Anal. Chem. 1983, 55, 981-983
to be intensity independent. The useful range for PAS measurements is thus extended by wavelength modulation. The detectability is comparable to those that were possible only in acoustically resonant cells (24),but without any of the critical requirernents in the operation of those cells. Finally, it may be noted that there are deviations in the experimental points from the line of unit slope in each of the last two figures. This reflects mainly the difficulty in preparing exact mixtures of gases a t thetge low concentrations, even under carefully controlled conditions. In summary, we show that a simple adaptation of a commerical COz laaier permits its operation in the wavelengthmodulated mode for PAS measurements, with good discrimination against background due to window absorption. We expect the same concept to be applicable to long-path absorption as well, with discrimination against scattering and atmospheric turbulence.
LITERATURE CITED “Cleaning Our Envlronment-A Chemical Perspectlve”, 2nd ed.; American Chemlcal Society: Washlngton, DC, 1978. Collls, R. T. H.; Russel, P. B. I n “Laser Monitoring of the Atmosphere”; Hinkley, E. D., Ed.; Springer-Verlag: New York, 1976; Chapter 4. Menzies, R. 1’. I n “Laser Monitoring of the Atmosphere”; Hlnkley, E. D., Ed.; Springer-Verlag: New York, 1976; Chapter 6. Byerly, R. IEEE Trans. 1975, NS-22, 856-869. Kobayaki, T.; Inaba, H. Appl. Phys. Lett. 1970, 17, 139-141. Golden. B. M.; Yeung, E. S. Anal. Chem. 1975, 4 7 , 2132-2135. Kerr, E. L.; Atwood, J. 6.Appl. Opt. 1966, 7 , 915-921.
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Kreuzer, L. B. J. Appl. Phys. 1971, 4 2 , 2934-2943. Rosengren, L. G. Appl. Opt. 1975, 14, 1960-1976. Deaton, T. F.; Depatie, D. A.; Walker, T. W. Appl. Phys. Lett. 1975, 2 6 , 300-303. Max, E.; Rosengren, L. G. Opt. Commun. 1974, 11 , 422-426. Krltchman, E.; Shtrlkman, S . ; Slatklne, M. J . Opt. SOC. Am. 11378, 6 8 , 1257-1271. Bonczyk, P. A.; Ultee, C. J. Opt. Commun. 1972, 6 , 196-198. Kaldor, A.; Olson, W. B.; Makl, A. G. Sclence 1972, 176, 506-510. Kavaya, M. J.; Margolis, J. S.; Shumate, M. S . Appl. Opt. 1079, 18, 2602-2606. Chang, T. Y.; Morris, R. N.; Yeung, E. S . Appl. Spectrosc. 1981, 3 5 , 587-591. Wake, D. R.; Amer, N. M. Appl. Phys. Lett. 1079, 3 4 , 379-381. Goff, D. A.; Yeung, E. S. Anal. Chem. 1078, 50, 625-627. Castleden, 5. L.; Kirkbright, G. F.; Spiilane, D. E. M. Anal. Chem. 1981, 53, 2226-2231. Welling, H.; L i i n , G.;Beigang, R. I n “Laser Spectroscopy 111”; Hall, J. L., Carlsten, J. L., Eds.; Springer-Verlag: New York, 1977; pp 370-375. Reid, J.; Garside, B. K.; Shewchun, J.; El-Sherbiny, M.; Ballik, E. A. Appl. Opt. 1978, 17, 1806-1810. Patel, C. K. N.; Burkhardt, E. G.;Lambert, C. A. Opt. Quantum Efectron. 1078, 8 , 145-154. Konjevic, N.; Jovicevlc, S. Spectrosc. Lett. 1079, 12, 259-274. Perlmutten, P.; Shtrlkman, S . ; Slatklne, M. Appl. Opt. 1979, 18, 2267-2274.
RECEIVED for review December 6, 1982. Accepted January 28, 1983. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-eng-82. This work was supported by the Office of Basic Energy Sciences.
Determination of Total Chromium in Seawater by Graphite Furnace Atomic Absorption Spectrometry S. N. Willie, R. E. Sturgeon,* and S. S. Berman Dlvision of Chemistty, National Research Council of Canada, Otfawa, Ontario K1A OR6, Canada
Reported concentrations of chromium in open ocean waters range from 0.07 to 0.96 Fg L-l, with a preponderance of values near the lower limit ( I , and references summarized therein). Methods used for the determination of Cr at this concentration have generally involved some form of matrix separation and analyte concentration prior to determination, such as coprecipitation ( I , 2), chelation-solvent extraction ( 3 , 4 ) ,electroreduction (5, 6), and ion-exchange (7, 8) techniques. Whereas it is desirous to utilize analytical schemes which permit elucidation of the various Cr species, particularlly since Cr(V1) is acknowledged to be a toxic form of this element, it is useful to have the capability of rapid, total Cr measurement where speciation is a matter of secondary importance. Determination of Cr by many of the methods cited earlier is problematic. Variable and nonquantitative recovery with chelation-solvent extraction techniques necessitates use of the method of additions ( 4 ) . Coprecipitation techniques require lengthy processing times and extensive sample manipulation. Ion-exchange suffers from slow uptake and release kinetics, necessitating total destruction and solubilization of the resin (8) or complex apparatus and multicomponent eluting solutions. Problems encountered with the routine determination of even total Cr by oceanographic laboratories are quite evident from the fact that only 7 participants of a total of 40 attempted determination of this element in a recent ICES intercalibration exercise (9). Overall accuracy and precision for Cr were unsatisfactory (9). 0003-2700/83/0355-098 1$01.50/0
This work reports on the use of an immobilized diphenylcarbazone chelating agent to provide a simple, rapid, and quantitative preconcentration procedure for the determination of total dissolved Cr in seawater.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model 5000 atomic absorption spectrometer fitted with an AS-40 automatic sampler and HGA-500 furnace with Zeeman background correction capability was used for all Cr determinations. Pyrolytic graphite coated tubes were exclusively used with peak height evaluation of signals. Reagents. All reagents were purified prior to use. Concentrated nitric and hydrochloric acids were prepared by subboiling distillation in a quartz still using reagent grade feedstock. A saturated solution of ammonium hydroxide (28%) was prepared by isothermal distillation according to the procedure recommended by Zief and Horvath (10). A saturated solution of SOz-water was prepared by bubbling anhydrous SO, from a lecture bottle through several water traps prior to dissolution in high-purity deionized, distilled water (DDW). Stock standard solutions of Cr(V1) and Cr(II1) were prepared from KzCrz07and KCr(SO,),, respectively. Serial dilutions were made with higb-purity DDW in order to prepare working standards. Silica gel (Fisher, 100-200 mesh) was acid leached with HNOB and HCl and washed with DDW prior to use. Diphenylcarbazone (Fisher) was used as supplied. Coastal seawater samples were obtained from the Atlantic Research Laboratory of the National Research Council, Halifax, Published 1983 by the American Chemical Society
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Nova Scotia. The samples had a salinity of 29.5%0and were filtered through a precleaned, nominally 0.45-pm membrane fiiter, acidified to pH 1.6 (with HN03) and stored in precleaned, polypropylene bottles. An open ocean, reference seawater, NASS-1 (11)was used as a second sample. Procedure. All sample preparations were carried out in a clean laboratory equipped with laminar flow benches and fume cupboards providing a class 100 working environment. Silica immobilized diphenylcarbazone (I-DCN) was prepared following the method outlined by Hill (12) for immobilized oxine except that DCN was substituted for oxine. The resulting product was brownish red in appearance. Following several cleanings in 2 N HCl, 600 mg of the product was slurry loaded into a borosilicate glass column supported by a coarse sintered glass frit. The column assembly and reservoir have been described earlier (13). Prior to use the entire assembly was precleaned by gravity feed of 100 mL of a solution consisting of 1N HCl/O.l N HNOBthrough the column. The column bed was then washed free of excess acid, using DDW. All Cr in the samples was first reduced to Cr(II1) by addition of S02-water and allowing them to stand for several minutes. One-half milliliter of SO2-water per 200 mL of seawater was sufficient for quantitative reduction of all Cr(V1) (including any added spikes). Aliquots of seawater were then adjusted to pH 9.0 i 0.2 by using high-purity ammonium hydroxide and gravity fed through the column at a nominal flow rate of 10 mL/min. Following passage of the sample, the column was washed free of interstitial seawater by using two 10-mL aliquots of DDW, allowing each to gravity drain. The sequestered Cr was then eluted from the column with 10.0 mL of 0.2 N HN03. This concentrate was stored in a 30-mL screw-capped polypropylene bottle prior to analysis. The column was then cleaned by further passage of 25 mL of a solution of 1N HC1/0.1 N HNOB. Following a wash with DDW to remove excess acid, the column was ready for the next sample aliquot. Blanks were also carried through this procedure. Initially, blank runs consisted of 200-mL aliquots of seawater which had been previously passed through the column and were therefore stripped free of Cr. Results from this procedure were compared to a simpler one involving only the acid elution of a cleaned column. Three such blanks were prepared for each analytical run. Analysis of blanks and concentrates was accomplished by calibration against a spiked aliquot of concentrate, thereby effecting an exact matrix match, and against a standard in the elution acid. RESULTS AND DISCUSSION Diphenylcarbazone (DCN) and diphenylcarbazide (DCD) have been widely used for the spectrophotometric determination of chromium (14). Only relatively recently, however, has the nature of the complexation reactions been elucidated. Cr(II1) reacts with DCN whereas Cr(V1) reacts (probably via a redox reaction combined with complexation) with DCD (15). Although speciation would seem a likely prospect with such reactions, commercial DCN is a complex mixture of several components, including diphenylcarbazide, diphenylcarbazone, phenylsemicarbazide, and diphenylcarbadiazone, with no stoichiometric relationship between the DCD and DCN (16). As a consequence, use of DCN to selectively chelate Cr(II1) also results in the sequestration of some Cr(V1). Total Cr can be determined with DCN following reduction of all chromium to Cr(II1). This approach was taken in these studies. Use of immobilized chelating agents for sequestering trace metals from aqueous and saline media presents several significant advantages over chelation-solvent extraction approaches to this problem (17, 18). With little sample manipulation, large preconcentration factors can generally be realized in relatively short times with low analytical blanks (13). Silica-immobilized diphenylcarbazone (I-DCN) was found to be stable over a wide p H range. No adverse effects were noted following repeated cycles of cleaning at pH 0 and sample
Table I. Analysis of Seawater for Total Cr concentration, ng/mL open-ocean, coastal water NASS-1 trial (salinity = 29.5 o/o,) (salinity = 35.0 o/oo) 1
0.100
2
0.096 0.095
3
4
ava 0.097 i: 0.003 accepted value 0.10 i: 0.01 a
0.19 0.15 0.18
0.19 0.18 t 0.02 0.184 f 0.016
Expressed as a mean and standard deviation.
loading at p H 9. Processing of more than 4 L of seawater, representing more than a dozen individual samples, resulted in no evident reduction in the efficiency of the I-DCN column. The relatively low exchange capacity exhibited by this material limits its use to separations involving trace or low level concentrations. The exchange capacity for Cr(III), as determined by a batch equilibration technique, was 0.45 mg/g at pH 9.0. Larger exchange capacities should result if a finer particle size silica substrate is used (e.g., 200-400 mesh). The small exchange capacity of this material does not present a problem for the processing of seawater due to the low levels of trace metals present in this sample. Substantial preconcentration factors can therefore be realized, particularly if the acid concentrates are further concentrated by simple evaporation. Recovery Efficiency. Elution of chelated Cr from the column bed was quantitative, using 10.0 mL of 0.2 N HN03. More than 93% of the Cr was recovered in the first 5 mL of eluant. Extraction of 80-ng spikes of Cr(II1) from 200-mL aliquots of seawater was quantitative. Neither Cr(II1) nor Cr(VI) could be quantitatively extracted from DDW (
