be adjusted by altering the position of the nebulizer tip in the spray chamber (in the present case it was adjusted to 2.5 mL/min); (ii) for given values of Fo and Fa the FIA system, disconnected from the AA spectrophotometer, has its own “natural” value of FM which can be adjusted by changing the lengths of the two tubes conducting liquids out of the phase separator (usually the natural FM is 1-2 mL/min) (19,20);(iii) the phase separator provides an open-ended parallel-branched flow circuit with flow through the membrane in parallel with flow to waste. When the FIA system is coupled to the AA spectrophotometer the natural FM and the aspiration rate should, ideally, be adjusted to the same value in a condition called “matched flow” (32, 33). In our experience, attempts to maintain matched flow at =2 mL/min as well as attempts a t using “flooded flow“ (34, 35) led to poor reproducibility in AA signals. The use of “starved flow” (36)resulted in the aqueous phase occasionally breaking through the porous Teflon membrane. An arrangement which yielded reliable and reproducible performance involved the use of flow compensation with MIBK, as shown in Figure 1. T o keep the consumption of MIBK from the compensating flask small during routine operation, the aspiration rate was adjusted to about 2.5 mL/min and the natural FM to about 2.0 mL/min. Attempts to use water as a compensating solvent in place of MIBK (37) led to reduced AA sensitivity, presumably because of the cooling effect of water on the flame, while attempts to use air as a compensating fluid (25, 38) yielded a variable base line and poor reproducibility of the AA signal. Extraction Characteristics. The following characteristics related to the solvent extraction chemistry were investigated systematically: extraction coil length, volume of sample injected, concentration of KSCN in the reagent, and concentration of ascorbic acid reductant. In all studies the sample solution injected contained 2 ppm Zn2+ (3 X M) in 0.1 M HCl, with no iron added. Extraction coil length was varied from 30 to 450 cm, and a fixed sample volume of 385 pL was repeatedly injected. FT was maintained at 10.5 mL/min and Fo/Faa t 0.4. A plot of peak height vs. extraction coil length showed a steep rise between 0 and about 100 cm followed by a nearly flat plateau at coil lengths above 100 cm. The peak width variation with extraction coil length was very similar to that observed previously (19) though displaced to larger values, partly because of the use of a 385-pL injection volume in this study compared to 44 pL in the previous one. Peak widths were 5.3, 6.7, and 7.4 s for coil lengths of 40, 200, and 450 cm, respectively. A coil length of 300 cm, on the peak height plateau, was used in all subsequent studies. Sample volume injected was varied from 135 to 1385 pL while FT and Fo/Fawere maintained at 13.8 and 0.44 mL/min, respectively. The flow rate Fcarrier was kept constant as the length of the 0.8 mm i.d. Teflon sample loop was increased, by removing an equal length of 0.8 mm i.d. Teflon tubing from between the carrier reservoir and the injection valve. As seen in Figure 2, peak height increases with sample volume until
0i
2h0
Ab0
.___ 630 800 1000
1200
J
1430
Sample Volume Injected (pL)
Flgure 2. Variation of peak height with volume injected: sample, 2 Fg/mL Zn2+ in 0.1 M HCI; F, = 4.8 mL/min; Freagem = 4.8 mL/min; F , = 4.25 mL/min; F,IF, = 0.44; F , = 13.8 mL/min; F, = 1.7 mL/min; carrier, 0.1 M HCI; reagent, 0.50 M KSCN in 0.1 M HCI; recorder speed, 2.50 cm/min; study performed on PE 4000 AA.
a plateau is reached a t about 500 pL. Peak width at halfheight approaches a nearly constant value of 5 s at sample volumes below about 200 pL and becomes linear with a slope of 1.6 X s/pL and a zero intercept for sample volumes above about 400 pL. Peak area, calculated from peak height and peak width at half-height, increases linearly with the volume injected from a zero intercept (not shown in Figure 2). The general aspects of the variations of peak height, width, and area with volume injected are similar to those previously described (19). An injection volume of 385 pL was chosen for all subsequent studies since it represents a favorable compromise between large peak height and small peak width. Potassium thiocyanate concentration in the reagent was varied from 0 to 0.5 M. Peak height increased rapidly until a limiting plateau at about 0.11 M KSCN. Peak height at 0.50 M KSCN was identical with that a t 0.11 M KSCN. Peakto-peak base-line noise, expressed as a percentage of the peak height obtained for 2 pg/mL Zn in the injected sample, increased only slightly from 3.1% a t 0.11 M KSCN to 4.1% at 0.5 M KSCN. A concentration of 0.5 M KSCN was selected for use in the determination of trace Zn in the presence of Fe. Sample solutions may contain u p to 2 % (0.36 M) iron, and the use of 0.5 M KSCN ensures an excess of this reagent over that required by the formation of the nonextractable complex FeSCN+ in the aqueous phase. Ascorbic acid was added as a solid in varying amounts to 25-mL portions of a solution containing 4 pg/mL zinc and 4% iron in 0.1 M HC1. The mixtures were then diluted to 50 mL with 0.1 M HCl. After the solutions stood for about 10 min, 385 pL of each solution was injected into the FIA/AAS system and the concentration of iron extracted into MIBK was measured by atomic absorption using an iron hollow cathode lamp set a t 296.7 nm. A 4% w/v concentration of ascorbic acid in the sample, which corresponds to about twice the stoichiometric amount of Fe(II1) initially present, was found to achieve maximum reduction of the iron. With this or higher concentrations of ascorbic acid the concentration of iron appearing in the MIBK extract was 110 pg/mL ( 2 X M). Flow Characteristics. The sensitivity, SF,of the AA spectrophotometer, expressed as signal height per unit concentration of absorbing element in the aspirated solution, depends on the flow rate of the solution into the nebulizer (39). The plot in Figure 3 is the sensitivity curve obtained by continuously pumping a 0.4 pg/mL solution of Zn(SCNI2 in MIBK into the AA spectrophotometer at various flow rates. (Flow injection was not involved in collecting these data.) Figure 3 is used as follows: The value of SFrelevant to an FIA/AAS experiment is read off of the curve at a flow rate equal to FM F,. Except in the nearly linear region of the curve at low flow rates, the nebulization efficiency decreases as the flow rate is increased, and in the limiting plateau region,
+
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
423
T a b l e 11. A s s a y of S y n t h e t i c S a m p l e s C o n t a i n i n g 2 % I r o n f o r T h e i r Zinc C o n t e n t b y D i r e c t AAS and by Extraction/FIA/AAS
added Zn, PPm 0.90
0.92 1.33
1.25
1.78
0.42 0.43 0.53*
1.60
2.25
0.6Eib
0.50
1
L
k
8
l'c
2
'
4
Flow Rate (mL/min )
Figure 3. Variation of sensitivity of the AA spectrophotometer with flow rate into the nebulizer: sample 0.4 pg/mL Zn(SCN), in MIBK pumped directly into nebulizer. The instrument conditions are in Table I, column 4.
SFactually becomes independent of the rate at which solution enters the nebulizer. An equation has been derived which expresses peak height in extraction/FIA/AAS as a function of SFand other system variables (31). Peak height of the AA signal obtained for the injected zinc is given by
where CznsAMpis the zinc concentration in the injected sample is the flow rate of the carrier stream, and R, solution, Fcarrier is the relative dilution at the center of the zinc zone resulting from band dispersion of the zone as it passes through the flow system. The value of R, is measured experimentally from a plot like that shown in Figure 2 as the ratio of the peak height corresponding to the injection volume (e.g., at 385 pL) to the peak height on the limiting plateau. Defined in this way, R, is analogous to the "dispersion" sometimes used in FIA (40). The quantity 4 is the fraction of zinc extracted, which can be calculated from the distribution ratio Dzn (20) and the phase ratio as
4=
&n
(Fo/ F a )
(2)
1 + Dzn(Fo/Fa) The quantity 4 depends on Fa,but if Dzn is large enough, then 4 = 1, independent of Fa. In the present case D -- 100 (29) so that 4 -- 1. It should also be noted that SFdepends on the value of F M + Fc, but it has been found that even when F M changes, the value of FM+ F, remains constant, equal to the aspiration rate as established by the nebulizer setting (31). Determining Zn in Fe. Synthetic samples containing 2 % Fe and various concentrations of Zn were assayed for Zn both by direct aspiration AAS and by extraction/FIA/AAS, using the P E 4000 spectrophotometer. Results are presented in Table 11. Results obtained by direct aspiration are always higher than the known value. Plots of peak height vs. micrograms per milliliter (ppm) Zn obtained in the direct aspiration technique become curved at concentrations above about 1 ppm Zn due to deviation from Beer's law a t the high absorbances. At every Zn concentration the sample peak height lies above the corresponding standard peak height obtained a t the same Zn concentration, by about the same amount. This height (or absorbance),which is necessarily the intercept of the sample plot on the peak height axis, corresponds to 0.42 ppm Zn in the linear part of the plots. In the curved part of the plots the same peak height difference corresponds to a greater difference between actual and found parts per million Zn (Table 11). The positive intercept of the sample plot, of course, arises from the absorbance of Fe at 213.9 nm. Thus, a Zn-free solution containing 2% Fe appears
direct AAS found Zn, difference," PPm PPm
FIAJAAS found Zn,' PPm
0.47 f 0.008 0.90 f 0.02 1.28 f 0.04 1.66 f 0.06
Added Zn - found Zn. Calibration plot is curved above about 1 ppm Zn, giving a greater difference in parts per million Zn for the same absorbance difference. Standard deviations (*) based on three replicates. to contain 0.42 ppm Zn, which means that a solid sample of pure Fe would appear to contain 21 kg of Zn per gram of Fe (Le., 21 ppm). This is consistent with the previously reported (28) spectral interference of Fe in the determination of Zn. In contrast, the assay values obtained for Zn in the samples by the extraction/FIA/AAS method are essentially free of interference from Fe (Table 11). The minimum detectable quantity of zinc is 0.2 pg/mL, taken as 3 times 3 of the peak-to-peak noise in the base line. Background correction techniques are frequently used to compensate spectral interferences in AAS, but they usually undercorrect in cases of spectral line interference where line overlap is extensive (41,42). Deuterium background correction was tested in the present case on the P E 4000 spectrophotometer using direct aspiration. The interference was reduced only by about 32%, indicating that, as expected, D2 background correction is not suitable for compensating Fe spectral interference on Zn. While spectral line interference is relatively rare in AAS, the extraction/FIA/AAS approach can be used routinely to eliminate all kinds of matrix effects in addition to spectral interferences. Also, use of the technique in conjuction with inductively coupled plasma (ICP) emission spectroscopy, where spectral interferences are more common, should provide an attractive means of eliminating them. Flow rates of the organic extract in FIA are in the same range as aspiration rates commonly used in ICP spectroscopy, and while the use of organic solvents is more problematic with the ICP than with AAS, they are now routinely employed (43, 44). Registry No. Zn, 7440-66-6; Fe, 7439-89-6;KSCN, 333-20-0.
LITERATURE CITED (1) West, A. C.; Fassei, V. A.; Kniseley, R. N. Anal. Chem. 1973, 45, 1586. (2) Kornblum, G. R.; DeGaian. L. Spectrochim. Acta, Parf B 1973, 288, 139. (3) Jones, H. A. At. Absorpt. News/. 1970, 9 . 1. (4) Siavin, S.; Sattur, T. W. At. Absorpt. Newsl. 1968, 7 , 99. (5) Lovett, R. J.; Welch, D. L.; Parsons, M. L. Appl. Spectrosc. 1975, 29, 470. (6) Roos, J. T. H. Spectrochim. Acta, Part 8 1972, 278, 473. (7) Manning, D. C.: Capacho-Delgado, L. Anal. Chim. Acta 1966, 3 6 , 312. (8) Koirtyohan, S.R.; Pickett, E. E. Anal. Chem. 1965, 3 7 , 601. (9) Stephens, R. CRC Crit. Rev. Anal. Chem. 1980, 9 , 167. IO) Fuller, C. W. A t . Absorpt. News/. 1972, 7 7 , 65. 11) Cresser, M. S. "Solvent Extraction in Flame Spectrscopic Analysis"; Butterworths: London, 1978. 12) Viets, J. G. Anal. Chem. 1978, 50, 1097. 13) Hannaker, P.; Hughes, T. C. Anal. Chem. 1977, 49, 1485. 14) Ailan, J. E. Spectrochim. Acta 1961, 1 7 , 467. 15) Ringbom. A. "Complexation in Analytical Chemistry"; Interscience: New York, 1963; Chemical Analysis Series, Vol. 16. (16) Morrison, G. H.; Freiser, H. "Solvent Extraction in Analytical Chemistry"; Wiley: New York, 1966. (17) Stary, J. "The Solvent Extraction of Metal Chelates"; Pergamon Press: New York, 1964. (18) Marcus, Y.; Kertes, A. S. "Ion Exchange and Solvent Extraction of Metal Complexes"; Wiley-Interscience: New York, 1964. Cantwell, F. F. Anal. Chem. 1982, 54, 1693. Cantwell, F. F. Anal. Chem. 1963, 55, 1882.
424 (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36)
Anal. Chem. 1985, 57, 424-427 Karlberg, B.; Thelander. S. Anal. Chim. Acta 1978. 9 8 , 1. Nord, L.; Karlberg, B. Anal. Chim. Acta 1981, 125, 199. Kawase, J.; Nakae. A.; Yamanka, M. Anal. Chem. 1979, 5 1 , 1640. Nord, L.; Karlberg, B. Anal. Chim. Acta 1983, 145, 151. Ogata. K.; Tanabe, S.; Imanari, T. Chem. Pharm. Bull. 1983, 3 1 , 1419. Taylor, M. S. Proc. Anal. Div. Chem. Soc. 1978, 342. Rosman, K. J. R.; Jeffery, P. M. Chem. Geol. 1972, 8. 25. Kelly. W. R.; Moore, C. B. Anal. Chem. 1973, 4 5 , 1274. Headridge. J. E.; Sowerbutts, A. Analyst (London) 1972, 9 7 , 442. Cantwell, F. F.; Sweileh, J. A. Anal. Chem., in press. Sweileh, J. A.; Cantwell, F. F. C a n . J . Chem., submitted. Yoza, N.; Ohashi, S. Anal. Lett. 1973, 6, 595. Thornburn-Burns, D.; Glockling, F.; Harriott, M. Analyst (London) 1981, 106, 921. Koropchak, J. A.; Coleman, G. N. Anal. Chem. 1980, 5 2 , 1252. Messman. J. D.; Rains, T. C. Anal. Chem. 1981, 53, 1632. Jones, D. R., IV; Tung, H. C.; Manahan, S. E. Anal. Chem. 1978, 4 8 , 7.
(37) Fukamachi, K.; Ishibashi, N. Anal. Chim. Acta 1980, 119, 383. (38) Yoza, N.; Aoyagi. Y.; Ohashi. S. Anal. Chim. Acta 1979, 1 1 1 . 163. (39) Treit, J.; Nielsen, J. S.; Kratochvil, B.; Cantwell, F. F. Anal. Chem. 1983, 55, 1650. (40) Ruzicka. J.; Hansen, E. H. Anal. Chim. Acta 1978, 9 9 , 37. (41) Yamada, H.; Uchino, K.; Koizumi, H.; Noda, T.; Yasuda, K. Anal. Lett. 1978, A 1 1 , 855. (42) Fernandez. F.; Giddings, R. A t . Spectrosc. 1982, 3 , 61. (43) Boumans, P. W. J. M.; Lux-Steiner, M. Ch. Spectrochim. Acta, Part 6 1982, 376,97. (44) Gilbert, T. R.; Penney. B. A. Spectrochim. Acta, Part B 1983, 388, 297.
RECEIVED for review May 31,1984. Accepted October 17,1984. This work was supported by the Natural Sciences and Engineering Research Council of Canada and by the University of Alberta.
Use of Air-Cored Solenoids for Zeeman Background Correction Guo Tie-Zheng and Roger Stephens* Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3
Design parameters are discussed for air-cored solenoids capable of generating pulsed magnetic fields at the strengths required by Zeeman-corrected AA spectrometers. Such coils are small and light enough to be mounted on an optical rail while their field volume is sufficient to avoid serious size ilmitations on the source or atomizer. The coil described here provides a combination of peak magnetic field strength and repetition rate which allows adequate signal to noise ratios to be maintained without requiring excessive power dissipation.
Zeeman background correction has been used successfully in AAS, either by direct means (1) or in the guise of magnetooptic rotation ( 2 ) . One of the most satisfactory variations of the technique is that described by de Loos-Vollebregt and de Galan (3-5), in which an alternating magnetic field is applied across the atomizer. In this case the background correction is highly effective unless the interfering species shows a Zeeman effect of its own ( 6 ) ;at the same time the degredation of signal to noise ratios, caused by anomalous Zeeman splitting and by the intensity losses associated with the introduction of polarizing components in the optical train, can be minimized. One disadvantage associated with the use of an alternating magnetic field is that the technique requires a careful design of the magnet and of its power supply if suitably high frequencies are to be attained, because of the large inductance of the electrical load. Conversely, if the modulation frequency is kept low, then the ability of the system to correct for fast transients is reduced, and in addition sensitivity to source flicker noise may become apparent. A second disadvantage, common to all Zeeman systems, is that their magnets are large, heavy pieces of apparatus which are not readily moved or adjusted and which can interfere with the operators’ choice of atomization system if inverse Zeeman correction is to be employed.
The use of air-cored solenoids offers a possible way to deal with both of these problems. However the associated power supply must be capable of delivering a heavy current a t very high power levels, and water cooling of the solenoid is essential ( 3 ,requirements which lessen the attractiveness of such an approach. These objections can be overcome by pulsed operation at a suitably low duty cycle. The use of air-cored solenoids driven by high current pulses allows useful fields to be generated with a very modest power supply and with a magnet assembly which is simple to build and which is small and light enough to allow its position to be adjusted as easily as that of any other component in the optical system. The present work was carried out to exame the feasibility of this approach.
THEORY The following equations describe the behavior of multilayer air-cored solenoids:
R = 8aNa/d2 L = 2 K a 2 P a 2 / ( 1X where H (T) is the magnetic field strength a t the center of the coil, n (m-l) is the packing density of the winding, i (A) is the current, 21 (m) is the length of the coil, ao,a x ,and a (m) are the inner, outer, and mean radii of the coil, R ( Q ) is the resistance of the coil, u ( Q m) is the conductivity of the wire, d (m) is the diameter of the bare wire, L (H) is the inductance of the coil, and K is the Nagaoka constant. Equation 1 is obtained by integration of Amperes’ law (8): eq 2 and 3 are of a standard form (9);the value of the Nagaoka constant, K, in eq 3 is given by 1/K = 1 + 0.45a/l + 0.32(a1 - a,)/a + 0.42(a1 - a o ) / l (9). Electrically the field coil can be treated as a resistance and inductance in series, driven by a voltage pulse of finite duration. A t the frequencies used here capacitance does not
0003-2700/85/0357-0424$01.50/0 1985 American Chemical Society
