Anal. Chem. 1990, 62, 107-111
Marburg for his helpful comments and for his kind providing of indicator betaine.
LITERATURE CITED (1) Bosch, E.; Roshs, M. Anal. Chem. 1086, 6 0 , 2008-2013. (2) Fritz, J. S. AcM-Base Titrations in Nonaqueous Solvents; Allyn and Bacon: Boston, MA, 1973. (3) Fritz, J. S.; Marple, L. W. Anal. Chem. 1062, 34, 921-924. (4) Marple, L. W.; Fritz, S. J. Anal. Chem. 1983, 35, 1223-1227. (5) Marple, L. W.; Fritz, S. J. Anal. Chem. 1983, 3 5 , 1431-1434. (6) Chantoonl, M. K.; Kolthoff, I . M. J . Phys. Chem. 1978, 8 2 , 994-1000. (7) Safarlk, L.; Stransky, Z. I n Comprehensive Analytical Chemistry; Elsevier: Amsterdam, 1986; Vol. XXII. (8) Kolthoff, I . M.; Chantooni, M. K. Anal. Chem. 1978, 50, 1440-1446. (9) K o b f f , I. M.; Chantoonl, M. K. J . Phys. Chem. 1979, 8 3 , 468-474. (10) Relchardt, C. Sdvents and Solvent Effects in &gnk Chemistry, 2nd
.
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ed.; VCH Verlagsgesellschaft: Weinheim, 1988. (11) Wlnkelmann, J.; Quitzsch, K. Phys. Chem. (Le@z@)1972, 250, 355-366. (12) Langhals, H. Angew. Chem., Int. Ed. Engl. 1082, 2 1 , 724-733. (13) B e k h k , V.; Jurina, J. Colkct. Czech. Chem. Commun. 1062, 47, 1060-1068. (14) Fuoss, R.; Shedlovsky, T. J . Am. Chem. Soc. 1949, 71, 96-1498. (15) Handbook of Chemistry and Physics, 59th 4.;CRC Boca Raton, FL, 1979. (16) Onsager, L. J. Am. Chem. Soc. 1938, 5 8 , 1486-1493. (17) Marcus, Y. Ion Solvation; J. Wiley: New York, 1985. (18) Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J . Org. Chem. 1903, 48, 2877-2887.
RECEIVED for review June 28, 1989. Accepted October 11, 1989.
Mass Spectrometric Studies on the Response Mechanism of Surface Ionization Detectors for Gas Chromatography Toshihiro Fujii* and Hitoshi Jimba National Institute for Environmental Studies, Tsukuba, Ibaraki 305, Japan Hiromi Arimoto Shimadzu Corporation, Nakagyo, Kyoto 604, J a p a n
Mass spectrometric technlques at atmospheric pressure are used to measure the Ionic specles generated when organic compounds from a gas chromatograph come into contact wtth a hot Pt emitter. The intensities and types of Ions formed against surface temperature, gas environments around the emitter, and emitter materlai were Investigated. I n almost every sample studied, the observed ion species in the mass spectrum are the same as those obtained wtth SIOMS (surface ionization organic mass spectrometry in vacuum condttion) with only the intenshy dlstrlbution showing differences. The results confirm that positive surface ionization is the mechanism of the production of charge carriers in the surface ionization detector.
INTRODUCTION In previous papers (1, 2), the surface ionization detector (SID) has been described for gas chromatography. The SID utilizes a platinum filament emitter which is heated electrically. The emitter is connected to the power supply and is positioned 2 mm above the nozzle of the carrier gas exit. The emitter is maintained at a positive potential with respect to the ion collector. This detector is extremely sensitive to organic compounds such as tri-n-butylamine which dissociates to species having low ionization potentials. It can be operated in any kind of carrier gas and the addition of air (oxygen) to the detector environment improves the performance. Previous work indicated that the ionization mechanism involves positive ionization of organic species generated on the hot surface. The molecules being detected decompose on the hot surface into radicals which have lower ionization energy (IE) than the molecules and are ionized efficiently. The ion current of the secondary species ( 8 ) is dependent ( 3 ) on the surface temperature, T
* To whom correspondence should b e directed.
where n is the number of molecules impinging on the surface, Ys(T)is the yield of chemical reactions on the surface, cp is the work function of the surface, and go/g+ is the ratio of the statistical weights of the ions and neutral species. The total ion current is J(T)= xjs(T). The positive surface ionization mechanism seems to be preferred so far. However, the exact identification of the charged species being formed in the SID has not been made. The produced ion should be identified, which directly reveah the response mechanism. In addition, the study furthers the understanding of the detector characteristics as well as improves its operation technique. Atmospheric pressure ionization mass spectrometry (APIMS) is a novel type of mass spectrometry ( 4 ) in which ionization is carried out in a reaction chamber (at atmospheric pressure) external to the low-pressure region of a mass analyzer. Ions present in the source enter the mass analyser region through a small aperture. By use of this technique, the details of various ion processes in atmospheric conditions have been determined by the direct mass spectrometric analysis with a 1 atm plasma. In studies by Horning et al. (5)and by Grimsrud et al. (6) in APIMS ion source was modified to be an actual electron capture detector (ECD), complete with a cell electrode, so that the ECD function of this ion source could be obtained simultaneously with negative ion measurements. This combination not only provides an ideal means of studying electron capture reactions but also appears to constitute a promising technique in itself for the analysis of trace amounts of organic substances. Because of the demonstrated success of APIMS for the sensitive measurement of ions formed within its ECD-like source, a similar approach was used for the mechanism study of SID. The instrument used is a specialized atmospheric pressure surface ionization mass spectrometer. Its ion source is an actual SID.
0003-2700/90/0362-0107$02,50/0@ 1990 American Chemical Society
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Flgure 1. Experimental setup of APIMS(S1D);a SID ion source attached to the front flange of the vacuum envelope of a mass spectrometer.
The authors began to explore the areas of application of the SID about 3 years ago by choosing various compounds. Recently the authors reported on several types of environmentally and biologically important compounds (7)to provide a broader understanding of the potential application of the SID in chemical analysis. Polyaromatic hydrocarbons, antidepressent compounds, and a-pinene were chosen for the present study, because the SID is clearly sensitive, being suitable for the qualitative and quantitative analysis of these compounds. Triethylamine (TEA), 1,3,5-~ycloheptatriene, Nfl-dimethylaniline, and piperidine were also used. These compounds were chosen because they were studied in detail by SIOMS (surface ionization organic mass spectrometry in vacuum conditions) (8, 9). The present reports include (1) the exact mass spectrometric identification of positive ions formed in the SID, (2) the effect of surface temperature, gas environments, and emitter material on the intensity and types of ions formed, and (3) the comparison of the SID ions with those obtained by SIOMS.
EXPERIMENTAL SECTION The main instrument used for the mass spectrometric studies on the response mechanism of the SID, is the modification of the apparatus specifically prepared by the Shimadzu Corp. for an atmospheric pressure ionization mass spectrometry. This is essentially a combination of mass spectrometry and SID for gas chromatography, which is referred to as APIMS(S1D) hereafter. With this instrument, the SID response can be monitored along with mass spectral measurement of the ions formed in the SID. The atmospheric pressure ionization mass spectrometer for sampling the ions under atmosphereic pressure has been described in detail elsewhere (10). Briefly, the components are a sampling interface, a three-stage differential vacuum system, an electrostatic ion lens system, and an ion detection system with the channeltron electron multiplier detector operated both in the pulse mode and in the analog mode. A detailed view of the SID atmospheric pressure ion source is shown in Figure 1. The SID ion source is an open-type modification (without any envelope) of a standard SID for gas chromatography. Therefore, we assume that the pressure at which the ions are formed is nearly 1 atm. It is noted that a precise pressure measurement was not made because of experimental difficulty. The coiled Pt emitter is positioned at the midpoint between the stainless steel nozzle and entrance cone of the atmospheric pressure ionization mass spectrometer. The repeller electrode around the nozzle is made of a 30 mm diameter stainless steel disk which is 5 mm behind the emitter. All the SID ionic measurements reported were obtained by applying 300 V to both the repeller and the emitter. The open-type SID ion source is fixed on the front flange of the vacuum envelope of the atmospheric pressure ionization mass spectrometer, where a 0.5 mm diameter orifice was drilled. This entrance cone, along with the skimmer (1mm diameter), provides a controlled leak of the ion source contents into the vacuum region.
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50
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Flgure 2. Mass spectra of a-pinene obtained with unt resolution where the ions are produced by either (a)surface ionization with the R emitter at atmospheric pressure, APIMS(SID), or (b) the Re oxide emitter
surface ionization at vacuum condition, SIOMS. The relative abundance is reported. Peaks having less than 2% base peak intensity were omitted. APIMS(S1D) is operated in the pulse mode of the muitichannel scaling (ref lo), while SIOMS is operated in the analog mode. Note the difference in relative intensity of the mass spectrum. It is observed that the 500 L/min fit-stage rotary pump is loaded to give a first chamber pressure of 2.6 Torr (see Figure 1). Thus, the magnitude of the total gas flow into the mass spectrometer is calculated as 1700 mL/min, most of which would be the air stream from the atmosphere. The grounded flange in Figure 1 serves as the cathode to which a positively charged species will migrate. Samples are introduced to the carrier gas stream via a temperature-controlled diffusion cell in which either a diffusion tube or a permeation tube is placed. A heated stainless steel transfer tube connects this cell with the nozzle. The SIOMS was performed with a Finnigan 3300 gas chromatograph-quadrupole mass spectrometer equipped with a home-made thermionic ion source. The oxidized Re ribbon was prepared and used as a surface ionization emitter. The sample substances were admitted to the mass spectrometer from a reservoir via a variable leak valve or gas chromatograph. Full experimental details have been reported previously (11). A Shimadzu gas chromatograph equipped with the SID (now commercially available) was used for the gas chromatography of the SID response, GC(S1D). All the test materials (10 compounds) were purchased from the Eeda Chemical, Tsukuba, and used without further purification.
RESULTS AND DISCUSSION Mass Spectra. Numerous experiments were conducted for each of the 10 tested compounds listed in Table I along with the formulas, molecular weights, and ionization energies. Table I also includes the following experimental results: (1) the GC(SID) responses, expressed in terms of sensitivity (C/g), under the optimum conditions ( I ) for the detection limit; (2) the APIMS(SID) mass spectrum by the Pt emitter (composed of an intense peak) with the ion current (A) referenced to the sample amount of 1 g/s (determined from the height of the lines in the mass spectrum) for five volatile compounds, data taken at the optimum emitter temperature give the maximum total ion signal to each of the sample; (3) the SIOMS mass
ANALYTICAL CHEMISTRY, VOL. 62, NO. 2, JANUARY 15, 1990
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Table I. Response Comparison of APIMS(SID), SIOMS in Vacuum Condition, and GC(S1D) compounds ( m / e ,IE) triethylamine (101, 7.50) (CzH&J N,N-dimethylaniline
GC(SID), C/g
1.2 x 10-3
1.6
8.6
A/(g/s)
X
lo-’
APIMS(S1D) mass spectruma ( m / z , % )
A/Torr
SIOMS mass spectrum’ ( m / z , %) (M - H), 100
86, 100 (M - H), 63.6
22.8
1.1 x 10-3
(M - H), 100
17.9
(M - H), 100
3.3 x 10-3
(M - H), 100 (M - 3H), 11.9
45.8
(M - H), 100
6.4
(M - H), 100
3.3
119,100 105, 100 91, 51.4 121, 34.3 (M - 3H), 20.0 (M - 5H), 15.1
9 x 10-2
86, 10
(121, ?)
CSHIIN piperidine (85, 8.7) C6H11N
1,3,5-cycloheptatriene (92, 8.28) C7H7 a-pinene (136, 8.07)
3.3 2.6
X
4.6 x 10-3
X
lo4
1.2 x 10-6
C10H16
107, 15.1 95,12
93,12 (M - H), 11.1 120, 11
anthracene pyrene (202, 7.48) ClBHlO
fluorene (166, 7.93) C13H10
imipramine (HC1)
(M - 3H), 100 (M- 5H), 77.8 (M - 7H), 55.6 93, 42.2
107, 38.9 105, 37.8 (M - H), 32.2 119, 26.7 91, 24.4 55,20 134, 12.2 M, 100
M, 100 (M + l),17.2
6.5 X lo-’
M, 100 (M + l),19.8
M, 100 (M + l),19.3
4.3 x 10-2
(M - H), 100 166, 16.0
(M - H), 100 166, 15.3
8.3 X 10-1
58, 100 84, 22.5
58, 100 84, 14
2.0
86, 100
86, 100
X
(280, ?) C19H24NZ
lidacaine
(M - 5H), 14 (M - H), 100
lo-’
2.7
(178, 7.41) C14H10
(M- 3H), 16
(234, ?)
(M + l),17
120, 23.3
CllH22N20 Spectrum is composed of an intense peak having more than 10% base peak intensity. spectrum for Re-oxide emitter with the ion current (A) referenced to a pressure of 1 Torr. Ion current expressed in amperes is output of the ion multiplier with the gain a t 5 x lo3 in both APIMS(S1D) and SIOMS experiments. Table I demonstrates parallels between sensitivity of GC(SID) and that obtained on APIMS(S1D). The order of sensitivity magnitudes observed by APIMS(S1D) is the same as that observed with the present SID for GC. This correlation provides support for the assumption inherent in this study that mass spectrometric study explains the responses observed in an actual SID of GC. In comparison of positive charge carriers produced from APIMS(S1D) with ion species obtained on conventional SIOMS using the oxide Re emitter, it was found that the observed ion species are exactly the same in both cases for all the compounds. The exact agreement of ion species confirms that the mechanism, which was initially considered in the previous study, need not be modified in order to account for the present results. The mechanism for the response of SID which detects positive-charge carriers is positive surface ionization. However, the relative intensity of the observed ion species is changed, except for some substances which yield a single ion peak. The example of this change is shown for a-pinene in Figure 2a, where molecular ions of (M - nH)+ are dominant in the spectrum obtained by the SIOMS. In Figure 2b where the APIMS(S1D) is done, ionic species having lower m / z values are produced more efficiently. The point is that the ionization observed by the APIMS(S1D) is more prone to
produce the extensively dissociated ion species than the SIOMS. This result was expected. In the vacuum environment of the SIOMS, sample molecules interact directly with the hot emitter surface, and the ions formed are the result of thermal pyrolysis. In the APIMS(SID), the hot emitter is placed in an atmospheric pressure “plasma” which contains substantial concentrations of chemically active species such as H atoms and 0 atoms, as well as substantial amounts of water vapor. This is a thermal and combustion environment as opposed to the thermal pyrolysis prevailing in the vacuum. It is certain that the differences are partly due to the difference in the emitter material which causes a different work function and different pyrolytic properties and hence a shift in the response. The mass spectrometric study on imipramine and lidocaine is especially interesting. These compounds were successfully examined in the previous study as a good example of drug analysis. The authors believe the SID technique can play a very important role in the analytical chemistry of many drugs in the near future. These compounds are also interesting from the view of surface ionization mass spectrometry, because they are nonvolatile compounds with a rather complex structure. The APIMS(S1D) spectra of these compounds are characterized by the dissociative surface ionization (DSI) ions, which are produced through the formation process of dissociative surface reaction product with a low ionization energy followed by the surface ionization. These DSI ions are the charge carrier causing the response in the SID. No indication is given that the molecular ions are formed
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Emitter Temperature (T)
Flgure 3. Emitter temperature dependence of triethylamine (TEA) response and background current for (left) GC(S1D) and (right) APIMS(S1D). For GC(S1D) the results are taken under a 30 mLlmin helium carrier gas mixed with additional gas of the dried air at 50 mllmin. Sample size was 2.6 ng of TEA in dichloromethane. For APIMS(SID), at the upper level, the resulting ion currents at m / e 86,100,23,39,and 41 were shown under the conditionsthat the TEA was introduced at the rate of 7.2 X lo4 g/min and was carried by the 50 mL/min air carrier gas. At the lower level, two curves of total ion currents either from the TEA sample or from alkali-atom impurities are drawn.
for both compounds in contrast to other substances. But the difference in relative intensity of the mass spectrum between APIMS(S1D) and SIOMS was observed again just like other compounds. The APIMS(S1D) spectrum of imipramine exhibits three intense peaks, m / z 58, 84, and 72, whose structures are probably (CH3)2NCH2+, (CH3)zNCHzCH2+, and (CH3)2NCH2CH2CH2+, respectively, via the dissociative reaction of a direct cleavage. The SIOMS spectrum has peaks at m / z 232, 186, and 70, whose composition has not been postulated thus far. The mass speectra of lidocaine comprise only two peaks at m l z 86 and 120. The most intense in the spectrum is the m / z 86 ion presumably with the structure of (CzH5)zNCHz+. Responses to Change. Emitter Temperature Effect. Figure 3 illustrates the response signal change caused by varying the emitter surface temperature (T)of the GC(S1D) and APIMS(SID). Emitter temperature is varied by changing the emitter heating current. The detailed operating conditions are described in the caption. On the left side, the temperature dependence of both and signal (i,) and background currents (ib) for the GC(S1D) is shown. The results of APIMS(S1D) are given on the right side. At the upper level, the temperature dependence of the respective sample ion species at m / z 86 and 100 as well as ions of alkali atoms at m / z 23,39, and 41 from the impurity in the Pt emitter material are shown. At the lower level, the temperature dependence of the total ion currents results either from the sample substance or from the
alkali atom impurity, I,(r) or Ib(n. No difference was observed in the shape of the is( T)curve for the GC(S1D) and the I,(r) curve for the APIMS(S1D). Both curves were increased with emitter temperature, and reached the maximum at a emitter temperature around 600 "C with a slight decrease at the higher temperature. Again, this agreement provides additional support for the assumption that the SID mechanism is the positive surface ionization. In a previous study, the background current of SID was initially considered to be the appearance of Na+ and K+ ions from Na and K impurity atoms in the Pt material of emitter. This assumption is essentially consistent with the present observation of ionic species Na+, K+, HzOK+,and H20Na+, which presumably are the adduct ions associated with the water vapor in the atmospheric environment. Effect of Field on APIMS(SID)Response. To demonstrate the effects of field, which is generated by applying the same voltage to the emitter and the repeller, the test sample of TEA was introduced through the diffusion tube. The response increased with the applied voltage and leveled off at about 150 V as expected. The applied voltage affects the desorption of the ion species from the surface as well as the ion transportation to the inlet of the mass spectrometer. Emitter Material. The effect on the response caused by varying the emitter surface was studied. Pt and Ir metals were examined because both provide excellent performance as an emitter material. Both emitters were formed into six-turn loops from wire of 0.25 mm diameter and then assembled into
Anal. Chem. 1990, 62, 111-115
the ion source housing so as to give the same length (area) and position. Since these emitter conditions were carefully controlled, it is certain that the observed large difference in i,(T) behavior is due to the differences in the properties of the emitter materials. It was found that the Ir emitter gave comparable results. The i vs T characteristics were essentially the same. The ion signal increased with heating current and leveled off at saturation values. Gaseous Environment around the Emitter. It could be concluded from the previous study that an oxidized emitter should be used in order to improve sensitivity and stability. Such an oxidized emitter can be obtained if air (oxygen) is added to the emitter surface from another gas line. The addition of 10 mL of air to helium as a carrier gas had effects on the signals observed by the APIMS(S1D). This effect was an increase in total ion intensity with a slight corresponding change in product ion distribution. This suggests that air serves the purpose of modifying the surface leading to a change in the work function as well as the chemical property of the surface, which is responsible for the difference in response.
CONCLUSION This study demonstrated the existence and theoretical basis of the SID mechanism; positive surface ionization is the mechanism of ion formation in SID. Thus, the probability of ion formation is related to the IE of the adsorbed species. On the hot surface, some of the compounds decompose through a series of unimolecular reactions. At each step of the decomposition newly formed species may be ionized. If the species has a low IE, its positive ion is formed. The charge carriers collected at the conventional SID collector may not be those originally formed. The ion of GC(S1D) moving through a gas in a longer electric field than the present experimental setup of the API(S1D)MS may ex-
111
perience charge stripping, charge transfer, ion/molecule reaction, etc. However, the SID response depends only on the total intensity of ion species initially formed. The incorporation of the SID into the ion source of an APIMS is very easily done. Since a heat Pt emitter reacts under atmospheric pressure condition as SID does to ionize compounds, the APIMS(S1D) may provide a powerful combination of functions which should be useful for trace organic analysis of specific substances. This is particularly the case in the parallel use of an ion counting system.
ACKNOWLEDGMENT The authors are grateful to M. L. Messersmith at Yokota Air Base for manuscript preparation. H.J. is a graduate student from Meisei University. Registry No. Pt, 7440-06-4; Ir, 7439-88-5.
LITERATURE CITED Fujii, T.; Arimoto, H. Ana/. Chem. 1985, 57, 2625. Fujii, T.; Arimoto, H. J . Chromatogr. 1988, 355, 365. Zandberg, E. Ya.; Ionov, N. I . Swface Ionization; Israel Program for Scientific Translations: Jerusalem, 197 1. McKeown, M.; Siegel, M. W. Am. Lab. 1975, 89. Horning, E. C.; Carroll, D. I.; Dzidic, I.; Lin. S. N.; Stiilwell, R. N.; Thenot, J. P. J . Chromatogr. 1977, 142, 481. Grimsrud, E. P.; Kim, S. H.; Gobby, P. L. Anal. Chem. 1979, 5 1 , 223. Fujii, T.; Jimba, H.; Ogura, M.; Arimoto, H.; Ozaki, K. Analyst 1988, 1134, 789. Fujii, T.; Kitai, T. h i . J . Mass Spectrom. Ion Processes 1986, 7 1 , 129. Fujii, T.; Suzuki, H Obuchi, M. J . Rtys. Chem. 1985. 89, 4687. Fujii, T.; Ogura, M.; Jimba, H. Anal. Chem. 1989, 6 1 , 1026. Fujii, T. Int. J . Mass Spectrom. Ion Processes 1984, 57, 63.
RECEIVED for review May 16, 1989. Revised manuscript received October 6, 1989. Accepted October 18,1989. Work was supported in part by the Ministry of Education, Science, and Culture of Japan; Grand-in-Aid for General Scientific Research (No. 63540452).
Determination of Chromium in Urine by Stable Isotope Dilution Gas Chromatography/Mass Spectrometry Using Lithium Bis(trifluoroethy1)dithiocarbamate as a Chelating Agent Suresh K. Aggarwal, Michael Kinter, Michael R. Wills, John Savory, and David A. Herold* Departments of Pathology, Biochemistry, and Medicine, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908
An Isotope dllutlon gas chromatography/mass spectrometry method uslng llthlum bls(trlfiuoroethyi)dlthiocarbamateas a chelating agent Is descrlbed for the determlnatlon of chromium In urine. A wet digestion procedure wlth "0,-H,O, Is used for oxldizlng the organic matter associated with urine samples. The Isotope ratios are measured by selected ion monitoring In a general-purpose mass spectrometer using a 10-m fused silica capliiary column. Memory effect, in sequential analyses of samples wlth different Isotope ratios, was evaluated by preparing a series of synthetic mixtures and was found to be negligible. The accuracy of the method was verlfied by quantitatlon of chromlum in the NIST freeze-dried urine reference material, SRM-2670, wlth a recommended chromlum concentration of 13 pg/L In the normal level and certlfled chromlum concentratlon of 85 f 6 pg/L In the elevated level. 0003-2700/90/0362-0111$02.50/0
INTRODUCTION Chromium (Cr) has been recognized as an essential micronutrient for humans that is involved in important biochemical processes such as glucose metabolism and the action of insulin (I). Current knowledge about the role of Cr in human nutrition has been reviewed in a recent article by Offenbacher and Pi-Sunyer (2). Nutrient Cr, Cr(III), is present in food in the trivalent form, while hexavalent chromium, Cr(VI), is considered to be an occupational hazard because of its allergenic and carcinogenic activities. Since the major pathway of elimination of absorbed Cr is excretion in urine, urinary Cr levels have been suggested as an indicator of total body burden and recent uptake (3). Blood Cr levels have been suggested to reflect long-term exposure to Cr (4). As with most other metals, Cr in biological materials is generally determined by electrothermal atomic absorption 0 1990 American Chemical Society
