Anal. Chem. 2004, 76, 3971-3978
Isotopic Variations of Zn in Biological Materials Anna Stenberg,*,† Henrik Andre´n,‡ Dmitry Malinovsky,† Emma Engstro 1 m,§ Ilia Rodushkin,§ and § Douglas C. Baxter
Divisions of Applied Geology and Chemistry, Luleå University of Technology, S-971 87 Luleå, Sweden, and Analytica AB, Aurorum 10, S-977 75 Luleå, Sweden
Variations in the isotopic composition of Zn present in various biological materials were determined using highresolution multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), following digestion and purification by anion exchange chromatography. To correct for differences in instrumental mass discrimination effects between samples and standards, Cu was employed as an elemental spike. Complementary analyses of Zn separates by sector field ICPMS instruments revealed that the concentrations of the majority of potentially interfering elements were reduced to negligible levels. Residual spectral interferences resulting from 35Cl16O2+, 40Ar14N2+, and 40Ar14N16O+ could be instrumentally resolved from the 67Zn, 68Zn, and 70Zn ion beams, respectively, during measurement by MC-ICPMS. The only other observed interference in the Cu and Zn mass range that could not be effectively eliminated by high-resolution multicollection resulted from 35Cl2+, necessitating modification of the sample preparation procedure to allow accurate 70Zn detection. Complete duplication of the entire analytical procedure for human whole blood and hair, as well as bovine liver and muscle, provided an external reproducibility of 0.05-0.12‰ (2σ) for measured δ66/64Zn, δ67/64Zn, and δ68/64Zn values, demonstrating the utility of the method for the precise isotopic analysis of Zn in biological materials. Relative to the selected Zn isotopic standard, δ66/64Zn values for biological samples varied from -0.60‰ in human hair to +0.56‰ in human whole blood, identifying the former material as the isotopically lightest Zn source found in nature to date. Zinc is the second most abundant trace element in the human body1 and, in varying amounts, strongly influences the function of the brain, reproduction,2 and the immune system.3,4 The total Zn content in man is about 2-3 g, with the largest fractions being found in muscle tissue, skeleton, and liver.1 Due to variations in the exchange rate of the element,3 as well as the lack of a * Corresponding author: (e-mail)
[email protected]; (tel) +46-920 28 99 76; (fax) +46-920 28 99 40. † Division of Applied Geology, Luleå University of Technology. ‡ Division of Chemistry, Luleå University of Technology. § Analytica AB. (1) Iyengar, G. V. Radiat. Phys. Chem. 1998, 51, 545-560. (2) Costello, L. C.; Franklin, R. B. Oncology 2000, 59, 269-282. (3) Crebs, N. F.; Hambidge, K. M. BioMetals 2001, 14, 397-412. (4) Tapiero, H.; Tew, K. D. Biomed. Pharmacother. 2003, 57, 399-411. 10.1021/ac049698f CCC: $27.50 Published on Web 06/18/2004
© 2004 American Chemical Society
specialized storage system5 and reliable indicators of Zn status,3 investigations often encompass various sample matrixes, e.g., body fluids,3,6-14 feces,3,9,10,12,13,15,16 tissues,17-19 and, in certain cases, hair,10,20,21 and teeth.22,23 Body fluids reflect the elemental level at the time of sampling and are readily acquired, transported, and handled, whereas long-term effects can be investigated by analysis of target organs.15 Hair presents an archive of exposure over time,17 but strict sample handling is of vital importance due to the risk for exogenous contamination by deposition on the hair strands.20 The concentration of Zn in teeth has been shown to be unaffected by diagenesis and, hence, may be a useful tool in archaeological research.22,23 The complexity and variability of biological matrixes, often with high organic contents and elevated levels of inorganic salts, poses challenges to be overcome concerning sample preparation and analysis.16 Over the last two decades, inductively coupled plasma mass spectrometry (ICPMS) has risen to meet these challenges, providing information concerning nutritional status,3,7,9,10,12,13,15 (5) Rink, L.; Gabriel, P. BioMetals 2001, 14, 367-383. (6) Mun ˜iz, C. S.; Marchante Gayo´n, J. M.; Garcı´a Alonso, J. I.; Sanz-Medel, A. J. Anal. At. Spectrom. 1999, 14, 1505-1510. (7) Sadagopa Ramanujam, V. M.; Yokoi, K.; Egger, N. G.; Dayal, H. H.; Alcock, N. W.; Sandstead, H. H. Biol. Trace Elem. Res. 1999, 68, 143-158. (8) Rodushkin, I.; O ¨ dman, F.; Branth, S. Fresenius’ J. Anal. Chem. 1999, 364, 338-346. (9) Stu ¨ rup, S. J. Anal. At. Spectrom. 2000, 15, 315-321. (10) Stu ¨ rup, S. Anal. Bioanal. Chem. 2003, 378, 273-282. (11) Ba´ra´ny, E.; Bergdahl, I. A.; Bratteby, L.-E.; Lundh, T.; Samuelson, G.; Schu ¨ tz, A.; Skerfving, S.; Oskarsson, A. Sci. Total Environ. 2002, 286, 129-141. (12) King, J. C.; Shames, D. M.; Lowe, N. M.; Woodhouse, L. R.; Sutherland, B.; Abrams, S. A.; Turnlund, J. R.; Jackson, M. J. Am. J. Clin. Nutr. 2001, 74, 116-124. (13) Manary, M. J.; Hotz, C.; Krebs, N. F.; Gibson, R. S.; Westcott, J. E.; Broadhead, R. L.; Hambidge, K. M. Am. J. Clin. Nutr. 2002, 75, 10571061. (14) Rodushkin, I.; O ¨ dman, F.; Olofsson, R.; Axelsson, M. D. J. Anal. At. Spectrom. 2000, 15, 937-944. (15) Ingle, C.; Langford, N.; Harvey, L.; Dainty, J. R.; Armah, C.; FairweatherTait, S.; Sharp, B.; Rose, M.; Crews, H.; Lewis, J. J. Anal. At. Spectrom. 2002, 17, 1502-1505. (16) Vanhoe, H. J. Trace Elem. Electrolytes Health Dis. 1993, 7, 131-139. (17) Bush, V. J.; Moyer, T. P.; Batts, K. P.; Parisi, J. E. Clin. Chem. 1995, 41, 284-294. (18) Panayi, A. E.; Spyrou, N. M.; Iversen, B. S.; White M. A.; Part, P. J. Neurol. Sci. 2002, 195, 1-10. (19) Krachler, M.; Radner, H.; Irgolic, K. J. Fresenius’ J. Anal. Chem. 1996, 355, 120-128. (20) Rodushkin, I.; Axelsson, M. D. Sci. Total Environ. 2000, 250, 83-100. (21) Rodushkin, I.; Axelsson, M. D. Sci. Total Environ. 2000, 262, 21-36. (22) Carvalho, M. L.; Marques, J. P.; Marques, A. F.; Casaca, C. X-Ray Spectrom. 2004, 33, 55-60. (23) Mansilla, J.; Solis, C.; Cha´ves-Lomeli, M. E.; Gama, J. E. Am. J. Phys. Anthropol. 2003, 120, 73-82.
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reference concentration ranges,11,21 and diseases18 related to Zn. It must be borne in mind that, despite having five stable isotopes, all are potentially affected by spectral interferences originating from the sample matrix and preparation steps, as well as the plasma and atmospheric gases, causing overlaps by isobaric species, polyatomics and doubly charged ions.6-10,15,16,24,25 Having a high ionization potential, Zn is also particularly susceptible to suppression or enhancement of the ion signal caused by the high levels of proteins and salts typically present in biological samples.16 Thus, care must be taken during sample preparation and analysis to ensure the integrity of the data obtained. This is of particular concern in tracer experiments,3,6,7,9,10,12,13,15 where at least two Zn isotopes must be amenable to interference-free measurement, using chemical separation techniques,7,10,12,13 high mass-resolution,6,9,10,15 and mathematical correction.9 During a short period of time, multicollector (MC) ICPMS has unleashed new possibilities for exploring Zn isotopic variations in biological matrixes.15,26-28 Nevertheless, the number of highprecision analyses of Zn isotopic composition remains limited, presumably a direct consequence of spectral interference problems. The aim of the work described here was to assess natural Zn isotopic variations in biological materials using MC-ICPMS. For future comparability and traceability purposes, several reference materials currently available in the laboratory have been included in the suite of samples analyzed. EXPERIMENTAL SECTION Instrumentation. Prior to the Zn isotope ratio measurements, elemental concentrations and potential interferents were determined using two single-collector types of sector field (ICP-SFMS) instrument (Element and Element 2, Thermo Electron, Bremen, Germany). General operating parameters can be found elsewhere,29 although additional measurements of 136Ba and 140Ce in low resolution and 27Al, 28Si,35Cl, 47Ti, 51V, and 73Ge in medium resolution were included in this study. The high-resolution MC-ICPMS instrument employed was the Neptune from Thermo Electron, as described in detail by Weyer and Schwieters,30 relevant instrumental operating conditions being summarized in Table 1. Isotope ratio measurements were performed using the medium-resolution entrance slit and with the guard electrode activated to maximize ion transmission efficiency. The Faraday cup array was configured as follows: 63Cu (L3); 64Zn (L2); 65Cu (L1); 66Zn (center); 67Zn (H1); 68Zn (H2); 70Zn (H3). Before every measurement session, baseline and amplifier gain calibration procedures were performed. Analysis of samples and standards consisted of nine blocks of five cycles each, with an integration time of 4.194 s per cycle. All samples and standards were analyzed in duplicate (or triplicate) in a sequence of isotopic (24) Burton, L. L.; Horlick, G. Spectrochim. Acta 1992, 47B, E1621-E1627. (25) Mason, T. F. D.; Weiss, D. J.; Horstwood, M.; Parrish, R. R.; Russel, S. S.; Mullane, E.; Coles, B. J. J. Anal. At. Spectrom. 2004, 19, 209-217. (26) Mare´chal, C. N.; Te´louk, P.; Albare´de, F. Chem. Geol. 1999, 156, 251273. (27) Ohno, T.; Shinohara, A.; Chiba, M.; Hirata, T. Geochim. Cosmochim. Acta 2003, 67, A352. (28) Zhu, X. K.; Guo, Y.; Williams, R. J. P.; O’Nions, R. K.; Matthews, A.; Belshaw, N. S.; Canters, G. W.; de Waal, E. C.; Weser, U.; Burgess, B. K.; Salvato, B. Earth Planet. Sci. Lett. 2002, 200, 47-62. (29) Stenberg, A.; Malinovsky, D.; Rodushkin, I.; Andre´n, H.; Ponte´r, C.; O ¨ hlander B.; Baxter, D. C. J. Anal. At. Spectrom. 2003, 18, 23-28. (30) Weyer, S.; Schwieters, J. B. Int. J. Mass Spectrom. 2003, 226, 355-368.
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Table 1. Instrumental Operating Conditions for the Neptune forward power accelerating voltage sample cone skimmer cone coolant auxiliary nebulizer magnet setting sample uptake rate
1300 W -10 kV nickel, 1.1 mm orifice diameter nickel, 0.8 mm orifice diameter 16 L of Ar min-1 0.70 L of Ar min-1 ∼1.2 L of Ar min-1 (optimized daily) 65.88-65.89 u (central cup; optimized daily) ∼100 µL min-1
standard, three samples, isotopic standard, and so on. Blank measurements were also interspersed in the sequence at regular intervals. Contamination between the samples was avoided by 5 min of aspiration of the sample solution before initiating data collection. An in-built 2σ outlier test was applied to all blocks and cycles. Reagents and Samples. The sample preparation was performed by using analytical reagent grade nitric acid (65%, Merck, Darmstadt, Germany) additionally purified by sub-boiling distillation in a quartz still. Hydrochloric acid of Suprapure grade (40%, Merck), analytical grade hydrogen peroxide (30%, Merck), and Milli-Q water (Millipore Milli-Q, Bedford, MA) were used without further purification. For the anion exchange procedure, Dowex 1×8, Cl- form, 100-200 mesh (Fluka, Steinheim, Switzerland) was used. Calibration and internal standard solutions were obtained by diluting single-element standard solutions (SPEX Plasma Standards, Edison, NJ). As isotopic reference, Zn metal wire of 99.9985% purity (metal basis) (Puratronic, Alfa Aesar, Johnson Matthey GmbH, Karlsruhe; lot NM00558) was utilized. A length of the wire was dissolved in distilled HNO3 and diluted to yield a Zn concentration of 1 g L-1 in a 0.17 M HNO3 matrix. This solution, hereafter denoted JMC Zn, was employed as the isotopic standard, adopting the abundances determined by Tanimizu et al.31 Seronorm Trace Elements, Whole Blood samples were obtained from SERO A/S (Asker, Norway; Batch No. OK 0336, MR9067 and 404108). Standard reference materials (SRMs) Bovine Liver 1577a and Bovine Muscle 8414 were purchased from the National Institute of Standards and Technology (NIST, Gaithersburg, MD). The certified reference material GBW07601 Human hair Powder was from the Institute of Geophysical and Geochemical Exploration (Langfang, China). By using stainless steel scissors, scalp hair samples were collected from subjects between 5 and 42 years of age, living in Luleå, Sweden. Sample Digestion. Reconstitution of the freeze-dried blood samples was performed in the original bottles by addition of 5 mL of Milli-Q water followed by gently (occasionally) mixing during 20 min. Two-milliliter aliquots of the blood solutions and equal volumes of 16.7 M HNO3 were than added to 50-mL acidwashed polyethylene tubes (Sarstedt, Nu¨mbrecht, Germany). For the bovine liver material, 100 mg was added together with 2 mL of Milli-Q water and 2 mL 16.7 M HNO3 to similar tubes. Digestion of the materials in open tubes was performed in a microwave oven (MARS-5 CEM, Matthews, MC) at 110 °C and a power of 1200 W (ramp time 30 min, hold time 45 min). (31) Tanimizu, M.; Asada, Y.; Hirata, T. Anal. Chem. 2002, 74, 5814-5819.
Table 2. Concentration Ranges of Potential Parent Elements of Interferences in Biological Materials before and after Anion Exchangea concentration range/ng g-1
concentration range/ng g-1
before
after
590-158000
0.02-2
isobars
Ca Cr
1.1 × 105-2.9 × 106
