37
Anal. Chem. 1981, 5 3 , 37-41
tremely useful for double bond location in such esters. However, it should be noted that in the one diolefinic methyl ester examined, methyl linoleate, no ions characteristic of the positions of the double bond were observed, although such ions may possibly be observed if the double bonds are more widely separated. Finally, we would note that the observation of the adduct ion, [M-VME’.], and the fragments, [M-VME+- - CH30H] and [M-VME+. - CH3], therefrom are not diagnostic for a carbon-carbon double bond since these ions are observed in the N2/CS2/VME CI mass spectra of saturated ketones and methyl esters. Thus, the CI spectrum of 2-hexanone showed peaks corresponding to [M-VME+.] and [M-VME+- CH30H] which were -35% and 15% of the M+. base peak. For methyl myristate the [M-VME+. - CH3] ion constitutes the base peak in the CI spectrum with [MeVME+.] (50% of base peak), [M-VME’. - CH,OH] (36%),and M+*(50%) aLS0 being of appreciable intensity. The CI spectrum of methyl nonanoate also showed peaks corresponding to [M-VME+- CH3] and [M-VME+. - CH30H] which were -20% and 15%,respectively, of the M+. base peak. The spectrum of methyl-d3 nonanoate showed that the methyl group lost in elimination of CH3 and CH3OH from the ester-VME complex did not originate from the methoxyl group of the ester and presumably originates from the methoxy group of the vinyl methyl ether. Mechanistic speculations are fruitless in the
-
absence of further isotopic labeling.
LITERATURE CITED (1) Budzsrbwkz, H.; Qerassi, C.; wwlams. D. H. “Mass Spectrometry of Orgenk compauds”;Holden-Day: San Francisco, CA, 1967; p 55. (2) F W , F. H. “IonMolecule Reactions”; Franklin, J. L., Ed., Mwn Press: New Yorlc, 1972; Vol. 1 , Chapter 6. (3) Audier. H.; Bary, M.; Fetizon, M.; LongeviaRa, P.; TouMana, R. N/, Soc. Chem. f r . 1984, 3034-3035. . 11, 835-838. (4) Hum, w.; mer.w. J. retrehedron ~ e t t1973, (5) FerTwCarete, A. J.; Jennings, K. R.; Sen shamre. D. K. olg. Mess Specbom., 1976, 11, 867-872. (6) FenercOneia. A. J.; Jennlngs, K. R.; Sen sherme,D. K. A&. Mess SpeCtrOm., 1978, 7, 287-294. (7) Hanison. A. (3,; U, Y.H.; hWbinb8, N. E. Paper presented at 27th A n f ~ aConfwence l on Mass Spectrometry, Seattle, WA, Jw1979; Abstract W W 2 . (8) He-, A. 0.; U, Y.H., A&. Mess Specbm.. In press. (9) MeOt-Nec, M.; Field, F. H. chem.m y s . Lett., 1976, 14,464-489. (IO) MeOtNec, M.; Hamlet, P.; Mwrter, E. P.; Field, F. H. J . Am. Chem. Soc.,1978, 100, 5486-5471. (11) E. “I-& Reactkns”; Franklin, J. L.. Ed.; M wn Press: New Ya(c, 1972; Vol. 2, Chapter 10. (12) Uas, S. Q.;Ausbos, P. “IorrMolecule Reactions: Their Role h ReBcttbn -try”; American chemicel Sodety, Washkrgton, DC, 1975; Chapter 6. (13) W e , P. D.; Fr)em,M. D.; (3ddhg, B. J.; Hal, D. R.; Jernlngs, K. R.; Stradlhg, R. S. Paper presented at 26th Annual Conference on Mass SpeCtrometry, St. LoUte, w ) , May 1978; Abstract P 3 8 .
m,
RECEIVEDfor review April 30,1980. Accepted September 25, 1980. This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada.
Elemental Analysis with a Microwave-Induced Plasma/Quadrupole Mass Spectrometer System D. J. Douglas’ SCIEX Inc., 55 Glencameron Road, Unit 202, Thwnhill, Ontarb, Canada L3T 1P2
J. B. French UniversnY of Toronto Institute for Aerospace Studks, 4925 Dufferln Street, Downsvlew, Ontarb, Canada M3H 5T6
Devekpmental work on the couplhg of a mlcrowaveJnduced plasma to a quadrupole ma88 spectrometer system for the elementaianatyskofsokrtknstedescrlbed lheatmospherlc pressure plasma has an excitation temperatwe of 54003900 K and an electron denshy of lo” ma. Solutions are nebulized, desolvated, and carried into the plasma where the dissolved solids are vaporized, dissociated, and Ionized. EL mental iom, are extracted from the plasma through a differentially pumped interface designed by uslng gasdynamk molecular beam techniques. No orme dogging or corrosion has occured. With the current UnOpHmlzed w c e , detectkn IhHs are in the range of 0.1-1 bg L-’ for many elements. Habgcm are readUy detected as mgatbe kns. The dynamic range covers 4-5 orders of magnitude of solution concentra tbn from the detection limit up to 1-10 ppm. An initlai Investigation shows the source b free from ionization interlerences for Na concentratlons up to 100 ppm.
Work is proceeding in several laboratories on the coupling of plasmas at atmospheric pressure to mass spectrometer systems for the elemental analysis of solutions. The concept of nebulizing solutions into a plasma and extracting the re0003-2700/81/0353-W37$Ol.W/0
sulting elemental ions from the plasma was pioneered by Gray some years ago (1-4). This early work, with a dc capillary arc plasma, demonstrated the inherent sensitivity of the method (1-3) and pointed out that the technique would be useful for the measurement of isotope ratios (4). Recently Houk et al. in collaboration with Gray (5)have extended Gray’s earlier work by using a higher power inductively coupled plasma (ICP) to overcome some of the limitations of the dc capillary arc. Our work centers on the use of a microwave-induced plasma (MIP) as an ion source. The results we report here, in accord with the earlier work, demonstrate that the technique promises elemental analysis with rapid sample throughput, broad dynamic range, and high sensitivity, as well as providing isotope abundance data. The plasma system and vacuum interface described in this paper overcome some of the difficulties encountered in the earlier work. All previous attempts to use the atmospheric pressure plasma/mass spectrometer technique have experienced, to varying degrees, clogging of the small orifice between the plasma and the mass spectrometer vacuum system. This has prevented the technique from developing into a useful method for routine analysis. We have overcome this problem by using substantially larger orifices. Orifice clogging has never occurred with our system despite lengthy runs with solutions 63 I980 American Chem!cal
Sodety
38
ANALYTICAL CHEMISTRY. VOL. 53, NO. 1, JANUARY 1981
1
-w J Flgure 1. Schematic of the apparatus: (1) sample: (2) peristaiiic pump: (3)nebulizer; (4) heatec?desolvation tube: (5) condenser; (6) microwave cavity: (7) plasma tail flame; (8) mechanical vacuum pump; (9) ion detection: (IO)argon supply. with salt contents up to 100 ppm. As an alternate to the dc capillary arc or inductively coupled plasmas used by Gray and Houk et al. we have investigated the use of a microwave-induced plasma, similar to the type described by Beenaker (6). A plasma system of this type is compact, with low argon consumption and minimal heat dissipation problems. Microwave-induced plasmas have been popular as element selective detectors for gas chromatography (7,8 ) but have also been used for the analysis of solutions (S23). Beenaker et al. (22) assessed the relative merits of the MIP and ICP for atomic emission analysis and concluded that while the detection limits of the MIP and ICP were comparable, the MIP experienced somewhat higher matrix effects. In some cases this could be attributed to the sample input system rather than the plasma itself. While these atomic emission results may act as a guide, the results do not necessarily carry over when the plasma is used as an ion source for mass spectrometry. As we show here, the sensitivity of our MIP ion source is high and matrix effects are small. EXPERIMENTAL SECTION An overall schematic of this apparatus is shown in Figure 1. Briefly, solutions are nebulized by an ultrasonic nebulizer, desolvated,and dried, and the resulting particulates are earried into the microwave plasma source where they are vaporized and ionized. Ions are extracted through a differentially pumped plasma to vacuum interface into the lens system and quadrupole mass fdter of a TAGA Zoo0 quadrupole mass spectrometer system (14).
Microwave Plasma Source. The microwave cavity used here was similar to that described in ref 6. The discharge was produced in argon in a quartz tube of 6 mm 0.d. and 1mm i.d. The plasma was run with 200 W of forward microwave power and less than 20 W of reflected power. The argon flow rate was 0.4 L min-'. A spectroscopic study of this discharge has shown an electronic excitation temperature of 540lL5900 K and an electron density of -10'' cm-3 (15). This electron density exceeds that of argon in thermal equilibrium in this temperature range by 1-2 orders of magnitude and indicates a degree of ionization above that of a plasma in local thermodynamic equilibrium. The excitation temperature of this plasma is at the high end of the range reported for other microwave plasma source9 (11). It is interesting ta note that the power density in the plasma core is 1 X lo" W c&, roughly 10 times that of an ICP, hut the residence time is calculated to he 0.2 ms, roughly one-tenth that of an ICP. Thus the energy available to excite the gas and vaporize aerosols is comparable in these sources. While sample penetration into microwave plasmas is relatively poor (I1), our plasma-mass spectrometer interface samples only the center line of the gas flow and hence gas which has passed throngh the hottest part of the plasma. The reduced residence time might be expected to lead to a limited dynamic range or severe matrix effects. However, as we report
here, this was not found to be the case. Sample Introduction. Aqueous solutions were nebulized with an ultrasonic nehulizer (Plasmatherm Inc., Model UNS-1) and desolvated by passing through a heated glass tube (20 em long, 1cm i.d.) and a 40 cm long Liebig condenser. The desolvation heater was operated at a temperature just sufficient to dry the aerosol (26). Unfortunately the nebulizer efficiency varied erratically on both long and short time scales, but with care and patience, it was pasible to a n y out experiments of several hours duration to map out the broad characteristics of the instrument Future work will be done with a more reliable nebulizer. All sample handling is at atmospheric pressure and sample h n g h p u t time is -1 min. Clear out time between samples is governed by the nebulizer unless the discharge tube is exposed to heavy loadings. Plasma-Vacuum Interface. Ions are extracted from the tail flame of the MIP into the quadrupole maas spectrometer system by means of a differentially pumped interface. The vacuum system and interface differ appreciably from those used by other workers in two ways. First, the quadrupole vacuum chamber is pumped by a high capacity cryopump with a nominal pumping speed of ZOO00 L 8'. (14). Second, the differentiallypumped region operates at a pressure of 1torr and is pumped by a mechanical pump (17,28). This system has been designed to sample the core of the plasma tail flame by forming a supersonic molecular beam free from boundary layer or shock-wave interferences. The plasma expands from atmospheric pressure to torr in -10 ps with little chance for ion recombination or clustering reactions (19). Because of the relatively large sampling orifice the plasma is pumped directly into the vacuum system without the formation of a cooler boundary layer in front of the orifce. Such a boundary layer introduces the possibility of interfering ion-molecule reactions with corresponding loss of signal and complication of the mass spectra 6 2 0 ) . The overall higher pumping speed of this apparatus allows the use of substantially greater orifice diameters both between the atmospheric pressure plasma and first vacuum chamber and between the first vacuum chamber and the mass filter chamber. The advantages are higher ion transmission and freedom from orifice plugging. Furthermore, the front orifice, between the plasma and 1-torrregion, can be more rugged,permitting adequate heat flow away from the critical region. Our first orifice is 0.41 mm diameter in the tip of a stainless steel cone. During operation the tip of the cone hecomes hot enough to glow red, but little corrosion other than discoloring of the steel is apparent after use for several months. The base of the cone wm mounted on a copper flange which could he cooled with either air or water. Air cooling was found to he adequate. Signal Detection. Ions were detected with a pulse counting system. Mass spectra were recorded by displaying the output of a rate meter on a chart recorder. For determination of detection limits and calibration curvea the mam fdter was tuned to the peak of interest and counts were totaled for 10 s. The average of five such measurements was used. Reagents. Reference solutions were prepared by dissolving reagent grade salts in deionized water (resistivity greater than 15 MR cm) to make loo0 ppm solutions and then diluting to the desired concentration with 0.1 M HNO, (reagent grade HNO,in deionized water). Copper and cadmium solutions were prepared hy diluting loo0 ppm (Cd in dilute nitric acid and CnO in water) atomic absorption standards (Fisher Scientific). Argon was Matheson prepurified grade, stated purity 99.998%. RESULTS AND DISCUSSION Figure 2A shows the mass spectrum recorded when a 0.1 M HNO, "blank"solution was nebulized into the plasma. The single largest peak in the spectrum at mass 30 is NO+. Also evident are the isotopic species of NO+ and sodium at mass 23. The nitric oxide is formed from entrainment of air in the plasma tail flame, from air dissolved in the sample and, to a lesser extent, from trace Nz and Ozin the argon. Figure 2B shows, on the same scale, the spectrum observed when a 20-ppm copper solution was nebulized into the plasma. The copper ions are readily seen at mass 63 and 65 with approximately the correct isotope ratio. For the spectra shown in
ANALYTICAL CHEMISTRY, VOL. 53, NO.
Flgure 4. Detection of Cd from a 1-ppm cadmlum solution.
.;LA+ IO
1, JANUARY 1981 39
XI
Counts 8
20pprn Cu
B
Table 11. Detection Limits Por Some Elements in Aqueous Solution, wg/L
4
2
0
iIO- - i
20
30
element this work
-cnp
50 c 60
40
Ag
Mars [ornu)
Flgure 2. (A) Mass spectrum obtained by nebulizing a 0.1 M nitric acid solution into the MIP source. (E)Mass spectrum obtained by nebulizing
a 20-ppm copper solution into the MIP source under the same conditions as (A). BLANK
lO6f
a+
O1
(01M
XlOO
io
io
30
40
50
MOSS
60
A
o i
SO
(omuj
18 19
23 27 30 31 32 39 40 46
47
H,O+ H, 0 ' Na
A1 NO
1 5 ~ 0
NL80 K
Ca NO2
mass
tentative ident
48
N1*OO
52
Wr
54 56 57 58 60
63 65 70
0.1 2 0.1 0.1
Ni
8
cu Pb
1
AAS a
0.1-1.0b 0.2-lb 0.2-0.4' 1-2b 1 2-23
1 2-3 1-2 2-5 10-11 20
T1 0.1 Reference 21. In 2% NaCl solution.
"03)
Figure 3. Spectra obtained by nebulizing a 0.1 M nitric acid solution into the MIP recorded at higher sensitivity than Figure 2: (A) X102; (B) x 104.
Table I. Background Peaks tentative mass ident
Cd Cr
ICP/AESa
CaO CaOH (NO), 6,CU
65cu
1 5 ~ 0 ,
Figure 2, the ion lens system was deliberately detuned to keep the peaks on scale. Figure 3 shows the background spectra recorded with the ion lenses set for maximum signal, giving an increase in sensitivity of 100. Other background peaks are evident. These are identified in Table I. The peak at mass 40 was determined to be @Carather than @Arbecause of the presence of the other isotopes of Ca (notably "Ca) and the absence of =Ar. The metal ions are trace impurities from the reagent grade nitric acid and deionized water and are present a t the parts-per-billion level. The other background peaks are principally oxides of nitrogen. Above mass 60 the spectrum is clean with no potential interferences (no element has a major isotope at mass 70). Similar spectra were seen by Gray (1-3) and Houk et al. (51, but the details of the background peaks differ in each case. Figure 4 shows the spectrum from a 1-ppm cadmium solution. Clearly evident is lo8Cdof 0.88% abun-
dance, indicating a sensitivity for cadmium in the partsper-billion range. Also evident is trace silver (at masses 107 and 109) present at the parts-per-billion level. Detection limits were determined by nebulizing solutions with concentrations of 10-100 times the detection limit and then calculating the concentration of the element in solution which would give a signal three times the standard deviation of the background noise for a 10-s counting period. The detection limits for some elements of environmental interest are shown in Table I1 and are compared with those of ICP atomic emission spectrometry (AES) and flame atomic absorption spectrometry (AAS) methods. (The detection limits of ref 21 were also defied as the concentration giving a signal 3X the standard deviation of the background.) The ICP data of ref 21 were obtained with an ultrasonic nebulizer and are therefore directly comparable with our results. Even with our current unrefiied source, the detection limits equal or improve on those of the best optical instruments for many elements. (The less abundant nickel isotope 62Niwas measured, owing to a background peak at mass 58 corresponding to 50 ppb nickel. This resulted in the somewhat higher detection limit for this element.) So far, little attempt has been made to reduce the background noise, typically 200-2000 counts s-l depending on operating conditions. This background noise arises from UV photons and metastable atoms striking the quadrupole rods and detector region. Reduction of this background noise would lower the detection limits up to a factor of 10. The response of the system to different elements depends on ionization potential. For investigation of this, a solution containing five elements of varied ionization potentials was nebulized into the plasma and the count rate for each recorded. The results are shown in Figure 5. For elements with ionization potential below 9 eV the sensitivity is high, the detection limit for cadmium (ionization potential 8.99 eV) being 2 ppb. For elements of higher ionization potential there is a dramatic drop in sensitivity. This arises from charge transfer from the elemental ions to nitric oxide (ionization potential 9.27 eV). Similar behavior was seen by Gray (1,4). The variation of response with ionization potential allows an estimate of the ionization temperature if a plot of In [M+] vs. ionization potential is linear. This, however, is not the case for our system, indicating a departure from equilibrium in the plasma tail flame. No attempt has been made yet to reduce the concentration of NO in the discharge. The NO in the discharge used by Gray was 2-3 orders of magnitude less than
40
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Counts
Counts
Per Sec.
lo3
I 1 -
OOOI
& -
dl
I'O
dl
d
Ibo
Concentration in Solutlon (pprn)
B
9
10
loniiolion P o f e n t ~ o l (eVJ
Flgwe 7. Analykal calibratbn w e s for '%r, '"Ag, %, 9, %, and 'hi. The superscript denotes the particular isotope measured.
Figure 5. Signal vs. ionization potential for five elements of varying ionization potential. I
0
0
10
Figure 8.
75 80 85 MASS (AMU)
1
0
1
I
90
Detection of bromide ions in the negative mode from a
10-ppm Br- solution.
in our source, as judged by the ratio of the NO+ peak to elemental ion peaks in his published spectra (compare our Figure 2 to Figure 4 of ref 1). Thus there is good reason to believe the NO can be reduced to a level which will offer useful sensitivity to elements with ionization potentials above 9 eV. Preliminary experiments using a shield gas to prevent entrainment of air in the plasma tail flame show that the NO concentration can be dropped substantially. This drop in sensitivity for elements of high ionization potential would, at first sight, seem to preclude the detection of halogens. Halogens, however, are readily detected as negative ions. Figure 6 shows Br- ions from 10-ppm bromide solution (NaE3r in deionized water). The plasma source has not yet been optimized for negative ions. There is a high background of 1.2 X lo4 counts s-l. Nevertheless, the sensitivity is high, and reduction of the background noise should extend the detection limits for halogens to the parts-per-billion range. The dynamic range of this plasma source can be judged from Figure 7 which shows analytical calibration curves (signal vs. solution concentration) for several elements. The most abundant isotope was measured for each, except nickel. The top line, passing through the points for copper, has been drawn with slope 1,indicating true linear response. Deviations from slope 1 were strongly suspected of being caused by drift in the nebulizing efficiency during the course of the experiment, either up (Cu and T1) or down (Ag, Ni, Pb). The dynamic range is at least 4 orders of magnitude, from the detection limit up to 1-10 ppm. The falloff in count rate above lo6 counts s-l is likely to result from pulse pileup in the counting system, rather than overloading of the plasma by excess aerosol. This is evident from the fact that the 62Nicount rate remains linear up to 10 ppm, while the 63Cu count rate levels off at this same
NQ [PPm]
Cadmium signal in the presence of varying concentrations of Na. The cadmium concentration was 200 ppb.
Figure 8.
concentration. The 62Nicount rate is roughly 3.5% of that of Cu, indicating that the count rate from the most abundant isotope, MNi, falls in with the group of elements of highest sensitivity. Thus the nebulization, desolvation, vaporization, and ionization of Ni and Cu are all similar, giving similar plasma loading. The drop in count rate for Cu, then, must result from pulse pileup in the counting electronics. The dynamic range at the high end could be extended somewhat with a faster pulse counting system. A further increase could be gained by measuring the signal from less abundant isotopes for elements of high concentration. Reduction of the background noise could extend the calibration curves to lower concentrations, and it is probable that the ultimate linear range of this plasma source will be 5-6 orders of magnitude. Use of a less efficient pneumatic nebulizer would shift these calibration curves to higher concentration (by about 1 order of magnitude) (5,221,but the dynamic range will remain the same. An experiment was performed to investigate possible ionization interferences in the source. The count rate of "'Cd (ionization potential 8.99 eV) from a 0.1 M HNOBsolution containing 200 ppb of Cd and varying amounts of Na was measured. The results are shown in Figure 8. There is no systematic shift in the Cd+ count rate within the 20% scatter of the data. The scatter arises from instability of the nebulizer. This is not an inherent difficulty with this technique of course, and work is proceeding on an improved nebulizer system for future work. The freedom from a sodium (Na) matrix effect is plausible in view of Figures 2 and 3. Assuming the sensitivity to sodium is comparable to that of Li and Cr, the analytical calibration curves indicate that a 100-ppm Na SOlution would give a Na signal of 108-109 counts s-l. On the
A ~ I Chem. . mi,53,41-47
same scale, the NO+ signal is 1O'O to 10" counts s-l. Thus assuming the collection efficiencies for NO+ and Na+ are comparable, the Na+ ions represent a small fraction of the total ionization, and no shift in ionization equilibrium is expected. It is possible that a reduction of the NO concentration will result in a decrease in the total ion concentration in the plasma. The mechanism of NO+ formation is complex, involving diffusion of air into the plasma with subsequent reaction and ionization. If, for example, NO+ is formed simply by charge transfer from Ar+ to NO, a reduction of the neutral NO level will lead to an increase in Ar+ but no net change in the total ion concentration. The data of Figure 5 and the spectroscopic study (15) indicate that the plasma is not in equilibrium and in fact has a higher ion concentration than an equilibrium plasma. Thus the MIP holds promise of freedom from ionization interference in the absence of NO. It is worth noting that the plasma mass spectrometer method opens up the exciting possibility of using the isotope dilution technique to overcome difficult matrix effects, regardless of the source of these matrix effects. While atomic emission analysts commonly spike samples to give an internal standard for absolute calibration, this can, at best, correct for only some matrix effects. The isotope dilution technique offers the possibility of correcting for all matrix effects from initial sample workup to the final data. Thus it may be possible to use a single less rigorous sample preparation designed to give acceptable recovery for many elements, even from difficult matrices, to provide convenient, sensitive multielement analysis.
ACKNOWLEDGMENT We wish to thank S. Wong for discussions of the spectroscopic measurements of the plasma and C. C. Poon for the
41
design and earlier development work on the plasma source.
LITERATURE CITED (1) Gray, A. L. Analyst(L&n) 1975, 700, 289-299. (2) Qray, A. L. Anal. Chem. 1075, 47, 600-601. (3) Gray, A. L. In "Dynamic Mass Spectrometry"; Price, D., Todd, J. F. J., Eds.; Heydon a Son Inc: Philadelphia, 1975; vol. 4, Chapter 10. (4) Gay, A. L. Roc. Anal. Dlv. Chem. Soc. 1976, 13, 284-287. (5) Houk, R. S.; Fassel V. A.; Flesch, G. D.; Svec, H. J.; Gay, A. L.; T a w , C. E. Anal. Chem., In press. (8) Beenaker, C. 1. M. Spectrochlm. Acta, Part B 1976, 378, 483-486. (7) Beenaker. C. I. M. Spectrochim. Acta, Part B 1077, 328, 173-187. (8) Qulmby, Bruce D.; Wen, Peter C.; Barnes, R a m M. Anal. Chem. 1978. 50, 2112-2117. (9) Uchte, F. E.; Skogerboe, R. K. Anal. Chem. 1073, 45, 399-401. (IO) Skogerboe, R. K.; Coleman, (3. N. Appl. Spectrosc. 1976, 30, 504-507. (11) Skogerboe, R. K.; Coleman. (3. N. Anal. Chem. 1076, 48, 8 l l A 622A. (12) Beenaker, C. I. M.; Bleneke, 8.; Boumans, P. W. J. M. Specfrochkn. Acta, Part B 1078, 338, 373-38 1. Heineman, W. R.; Caruso. J. A.; Friclce, F. L. Analyst(Lon(13) Rose, 0.; don) 1976, 103. 113-121. (14) Lovett, A. M.; Reid, N. M.; Buckley. J. A.; French, J. 6.; Cameron, D. M. Bbmed. Mass Spectrom. 1070, 8 , 91-97. (15) Wong, S. W. Technical Note No. 225; Unhrerslty of Toronto InstlMe for Aerospece S W k Downsvlew, Ontarb, Canada, 1980. (16) Berman, S. S.; Mclaren, J. W.; WIWle, S. N. Anal. Chem. 1060, 52, 488-492. (17) Campargue, R. Entrople 1969, No. 30, 16-21. (18) Campargue. R. Rev. Sci. Insfrum. 1084, 35, 111-112. (19) Mllne. T. A.; Qreene, F. T. J . Chem. phys. 1066. 44, 2444-2449. (20) Hayhurst. A. N.; Kittelson, D. B. Combust. Flame 1077, 28, 137-143. (21) Fassel, V. A. Sclence 1076, 202, 183-190. (22) Olsen, K. W.; Haas. W. J.; Fassel, V. A. Anal. Chem. 1077, 49, 832-837.
RECEIVED for review July 7,1980. Accepted September 17, 1980. This work was carried out under contract to the National Research Council of Canada. Presented in part at the Annual Conference of the American Society for Mass Spectrometry, New York, NY, 1980.
Electrochemical Behavior of the Vitamin B,,,/Vitamin Couple on Mercury and Platinum Electrodes
B,2r
C. L. Schmidt, C. F. Kolpin, and H. S. Swofford, Jr.' Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
The electrochemicalbehavior of the vitamin BlzJBla couple on mercury and platinum is investigated by use of standard electrochemical methodology. The effect of the surface adsorption of vitamin B l h on the observed electrochemistry in dropping mercury electrode polarographic experiments is discussed in terms of a rearrangement on the electrode surface.
Earlier work on the electrochemistry of vitamin Blz can be found in ref 1-11. Some of these previously reported voltammetric studies have shown the first reduction wave for vitamin Blz to be quite distorted; absorption of the vitamin on the electrode surface has often been cited as the possible cause of this distortion (7, 8, 12). It should be noted that most of the early studies reported were carried out in electrolytes containing ethylenediaminetetraacetic acid (EDTA) or chloride salts. Anions such as these are capable of either complexing or precipitating mercury, thus, drastically reducing the positive potential range available for conducting voltammetric experiments with mercury
electrodes (13). The use of these types of electrolytes has obscured some important aspects of the electrochemistry and surface electrochemistry related to vitamin BlZa(14). Some of the problems associated with voltammetry, a t potentials positive of 0 V vs. the saturated calomel electrode (SCE), were overcome by the use of a platinum electrode. Two reasonably complete studies involving the electrochemistry of vitamin Blza using solid electrodes have appeared in the literature (8,151.This work, done by Lexa et al. has shown the redox chemistry of vitamir Blza on platinum, gold, and carbon electrodes to be irreversible in the range of pH from 3 to 8. Their studies have also included the effect of pH on the half-wave potential by using well-buffered solutions and a spectroelectrochemical technique which was not dependent on electron-transfer kinetics. The half-wave potential of Bla/Bla couple is reported to be independent of pH from pH 3 to pH 8, indicating that there is no net difference in the number of protons associated with vitamin Blza and B12rin this pH range. Below pH 2.9, the half-wave potential shifted positive by 60 mV per pH unit, and above pH 7.8, the halfwave potential shifted negative by 60 mV per pH unit. This
0003-270018110353-004 1$01.0010 0 1980 Amerlcan Chemical Society
