Anal. Chem. 1085, 57, 2055-2059 (46) Goldstein. S.A.; Waiters, J. P. Spectrochlm. Acta, Part8 1978, 378, 201. (47) Goldstein, S. A.; Walters, J. P. Spectrochlm. Acta, Part B 1078, 378, oafi
-11.
blhaye, M.; Dhamellncourt, P. J . R a m n spectrosc. 197% 3, 33. Dhamellncourt, P.; Blsson, P. Mlcrosc. Acta 1977, 79, 267. Hieftje, G. M.; Malmstadt, H. V. Anal. Chem. 1088, 4 0 , 1860. Bastiaans, G. J.; Hleftje, G. M. Anal. Chem. 1074, 46, 001. Boss, C. B.; Hieftje, G. M. Anal. Chem. 1970, 51, 1897. (53) Boss, C. B.; Hleftje, 0. M. Appl. Spectrosc. 1978, 32, 377.
(48) (49) (50) (51) (52)
RECEIVED for review November 29, 1984. Resubmitted May
2055
13, 1985. Accepted May 13, 1985. This work was supported by the National Science Foundation through Grants CHE 82-14121 and CHE 83-20053 and bv the Office of Naval Research. Portions of this work were presented at the Tenth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies (Sept 1983, Philadelphia, PA) and the Thirty-fifth Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy (March 1984, Atlantic City, NJ).
Controlled Potential Electrolysis Coupled with a Direct Sample Insertion Device for Multielement Determination of Heavy Metals by Inductively Coupled Plasma Atomic Emission Spectrometry Magdi M. Habib and Eric D. Salin* Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada
The appilcatlon of controlled potential electrolysis with both graphite electrodes and a hanglng mercury drop electrode as a separatlon and preconcentratlon technique for Inductively coupled plasma (ICP) atomlc emission spectrometry uslng the direct sample lnsertlon device (DSID) is described. Heavy metal Ions in aqueous solution are determined. With a deposltlon time of 5 min the detection llmlts under compromise condHions are 2.4, 680, 2.0, 175, 25, and 259 ng/mL for Cu, Pb, Zn, Cd, Ni, and Co, respectively. A determination of Cu at the 63 ng/mL level In artlflclai seawater (3.5% salinity) was made with a 4% error.
The application of inductively coupled plasma atomic emission spectrometry (ICP-AES) to the simultaneous determination of major and minor and trace level elements in various matrices has been well documented (1-5). Pneumatic nebulization appears to be the most popular method of sample introduction although the sensitivity attainable is not sufficient for the ICP analysis of many elements which are present in the nanogram per gram range (6). A number of studies have concentrated on developing methods for isolating trace elements from complex matrices including coprecipitation (7), chelation (8, 9), chromatography (IO),and conversion into hydrides (11). In all of these isolation methods large volumes of additional chemicals are brought into contact with the samples and thus may introduce contaminating or interfering species. In addition, some of these techniques are time-consuming and tedious. Ultrasonic nebulizers have demonstrated improvements in working range by factors of 1.1-12 (12-14) in various matrices; however, considerable question still exists about the general reliability and freedom from interferences of these devices (15). Separation of heavy metal ions from various matrices by controlled potential electrolysis is often a very useful method in trace analysis (16-19). By the use of mercury electrodes, a number of heavy metals can be deposited even from acidic aqueous solution because of the broad cathodic potential range 0003-2700/85/0357-2055$01.50/0
(20). This separation technique has found wide application in the field of atomic spectrometry. Higher sensitivity than conventional techniques was obtained because of the sample preconcentration during electrolysis and the potential power of the technique to separate trace elements from complex interfering matrices. Electrolysis has been performed on metal wires (21-25), carbon rods (26, 2 3 , hanging mercury drop electrodes (28, 29), and tubular pyrolytic graphite-coated electrodes (30)for spectrochemical applications. The technique has also been applied to flame AA, using a thin film of mercury deposited on a wax-impregnated graphite rod (31), to a direct current arc using a hanging mercury drop electrode (HMDE) (32) and to a helium microwave induced plasma (He-MIP) (33). A wall-jet electrochemical cell for preconcentration of trace metals from flowing streams prior to their determination by ICP with conventional pneumatic nebulization was also described (34). We previously reported preliminary results on controlled potential electrolysis coupled with ICP-AES for the determination of copper in aqueous solutions (35). The method involved the electrodeposition of copper from an aqueous solution of copper nitrate onto a piece of spectrographic graphite electrode previously coated with mercury. After the completion of the electrolysis the electrode was demounted from its holder and mounted on the top of the quartz rod of the direct sample insertion device (DSID) (36). The electrode was then inductively dried for 1 min at a forward power of 30 W prior to ICP analysis. This study is an evaluation of this technique for the simultaneous determination of heavy metals (Cu, Pb, Cd, Zn, Ni, and Co) in aqueous solution as well as an evaluation of the performance with a difficult sample matrix.
EXPERIMENTAL SECTION Table I lists the principal components of the instrumentation. Figure 1 is an illustration of the electrochemical cell. The cell body (a) is made from Teflon and has a volume capacity of 40 mL. The cell lid (b) contains three holes to fit the reference electrode (c),the working electrode (d), and the auxiliary electrode (e). The lid also contains two small holes (f) for nitrogen input 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
Table I. Experimental System component
description and supplier
inductively coupled plasma system
Model HFP-2500 D RF generator with a rated power output of 2500 W, and a crystal controlled frequency of 27.12 MHz, a Model AMN-2500 E automatching network, and a water-cooled load coil made from 3 turns of l/s-in. copper tubing (Plasma Therm, Inc., Kresson, NJ) torch design and sample introduction system demountable torch designed for direct sample insertion device (DSID) spectrometer Jarrell Ash, 0.5-m Ebert monochromator with rectangular slits (25 pm), with a 1180 grooves/mm plane grating replica blazed for 190 nm and with a spectral band-pass of 0.04 nm plasma imaging spherical plano-convex lens (1:l image) having a diameter of 5 cm and a focal length of 20 cm photomultiplier tube 1P28 photomultiplier tube operated at 600 V amplifier AD (Analog Devices) 542 KH Model operational amplifier to convert from PMT to voltage with a time constant of 0.5 ns; an AD 757 P Model logarithmic amplifier used in conjunction with the operational amplifier as a second stage for voltage conversion and data acquisition recorder Heath Schlumberger Model strip chart recorder Model SR-204 with a 0.2-9 full scale deflection data acquisition system a 12-bit analog-to-digital converter, AD Model 8074 KD, with a 10 V full range, interfaced to an AIM-65 microprocessor based system expanded to 24K RAM graphite rods Ultra Carbon Corp., Bay City, MI, lot no. 120481, 1/4-in.necked crater, part no. HS-5 ion exchange resin Branstead, Ultra Pure (mixed bed), D8902 digital coulometer Model 179 digital coulometer with an absolute accuracy of 0.1% of full scale hanging mercury drop electrode Model 9323 (Princeton Applied Research) potentiostat laboratory potentiostat built from operational amplifiers AD Model 542 KH
c
I
h
I
Flgure 1. Electrochemical cell. c is a saturated calomel reference electrode and e Is a platinum wire auxiliary electrode.
and (8) exhaust. The solution was stirred with a magnetic stirrer (h) and a stirring bar 6). The electrode holder (Figure 2) was constructed from a Teflon cylinder (a) threaded at its end to accept a threaded Teflon ring (d) with three nylon screws located at 120° (e). Electrical contact to the electrode (0 is made through a spring (c) to maintain a constant coptact between the electrode and the binding post (b). This configuration provided good electrical contact, a tight electrode fit during electrolysis, easy removal with the Teflon ring after electrolysis, and convenient mounting on the top of the quartz rod of the DSID apparatus. Staridard solutions were prepared from reagent-grade nitrate salts. Water was distilled and then deionized with a Branstead Ultra Pure (Mixed bed) ion exchange resin. The stock sample M of each of the metal ions in 0.1 M solution was made to perchloric acid. Dilutions were made before each set of experiments with 0.1 M perchloric acid. The mercury stock solution M) was prepared from reagent grade mercuric nitrate dissolved in deionized water. A synthetic seawater preparation consisted of 30.2 g/L NaCl and 3.8 g/L Na2S04corresponding to 3.5% salinity seawater (37, 38). All solutions were stored in polyethylene bottles. In most of the experiments two procedures were followed. The first method (predeposition) involved a precoating of mercury at -0.900 V vs. SCE from a 25-mL solution of M Hg onto a piece of graphite electrode for 30 s, unless otherwise stated, followed by electrolysis of the sample solution at -0,900 V vs. SCE for 5 min. In the second method (codeposition), 0.3 mL of lo-' M Hg was added to a 25-mL aliquot of the sample, and electrolysis took place at -1.300 V vs. SCE for 5 min. In both methods the sample solution was dearated with nitrogen for 5 min prior to electrolysis, and nitrogen was passed over the solution during
Flgure 2. Electrode holder. See text for discussion.
electrolysis. The type of electrode used as well as its exposed surface area was varied in the experiments. The analysis of the multielement solution mixture was performed by the codeposition M of each of the method on a standard solution containing metal ions. For the system characterization as well as the elemental analysis in a high salt content matrix, copper was used, and the analysis was performed by the predeposition method. Electrodes were dried by positioning the electrode so that its top was 2 cm above the top of the load coil. Forward power was adjusted to 30 W for 1 min, unless otherwise stated, and argon was flushed through the plasma gas tubing at 16 L/min. The electrode temperature at this stage was determined with a chromel-alumel thermocouple. The electrode was then lowered to a height 1cm below the auxiliary tube. The ICP was initiated and operated at a forward power of 2.0 kW with an argon plasma gas (outer) flow rate of 18 L/min and an auxiliary gas flow rate of 0.2 L/min. Normally oxygen was introduced into the plasma gas at a flow rate of 1.4 L/min. After the plasma stabilized, in approximately 30 s, the electrode was inserted rapidly to a position 5 mm above the top of the load coil. Spectral observation were made at a height of 15 mm above the load coil. Experiments using the hanging mercury drop electrode (HMDE) were carried out in the following manner. The HMDE provided a mercury drop (MD) with an area of 0.03 cm2 and a weight of 0.007 g. Electrolysis was performed on 25 mL of the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
I
6.0
I
2057
-*
5.0
* 3 (d
U M .*
4.0
3.0 0
1
2
3
4
5
6
T I M E , min
2.0
Emission signal for copper vs. drying time using an LTIC
Flgure 4.
5 .*
electrode.
i W
0 0
1
2
3
4
5
T I M E , rnin
Emission signal for copper and stripping signal for mercury vs. drying time. See text for discussion. Figure 3.
analytical copper solution at -0.900 V vs. SCE for 5 min. After electrolysie, the whole HMDE assembly was raised from solution, followed by rinsing of the mercury drop with approximately 1mL of deionized water from a dropper. The mercury drop was then transferred directly to the crater of a conventional dc arc graphite electrode and vaporized completely by thermally heating the electrode in an oven at approximately 120 O C for 20 min. The electrode was then mounted in the DSID apparatus for the ICP spectral analysis and treated with the normal DSID procedure. Data acquisition began when the observed spectrometric signal dropped by a certain fraction after electrode insertion. Twelve bit daCa points were collected at a rate of 33 Hz and stored in the memory of an AIM-65 computer. The system has been discussed previously (39). A BASIC program allows the calculation of peak heights and peak areas.
RESULTS AND DISCUSSION Electrode drying prior to the ICP analysis was found to be essential to vaporize excess mercury and water which otherwise could result in sputtering and sample loss during insertion into the plasma. The electrode was heated inductively (35, 36). The variation of the electrode temperature with drying time at a forward power of 30 W was found to follow the relationship
T = TL(l- e-t/T) where Tis the electrode temperature after a drying time t and TLis the limiting temperature (120 "C) at this forward power. The time constant (7)was found to be 30 s at a forward power of 30 W. Experimental optimization of electrode drying time prior to the ICP analysis was performed by studying the effect of drying time (min) on the copper emission signal. In this experiment, mercury was deposited from a solution of mercuric nitrate M Hg) for 30 s onto a conventional graphite rod electrode, and then a solution of copper nitrate M Cu) was electrolyzed for 1min. Electrodes were then inductively dried at a forward power of 30 W over various time intervals, and the copper emission signal was then observed using the normal insertion technique. Figure 9 (curve A) shows the variation in the copper signal with drying time. The initial increase in the copper emission signal to a maximum value is due to the vaporization of water and mercury from the electrode ~urface. Since elemental diffusion occurs only at very high temperatures (401,it is unlikely that the subsequent decrease is due to copper diffusion. The completion of the mercury vaporization step under these conditions was then studied. This was done by obtaining different anodic stripping voltammograms for mercury after drying the electrode in-
0
Flgure 5.
1
2 3 LOG (C)
Signal-to-noiseratio vs. logarlthmic concentration for copper.
ductively over various time intervals with a forward power of 30 W. The anodic potential was initiated at +0.200 V, and the scan was carried out at 2 mV/s. Figure 3 (curve B) shows the anodic peak voltage for mercury with drying time. This plot indicates that a small amount of mercury was present even at longer drying times. The graphite electrode used in these experiments is porous and mercury can diffuse by capillary action into the pores of the graphite. As the mercury vaporizes, its volume is reduced so that diffusion of the amalgam can take place. Two experiments were performed to test the theory that the drop in signal at longer drying time was due to the diffusion of the amalgam. The first experiment involved electrodeposition of copper for 1 min directly on graphite and observing the effect of drying time on the copper emission signal (curve C). The signal increases to a maximum limiting value without any observable drop at longer drying times. The second experiment was performed by electrodepositing copper on a nonporous low temperature isotropic carbon electrode (LTIC) coated with mercury. Again, a plateau was reached with no appreciable drop in the signal (Figure 4). We have suggested previously (35) that the nonlinearity (log-log slope of 1.05, and r = 0.9998) observed in the calibration curve for copper by this technique was due to the variation in the standard solution pH over the range of concentrations studied. In these experiments all standards (10-6-104 M) were prepared in 0.1 M perchloric acid (pH 1.72), and the calibration curve was again constructed over the range (0.063-6.3 MgImL). The log-log plot was linear with a slope of 1.003 and a regression coefficient ( r ) of 0.999934. This indicates that a constant pH is of prime importance when using this method. The signal-to-noise ratio (SIN)was studied over a copper concentration range of 0.0063 Kg/mL to 0.315 pg/mL with the predeposition method. Figure 5 shows a plot of S I N vs. concentration. Several regions are observed. Close to the
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
Table 11. Compromise Condition Detection Limits for 5 rnin
Electrolysis
"
O
c
detection limit, ng/mL (Sf N = element
wavelength, nm
3)
cu
324.7 405.8 206.2 361.1 352.5 345.3
2.4 680 2.0 175 25 259
Pb
Zn Cd Ni
co
o
20
40
60
a0
io0 120
T I M E , min
detection limit (2.4 ng/mL) the system is background noise limited. Improvement in the performance of S I N in this region is possible with longer analyte deposition time and larger electrode surface area as demonstrated in our previous work (35) where an improvement in the detection limit by a factor of 17 for a 120-min deposition was demonstrated for copper. Improvements may also be possible with a high-purity graphite electrode as we observed some copper emission from the electrode in the blank signal. In the higher concentration region, the plot plateaus indicating an analyte signal flicker noise limited situation. In this region the reproducibility of the sample introduction and sample deposition techniques is limiting. We consider the 3% relative standard deviation to be a good level of performance for a nonautomated transient sample introduction method. Heavy Metals Analysis. Two groups of elements were examined. The first group includes the volatile elements Cd and Zn, and the second group comprises the elements Cu, Pb, Ni, and Co. In the determination of cadmium and zinc, two adjacent emission peak signals of nearly the same magnitude were observed. The possibility of spectral overlap from other elements in the sample solution mixture was eliminated when a single element solution for Zn was analyzed and the two peaks were reproduced. Since Cd and Zn form oxides when burned in air and their oxides sublime without decomposition (41),the possibility of oxide formation is probable due to the oxygen introduced into the plasma gas. A single, well-defined emission peak was obtained when the determination was carried out in an argon plasma atmosphere free from oxygen. The SIB of the first peak in this case is 20 times higher than that in the presence of oxygen. Therefore, the detection limits for Cd and Zn are reported for an oxygen-free argon plasma. In the analysis of Cu, Pb, Ni, and Co, the presence of oxygen was found to be advantageous for their determinations, bemuse oxygen increased the graphite electrode heating rate and allowed the element to be vaporized quickly from the surface. In the determination of cobalt, a small peak appeared prior to the main peak. These two peaks were not related to the electrochemical irreversibility of cobalt on mercury. A single element solution of cobalt was pipetted directly into an electrode crater and inductively dried. Two peaks were present. When the analysis was performed in the absence of oxygen in the plasma gas, the second peak became 2.5 times broader compared to the first one. The detection limit for cobalt by this technqiue was determined by considering only the second peak emission since the first peak was much smaller at lower concentrations (ratio of 123). The detection limits for the elements studied with a 5-min deposition time are given in Table 11. The detection limit was calculated by applying the definition "The concentration which would provide a signal-to-noise ratio = 3." Lower relative and absolute detection limits are possible by increasing the electrode surface area and the deposition time (35). Effect of Deposition Time. The dependence of detection limit for copper on deposition time was demonstrated previously (35). An improvement by a factor of 17 was achieved
Flgure 6. Signal (arbitrary units) vs. deposition time text for discussion.
for nickel. See
by increasing the deposition time from 5 min to 120 min. The electrolysis efficiency with a 5-min deposition was 3% when determined coulometrically. Even though nickel is irreversibly reduced on mercury, the sensitivity of its determination can be increased by lengthening the electrolytic deposition time. The variation of the peak height signal with deposition time for 59 ng/mL Ni is shown in Figure 6. The solid points are the average of two experimental values at each time and the circles are calculated signals, for a 6% electrolysis efficiency with a 5-min deposition time, normalized, so that the highest experimental and calculated signal value are equal. An improvement in the nickel detection limit (5-min deposition) by a factor of 13 is possible after a 120-min electrolysis. Matrix Effects. The practical utility of the electrodeposition technique was tested with the determination of copper in a synthetic seawater sample. The analysis was performed by the method of standard addition. Sample and standards were diluted with 0.1 M perchloric acid, and electrolysis was performed by predeposition. Since small amounts of NaCl remaining on the graphite electrode after deposition were sufficient to cause stray light spectral interferences in the ICP determination, an electrode washing step was necessary. A 3-min wash of the electrode with 0.1 M perchloric acid while the potential was still applied, interupted each 1min with a fresh washing solution, was found to be satisfactory. All measurements were made in duplicate, and the best fit of the standard addition curve was calculated by the method of least squares. The curve was linear with a regression coefficient ( r ) of 0.9994. The determined value was within 4% of the correct value (63 ng/mL) of the artificial sample. Hanging Mercury Drop Comparison. Electrolytic reduction of metal ions onto a HMDE was also investigated as another sampling technique. The detection limit wm obtained from the slope of two standards analytical copper solutions (0.63 and 6.3 pg/mL) and the blank noise was 104 ng/mL. The relative standard deviation of five replicate measurements at the 6.3 pg/mL level was 6%. The higher detection limit observed with this technique is due to the smaller electrode surface area (0.03 cm2) compared to a surface area of 4.3 cm2 using electrodeposition on a mercury plated graphite electrodes and the slower stirring rate which was required using a HMDE because of the tendency of the drop to fall during electrolysis. While longer deposition time could improve the sensitivity of the technique, metallic diffusion into the glass capillary of the mercury electrode would result in a partial loss of the metal electrolyzed. Registry No. H20, 7732-18-5; Cu, 7440-50-8; Pb, 7439-92-1; Sn, 7440-31-5; Cd,7440-43-9; Ni, 7440-02-0; Co, 7440-48-4. LITERATURE CITED (1) Nygaard, D. D. Anal. Chem. 1979, 5 7 , 881-884. (2) Ward, A. F.; Marciello, L. F. And. Chern. 1979, 51, 2264-2272. (3) Mc Quaker, N. R.; Kluckner, P. D.; Chang. G. N. Anal. Chern. 1979,
51.888-ag5. (4) Nadkarni, R. A. Anal. Chem. 1980, 52, 929-935.
Anal. Chem. 1985, 57, 2059-2064 (5) Mc Laren, J. W.; Berman, S. S.; Boyko, V. J.; Russel, D. S. Anal. Chem. 1981, 5 3 , 1802-1806. (6) Boumans, P. W. J. M.; Barnes, R. M. ICP Inf. News/., 1978, 3 , 445. (7) Hlrarlde, M.; Ito, T.; Baba, M.; Kawaguchl, H.; Mlzulke, A. Anal. Chem. 1980, 5 2 , 804-807. (8) Barnes, R. M.; Genna, J. S. Anal. Chem. 1979, 51, 1065-1069. (9) Mlyazak, A.; Barnes, R. M. Anal. Chem. 1981, 5 3 , 364-365. (10) Carnahan. J. W.: Mulllaan. K. J.: Caruso, J. A. Anal. Chlm. Acta 1981,
(12) Boumans, P. W. J. M.; de Boer, F. J. Spectrochlm. Acta, Part 6 ‘ 3 0 , 309-334. (13) Taylor, C. E.; Floyd, T. L. Appl. Spectrosc. 1981, 3 5 , 408-413. (14) Berman, S. S.; Mc Laren, J. W.; Willie, S. N. Anal. Chem. 1980, 5 2 , 488-492. (15) Boumans, P. W. J. M.; de Boer, F. J. Spectrochlm. Acta, Part 6 1978, 31, 355-375. (16) Llngane, J. J. “Electroanalytical Chemistry”, 2nd ed.; Intersclence: New York, 1958;pp 416-433. (17) Tanaka, N.; Kolthoff, 1. M.; Elving, P. J. “Treatlse on Analytical Chemistry”; Intersclence: New York, 1963;Part 1, Vol. 4, p 2417. (18) Ashley, S. E. Q. Anal. Chem. 1950, 2 2 , 1379-1385. (19) Rogers, L. B. Anal. Chem. 1950, 2 2 , 1386-1367. (20) Lundell, G. E. F.; Hoffman, J. I.“Outllnes of Methods of Chemical Analysls”; Wiley: New York, 1938;p 94. (21) Brandenberger, H. Chlmia 1988, 2 2 , 449. (22) Brandenberger, H.; Bader, H. At. Absorp. News/. 1987, 6 , 101. (23) Lund, Waiter; Larsen, Bjorn V. Anal. Chim. Acta 1974, 7 0 , 299-310. (24) Lund, Walter; Larsen, BJorn, V. Anal. Chim. Acta 1974, 72, 57-62. (25) Newton, M. P.; Davis, D. G. Anal. Chem. 1975, 47, 2003-2009.
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(26) Fairless, Charles; Bard, Allen J. Anal. Left. 1972, 5 , 433-438. (27) Batley, Graeme E.; Matousek, Jaroslav P. Anal. Chem. 1980, 5 2 , 1570-1 574. (28) Fairless, Charles; Bard, Allen J. Anal. Chem. 1973, 45, 2289-2291. Dolezal, Jan; Langmyhr, F. J. Anal. Chim. Acta 1974, (29) Jensen, B. 0.; 72, 245-250. (30) Batley, Graeme E.; Matousek, Jaroslav P. Anal. Chem. 1978, 49, 2031-2035. (31) Edwards, Lawrence L.; Oregioni, Beniamino Anal. Chem. 1975, 47, 2315-2316. (32) Matuslewicz, H. Presented at the Annual Plttsburgh Conference on
Analytical Chemistry and Applied Spectroscopy. Atlantic Clty, NJ, March 9-13, 1981;Paper 237. (33) Volland, 0.; Tschopel, P.; Tolg, G. Spectrochim. Acta, Part 8 1981,
368,901-917. (34) Long, S. E.; Snook, R. D. Analyst (London) 1983, 108, 1331-1338. (35) Hablb, M. M.; Salin, E. D. Anal. Chem. 1984, 5 6 , 1186-1188. (36) Salln, E. D.; Horlick, G. Anal. Chem. 1979, 51, 2284-2286. (37) Segar, D. A.; Gonzelez, J. G. Anal. Chim. Acta 1972, 5 8 , 7-14. (38) Vonarx, W. S. “Introduction to Physical Oceanography”; AddisonWesley: Readlng, MA, 1962;pp 1-120. 805-809. R. L.; McGeorge, S. W.; Salin, E. D. Talanta 1983, 30 (IO), (39) Sing, (40) Sturgeon, R. E.; Chakrabartl, C. L. Anal. Chem. 1977, 4 9 , 90-97. (41) Cotton, F. A.; Wilkinson, 0.“Advanced Inorganic Chemlstry”, 2nd ed.; Wlley-Interscience: New York, 1966;pp 600-604.
RECEIVED for review October 29,1984. Resubmitted April 17, 1985. Accepted April 17, 1985.
Computer-Controlled Graphite Cup Direct Insertion Device for Direct Analysis of Plant Samples by Inductively Coupled Plasma Atomic Emission Spectrometry Mohammad Abdullah’ and Hiroki Haraguchi* Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
A computer-controlled graphite cup direct insertion device which allows ashing and atomization of 1-5 mg of powdered plant samples In an Inductively coupled plasma has been developed for direct solid sample analysis. The use of a graphite lid on the top of the cup prevented loss of sample by spitting and allowed introduction of atomic vapor through a central hole Into the plasma at the atomization stage. The system was optimized by a simplex method. The effects of ashlng time and of Ca, K, Mg, and Na on the analyticai emissions of other elements were examlned. The absolute detection ilmlts for 18 elements in a cellulose matrix were at the nanogram level with better than 13% precision. The system was then tested wlth a series of standard reference plant materials. The analytical results of NBS Orchard Leaves (SRM 1571) showed good agreement wlth the certlfled vaiues.
Atomic emission spectrometry with an inductively coupled plasma (ICP) as an excitation source has now been well established as a powerful analytical tool for multielement analysis of solution samples. But the conventional sample introduction utilizing a pneumatic nebulization technique to On leave f r o m Bangladesh A t o m i c Energy Commission, A t o m i c Energy Center, Ramna, Dhaka, Bangladesh.
generate and introduce the aerosol into the plasma has some limitations. For example, the sample should be a solution with less than 3% total solid content, and a solid sample must be converted to a solution by an appropriate digestion procedure which may lead to loss of some volatile elements or contamination from the reagents and glassware used. Moreover, dilution may reduce the detectability of the trace elements. Therefore, it is highly desirable to be able to introduce solids directly into the plasma in order to overcome the abovementioned limitations and also to extend the analytical capabilities of ICP-AES. In this context several approaches have been made including laser ablation (1,2), spark vaporization (3, 4), electrothermal atomization (5-71, and ultrasonic atomizing (8). In these sample introduction devices the generated aerosol was carried into the plasma with a stream of carrier gas through tubing in which transport losses might occur. To avoid transport losses, it is also desirable to introduce the solid sample directly into the plasma. Injection of coal fly ash powder with argon gas (9) has drawn little attention due to interrupted plasma operation. Direct introduction of solid sample contained in a graphite electrode via an injector tube of a Fassel type torch into a 2.5-kW Ar-ICP was first reported by Salin and Horlick (10). Salin and Habib (11)also preconcentrated copper electrochemically by cathodic deposition onto a graphite electrode before insertion into the ICP. They ignited the plasma on each insertion. Sommer and 0% (12) reported continuous operation to introduce the samples contained in a graphite crucible into
0003-2700/85/0357-2059$01.50/00 1985 Amerlcan Chemical Society
