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A M Chem. 1889, 65, 2485-2488
Determination of Mercury in Environmental Samples by Isotope Dilution/ ICPMS Ralph G. Smith Skidaway Institute of Oceanography)P.O.Box 13687, Savannah) Georgia 31416
The application of isotope dilution inductively coupled plasma mass spectrometry for the determination of mercury in sediment and water samples using an enriched zolHgspike is described. Sediment samples are determined directly after spiking with glHg and a 1-h acid digestion. The procedural detection limit is 2 ng/g using a 0.5-g sample. Quantitativerecovery is obtainedon NRC reference standard (BEST-l),which is certified at 92 f 9 ng/g. Natural levels of mercury in water samples require preconcentration onto gold traps and subsequent electrothermal heating and purging of the traps with argon directly into the torch of the ICPMS. The procedural detection limit is 0.2 ng/L using a 200-mL sample. The accuracy of the method was evaluated using a river water standard (ORMS-1) from the National Research Council of Canada, certified at 6.8 f 1.3 ng/L. Results using this technique yield a value of 6.3 f 0.4 ng/L.
INTRODUCTION The toxicity of mercury in the environment has been well established. While there have been many different analytical techniques developed over the last two decades,the technique with the most widespread usage has been cold vapor atomic absorption. Improvements in the technique such as preconcentration onto gold traps and subsequent purging into an absorbance cell of an atomic absorption system or other measuring devices have helped to improve detection limits.14 Also, the use of cold vapor atomic fluorescence for the determination of mercury species in natural samples ha^ been reported.616 The emergence of inductively coupled plasma mass spectrometry (ICPMS) over the past 10 years has resulted in a multitude of technique papers. Recently, a compiled bibliography containing152references to publications on ICPMS techniques was reported.' However, specific applications to mercury determination in natural samples have been few. The direct determination of mercury in drinking water by ICPMS using direct injection nebulization(DIN) has recently been reported.8 The method has a detection l i i i t of 30 ng/L, which may not be suitable when accurate determinations of concentrations in natural waters approaching 1 ng/L are needed. The use of ICPMS for the cold vapor determination of mercury in natural samples using calibration curve (1) Bloom, N.5.; Crecelius, E. A. Mar. Chem. 1983,14,49-59. (2)W e l , J. A.; Yeata, P. A. Mar. Chem. 1985,15,357-361. (3)Fitzgerald, W. F.;Gill, G.A. AM^. Chem. 1979,51,1714-1720. (4)Gill,G.A,;Fitzgerald, W.F. Mar. Chem. 1987,20,227-243. (5) Bloom, N.;Fitzgerald,W. F. AM^. Chim.Acta 1988,208,151-161. (6)Bloom, N.Can. J. Aqwrt. Sci. 1989,46,1131-1140. (7)McLaren, J. W. Atom. Spectrosc. 1992,13,81-88. (8)Powell, M.J.; Quan, E. S. K.; Boomer, D. W. AM^. Chem. 1992, 64,2253-2257. 0003-2700/93/0385-2485$04.00/0
Table I. Instrument Parameters maw range number of channels number of scan sweeps dwell time (pa) collector type skipped masa regions isotopes selectad nebulizer flow (L/min)
200.4-204.9 512 600/4W
160 pulse 202.6-204.9 "'Hg, q g 0.7
a Sediment: 600 scans, 49 s (aquisitiontime). Water: 400 scans, 33 s (aquisition time).
comparisonand the capability of isotope dilution techniques was reported.0 The reported instrument time required per sample is 10 min. A general review of isotope dilution maas spectrometry was done by Fasset and Paulsen.10 The work described in this paper represents a significant improvement over earlier reports on the determination of mercury in natural samples using ICPMS. Details on the determination of mercury in sediment and water using isotope dilution techniques are presented. Detection limits are comparable with other techniques and less than 1 min of instrument time is required per sample. The optimal spiking ratios of 202Hg/m1Hg for the best accuracy and precision are reported. An unexpected loss of sensitivity in sediment analyses associated with purge time is also reported.
EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used in this work is a FisonsModel PQII+ equipped with a standard interface and oil diffusion pumps. The modificationsto the instrument include replacement of the glass elbow between the torch and the sample chamber with a short 12/2mm ball and socket fitting. This fitting is connected with a Ylein. Teflon elbow which reduces to a '/*-in. tube fitting. Teflon tubing (l/g in.) is used between the Teflon elbow and the sample input source via a 1/8-l/4-in. Teflon connector. Also the nebulizer supply is detached from the torch box and fitted with an extra length of tubing and a V4-in. Teflon connector for mating with the sample purge input. The operating conditions used for data collection are shown in Table I. The instrument is tuned before the above input modifications are made, using an aqueous tuning solution, and the tuning is rechecked about every 3 days. The maas calibration and resolution are also checked each time the instrument is tuned. The importance of proper mass calibration and resolution when ions with adjacent masses such as alHg and mHg are measured is obvious. Reagents. Baker Instra-Analyzed nitric and hydrochloric acids are used without further purification. A 5% solution of sodium borohydride is made in a 0.1% KOH solution. This solution is purified by purging with nitrogen overnight at a flow rate of 200 mL/min. A 10% solution of stannous chloride is prepared in 3% HfiO,, and the solution is purified by purging with nitrogen at 200 mL/min overnight. (9) Haraldeson, C.; Weeterlund, S.; O h m , P.Anal. Chim.Acta 1989, 221,77-84. (10)Fassett, J. D.;Paulaen, P. J. AM^. Chem. 1989,61,643A-649A.
Q 1993 Amerlcen Chernloal Society
2488
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993 Au Tube
N2 106-
-
0
Cnm-
w
Sample
Flgurs 1. Sample cell (300 mL) for mercury reduction and trapping for water samples. The side arm near the bottom of the cell Is fitted with a '/,-in. Teflon connector and a silicon septum. All tube fittings
are I/, in.
Water. Our laboratory tap water which is used as a diluent was found to contain approximately 0.2 ng/L total Hg, which is about one-fourth the concentration of our best distilled water. Bloom6 reported similar circumstances between laboratory tap water and distilled water. Gold Traps. The gold traps are similar to those described by Bloom and Crecelius' consisting of 1 cm X 10 cm X 0.25 mm thickness gold foil which was rolled and placed in in. X 15 cm quartz tubes. The gold foil is held in place by quartz wool at either end of the tube. The tubes were initially blanked by electrothermal heating using nichrome wire wrapped around the tube and connected to a variable transformer while purging the tube with nitrogen. The tubes are stored in an air-tight Pyrex culture tube when not in use. Aerosol Trap. A moisture trap as described by Gill and Fitzgerald' is placed in l i e between the sample cell and the gold trap to prevent acid vapor from attacking the gold foil. These traps consist of a quartz tube packed with KzCOa, which is held in place with quartz wool on either end of the tube. The aerosol traps were blanked by heating in a muffle furnace for 1h at 500 "C. These tubes are also stored in air-tight Pyrex centrifuge tubes. Certified Standards. Certifiedstandards were obtained from the National Research Council of Canada. The sediment standard is BEST-1 and the river water standard is ORMS-1. These samples are certified at 92 f 9 ng/g and 6.8 f 1.3 ng/L, respectively. The enriched isotopic mercury standard (m1Hg) was obtained from Oak Ridge National Laboratory and has a certified abundance of 91.2%. Procedure (Sediment Samples). The sediment samples are weighed,spiked with mlHg, and digestedwith 5.0 mL of 1:lHNO, and HCl on a steam bath for 1h using Teflon (Savillex No. 561) containers. After the samples have cooled for 30 min in an ambient temperature water bath, they are poured into 20-mL precleaned polyethylene scintillation vials. The sample cell used for mercury reduction of sedimentdigests is similar to that shown in Figure 1, but with a total volume of 100 mL. The moisture trap and sample collector tube shown in Figure 1are not used with sediment analyses. The nitrogen purge gas shown in Figure 1is replaced with argon from the nebulizer supply. A 0.5-mL aliquot of the digest is diluted with approximately 40 mL of water in the sample cell. The sample cell is purged with argon (0.7mL/min), from the nebulizer supply, for 15s before the output from the cell is connected to the torch. The 15-sdelay is necessary to remove atmospheric oxygen from the sample cell, which would quench the plasma. After the sample cell output is connected to the torch, borohydride is immediately and continuously injected into the side arm of the sample cell via a hypodermic needle connected to Teflon microbore tubing and a peristaltic pump. A suitable flow rate for the reductant was found to be approximately0.5 mL/min. The instrument is initializedto begin scanning as soon as the addition of borohydride begins. After the scan is completed, the addition of reductant is interrupted and the sample cell is prepared for the next sample. Water Samples. Because the concentrations of natural unpolluted waters are in the range of a few nanograms per liter to less than 1 ng/L, a preconcentration step onto gold traps is used. Two hundred milliliters of sample, or an aliquot containing
cn
= 807 7 60 0
0
0
0
0
0
a 0
I
I
I
0.5
1.a
15
2.0
202Hg I201Hg Flguro 2. Effect of measured ratios upon the observed concentration of mercury in certified sediment standard, BEST-1.
1100 ng of mercury, is injected with 5 mL of the SnClz solution through the side arm of the sample cell (Figure 1). The sample is purged for 15min with nitrogen at a flow rate of 500 mL/min with an aerosol trap and a gold trap downstream from the sample cell. Quantitative removal of the mercury was found to occur in 15 min. In order to remove any Hg present in the nitrogen gas a gold trap is placed in line, upstream of the sample cell. After purging for 15min, the gold tube containingthe sample is removed and placed in a Pyrex culture tube as described above. After the amalgamation of 15-20 samples onto individual gold tubes, they are prepared for electrothermal heating and purging into the ICPMS. The argon purge gas from the nebulizerflowis connected to a "clean" gold trap on the upstream side of the sample tube to remove any mercury present in the argon. A coil of nichrome wire is placed on the sample tube covering the entire length of the gold foil. The sample tube is connected to the '/,-in. Teflon connector leading to the torch. As heating commences the instrument is initialized to begin scanning. After the scan is completed, the power supply to the nichrome wire is defeated and the wire is allowed to cool. The nichrome wire is then removed from the gold tube and placed on the next sample tube and the process is repeated.
RESULTS AND DISCUSSION Calculations. The isotope dilution calculations are accomplished using an equation similar to that described by Klinkhammer.11 The equation is as follows:
where VI is the volume of spike, V2 is the volume of sample, R, is the measured sample + spike m2Hg/mlHg ratio, R,is the spike ratio, Rmt is the natural ratio of mHg/mlHg (2.25416), A202 is the natural abundance of m2Hg (0.29801, and (m1Hg) is the concentration of mlHg in the spike. Optimum Spiking Ratio. Theoretically the optimum spike ratio for highest accuracy in isotopic measurements is near the geometric mean of the natural and spike isotopic ratios.12 It follows then that as measured ratios approach either end member the imprecision of their measurment increases. An empirical evaluation of ratios vs accuracy using sediment standard BEST-1,certified at 92 f 9 ng/g, is shown in Figure 2. Discrete fractions of the sediment standard were spiked with enriched m1Hg to obtain ratios from 0.2 to 2.0. Ratios between 0.2 and 1.5 yield concentrations within the limits of uncertainty of the certified value. Sensitivity vs Purge Time for Sediment Samples. Sensitivity (area counts per second, ACPS) was observed to decrease as the purge time, prior to the addition of borohy(11) Klinlrhammer, G. P. Anal. chi^. Acta 1990,232,323-329. (12)Webeter, R. K.; Smalee,A. A.; Wagner, L. R. Methods in Geochernrstry; Interscience: New York, 1960;Chapter 7.
ANALYTICAL CHEMISTRY, VOL. 05, NO. 18, SEPTEMBER 15, 1993
Table 11. Precision and Accuracy sediment BEST-1 sediment reagent 97' 91 96 92 95
4.0 3.1 3.9 2.6
mean 94 i 2.6 certified 92 9 0
0.3
tap water
tap water
0.3 0.3 0.3 0.3 0.4 0.2
3.4 f 0.67
*
+ reagents (ng/L)
reagent blank (ng/L)
blank (ng/g)*
(ng/g)
0.5 0.4 0.5 0.5 0.6 0.5
water ORMS-1 (ng/L)
10.2 11.2 10.0 8.3 11.0 9.4 9.4 9.6 9.8 9.8 i 0.84
0.5 i 0.06
0.06
+ spike, blank
corr (ng/L)
2467
6.8 6.3 6.3 5.9 6.3
0.4
6.8 i 1.3
Found. b Based on 0.5-g aample weight.
Measured isotopic ratio (Tapwater with 10 ng I I spike) goo0
8000
-
7000
-
6000
-
ul
5 SO00 -
L I
CI
Y
8
I
rn 4000
-
3000
-
2000
-
1000
-
K
I 0 1
I 10
I 20
I I 3 0 4 0
T I 50
I 60
I I I 7 0 8 0
1 S4
Purge Tlme (seconds) Flgurs 3. Observed senskivlty decrease with increasing purge times for sediment analyses. dride, is increased. An experiment was conducted using 40 mL of water spiked with 5 ng of natural mercury (HgClZ), 0.082 ng of enriched MlHg, and 0.5 mL of "03. This was chosen to approximate the concentration of mercury in a 0.5mL aliquot of the digest from certified sediment standard BEST-1. As shown in Figure 3, the ACPS for an aqueous sample is approximately 35 X 109if the sample is purged only 15s prior to addition of borohydride and this value decreased to 1.4 X 109 by purging for 120 s. It was also observed that the purge time had no effect on sensitivity if the sample were allowed to equilibrate for 15min undisturbed prior to purging. Choice of Reducing Agents. Harldsson et al.9 reported that the use of SnClz as a reducing agent for mercury reduction directly into the ICPMS resulted in severe tin contamination in the instrument. Therefore, in this work stannous chloride is only used to reduce mercury in water samples for amalgamation onto gold tubes and sodium borohydride is used to reduce aliquots of the sediment digests directly into the ICPMS. The subsequent electrothermal heating and purging of the gold tubes into the instrument has produced no observed increase in the tin background. Analytical Figures of Merit. The results of replicate analyses of certified reference standards and blank determinations are shown in Table 11. The detection limit for
0.00
1
200
ACPS 23,368
ACPS 26,300
201
202
1 203
Mass Flguro 4. Typical isotopic spectrum of a 10 ng/L spike in a tap water sample with relative senskivities In ACPS.
sediment analysis based on three times the standard deviation of the reagent blank is 2 nglg for a nominal sample weight of 0.5 g. Replicate analyses of SRM (BEST-1) are well within acceptable values. Water samples are somewhat more difficult to distinguish the blank contribution of the reagents from the mercury present in the water used for determining the blank concentration. This was accomplished using tap water, which is initially reduced with SnClz to determine the mercury concentration in the water plus reagents. The tap water is reacidified with "03 and reduced as before in order to determine the reagent contribution. Replicate analyses used for this determination are shown in Table 11. Spike recoveries from tap water spiked with 10 ng/L mercury are also shown in the table. An instrument detection limit of 5 pg/L could be reported using the square root of the background counts (20 CPS) as the standard deviation and comparing this to a sensitivity of 26 OOO CPS for a 10 ng/L spike (Figure 4). However, the counting statistics at this levelwould prevent accurate isotopic ratio determinations. The procedural detection limit as determined from 3a of the reagent blank is 0.18 ng/L using a 200-mL sample. The absolute detection limit then is 36 pg,
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
which is significantly higher than (8 pg) reported by Haraldsson et al.9 This is puzzling, given their relative sensitivity of 20 000-40 000 CPS for a 20 ng/mL tuning solution, which is one-fourth to one-half of the sensitivity of the instrument used in this work. However,since detectionlimitsare usually blank-limited, the lower detection limit reported by these workers may be attributable to their lower total blank. Peak jumping was used to assess the sensitivity attainable in this more efficient mode of counting. A 10 ng/L spike, identical to that shown in Figure 4, was used for comparison. A count rate of over 40 000 CPS was obtained, which would lower the detection limit to 0.13 ng/L; however, the inability to observe the spectra using this counting mode must be considered. Conclusions. The accuracy of isotope dilution techniques is shown to be superiorto other techniqueswhen the optimum isotopic ratio for analyses is maintained. The application of this technique to tissue samples may prove successful by
following the procedure described for sediment samples although some modificationsmay be necessary. Also, while this work has demonstrated the ability to determine mercury concentrations at or below 1ng/L in drinking water and river water, its application to seawater should also prove useful. It may be necessary to increase the sample volume to 500-1000 mL in order to lower the detection limit.
ACKNOWLEDGMENT This work was partially funded by Florida Dept. of EnvironmentalRegulations Contract CM-277 and New Jersey Dept. of Environmental Protection and Energy Contract P33492. A special thanks to Debbie Wells for her assistance in sample preparations. RECEIVED for review April 7, 1993. Accepted June 10, 1993.
