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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Determination of Metals in Waters and Organic Materials by Flameless Atomic Absorption Spectrometry with a Wire Loop Atomizer M. H. West,' J. F. Molina,*2 C. L. Yuan, and D. G. Davis3 The Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70122
J. V. Chauvin The Department of Chemistry, Nicholls State University, Thibodeaux, Louisiana 7030 I
Tungsten-rhenium (3 YO)wire loops are utilized as atomizers for nonflame atomic absorptlon spectrometry. The wire loop atomizer, uniformly constructed with a template, is mounted on a brass atomizer head. The atomizer head replaces the burner head on the commercially available burner base of a Varian Techtron AA-5 atomic absorption spectrophotometer allowing for a significantly improved optical alignment of the wire loop atomizer with respect to the hollow cathode lamp beam. The atomizer is heated electrothermally with an lnexpensive variable transformer. For the majority of the present work, a line voltage ramp, provlded by a variable transformer driven by a mofor, was applied to the wire loop for atomization of the analyte. The technique of linear voltage programming (LVP) had several advantages including the ability to separate, in time, the analyte peak from matrix peaks arlsing from nonatomic absorption. The wire loop atomizer has been successfully applied to the analysis of iron, zinc, chromium, manganese, silver cadmium, copper, and lead In water samples, Environmental Protection Agency standard water samples, and National Bureau of Standards Standard Reference Materials 1571 (orchard leaves) and 1577 (bovlne liver). A limited study of the effect of the presence of various diverse ions on the analyte peak absorbance signal is also presented.
T h e use of graphite or various metals and their alloys as nonflame atomizers for atomic absorption spectrometry (AAS) has been extensively investigated in the past two decades (1). Nonflame atomizers require far smaller sample volumes and are more efficient than flame atomizers a t converting the analyte species to atoms in the vapor phase. However, poor precision and accuracy are often problems with nonflame atomizers (2). Molecular absorption and/or light scattering of radiant energy are classic obstacles to nonflame AAS and must be surmounted by the use of background correction systems and/or minimization of matrix effects by ashing, chemical separation, or other appropriate means ( 3 ) . Numerous graphite atomizers such as carbon rods, braids, tubes, cups, and furnaces have been employed with varying degrees of success in trace analysis (4-10). In general, graphite atomizers require a sophisticated, relatively expensive power supply in order to electrothermally produce the requisite atomic vapors in or above the atomizer. Since metals and their alloys have an intrinsically lower resistivity, metal filament atomizers can be powered by inexpensive variable transformers and the electrode terminals need not be water-cooled. TanPresent address: Coors Snectro-Chemical 1,ahnratorv. P.O. Box
Summit, N.J. 07! Deceased. 0003-2700/79/0351-2370$01 .OO/O
talum ribbon analyzers have been developed (11) but become brittle after repeated heating and its surface reacts with nickel and cobalt compounds (12). Wire loops made of tungsten and platinum (13) as well as platinum-rhodium alloy (14) have been suggested as flameless atomizers for atomic fluorescence studies. Lund and co-workers took advantage of the useful electrochemical properties of tungsten and platinum filaments to preconcentrate cadmium from seawater (15) and urine (16) by electrodeposition prior to atomization. In our laboratory, we have employed wire loop atomizers, constructed from a tungsten-rhenium (3%) alloy, for the analysis of a number of elements (17-19). We report here significant improvements in the design of the atomizer head which allow for an improved optical alignment of the wire loop atomizer with respect to the hollow cathode lamp beam and demonstrate the adaptability of the atomizer head to a commercially available atomic absorption spectrophotometer. In addition, the wire loop atomizer has been employed to analyze a variety of samples of environmental and biological interest including finished water samples from the United States Army Environmental Hygiene Agency (USAEHA), standard water samples from the Environmental Protection Agency, and NBS SRM 1571 (orchard leaves) and SRM 1577 (bovine liver). In this manner, the utility of the tungsten-rhenium wire loop atomizer has been assessed for the first time for the trace elemental analysis of iron, zinc, chromium, silver, manganese, cadmium, copper, and lead.
EXPERIMENTAL Instrumentation. A Varian Techtron AA-5 atomic absorption spectrophotometer (AAS) equipped with a BC-6 background corrector was used for the majority of this work and a PerkinElmer Model 306 AAS for the remainder. Lamp currents, monochromator slit widths, and analytical wavelengths used, were those recommended by the instrument manufacturers. Slit height adjustments to compensate for the light emitted by the atomizer were not necessary for any elements examined in this study. Since the atomic absorption signals have a very sharp profile, the peak absorbance was employed as a measure of the absorbance at each analyte concentration and in establishingworking curves for each element. The atomizer head, designed and constructed in the University of New Orleans machine shop, is illustrated in Figure 1. The atomizer head is a distinct modification of those employed previously (27-19). The previously designed wire loop atomizer of Newton and Davis required the use of the fluorescence optical railing (20). Optical alignment of the wire loop atomizer was difficult as there were no coarse or fine controls. Optical alignment was accomplished by manual horizontal movement of the atomizer support brace attached t o the fluorescence railing, and manual vertical and rotational movements of atomizer about this brace. Once optical alignment was obtained,there was no way of ensuring that this alignment could be duplicated. It was observed that the atomizer was always at a slight skewed position with respect t o the optical path. 1979 American Chemical
Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Flgure 1. Wire loop atomizer head and configuration for the aliquot
method 7
Flgure 2. Alignment of the atomizer head on the optical railing of the Varian AA-5. (A) Hollow cathode lamp turret. (6)Hydrogen continuum source. (C)Front focusing lens and beam splitter. (D) Wireloop atomizer. (E) Rear focusing lens. (F) Monochromator and photomultiplier tube
The newly designed brass atomizer head was machined so that it could be mounted on the commercially available Varian Techtron burner base, thus replacing the burner head. As a result, an improved and facile optical alignment of the wire loop atomizer with respect to the hotlow cathode lamp beam is possible by using the horizontal, vertical, and rotational adjustment dials of the burner. The wire loop atomizer is aligned such that the hollow cathode lamp beam is concentric with the center of the wire loop (Figure 2). Background and interference signals do not increase when the wire loop atomizer is misaligned; however, the analyte signal drops as much as 11%when the atomizer is only slightly misaligned. The burner base was placed on the optical rail at 19.9 cm from the monochrornotor entrance slit. The leads from the power supply unit were attached to the atomizer head with screws mounted in phenolic blocks which replaced the asbestos formerly used as an insulating material (18, 19). This reduced the amount of electrical shorting to the brass block. An inert sheathing gas of either dry nitrogen or argon, entering through gas inlets attached to the atomizer head, was passed over the loop at 1.95 or 1.63 L/min to maintain an inert atmosphere in the region surrounding the wire loop. The inert gas was supplemented by the presence of hydrogen at 0.225 L/min during the atomization process. The hydrogen gas extended considerably the useful lifetime of each loop by reacting with any entrained oxygen present. Once beyond the ignition temperature of H2,an entrained air-H, flame persisted. Owing to the heating of the brass block, a 15-s cool-down period was required before the next sample was introduced. To assist in maintaining a reducing atmosphere about the wire loop atomizer, a brass enclosure was moved over the loop prior to the atomization step. Without the brass enclosure, the analyte atoms would be forced out of the absorption path more rapidly by the sheathing gases in addition to the increased likelihood of loop oxidation. Since no memory effects were ever noted and the brass enclosure was heated by the entrained air-H, flame during atomization, the enclosure is unlikely to act as an atom sink.
2371
Aliquots of the desired solution were placed on the wire loop utilizing a fixed volume (5 pL) Eppendorf pipet. Owing to the surface tension of the aqueous solution applied to the wire loop atomizer, even larger volumes may be successfully employed. The reproducibility of sample application is quite good, judging from the small relative standard deviation based on at least five replicate determinations (vide infra). The power supply used to provide a current sufficient to evaporate the solvent and atomize the sample has been described previously ( I 7). Variable transformer settings appropriate for drying and atomization are different for each element and were determined empirically. Since it was noticed that “aged” or oxidized loops had slightly different resistances and, therefore, temperatures, a photocell detector (20) was constructed to measure the temperature as a function of applied voltage. An optical pyrometer (Leeds and Northrup 8632C Optical Pyrometer) was employed to calibrate the device. A wire loop was considered “aged” after the first 10 atomizations. Thereafter the wire loop maintained a constant atomization temperature at the same applied voltage as well as hydrogen and sheathing gas flow rates for the lifetime of the loop. Therefore, it was not necessary to construct a feedback circuit whereby the applied voltage was controlled by the photodiode output. The drying temperatures were below the range of the optical pyrometer but varied approximately over the range of 200-600 “C determined by extrapolation of temperature-applied voltage profiles. The observed atomization temperatures varied from 890 “C (that used to atomize lead in this study) to 1700 “C. The latter temperature is high enough to effectively atomize any of the elements examined with the wire loop atomizer technique. In approximate agreement with our results, Agget and Sprott found a temperature range of 300-1560 “C for the tantalum ribbon atomizer (22). However, they reported a temperature for each element corresponding to the minimum temperature at which the analyte could be observed (appearance temperature). The voltage, applied t o the wire loop for atomization, can be linearly ramped by using a variable transformer driven at 4 rpm by a motor. Since the optimum adjustment of the variable transformer for the atomization step varies with the time-dependent properties of the wire loop, linear voltage programming (LVP) nicely overcomes this difficulty by simply ramping to voltages higher than thoise ordinarily employed for atomization of the analyte. The lifetime of the wire loop was not noticeably shortened by this method. This difficulty did not exirt with “aged’ wire loops. In addition, the analyte and less volatile components of the sample can be routinely cleaned from the wire loop surface by the LVP technique. The ramping technique could potentially lead to the separation of the analyte peak from any matrix peak (nonatomic absorption), thereby reducing potential interferences resulting from gas phase interactions. The release of an analyte of interest present as an iimpurity in the atomizer, as in the case of iron (vide infra), can be separated from the true analyte peak via the LVP method. Reagents and Container. Doubly-distilled, deionized water was produced by distillation of singly-distilled water with a Corning Mega-Pure 1-L still followed by passage through a Corning LD-3 general purpose demineralizer. All containers were soaked in either 1:l concentrated nitric acid/water or 1:1 concentrated nitric acid/concentrated sulfuric acid for at least three days, thoroughly rinsed several times with tap water, and finally rinsed several times with doubly-distilled, deionized water (22, 23). Stock solutions were prepared from the best available reagent grade metals or their salts at a concentration level of 1000 ppm using doubly-distilled, deionized water. In most instances, dilute solutions prepared from the 1000 ppm stock solution were used only on the day of preparation. Sample Preparation. The small surface area, localized heating, and geometric design of the wire loop atomizer prevent the effective destruction of large quantities of organic and inorganic matrix components during the drying and ashing steps. Therefore, it is necessary to digest a sample of high organic content. A General Electric microwave oven, model JET 84 with 580and 360-W outputs, in the high and defrost settings, respectively, was employed for digestion of biological materials (24). In order
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Table I. Results of Microwave Oven Digestion Procedure for NBS Standard Reference Materials
06
NBS SRM 1571, Orchard Leaves, Mg/g
05
NBS value
average
Mn
r
91 i 4 (7)Q
04
91 t 4 0 5 0
NBS SRM 1577, Bovine Liver, Mg/g
03
NBS value
average
cn
n
Mn 1 1 . 7 * 0.7 ( 2 ) 1 0 . 3 t 0.13 Pb 43 2 4 ( 5 ) 45 2 3 The number of determinations is given in parentheses.
02
01
Table 11. Sensitivities and Absolute Sensitivities for Analysis by Flame and Nonflame Atomic Absorption Spectrometry sensitivity, ppm ( X 103) element Zn Fe Cr Ag Mn Cd cu Pb
flamea 9 62 55 36 24 11
40 110
absolute sensitivity, g(x
wire loop
wire
0.17 1.9 2.45d 1.72 1.9 0.88 9 5.4
0.85 9.4 32.6d 8.59 9.6 4.4 45 27
a Ref. 25. Ref. 11. termined in this study.
00
loop
1 0 1 2 )
20
20 7
20
30
I
IO
I
I
20 30 Z i n c , ppb
I
40
I 50
Figure 3. Calibration plot for the determination of zinc by the aliquot
tanta- carbon lum tube, ribbonb 5 @ L C 4 200 40
I 0
0.185 3.6 6.2je 1.35 1.4 _--
3.3 _._
Varian Model 63-CRA as deFor Cr (VI). e With H2 gas.
t o evacuate the interior of the oven of corrosive fumes, a onequarter horsepower vacuum cleaner motor was attached to the already present louvers at the rear of the oven. A procedure found to be satisfactory involves the addition of 20 mL of concentrated HN03 to 0.5 g of solid sample and heating the solution in a microwave oven for 3 min on the high setting. Then 10 mL of 30% HzOZare added and the solution is heated on the “defrost” setting for 3 min. An additional 2 mL of Hz02 are added to the sample mixture and the heating is continued for 1 min on the defrost setting. The sample is then filtered directly into a volumetric flask. This method was tested by determining two elements in NBS SRM 1571 (orchard leaves) and NBS SRM 1577 (bovine liver) using a standard calibration curve (Table I). R E S U L T S AND DISCUSSION The sensitivities and absolute sensitivities of each of eight elements determined by the aliquot method are reported in Table 11. Sensitivity is defined as the concentration of an element needed to cause absorption of 1% of the incident radiation. Sensitivities are similarly tabulated in Table 11for flame atomic absorption spectrometry and the absolute sensitivities are shown for the tantalum ribbon and the Varian Model 63-CRA (actually a carbon tube atomizer). In general, nonflame atomic absorption methods provide superior sensitivities to the corresponding flame methods. The sensitivities for the aliquot method are generally a t least 2 orders of magnitude better than those for flame analysis. The absolute sensitivities for the wire loop atomizer are somewhat better than for a comparable metal atomizer, the tantalum ribbon. Work carried out in this laboratory with the Varian Model 63 Carbon Rod Atomizer indicates the carbon tube may provide greater absolute sensitivity in comparison with the wire loop atomizer for a specific analyte. On the other hand, it is our experience that the lifetime of a single wire loop atomizer is of longer duration (- 300 atomizations) than the carbon tube
method atomizer (-50 atomizations), the cool-down time between successive atomizations is negligible (- 15 s), and the cost of each wire loop atomizer is a t most a few cents. The optimum analytical range varies considerably from element to element resulting from nonlinearity in the working plots. Such plots for zinc (Figure 3), silver, cadmium, and lead produced only narrow linear ranges, all much below 100 ppb concentration. The plot for manganese was noticeably nonlinear throughout the 0-100 ppb concentration range. The calibration plots for copper, trivalent iron, and trivalent chromium were linear up to 250, 220, and 100 ppb, respectively. The volatility of the metals is probably partly responsible for the degree of linearity or nonlinearity exhibited in the working plot. Zinc, silver, cadmium, and lead are very volatile elements while iron and chromium are relatively nonvolatile (26). The gases flowing through the wire loop absorption cell undoubtedly decrease the residence time of the analyte in the cell. The decreased residence time and the short atomization time, characteristic of the volatile elements, makes peak absorbance measurements less satisfactory in terms of obtaining working plots with large linear concentration ranges. In many cases, the use of peak areas leads to calibration plots with greater linearity. Sturgeon et al. found that the use of integrated absorbance increased the linear range of the working plot for zinc and cadmium by factors of 16.7 and 4.0, respectively (27). The rate of evaporation of iron and chromium from the wire loop is slow enough in comparison with the more volatile elements to allow the recorder to follow the peak absorbance signal more accurately, and this likely leads to the observed increase in the linear range for the working plots. For most of the elements examined, some limited study was made of the effect of various diverse ions on the peak absorption signal, the results are shown in Table 111. Most of the diverse ion studies were carried out using a single analyte and a single diverse ion, with the latter being present in much larger amounts than would usually be found in natural water samples. Smeyers-Verbeke et al. have shown the direction of an interference may occasionally change with its concentration (26). Thus, a complete evaluation of diverse ion effects would require determining their effect a t various concentrations. Since the intent of this report is only to show the general utility of the wire loop atomizer, such detailed studies were not made. Iron, as ferric chloride, caused a rather severe depression in the zinc and chromium peak absorbance signals. Since iron salts have a relatively low volatility compared to zinc and other
ANALYTICAL CHEMISTRY,VOL. 51, NO. 14, DECEMBER 1979
2373
Table 111. Evaluation of Various Diverse Ions on Analysis with the Wire Loop Atomizera analyte
interferent,
M salt
Fe FeCl, (HCL matrix) CU CuSO,.5HZO Mn MnSO;H,O MnC1;4 H, 0 Cd 3CdS04.8H,0 Cd C1,.2.5H, 0 Pb Pb(NO,), Zn Zn(NO,), ("0, matrix) Ca Ca(N0,),,4HZ0 NO; NaNO, C1- NaCl SO; NaZS04 phosphate Na,HP04 Na H PO H ,0 OAc- NaOAc,3H2O F- NaF 1- NaI carbonate Na,CO,.H,O NaHCO, Br- NaBr
Zn cr CrCl,, Zn(NO,),, 0.1 ppm 1.0 ppmC 1.1 ppin 59 -19 0
~
0.75 ppm
Mn Ag Pb MnCI,, AgNO,, E'b(NO,),, 0.2 ppmC 1.1)ppmd 1.0 ppmc
30b 54
31 15
41
0 -48'-58
0
12
-19
0
-16
-6
-16
0
-78 - 29
--62 -1 6
0 0 --11 0 -1 3
-28
12 -48
- 5 c -19 -83
-32 -1 6
-15 6
-34 -6lC-88 -6lC-89 -65' - 85
-100
0
-23 -19
0
-40
-1
-30
-36 -30 -95 --66
-15 0
Negative 70 deviations for the ancd7tesignal in the presence of the interferent are ini~-cated b y a ininus sign. zinc. Linear Voltage Programming (LVP) employed. Soaking method.
2.2 ppm
Table IV. Results of EPA Sample Analysis with Relative Standard Deviations, Results in ppb metal
true value
found, %
Zn
11
Fe Cr Pb
26 16 22
9?0 24 i 2.6 1 6 i 5.4
Mn Cd
26 5.2
cu
16
1 2 5 10
31 i 0.9 6 I 6.7 1 6 i 1.9
true value 30 41 7
154 29 8 45 23 72
salts, the interference may have occurred on the wire loop by preventing effective volatization of the analyte. Interestingly enough, cupric sulfate a t 1-mM levels enhanced the zinc and sliver signals, but suppressed the chromium and manganese signals. Perhaps the rate of evaporation of zinc and silver or their salts from the loop was reduced in the presence of cupric sulfate, allowing the detection system to follow the peak absorbance signal more efficiently for these volatile elements. It is more difficult to explain the diminished signals for chromium and manganese since they are less volatile than copper. More work is needed to shed light on the mechanism by which these interferences occur. Sensmeier, Wagner, and Christian (28),as well as Maruta and Takeuchi (29),commented on the complex pattern generally observed for interferences and the lack of a coherent theory to explain their origin. A solution of 0.200 ppm manganese containing each of the interferents in Table I11 produced a decrease in the signal (40%) which obviously could not be deduced from the results of individual interferents. Obviously, the most important inference to be made from the interference study is that since a multitude of interferences may exist when the aliquot method is employed, the use of calibration curves for the determination of unknown concentrations must be viewed with caution. Assuming the existence of a linear working plot for a particular element, both the method of standard additions and an external calibration plot method should be used for any matrix suspected of enhancing or suppressing the analyte signal. If the standard additions plot and the calibration plot paralled one another, then no observable matrix effect exists. Linear voltage programming was used in some of the interference studies for
found, 70
true value
20 I 2.2 420 I O 148 ?- 1.1 298 2 6.2 18 i 1.1 24 ? 4.6 64 i 2 . 7
174 67 8 209 352 39 7 73 102
found, ?C 160 t .l.5 653 f 2.5 185 i 3 . 0 338 z 11.9 390 I4.6 76 i 3.0 78
z
2.8
reasons delineated in the Experimental section. A special comment is due on the subject of iron determinations. After some peculiar results were obtained it was found that the wire itself contained iron. This was discovered by examination of the wire using an X-ray fluorescence method. By use of a high recorder speed (5 in./min) and the LVP technique, the analyte peak was observed first, followed by a second large iron peak from iron metal present as an impurity in the tungsten-rhenium alloy. Prolonged heating also reduced the iron signal from the wire to a minimal background. It was thus possible to obtain excellent linear standard addition plots for iron, up to a t least 450 ppb using the LVP method. Step atomization of the desolvated sample at a fixed transformer setting cannot lead to a separation of the two signals. In order to better assess the analytical reliability of the aliquot sampling method for trace elemental analysis, three sets of Environmental Protection Agency quality control samples, generously provided by the Environmental Monitoring and Support Laboratories in Cincinnati, Ohio, were analyzed for zinc, iron, chromium, lead, manganese, cadmium, and copper. All of the diluted EPA samples and the standard metal ion solutions for establishing calibration plots contained 1.5% nitric acid (Baker Ultrex grade) in order to match sample and standard matrices as closely as possible. The results are shown in Table N for the EPA samples along with the relative standard deviation (rsd) for each elemental determination. In general, the precision (rsd) for five replicate determinations was better the 5% except a t very low analyte concentrations (sample 1). Quite good agreement was observed between the values determined for each metal ion by the aliquot method and the values provided by the Environmental Protection
2374
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Table V. Results €or the Analysis of the USAEHA Water Samples with the Wire Loop AtomizeP sample ment code ele-
Cd
cu Ag
zI1
hl n
CRA-
flame
63
AAS
USAEHA~
13'
15
15
14'
14 124
14
60 79 92 94 60 65
178' 2's 24
79 19
25 24
60 65 79 75
174 173
196
111
184
169
196 21 6
136 183 202
10 58
78 39 65 Fe
10 90 58
Pb
59 60 65
123'
198
28 31 30 32
29 31 30 31
157 14
132
132
28 181
15
20
28 22
38
17
20
1835 900, 1800c 50 1800, 3900' 37
1820 2200
7 50 2051
4900
285 3925 19
18
19
59 79
11
8 19
c -
14 ..
10
Cr
wire loop atomizer
19 60 6.5 79 19
26
6 19
i
18 18 18
16
38 35 35 39
23
Data proAll concentrations a r e expressed in ppb. vided f o r these samples b y the L'nited States Army Method of standard Environmental Hygiene Agency.
addition. Agency which incidentally does not list any error limits for their pooled data on the Quality Control samples. Any variations between the experimentally determined elemental concentrations and the Environmental Protection Agency values appeared to be random with the exception of zinc where the wire loop atomizer gave consistently lower results. Drinking water samples, provided by the United States Army Environmental Hygiene Agency (USAEHA) and previously analyzed for ten elements by flame and nonflame atomic absorption spectrometry on a Perkin-Elmer 503 atomic absorption spectrophotometer (30),were analyzed in our laboratory using the wire loop aliquot method in addition to the Varian Model 63 Carbon Rod Atomizer, in order to provide some measure for comparison of methods. The results are presented in Table V. Excellent agreement was found for the determination of cadmium, copper, and silver in the finished water samples. The results for the determination of the other metals are discussed below in some detail. The zinc concentrations, determined by the aliquot method were neither consistently lower nor higher than those reported by the USAEHA. For the four water samples having zinc concentration between 100 and 200 ppb, two samples, analyzed by the aliquot method, gave values intermediate between those obtained in our laboratory by flame atomic absorption and those provided by the USAEHA. The remaining two samples were found to have zinc concentrations slightly lower than those predicted by the other two atomic absorption techniques. Because of nonlinearity in the working plots, Figure 3, it was not possible to check for matrix effects by the method of standard additions. Zinc was the only element in our study to give a nonzero absorbance for a doubly-distilled, deionized
water blank. The lead concentrations were, in general, higher than the USAEHA values. Severe matrix effects were observed for the iron analyses. The concentrations of iron found in samples 58, 59, and 90 by the aliquot method were much lower than expected. Our preliminary results, obtained with the carbon rod atomizer, were in substantial agreement with the USAEHA values. For samples 90 and 59, the iron concentrations were determined by flame atomic absorption analysis of the undiluted samples and by the aliquot method employing both calibration plots and the method of standard additions. The concentration obtained by flame analysis, aliquot standard additions analysis, and the USAEHA analysis were in good agreement. However, the results derived from the aliquot method working plots were approximately one half of those found by standard additions (Table V). The reason for signal suppression by the matrix is not known and further study is necessary to evaluate possible interferences for the determination of iron with the wire loop atomizer. Wherever the manganese analyses, provided by the USAEHA were not in agreement with the aliquot method determinations, analyses in our laboratory, employing the carbon tube atomizer, corroborated the results from the latter method. For samples 78 and 65, the lower manganese concentrations obtziined with the aliquot sampling relative to the carbon tube atoniizer indicated a possible signal suppression by the matrix for the former techniques.
CONCLUSIONS As evidenced by the successful analysis of the EPA standard water semples and the digested NBS Standard Reference Materials, the tungsten-rhenium wire loop atomizer shows Considerable promise as a means of electrothermally volatilizing analytes for atomic absorption spectrometry. The cost per atomizer is a t most a few cents and the associated power supply construction costs are minimal ($23.00). The atomizer also requires a negligible waiting period to reach thermal equilibrium after completion of the atomization process. The technique of linear voltage programming has been shown to be of considerable value in separating the desired analyte peak for iron from a peak arising from the atomization of iron impurities within the atomizer. As with other electrothermal atomizers, matrix effects can exert considerable influence upon the analytical results, as the use of calibration curves for the determination of analyte concentrations without the parallel use of the method of standard additions must be viewed with considerable caution. In the future, we plan to report further improvements in the atomizer design, demonstrate its use for barium analysis, and expand upon the analysis of chromium in different valence states using the aliquot sampling method. A new wire loop atomizer design will be described for arsenic and selenium determinations. Finally, adaption of the CRA-63 power supply to the wire loop atomizer will be demonstrated. ACKNOWLEDGMENT We wish to acknowledge the support of this work by the U S . Army Medical Research and Development Command under contract DAMD 17-75-C-5048and for analyzed water samples. Thanks are due to Bud Schuler of the University of New Orleans machine shop for his help in the design and construction of the atomizers used in these studies, and to Harry Rees and Frank Stone for electronic assistance. We also thank Gary Allen of the Department of Earth Sciences at the University of New Orleans for assistance with the X-ray fluorescence spectrometer. LITERATURE CITED (1) W.
G.Schrenk, "Analytical Atomic Spectroscopy", Plenum Press, New
York, 1975, Chapter 10.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
(2) H. L. Kahn, M. Bancrofl and R. H. Emmel, Res./Dev., 27, 30 (1976). (3) 6. R. Culver and T. R. Surles, Anal. Chem. 47, 920 (1975). (4) G. Lundgren, Talanta, 23, 309 (1976).
(19) J. V. Chauvin, M. P. Newton, and D. G. Davis, Anal. Chim. Acta, 65, 291 (1973). (20) 6.Watne and R. Woodriff, Appl. Spectrosc.. 30, 7 1 (1976). (21)J. Agget and A. J. Sprott, Anal. Chim. Acta, 72, 49 (1974). (22)R. E. Thiers in "Methods for Biochemical Analysis" D. Glick, Ed., Vol. 5, Interscience. New York, 1957,pp 274-309. (23) R. W. Karin, J. A. Buono, and J. L. Fasching, Anal Chem., 47, 2296 (1976). (24)Adel Abv-Samra, J. Steven Morris, and S.R. Koirtyohann, Anal. Chern., 47, 1475 (1975). (25) "Analytical Methods for Flame Spectroscopy", Varian Instrument Division, 61 1 Hansen Way, Palo Alto, Calif. 94303. (26) J. Smeyers-Verbeke, Y. Michotte, P.Van den Winkel, and D. L. Massart, Anal. Chem., 46, 125 (1976). (27)R. E. Sturgeon, C. L. Chakrabarti, I. S. Maines, and f'. C. Bertels, Anal. Chem., 47, 1240 (1975). (28)M. R. Sensmeier, W. F. Wagner, and G. D. Christian, Fresenius' 2. Anal. Chern., 277, 19 (1975). (29) T. Maruta and T. Takeuchi, Anal. Chim. Acta, 66,5 (1973). (30) Eugene L. Meier, U S . Army Medical Research and Development Command, Fort Detrick, Frederick, Md. 21701,personal communication, 1976.
(5) T. Surles, J. R. Tuschall. and T. T. Collens, Environ. Sci. Techno/.,9,
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RECEIVED for review May 31, 1977. Resubmitted September 8, 1978. Accepted August 6, 1979.
Sampling at Constant Temperature in Graphite Furnace Atomic Absorption Spectrometry D. C. Manning, Walter Slavin," and S. Myers The Perkin-Elmer Corporation, Main A venue, Norwalk, Connecticut 06856
When using graphite furnace sampling in atomic spectrometry, it is advantageous to introduce the sample into a hot environment that is at thermal equilibrium (constant temperature). One means of doing this is to dry an aliquot of the sample on a wire made of a high melting point material, and then l o place the wire and dried sample into the hot furnace. Using this procedure, matrix interferences are eliminated or greatly reduced compared to the conventional procedure of introducing the sample onto the cold wall of the furnace for the determination of Pb and TI. High concentrations of chloride matrix provide a vapor phase interference.
Several papers dealing with graphite furnace atomic spectrometry have pointed out the advantages of introducing an analytical sample into a hot environment, isothermal in time and space (1-7). Such an environment reduces or eliminates the analytical interferences associated with the variations in atomization conditions due to differences in sample matrix. In his early work, L'vov dried the sample on the end of a graphite rod which was then introduced into a preheated graphite tube (6). Woodriff and Ramelow ( 7 ) introduced the sample into a preheated tube using a small graphite cup. Littlejohn and Ottaway (8)used a furnace tube for emission. Their tube was designed to become hotter on the ends than in the center. When the sample was vaporized from the tube center, the vapor diffused toward the ends, encountering a hotter region that dissociated molecular bonds and excited the atoms more efficiently. In the furnace design described by Massmann, the sample is introduced to the inside wall of a cold furnace tube (9). The tube is then heated, and the analyte is volatilized at a tem0003-2700/79/0351-2375$01 .OO/O
perature that is at least partially dependent on the matrix composition of the sample. L'vov (2) points out that some of the potential difficulties inherent in heating the tube and sample simultaneously can be avoided if the sample is placed on a platform within the graphite tube. Slavin and Manning ( 4 ) have demonstrated the advantage of this idea for the determination of Pb, using a commercial graphite furnace based on the Massmann design. However, use of the L'vov Platform delays the vaporization of the sample for only a limited time, which is not under the control of the analyst. The atmosphere within the graphite tube may not have reached constant temperature by the time the platform surface reaches the temperature at which the sample vaporizes. This will depend, among other things, on the analyte, the matrix and the heating rate. It would be preferable to introduce the sample after constant temperature conditions have definitely been established. L'vov (10) has suggested this might be done hy drying the sample on a wire, and then plunging the wire and dried sample into the preheated graphite tube. Since the time when the sample is introduced can be controlled, introduction into an environment at constant temperature is assured. Drying the sample on a wire was used in early quantitative analytical spectroscopy. J. Janssen (11) used a platinum wire to introduce the sample into the flame of a Bunsen burner in 1870, attempting to determine sodium quantitatively. Since that time, many other workers have used the method (12), although not associated with the graphite furnace. Very recently Garnys and Smythe (13)have described a tungsten wire for introducing the sample into the graphite furnace. Although their arrangement is very similar to the one described in this paper, their goal appears to relate to sampling from a tungsten surface and to increasing sample throughput while this paper t2 1979 American Chemical Society
