2258
Anal. Chem. 1989, 6 1 ,
From the data presented, it is clear that the separation of different sized particles from -200 mesh dry standards over years of time is-certainly less than the precision with which such differences can be measured using the most precise measurement techniques available. This of course requires that the standard be homogeneous to begin with and does not contain high specific activity particles. Perhaps the most important recommendation in the previous procedure ( I ) is to ensure that the spike activity is uniformly distributed over a very large number of particles by physically stirring the spiked mud continuously until dried to immobility, permitting no separation of liauid Dhases that could evaDorate and form "hotspots". Howeier, it would probably be irudent to shake solid standards moderately each time they are used as an added precaution.
2258-2263
Registry No. 13'Cs, 10045-97-3; 6oCo, 10198-40-0; 241Am, 14596-10-2;2 3 9 h ,14269-63-7. LITERATURE CITED (1) Sill, C. W.: Hindman, F. D. Anal. Chem. 1974, 4 6 , 113-118. (2) Trahey, N. M.. Department of Energy, New Brunswick Laboratory, Argonne, IL, private communication, 1982. (3) Bowen V. T.; Volchok, H. L. Envkon. Int. 1980, 3 , 365-376. (4) Olson, D. G.; Bernabee, R. P. Health Phys. 1988. 5 4 , 451-459. ( 5 ) Sill, C. W. Health Phys. 1989, 5 7 , 207-208. (6) Sill, C. W. Waste Manage.,in press. (7) Donivan, S.: Hollenbach, M.; Costello, M. Anal. Chem. 1987, 5 9 , 2556-2558. ( 8 ) Sill, C. W. Health Phys. 1977, 3 3 , 393-404,
RECEIVED for review June 19,1989. Accepted July 21,1989. Work performed under the auspices of the U.S. Department of Energy, DOE Contract No. DE-AC07-76ID01570.
Electrocatalysis and Detection of Amino Sugars, Alditols, and Acidic Sugars at a Copper-Containing Chemically Modified Electrode Sunil V. P r a b h u a n d Richard P. Baldwin*
Department of Chemistry, University of Louisville, Louisville, Kentucky 40292
Chemically modified electrodes (CMEs) produced by deposition of a Cu/CI-containlng crystalline specles onto glassy carbon exhibited stable electrocatalytic oxidation of numerous polyhydroxyl compounds lncludlng carbohydrates, amino sugars, aldltols, and aldonlc, uronlc, and aldarlc acids. I n cycllc voltammetry, the electrocatalysls appeared as an lrreversible anodlc wave that occurred only at hydroxide concentratlons of at least IO-' M and was centered at +0.5 V vs Ag/AgCI. The CMEs were readlly adapted for constant-potentlal amperometrlc detectlon of these compounds In flow Injection analysis and llquld chromatography. When used In this fashlon, the electrodes provided detectlon llmlts In the nanomole-to-plcomole range and were compatible wtth a relatively wide range of anion-exchange chromatography mobile phases. Examples of possible applicatlons included the separation and quantltatlon of amino sugar mlxtures, antlblotlcs, and mono- and disaccharides In tobacco.
INTRODUCTION The development of novel electrode materials for use in the detection of carbohydrate compounds is currently an area of very active investigation. The most important of the approaches reported to date have utilized either metallic electrode substrates, such as platinum (1-3),gold (4-8),and nickel (9-1 I ) , or surface-attached metal electrocatalysts, such as cobalt phthalocyanine (CoPC) (12-14), to obtain enhanced carbohydrate oxidation over that seen at conventional carbon electrodes. Thus far, all of these electrodes have been shown to enable direct detection of carbohydrates, without derivatization, at the nanomolar level or below; and the platinum and gold systems form the basis of the commercially available liquid chromatography/electrochemical detection (LCEC) units currently recommended for carbohydrate analysis. The primary drawbacks common to these electrode systems are (1)their requirement for strongly alkaline solution conditions
in order for appreciable carbohydrate oxidation to occur and (2) the need for a pulsed potential waveform in order for stable and reproducible operation to be maintained over an acceptable length of time. Very recently, our group has described the construction and use of a new Cu-based chemically modified electrode (CME) whose performance in carbohydrate oxidation and detection appears to be superior to these systems in nearly every respect (15). For example, the Cu CME was shown to provide detection limits on the order of 1-5 pmol for most mono- and disaccharides. Furthermore, because the detection was performed at a constant potential with no need for specialized pulse sequences to enhance electrode stability, the CME was completely compatible with the simple constant-potential LCEC units already in common usage among chromatographers. In this work, we have characterized more fully the nature of the Cu CME surface and have extended its range of application beyond the simple sugars previously considered. In particular, the Cu CME has been shown to offer sensitive and selective constant-potential detection for alditols, acidic sugars, and amino sugars as well. EXPERIMENTAL SECTION Reagents. Carbohydrates and related compounds were purchased from Sigma or Fisher, and stock solutions were prepared fresh daily in deionized water. Just prior to use, the stock solutions were adjusted to the desired concentration and pH by addition of the appropriate hydroxide-containing diluent. Mobile phases used for flow injection and liquid chromatography were prepared from carbonate-free NaOH and thoroughly degassed deionized water. Sugar samples were obtained from flue-cured McNair No. 944 tobacco leaves (provided by Brown & Williamson Tobacco Corp. Research and Development, Louisville, KY) by extracting a known weight of tobacco with a 4060 acetonitrile/deionized water solvent adjusted so that the tobacco concentration was approximately 0.1 g/L. The extraction was allowed to continue for 60 min with frequent manual stirring during this period. The sample used for injection onto the chromatograph was obtained by passing
0003-2700/89/0361-2258$01.50/0 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
the acetonitrile/water solution through a 0.22-pm Millex-GS filter to remove particulates and diluting with deionized water to bring the sugar concentrations down into the linear range of the CME/LCEC method. Electrodes. Electrode modification procedures were similar to those described in a previous communication (15). A freshly polished thin-layer glassy carbon electrode (BioanalyticalSystems, West Lafayette, IN) was immersed in a 0.050 M CuC12solution for 5 min. A t this point, a white deposit appeared on the glassy carbon surface and the CME was ready for use. During exposure to the CuC12,it was necessary to immerse the entire electrode assembly, including the metallic leads, in order for the catalytically active deposit to develop. Although this remains to be verified, it appears likely that the initial surface modification results from interaction of the Cu(I1) solution with metallic Cu from the electrode contacts to form a Cu(1) salt such as CuC1. Initial exposure of the electrode to strong base, used previously (15)to condition the glassy carbon surface prior to deposition of the copper film, was not essential for the formation of an electrocatalytically active CME in flow experiments but did appear to enhance the electrode’s cyclic voltammetry response. The activity of the modified electrode could be restored to that of the original glassy carbon by polishing with alumina. Apparatus. Cyclic voltammetry was performed with a Bioanalytical Systems Model CV-1B potentiostat with a Model MFlOOO glassy carbon working electrode (modified or unmodified), an Ag/AgC1(3 M KC1) reference electrode, and a platinum wire auxiliary electrode. Flow injection and liquid chromatography experiments were carried out with either a Waters Model M-45 or a Beckman Model llOB pump, a Rheodyne (Berkeley, CA) Model 7125 injector with a 20-pL sample loop, an SSI Model LP-21 pulse dampener, and an IBM Model EC/320 electrochemical detector. AU chromatographic separations utilized either a 25-cm-long, 4-mm4.d. Dionex CarboPac PA1 or a 15-cm-long, 4-mm-i.d. Dionex HPIC AS6A-5p anion-exchange column maintained at room temperature. Surface imaging and X-ray fluorescence measurements were performed with an International Scientific Instruments Model SS60 scanning electron microscope equipped with an energy dispersive EDAX Model 9900 detector.
RESULTS AND DISCUSSION Electrochemistry. In our initial report (15),the Cu CME itself was shown to exhibit one principal redox wave in strongly alkaline solution. I t was suggested that this wave, which occurred a t +0.45 V vs Ag/AgCl and was seen only on the initial cyclic voltammetry (CV) scan for each CME, might correspond to a Cu(II)/Cu(III) oxidation process similar to that obtained earlier by Miller (16) a t a metallic copper electrode under similar conditions. The catalytic oxidation of glucose and other simple mono- and disaccharides at the CME was observed as a long-lived increase in the current at this potential that was directly proportional to the sugar concentration employed and to the square root of the potential scan rate (up to 50 mV/s). In the course of this work, analogous electrocatalytic currents were seen for a variety of polyhydroxy1 compounds in addition to the carbohydrates originally reported. Representative examples are illustrated in Figure 1, which shows CVs obtained a t the Cu CME for 1.0 mM solutions of glucose, glucitol, gluconic acid, glucuronic acid, glucaric acid, and glucosamine. All the CVs were quite similar, containing a broad oxidation in the +0.4-+0.6-V region. The exact peak potentials seen for each compound varied slightly; but other than this, the CVs were virtually the same. Similar electrocatalyses obtained a t the CME for an extensive series of carbohydrates and related polyhydroxy compounds are summarized in Table 1. Included in this compilation are not only larger oligosaccharides up to maltoheptose (no larger polysaccharides were examined) but also all alditols, aldonic acids, uronic acids, aldaric acids, and amino sugars examined. None of these compounds exhibited any significant current in CV when an unmodified working electrode was employed in place of the Cu CME.
2259
Table I. Electrochemical Behavior of Polyhydroxy1 Compounds at the Cu CME CV peak potential, FIA detection V vs Ag/AgCl limits,” ng
compound g1u cose
galactose fructose ribose
I. Monosaccharides +0.50 +0.52 +0.53
+0.53 +0.50 +0.40
xylose
arabinose
0.2 0.2 0.2
0.2 0.2 0.2
11. Disaccharides lactose maltose sucrose trehalose
+0.60 +0.60 +0.60 +0.60
0.4 0.7 0.7 0.7
111. Oligosaccharides maltotriose maltotetrose maltopentose maltohexose maltoheptose
+0.60 +0.60 +0.60 +0.60 +0.60
IV. Amino Sugars glucosamine +0.53 galactosamine +0.50 N-acetylglucosamine +0.60 N-acetylgalactosamine +0.60 V. Alditols NR~ NRb
methanol ethylene glycol glycerol erythritol ribitol glucitol
0.6 1 1
2 2
0.3 0.3 0.9 0.9
NRb NRb
+0.40
0.2
+0.45 +0.45 +0.45
0.2 0.1
0.2
VI. Aldonic Acids gluconic acid galactonic acid gulono-y -lactone gluconolactone
+0.43 +0.45
0.3 0.3 0.2
+0.45
0.2
+0.45
VII. Uronic Acids glucuronic acid galacturonic acid
+0.50 +0.50
0.2
0.3
VIII. Aldaric Acids tartaric acid glucaric acid galactaric acid
+0.60 +0.50
0.3
+0.50
0.3
0.3
IX. Antibiotics streptomycin sulfate kanamycin sulfate digitoxin erythromycin
+0.60 +0.60
6 2
NRb NRb
NRb NRb
“Potential applied, +0.50 V vs Ag/AgCl; sign€ /noise, 3.
J
no resnonse.
A critical feature of these CVs, from an analytical viewpoint, was that the catalytic oxidations seen a t the Cu CME all occurred reproducibly at modest positive potentials. In fact, the electrode was sufficiently well-behaved toward the compounds examined that CV could typically be continued for several hours (encompassing hundreds of individual cycles) with no decrease in the current levels obtained. An important consequence of these observations was that the CME was well-suited for stable, constant-potential detection of these compounds in liquid chromatographic and flow injection schemes. This is unlike the situation with electrodes such as Pt (1-3), Au (4-8), and CoPC-containing carbon paste (12-14), which, when used for carbohydrate detection in LCEC, require the application of a pulsed potential waveform for stable, long-term operation to be realized. As a result, it was an easy
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
2260
Table 11. FIA Response of Cu CME to Various Alditolsn
i
Glucosamine
'Gluconic
no. of
Acid
/'
Glucaric Acid
I
,//__7c-Glucitol
hydroxyls
methanol ethylene glycol
1
4.5 x 10-5
2
2.3
3 4 5 6
0.098
erythritol ribitol glucitol
,.I--
T
1 i
t0.85 t0.65 +0.45 +Ob5 +0:05
Potential, Volts vs. A g / A g C I
Steady-state cyclic voltammograms of 1.0 mM glucose, glucitol, gluconic acid, glucuronic acid, glucaric acid, and glucosamine at Cu CME: electrolyte, 0.15 M NaOH; scan rate, 20 mV/s.
0.02 0.11
10-4
47 75
0.16
0.17
83 100
0.21
0
WO-, 250..
2 200.. E + O 85 +0.65 +0.45 +0.25 tO.05
x
Potential applied, +0.50 V vs Ag/AgCl.
Glucose
Glucuronic
current, nA, % response with for 10" M respect t o glucitol
compound
glycerol
,/--
-'
/
3
150..
k
k
Glucosamine
*
3100..
n
Figure 1.
matter to transfer the Cu CME to a thin-layer flow cell arrangement and characterize its performance as a detector element for polyhydroxyl compounds in flow injection analysis. The qualitative outcome of such flow experiments was exactly what would be expected on the basis of the voltammetric behavior seen in Figure 1. In particular, hydrodynamic voltammograms obtained under flow conditions for compounds from each of the families of carbohydrate derivatives were nearly identical, exhibiting oxidation currents starting a t potentials in the neighborhood of +0.2 V vs Ag/AgCl and reaching maximum levels between +0.5 and +0.6 V. This is essentially the same hydrodynamic current-voltage behavior as that seen previously for glucose and other simple sugars a t the Cu CME (15). The quantitative results of the flow experiments, shown as the detection limits (signal/noise = 3) obtained a t the CME for the same compounds, are also provided in Table I. In nearly all cases, promising detection limits, nearly always below the nanogram level, were obtained. An applied potential of +0.5 V vs Ag/AgCl was chosen for use, even for compounds whose CV peak potential was somewhat more positive, because the high background currents due to solvent oxidation at higher potentials more than offset any increase in analyte current that could be so obtained. Finally, the stability of the CME's response in the flow system, though not perfect, appeared to be adequate for routine quantitative applications. For example, 60 repeated M gluconic acid over a period of apinjections of 1.0 x proximately 1 h produced only an 8% decrease in the peak height observed at +0.5 V. Furthermore, the relative standard deviation of these measurements was only 4.9%. In general, the Cu CMEs did show a gradual decrease in response during continuous usage over extended periods-with the rate of the decrease dependent largely on the mobile phase flow rate in effect. However, as long as their response was periodically recalibrated, the electrodes could typically be placed in the flowstream and used continuously for periods as long as a few days before renewal of the modified surface was required. The common trait shared by the compounds in Table I is that they all contain multiple hydroxyl groups. In fact, in the case of the alditols, the hydroxyl group is the only functional group present. Thus, it was reasonable to consider in particular the role played by this functionality in the electroca-
50-.
"
00
0.05
0:10
0115
NaOH Concentration,
0:no M
Plots of peak current at Cu CME vs NaOH concentration for FIA of glucose (0).glucitol (0),and glucosamine (0): potential, 4-0.48 V vs Ag/AgCI; flow rate, 0.5 mL/min. Flgute 2.
talysis. This was first done by examining the homologous series of alditols from methanol up to glucitol. The results obtained for this group under flow injection conditions are summarized in Table 11. Interestingly, the electrocatalytic current (per mole) was at a maximum for glucitol and steadily decreased down to glycerol, which still produced roughly half the current that was seen for the compounds containing six hydroxyl groups. Further decreases in molecular size down to ethylene glycol and then methanol resulted in a drastic decrease in catalytic response. Thus, it appears that the presence of at least three contiguous hydroxyl groups in the target compound was a necessary condition for the Cu CME response to approach a useful level. Of course, some exceptions to this generalization did occur among the various compounds examined. For example, glyceraldehyde, tartaric acid, and 2-deoxyribose all exhibited excellent electrocatalytic activity and low detection limits despite lacking three adjacent hydroxyls. One definite requirement of the Cu CME-which is shared by all of the various electrode systems used thus far for carbohydrate detection-was the requirement for strongly alkaline solution conditions in order for the electrocatalysis to proceed efficiently. The extent of this requirement for the Cu CME is given explicitly in Figure 2, which shows the peak current response in flow injection as a function of pH for glucose, glucitol, and glucosamine. The pH was varied in these experiments by decreasing the NaOH concentration and keeping the ionic strength constant (w = 0.4)by appropriate addition of NaCl. For each of the compounds examined, excellent signals could be maintained for NaOH concentrations down to M before significant decreases in the current levels started to occur; similar behavior was observed for the aldaric, uronic, and aldonic acids as well. This pH dependence turned out to be somewhat less limiting than what has been previously seen with Pt, Au, and CoPC electrodes, which functioned well only down to OH- concentrations of 0.01 M.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
0.0
0.2
0.4
0.6
0.8
1.0
2261
7
1.2
Flow Rate, mL/min
Figure 3. Plot of peak current vs mobile phase flow rate at Cu CME: potential, i-0.48 V vs Ag/AgCI; glucose concentration, 10 pM; mobile phase, 0.15 M NaOH.
a 0
IT
2 4 6 Retention Time, min
8
Figure 5. Chromatogram of 0.20 mM kanamycin: mobile phase, 0.15 M NaOH; stationary phase, Dionex AS6A-5p HPIC column; other conditions, same as in Figure 4.
I
9
0
8
16 24 Minutes
32
T
50nA
I
Figure 4. Chromatogram of 30 pM galactosamine (a), 30 pM glucosamine (b), and 100 pM N-acetylgalactosamine (c): mobile phase, 10 mM NaOH; stationary phase, Dionex Carbopac PA1 column; potential, +0.50 V vs Ag/AgCI; flow rate, 0.5 mL/min.
As will be seen below, this ability to function usefully at lower pH values was important because it served to allow greater freedom to the chromatographer for the selection of an effective mobile phase. Figure 3 illustrates the effect of variation in mobile phase flow rate on the Cu CME response for glucose in flow injection. The exponential decrease observed here for increasing flow rates contrasts sharply with the direct dependence typically seen at unmodified electrodes (17-20). A t this point, the specific reasons for decreased CME response at higher flow rates are not known. One possibility, of course, is that the rate of the catalytic reaction between the CME and the polyhydroxy solute is simply too slow to produce large currents for the faster flow rates. In any case, the choice of flow rate to be used in any instance involves a compromise between electrode sensitivity and sample throughput. The flow rates that gave optimum results in chromatographic experiments described below and therefore were employed in all subsequent work were in the 0.3-0.7 mL/min range. Liquid Chromatography. The most interesting feature of the Cu CME was the prospect that it provided for direct constant-potential monitoring of a wide range of important compounds not ordinarily considered for LCEC. In its initial use for detection of simple sugars, the CME had proven to
b
4
a
12
16
20
24
28
Minutes
Figure 6. Chromatogram of flue-cured McNair No. 944 tobacco leaf extracted in CH3CN/H20: mobile phase, 0.15 M NaOH; stationary phase, Dionex Carbopac PA1 column; flow rate, 0.3 mL/min; other conditions, same as in Figure 4.
be not only extremely sensitive but also quite stable and very easy to operate (15). It was hoped that these properties would also be displayed for all of the families of related compounds listed in Table I. That this was in fact the case is demonstrated at least in part by the chromatograms contained in Figures 4-6. It is important to note that no response was obtained for any of these carbohydrate-related compounds when unmodified glassy carbon electrodes were employed. LCEC results obtained for a mixture of amino sugars containing galactosamine, glucosamine, and N-acetylgalactosamine are shown in Figure 4. The separation, carried out with an anion-exchange column and 0.010 M NaOH as mobile phase, has presented particular problems for previous LCEC efforts because the relatively low OH- concentration needed
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ANALYTICAL CHEMISTRY. VOL. 61. NO. 20. OCTOBER 15. 1989
Table 111. LCEC Assay for Tobacco Leaf Extract
method Cu CMEO LC/RI’
zlucose mean, SD,
fruaose mean, SD,
sucrose mean, SD,
%
%
%
%
%
%
3.7 3.7
0.2
5.2 5.0
0.02 0.5
1.4 1.4
0.2 0.4
0.5
‘Reported results represent the mean and standard deviation of two separate assays obtained via LCEC with the Cu CME. Conditions were the same as in Figure 6. “From liquid chromatographylrefractive index detection results provided by Brow & Williamson Tobacm Cow. Research and Development, Louisville, KY, and based on more than 50 senarate assavs.
for the separation of these compounds is not well-suited for eleetrocatalytic detection at F’t or Au electrodes (21). Rather, postcolumn addition of concentrated base was required in order to avoid severe tailing caused by poor adsorption/desorptioncharaeteristiar of the amino sugars on these eledrode materials. However, the Cu CME was able to operate quite efficiently at these lower OH- concentrations (see Figure 2) and had no difficulty quantitating the amino sugars a t quite low levels. Detection limits obtained for the above compounds under these chromatographic conditions were 5 ng for galactosamine, 8 ng for glucosamine, and 0.1 pg for N-acetylgalactosamine. Chromatograms obtained for a standard solution of the antibiotic kanamycin and for the glucose-, fructose-, and sucrose-containing extract from a tobacco leaf are shown in Figures 5 and 6, respectively. Kanamycin is an antibiotic c o m w of three variously substituted monosaccharide units. Although none of the three rings possessea the intact structure of an actual sugar residue, an excellent response was still obtained at the CME, with a detection limit of 5 ng able to be reached. The tobacco leafsample employed was obtained from standard flue-cured McNair No. 944 tobacco that had been soaked for 60 min in a 4@60 acetonitrile/water mixture. Despite the possible complexity of the resulting extract, the anion-exchange chromatogram produced with 0.15 M NaOH as mobile phase was relatively simple, exhibiting well-resolved peaks with the same retention times as glucose, fructose, and sucrose. Quantitation of these species by comparison to a dibration curve generated from standard solutions gave the assay results shown in Table 111. The results derived from CME/LCEC approach compared very favorably with those from liquid chromatography/refractive index detection techniques, also summarized in the table. Nature of the Cu CME. In order to obtain further insight into the Cu CME and its mechanism of action, the physical and chemical nature of the CME surface was examined by X-ray fluorescenceand scanning electron microscopy. These experiments were carried out for glassy carbon surfaces a t three different stages of the modification procedure: (A) after alumina polishing and rinsing with water, (B)after treatment with 0.15 M NaOH and rinsing with water, and (C) after exposure to 0.050 M CuCI, to form the working CME. As would be expected for what essentially were just clean carbon surfaces, micrographs obtained for surfaces A and B were nearly featureless. The NaOH-treated surface was n o ticeably rougher than the freshly polished surface; but a t magnifications of up to 5OO0, no further differences were apparent. For the CME, however, the growth of the green deposit noticeable visually following the CuCI, treatment appeared clearly as well-defined hexagonal crystalline strue tures, 1-2 pm in diameter, aeemingly embedded randomly into and across the glassy carbon surface (see Figure 7). X-ray fluorescencespectra for the fmt two surfaees also were nearly featureless, with no real differences to be seen between the two. The spectrum for the freshly polished surface is shown
f Scanning electron micrographs of clean glassy carbon electrcde (a)and Cu CME (b). Magnification: 4900.
Flgure 7.
.-t Energy. KeV
sm :
4
1
4
CuW CYE L
138
0
2.0
4.0
8.0
8.0
Energy. KsV
8. X-ray fh”specba of dean gksycarbon eleclmde (a) and Cu CME (b).
in Figure 8a. For the fully treated CME, however, distinct Cu and C1 peaks were seen. The only additional peaka present were small ones occurring a t energies corresponding to Al and
Anal. Chem. 1989, 61, 2263-2266
Si and presumably representing surface contaminants left from the initial alumina polishing process. At this point, a definitive identification of the Cu/C1 surface microstructure has not been possible. X-ray diffraction spectra obtained for the crystals either intact on the glassy carbon surface or scraped off and collected for separate analysis have not yet been able to be matched with diffraction patterns of known Cu- and C1-containing crystalline species. At present, more extensive characterization of the modification mechanism and specific surface structure of the Cu CME is continuing, and it is hoped that these investigations may lead to an improved understanding of the chemistry involved in its formation and operation. In the meanwhile, it is apparent that the CME offers impressive capabilities for the constant-potential flow detection of a wide variety of polyhydroxy compounds and an improved compatibility with a useful range of anion-exchange chromatography mobile phases.
ACKNOWLEDGMENT We thank P. Luo for providing useful insights regarding the electrode modification procedure and J. R. Richardson and G. A. Lager for advice and assistance in carrying out X-ray diffraction characterization of the Cu CME surface.
LITERATURE CITED (1) Hughes, S.; Johnson, D. C. Anal. Chim. Acta 1981, 132, 11-22. (2) Hughes, S.; Johnson, D. C. J. Agric. Food Chem. 1982, 3 0 , 7 12-714.
(3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
(15) (16) (17) (18) (19) (20) (21)
2283
Hughes, S.; Johnson, D. C. Anal. Chlm. Acta 1983, 149. 1-10. Neuberger, G. G.; Johnson, D. C. Anal. Chem. 1987, 5 9 , 203, 204. Edwards, P.; Haak, K. K. Am. Lab. 1983, (April), 78-84. Rocklln, R. D.; Pohl, C. A. J . Ll9. Chromefogr. 1983, 6 , 1577-1590. Neuberger, G. G.; Johnson, D. C. Anal. (2”. 1987, 59, 150-154. Neuberger, G. G.; Johnson, D. C. Anal. Chim. Acta 1987. 192, 205-213. Schick, K. G.; Magearu, V. G.; Huber, C. 0. Clln. Chem. 1978, 2 4 , 4480-450. Buchberger, W.; Winsauer. K.; Breitwieser, C. H. Fresenius’ Z . Anal. Chem. 1983, 315. 518-520. Reim, R. E.; Van Effen, R. M. Anal. Chem. 1988, 58, 3203-3207. Santos, L. M.; Baldwin, R. P. Anal. Chem. 1987, 5 9 , 1766-1770. Santos, L. M.; Baldwin, R. P. Anal. Chim. Acta 1988, 206, 85-96. Tolbert, A. M.; Baldwin, R. P.; Santos, L. M. Anal. Lett. 1989, 2 2 , 683-702. Prabhu, S.V.; Baldwin, R. P. Anal. Chem. 1989, 67, 852-856. Miller, B. J . Nectrochem. SOC. 1989, 116, 1675-1680. Swartzfager, D. C. Anal. Chem. 1976, 48, 2189-2192. Weber, S. G.; Purdy, W. C. Anal. Chim. Acta 1978, 100, 531-544. Meschi, P.; Johnson, D. C. Anal. Chfm. Acta 1981, 124, 303-320. Prabhu, S. V.; Anderson, J. L. Anal. Chem. 1987, 5 9 , 157-163. Olechno, J. D.; Carter, S. R.; Edwards, W. T.; Gillen, D. G. Am. Bbtechno/. Lab. 1987, (Sept.-Oct.), 38-50.
RECEIVED for review May 19, 1989. Accepted July 21, 1989. This work, which was presented in part at the 197th National Meeting of the American Chemical Society in Dallas,TX, was supported by the National Science Foundation through EPSCoR Grant 86-10671-01 and by Burdick & Jackson Research Grant BJ8807.
Determination of Osmium and Osmium Isotope Ratios by Microelectrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry Takafumi Hirata, Tasuku Akagi, Hiroshi Shimizu, and Akimasa Masuda* Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo 113, Tokyo, Japan
A new merging lntroductlon technlque has been developed for Os analysk wtth lnductlvely coupled plasma mass spectrowetry (ICP-MS). The sample was placed In a mkroheater cell In a merglng chamber and vaporized OsO, was carried to the ICP wlth a blank matrlx mlst flow sprayed from a nebulizer. I n the merglng lntroductlon, the best operatlonal parameters could be obtalned by the usual optlmlzation using a standard solution. The 1870s/18eOsratio and the Os abundance were measured slmuttaneausly by spiklng 1020s. The preclslons of the ratlo and abundance measurements using 0.8 ng of Os were 5 and 4 % , respectlvely. The detection llmlt of Os by this method was lower than 100 fg, which Is almost onetwelfth of that obtalned by conventlonal nebulization lntroductlon.
Mass spectrometry using inductively coupled plasma as an ion source (ICP-MS) is a new technique for elemental and isotopic analysis. However, nebulizers commonly used in ICP-MS transport only 2-370 of a sample solution into the ICP. Several techniques have been developed to enhance the efficiency of sample introduction to the ICP torch; e.g., the use of a recirculating nebulizer, a microconcentric nebulizer 0003-2700/89/036 1-2263$01.50/0
(I), direct insertion (Z),electrothermal vaporization (ETV) techniques (3,4),and vapor generation introduction (5-8). The 1s7Re-1s70sisobaric pair is a promising isotopic system in the fields of geo- and cosmochemistry. However, the difficulty in Os isotopic analysis, due mainly to the high ionization potential of Os and the low abundance in common silicate rocks, has limited the use of this system. Methods with high performance in obtaining detection limits, including secondary ion mass spectrometry (9-1I), accelerator mass spectrometry (IZ),and resonance ionization mass spectrometry (13),have been applied to Os isotopic analysis. With ICP-MS, we have succeeded in measuring the Os isotope ratios for natural metallic samples containing Os at parts per million (ppm) levels (14).A further enhancement of sensitivity is, however, required in order to measure the isotope ratio of Os for silicate rocks. In this study, we put forward a new introduction technique using a miniature heater placed in a merging chamber (abbreviated to “microheater/merging introduction”). This method assures the effective introduction of Os to the ICP torch. EXPERIMENTAL SECTION The ICP-MS used in this study is a VG PlasmaQuad type I. The operational conditions are listed in Table I. Ion optical 0 1989 American Chemical Society
