Anal. Chem. 1990, 62,2522-2526
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Determination of Rhenium in Groundwater by Inductively Coupled Plasma Mass Spectrometry with On-Line Cation Exchange Membrane Sample Cleanup Michael E. Ketterer United States Environmental Protection Agency, National Enforcement Investigations Center, Box 25227,Building 53, Denver Federal Center, Denver, Colorado 80225
A scheme Is presented for determlnlng rhenium In groundwater samples that contain up to 4000 mg/L dtssolved sdids by lnductlvely coupled plasma mass spectrometry. A commercially available catlon exchange membrane cartridge Is used on-line to exchange cationic specles for equlvalent quantities of hydrogen lon, rhenlum, which Is present as the perrhenate anion, remains on the upstream slde of the membrane and Is transported dlrectly Into the inductively coupled plasma. Thls arrangement successfully alleviates the matrlx-related sample introductlon dltflcultles and permits direct determination of rhenium In water with a detection Ilm%of 0.03 Fg/L uslng a Melnhard-type nebulizer. Removal efficiencies of up to 100% are achleved for sodlum, m a g " , aluminum, potassium, and calcium Ions, whlle perrhenate is transmitted with 100% efficiency. Results are presented for the determlnatkn of rhenkm In groundwater samples from the vkinlty of a metal sulfide talllngs Impoundment in the western United States.
INTRODUCTION Rhenium is one of the rarest elements in the Earth's crust, possessing an abundance of approximately 1&kg (1-3). This element is enriched in porphyry copper and molybdenite ores and is present in these sources at 1-2000 mg/kg (4-8). The seawater abundance of rhenium has been established at about 0.01 +g/L (9-11). It has been established (7-11) that the perrhenate ion, ReW,, is the only significant rhenium species present in most aqueous environments. Data on the occurrence and levels of rhenium in fresh surface waters and groundwaters are minimal (12). Interest in the chemistry and measurement of rhenium in the author's laboratory has been aroused by recent findings of elevated levels of rhenium in groundwaters underlying a copper, zinc, and lead sulfide ore tailings impoundment in the western United States. This element's presence is believed to associated with the entry of tailings-related contaminants into heretofore uncontaminated groundwater systems. Methods were thus sought for the determination of rhenium in these samples, which typically contained 0.1-0.4% total dissolved solids, chiefly sodium, potassium, magnesium, and calcium chlorides and sulfates. Levels of rhenium were found to range from less than 0.03 +g/L to about 3 gg/L. Analytical methods for the determination of low levels of rhenium in water and solid sample digests include electrothermal vaporization atomic absorption (AA) spectroscopy (II), resonance ionization mass spectrometry (13),and neutron activation analysis (9). The electrothermal AA methods generally have poor sensitivity, with detection limits on the order of 0.5 ng of Re injected; additionally, matrix effects are severe in the presence of elements such as Mo. For a relatively large sample injection of 100 gL, the corresponding solution detection limit is 5 pg/L. Owing to the high concentration-
based detection limit and the severity of matrix effects due to the presence of concomitant elements, Goldberg and coworkers (11)developed a preconcentration scheme for isolation of rhenium from seawater by using concentration of large sample volumes on chloride-form anion exchange resins, followed by elution with 8 M nitric acid, evaporation, fusion of the residue, and solvent extraction of tetraphenylarsonium perrhenate. Overall recoveries were monitored through the addition of a radioactive lMRetracer, and ranged from 20 to 76%. Walker (13) has demonstrated the utility of resonance ionization mass spectrometry (RI-MS) for the determination of rhenium in aqueous digests of rock samples; acid digestions were followed by solvent extraction with tribenzylamine in chloroform, followed by back-extraction with aqueous ammonia. Overall recoveries were about 70%;isotope dilution was used for quantitation. While sensitivities in the 20-40 pg range were realized for Re, the disadvantage of the RI-MS method is in the tedious filament loading steps required prior to mass spectrometric measurement. Inductively coupled plasma mass spectrometry (ICP-MS) is now a well-established analytical technique for the determination of many elements a t detection limits of 1gg/L or less (14). The technique lends itself well to the direct measurement of elemental concentrations and isotopic ratios using solution-phase sample introduction; for rhenium, favorable analytical figures of merit, such as a lack of significant polyatomic interferences, facile sample introduction, and detection limits of less than 0.1 pg/L, would be expected. Previous work by Thompson and Houk (15), and by Van Heuzen and co-workers (16)demonstrated the feasibility of using ICP-MS for the determination of RE. The latter study (16) focused on the determination of Re in oil refining catalysts using ICP-MS with isotope dilution quantitation. The ICPMS technique had been used in preliminary identification of rhenium in the groudwater samples of interest; a practical approach for its routine determination in large numbers of samples was desired. Analytical schemes requiring extensive separation and preconcentration are generally disfavored over direct analysis of the sample, as losses and contamination problems frequently occur. These approaches also greatly increase the workload of the laboratory per unit of sample throughput. Thus, for the analytical problem mentioned herein, an ICPMS scheme involving direct determination of Re in aqueous solutions at the 0.03-3.0 pg/L level was sought. ICP-MS with direct introduction of undiluted water samples was first tried; many samples produced problems with nebulizer and sampler cone clogging. The high matrix content of the samples also produced reductions of 10-20% in analyte signal sensitivity; while 1931r+was found to be an effective internal standard, real-sample detection limits were proportionally degraded due to signal suppression. Therefore, a method was sought that would circumvent these problems without requiring separation schemes of the types used previously (11, 13). The on-line cation exchange sample cleanup method was developed spe-
This article not subject to U S . Copyright. Published 1990 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
cifically based upon the needs of this problem. Ion exchange membranes have been utilized in analytical chemistry in order to achieve analytical separations, and to act as separators of electrochemical cell compartments. A widely used application is in anion chromatography (AC), where a cation exchange membrane is used to quantitatively remove sodium ion at approximately 100 mg/L from eluents prior to conductometric detection ( I 7). In the AC application, a 0.025 M aqueous sulfuric acid solution ("regenerant") is pumped countercurrently on the downstream side of a sulfonated polymeric membrane; the sodium ions are exchanged for an equivalent quantity of hydrogen ions. Anionic constituents in the upstream (sample) stream are unable to permeate the membrane because of the high density of fixed anionic sites in the membrane (Donnan exclusion). Flow rates used in AC are on the order of 0.5-3 mL/min for the sample stream and 1-15 mL/min for the regenerant stream. Because the process operates in a continuous-flow fashion, unlike a cation exchange resin bed, no interruption of operation is required for regeneration, as was required in the first implementation of chemically suppressed anion chromatography (18).
In the present study, a commercially available cation exchange membrane cartridge, designed for the anion chromatographic application, has been utilized without modification for on-line removal of cationic constituents of water samples prior to ICP-MS. Described herein are results for the determination of rhenium, which exists as the anionic perrhenate species, in groundwater samples containing 0.1-0.4 ?% dissolved solids. The removal efficiencies of major cationic constituents have also been investigated for a range of sample flow, regenerant flow, and matrix concentration conditions.
EXPERIMENTAL SECTION Reagents. Deionized-distilled water was prepared in-house by distillation in glass following mixed bed ion exchange purification. Trace-metal grade nitric acid was used without further purification. Stock solutions of lo00 mg/L rhenium and iridium were obtained from commercial sources. Rhenium standard solutions were prepared by serial dilution of lo00 mg/L stocks into 0.5% (volume/volume) aqueous nitric acid. Test solutions of sodium, magnesium, aluminum, potassium, and calcium were prepared by dilution of loo00 mg/L stock solutions obtained from commercial sources; the test solutions had a nitric acid content of 0.5% (v/v). A test solution containing 0.1 mg/L of 38 elements in a matrix containing 100 mg/L of Na, Mg, Al, K, and Ca was prepared by using 1000 mg/L stock solutions, obtained from commercial sources. Ion Exchange Apparatus. A Dionex AMMS-1 cation exchange membrane cartridge (part no. 037072),was used without modification. This device possessed upstream and downstream solution inlets and outlets; for routine analysis, 1.3 mL/min of analyte solution was pumped upstream of the membrane, and 3.9 mL/min of 1 M aqueous nitric acid was pumped countercurrently downstream. A block diagram of the analytical configuration is shown in Figure 1. The internal standard addition arrangement shown therein was used in the cation exchange transmission studies (vide infra) but not for routine analysis. Inductively Coupled Plasma Mass Spectrometry. A Sciex Elan Model 250 inductively coupled plasma mass spectrometer, equipped with mass flow controllers and a water-cooled spray chamber, was used in this study. Operating conditions for routine analysis are shown in Table I. Rhenium standards of 0,0.001, 0.003, and 0.010 mg/L in 0.5% (v/v) nitric acid were used; the elemental equation shown in Table I, which contains a correction for le70s+,was used. For routine analysis, 0.1 mg/L Ir was used as an internal standard; this was added manually to each sample prior to passage of the sample through the AMMS-1 membrane cartridge. Iridium was not removed by the cation exchange membrane cartridge due to its presence as [WC&]". The possible effects of polyatomic ions derived from ls9Tm+or 171Yb+were neglected; preliminary mass spectra of the samples demonstrated the absence of these elements.
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INTERNAL STANDARD SOLUTION
I
t1 AMMS-1
TO ICP
I
=
PUMP
REGENERANT SOLUTION
Figure 1. Block diagram of apparatus for on-line sample cleanup with a cation exchange membrane cartridge.
Table I. Operational Parameters for Routine Determination of Rhenium by ICP-MS with On-Line Cation Exchange Membrane Sample Cleanup forward rf power = 1150 W; reflected power = 5 W plasma Ar flow = 13.5 std L/min auxiliary Ar flow = 1.3 L/min nebulizer Ar flow = 1.2 L/min sample solution flow rate = 1.3 mL/min sampling depth = 25 mm spraychamber temperature = 10 "C mass spectrometer pressure = 2.6 X Torr interface pressure = 1.0 Torr electron multiplier voltage = 3930 V deflector voltage = 4030 V B lens setting = 20; El lens setting = 37 P lens setting = 02; S,lens setting = 37 resolution = low (1.0-1.1 m / z at 10% height) measurement mode = multichannel measurement time = 10 s dwell time = 0.050 s cycle time = 0.100 measurements/peak = 1 reueatslintearation = 3
Cation Exchange Transmission Studies. Experiments were conducted to determine the AMMS-1 device's transmission of matrix ions (Na, Mg, Al, K, Ca) and perrhenate. The ICP-MS was first calibrated with the AMMS-1 bypassed; standards contained 0-10 mg/L matrix elements and 04.02 mg/L Re. For this study, the ion optics were adjusted to provide good sensitivity for Re and high linear working ranges for the light matrix elements. The transmission of matrix components for solutions containing 10, 100, and 500 mg/L of each matrix element and 0.01 mg/L Re were determined by on-line analysis of the effluent using 1.3 mL/min sample flow and 3.9 mL/min 1M aqueous nitric acid regenerant. The 500 mg/L Na/Mg/Al/K/Ca matrix's transmission was also studied by using the same sample flow rate with (a) 3.9 mL/min 3 M nitric acid regenerant and (b) 6.3 mL/min 3 M HN03. A 2000 mg/L Na matrix containing 0.02 mg/L Re was also investigated with sample flow rate of 1.3 mL/min and a 3 M nitric acid regenerant at 6.3 mL/min. In all of these transmission studies, the sample stream was modified to contain 0.25 mg/L Sc and Ir by blending in 0.03 mL/min 10 mg/L Sc/Ir solution as shown in Figure 1. &Sc+was used as an internal standard for the light matrix elements, and ls31r+was used as an internal standard for rhenium. Sodium, magnesium, aluminum, potassium, and calcium were monitored at masses 23, 24, 27, 39, and 44, respectively. Also studied was the transmission of components in a solution containing 100 mg/L each of Na, Mg, Al, K, Ca, and 0.1 mg/L of 38 elements (Li, Be, Ti, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Rb,
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 23, DECEMBER 1, 1990
Table 11. Cation Exchange Membrane Cartridge Transmissions of Sodium, Magnesium, Aluminum, Potassium, Calcium, and PerrhenateO matrix 10 mg/L Na/Mg/Al/K/Ca 0.0100 mg/L Re
100 mg/L Na/Mg/Al/K/Ca 0.0100 mg/L Re
transmission
regenerant 3.9 mL/min 1 M HN03
3.9 mL/min 1 M HNOB
Na Mg
A1 K Ca Re Na Mg A1 K
Ca Re 500 mg/L Na/Mg/Al/K/Ca 0.0100 mg/L Re
3.9 mL/min 1 M HN03
Na Mg
A1 K
Ca Re
500 mg/L Na/Mg/Al/K/Ca 0.0100 mg/L Re
3.9 mL/min 3 M HN03
Na Mg
A1 K
Ca 500 mg/L Na/Mg/Al/K/Ca 0.0100 mg/L Re
2000 mg/L Na 0.0200 mg/L Re
6.3 mL/min 3 M HNO,
6.3 mL/min 3 M HNO,
Re Na Mg A1 K
Ca Re Na Re
2.1 f 0.5%b
