Anal. Chem. 1993, 65, 1207-1272
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Chemical Modification by Ascorbic Acid and Oxalic Acid in Graphite Furnace Atomic Absorption Spectrometry John P. Byme,+Chuni L. Chakrabarti,'**Glen F. R. GilchristJ Marc M. Lamoureux,t and Peter Bertelsj Department of Chemistry, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia, and Centre for Analytical and Environmental Chemistry, Department of Chemistry, Carleton University, Ottawa, Ontario K1S 5B6, Canada
The effects of addition of ascorbic and oxalic acids on the atomic absorption signal for lead are explained by a gas-phase thermodynamic equilibrium model. The gas-phase composition of the graphite furnace was determined as samples containing ascorbic or oxalic acid were pyrolyzed. Hydrogen and carbon monoxide were identified as two of the major pyrolysis products. The amounts of these reducing gases, formed in the atomization cycle, varied with the modifier used, the charring temperature employed, and the condition of the surface of the pyrolytically coated graphite tube. The extent of the shift in the appearance temperature showed correlation with variations in concentrations of hydrogen and carbon monoxide produced by pyrolysis. Results for oxalic acid clearly showed that the appearance temperature shifts for lead in pyrolytically coated tubes cannot be explained by a surface reaction mechanism. INTRODUCTION The effect of ascorbic acid on the graphite furnace atomic absorption signalfor lead has been reported in the literature.14 In the presence of ascorbic acid, the absorbance signal is shifted to lower appearance temperatures,',4-6 and double peaks may appear.133 Gilchrist et a l . 5 have shown that pyrolysis of ascorbic acid in the graphite furnace produces significant amounts (up to 1% v/v) of hydrogen and carbon monoxide and have used a gas-phase equilibrium model' to explain the shift in the appearance temperature. They7 attributed this shift to the effect of these reducing gases on the oxygen concentration in the furnace during the atomization step. As the oxygen concentration is reduced, the equilibrium dissociation reaction of gaseous lead oxide
is shifted to the right, resulting in formation of lead atoms at lower temperature. Values for the shift in the appearance temperature, calculated from this model, correlated well with the experimental1 observed shifts in the appearance temperature.
* To whom all correspondence should be addressed.
' University of Technology, Sydney. 1
Carleton University.
(1) McLaren, J. W.; Wheeler, R. C. Analyst 1977, 102, 542-546. (2) Regan, J. T. G.;Warren, J. A t . Absorpt. Newsl. 1978, 17, 89-90. (3) Tominaga, M.; Umezaki, Y. Anal. Chim. Acta 1982,139,279-285. (4) Sturgeon, R. E.; Berman, S. S. Anal. Chem. 1985,57, 1268-1275. ( 5 ) Gilchrist, G. F. R.; Chakrabarti, C. L.; Byrne, J. P. J. Anal. A t . Spectrom. 1989, 4 , 533-538. (6) Gilchrist, G.F. R.; Chakrabarti,C. L.; Byrne, J. P.; Lamoureux, M. J. Anal. At. Spectrom. 1990, 5, 175-181. (7) Byrne, J. P.; Chakrabarti, C. L.; Chang, S.B.; Tan, C. K.; Delgado, A. H. Fresenius' 2.Anal. Chem. 1986, 324, 448-455. 0003-2700/93/0365-1267$04.00/0
Recently, Imai and Hayashis reported similar shifts in the lead appearance temperature using 1% (m/v) ascorbic acid, but they could only observe these shifts when nonpyrolytically coated graphite (NPG) tubes were used. The authors questioned the validity of the gas-phase equilibrium model5 and explained their observations in terms of surface effects arising from the carbonaceous char; deposited on the tube wall during the pyrolysis of ascorbic acid. They claimed that this resulted in graphite sites pf different activity-one type a pyrolytic graphite site produced by the ascorbic acid char, and the other type a nonpyrolytic graphite site of the tube wall itself. The shift in the appearance temperature then occurred as the lead atomization process shifted from one type of site to the other. Their main evidence for this mechanism was the absence of any shift in the appearance temperature when lead was atomized in a pyrolytic graphite (PG) coated tube, in which ascorbic acid had been previously pyrolyzed; they concluded that the absence of any shift in the appearance temperature was because the addition of pyrolytic graphite (PC) sites by pyrolysis of ascorbic acid to a pyrolytically coated graphite tube should have no effect. The validity of the above conclusions hinges on this single experimental observation. However, the above conclusion and their consequent rejection of the gas-phase equilibrium model5 are questionable since shifts in appearance temperature with ascorbic acid have been observed in PC tubes, as reported below. Bothsturgeon and Berman4and Gilchrist et al.596observed shifts in the appearance temperature of lead and other elements in PC tubes, but only when the lead solution and ascorbic acid were pyrolyzed together in the graphite furnace. The reason Imai and Hayashis observed no shift in a PC tube was because, in their experiments, the lead was injected into the graphite furnace only after the ascorbic acid had been already pyrolyzed in it by repeated atomization cycles. Under these conditions, the products of pyrolysis of ascorbic acid, H2 and CO gases, had been completely expelled from the graphite furnace before the lead solution was injected into the graphite furnace. All that remained when the lead solution was added was the surface char, and as the gas-phase equilibrium model5 would predict, this was ineffective in causing any shift. This point was confirmed by Gilchrist et al.? who showed that the shift in the appearance temperature was only observed when (i) the ascorbic acid and the lead solution were injected into the furnace simultaneously and (ii)the charring temperature was kept below -650 OC. 'Above this temperature no shift was observed as the pyrolysis gases were expelled out of the furnace prior to atomization. The above arguments suggest that the effect of ascorbic acid on the atomization of lead cannot be explained by a surface char mechanism; moreover, the effect is dependent on factors such as the charring temperature, the tube type, and other experimental conditions. The objective of this (8) Imai, S.;Hayashi, Y. Anal. Chem. 1991, 63, 772-775.
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paper is to investigate these factors, using both ascorbic acid and oxalic acid as chemical modifiers. Oxalic acid was chosen because it is kn0~113~9 to produce effects similar to ascorbic acid but pyrolyzes without leaving a surface carbonaceous char.9 This would permit a clear distinction between the gas-phase equilibrium model5 and the surface mechanism suggested by Imai and Hayashi.8 This study reports the shift in the appearance temperature for lead, with both ascorbic acid and oxalic acid, in new and used PC tubes. The concentrations of the gaseous pyrolysisproducts from ascorbic acid and oxalic acid at different temperatures, and the nature of the graphite tube surface before and after such pyrolysis, are also reported.
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(9) Hydes,
D.J. Anal. Chem. 1980,52, 959.
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EXPERIMENTAL SECTION Apparatus. An atomic adsorption spectrophotometer, Model AAS (Varian Techtron) fitted with a Hamamatsu R213 photomultiplier tube and a heated graphite atomizer (HGA), Model 76B (Perkin-Elmer Corp.),and pyrolyticallycoated graphite tubes (Perkin-Elmer,part No. 091504) were used together with a lead hollow cathode lamp modulated at 843 Hz and a laboratorymade, synchronous detection system (lock-in amplifier) with a time constant of 8 ms. The graphite furnace AAS was performed by atomizing the samples from the graphite tube wall. The scanning electron micrographs were of the surface of pyrolytic graphite platforms (Perkin-Elmerpart no. B012-1091). Analysis of the furnace gases was performed with a gas chromatograph (GC),Model Sigma-300(Perkin-Elmer Corp.). The GC wm fitted with a stainless-steel column, 72 in. long, 0.125-in. outside diameter, packed with 100-120 mesh Carbosieve S-I1 (Supelco, Bellefonte, PA) and with a thermal conductivity detector. Hydrogen, carbon monoxide, methane, and carbon dioxide gases were separated under isothermal conditions at 100 "C. The following carrier gases were used: argon for the determination of hydrogen and helium for the determination of all other gases. Gas samples were collected using a laboratory-made, computerized sampling system described by us in an earlier paper.5 For collection of gas samples the right-side quartz window of the heated graphite atomizer was replaced with a high-temperature siliconrubber septum through which sampleswere injected. The left-side window mount was removed and replaced with a stainless-steel,water-cooled gas extraction head which tapered from 0.6 (the inside diameter of the furnace) to 0.09 cm (the inside diameter of the gas transfer line). The injection hole of the graphite furnace was sealed with a plug of solid pyrolytic graphite. Prior to the start of the atomization cycle, a trigger pulse would activate a timing program on an Apple IIe computer (AppleComputer, Cupertino, CA). After a user-selected interval, the computer would send a trigger pulse to an optically isolated relay (5mV dc, 120V ac), which would in turn activate a solenoid located at the input terminals of an air-actuated sampling valve (Valco Instruments, Houston, TX). By changing the timer interval, the user could obtain time-resolved samples of the furnace gas. Reagents and Sampling Procedure. A stock solution of lead (taken as the nitrate) was dissolved in 10% (v/v) HNOs (Ultrex) and diluted with ultrapure water. Test solutions were made up daily by serial dilution of the stock solution with ultrapure water which was obtained direct from a Millipore water purification system, Model Milli-Q2 (Millipore Corp.). All test (Ultrex).For solutions were prepared to contain 1% (v/v)"03 measurement of the Pb atomic absorption (AA) signals, a 10-pL aliquot of the test solution was deposited with an Eppendorf pipet fitted with disposable plastic tips. Aqueous solutions of ascorbic acid, 1% (m/v), and oxalic acid, 1%(m/v), were used. All test solutions were dried inside the furnace at 380 K for 30 s, charred at either 540 or 680 K for 30 s, and atomized using the maximum power heating mode. Gases and Gas Flow Control. Ar gas of 99.9998% purity was used as internal purge gas and as sheath gas. A Matheson gas proportioner, Model 7351, fitted with two Model 610A rotameters, was used to set manually the purge gas flow to 50 mL
C
Flgure 1. Absorbance-time profiles for 1 ng of Pb (taken as the nitrate),atomized In a used PC tube in test solutlons contalnlng(A) 1% (v/v) nitric acid alone or with (e) 1% (m/v) oxalic acid or (C) 1% (m/v) ascorbic acid. Charring temperature 680 K. OSO
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Time, rns Figure 2. Absorbance-time profiles for 1 ng of Pb (taken as the nitrate), atomized in a new PC tube in test solutions Containing (A) 1 % (v/v) nitric acid alone or with (B) 1 % (m/v) oxalic acid or (C) 1 % (m/v) ascorbic acid. Charring temperature 680 K.
mind, which was maintained throughout all heating cycles. GC standards were prepared by dilution of commercially available certified standards, using a rotameter/mixer. The following detection limits were obtained using the above gas chromatograph: hydrogen, 20 ppm; carbon monoxide, 100ppm; methane, 100 ppm; carbon dioxide, 500 ppm. Measurement of Graphite Furnace Temperature. The temperature of the inside surface of the graphite tube directly below the sample injectionhole was measured with an Ircon Model 300 automatic optical pyrometer. The voltage of the pyrometer was recorded with a digital oscilloscope, Model 4094 (Nicolet Instrument Corp., Madison, WI). The output voltage of the pyrometer was converted into temperature using the calibration table provided by the manufacturer.
RESULTS AND DISCUSSION The effects of chemical modifiers, ascorbic acid and oxalic acid, on the appearance temperature of the lead atomic absorption signal were investigated by atomizing samples of lead prepared in aqueous solutions containing either 1% ! (m/ v) ascorbic acid or oxalic acid. T o determine the influence of graphite tube condition (used or unused) on the appearance temperature, these samples were atomized in both new (
