Environ. Sci. Technol. 1991, 25, 1704-1708
supported in part by the Ocean Assessments Division, National Oceanic a n d Atmospheric Administration (NOAA); the Office of the Chief of Naval Operations, Department of the Navy; the Minerals Management Service, Department of the Interior; and Environmental Monitoring Systems Laboratory (Las Vegas), Environmental Protection Agency. Certain commercial equip-
ment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are the best available for the purpose.
Determination of Trace Metals in Saline Irrigation Drainage Waters with Inductively Coupled Plasma Optical Emission Spectrometry after Preconcentration by Chelation-Solvent Extraction Gordon R. Bradford* and Darlush Bakhtar Department of Soil and Environmental Sciences, University of California, Riverside, California 9252 1
Preconcentration of Fe, Mn, Cu, Zn, Cd, Pb, V, Mo, Ni, Co, Cr, T1, Ga, Au, U, Hg, Se, As, Sn,Sb, Bi, and Te from saline water is described using multielement chelation with ammonium pyrrolidinedithiocarbamate extracted into chloroform. Extract residues were taken up in dilute nitric acid solution for analysis by simultaneous multielement inductively coupled plasma optical emission spectrometry (ICP-OES). Recovery percentages of elements at low microgram per liter levels in spiked saline samples ranged from 92 to 102 5%. Saline agricultural drainage and evaporation pond water samples from the San Joaquin Valley, CA, were analyzed by this method. In the case of U, the accuracy of the combined procedure was confirmed by an independent method. Introduction
Direct determination of low microgram per liter concentrations of many trace elements by inductively coupled plasma optical emission spectrometry (ICP-OES) in saline waters is not feasible due to insufficient instrumental sensitivity and/or interferences from a highly saline matrix. Liquid-liquid extraction, (1)coprecipitation, (2) chelating ion exchange, (3)solvent evaporation, ( 4 ) hydride generation, (5) and chelation-solvent extraction (6-10) have been used for preconcentration. Cresser (11) cited several hundred papers published between 1955 and 1975 on single-element analyses by atomic absorption spectrometry (AAS) following chelation and solvent extraction. More recently, multielement extractions have been reported (7-9) for analyses by AAS. Sugiyama et al. (12) reported on chelation of 13 elements and direct analyses of their organic solvent extract by ICP-OES. However, interchanging ICP-OES parameters from pneumatic nebulization of aqueous to organic solvents is difficult. Therefore, a method fully adapted to simultaneously concentrate as many as 22 trace elements for analyses by ICP-OES using direct pneumatic a q u e o u s nebulization is attractive. As recently as 1986, Thompson (13) wrote, "Several applications of pre-concentration to geochemical analyses by ICP-OES have been reported, the materials being water, rocks, soils and sediments. However, it is clear that a good, general-purpose, rapid, multi-element pre-concentration method which is selective against interfering matrix elements (Al, Ca, Mg, Fe, Na, K) is still lacking and would easily repay the considerable thought and investment in development time". Particularly, dithiocarbamates have been used to complex many metals for extraction into an organic phase with 1704
Environ. Sci. Technol., Vol. 25, No. 10, 1991
Table I. ICAP-OES Wavelengths and Instrumental Detection Limits" element
wavelength, nm
arsenic antimony barium bismuth boron cadmium chromium cobalt copper gold gallium iron lead lithium manganese mercury molybdenum nickel selenium tellurium thallium tin strontium uranium vanadium zinc
193.69 206.83 493.40 249.67' 223.06a 228.80" 267.71 286.61 324.25 242.80 417.21 259.94 220.35 670.70 257.61 253.65 202.03" 231.60 196.02 214.20" 190.86" 284.00 421.50 385.96 292.40 2O6.2Oa
Second-order lines. NA, not applicable. a
detection limits, m d L instrumental preconcn 0.001b 0.001b 0.002 0.001b 0.005 0.004 0.002 0.005 0.010 0.005 0.020 0.005 0.020 0.005 0.005 0.001*
0.008 0.010 0.001b 0.001b 0.100 0.10 0.20 0.10 0.010 0.005
0.001 0.001 NAc 0.001 NAc 0.001 0.001 0.001 0.002 0.001 0.001 0.003 0.003 NAc 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.001 NAc 0.003 0.001 0.002
Continuous-flow hydride generation.
high distribution ratios.(6-10, 12, 14, 15) A typical structural formula for a metal (M) complex of ammonium pyrrolidinedithiocarbamate (APDC) is shown in Figure 1. In this paper, a simultaneous preconcentration method for 22 elements with large enrichment factors by extraction of APDC metal complexes into chloroform is reported. It has application for trace element analyses of waters, soil extracts, soil-exchange solutions, and agricultural drainage waters containing a wide concentration range of cations (Ca, Mg, Na, K)and anions (Cl-, SO4"). The procedure is relatively simple, rapid, and adaptable to aqueous pneumatic nebulization with ICP-OES. Accordingly, one laboratory worker can extract 20-25 samples per 8-h working period. Experimental S e c t i o n Apparatus. A Jarrell-Ash Atomcomp 800 series spectrometer with computer-controlled background correction including spectral line overlap correction and other timing
0013-936X/91/0925-1704$02.50/0
0 1991 American Chemical Society
High Salt Matrix Test Solution. Naturally high salt water used for APDC solvent extraction testa was collected from the Salton Sea, a large inland body of water in southern California maintained by surface inflow of saline irrigation water. The water was filtered through a 0.45-1.tm Millipore filter to remove suspended material. Two hundred milliliter portions of water were purified by the APDC extraction procedure described below. Procedure. Aliquots of test solutions to be used for extraction recovery experiments, ammonium acetate buffer solution and saline water samples were preadjusted to pH 5.0 with acetic acid or ammonium hydroxide to ensure accurate buffer pH control during extraction. A 100-mL aliquot of purified test solution (pH 5.0) was spiked with trace element standard solutions and transferred to a 250 mL Teflon separatory funnel held upright in a funnel rack. Trace element concentrations in spiked test solutions varied between 10 and 120 pg/L for different elements depending on their sensitivity when measured by ICPOES. Then, 10 mL of purified 1M ammonium acetate solution (pH 5.0) were added, followed by 5 mL of 3% solution of purified APDC. After 5 min, 10 mL of chloroform were added and the separatory funnel shaken briefly by hand, and subsequently by a horizontal mechanical shaker (3 min). After phase separation the chloroform layer was drained off slowly into a 30-mL Teflon beaker. Another 5 mL of APDC solution was added, the separatory funnel (uncapped) was transferred to a microwave oven, and the contents were heated to incipient boiling (approximately 1.5 min on high). The separatory funnel was removed from the microwave oven and allowed to cool to room temperature (approximately 20 min) before recapping and extracting with another 10 mL of chloroform. This chloroform extract was combined with the previous extraction. Another 10 mL of chloroform was added (no APDC), extracted, and combined with the previous extractions. Any visible water droplets in each chloroform extract were removed by drawing into a disposable polyethylene dropper and adding back to the aqueous phase. The combined volume of chloroform was slowly evaporated to dryness on a temperature-controlled hot plate a t
