Art ides Anal. Chem. 1994,66,193-199
Mass Spectral Analysis of Electrochemical Products Generated Directly within the MS Source Vacuum Stephen D. Houset and Larry B. Anderson' Department of Chemistv, The Ohio State Univers&, Columbus, Ohio 43210 Volatile molecules are generated for mass spectral analysis at anelectrodeplaceddirectly withintheionsource. Poly(ethylene glycol) (PEG, average molecular weight 400) is used as the solvent due to its extremely low volatility, while hexamethylphosphoric triamide (HMPA) provides a wider potential range. A twin, interdigitated electrode pair is used, one side of which is coated with the redox polymer Prussian blue to act as an auxiliary/pseudoreference. Mass spectral peaks characteristic of ferrocene are seen during the reduction of ferrocinium, including the parent at m/z 186. Following the mass spectral signal at m/z 186 as a function of time during electrolysislends insight into the transport properties of material from the electrodesurface to the mossspectraldetector. S i r qualitative results are shown for the oxidation of carboxylic acids in PEG and the reductionof dibromododecanein HMPA. Several approaches have been described for mass spectral analysis of electrode reaction The most serious challengefor such methods has been the separation of analyte molecules from the great excess of electrochemical matrix (solvent plus electrolyte) prior to MS analysis in the lowpressure gas phase of the spectrometer. This separation and change of phase also causes special problems in the quantitation of millimolar concentrations of electrochemical products. A unique solution to efficiently capturing the electrochemical products in the MS source was described by B a r t m e s ~ ,who ~*~ electrolyzed substrates in a liquid phase positioned directly in t Prescnt address: General Mills, Inc., JFBTC, Minneapolis, M N 55427. (1) Bruckenstein. S.;Gaddc, R. J . Am. Chcm. Soc. 1971, 93, 793. (2) Bruckcnstcin, S.;Comeau, J. Faraday D i s w s . Cham. Soc. 1973, 56, 285. (3) Bruckenatein, S.;Grambow, L. Elccfrochim. Acta 1977, 22, 377. (4) Anderson, L.;Pinnick, W.; Levine, B.; Weiacnbergcr, C. Anal. Chrm. 1980, 52, 1102. (5) Anderson, L.; Brwkman, T. Anal. Chcm. 1984,56, 207. ( 6 ) Heitbaum, J.; Hambitzer, G. Anal. Chcm. 1986, 58, 1067. (7) Heitbaum, J.; Eggert, G. Elccfrochim. Acta 1986, 31, 1443. (8) Bartmess, J.; Phillips, L. Anal. Chrm. 1987,59, 2014. (9) Phillip, L.;Bartmess,J. Enuiron. Moss Spcrrom. 1989, 18, 878. (10) Brajter-Toth, A.; Yost, R.;Lee,M.; Volk, K.Anal. Chrm. 1988, 60, 722. (11) Brajter-Toth, A.; Yost, R.; Volk, K. A M I . Chcm. 1989, 61, 1709.
0003-2700/94/0306-0193t04.50/0 @ 1994 American Chemlcal Soclety
the MS source. He did not address fully the problems of electrochemical potential control and product quantitation. Controlled-potential electrolysis is entirely feasible in a liquid matrix positioned in a moderate vacuum. Many lowvolatility liquids have been described in the fast atom bombardment literature, and several of these are suitable for use as electrochemical solvents. Interdigitated microelectrode arrays are compatible with the restricted space in an MS source, and they are especially capable of conducting electrolysis in highly resistive media. They also are capable of attaining microampere to milliampere currents, sufficient to produce detectible quantities of electrochemical products. The interdigitated pair described here consists of a working electrode (Pt or Au) and a Prussian blue-coated electrode which functions as both counter and reference electrodes. An electrode coated with a layer of Prussian blue (PB) contains several hundred microcoulombs per square centimeter of oxidizable and reducible charge centers. These charge centers behave in a nearly nernstian manner. Used as a counter electrode, PB does not produce freely diffusing or volatile products which might contaminate the small volume of analytical solution. Because its potential is stable and predictable, PB also behaves as a pseudoreference electrode, available to poise the working electrode. The impervious nature of the PB layer isolates the pseudoreference from many sources of disruption and contamination present in the bulk solution. We have performed electrolysis of two nonvolatile species in poly(ethy1ene glycol) matrix, ferrocinium ion and benzoate ion, to form volatile small molecule products, ferrocene and benzene/carbon dioxide, respectively. Interdigitated filar microelectrodes initiate and control the electrolysis, and the products evaporate from the solvent surface, ionize in the electron impact source, and are mass analyzed. We have used similar methods to identify and measure reduction products of nonvolatile 1,Zdibromododecane in hexamethylphosphoramide solvent. AnalytlcaIChemlstty, Vol. 66, No. 2, January 15, 1004 193
- F4-1 MS analog output
Multimeter
spectrometer
MS trigger input
I
potentiostat
I
Flguro 1. EClMS instrument schematic. Flgure 2. Twin interdigitated electrode: center line separation (W); 200 pm; total area (excluding lead wires), 0.15 cm2.
EXPERIMENTAL SECTION Reagents. Ferrocene (Fc), poly(ethy1ene glycol) (PEG-n, where n indicates the average molecular weight quoted by the manufacturer), sodium benzoate, and 1,12-dibromododecane were purchased from Aldrich. Hexamethylphosphoric triamide (HMPA), was dried by dissolving metallic sodium in the solvent and distilling under vacuum immediately prior to use. Ferric nitrate, potassium nitrate, and nitric acid were purchased from Mallinckrodt, potassium ferricyanide and sodium perchlorate were purchased from Baker, tetrabutylammonium perchlorate (TBAP) was purchased from Southwestern Analytical (dried under vacuum at 80 OC before use) and, unless specified otherwise, all purchased reagents were used as received. Ferrocinium hexafluorophosphate was prepared according to the procedure of Duggin.12 Ferrocinium solutions were maintained under an inert atmosphere. Solutions. Electrochemical characterization of the PB system was carried out in aqueous saturated KNO3 to minimize uncompensated resistance effects. The potential of the PB reference showed no dependence on K N 0 3 concentration. Solutions of ferrocinium ion in PEG were prepared under an inert atmosphere, typically by adding a weighed amount of FcPF6 to a measured volume (- 10 mL) of PEG-400 containing 0.1 M KN03and diluted with a measured amount (1-3-fold excess) of methanol. Measured volumes of these solutions were placed onto the electrode substrate, and the methanol was evaporated to produce a known (measured) area of liquid. After evaporation of methanol, the PEG layer thickness could be calculated directly. Solutions of 0.010 M sodium benzoate in PEG were prepared as above, but no additional electrolyte was added. TBAP was used as the electrolyte in HMPA solvent because it is soluble in HMPA and inert at large negative potentials. The concentrations of analytes and electroactive species for the HMPA solutions are quoted for the solution when it was placed on the MS probe tip. Since HMPA evaporates continuously during the EC/MS experiment, concentrations must be considered only approximate. Hardware. Mass spectra were acquired using a Balzers QMG-511 quadrupole mass spectrometer equipped with a secondary electron multiplier (SEM) and a Keithley Model 199 multimeter. Electrochemistry was done with a Princeton Applied Research Model 173 potentiostat/galvanostat with a Model 276 IEEE 488 (GPIB) interface. A Hewlet-Packard Model ES/ 12 personal computer equipped with a HPIB card allowed computer-controlled mass spectral data acquisition, as well as control of the potentiostat. Figure 1 shows a schematic of the hardware setup. (12) Duggan, M.; Hendrickson, D. Inorg. Chem. 1975, 14. 955.
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Analytlcal Chemlstry, Vol. 66, No. 2, January 75, 1994
Gold Interdigitated Electrodes. Interdigitated electrode pairs (Figure 2) were made photolithographically using procedures described by Heiger13 and Harrington;14 fabrication methods are described in the thin-film 1iterat~re.l~ Electrodes were fabricatedon l-in.2Superstrates(Materials Research Corp., Orangeburg, NY) consisting of a polished ceramic substrate coated with layers of vapor-deposited chromium (200 A) and gold (2000 A). A Superstrate was covered with a thin, spin-coated layer of Kodak KPR photoresist. A negative film mask in the desired pattern, emulsion side up, was placed on the glass plate of a UV light box 5 cm from the light source. The Superstrate was placed photoresist-side down on the mask and weighted, and the photoresist polymerized by exposure to UV light for a measured time appropriate to the mask pattern. The unpolymerized photoresist on thesuperstrate was then washed away in Kodak photoresist developer for 30-50 s and rinsed with 2-propanol. Exposed gold (not covered with polymerized photoresist) was etched in aqueous 0.3 M I-/0.05 M I2 until the chromium underlayer was visible. Chromium was etched in aqueous 0.2 M Ce(IV)/0.075 M HC104, leaving only the photoresistcovered gold image. The polymerized photoresist was removed finally by sonication in Kodak photoresist stripper, leaving a bare gold surface in the pattern defined by the mask. Several electrode arrangements have been suggested which isolate reference, working, and auxiliary electrodes in a smallvolume microelectrode cell.'619 However, most of these arrangements lead to severe iR drop in EC/MS because, in any solvent with vapor pressure low enough for use in the MS vacuum (glycerol, PEG, HMPA), the solventviscosity is much higher than that of water. One solution to this iR drop problem is provided by a two-electrode interdigitated arrangement (Figure 2), with one electrode serving as both auxiliary and reference. The combined auxiliary/reference electrode should provide a near-nernstian electrode reaction which does not produce products that interfere with electrochemistry at the working electrode, and in the present application, it must not produce volatile products that could interfere with mass spectral monitoring. A gold auxiliary/reference coated with Prussian blue satisfies all of these restrictions.
-
(13) Heiger, D. Master's Thesis, The Ohio State University, 1987. (14) Harrington, M. S.Ph.D. Thesis, The Ohio State University, 1990. ( 1 5) Maissel, L.; Glang, R. Handbook of Thin Film Technology; McGraw-Hill, Inc.: New York, 1980. (16) Wooster. T. T.; Longmire, M . L.; Zhang, H.; Watanabe, M.; Murray, R.W. Anal. Chem. 1992, 64, 1 1 32. (17) Pickup, P. G.; Murray, R. W . J . Am. Chem. SOC.1983, 105, 4510. (18) Fritsch-Faules, 1. F.; Faulkner, L. R. A n d . Chem. 1992, 64, 1118. ( 1 9) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; J. Wiley and Sons Inc.: New York. 1980; pp 569-573.
$
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E,V vs. SCE Fbure 5. Steady-state cyclic voltammogam of a Russian bluecoated goid electrode hnmerwdIn aqueous saturatedKW3: auxllary electrode, Pt flag; reference, SCE; sweep rate, 100 mV/s. 0 = 195 pC. Colors
Figure4. Electrode potential (9 vs added charge (a)for the reductbn of the ferrate Iron of a Russian blue fllm, 0' = 654 pC. Equilibrium experlmental(0) and calculated (-) from eq 4. 1000 t
2 840.
of the varkus forms are Indicated.
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Prussian Blue-Coated Gold Electrodes. Prussian blue films on gold can act as auxiliary/pseudoreferences in two electrode cells. These films provide oxidative and reductive chargetransfer reactions that balance the current flowing at the working electrode and potentials that are stable as long as the net charge passed does not exceed the charge capacity of a given film. The charge-transfer reaction at the PB films produces neither solution soluble electroactive nor volatile products, making a PB-coated gold electrode a suitable candidate for use in the mass spectrometer vacuum. The electrochemicalformation of Prussian blue films has been described previously.20u21 Prussian blue was electrochemically deposited on the gold filaments by a procedure suggestedby Itaya.zo The electrode to be coated was immersed in a solution of 20 mM Fe(NO3)3, 20 mM &Fe(CN)6, and 0.1 M "03; the potential of the electrode was stepped to +550 mV vs Ag/AgCl, causing a reduction current to flow for 3&180 s. Once deposited, the film acts as a source or a sink for electrons, and its properties, which were evaluated by cyclic voltammetry and chronopotentiometry, are a direct result of the structure of the film. ItayaZO reported the structure of electrochemically made Prussian blue (the so-called "water-insoluble" type) to be nearcrystalline with the formula Fe,"'[Fe"(CN),]3 I where Fe is ferrate iron and Fe is cyanoferrate iron. The polymer is electrochemically converted to the colorless form (Prussian white) by reduction of Fe, or to the green form (Berlin green) by oxidation of Fe. Figure 3 is a steady-state cyclic voltammogram of a PB film on gold, showing the potential regions where the polymer exists in the green, blue, and colorless forms. The CV waves confirm that the polymer provides both oxidizable and reducible iron and that the ratio of Fe to Fe is -4:3. The CV waves in Figure 3 define two regimes where a PB-coated electrodeexhibits nearly nernstian potential changes as a function of chargeadded. This is analogous to the addition of charge to a layer of adsorbed depolarizer, which can be reversibly converted from oxidized to reduced form.lg If Go (20) Itaya, K.; Atah, T.; Tmhima, S. J. Am. Chem. Soc. 1982, 104,4161. (21) Ncff,V.; Eckoff, M.; Ellis, D. J. Phys. Chem. 1981, 85, 1225.
I
P
O
0
Co
>
E 520 3
-A*
>
= Q/Fand GR = [kQ*- Q]/F is substituted into Bard and Faulkner's equation (12.5.9), then
for each separate wave in Figure 3. In eq 1, k = '13 when = and k = 1 when Exof= EFeof, corresponding to the stoichiometryof structure I. The difference (EPB-E& is the potential of the Prussian blue-coated electrode, and Q ' is the coulometric capacity of the film, approximately q u a l to the charge passed in depositing the original film. Q is the total charge passed in changing the potential from (Exofi ! 120 mV) to E: &Of
We have not fully investigated the behavior of PB films, but damage has been observed at potentials more negative than -0.30 V or more positive than +1.0 V vs SCE. Equation 1 was verified experimentally for both regimes. Figure 4 shows the comparison between equilibrium potentials calculated from eq 1and experimentally measured equilibrium potentials, as a function of added charge for the reduction of ferrate iron in a freshly prepared PB film. The initial potential of the electrodewas 574 mV vs SCE, indicating that virtually all of the ferrate iron began in the oxidized (+3) state. The value for EF~O'calculated from eq 1 using the data in Figure 4, is 0.20 V, a value in agreement with the observed voltammetric peak potential in Figure 3. Figure 5 shows the analogous comparison for the oxidation of Fe; charge could be added only up to 50% kQ*.EF~"is calculated to be 0.80 V vs SCE, significantly different from the voltammetric peak potential of 0.90 V, which indicates non-nernstian behavior of Fe in the CV. Equation 1 can be used to calculate the potential of a working electrode with respect to SCE from the two-electrodeAnalytical Chemkity, Vd. 66, No. 2, Jenuery 15, 1994
181
Table 1. Summary of Electrode Materlalr, Solvents, and Condltionr Used for In SHu Electrochemlcal Studler In the Mess Spectrometer
experiment
electrochemistry
MS mode@)
reduction of constant i, 5-20 r A Fc+ at gold in PEG oxidation of constant i , 20-200 FA benzoate a t PtJAu in PEG reduction of constant E , -3.0 V 1,12-DBDD a t vs P B gold in HMPA
Ion Source
m/z
ES SI
1-200 186
ES SI
1-200 44,78
SI
168,248
Top View ~~
~
~
nrs VACUUbl Flgure 6. Schematic of EC/MS probe placed within the mass spectrometer source vacuum. Exploded views show side view and top view of the electrode.
cell voltage, the initial potential and the total charge passed through the auxiliary/reference electrode. Thevoltage applied between the working electrode and a PB-modified auxiliary/ reference is VCea = E , - EPB,or =
(E, -
- EpB)
+ (EPB
-
(3) = c' ell + (EPB- ESCE) Combining eqs 1 and 3 yields an expression which can be used to calculate the working electrode potential or control Vceuto establish the desired value for E,:
Mass Spectrometry. Figure 6 shows schematically the probe used to introduce the electrochemical cell into the modified MS source. A 4-cm extension was inserted between the conventional electron impact source and the pinhole leading to the quadrupole. This modification greatly decreases the efficiency of capture of ions produced in the E1 source and leads to reduction of the resolution and sensitivity normally expected from an instrument of this type. Perhaps a redesign of the source could improve performance, but such a task was beyond the needs or scope of the present work. A 1-3-~L volume of PEG containing the analyte and supporting electrolyte was spread to cover the electrode surface to form a film with a thickness of -0.10 mm. The probe was then placed within the MS source, and the mass spectrometer was evacuated. After 2 min., sufficient atmospheric gases and other volatiles had been pumped away that a steady-state pressure mbar) was attained, and the EC/MS experiments were performed. All mass spectral ion currents discussed here were produced by SEM detection of positive ions created from uncharged molecules in the somewhat modified electron impact source of a conventional quadrupole mass spectrometer. No solvated ions, generated at the electrode surface, could escape from the solvent surface into the source vacuum to be detected in the SEM. The electroactive starting materials used here were nonvolatile,so the background spectra were essentially constant for many minutes to several hours and showed only solvent peaks (see below) and contaminants desorbed from the
-
196
Analytica/Chemi..try,Vol. 66,No. 2, January 15, 1994
0
i 00
23c
m/z, a m u Flgure 7. Mass spectrum of HMPA. Noninterference regions are indicated in box.
spectrometer walls. Therefore, the spectra obtained from EC/ MS experiments should be directly comparable to E1 spectra from the literature or from known samples introduced into the source vacuum through conventional methods, with appropriate allowance for the differences between spectra obtained on different instruments. Mass spectral data were collected in two modes: single ion monitoring (SI), where the mass spectral ion current for a single m / z value was recorded as a function of time, and entire spectrum monitoring (ES), in which a range of m / z values were repeatedly scanned. MS data were collected throughout electrolysis. Products of electrochemical reactions diffused through the thin solvent layer to the solvent/vacuum interface and evaporated. Thevaporized products were ionized in the electron impact source, and the positive ions mass separated and detected by a 90° off-axis SEM. Table 1 gives the electrochemical and mass spectrometric parameters for the EC/MS experiments performed in this work. Proper choice of solvent is necessary to ensure that solvent MS peaks do not overlap analyte peaks. For example, glycerol, a widely used matrix for FAB-MS, gives major mass spectral peaks at m / z 61, 43, 44, and 1522and thus provides several regions (windows) free from solvent peaks. Figures 7 and 8 show the MS windows for HMPA and PEG-400, respectively.22 The spectra of both HMPA and PEG provide broad windows below m / z 100 and little interference above m / z 100. RESULTS AND DISCUSSION Mass Spectrometry of Ferrocene Produced Electrochemically. Positioning of the electrode and solution within the source vacuum of the mass spectrometer allows detection of specific molecular species participating in the electron-transfer (22) Stenhagen, E.; Abrahamsson, S . ; McLafferty, F. Allus ofMussSpenralDaro; John Wiley and Sons: New York, 1969.
C 0 aJ
'0 01 I 0
200
100
m/z, amu F i p m 8. Mass spectrum of PEG. Noninterferencereghsare Indicated in box.
~~
100
150
200
m/z, a m u Flguro 0. ES mass spectra acqulred during the reduction of 5.0 mM FcPF8at gold In 0.1 M KNOs/PEG. The electrochemical event was a 5-PA cathodic current step Inltiated at time t = 0. For comparlson, the reported spectrum for ferrocene Is given as A,**
reaction, by monitoring the mass spectral ion currents produced by E1 ionization of those species in the low-pressure gas phase above the solution. This simple connection is illustrated in Figure 9, where ferrocene is generated and volatilized and the mass spectrum of the volatile species (in the 1O-4-mbarvacuum above a 0.10-mm layer of the PEG solvent) is monitored during reduction of 1.O X l k 3 M FcPFa in PEG. The characteristic peaks for the ferrocene parent ion ( m / z 186) and the fragment obtained by loss of C5H5 from the parent ( m / z 121) were observed within 30 s of initiation of a reductive electrochemical current step of 5.0 mA. A relatively low current was chosen for this experiment to allow for measurable production of product over the time period of 120s. Because the ferrocinium ion was never depleted at the electrode surface, the potential remained at ca. + l o 0 mV vs E p g throughout the experiment. The generation of a particular volatile species was monitored during electrolysis by recording the ion current at the m/z value characteristic of that species. In Figure 10, the single ion current at m/z 186 is shown during the constant-current reduction of FcPFa in a thin film of PEG containing 0.10 M KNO3.' The electrolysis was continued at this current for an ' extended time, allowing measurement of the time necessary to rise to a steady-state flux of ferrocene at the solution/ vacuum surface (as indicated by 1186), as well as the time to deplete the solutioncompletely of ferrocinium ion. Controlledpotential electrolysis would not have allowed separation of these two processes and thus not allowed estimation of the diffusion coefficient. After ferrocinium ion is depleted at the electrode surface, the potential of the working electrode moves positive to a
0.0
1
0
'
'
400
800 time, s
1200
'
6CO
Flguro 10. Mass spectral Ion current response at mlr 180 to the In situ reduction of 5.0 mM FcPFe In PEG. The electrochemical event was a 5-pA cathodlc current step at time t = 160 s. The PEG film height was 1.3 X lo2 pm.
value between + 1.2 and 1.5 V vs PB, where electrolysis of the solvent occurs (vide infra). Because solvent electrolysis does not produce molecules with fragment ions at m / z 186, these background processes are not seen to influence MS monitoring of ferrocene produced. An estimate of solvent film thickness was made using the volume of PEG deposited on the filar electrode Superstrate and the measured area covered by the resulting film. The MS ion current indicates that ferrocene evaporation from the PEG surface can be detected within 10 s of initiation of the current step and continues for at least 1100 s, until virtually all of the ferrocene is depleted from the solution. The ion current, Z M S , ~in ~ , Figure 10 indicates that the flux of ferrocene molecules from the PEG/vacuum interface increases during constant-current generation, until it equals approximately the flux of ferrocene generated at the electrode surface, and virtual steady-state mass transport of Fc is achieved. At 500 s, the rate of diffusional transport of Fc+ to the electrode is insufficient to sustain the applied current, and the flux of Fc at the electrode and the PEG/vacuum interfaces decays exponentially as Fc+ and Fc are depleted from the liquid phase. The time for 1186 to reach half its estimated steady-state value, t i p , is related to the film height, h, and the diffusion coefficient of ferrocene in the PEG phase, D:23
-
t l I 2= 0.38h2/D (5) The time constant for decay of 1186 after the transition time, t (=500 s), may be represented approximately as
In (z,&j - 1186,ss) - a 2 D / h 2 (6) These two equations applied to the data in Figure 10 yield estimates of D of 2 X lo-' cm2/s and I X cm2/s, respectively. While these D values are quite uncertain, they are of a magnitude expected for a small molecule in a solvent with the viscosity of PEG (13 cP) and indicate that our qualitative interpretation of the EC/MS experimental data is valid. The mass spectrometer can detect very small quantities of product; electrochemical currents in the range 5-20 mA generate tens of picomoles of product per second. The estimated detection limit (the amount of product which can (23) Carlslaw, H. Conducrion of Hear in Solids; Oxford: New York, 1947.
100
0
23:
m/z, a m u
=
be detected per unit time with S / N 2) is -20 pmol/s. The mass spectral time response is governed largely by diffusion of products from the electrode surface to the solvent/vacuum interface through eq 5 . With a time response of 10-30 s, this method is suitable for detection of products which are chemically stable for 1 min or longer. We have identified primary products of several electrochemical reactions by mass spectrometry. Quantitative control of the electrochemical current generation of a known number of moles of product results in a proportionate mass spectral ion current. We have used this relationship to calibrate mass spectral ion currents. EC/MS of the Products of Benzoate Oxidation in PEG. The oxidation of benzoate at platinum in methanol is reported to yield benzene and C02 with low current efficiency.24 Benzoate ion was oxidized in the source of the mass spectrometer at a Pt-coated filar electrode in PEG, using a constant current of 40 HA. A larger current density is needed for detection of benzene and carbon dioxide, because the background and noise at these lower m / z ratios are greater than at m / z 168, the parent ion of ferrocene (vide supra). Mass spectra of volatile products above the thin solution layer, shown in Figure 11, indicate that several new peaks appear in the spectrum after initiation of electrolysis. Ion currents which increase in Figure 11B include m / z 51 /52,77/18,106, and 122, indicating spectral signatures of electrochemical products. In addition, SI monitoring at m / z 44 indicates a significant increase (on top of a large solvent background) simultaneous with the electrochemically initiated increase in 178 (Figure 12). Electrolysis produces CO2 and benzene, apparently by the reactions suggested by Fichter and M e ~ e r : ~ ~
-
e- +
-
+ [Ph']
(7)
PhH + [PEG'] (8) m / z 78 where the species in brackets are hypothetical and assumed to be short-lived. The peaks at m / z 106 (base peak), 122 (parent), 77/78, and 51/52 indicate that benzoic acid is also a product of electrolysis and it diffuses to and evaporates from the solvent surface. Protons are produced from oxidation [Ph']
+ PEGH
c02
m/e 44
(24) Fichter, F . ; M e y c r ,
198
R. H e / u .
502
360
I 5 0 time, s
Flguro 11. Mass spectra acquired prior to (A) and subsequent to (e) the in situ oxldation of 0.010 M sodium benzoate at platinum in PEG. The electrochemical event was a 4 0 - ~ Aanodic current step.
PhCOO-
I
Chim. A c t o 1934, 1 7 , 5 3 5 .
Analytical Chemlstty, Vol. 66, No. 2, January 15, 1994
Figure 12. Mess spectral ion current responses at mlr 44 and 78 to the in situ oxidation of 0.010 M sodium benzoate at platinurn in PEG. The electrochemical event was a 100-/.tA anodic current step initiated at t = 120 s.
of the solvent and of adventitious water (background processes), 2HOH Q 4H' ROH
Q
+ 0, + 4e-
H+ + [RO']
+ e-
(9)
Hydrogen ion rapidly neutralizes benzoate ion to produce the benzoic acid detected: H+ + PhCOO- * PhCOOH m / z 122
(10)
The spectrum in Figure 1 1B shows measurable ion currents at nominal masses 77 and 51. The ratios 177/1106 and 151/1106 are 3-times larger for our experimental data than for the reported spectrum for benzoic acid,22while the ratio 1122/1106 in Figure 1113 is comparable to the literature value. The ion currents at nominal m / z 77 and 5 1 may include contributions from benzene, with parent at m / r 78 and fragments at m / z 52 and 51, which we have demonstrated is produced by decarboxylation of benzoate (eqs 7 and 8), which would account for the enhancement observed. The carbon-centered radicals produced in Kolbe electrolysis often undergo further oxidation to the carbocation. We find no evidence for the dimer biphenyl ( m / z = 154) nor for any carbocation products, such as phenol (mlz = 94). Other carbocation products such as esters and aldehydes would be readily detected by EC/MS, but none were found. A comparison of literature spectra for benzene and benzoic acid to our product spectrum indicates that benzene accounts for -90% of the ion current magnitudes at nominal masses 78 and 52. The diffusion coefficient of benzene in PEG was estimated using eq 5 to be 2.1 X 10" cm2/s. These EC/MS studies of ferrocinium reduction ana carboxylate ion oxidation show that PEG is a versatile electrochemical solvent whose low volatility allows electrolysis to be performed in the source vacuum for several hours without significant loss of matrix. But PEG is a poor electrochemical solvent in the potential region negative of SCE. In order to extend our studies over the entire potential region negative of SCE, we have chosen HMPA as an EC/MS matrix, although this solvent is much more volatile than PEG. EC/MS of the Reduction of 1,12-Dibromododecanein HMPA. Electrochemical reductions of dibrominated alkanes
..
160 128 -
2 96-
-
lli
2
64-
...
32'-
"0
followed by a decrease after -200 s, due to depletion of the precursor by continuing electrolysis. No comparable change is seen in 1246/8, the parent ion@)of bromododecane;however, the parent ion abundanceof most bromoalkanes is rather small and its absence is not definitive. Prominent HMPA fragments at m / z 135-137 prevent detection of two distinctive bromododecane fragments, which would appear at 25% relative abundance, if present.30 No peak (greater than -5% of 1168) could be detected a m / z 170, the parent mass of dodecane, or at m / z 166, parent mass for a diene product. In the electron impact spectra of most dodecane and dodecene isomers, the most abundant peaks are quite similar and do not contain fragments larger than 80 u , making ~ identification by other than the parent ion quite uncertain. We see two likely candidates for the mechanism of olefin formation from dibromide reduction. One is that suggested by Bart and Peters,27in which successive reduction, proton abstraction, and E2 elimination produces a monoolefin. A second possibility is migration of the generated intermediate radical, to the opposite end of the chain, perhaps through several steps involving five- or six-member transition states. The great uncertainty in hypothesis of a radical (or diradical) process is whether radical lifetime is sufficient, even on the electrode surface, to allow the required migration to occur. The fact that no alkane or diene is detected, during the time analyte and solvent are present in the MS source, would appear to favor the migration mechanism for formation of the monoolefin wedetect. If an intermediatecarbanionabstracted protons rapidly from solvent or adventitious contaminants, concomitant alkane formation would be expected. The E2 elimination,clearly demonstratedby Bart and Peters in DMF, apparently does not occur on the 100-s time scale in HMPA. These, somewhat equivocal, results of dibromide reduction point out potential strengths and weaknesses to electrochemical studies in solvents directly exposed to the MS sourcevacuum. Product analysis is possible within seconds of electrochemical formation, and the direct connection of volatile species to the electrode process can be confirmed by correlation with the electrochemical current program. On the negative side, interferences from solvent can seriously mask ion currents in some regions of the mass spectrum. In addition, suitable electrochemical solvent systems are extremely limited, particularly in the potential region negative of SCE. At the present time, PEG appears to be the best compromise for an electrochemical solvent that is thoroughly compatible with sustained residence in the source vacuum.
72
216 time, s
144
288
360
F l g m 1s. Mass spectral Ion current responses at mlz 168 (0)and 248 ( 0 )to the in situ reduction of 1.0 mM DBDD at gold In HMPA contalnlng 0.10 M TBAP. A potentlei step to from 0.0 to -3.0 V vs. PB was applled at t = 120 8.
have been reported p r e v i o ~ s l y . 2Vicinal ~ ~ ~ dibromides produce alkenes almost exclusively, even in the presence of added proton s0urces,2~but Rifi2' and Casanova26reported that reductionsof several 1,o-dibromidesat mercury in DMF yield a mixture of the corresponding n-alkanes and cycloalkanes, in addition to organomercurials. Finally, Bart and Peters2' have described electroreduction of 1,lO-dibromodecane at mercury in DMF, with positive identification of several hydrocarbon products, including alkanes and alkenes. We therefore undertook an EC/MS survey study of the reduction of 1,12-dibrornododecane (DBDD) in HMPA solvent containing TBAP as a background electrolyte, in order to develop techniques adaptable to reduction in the source vacuum at very negative potentials and to study the mechanism of these interesting reactions. Cyclicvoltammetryof DBDD at a gold electrode in HMPA shows a broad, irreversible two-electron reduction of about -2.5 V vs PB (-1.7 V vs SCE). A gold electrode covered with a thin film of HMPA/TBAP containing DBDD was positioned in the MS source, which was then evacuated. Mass spectra were recorded before and after the potential was stepped to -3.0 V vs PB (at the top of the CV wave to achieve limiting current without causing significant electrolysisof the solvent), and a significant increase in the ion current was observed at m / z 168. Figure 13 shows that a rise in 1168, the parent ion of dodecene, is observed 100 s after initiation of electrolysis,
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(25) Rifi, M. Tetrahedron Lett. 1969,13, 1043. (26) Casanova, J.; Rogers, H. J. Am. Chem. Soc. 1974, 96,1942. (27) Bart, J. C.; Peters, D. G. J. Efectroanaf. Chem. 1990, 280, 129. (28) Evans, D.; O'Connell, K. J . Am. Chem. Soc. 1983, 105, 1473. (29) Brown, 0.;Middleton, P.; Threlfall, T. J . Chem. Soc., Perklns Trans. 2 1!M, 955. (30) Eight Peak Index of Mass Spectra; The Royal Society of Chemistry: Nottingham, UK, 1983.
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Received for review April 13, 1993. 1993."
Accepted October 28,
Abstract published in Adoance ACS Abstracts. December 1, 1993.
Analytlcel Chemlstiy, Vol. 66,No. 2, January 15, 1994
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