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Anal. Chem. 1987, 59. 2441-2444
reaction was eluted and its quantity determined by chemical analysis. Table I lists the relevant information. In each case the amount of the ion eluted from the resin measured at “breakthrough” was within 20% of the nominal capacity of the resin used. While these experiments indicated that the traps did indeed efficiently capture the gases actually swept from the reaction vessel, the original ions were not quantitatively recovered from the discrete salt samples introduced into the test tube. The reason for this is that all of these gases have much greater water solubility than does carbon dioxide and are therefore much more difficult to completely sparge from water (7). Any apparatus developed for determining these species would have to provide more rigorous sparging conditions than are provided by the simple apparatus shown in Figure 1. In conclusion, ion exchange resin beads have a number of features which make them potentially useful in gas analyses. First, moist resin bead beds are extremely efficient a t capturing any gases capable of undergoing acidlbase type reactions with the counterion occupying the exchange sites. Second, the capacities of the resin beds for these gases are both quite high and readily predictable from simple stoichiometry. Third, ions formed on the resins by the absorption reactions can be readily eluted and then determined by classical chemical methods. Finally, the resins themselves are relatively inexpensive, easily “regeneratable” after use if so
desired, and commercially available in a wide range of uniform particle sizes. Probably the most serious practical limitation is that the resin must not totally dry out.
ACKNOWLEDGMENT The author expresses his appreciation to Arnold Lewis who developed the automatic titration system used. He also thanks Knut Irgum who pointed out the two previous references to related applications of ion exchange materials. Registry No. CO?-, 3812-32-6;COz,124-38-9;NH3,7664-41-7. LITERATURE CITED (1) Laitinen, H. A. Chemical Analysis: McGraw-Hill: New York, 1960; p 436. (2) Conway, E. J. Microdiffusion Analysis and Volumetric Error; Crosby, Lockwood 8 Son, Ltd.: London, 1947. (3) Siemer, D. D. Analyst (London) 1986, 111(9), 99.1013-1015. (4) Vulikh, A. I., Zagorskaya, M. K., Alovyainikov, A. A. Sb. Nauchn. T r . NIITsvet. Met 1970, 37, 120-133. (5) Validyanathan, A. S.;Youngquist, G. R. Ind. Eng. Chem. Prod. Res. Dev. 1973, 72,288-293. (6) Colorimetric Determination o f Non-Metals ; Boltz, D. F.. Ed.; Chemical Analysis Series; Interscience: 1958, Vol. 8, pp 84-91. (7) CRC Handbook of Chemistry and Physics, 44th ed.; CRC Press: Cleveland, OH, 1963.
RECEIVED for review December 8, 1986. Accepted May 18, 1987. Work supported by the U.S. Department of Energy Assistant Secretary for Defense Programs under Contract No. DE-AC07-84ID12435.
Atomic Absorption Determination of Tin Using in Situ Concentration of Stannane in a Graphite Furnace R. E. Sturgeon,* S. N. Willie, a n d S. S. B e r m a n Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9 Tin is one of the more difficult elements to determine in environmental materials. Extremely low concentrations (nanograms per gram) in food products, plant and animal tissues, and aquatic samples (1)coupled with the ubiquitous nature of this element (2)and the ease with which laboratory reagents and vessels can become significant sources of contamination necessitate constant vigilance on the part of the analyst. Atomic absorption spectroscopy (AAS) is the most widely used technique for such determinations ( 1 )and considerable improvements in detection capability can be gained by conversion of the element to stannane prior to atomization (3-6). Further improvements in concentration detection limits may be realized when the hydride generation technique is coupled with cryogenic trapping (7-9), thereby effecting a preconcentration of the analyte prior to detection. This latter approach is particularly attractive when alkyltin speciation is desired. Equally effective for the preconcentration of the volatile hydrides of several metals has been use af a graphite furnace to serve as both the hydride trapping medium and atomization cell (1&13). This methodology offers substantial advantages over conventional furnace and hydride generation techniques including simplicity of operation and use of small sample volumes, high sensitivity, and a substantial increase in detection power. While it is not possible to retrieve speciation information by this approach, its use as a simple and rapid technique for total tin determination is an attractive alternative to other schemes which may require analyte preconcentration with their attendant sample manipulation and the
danger of ensuing adventitious contamination. This report details the application of such in situ metal trapping to the determination of total Sn in marine biological tissues and sediments by hydride generation graphite furnace AAS and its potential use as a rapid screening technique for Sn in seawater.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model 5000 atomic absorption spectrometer was fitted with an HGA-500 graphite furnace and Zeeman effect background correction. A Perkin-Elmer Sn hollow cathode lamp operated at 30 mA was used as the line source. Standard Perkin-Elmer pyrolytic graphite coated tubes were used with minor modification. The coating of pyrolytic graphite was removed from the interior surface by passage of a 0.242-in. reamer through the tube. Additionally, the diameter of the sample introduction port was increased to -3 mm with a drill bit. Following about a dozen runs, during which the tube surface was conditioned, it was suitable for use over the next 400-500 firings. Alternatively, extremely well-used pyrolytic graphite coated tubes, normally considered to be beyond their useful lifetime, can be employed. A custom-made Pyrex cell (22) was used to generate SnH, which was transferred, via a quartz delivery tube, into the sample introduction port of a preheated furnace tube. The design and operation of the cell have been detailed elsewhere (22-13). Reagents and Standards. A 1000 mg/L stock solution of inorganic tin was prepared by dissolving high-purity tin shot (Spex Ind., Metuchen, NJ) in concentrated HCl and diluting t o 1 M HC1. This solution has been found to be stable for more than 1 year, as has been a working standard of 1 mg/L prepared by dilution of the above stock with 1 M HCl.
Published 1987 by the American Chemical Society 0003-2700/87/0359-2441$01.50/0
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
Table I. Furnace Program time, s program step temp, "C ramp hold generationcollection
atomization
1
2
1 2 3a 4 1 2
3
internal gas,
mL/min
20
1
19
800
1
800 800
1 1
9 29
150
119
150
20 2700 2700
1
4 4 1
0 0 300
0 1
U
150 150
wr %
1.0
a
'Addition of NaBH, at 4 mL/min. Several organotin compounds (Alfa Products, Danvers, MA) were studied. Stock solutions of approximately 1000 mg/L of dimethyl- and trimethyltin chloride were prepared in 1% (v/v) HC1. Solutions of dibutyltin dichloride, phenyltin trichloride, and tri-n-butyltin oxide were prepared in methanol. Concentrations of working standards of these compounds were obtained by calibration against inorganic tin standards (in 1% HCl or methanolic solutions) using flame AAS. High-purity subboiling distilled HC1, "OB, HF, and HC104, prepared in-house, were used for sample decompositions. A 1%(m/v) solution of NaBH, (Alfa) was prepared daily, or more frequently if required, by dissolution of 0.25-g pellets in distilled deionized water (DDW). Several marine samples were analyzed for total Sn including National Research Council of Canada (NRCC) marine sediments (BCSS-1 and MESS-1) and NRCC lobster hepatopancreas (TORT-1). Recovery of inorganic Sn spikes from NRCC opean ocean seawater (NASS-1) was studied. Procedures. All sample and analytical manipulations were conducted in a class 100 clean room environment. Aliquots of NASS-1 (30 mL) were transferred directly to the hydride cell and acidified to 0.1 M in HC1. Sediment samples were solubilized in LORRAN Teflon pressure decomposition vessels (H. K. Morrison and Sons, Ltd., Mount Uniacke, NS). Nominal 0.5-g samples were mixed with 3 mL of HNO,, 3 mL of HF, and 2 mL of HCIOl and the vessels submerged for 3 h in a water bath at 100 "C. After cooling, the contents were quantitatively transferred to 50-mL Teflon PFA beakers and evaporated to incipient dryness. A further 1mL of HF and 1mL of HC104were added and the samples again taken to near dryness. A final addition of 1mL of HCl and 1 mL of HNO, followed by gentle warming and dilution to 50.0 mL with DDW resulted in complete solubilization of the sample. An initial white sediment (presumably slightly soluble perchlorate salts) completely dissolved after the samples were permitted to stand for 12 h. Duplicate samples were taken from each of four bottles of both sediments. Two blanks were run concurrently with each set of four sample decompositions. Total Sn was determined by using 20-pL aliquots of dissolved sediment solution delivered into the hydride cell containing 20 mL of 0.1 M HC1. The biological reference material TORT-1 was solubilized by addition of 20 mL of HNO, to nominal 0.5-g samples in PFA beakers. The vessels were covered with Teflon watch covers and heated under reflux conditions for at least 4 h. The covers were then removed, 1 mL of H F and 1 mL of HClO, added to each, and the samples evaporated to near dryness. A further 5 mL of HNOB and 1mL of HClO, were then added and the solutions again taken near dryness. Following a final 1 mL addition of HC10, with heating to near dryness, the residues were dissolved in 1mL of HC1 and diluted to 25.0 mL with DDW. Duplicate samples were taken from each of four different bottles of TORT-1. Two blanks were run concurrently with each set of four sample decompositions. Total Sn was determined by using 200-pL aliquots of dissolved tissue solution delivered into the hyride cell containing 20 mL of 0.1 M HCl. The sequence of operations describing generation, collection, and atomization of SnH, is similar to that reported for As (22) and Se (13) and will not be repeated here with the exception of
a 0
200
400 600 800 TRAP TEMPERATURE, O C
I000
z
Figure 1.
Signal recovery as a function of system parameters.
details pertinent to the Sn system. The graphite furnace program is given in Table I. The furnace is preheated to 800 "C in order to deposit the Sn onto its surface. Prior to hydride generation, the cell, transfer line and furnace must be purged free of air in order to prevent the formation of a combustible mixture in the preheated tube as the excess H, cogenerated with the SnH, can be ignited by the hot graphite surface. A flow of Ar at 150 mL/min for 20 s is sufficient for this purpose. Two milliliters of NaBH, were metered into the cell a t a flow rate of 4 mL/min with a peristaltic pump. The cell was continuously purged with Ar a t a flow rate of 150 mL/min during this time and for a further period of 2 min following the NaBH, addition. After removal of the quartz transfer line from the sample introduction port, the furnace was rapidly heated to 2700 "C and the peak-height absorbance measured. Standard calibration curves prepared from spikes of inorganic tin standard added to 20 mL of 0.1 M HC1 were used for sample analyses.
RESULTS AND DISCUSSION Optimization of Signal. Efficient retention of tin in the furnace required use of a graphite tube with a well-developed surface structure or area. Relative t o the pyrolytic graphite coated tube, the uncoated surface used in this study enhanced the signal by a factor of 5. Figure 1 shows the relative peak height signals obtained in response t o changes affecting generation, trapping and atomization of tin. Although enhanced sensitivity (20% higher) could be achieved by generating the stannane from 0.01 M HC1, a 0.1 M HC1 medium was used because it proved to be more compatible with the generation of SnH, from real samples. Addition of 2 mL of 1% NaBH, to 20 mL of 0.01 M HC1 increased the p H to a final value of 9.4. When a 20-pL aliquot of dissolved sediment was present, a heavy black precipitate formed due to hydrolysis of major constituents. No pH change occurred upon addition of NaBH, to 0.1 M HCl solutions and, correspondingly, no precipitates were formed with sample present. Figure 1 shows 800 "C to be the optimum tube temperature for trapping the tin in the furnace. Temperatures in this figure refer only to those programmed into the furnace power supply. Stannane is rapidly decomposed to elemental tin at 800 "C (14) and subsequent conversion of Sn,,, to Sn02(s)may occur
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
in the presence of the relatively high oxygen partial pressure present during the stannane generation cycle. Above 850 "C,retention of Sn in the furnace rapidly decreases. This is comparable to data reported by Frech et al. (15) and Lundberg et al. (16) for the atomization of tin originally introduced into the furnace as an aqueous solution. Analytical Blanks. The primary source of the blank was determined to be the NaBH,. Although it is possible to clean this reagent (e.g. ref 8), no effort was made to do so and the long-term absolute blank was found to be 0.110 f 0.024 ng. Dissolution blanks for the sediment samples averaged 180 f 30 ng/g while those for TORT-1 were 12 f 4 ng/g. These represent approximately a 5-10% correction to the analytical results for the sediment and a 9% correction to the total tin found in TORT-1. It should be noted that dissolution blanks in excess of 150 ng/g were obtained when samples of TORT-1 were acid solubilized in clean quartz Erlenmeyer flasks as a consequence of leaching of Sn from the quartz surface. Attempts to dry ash TORT-1 in a furnace a t 450 OC were successful in that the organic matrix was completely oxidized, but no Sn was recovered, presumably due to formation of insoluble SnOz. Organotin Response. Alkyltin compounds react with NaBH4 under acidic conditions to yield the corresponding hydrides (7-9). Attempts to run a number of organotin hydrides were relatively unsuccessful. The efficiencies of generation, trapping, and atomization relative to inorganic tin were 100% for dimethyltin, 51% for trimethyltin, 75% for phenyltin, 84% for dibutyltin, and 46% for tri-n-butyltin. Use of a Tris buffer at pH 5-6, as suggested by Braman and Tompkins (7j, did not improve overall recovery. Since other researchers have been successful in quantitatively generating the alkyltin hydrides (7-9), the problem encountered in this study is most likely the deposition or trapping step which involves decomposition of the hydride on the graphite surface (17). As a consequence of these difficulties, analysis of aqueous environmental samples for total tin is dubious with this technique since the contribution to the signal by alkyltin compounds is unknown. It is of interest to note, however, that 1-ng spikes of inorganic tin added to 30-mL volumes of deep ocean seawater (salinity %%), coastal seawater (salinity 29.5%0),and river water could be completely recovered yielding a detection limit of -2 pg/mL. Although quantitative measuements of total tin concentrations in such samples cannot be undertaken, an estimate may be made with an error of no greater than 50% (assuming all tin to be present as tri-n-butyltin oxide). The methodology may thus prove useful as a rapid screening technique since, as will be evident, the detection capability is adequate for all but the cleanest of samples. Alternatively, organotin species in aqueous samples can be digested by UV irradiation in the presence of HzOz prior to quantitative analysis. Decomposition of organotin compounds with this procedure is reported to be complete, even in complex samples ( I , 18). Destruction of organotin compounds present in biological or sediment material is easily effected by using acid mineralization procedures, with quantitative conversion to inorganic tin (3,19). Thus, accurate total tin determinations in these materials is straightforward. Analytical Results. Analytical results, reported on a dry weight basis, are summarized in Table I1 and were obtained by calibration against simple working curves prepared by generating SnH, from 20-mL aliquots of 0.1 M HCl spiked with Sn(1V). The accuracy of the methodology is evident from a comparison of these results with those obtained by direct graphite furnace AAS analyses and by stable isotope dilution inductively coupled plasma mass spectrometry.
2443
Table 11. Analytical Results" total tin, wg/g technique
MESS-1
BCSS-1
TORT-1
HG-GFAAS
4.0 f 0.2 3.9 f 0.3 3.9 f 0.2
1.9 f 0.2 1.5 f 0.3 1.78 f 0.06
0.144 f 0.016
GFAAS
ID ICP-MSd
ND' 0.141 f 0.006
Mean and standard deviation of replicate analyses. *Direct injection conventional graphite furnace AAS. Not determined, below limit of detection. Isotope dilution inductively coupled plasma mass spectrometry.
Table 111. Figures of Merit sensitivity, matrix
AU/ng
sediments biological 0'099
]
* O'Oo7
LOD,O precision,* ng/g
18
%
linear
range, ng
6 (6)
a LOD = 3abbk *Numberin parentheses is concentration factor above LOD at which samale analvsis comaleted.
Interference Study. High concentrations of several metal ions have been reported to inhibit the formation of stannane (7). With the present methodology, no interferences were observed (as is evident from the accuracy of the data). Quantitative recovery of stannane was obtained from solutions containing 200 ppm Fe3+,2 ppm As3+, 1 ppm Ni2+,0.1 ppm CuZ+,and 0.1 ppm Se4+. These levels of potential interferences are, in most cases, 100-fold greater than the respective concomitant concentrations of these elements in the dissolved sediment solutions placed in the generation cell and exceed those in the dissolved TORT-1 solutions taken for analysis. Analytical Figures of Merit. Figures of merit are presented in Table 111. Absolute peak-height sensitivity, as determined from the slopes of calibration curves run in a number of tubes over several months, is 0.099 f 0.007 AU/ng (Le., 44 f 3 pg/0.0044 AU), comparable to that obtained by direct injection of aqueous solutions (31 pg/0.0044 AU (20)). This suggests that the generation-trapping process is better than 75% efficient. Estimated procedural detection limits for inorganic tin, based on the variability of the blank (3a) and the sizes of the sample portions used are 2 pg/mL in seawater, 360 ng/g in sediments, and 18 ng/g in biological tissue. Precision of determination is better than 10% (relative standard deviation) on determinations 10-fold above detection limits. The linear working range spans nearly 2 decades, extending to 4 ng. Higher sample analyte concentrations are accessible by working with smaller sample volumes, introducing a purge gas flow during atomization, or making measurements at the less sensitive 235.5-nm line.
CONCLUSION The combination of hydride generation with subsequent trapping of stannane in the graphite furnace gives the analytical chemist access to the determination of extreme trace concentrations of total Sn in diverse materials without resorting to complex cryogenic trapping arrangements. Although the method is suitable for the quantitative determination of total Sn in samples containing only inorganic Sn, it may prove useful as a rapid screening technique for environmental aqueous samples. Alternatively, the latter may be digested or photolyzed (with a possible degradation of detection limit) to yield a total Sn determination.
Anal. Chem. 1987, 59,2444-2446
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Note Added in Proof. Addition of 40 Kg of P d to the furnace with subsequent drying of this deposit a t 120 "C permitted equally efficient trapping of SnH, from ambient to 800 O C . ACKNOWLEDGMENT The assistance of J. W. McLaren in obtaining the isotope dilution inductively coupled plasma mass spectrometry data is gratefully acknowledged. Registry No. Sn, 7440-31-5; water, 7732-18-5. LITERATURE CITED Weber, G. Fresenius' Z . Anal. Chem. 1985, 321,217-224. Thompson, J. A. J.: Sheffer, M. G.; Pierce, R. C.; Chau, Y. K.; Cooney, J. J.; Cullen, W. R.; Maguire, R. J. "Organotin Compounds in the Aquatic Environment", NRCC 22494; National Research Council of Canada: Ottawa, ON, Canada, 1985. Maher, W. Anal. Chim. Acta 1982, 138,365-370. Togrul, S.; Balkas, T. I.; Goldberg, E. D. Mar. Pollut. 1983, 14, 297-303. Woollins, A.; Cullen, W. R. Analyst (London) 1984, 109, 1527-1529. Chan, C. Y.: Baig, M. W. A. Anal. Chlm. Acta 1982, 136, 413-419. Braman, R. S ; Tompkins. M. A. Anal. Chem. 1979, 5 1 , 12-19.
(8) Andreae, M. 0.; Bryd, J. T. Anal. Chim. Acta 1984, 156, 147-157. (9) Donard, 0. F. X.: Rapsomanikis, S.:Weber, J. H. Anal. Chem. 1988, 5 8 , 772-777. (IO) Lee, D. S.Anal. Chem. 1982, 5 4 , 1682-1686. (11) Sturgeon, R. E.; Willie, S. N.; Berman, S. S. Anal. Chem. 1985, 5 7 , 231 1-2313. (12) Sturgeon, R. E.;Willie, S. N.; Berman. S. S. J . Anal. A t . Spectrosc. 1986, 1 , 115-118. (13) Willie, S. N.: Sturgeon, R. E.; Berman, S. S. Anal. Chem. 1986, 58, (14) Tamaru, 1140-1143. K, J , phys, Chem, 1958, 60, 610-612, (15) Frech, W.; Lundberg, E.;Cedergren, A. Prog. Anal. A t . Spectrosc. 1985, 8 ,257-370. (16) Lundberg, E.; Bergmark. E.;Frech, W. Anal. Chim. Acta 1982, 142, 319-324. (17) Sturgeon, R . E.: Willie, S N.; Sproule, G. 1. J . Anal. A t . Spectrosc. 1987, 2 , 0000. (18) Burba, P.: Willmer. P. G. Fresenius' Z . Anal. Chem. 1982, 317, 222-231. (19) Thorburn Burns, 0.; Glockling, F.; Harriott, M. Analyst (London) 1981, 106, 921-930. (20) Fernandez. F. J.; Iannarone, J. A t . Abs. News/. 1978, 7 7 , 117-120.
Received for review January 29, 1987. Accepted June 4,1987. This paper is NRCC No. 28027.
Fiber-optic Probe for Measurement of I nterfaclal Area in Vigorously Stirred Solvent Extraction Systems Mark L. Dietz and Henry Freiser* Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson. Arizona 85721 The quantitative determination of interfacial area in dispersions of immiscible liquids is of considerable importance in the analysis of mass transfer across phase boundaries and, therefore, in the understanding of the mechanisms of solvent extraction processes. Recent work in this laboratory ( I ) has shown that in dispersed systems containing some surfaceactive species, interfacial tension measurements, in combination with measurements of the reversible changes in the organic phase concentration of the surfactant induced by vigorous stirring, can be used to calculate the interfacial area and mean drop size of the dispersed liquid phase. This interfacial adsorption method, while simple and convenient, is obviously directly applicable only to systems in which some interfacially active species is present. In the course of studies concerning the role of the interface in the kinetics and mechanisms of solvent extraction processes, we have, in fact, encountered several systems in which the interfacial activity exhibited by all species present is so low as to make a reliable measurement of the reversible absorbance change due to stirring difficult, thereby precluding an accurate determination of interfacial area by this approach. In an effort to obtain area data in such systems, we have turned to light transmission methods. Several studies (2-5) have demonstrated that specific interfacial areas in liquidliquid dispersions can be readily determined by measurements of the amount of light transmitted by the dispersion relative to the pure continuous phase. However, published procedures for these measurements suffer from several shortcomings which make them not entirely satisfactory for our purposes. First, the light transmission probes employed have typically been calibrated by photomicroscopy, that is, by determining the response of the probe in a series of dispersions whose interfacial areas were determined photographically. This method of calibration, while accurate, is both tedious and time-consuming. In addition, these probes have typically been constructed of metal and have been relatively large, consistent
with their use by engineers in large mixing vessels. In our studies, in which chelating extractants are normally present in the organic phase, a probe without exposed metal parts is clearly desirable. Moreover, since our studies are ordinarily carried out in 250- or 500-mL Morton flasks ( 6 ) , a probe considerably smaller than those previously described is required. In this report, we describe a simple fiber-optic light transmission probe without exposed metal parts, suitable for use in small reaction vessels. In addition, we demonstrate that the interfacial adsorption method ( I ) provides a simple means of calibrating the device, eliminating the need for photographing the dispersions. Finally, we present data demonstrating the accuracy of the probe system and its range of applicability.
EXPERIMENTAL SECTION Reagents. Triton X-100 (TX-100) and Triton X-45 (TX-45) were obtained from Sigma Chemical Co. and were used as received. 8-Quinolinol (Aldrich) was recrystallized twice from absolute ethanol. Stock solutions of its copper(I1)and nickel(I1) chelates in chloroform were prepared by shaking 200 mL of an aqueous solution of the appropriate metal perchlorate with a slight excess of the ligand dissolved in an equal volume of chloroform. Prior to use, the chloroform was washed three times with water. Distilled, deionized water was used throughout. All other chemicals were reagent grade. Apparatus. A schematic diagram of the light transmission probe is shown in Figure 1. The probe is simply inserted into one of the necks of a 500-mL Morton flask. Dispersions are produced in this flask by means of a high-speed stirrer (0-20000 rpm) supplied by Cole-Parmer Instrument Co. Light (632.8 nm) from a 5-mW HeNe laser (Hughes Aircraft Co.) is directed into the dispersionsvia a 1-ft length of in. diameter image conduit, a bundle of 3050 5-pm diameter glass optical fibers fused to form A rod (Edmund Scientific Co.). Transmitted light is directed to the monochromator/detector (of a Varian Techtron Model AA-6 ipectrophotometer) by a flexible 1/8 in diameter fiber-optic light
0003-2700/87/0359-2444$01.50/0 C 1987 American Chemical Society
