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A N A L Y T I C A L CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
analyzing combustion effluents.
ACKNOWLEDGMENT The authors gratefully acknowledge the helpful suggestions of their colleague, Karl R. Westberg.
LITERATURE CITED (1) G. A. Capelle and D. G. Sutton, Appl. Phys. Let?., 3 0 , 407 (1977). (2) A. N. Wright and C. A. Winkler, "Active Nitrogen", Academic Press, New York. N.Y.., 1968. . . ~ (3) F.-C.'Fehsenfeld, K. M. Evenson. and H. P. Broida, Rev. Sci. Insbum., 36,294 (1965).
(4) A . H. Shapiro, "The Dynamics and Thermodynamics of Compressibie Fluid Flow", Vol. I , Ronald Press, New York, N.Y., 1953. (5) L. B. Loeb, "The Kinetic Theoty of Gases", Dover Publications, New York, .,I,
N.Y.,
.**I
IYOI.
(6) H. A. Dewhurst and G. D. Cooper, J. Am. Chem. SOC., 82, 4220 (1960). (7) R. Storr. A. N. Wright, and C. A. Winkier, Can. J . Chem., 40, 1296 (1962). (8) C. J. Duthler and H. P. Broida, J . Chem. Phys., 59, 167 (1973). (9) D. H. Stedman. J. A. Meyer, and D. W. Setser, J. Chem. Phys.,48, 4320 (1968). (10) W.R. Brennen and G. B. Kistiakowski, J , Chem. Phys., 44, 1695 (1966). (11) L. F. Phillips, Can. J . C h e m , 41, 73 (1963): also, 41, 2060 (1963).
RECEIVED for review April 14, 1978. Accepted May 22, 1978.
Determination of Microgram Amounts of Some Transition Metals in Seawater by Methyl Isobutyl Ketone-Nitric Acid Successive Extraction and Flameless Atomic Absorption Spectrophotometry Tsu Kai Jan" and David R. Young Southern California Coastal Water Research Project, €1 Segundo, California 90245
A method Is presented for the analysis of trace metals in seawater at concentrations below 1 pg/L. The method utilizes a methyl isobutyl ketone (MIBK) extraction procedure followed by a nitric acid back extraction step (to stabilize the metal complexes), and analysis by flameless atornlc absorption spectrophotometry. The detection limits for Ag, Cd, Cr, Cu, Fe, Ni, Pb, and Zn are 0.02, 0.003, 0.05, 0.05, 0.20, 0.10, 0.03, and 0.03 pg/L, respectively. For those metals occurring below 1 pg/L, triplicate analyses of three different seawater samples yield mean relative standard deviations ranging from 18 to 25%.
Flameless atomic absorption spectrometry is being widely utilized for metals determinations in aqueous samples because of its low detection limits (on the order of 1 bg/L) and its relative ease of operation. However, concentrations of most trace metals in seawater are often considerably below this level. In addition, certain of the dissolved solids in seawater (totaling about 3.5 g/L) may cause serious interferences in the determination of such elements as Ag, Cd, Cu, P b , and Zn. Hence, a sample preparation technique is necessary to preconcentrate trace elements from seawater and separate them from the interfering components. Many studies (1-6) have been reported to serve this purpose, but ion exchange and MIBK extraction are t h e two methods most generally used. T h e Chelex-100 cation-exchange method of Riley and Taylor ( I ) requires a large sample size and gives low recovery efficiencies for Cr and Cu, two metals of great interest to marine ecologists ( 7 ) . Brooks e t al. (3) developed a n APDC-MIBK extraction technique for the determination of six elements in saline water by flame atomic absorption spectrophotometry. However, their work did not include three metals of primary concern (Ag, Cd, Cr); i t also was limited by the instability of the metal complexes formed (less than one day). Therefore, we have attempted to overcome these difficulties by developing a revised procedure for the determination of Ag, Cd, 0003-2700/78/0350-1250$0 1. O O / O
Cr, Cu, Fe, Ni, P b , and Zn in seawater. This procedure involves a n APDC-MIBX extraction step followed by back extraction with 4 N nitric acid. T h e metals complexes in the acid extract were found to be stable for more than one week, with the exception of the Ag complex, which is stable only for about 3 days. Here we present the details of this procedure, and compare the results with those obtained by the Chelex-100 ion exchange and APDC-MIBK single extraction methods.
EXPERIMENTAL Apparatus. A Varian-Techtron atomic absorption spectrophotometer (model AA-6), equipped with a carbon rod atomizer (model 63) and a background corrector (model BC-6), was used for all trace metal measurements. Two and one-half gL of treated sample solution was injected into the graphite tube using a Unimetrics micropipetter (model Bolo), and absorbance peaks were recorded in the peak-read mode. The instrumental settings for each element are given in Table I. A Beckman Model H-5 pH meter with glass electrode was used for pH adjustments and a Burrell wrist action shaker was used for the extractions. Reagents and Materials. The following analytical reagent grade chemicals were used without further purification: Ammonium hydroxide, concentrated and 1 N solutions; 5% ammonium acetate buffer solution, pH adjusted to 5.5 with acetic acid; metal standard solutions, which were made from dilutions of 1000 ppm stock solutions (Varian-Techtron). Nitric acid was double distilled from Vycor (G. Frederick Smith Chemical Co.). MIBK was double distilled and 1% ammonium pyrrolidine dithiocarbamate aqueous solution (APDC) was freshly prepared and then purified by redistilled MIBK. Distilled deionized water (DDW) was used for all rinses and dilutions. Chelex-100 resin (Bio-Rad Laboratories) was purified with 1 N HN03solution and then successively treated with 1 N NH40H solution and CH,COONH, buffer to convert it to an ammonium form. The seawater samples were collected in Nalgene polyethylene bottles and filtered through a 0.4-pm Nuclepore membrane filter packed with polypropylene separators. The filtered samples were acidified with concentrated HN03 to a pH of 2 and preserved by refrigeration. Nalgene Teflon beakers with Teflon covers and Teflon separatory funnels were used for sample preparations. Glass chromatography columns, 2 cm in diameter fitted with 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO 9, AUGUST 1978
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Table I. Instrumental Settings for AAS of Metals in Seawater Extracts element
Ag
Cd
Cr
cu
Fe
Ni
Pb
Zn
wavelength, nm lamp current, mA spectral slit, nm dry,a 50 s ash,a 20 s atomize' 2 s N, gas, L/min H, gas, L/min
328.1 3 0.5 3.5 5.0 6.5 4
228.8 5 0.5 3.5
357.9 5 0.2 3.5
248.3 5 0.2 3.5
232.0 5 0.2 3.5
217.0 5
4.0
6.0
324.7 3 0.5 3.5 6.0
6.0
6.0
7.0 4
7.5 4
7.0
4
8.0 4
8.0 4
213.9 5 0.5 3.5 4.0 6.5 4
-
-
-
1
Arbitrary dial settings o n the M-63 power supply.
Ag Cd Cr cu Fe Ni Pb Zn
Crg added'
relative recoveryb 4N
6N
0.020
1.0 1.2
1.5 1.1
0.025
1.2
0.100 0.100
1.0 0.61
1.0 1.0 1.0 1.0
0.100 0.050 0.250
0.88 0.21 0.79
0.037
1.0
1.0 1.0 1.0
6.5
4
-
-
-
Table 111. Analytical Blanks and Detection Limits element
2 N
3.5 4.0
-
Table 11. Effect of Nitric Acid Concentration o n Metals Determination element
-
1.0
Ag
Cd
cr
cu Fe Ni Pb Zn
0.83 1.3 0.75 0.84
0.81 0.75
Filtered island control seawater (200 mL) spiked with metal standard solution. Results are mean values obtained from 3 replicates (corrected for unspiked values) and are determined relative to the recovery obtained using 4 N HNO, as the back extraction reagent.
analytical blank,a pg/L
detection limit,b pg/L
< 0.02
