Anal. Chem. 1997, 69, 2464-2470
Low Blank Preconcentration Technique for the Determination of Lead, Copper, and Cadmium in Small-Volume Seawater Samples by Isotope Dilution ICPMS Jingfeng Wu* and Edward A. Boyle
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A simple low-blank method is described for the analysis of Pb, Cu, and Cd in seawater using Mg(OH)2 coprecipitation and isotope dilution inductively coupled plasma mass spectrometry (ICPMS). Here, 20-40 µL of 9 M aqueous NH3 is mixed into a 1.3 mL seawater sample spiked with enriched isotopes of Pb, Cu, and Cd. After centrifugation, the supernatant is discarded and the Mg(OH)2 precipitate dissolved in 100 µL of 5% HNO3 for ICPMS analysis. This method is simple, accurate, and precise, with detection limits of Pb ) 1.3 pM, Cu ) 39 pM, and Cd ) 5.0 pM and blanks of Pb ) 0.62 pM, Cu ) 27 pM, and Cd ) 6.0 pM. The method is demonstrated by oceanographically consistent profiles of these trace metals at an ocean station in the eastern North Atlantic. Lead, copper, and cadmium exist at picomolar (1 × 10-12 mol/ L) to nanomolar (1 × 10-9 mol/L) levels in oceanic waters.1,2 Accurate and precise measurement of Pb, Cu, and Cd is desirable for studies of the penetration of anthropogenic Pb into the ocean,3-6 seawater Cu speciation and its toxic effect for phytoplankton,7,8 and the use of Cd/Ca as a paleooceanographic tracer.9 The major difficulties in seawater Pb, Cu, and Cd analysis come from their extremely low concentrations and the high salt matrix (3.5%). The most common analytical methods for the determination of Pb, Cu, and Cd in seawater are anodic stripping or absorptive cathodic stripping voltammetry direct measurements10,11 and graphite furnace atomic absorption spectrometry (AAS) and thermal or inductively coupled plasma mass spectrometry (MS) detection after some forms of preconcentration steps
Table 1. Pb, Cu, and Cd Blanks per Liter of Reagents
Vycor-distilled H2O Vycor-distilled HNO3 Q-NH3
Pb (pM/L)
Cu (nM/L)
Cd (pM/L)
1.2 5.6 3.9
0.05 0.38 0.14
0.07 4.28 0.07
such as solvent extraction,12,13 Co-APDC coprecipitation,14 and Chelex-100 ion exchange15 or on-line preconcentration.16-19 All of these techniques are relatively time consuming and subject to contamination, especially for Pb and Cd at picomolar levels. We present here a simple, rapid, and highly sensitive technique for Pb, Cu, and Cd in seawater using low-blank Mg(OH)2 coprecipitation, with subsequent determination of these trace elements by inductively coupled plasma mass spectrometry (ICPMS). The total analytical blanks by this method are low because the preconcentration procedures are simple and because of the ease with which NH4OH and HNO3 can be purified. To ensure accuracy and precision regardless of recovery efficiency or analytical interferences, the isotope dilution (ID) method is employed prior to the separation step. EXPERIMENTAL SECTION Reagents. High-purity water was prepared by redistillation of “ultrapure” deionized water in a Corning “Mega-Pure” Vycor/ borosilicate still. Nitric acid and hydrochloric acid were triply distilled in a Vycor still. Ammonium hydroxide was prepared by vapor phase transfer from concentrated aqueous NH3 into Vycordistilled H2O. The reagent blanks (reported as per liter of reagents, Table 1) were individually determined by evaporation of a large volume down to a small volume followed by ID-ICPMS. To examine isobaric ICPMS interferences, trace metal-free Mo and Mg standards were prepared by passing concentrated solution
(1) Bruland, K. W.; Franks, R. P. In Trace Metals in Seawater; Wong, C. S., et al., Eds.; Plenum: New York, NY, 1983; pp 395-414. (2) Schaule, B. K.; Patterson, C. C. In Trace Metals in Seawater; Wong, C. S., et al., Eds.; Plenum: New York, NY, 1983; pp 487-504. (3) Boyle, E. A.; Chapnick, S. D.; Shen, G. T.; Bacon, M. P. J. Geophys. Res. 1986, 91, 8573-8593. (4) Flegal, A. R.; Patterson, C. C. Earth Planet. Sci. Lett. 1983, 64, 19-32. (5) Sherrell, R. M.; Boyle, E. A.; Hamelin, B. J. Geophys. Res. 1992, 97, 1125711268. (6) Helmers, E.; Rutgers van der Loeff, M. M. J. Geophys. Res. 1993, 98, 2026120273. (7) Moffett, J. W.; Zika, R. G. Environ. Sci. Technol. 1987, 21, 804-810. (8) Sunda, W. G. In Chemistry of Aquatic Systems: Local and Global Perspectives; Bidoglio, G., Stumm, W., Eds.; Kluwer: Dordrecht, 1994; pp 213-247. (9) Boyle, E. A. Paleoceanography 1988, 3, 471-489. (10) Bruland, K. W.; Coale, K. H.; Mart, L. Mar. Chem. 1985, 17, 285-300. (11) Capodaglio, G.; Coale, K. H.; Bruland, K. W. Mar. Chem. 1990, 29, 221233.
(12) Bruland, K. W.; Franks, R. P.; Knauer, G. A.; Martin, J. H. Anal. Chim. Acta. 1979, 105, 233-245. (13) Patterson, C. C.; Settle, D. M. Nat. Bur. Stand. Spec. Publ. 1976, 422, 321351. (14) Boyle, E. A.; Edmond, J. M. Anal. Chim. Acta 1977, 91, 189-197. (15) Kingston, H. M.; Brady, I. L.; Rains, T. C. Anal. Chem. 1979, 50, 20642070. (16) Miyazaki, A.; Reimer, R. A. J. Anal. At. Spectrom. 1993, 8, 449-452. (17) McLaren, J. W.; Lam, J. W. H.; Berman, S. S.; Akatsuka, K.; Azeredo, M. A. J. Anal. At. Spectrom. 1993, 8, 279-286. (18) Petty, J. R.; Blubaugh, E. A.; Evans, E. H.; Caruso, J. A.; Davidson, T. M. J. Anal. At. Spectrom 1992, 7, 1131-1137. (19) Heithmar, E. M.; Hinners, T. A.; Rowan, J. T.; Riviello, J. M. Anal. Chem. 1990, 62, 857-864.
2464 Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
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© 1997 American Chemical Society
Table 2. ICPMS Data Collecting Conditions
Table 3. Target Spike Ratios
Aquisition Method peak jump mode 3 points/peak 5 DAC between points 30 s aquisition 512 channels
element Pb
Cu
Cd
Element Menu mass (amu) 202 204 208 220 63 65 66 68 220 95 106 110 114 118 220
dwell/peak (ms) 4.4 10.2 10.2 2.2 4.4 4.4 4.4 4.4 1.2 1.2 10.2 4.4 10.2 10.2 4.4
through a Chelex-100 ion exchange column and then diluting with Vycor-distilled H2O to make working standards. Isotopically enriched 204Pb (20 ppm), 65Cu (1000 ppm), and 110Cd (20 ppm) primary standards were prepared by dissolution of 204Pb2CO3, 65CuO, and 110CdO (purchased from Oak Ridge National Laboratories) in 1% Vycor-distilled HNO3. Working standards were prepared from the primary standards as needed by further dilution in dilute Vycor-distilled HNO3 (pH 1.5). The exact concentrations of the enriched isotope spike solutions were determined by isotope dilution calibration against known standards of natural abundance, after correcting for ICPMS mass fractionation using natural abundance standards. All reagent and sample preparations were carried out in a Class 100 clean flow bench. Seawater Samples. Seawater samples for Cu and Cd vertical profile were collected using modified Niskin bottles (with rubber O-rings replaced by silicone and the internal rubber spring replaced by an epoxy-coated steel spring), suspended on hydrowire. Samples for Pb vertical profile were collected by a specially designed weather-vaning sampler made of polypropylene and Teflon.3 All samples were left unfiltered and acidified to pH 1.5 with vycor-distilled HCl soon after collection. In addition, a largevolume surface seawater sample (“SS”) was collected by the “pole sampling” method.20 This sample was used throughout the course of the method development work. A portion of this sample was acidified, UV irradiated, and passed through a Chelex-100 ion exchange column to make a trace metal-free seawater. Instrumentation. The inductively coupled plasma mass spectrometer used for this work was the VG Plasma Quad II + (VG Element, Denver, MA), with a typical sensitivity of 200 000 (counts/s/ppb of uranium. A standard concentric glass nebulizer (for 10 mL sample size) or CETAC microconcentric nebulizer (for 1.3 mL sample size) was used in conjunction with a water-cooled spray chamber. A Gilson four-channel peristaltic pump (Gilson Minipulse 3) was used with the standard nebulizer. A free-draw method was used for the microconcentric nebulizer. Details of (20) Boyle, E. A.; Huested, S. S.; Jones, S. P. J. Geophys. Res. 1981, 86, 80488066.
isotope 1 isotope 2 “1”/“2” of sample (Rs) “1”/“2” of spike (Rt) geometric mean of Rs,Rt “1”/“2” target ratio error magnification factor at target ratio
Pb
Cu
Cd
204 208 0.02615 5.837 0.3949 1 1.2
65 63 0.4472 233.7 10.2 15 1.1
110 114 0.4314 77.38 5.778 8 1.2
data collection parameters are given in Table 2. Typically, signals were acquired for 30 s in peak jump mode beginning 3-5 s after the signal reached the detector. General Procedure. Acidified seawater samples (1.3 mL) in 1.5 mL polypropylene microcentrifuge vials were spiked with enriched isotope 204Pb, 65Cu, and 110Cd spikes (50 µL) and allowed to equilibrate overnight. Aqueous NH3 (20-40 µL, depending on its concentration and the sample acidity) was mixed into the sample to precipitate Mg(OH)2. The amount of NH4OH used for Mg(OH)2 precipitation was determined empirically each day, which will be discussed in detail below. After the mixture was centrifuged, the supernatant was discarded and the Mg(OH)2 precipitate washed three times with 200 µL of pH 10 dilute NH4OH prepared with Vycor-distilled H2O and concentrated purified NH4OH. The precipitate was then dissolved in 100 µL of 5% triply Vycor-distilled HNO3 and analyzed for Pb, Cu, and Cd by ICPMS with a CETAC microconcentric nebulizer. To estimate the procedural blank, a few replicates of 50 µL trace metal-free seawater samples were processed in the same way as samples (spiked with same amount of the reagents as for the samples). The metal content of these small samples is negligibly small, so the measured metal content of these samples was attributed to the procedural blank. The washing step is necessary for Cu analysis but not for Pb and Cd (see explanation below). A standard concentric glass nebulizer (uptake rate of 0.6-1.0 mL/min) can be used to replace the microconcentric nebulizer (uptake rate of 20-40 µL/min) if sample and reagent sizes in the above procedure are scaled up by 10-fold. The procedure separates Pb, Cu, and Cd from the 3.5% matrix salts present in the seawater sample and provides a 7-12-fold preconcentration ratio, depending on recoveries for each element (see below for recovery details). Higher concentration ratios can be achieved by repeating the precipitation procedure, although this additional step usually is not necessary for routine seawater analysis. A double precipitation method is needed to achieve a detection limit of less than 1 pM for Pb, which will be necessary for old, deep water, especially in the Pacific Ocean. After the Mg(OH)2 from the first precipitation step was redissolved in dilute HNO3, aqueous NH3 was added into the solution to reprecipitate 5-10% of the Mg in the solution. The second precipitate was then dissolved in 5% HNO3 having a volume 1.0% of the original sample, leading to a 40-90-fold concentration ratio depending on recoveries of each element (see below for details). RESULTS AND DISCUSSION An isotope dilution (ID) quantitative method (equilibrating a known amount of spike of known isotopic composition with a known amount of sample and measuring the isotopic ratio of the mixture) was used to avoid corrections for the Mg matrix effect Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
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isotope “1”, such as 204Hg for Pb, 110Pd and 94Mo16O for Cd, and 40Ar25Mg for Cu, increasing “1”/“2” spike ratios decrease the relative standard error during the correction for these isobaric interferences (see below for details). Thus, we chose target spike: sample ratios higher than geometric means of “1”/“2” (see Table 3). To obtain the target ratio, however, one would have to know the exact Cu, Cd, and Pb concentrations in the sample before adding the enriched isotope spike. It is more practical to limit the spike ratio in a range (“1”/“2” ) 1-2 for Pb, 8-30 for Cd, and 15-80 for Cu) within which the error magnification factor estimated from the isotope dilution equation (defined as the ratio of relative error in calculated concentration to the relative error in measured ratio) is below 1.6. Isotope Equilibration Time. Equilibration times from 2 min to 2 days were tested with a surface seawater sample collected near Bermuda and acidified to pH 1.5. It was found that there was no detectable difference in analytical outcome during this period, suggesting rapid equilibration between added enriched isotope spikes and their natural isotopes present in the sample at pH 1.5. Although Pb, Cu, and Cd have been found to be complexed by strong natural organic ligands at seawater pH (8.1),11,21,22 these complexes would not be as stable at pH 1.5. As a result, the metal ions can be quickly equilibrated by spike isotopes. Mg(OH)2 Precipitation. When a complete isotope equilibration has been achieved, a loss of Pb, Cu, and Cd during coprecipitation with Mg(OH)2 will not affect the analytical result. However, analyte loss may affect the detection limit, and excess residue seasalt and Mg matrix could suppress the ICPMS signal,
Table 4. Isobaric Interferences in ICPMS element
isotope
abundance (%)
Pb Cu
204 63 65 110
1.4 69.1 30.9 12.4
114
28.8
Cd
interference 204Hg 40Ar23Na 40Ar25Mg 110Pd 94Mo16O 114Sn 98Mo16O
abundance (%) Hg, 6.8 Na, 0.0001 Mg, 0.00001 Pd, 11.8 Mo, 0.007 Sn, 0.65 Mo, 0.04
and other interferences, ICPMS sensitivity fluctuations, and recovery variations in the Mg(OH)2 coprecipitation and washing steps. To achieve accurate results by ID, background variations and ICPMS mass biases must be considered, contamination must be eliminated in sample handling and analysis, and isotope equilibration and isobaric interferences must be taken into account. In addition, the precision of the analysis is affected by factors such as sample and spike pipetting precision, ICPMS stability, spike ratio, isotopic abundances of spike and sample, counting statistics, reproducibility of procedural blanks, and error propagation in isobaric interference corrections. Spike Ratio. Table 3 lists spike ratios used for Pb, Cu, and Cd analysis. The target spike ratios are chosen by compromise among several factors. To minimize the uncertainty introduced by error propagation through isotope dilution equation, one would like a ratio close to the geometric mean of the “1”/“2” ratios of the spike and sample, where “1” and “2” refer to the two isotopes of the element and the spike is enriched in isotope “1”. On the other hand, when there are isobaric interferences occurring at Table 5. Mo Interference in Cd Analysis by ICPMS count rate (counts/s) sample 1% HNO3 5 nM Mo
10 nM Mo
12 nM Mo 40 nM Mo
95 amu
106 amu
110 amu
114 amu
118 amu
95Mo/94Mo16O
95Mo/98Mo16O
85 52572 51373 49993 53123 53563 93751 95809 98170 97762 121096 121140 124480 363447 349961 355142
41 44 39 33 39 44 46 45 48 45 48 48 39 42 52 45
42 86 87 93 94 91 158 150 161 152 172 166 182 452 467 477
32 200 231 209 203 193 357 376 355 345 450 466 448 1285 1242 1304
200 239 240 195 189 198 357 397 357 345 184 187 197 202 174 193
324 266 289 318 342 296 287 312 321 293 282 302 291 290 280
1714 1644 1443 1508 1598 1310 1430 1340 1434 1523 1592 1461 1524 1417 1406
300 7
1490 6
average SD (%)
Table 6. Mg Interference in Cu Analysis by ICPMS count rate (counts/s) sample
63 amu
65 amu
66 amu
68 amu
220 amu
40Ar26Mg/40Ar25Mg
1% HNO3 4 mM Mg 18 mM Mg 35 mM Mg 80 mM Mg 160 mM Mg
475 432 432 451 385 385
349 6052 23818 32099 41605 48209
167 8263 29150 36906 47405 53912
285 1387 1770 1488 1125 988
31 39 24 38 34 29
1.07 1.12 1.09 1.11 1.09
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Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
Figure 1. ID-ICPMS recoveries of Pb, Cu, and Cd added to a trace metal-free seawater. The linear regressions of the data are Pbmeasured (pM) ) 0.004 + (0.9997 ( 0.004)Pbadded (pM), r2 ) 0.9989 (n ) 39); Cumeasured (nM) ) 0.017 + (0.9962 ( 0.017)Cuadded (nM), r2 ) 0.9948 (n ) 11); and Cdmeasured (pM) ) 0.022 + (0.9974 ( 0.002)Cdadded(pM), r2 ) 0.9998 (n ) 22).
Figure 2. Profiles of Pb, Cu, and Cd in the northeastern Atlantic (AII 123, Station 3, 26°25′ N, 33°40′ W and station 8, 31°00′ N, 31°00′ W) analyzed by ID-ICPMS after coprecipitation with Mg(OH)2 (9) as compared with the profiles from the northwestern Atlantic1,2 measured by TIMS and AA methods (O).
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Table 7. Data from AII 123 Cruise in October 1989
depth (m)
Cu (nM)
24 61 72 102 113 145 170 257 342 425 489 558 637 802 892
1.15 1.20 1.26 1.12 1.35 1.70* 1.31 1.56* 1.22 1.34 1.22 1.33 1.39 1.26 1.32
depth (m) 0 17 55 75 95 117 133 154 189 208 231 251
(A) Cu and Cd Data at Station 3 (26°25′ N, 33°40′ W) Cd (pM) P (µM) depth (m) Cu (nM) 13 9 5 8 7 5 49 75 122 171 215 236 298 386 392
