1099
Anal. Chem. 1989, 67, 1099-1103 (31) Kassei, D. 6.; Kayganich, K. A.; Watson, J. T.; Allison, J. Anal. Chem. .1988, 60, 911. (32) Karasek, F. W.; Betty, K. R.; Kim, S. H. Anal. Chem. 1978, 50, 1278. (33) Leasure, C. S.; Vandiver, V. J.; Rico, G.; Eiceman, G. A. Anal. Chim. Acta 1985, 175, 135. (34) "Instruction Manual for the Installation, Operation, and Maintenance of Dilution Apparatus, Q 5 " ; Report No. 136-300-52E, Product Assurance Directorate, Edgewood Arsenal, Aberdeen Proving Ground, MD, May 3. 1976.
(35) Spangler, G. E.; Carrico, J. P. Int. J . Mass SDectrom. Ion Phvs. 1983;52, 267. (36) Vandiver. V. J. Gas Phase Reaction Rate Constants for Atmospheric Pressure Ionization in Ion Mobility Spectrometry. Ph.D. Dissertation, New Mexico State University, Las Cruces, NM, May 1987. pp 62-75.
for review
28$ lg8%Accepted February 22,
1989.
Isotope Dilution Gas Chromatography/Mass Spectrometry for the Determination of Nickel in Biological Materials Suresh K. Aggarwal, Michael Kinter, Michael R. Wills, John Savory, and David A. Herold* Department of Pathology, Biochemistry and Medicine, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908
Precise and accurate methods are requlred to measure nlckei in urine and serum samples to ldentlfy clinical states of elther deficiency or toxlclty. This paper presents an isotope dllutlon gas chromatography/mass spectrometry method for the measurement of nlckel In biological samples. The method involves the preparation of a thermally stable and volatile nickel chelate uslng iHhlum bis(triflu0roethyl)dithlocarbamate as the chelating agent. Conditions were optlmired for the dlgestbn of the sample and quantltative Preparation of chelate as well as the preclse and accurate measurements of the Isotope ratios using a capillary column gas chromatograph wlth a general purpose mass spectrometer. The memory effect between samples of different isotope ratlos was evaiuated and was found to be negligible. The quantitative accuracy of Isotope dilution was vaildated by measurlng nickel In the NIST freeze-dried urine reference material, SRM 2670, wlth comparison to the recommended value.
INTRODUCTION Interest in the quantitative determination of nickel has increased due to the important clinical, nutritional, and toxicological effects of this element ( I , 2). For example, it has been found that nickel is an essential trace nutrient that is involved in enzyme activity, hormonal action, structural stability of biological macromolecules, and general metabolism. However, excessive exposure to certain nickel compounds can cause dermatitis or respiratory disorders, including cancer of the respiratory system, and reduce activities of the enzymes cytochrome oxidase, isocitrate dehydrogenase, and maleic dehydrogenase. I t has also been reported that nickel can accumulate in patients undergoing chronic hemodialysis for end-stage renal disease (3, 4 ) . As points of reference, Ni concentrations in serum and urine of nonexposed healthy human subjects have been found to be as follows: serum 0.25 h 0.24 pg/L, n = 30 ( 5 ) ,and spot urine collection 2.0 f 1.5 pg/L, n = 34 (61, with biological tolerance values of 8 pg/L in serum and 30 pg/L in urine having been proposed (7). Therefore, accurate and precise quantitative methods are needed to determine nickel in urine and serum samples to identify normal, deficient, and toxic clinical states. As with most metals, nickel in biological materials is usually measured by electrothermal atomic absorption spectrometry (8,9). Nickel has also been measured by dimethylglyoximesensitized differential pulse polarography a t a dropping 0003-2700/89/036 1-1099$0 1.50/0
mercury electrode (10). Both methods possess sufficient sensitivity and precision for biological applications; however, it is interesting to note that reference values have decreased steadily over the years (11). In 1981 the IUPAC Subcommittee on Environmental and Occupational Toxicology of Nickel identified the need to develop a definitive method for the analysis of nickel in biological materials by stable isotope dilution mass spectrometry (8). Since isotope dilution employs an ideal internal standard, i.e. an enriched isotope of the same element, its accuracy does not depend on quantitative sample preparation and external standardization. Isotope dilution mass spectrometry is, therefore, ideal for use in definitive analytical methods. We have recently developed methods for the determination of isotope ratios in various metals using their thermally stable, volatile chelates by gas chromatography/mass spectrometry (GC/MS), which are suitable for detection of nanogram amounts of the metal (12). The method involves the use of a general purpose organic mass spectrometer and offers the high sensitivity needed for trace metal determination in biological samples. The use of an organic mass spectrometer is important because these instruments are more widely available than the thermal ionization, spark source, or inductively coupled plasma mass spectrometers commonly used in the nuclear and geological fields. Further, the use of GC/MS with chelate formation involves the measurement of isotope ratios in the high mass range, for example m / z 550-600 for the nickel chelate used here, where discrimination effects in the ion source and at the detector are less pronounced. Also, the use of capillary column gas chromatography allows the separation of different metal chelates (13). Lastly, the measurements can be carried out by using the molecular ion as well as any metal-containing fragment ions. This enhances the confidence in the quantitative data in terms of evaluating possible isobaric or other interferences. These advantages, as well as the need for a definitive method for nickel determination in biological samples, prompted us to develop this stable isotope dilution GC/MS method. The potential of organic mass spectrometers for trace metal determinations has been demonstrated in a few published studies (14-17). One of the problems with using metal chelates in GC/MS is cross-contamination of samples due to a memory effect between samples of different isotope ratios (16,18). This can be a serious problem for some metals and must be evaluated for the metal and the chelating agent under investigation, as well as for the GC/MS system being used. In this work, it is shown that for the nickel chelate used, there 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
is a minor amount of carry-over when samples of widely different isotope ratios (e.g., natural and enriched isotope composition) are analyzed. This effect is negligible under conditions that would be used in a n isotope dilution experiment, where good experimental design would keep the isotope ratios within a factor of 10, preferably approaching 1. This paper presents data from studies carried out for the preparation of thermally stable and volatile nickel chelates and the optimization of the gas chromatographic/mass spectrometric parameters for maximum sensitivity with high precision and accuracy in the isotope ratio measurements. The accuracy of the isotope dilution quantitation has subsequently been validated by the quantitation of nickel in a standard reference material, National Institute of Standards and Technology (NIST) freeze-dried urine SRM 2670, which has a recommended value for nickel of 70 gg/L. This method offers sensitivity to the parts-per-billion level (gg/L) with the inherent advantages of isotope dilution analyses, most notably that the accuracy and precision of the analysis are not affected by incomplete recovery. In this work, 62Niwas used as a spike, and the concentration was determined by monitoring the molecular ion and using two sets of isotope ratios.
EXPERIMENTAL SECTION Instrumentation. The mass spectrometer used was a double-focusing, reverse-geometry instrument (Model 8230, Finnigan MAT, San Jose, CA) with a SpectroSystem 300 data system. The instrument was operated in the electron ionization (EI) mode using 70-eV electrons with a source temperature of 200 "C, the conversion dynode at -5000 V, and the secondary electron multiplier at 2200 V. Data were acquired in the selected ion monitoring mode (SIM) using voltage peak switching. A Varian 3700 gas chromatograph was equipped with a DB-1 (J. W. Scientific, Rancho Cordova, CA) poly(dimethylsi1oxane) bonded-phase fused silica capillary column, 10 m X 0.32 mm, with a 0.25-gm film thickness. Samples were injected by using an on-column injector (OCI-3, Scientific Glass Engineering, Austin, TX) at an oven temperature of 100 "C followed by a 15 OC/min ramp to 300 O C . High-purity helium was used as a carrier gas. Reagents. "Ni (>98% enriched) was obtained from Oak Ridge National Laboratory (Oak Ridge, TN). Certified Atomic Absorption Standard purchased from Fisher Scientific (Fairlawn, NJ) was used as the primary standard. Double sub-boiling quartz-distilled HNO, in Teflon bottles was obtained from the National Institute of Standards and Technology (NIST, Gaithersburg, MD). The standard reference material, freeze-dried urine SRM 2670, was also purchased from the NIST and was prepared according to their directions. Sodium diethyldithiocarbamate (Na(DEDTC))was purchased from Aldrich Chemical Co. (Milwaukee, WI). Lithium bis(trifluoroethy1)dithiocarbamate (Li(FDEDTC))was synthesized by using bis(trifluoroethy1)amine from PCR Research Chemicals (Gainesville, FL), n-butyllithium, and carbon disulfide in an inert atmosphere a t -70 "C (19). Several precautions were taken to minimize the potential for contamination by environmental nickel. Prior to the sample preparation, the acetate buffer was treated with a 0.1 M solution of Na(DEDTC) and extracted with CH,Cl, to remove any Ni contamination. The organic extract was discarded. Hydrogen peroxide and ammonia solutions were checked for the presence of nickel by electrothermal atomic absorption spectrometry. No detectable amounts of Ni (
,
i
'
I
2
Anal. Chem. 1989, 67, 1103-1108
Aggarwal, S. K.; Duggal, R. K.; Rao, R.; Jain, H. C. Int. J . Mass Spectrom. Ion Processes 1988, 7 7 , 221-231. DeBievre, P.; Gallet, M.; Holden, M. E.; Barnes, I. L. J . Phys. Chem. Ref. Data 1984, 13, 809-891.
(7) Sunderman, F. W., Jr. I n Handbook on Toxlclty of Inorganic Compounds; Seiler, H. G., Sigel, H., Eds.; Marcel Dekker: New York,
1988;p 460. (8) Brown, S.S.;Nomoto. S.;Stoeppler, M.; Sundermann, F. W., Jr. Pure Appl. Chem. 1981, 53, 773-781. (9) Anderson, I.; Torjussen, W.: Zachariasen, H. Clin. Chem. 1978. 24,
1198-1202. (10) Versieck, J. Crlt. Rev. Clln. Lab. Sci. 1985, 22, 97-184. (11) Flora, C. J.; Nieboer, E. Anal. Chem. 1980, 52, 1013-1019. (12) Kinter, M.; Aggarwal, S. K.; Wills, M. R.; Savory, J.; Herold, D. A. Abstr. No. 744,40th National AACC Meeting, July 24-28, 1988,New Orleans; Clin. Chem. 1988, 34, 1305. (13) Schaller, H.; Neeb, R . Fresenius 2. Anal. Chem. 1986, 323, 473-476. . . - .. . . (14) Veillon, C.; Wolf, W. R.; Guthrie, B. E. Anal. Chem. 1979, 57, 1022- 1024. (15) Hachey. 6. L.; Biais, J. C.; Klein, P. D. Anal. Chem. 1980, 52, 1131-1 135. (16) Buckley, W. T.; Huckin, S. N.; Budac, J. J.; Elgendorf, G. K. Anal. Chem. 1982, 5 4 , 504-510. (17) Reamer, D. C.; Veillon, C. Anal. Chem. 1981, 53, 2166-2169. (18) Siu, K. W. M.; Fraser, M. E.; Berman, S. R. J . Chromatogr. 1983, 256, 455-459. (19) Sucre, L.;Jennings, W. Anal. Lett. 1980, 13,497-501.
1103
RECEIVED for review October 17, 1988. Accepted February 21,1989. Funding for the purchase of the high-resolution mass spectrometer was obtained from the National Institute of Health, Division of Research Resources Shared Instrumentation Grant Program, Grant No. 1-S10-RRO-2418-01. Additional funding from the John Lee Pratt Fund of the University of Virginia and Grant ESO 4464 of the National Institute of Environmental Health Sciences is also gratefully acknowledged. S.K.A. thanks the Division of Experimental Pathology, Department of Pathology, University of Virginia Health Sciences Center, for a postdoctoral fellowship and the authorities a t Bhabha Atomic Research Center, Trombay, Bombay-400 085, India, for granting leave.
Assessment of the Relative Role of Penning Ionization in Low-Pressure Glow Discharges Rebecca L. Smith, David Serxner, and Kenneth R. Hess* Department of Chemistry, Franklin and Marshall College, Lancaster, Pennsylvania 17604
Optical investigations into the relative role of Penning ionization as a mechanism for the ionization of sputtered species in low-pressure argon glow discharges were carried out. These studies employed methane as a quenching agent to effectively reduce the argon metastable population. This reduction of metastable atoms significantly limits the extent of Penning ionization present in the discharge, which wlll decrease ion emission signals from the sputtered species. The magnitude of the decrease in these ion emission signals may then be related to the relative importance of the Penning Ionization process In overall discharge ionization of the sputtered species. These studies show Penning ionization to account for approximately 40-80% of the ionlzation of sputtered species, depending upon dlscharge conditions of current and pressure.
impact, where there is a kinetic energy transfer from an energetic electron to the atom resulting in ionization, and Penning ionization in which ionization results from a transfer of potential energy from a metastable state of the discharge gas. electron impact:
-
Mo + ePenning ionization:
Mo + Ar*
M+ + 2e-
M+ + ArO + e-
For argon, these metastable levels are the 3P2a t 11.55 eV and the 3P0a t 11.72 eV. If the atom or molecule of interest has an ionization potential lower than these metastable levels, then ionization may occur with generally uniform cross sections (19, 20).
INTRODUCTION Glow discharge devices (1) are finding broad analytical use as sources of atoms and ions for atomic absorption (2-4), atomic emission (5-7), atomic fluorescence (8-10, resonance ionization (12), and mass spectrometry (13-16). With continuing research and development into quadrupole based instruments for glow discharge mass spectrometry, along with the installation of a commercial magnetic sector system (17) in an estimated 20 user laboratories (18),applications of glow discharge mass spectrometry appear to be on the verge of a significant expansion. In order to develop adequately the technique of glow discharge mass spectrometry, information on the fundamental processes that result in the formation of sampled ions is of interest. The two major mechanisms of ionization believed to be present in low-pressure glow discharge devices are electron
A quantitative assessment of the relative roles of Penning ionization vs electron impact ionization has not been demonstrated, although Penning ionization is believed to play an important role in the ionization of discharge species for a variety of reasons. First, mass spectra from discharge sources similar to that used in this work exhibit ion signals from the sputtered cathode material generally larger than ion signals from the discharge gas, even though the discharge gas is present in a much greater concentration. This would imply preferential ionization of the sputtered species, possibly through Penning ionization, which will ionize the sputtered species but not the discharge gas. In addition, relative sensitivity coefficients for ion signals of the sputtered species are generally within a factor of 3-5 in glow discharge sources. Sputter yields are known to vary by this amount (21, 22), indicating a uniform method of ionization for the sputtered species. Laser ablation mass spectra from redeposited sputtered material have shown elemental ion ratios for copper
0003-2700/89/0361-1103$01.50/0 0 1989 American Chemical Society
