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information obtained. Apparently, only two other reported studies combined the use of both solid surface RTP and RTF for qualitative analysis (4,6). Thus, this report also shows the advantages of using both solid surface RTF and RTP for organic trace analysis. LITERATURE CITED (1) Green, G. L.; O'Haver, T. C. Anal. Chem. 1874, 46, 2191. (2) Vo-Dinh, T.; Gammage, R. B. Anal. Chim. Acta 1878, 707, 261. (3) Grifflths, T. R.; King, K.; St. A. Hubbard, H. v.; Schwing-Weill, M. J.; Meuiiemeestre, J. Anal. Chim. Acta 1982, 743, 163. (4) DaRerio, R. A.; Hurtubise, R. J. Anal. Chem., in press. ( 5 ) Talsky, G.: Mayring, L.; Kreuzer, H. Angew. Chem., Int. Ed. Engl. 1978, 17,785.
(6) Ford, C. D.;
Hurtubise, R. J. Anal. Lett. 1880,
13 (A6), 485.
R. A. Dalterio R. J. Hurtubise* Chemistry Department University of Wyoming Laramie, Wyoming 82071
RECEIVED for review November 17, 1983. Accepted February 15,1984. Financial support for this project was provided by the Department Of Division Of Basic E n e r a Sciences, Contract No. DE-AC02-80ER10624.
Electrochemical Preconcentration and Separation for Elemental Analysis by Inductively Coupled Plasma Emission Spectrometry Sir: In a previous paper to this journal Salin and Horlick (I)described a device which was designed to allow the introduction of solid, powder, and liquid samples directly in the inductively coupled plasma (ICP) for simultaneous multielement analysis. This device is called the direct sample insertion device (DSID) and utilizes a torch essentially identical with the conventional design with the exception that the central aerosol tube was replaced with a movable quartz rod. The sample is placed inside a conventional dc arc electrode (I) which has been drilled in its base to accept the quartz rod. The rod may then be drawn down to a level which places the electrode in the volume immediately surrounded by the coils. In this region low forward power can be applied and the temperature of the electrode can be raised by inductive heating. This procedure can be used to dry and ash samples. The plasma is ignited when the electrode is well below the volume occupied by the plasma and the electrode is inserted into the plasma by pushing the quartz rod upward. Continuous operation has been reported by Sommer and Ohls (2), running a t 3 kW with an argon-nitrogen ICP, and by Horlick (3)and Kirkbright (4),running lower powers and lower proportions of mixed gases. We have used an apparatus functionally identical with the one used by Salin and Horlick (I) with the only major difference being that we used a demountable torch of our own design. Our goal was to preconcentrate the sample electrochemically by cathodic deposition onto a graphite electrode before insertion into the ICP. We envision two major advantages of this method of sample introduction. Calculations indicate that an improvement in detection limits should be possible with this as a preconcentration technique. We also believe that the selective separation of elements or elemental species from difficult matrices will be possible thereby providing a separation as well as preconcentration step for certain sample types. The ICP should provide greater stability than the microwave inductively coupled plasma (MIP) ( 5 ) as well as a simultaneous multielement capability which is not present in atomic absorption (AA) systems. We expect the ICP to provide greater precision, linearity, and freedom from matrix effects when compared to a dc arc. Controlled potential electrolysis has been widely used for electrothermal atomic absorption spectrometry. Electrolysis has been performed on metal wires (6-9),carbon rods (IO), hanging mercury drop electrodes (HMDE) (II,I2), and tubular pyrolytic graphite-coated electrodes (13). The technique
has also been applied to flame AA, using a thin f i i of mercury deposited on a wax-impregnated graphite rod (I4),to direct current arcs using a HMDE (15)and with a He-MIP ( 5 ) . EXPERIMENTAL SECTION The plasma is generated by a conventional source (PlasmaTherm Inc., Kresson, NJ, Model HFP-2500D with an AMN 2500E Automatic Matching Network). No mass flow control devices are used. A Jarrell-Ash 0.5-m Ebert spectrometer with a spectral band-pass of 0.04 nm was equipped with a 1P28photomultiplier tube operated at 600 V. The signals were amplified by conventional means and displayed on a strip-chart recorder with a 0.2-5 full scale deflection capability. Data were extracted manually from the strip-chart recorder or from an AIM-65 microprocessor-based system. The controlled potential electrolysis was performed with a potentiostat (Bioanalytical Systems, Inc., Model DCV-4). The electrochemicalmeasurements of charge were done by a digital coulometer (Model 179 Princeton Applied Research digital coulometer) with an absolute accuracy of 0.1% of full scale. A three-electrode electrochemical system was used. The working electrode was a conventional undercut spec-puregraphite electrode as described in the work of Salin and Horlick ( I ) . The electrode was immersed in the analyte solution to a depth of 2.5 cm and provided an exposed surface area of 4.3 cm2to the solution. The reference electrode was a saturated calomel electrode (SCE) and the auxiliary electrode was a platinum electrode. Our preliminary work indicated that the analysis was much more effective after precoating the working electrode with Hg in a separate solution rather than adding mercury(I1) ions to the sample solution prior to analysis. Separate precoating of Hg was accomplished in the following manner. A 40.0-mL solution of mercuric nitrate (2 g of Hg/L) was pipetted into an 80-mL beaker and then deaerated with nitrogen for 1 min. Electrolysis took place for 30 s at -0.9 V vs. the SCE with stirring as nitrogen was passed above the solution surface. Reactions 1 and 2 occur simultaneously. M"+ Hg2++ 2e- = Hg(1) (1) Mn++ ne- = M(Hg)
(2)
represents any trace impurities that might exist in the electrolyte solution during electrolysis. The potentiostatic circuitry was then disconnected and the working and reference electrodes remained connected to a high input impedance voltmeter. The potential changes due to the reaction M(Hg) + n/2Hg2+= M"+ + n /2Hg(l) (3) could be observed. These steps are based upon the potentiometric stripping analysis technique (16-19). Step 3 allows the oxidation of any trace impurities of dilute amalgams by mercury(I1) ions. We used a 1-minoxidation period to remove trace impurities from
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Table I. Relationship between the Preheating Time and the Electrode Temperature Using a &romel-Alumel Thermocouple time, s temp, "C time, s temp, "C 15 30 45
55.0 76.5 88.0
60 75
97.0 103.0
90 105 120 180
108.0 112.0 115.0 119.0
the deposited mercury. The electrode was then rinsed several times with deionized water. Sample electrolysis began after pipetting 40.0 mL of the copper nitrate analytical solution into an 80-mL beaker. The solutions were not buffered, and the pH ranged from 4.7 to 5.7 for solutions ranging from to lo-' M, respectively. After deaeration for 1min, the solution was then electrolyzed as described above for 5.0 min. The electrode was then raised from solution and rinsed while the potential was still applied. The electrode was then demounted from its support and mounted on top of the quartz rod of the DSID apparatus.
0
30 60 Time(seo)
Figure 1. Voltage vs. time for a typical insertion. .
7
L
RESULTS AND DISCUSSION Experimentation indicated that if the Hg layer was not removed prior to insertion into the ICP, it and its amalgamated elements would be violently and rapidly vaporized before the electrode entered the plasma. The Hg was removed from the electrode by inductively heating the electrode when it has been positioned so that its top is 2 cm above the top of the load coil. Forward power was adjusted to 30 W for 1 min and argon was flushed through the plasma gas tubing at 16 L/min during this operation. Electrode temperatures were determined with a chromel-alumel thermocouple. One of the junctions was placed inside the graphite electrode, and the other junction was placed in an ice bath. The thermocouple lead was connected to a toggle switch, the output of which was connected to a strip-chart recorder. This experiment was done under various conditions. During the heating step, the switch was turned off, to prevent loading of the measuring device by the alternating current flowing in the load coil. After the selected time, the plasma gas was turned off and the toggle switch was turned on. The temperature trace showed an exponential decay due to the cooling of the electrode. The maximum value in the temperature curve was considered to be the highest temperature. These values are presented in Table I. Experiments are in progress to study the effect of the preheating process; however our initial investigation indicates that a 1-min preheat at 30 W is optimal. After this step the electrode was lowered to a position 1cm below the tip of the auxiliary plasma-gas tube. For the spectral analysis, the ICP was operated with 16 L/min of plasma gas, 0.41 L/min of auxiliary flow, and a forward power level of 2.0 kW. Oxygen was introduced at a rate of 1.7 L/min in the plasma (outer) gas flow to aid in the heating of the electrode and to ensure that the outer electrode layer was removed. After the plasma has stabilized, approximately 30 s, the electrode was rapidly inserted to ita final position with the head of the electrode level with the top edge of the load coil. Before insertion of the electrode, the signal may be quite large, because of the absence of the central hole in the plasma. On insertion the signal drops dramatically, then a peak appears, and the signal settles to a new base line. This sequence is illustrated in Figure 1. The peak height signal was calculated by extrapolating A (the new base line voltage) and drawing a perpendicular from the peak, B, to meet A. The peak half-width for Cu was approximately 5 s. Cu was selected for our initial experiments because its electrochemicaland spectral properties are predictable. The Cu I 324.754-nm line was observed at a height of 1.5 cm above the load coil. A deposition time of 5.0 min was used for all
0
20
40
60
80
100
120
140
160
180
Time ( m i n )
Flgure 2. Relative signal vs. deposition time for Cu. The solid points
are experimental data points. The circles are predicted normalized signal levels. See text for further explanation. concentrations with the exception of 3.15 pg/mL Cu, which was deposited for only 2.0 min, because it generated an inconveniently large signal. A calibration curve of log signal vs. log concentration was constructed with concentrations ranging from 3.15 to 0.00315 pg/mL. The 3.15 pg/mL solution signal was corrected for the 2.0-min deposition time by multiplying by the factor 5.0/2.0. The log-log curve was linear with a slope of 1.055 and a coefficient of variation (r2value) of 0.9996. The average difference between the regression calibration curve concentration value and the true concentration was 6.0% with a maximum error of 11.5% over the 4 orders of magnitude. The nonlinearity in the calibration curve may be due to changes in pH of the solutions, since this parameter was not controlled. Experiments were under way to determine the cause of nonlinearity; nonetheless, the technique presently is quite usable for many applications. The precision was evaluated by measuring the background-corrected peak heights of five successive determinations at the 0.0315 pg/mL level by two methods. In the f i s t method, peak heights were calculated by extracting the numbers manually from the tracing of the strip chart recorder. The relative standard deviation was 2.5% for this method. In the second method, a BASIC computer program was written on an AIM-65 microcomputer, and peak heights as well as peak areas could be calculated by recording the transient signal with an 8 bit AID converter. The relative standard deviation was 3.1%. The detection limit of the present method depends on deposition time, electrode design, and stirring rate. The dependence between the peak height and the deposition time is illustrated in Figure 2. The peak height appears to increase initially linearly with the deposition time. For deposition times
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above 2 h, deviation from linearity becomes pronounced. With a deposition time of 5 min the detection limit (signa1:noise = 31) was found to be 1.2 ng/mL Cu. With a deposition time of 2 h, the detection limit was found to be 0.07 ng/mL. The detection limit was also determined for aqueous copper solutions, by conventional nebulization using the same demountable torch system, and was found to be 23 ng/mL. Therefore, a significant improvement in detection limit is gained by the present technique. To determine the absolute detection limit of the method, an estimate must be made of the fraction of copper which is deposited on the graphite electrode during the electrolysis. From the coulometric studies, it was concluded that approximately 3.0% of the copper was deposited per 5 min during the electrolysis. This is confirmed by the shape of Figure 2. The solid points are the experimental points and the circles are calculated signals normalized so that the highest experimental and calculated signal value are equal. The fit is significantly better than that of either 2.5 or 3.5%. With a sample volume of 40 mL and a deposition time of 5 min, the absolute detection limit was then determined to be 1.9 ng. Direct addition of standard copper solutions to the graphite electrode was carried out, for comparison purposes, by pipetting 20 p L of sample, using an Eppendorf pipet, followed by vaporization of the solvent at a forward power of 30 W for 1 min with an argon flow rate of 18 L/min. The detection limit (signahoise = 3:l) was found to be 0.16 ng. The order of magnitude difference observed in the absolute detection limit is probably due to a higher atom population in the viewing region when the analyte is expelled from the electrode crater rather than from the electrode exterior surface. The RSD of the electrochemical deposition method was 3%, which is significantly better than the direct liquid additions method where an RSD of 12% was obtained for nine determinations carried out on three standard copper solutions (1260,126,and 12.6 ng). The precision improvement may be due to a more regular deposition of the analyte on the electrode surface than that which was obtained by placing liquids in the crater. This method should be well suited for the analysis of trace and ultratrace elements that are soluble in Hg (20). Sample pretreatment may be required in cases where direct electrodeposition is not possible; however, the method should provide a good simultaneous separation-preconcentration step prior to the ICP analysis. It should also be pointed out that although small sample volumes (in the order of microliters) cannot be analyzed by the present technique, it may be possible to carry out the electrolysis within the graphite electrode, which would serve as the sample container as well as the working electrode. The use of a flow-through deposition cell may (21) allow the determination of some elements, notably cobalt, nickel, manganese, and chromium even though the anodic stripping voltammetry reduction is irreversible. Their electrochemical irreversibility should not affect their spectrochemical analysis. The use of a cylindrical graphite flow-through cell system should make possible the generation
of a conventional annular shaped plasma by the introduction of an injector gas flow through the tubular electrode. This could eliminate the odd transient signal shape (Figure 1) obtained in these experiments and enhance the ease of automating the data processing. An efficient flow-through cell may also result in a significant decrease in detection limits by increasing the deposition rate as well as introducing the analyte atoms into the central viewing zone of the plasma. We also expect that the technique will be quite capable of simultaneous multielement analysis within the constraints of the electrochemical system. There may be additional improvements to both precision and accuracy by using internal standards or ratioing techniques.
ACKNOWLEDGMENT The authors acknowledge the assistance of Jitka Kirchnerova during the conceptual stage of this project and W. C. Purdy during the evaluation phase. Registry No. Cu, 7440-50-8.
LITERATURE CITED ( I ) Salin, E. D.;Horlick, G. Anal. Chem. 1978, 5 1 , 2284-2286. (2) Sommer, D.;Ohls,K. Fresenlus' 2.Anal. Chem. 1980, 9 7 , 304. (3) Horlick, G.; Pettii, W. E.; Todd, B. Presented at Annual Conference of the Spectroscopy Society of Canada, September 26-29, St-Jovite, Quebec, Canada, 1982; Paper No. 10. (4) Kirkbrlght, G. F.; Walton, S.J. Ana&st(London) 1982, 107, 276-281. (5) Volland, G.; Tschopel, P.; Tolg, 0 . Spectrochlm. Acta, Part 6 1981, 36, 901-917. (6) Brandenberger, H. Chlmia 1988, 2 2 , 449. (7) Brandenberger, H.;Bader, H. At. Absorpt. Newsl. W67, 6 , 101. (8) Lund, Walter; Larsen, Bjorn. V. Anal. Chlm. Acta 1974, 7 0 , 299-310. (9) Lund, Walter: Larsen, Bjorn. V. Anal. Chim. Acta 1974, 7 2 , 57-62. 10) Falrless, Charles; Bard, Allen. J. Anal. Left. 1972, 5 , 433-438. 11) Falrless, Charles; Bard, Allen. J. Anal. Chem. 1975, 4 5 , 2289-2291. 12) Jensen, B. 0.;Dolezal, Jan; Langmyhr, F. J. Anal. Chlm. Acta 1974, 7 2 , 245-250. 13) Batley, Graeme E.; Matousek, Jaroslav. P. Anal. Chem. 1977, 4 9 , 203 1-2035. 14) Edwards, Lawrence L.; Oreglonl, Beniamino Anal. Chem. 1975, 4 7 , 23 15-2316. (15) Matuslewicz, H. Presented at Annual Plttsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 9-13, Atlantic City, NJ, 1981; Paper No. 237. (16) Jagner, Daniel; Granell, Anders Anal. Chlm. Acta 1976, 83, 19-26. (17) Jagner, Daniel Anal. Chem. 1078, 5 0 , 1924-1929. (18) Jagner, Daniel; Aren, Kerstin Anal. Chlm. Acta 1978, 100, 375-388. (19) Jagner, Danlel Anal. Chem. 1979, 5 1 , 342-345. (20) Lundell, G. E. F.; Hoffman, J. I. "Outlines of Methods of Chemical Analysls"; Why: New York, 1938; p 94. (21) Long, S. E.; Snook, R. D. Analyst (London) 198% 108, 1331-1338.
Eric D. Salin* Magdi M. Habib Department of Chemistry McGill University 801 Sherbrooke St. West Montreal, Quebec H3A 2K6, Canada RECEIVED for review November 18, 1982. Resubmitted October 7, 1983. Accepted February 3, 1984. This work was made possible by grants from the Natural Sciences and Engineering Research Council of Canada (Grant A1126) and the Government of Quebec (Fonds F.C.A.C. EQ1642).
Standard Addition Method in Flow Injection Analysis with Inductively Coupled Plasma Atomic Emission Spectrometry Sir: The introduction of sample solutions to various detectors by flow injection analysis (FLA) is a valuable approach and is utilized for rapid routine analysis employing diverse instrumental techniques (1,2). Tyson and Idris (3) suggested that flow injection sample introduction offers more than re-
producible passage of sample to instruments. Tyson et al. (3-6) devised a simplified model for the dispersion effects observed in FIA with atomic absorption spectrophotometry (AAS), which enabled the development of a novel flow injection analogue of the standard addition method. This ap-
0 1984 American Chemical Society 0003-2700/84/0356-118S$01.50/0
