Anal. Chem. 1990, 62, 1477-1481 (51) Yang. Y.; D'Silva, A. P.; Fassel. V. A. And. Chem. 1981, 53, 2107-2109. (52) Dankovic, D. A.; Springer, D. L.; Mnn, D. 6.; Smith, L. G.; ~ hB. L.; Bean, R. M. Carcinogenesis 1989, 10, 789-791.
RECEIVFXI for review January 3,1990. Accepted April 5,1990.
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Ames Laboratory is operated by Iowa State Univerisity for the Department of Energy under Contract No. ~ ~U.S. ~ , 7405-ENG-82. This research was supported by the U S . Department of Energy, Office of Health and Environmental Research, Physical and Technological Studies.
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Determination of Gold at Femtomolar Levels in Natural Waters by Flow Injection Inductively Coupled Plasma Quadrupole Mass Spectrometry Kelly Kenison F a l k n e r * a n d John M. Edmond Department of Earth, Atmospheric a n d Planetary Sciences, E34-246, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A method for the determination of Au In seawater at femtomolar moi/L) levels Is presented. The technique involves preconcentration by anlon exchange of Au as a cyanide complex, [Au(CN),-1, using lgSAuradiotracer (f,,, = 183 days) to monitor recoverles. Samples are then Introduced by flow injection into an Inductively coupled plasma quadrupole mass spectrometer for analysis. The method has a detection limit of 4 0 fM for 4 L of seawater preconcentrated to 1 mL and a relative preclslon of 15 % at the 100 fM level. With slight modifications, It can also be applied to the analysis of hydrothermal flulds and freshwaters.
Koide and co-workers have recently reported concentrations of Au in Pacific seawater samples on the order of 100 fM (I). Using the analytical method described here, we have independently confirmed levels of 10-100 fM Au in seawater a t Atlantic and Pacific Ocean and Mediterranean Sea locations (21, approximately 3 orders of magnitude lower than values reported prior to 1988 ( 3 , 4 ) . As was the case for other traces metals in seawater (5), earlier workers appear to have been hampered by contamination and limitations of available instrumentation. Accordingly, very little is presently known about Au in the marine environment. Gold is predicted to be present in oxic seawater in the +I state as the neutral species [Au(OH)(H,O)] (6),but if thermodynamic equilibrium is not attained, it may also be present in other soluble forms including complexes of the +I11 state and reduced elemental colloids. Under anoxic conditions, in the presence of sulfide, [Au(HS),]- and the metallic form are predicted to be the major species (6). The speciation of Au in the marine environment has yet to be determined analytically. Progress in understanding the behavior of Au in the weathering cycle and in the oceans will require further mapping of its distributions in natural waters combined with an assessment of its redox behavior. While a variety of techniques for the preconcentration and analysis of Au in natural waters have recently been reported in the literature (7-13), these do not provide low enough detection limits for the determination of Au in open ocean waters. Koide and co-workers ( 1 ) were able to obtain sufficiently low detection limits by preconcentrating 2-L seawater samples via anion exchange of Au chloride complexes followed 0003-2700/90/0362-1477$02.50/0
by uptake of the samples onto single-anion-exchange resin beads. Individual beads were injected and combusted in a graphite furnace for atomic absorption spectrophotometric analysis. This technique allowed them to first approximate the true concentration of Au in seawater; however, it appears from replicate analyses that their relative precision varied considerably (up to 80% at 3140 M),making it difficult to interpret variations in their Pacific profile. In addition, overall Au recoveries, as monitored with Au-195 radiotracer, were only - 5 0 % . Here we present an analytical technique for the determination of Au in seawater using anion-exchange preconcentration and flow injection inductively coupled plasma quadrupole mass spectrometry (FI-ICPMS). It offers consistent, nearly quantitative Au recoveries in the preconcentration step (90 f 5%) and improved overall repeatability (15% a t the 100 fM level). EXPERIMENTAL SECTION Apparatus. Yield monitoring of Au-195 radiotracer ( t l j z = 183) was performed on a 1.5 in. by 1.5 in. NaI(T1) crystal (Harshaw, Type 12S12/E) with an attached amplifier (Canberra,Model 2007P) connected to a 4096-channel MCA (Canberra,Model 1004, Series 10). Spectra were reduced by conventional background stripping and integration of the combined peak areas of Pt X-rays ( K a at 67 keV and K(3 at 76 keV), the 99-keV y-ray, and their iodine escape peaks (14). Lead brick shielding was employed to reduce the background and Cu-Cd-A1 sheeting was placed between the Pb and NaI crystal to eliminate the interference of the 77-keV Pb K a X-ray peak that arises from interaction of Pb with background and cosmic radiation (14). A VG Plasmaquad ICPMS was used for determining Au concentrations. The sample introduction system was modified by insertion of a flow injection valve (Rheodyne type 50 Teflon four-way) with a 120-pL sample loop in the tubing (Teflon, 0.022-in. i.d.) downstream of a peristaltic pump (Gilson Minipuls 2) and upstream of the nebulizer (concentricMeinhard, TR-30-A3 glass). The length of Teflon tubing between the valve and nebulizer was minimized to reduce diffusive broadening of sample peaks. The aerosol created was subsampled and directed to the ICP through a spray chamber (Scott double bypass,borosilicate glass) water-cooled to 15 "C. A power of 1350 kW for the ICP was supplied at an rf frequency of 27.12 MHz (2-kW Henry Model 2000D rf generator) via a three-turn load coil grounded at the end closest to the mass spectrometer. Argon gas flows for the ICP, controlled by mass flow controllers, were 13.70,0.700, and 0.500 L/min for the coolant (outer), nebulizer (inner), and auxiliary (intermediate) lines, respectively. The standard VG interface 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
having high-purity nickel cones with a 1.0-mm-diameter orifice for the sampler and 0.75 mm for the skimmer was employed. The rf generator for the quadrupole (VG Model 12-12s)operated at a frequency of 2.631 MHz. Ions were detected with a continuous dynode electron multiplier (Galileo Model 4870), and the signals were collected and processed with a VG MCA and IBM XT-286 computer. Reagents and Standards. Analysis at the ferntomolar level is possible only by purification of reagents and careful tracking of blanks. Reagent grade HNOB(16 N) and HCl(6 N) were triply distilled in a Vycor still with collection into aqua regia cleaned polyethylene or Teflon containers. Aqua regia (AR) was prepared with distilled acids at a 2.3 HCl/HNO, molar ratio and concentrations are expressed herein as percent dilution of conventionally defined concentrated A R 3 parts 12 N HCl to 1 part 16 N HN03 by volume. Reagent grade H F (28.9 N) was triply distilled in a two-bottle Teflon still. Bromine was triply distilled in a Pyrex still and sealed until used in glass ampules that had been precleaned in hot AR. Ammonium hydroxide was purified by allowing open containers of reagent grade ",OH (-14 N) and distilled dionized water (DDW) to equilibrate for 2 days within a closed polyethylene vessel that was gently agitated on a shaker table. The 4% KCN solution was made by dissolving 40 g of reagent grade KCN in lo00 g of DDW and was purified by passing this solution (2 mL/min) through AR-precleaned disposable 15-mL polyethylene ion exchange columns, containing =2.0 mL of pretreated (see below) anion exchange resin (Bio-Rad analytical grade AG 1-X2, 50-100 mesh). Stable Au standard was obtained commercially (VWR scientific atomic absorption reference standard) as a 1000 ppm solution in dilute AR. The concentration of this standard was found by Zeeman graphite furnace atomic absorption spectrophotometry to be consistent within analytical uncertainties ( 1 5 % ) with a standard prepared in the laboratory by dissolving high-purity Au wire in a small amount of AR and diluting it with DDW. The primary standard was stored in an amber bottle. However, unlike silver, which is very photosensitive, Au does not appear to undergo adverse photoreactions. Working standards (=0.1-50 nM) in 5% AR solutions stored in glass volumetric flasks exposed to laboratory lighting for up to a year showed no signs of diminished Au contents with respect to freshly prepared working standards. Gold-195 radiotracer was obtained from New England Nuclear in 100-pCi aliquots in -4 M HCl that contained >20 Ci/g Au. The tracer was diluted to a working range (=20 mL) with 5% AR. Sample Handling a n d Storage. Loss of significant amounts of Au by adsorption onto container walls can pose a serious problem for quantification of Au in natural waters; indiscriminate acid additions can actually promote Au losses in glass and polyethylene containers (15-177. On the basis of a series of seawater storage experiments, using Au-195 as a tracer (see ref 18), the following procedures were adopted. To avoid contamination or isotope exchange of Au-195 with contaminant Au, polyethylene sample containers were precleaned by leaching in 10% AR at 60 "C overnight. Gold-195 radiotracer (-0.1 pCi as [Au(Cl),]-) was added to each sample as soon as possible after collection (and filtration, if done). Whenever possible, 0.5 mL of a 4% KCN solution/L of sample was added to a container before a sample was drawn, and samples were processed shipboard. When there was insufficient time or space to process samples at sea, samples were spiked with Au-195 and acidified to pH 1.5 with AR, shown by the radiotracer experiments to retain Au in seawater solutions for at least 1 year. Recommended Procedure. Column Preparation. Columns were prepared for each sample as follows: Approximately 1.8mL of anion-exchange resin (Bio-Rad analytical grade AG 1-X2, 50-100 mesh) was added as a slurry in DDW to a 12-mL polyethylene column (Bio-Rad), precleaned in the same way as sample containers. The resin was eluted with 15 mL of 90 "C 16 N HNO, to strip contaminant Au and then rinsed with 20 mL of DDW and 20 mL of 6 N HCl. Columns were capped and stored in sealed plastic bags until use. Preconcentration. To facilitate the processing of 4-8-L seawater samples, 4-L polyethylene bottles equipped with bottom tubulations (Nalgene) were set up to accommodate the ion-exchange columns (Figure 1). These elution rigs were then precleaned in the same way as sample containers.
A I
*----------plastic
n
+-----------------Tygon
crale
tubing waste drain
Figure 1. Side view, schematic of apparatus used to preconcentrate 2-8-L seawater samples: pa = polyethylene. The metal hose clamp was wrapped in Parafilm to prevent m & n . Exposure of the sample to flexible Tygon tubing was minimized since Au can be lost by adsorption onto it (20). At sea, the plastic crates were fastened with tie-wraps, the sample and waste containers secured with bungy cords, and the rigs covered with plastic sheeting.
Before they were passed through the columns, the pH of stored samples was first adjusted with NHIOH to 8, as determined by indicator paper, and 0.5 mL of a 4% KCN solution/L of sample was immediately added. Cyanide was chosen for this work because of its extremely strong Au complexing ability (19) and good retention of the Au cyanide complex by the resin. Samples processed at sea received the same amount of KCN without pH adjustment, and at least 1 h was allowed for Au cyanide complex formation. Flow rates were adjusted with the Teflon valves to 2-6 mL/min. When the seawater had passed through a column, salt was rinsed from its exterior with DDW and the columns were eluted or capped and sealed in plastic bags for transport to the laboratory. Since elution involved handling hot concentrated acid, for safety's sake, it was carried out in a fume hood in the laboratory. Exposure of the samples to unfiltered air was minimized, however, to avoid contamination. Columns were rinsed with 40 mL of 0.12 N HC1 and 40 mL of DDW, followed by elution intoquartz tubes with 40 mL of 90 "C 16 N "0% After 40 mL of 6 N HC1 was added, the eluants were UV-oxidized for 24 h to eliminate most of the organic matter carried through from the samples and harsh treatment of the resin. Samples were next transferred to 100-mL Teflon beakers and evaporated under filtered air with gentle heating ( 4 0 "C) under IR lamps. When reduced to =l mL, the sample solutions were transferred to 5-mL conical Teflon vials (Savilex). Concentrated H F (1 mL) was added to volatilize any Si present, and the samples were evaporated in a Teflon-coated heating element (75 "C) housed in a fiitered air-laminar flow box until only 20-50 pL remained. At this point, 250 KLof 56% AR was added to resolubilize the Au, and the vials were capped and placed in a 60 "C oven for at least 24 h. Finally, the samples were diluted to 1.1mL with DDW. Reagent blanks were prepared by preconcentrating a reagent plus spike solution in the same way as for the samples. Yield Monitoring. One milliliter (*l%) of preconcentrated samples and reagent blanks was transferred to 1.5-mL polyethylene vials with caps for NaI(T1) y-counting. Standards were made in the same type of vial by diluting aliquots of the radiotracer, equal to that added to the samples ( ~ 0 . pCi), 1 to 1 mL such that the final AR concentration was also 10%. Samples, reagent blanks, and standards were each counted three times in the same geometry, and recoveries were calculated from averaged
ANALYTICAL CHEMISTRY, VOL. 62,NO. 14,JULY 15, 1990 aulOO I n t
I
640
C
0
"
2
4
6 8 IO time in minutes
12
14
16
Flgwe 2. Typical FI-ICWS spectra: mass 197,640counts full scale, 120-pL loop size, 1 mLlmin flow rate. Peaks numbered sequentially from left to right are 1, 2,7,8 = Au standard for drift monitoring; 3,
4 = sample (21of filtered Black seawater);5,6 = sample + standard added. Average background corrected peak areas (1 standard deviation) are l, 2 = 5680 (50);3,4 = 7250 (91);5,6 = 13226 (149); 7, 8 = 5722 (15).The standard addition was 0.489 nM Au, and the drift monitor concentration was 0.637 nM Au. The sample was quantitatively preconcentrated and contained 110 f 20 fM Au. See text for details.
counts. Through thermodynamic considerations and empirical observations, it has been shown that the Au-195 recovery represents the total soluble Au contents of the sample regardless of its oxidation state (18). If particulate Au is present in the sample, some of it may be physically trapped on the column and solubilized by the hot acid eluant. Since this is not necessarily a quantitative process, Au-195 recoveries would be misleading; samples with significant particulate loads must be filtered. FI-ICPMS Determination. The spray chamber, elbow, torch, and nebulizer of the ICPMS were cleaned in 100% AR before each run. With clean glassware in place, the ion optics settings, gas-flow rates, and torch box position were optimized on the Pb-208 peak of a 100 ppb Pb solution. Optimal settings are the same for P b and Au; use of Pb circumvenb memory effects that occur when solutionswith high concentrationsof Au are aspirated (18). To further maximize sensitivity, resolution was reduced until tailing from Hg-198 (a persistent background peak) was at background levels at mass 197.5. The quadrupole was then set to transmit at mass 197, and 10% AR was aspirated through the flow injection valve until the background for Au dropped to 510 cps. A flow-injection sample introduction system was employed, rather than conventional steady-statenebulization, for two reasons. First, due to practical limitations on the seawater sample size that can be handled routinely without contamination and the very low concentrations of Au in seawater, the analysis is sample limited. Flow injection allowed complete integration of the aliquot of sample presented, whereas steady-state conditions can be achieved only with much larger sample volumes (=2 mL) at the expense of sample wasted in achieving the rise to steady-state signals. Second, the flexible tubing used in peristaltic pumps used to control sample flow rates adsorbs Au, causing diminished signals and memory affects that severely degrade precision a t low levels (20). By placing the flow injection valve downstream of the peristaltic pump, the flexible tubing can be bypassed, and yet control over flow rates maintained. In preparation for injection, 250-pL aliquots of each sample were transferred from the counting vials to four 500-pL polyethylene centrifuge vials, and 10 pL of 10% AR was added to two of each vial set. The remaining two vials were reserved for standard additions, which were carried out to compensate for variations in the matrix composition and viscosity. Samples were then loaded and injected by hand in a 10% AR carrier stream, and their transient signals monitored (Figure 2). A single analysis consisted of a standard injected in duplicate before and after the sample suite as a check on instrumental stability. The sample (with 10 pL 10% AR) was injected in duplicate. After its signal was observed, 10 pL of the appropriate standard to approximately double the signal (generally 0.5-25 nM Au in 10% AR) was added to each of the two remaining vials and mixed with a vortex mixer prior to injection. Although the sample aliquots (260 pL) exceeded the flow injection loop volume (120 pL), the excess volume was
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used for rinsing and ensuring complete filling of the sample loop, so the entire sample was consumed in the analysis. Each sample suite consisting of eight injections took about 15 min to run with a fixed interval, typically of 2 min, required between injections for the signals to decay to background levels. It should be noted that there are a few possible isobaric interferences at mass 197. Theoretically, a hydride of 25% abundant Pt-196 or oxide of the 99.9% abundant Ta-181 could interfere if these elements were present in high enough levels in preconcentrated samples. Scanning of the 181 and 196 mass regions of representative samples can be done, and assuming that their molecular species were present at 51% and 55%, respectively, of their free ion concentrations (21),the levels of Pt and T a can be assessed. Their count rates did not exceed background levels (
