Anal. Chem. 1983, 5 5 ,
may have to be used to recover the shorter chain alcohols. Nevertheless, long chain alcohols 2C8 should be quantitatively collected and recovered by using the above procedure. The method has been tested in the field and appears to produce satisfactory results. The methodology given here can be applied to evolve a personal collection, recovery, and analytical method for most compounds with vapor pressures >lo5mmHg. The use of gas bags to contain known atmospheres of interest is a relatively cheap and quick way to test a potential method. The methodology allowing some discrimination of humidity effects via water presaturation of the sampling medium is novel, as is the manner in which the most suitable sampling medium under dry conditions is selected. Registry No. 2-EH, 104-76-7;Chromosorb 102, 9003-70-'7.
403
403-405
LITERATURE CITED (1) Treon, J. F. "Industrial Hygiene and Toxicology Vol 11: Toxicology", 2nd ed.: Patty, F. A., Ed.; Wiley: New,,York, 1963: pp 1462-1463. (2) Kirk, R. E., Othmer, P. F., Eds.; Encyclopedia of Chemical Technology", 3rd ed.; Wlley: New York, 1978; Vol. I, p 716. (3) Scala, R. A,; Burtis, E. G. Am. Ind. Hyg. Assoc. J . 1979, 3 4 , 493. (4) Amer. Conf. Governmental Industrial Hygienists, TLVs (Threshold Limit Values) for Chemical Substances in Workroom Air adopted by the A. C.G.I.H. for 1980, A.C.G.I.H., Cincinnati, 1980. (5) Pouchert, C. J. "The Aldrlch Library of Infrared Spectra", 2nd ed.; Aldrlch Chemical Co.: Milwaukee, WI, 1975; p 67. (8) Taylor, D. J., Ed. "NIOSH Manual of Sampling Data Sheets", Supplement to 1977 Ed.; DHEW (NIOSH): Cincinnati, OH, 1978; Publication Number 78-189.
RECEIVED for review March 22, 1982. Accepted October 28, 1982. We gratefully acknowledge the financial support of NIH-ES-00159.
Analysis of High-Purity Graphite for Trace Elements by Inductively Coupled Plasma Atomic Emission Spectrometry after Chelating Resin Preconcentration Hlmansu S. Mahantl' and Ramon M. Barnes" Department of Chemistiy, GRC Towers, University of Massachusetts, Amherst. Massachusetts 0 1003-0035
Graphite is a critical material in nuclear reactors and has widespread industrial applications for which knowledge of the trace element content is essential. Both dc arc emission spectrometry (1-5) and neutron activation analysis (6-8) are the primary instrumental techniques employed for graphite analysis. Inductively coupled plasma atomic emission spectrometry (ICP-AES) only recently was adapted for the analysis of activated carbon after sample ashing followed by sodium peroxide fusion (9) and for the analysis of graphite after sample dissolution in ]perchloric and periodic acids (10). Although ICP-AES exhihits excellent powers of detection, the concentrations of most elements in high-purity graphite lie below the limits of quantitative determination obtained with ICP-AES. Therefore, a preconcentration step is required t o apply ICP-AES for graphite analysis. Recently two chelating resins were shown to be effective means to separate and concentrate trace elements from complex matrices prior to spectrochemical determinations (11-20). This paper describes the determination of six elements in high-purity graphite samples by sequential ICP-AES after the samples are ashed and dissolved and sought elements are concentrated by either a poly(dithi0carbamate) or poly(acrylamidoxime) resin.
EXPERIMENTAL SECTION Apparatus. Experimental facilities and operating conditions are listed in Table I, and analysis wavelengths are indicated in Table 11. Both conventional pneumatic nebulization and electrothermal vaporization (ETV) sample introduction are applied in these determinations. Prior to the ETV-ICP copper determination, the ETV graphite rod is coated first with pyrolytic graphite and then with tantalum (21). Operating conditions were determined for each element by Simplex optimization (22). For the determination of six elements, the ICP operating conditions are argon outer gas flow 16 L/min, nebulizer gas flow 0.8 L/min at a back-pressure of 24 psig, observation height above the in. duction coil 16 mm except for Si (14 mm), inonochromator slit widths, 50 pm, and power Al, Ti, V (0.7 kW), Cu and Fe (0.8 kW), and Si (0.5 kW). Sample Preparation. High-purity graphite (National SP-2 and TB-6, Union Carbide, New York) is dried in an oven at 100 'On leave from the Naliional Institute of Foundry and Forge Technology, Hatia, Ranchi-834003, India.
Table I. Instrumentation and Operating Conditions generator nebulizer plasma torch detection
electrothermal vaporizer
A. Instrumentation Plasma-Therm Model HFS-5000D, 40.68 MHz with three-turn ( in. copper) load coil Babington with double-barrel glass spray chamber Conventional 1 8 mm i.d. quartz with 1.5 mm i.d. injector orifice except 0.8 mm i.d. orifice for Si Minuteman monochromator, Model 310SMP, 1-m Czerny-Turner with 1200 groove/" grating; 1:1 image formed by quartz lens (Oriel A-11-661-37); RCA 1P28 photomultiplier (-700 V ) Keithley 411 Picoammeter, Heath EU201V log/linear recorder Varian carbon rod atomizer (CRA-90) with laboratory fabricated graphite rod electrode and enclosed quartz chamber (21 )
B. ICP-AES Operating Conditions for Cu with ETV-ICP 0.55 power, kW outer gas flow, L/min 16 intermediate gas flow, L/min 1 chamber flow rates, L/min inner 1.6, outer 4.6 observation height, mm 16 slit height, mm 5 monochromator slit widths, mm 0.05 temperature drying, 100 "C for 1 0 s ashing, 200 "C for 10 s vaporization, 2100 "C heating rate, "C/s 800 "C for 2 h. One to five grams of high-purity graphite is weighed into a platinum crucible, and 2 mL of 5% magnesium nitrate solution is added as an ashing aid, especially t o prevent volatilization loss of V. Prior to its use, the magnesium nitrate solution was purified by shaking for 24 h with 500 mg of the poly(dithiocarbamate) resin. The graphite sample is then dried on a hot plate and kept in a muffle furnace at 800 OC for 12 h until a complete ash is obtained. The ash is dissolved in 10 mL of hydrochloric and nitric acids (3:l) and diluted with distilled water to a final volume of 100 mL. Stock solutions are prepared from high-purity metal or reagent-grade chemicals,and high-purity acids and distilled, deionized
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 2, FEBRUARY 1983
Table 11. Wavelengths and Figures of Merit
wavelength, nm
AJ 396.152 Cu 324.754 Fe 259.94 Si 251.61 Ti 334.94 V 292.40
detection limit,a ng/mL B E C , ~ acid matrix mg/L 8.5 5 8
20 4 8
0.4 0.2 0.3 0.7 0.17
0.28
LQD,' ccg/g in graphite 0.042 0.025 0.040 0.100 0.020 0.040
a Concentration giving a signal equal to three times the relative standard deviation of the background. Background level expressed as analyte concentration equivalent. Lowest quantitatively determinable concentration calculated from five times the limit of detection and for 5 g of sample in 5 mL of acid digested resin solution (24fold concentration factor).
water are used throughout. ACS Reagent-grade ammonium hydroxide (Fisher A-669) and 30% hydrogen peroxide (Fisher H-325) are used. Reagent blanks are evaluated and subtracted when necessary. Poly(dithi0carbamate) Separation. Earlier pH studies indicate that at pH -5 Cu, Fe(III), Ti, and V and at pH 8 Si are quantitatively complexed by the poly(dithi0carbamate) resin. After dissolution of the ash, the sample solution pH is increased to 5 or 8 with ammonium hydroxide, and the solution is passed through a resin column containing 70 mg of 70-80 mesh poly(dithiocarbamate)resin. The resin is digested with 30% hydrogen peroxide at room temperature for 5 min followed by 2 mL of concentrated nitric acid heated slowly at -100 "C for 15-20 min. The final volume (5 mL) is obtained with 0.1 N nitric acid. The concentration of each element is determined by using aqueous metal standards containing similar acid concentrations. From an initial solution volume of 120 mL, the final digested resin volume of 5 mL provides a concentration enrichment of 24 times. Poly(acry1amidoxime)Separation of Aluminum. Previous investigations (17)indicated that aluminum is strongly complexed by the poly(acry1amidoxime)resin at pH 6. Although the poly(dithiocarbamate)resin also chelates with aluminum, complexation is relatively weak and aluminum recovery is lower than that obtained with the poly(acry1amidoxime)resin. For the determination of Al, the graphite ash dissolved in aqua regia is transferred to a 125-mL polyethylene bottle, and the solution pH is adjusted to 6 with ammonium hydroxide. The poly(acry1amidoxime) resin (100 mg) is added, and the solution is shaken for 10 h. The solution is then filtered and washed with distilled water, and the resin with its sequestered metals is shaken with 2 mL of 20% hydrochloric and 20% nitric acids for 3 h. The solution is filtered, the resin washed, and the filtrate diluted to 5 mL. Aluminum is determined by using acid-matched standard solutions. Copper by ETV-ICP. Copper is also determined in the poly(dithiocarbamate)resin digested solution using ETV-ICP (21). Five microliters of the digested resin solution is placed into the graphite electrode cup, and the ICP emission signal is measured under the conditions given in Table I. Aqueous copper standards containing the same acid concentration as the digested resin are used for calibration.
RESULTS AND DISCUSSION Limits of detection and background equivalent concentration (BEC) values for the six elements determined in aqueous reference solutions containing nitric acid are given in Table 11. These are comparable to literature values (23-25), and on the basis of a concentration enrichment resulting from resin chelation, a 24-fold improvement in the limits of detection is obtained for graphite. The lowest quantitatively determinable concentration (LQD) values in graphite are also presented assuming a 5-g initial sample weight. High-temperature ashing of graphite samples using magnesium nitrate as an ashing aid followed by dissolution in a mixture of hydrochloric and nitric acids is found to be the
Table 111. Analysis of Graphite withPoly(dithiocarbamate) and Poly( acrylamidoxime) Resins (All Values in p g / g )e National SP-2 ele- presment ent found
National TB-62 present found
Ale
Cu
a a
b b
0.5 0.5
0.3 0.2 a
0.29 + 0.01 0.21 c 0.01 0.10 i 0.008 0.08 c 0.008
a a
b
a
b b
0.5
Fe Si Ti V
a
a
0.47 i 0.01 0.53 c 0.01 (0.48
* 0.0E1)~
b
a Not reported. Not determined. CPoly(acrylamidoxime) resin at pH 6. Obtained by ETV-ICP. e Means of triplicate analysis reported.
Table IV. Element Recovery from Graphite with Poly(dithiocarbamate) and Poly( acrylamidoxime) Resinsa amt added, element
ccg
Alb
2 2 2 2 2 2
cu
Fe Si Ti V
concentration, Fg expected found 3 3 2.6 2.8 2.4 2.32
2.9 i 0.03 2.9 f 0.03 2.56 * 0.07 2.9 c 0.1 2.3 I 0.02 2.25 i 0.02
recovery, %
96.7 96.7 98.5 103.5 95.8 97.0
a Mean of three analyses in sample TB-62 for Al and Cu and in SP-2 for Fe, Si, Ti, and V. Sample weight 2 g for Al, Cu, and Fe and 4 g for Si, Ti, and V. With poly(acrylamidoxime) resin.
preferred method for sample preparation without loss of the elements considered. Radioactive trace studies previously established the absence of element loss during high-temperature ashing (25),and magnesium nitrate used as an ashing aid in this experiment does not affect the ICP-AES results (26-28), because the analytes are separated completely from the magnesium solution by the chelating resins. The concentrations of six elements determined in two high-purity graphite samples by the present method (Table 111) agree well with the manufacturer's certified concentrations. The relative standard deviation of the method ranges from two to several precent. Quantitative recoveries were obtained for each of the five elements a t the 2 pg concentration level added to the graphite samples prior to ashing (Table IV). The Cu concentration was verified by ETV-ICP (Table 111). The ICP-AES determination of six elements in high-purity graphite samples by means of high-temperature sample ashing with an ashing aid followed by separating and concentrating analytes by either the poly(dithi0carbamate) or poly(acry1amidoxime) resins enhances the capability of ICP-AES. Application of ETV-ICP in this procedure permits analyses with a smaller final solution volume than nebulization, and thus larger concentration factor. Since neither resin will complex B or the group 1A or 2A elements, the approach is not suitable for the enhanced determination of B or Ca; however, the determinations of Co, Mn, Ni, Ag, and rare earth elements are possible. Loss of Cd, Pb, and Sn during ashing must be examined prior to their determination. Registry No. Al, 7429-90-5; Cu, 7440-50-8; Fe, 7439-89-6; Si, 7440-21-3; Ti, 7440-32-6; V, 7440-62-2; graphite, 7782-42-5; poly(acrylamidoxime), 27939-99-7. (1)
LITERATURE CITED Gorbunova, L. B.; Kutelnikov, A. F.; Avdeenko, M. A.; Murashkina Zavod. Lab. 1975, 4 1 , 178.
Anal. Chem. 1983, 55, 405-407 (2) Lordello, A. R.; Tognlrii, R. P. Relat. Inst. Energ. Atom. S. Paul0 1975, No. IEA-396. (3) Maillard, P.; Ades, C. Can. Spectrosc. 1969, 74, 17. Janga, I.;Spatzek, H. Mikrochim. (4) Schroll, E.; Huber-Schausberger, I.; Acta l W 8 , 3, 649. (5) Krishnamurty. G.; Marillhe, S. M. Rep. Bhabha Atom. Res Centre 1971 BARC-575, 20. (6) May, S.;Pinte, G. J . Radioanal. Chem. 1969, 3, 329. (7) Okada, M.; Tamura, N. Rep. Jpn Atom. Energy Res. Inst. 1972, JAERE-M-4900, 29. (8) Mukhamedshlna, N. M.; Yankovskii, A. V. Zavod. Lab. 1972, 38, 1099. (9) Balaes, G. E. E.; Dixon, K.; Russell, G. M.; Wall, G. J. S. Afr. J . Chem. 1982, 4 , 35. (IO) Kato, K. 17th Summer Symposium, Japan Society of Spectroscopy, Shirakaba-ko Lake, Japan, Aug 1981; ICP I n f . Newsl. 1982, 8 , 138. (11) Hackett, D. S.;Siggia, S. "Environmental Analysis"; Academlc Press: New York, 1977; p 253. (12) Barnes, R. M.; Genna, J. S.Anal. Chem. 1979, 57, 1065. (13) Mlyazakl, A.; Barnes, FI. M. Anal. Chem. 1961, 53,299. (14) Miyazakl, A.; Barnes, Fi. M. Anal. Chem. 1981, 53, 364. (15) Colella, M. B.; Siggia, S.;Barnes, R. M. Anal. Chem. 1980, 5 2 , 967. (16) Colella, M. B.; Siggla. S.;Barnes, R. M. Anal. Chem. 1980, 5 2 , 2347. (17) Barnes, R. M.; Lu Shang Jlng, unpublished work.
405
(18) Barnes, R. M.; Fodor, P.; Inagakl, K.; Fodor, M. Spectrochim. Acta, Part 5 ,in press. (19) Fodor, P.; Barnes, R. M. Spectrochim. Acta, Part B , in press. (20) Zhung Mlanzhl; Barnes, R. M. Spectrochim. Acta, Part 6 ,in press. (21) Barnes, R. M.; Fodor, P., unpublished work. (22) Cave, M.; Barnes, R. M.; Denzer, P. 1982 Winter Conference on Plasma Spectrochemistry, Orlando, FL; ICP Information Newsletter: Amherst, MA, 1982; Abstr 23. (23) Winae, R. K.: Peterson, V. J.; Fassel, V. A. A m / . Spectrosc. 1979, 33,-206. (24) Boumans, P. W. J. M.; Barnes, R. M. ICP I n f . Newsl. 1978, 3 , 445. (25) Gorbunova, L. 8.; Kuteinikov, A. F.; Marunina, N. I.; Sukhov, G. V. Zh. Anal. Khim. 1976, 31, 2061. (26) Larson, G. F.; Fassel, V. A,; Wlnge, R. K.; Knlseley, R. N. Appl. Spectrosc. 1976, 30, 384. (27) Fassel, V. A.; Katzenberger, J. M.; Winge, R. K. Appl. Spectrosc. 1979, 33, 1. (28) Larson, G. F.; Fassel, V. A. Appl. Spectrosc. 1979, 33, 592.
RECEIVED for review September 1, 1982. Accepted October 19, 1982. Research supported by Department of Energy Contract DE-AC02-77EV-0432,
Battery-Powered Coulostat for Low-Noise Photoelectrochemical Measurements with a Pulsed-Laser Source S. J. Parus and S. P. Perone*' Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Pulsed illumination of electrode/solution interfaces combined with electrochemical monitoring provides dynamic information about photoelectrochemical processes. For examination of rapid events, short light pulses are used. The electrochemical measurement technique must then be capable of fast, undistorted response. Potentiostatic methods yield current vs. time information under controlled potential conditions. However, there is an inherent resistancecapacitance (RC) time constant present resulting from the solution uncompensated resistance and electrode double-layer capacitance. This RC time constant limits the rate at which potential control a t the double layer can be achieved. In addition, for sufficiently Bhort perturbations, the resulting current response reflects the exponential recharging of the double-layer capacitance and not the time variation of the electrode process. Thus$,a t short times, potential control is lost and the current output is distorted. A typical response time limit is 50 ps. With extreme care this can be extended to 1 ps ( I ) . Coulostatic techniques give time resolution several order8 of magnitude shorter. As Bed here, the term coulostatic refers to measurement of the electrode potential at open circuit following photoinduced charge transfer using a light pulse. The potential is not controlled, instead its change with time is monitored. Limitations imposed on the total electrochemical cell response time by electrode capacitance are not significant. The source rlesistance of the electrode and cell combined with stray capacitance in the monitoring circuit and cell determines the coulostatic response time (2). The general coulostatic-flash experimental procedure involves first adjusting the electrode to the desired initial potential with a conventional potentiostat. A trigger signal then opens a switch between the potentiostat and auxiliary elec. trode connection, placing the cell under open circuit conditions (see Figure 1). A short time later (typically 10 ps), a light Present address: Chemistry & Materials Science Department, Lawrence L i v e r m o r e N a t i o n a l Laboratory, P.O. Box 808, L-326, Livermore, CA 94550.
source is triggered. The potential difference between the working and reference electrodes is then monitored with time after the flash. Variations of the experimental coulostatic arrangement have been studied. Measurements have been made on cells at their equilibrium potential with no external applied potential bias (3). With an external potential applied, but the cell not switched to open circuit, the photopotential can still be followed for times much shorter than the cell time constant. Measurements will thus be made under virtual coulostatic conditions before the cell can respond to the potential perturbation (4-7). Alternatively, the electrode connections have been disconnected manually prior to the flash (8). Recently, solid-state switches and pulsed lasers have allowed very rapid photoelectrochemical processes to be studied (2,9-11). Rise times as short as 12 ns have been observed at semiconductor electrodes ( 2 ) , and even shorter response times appear achievable. With the coulostatic technique, it is possible to accurately monitor photopotential signal changes commencing with the beginning of the light pulse. Pulsed light sources generate electronic noise however, which interferes with electrochemical measurements at short times. Although attempts have been made to reduce such noise levels when utilizing potentiostatic techniques, it is not very important to do so since the signals are distorted on this short time scale. For full advantage of the capabilities of the coulostatic technique, noise levels at all times must be reduced to a minimum. This report describes results of these noise reduction attempts. Transient flash induced noise can originate from two sources. The cell's electrode leads themselves and their shielding and grounding may pick up noise. Noise may also enter the system through the coulostat's electronics and its corresponding shielding and grounding. These may be differentiated from each other by totally disconnecting the coulostat (including its electrode leads) from the cell. The noise pickup on the cell alone under true open circuit conditions is then measured and optimized. The coulostat system (electrode leads, switches, etc.) can then be reconnected in-
0003-2700/83/0355-0405$01.50/00 1983 American Chemical Society
