1102
Anal. Chern. 1980, 52, 1102-1105
trates two combinations of cutsets on the decalin system; the first combination is valid and represents a two-step process cleaving each ring in succession and leaving the charge on the ring-junction and neighboring atoms. T h e second cutset combination illustrated is invalid, for the "ionic part" selected is in fact disconnected. Since it is possible t o generate such disconnected graphs by combination of individually valid cutsets, the generating algorithms have to include path-tracing checking functions that can determine if the "ion" is a single connected substructural fragment. Another invalid combination would use (step 1 ((1,2) (3,4)), step 2 ( ( 1 , l O ) (4,5)) charge on the 5-6-7-8-9-10 portion) for here the "fragment" lost in the second step would be disconnected; the program must check that the atoms of the fragment lost in the second or third step of a multistep process are all present in the "ion" resulting from earlier steps. Another invalid combination of cutsets would be the formation of a fragment ion containing atoms 5-10 by a two-step process involving first the loss of atoms 1 and 2, then in a second step losing atoms 3 and 4; this two-step process is not considered because all ions that it can serve to rationalize can be explained in terms of simpler one-step processes (such invalid combinations can be simply detected by seeing that they involve cleavage of the same bond at different steps, in the example this would be the 2-3 bond).
G. M. Pesyna, and F. W. McLafferty, "Determination of Organic Structures by Physical Methods", Vol. 6, F. C. Nachod, J. J. Zuckermann, and E. W. Randall, Eds., Academic Press, New York, 1976 p 91. R. E. Carhart, D. H. Smith. H. Brown, and C. Djerassi, J . Am. Chem. Soc., 97, 5755 (1975). H. M. Rosenstock. "Advances in Mass Spectrometry", Voi. 4, E. Kendrick, Ed., Institute of Petroleum, London, 1968. C. Lifshitz, "Advances in Mass Spectrometry", Voi. 7A, N. R. Daly, Ed., Heyden, London, 1978. G. S. Zander, and P. C. Jurs, Anal. Chem., 47, 1562 (1975). D. H. Smith, B. G. Buchanan, W. C. White, E. A. Feigenbaum, C. Djerassi, and J. Lederberg, Tetrahedron, 29, 31 17 (1973). H. Budzikiewicz, C. Djerassi, and D. H. Williams. "Structure Elucidation of Natural Products by Mass Spectrometry, II", Holden-Day. San Francisco, California, 1964. D. H. Smith, and R. E. Carhart. "High Performance Mass Spectrometry: Chemical Applications", ACS Symp. Ser. 70, 325 (1978). L. Tokes, R. T. LaLonde, and C. Djerassi, J . Org. Chem., 32, 1012 (1967). N. A. B. Gray, D. H. Smith, T. H. Varkony, R. E. Carhart, and B. G. Buchanan, "Biochemical Applications of Mass Spectrometry", G. R. Walier, Ed., Interscience, New York. in press. B. G. Buchanan. D. H. Smith, W. C. White, R . J. Gritter, E. A. Feigenbaum, J. Lederberg, and C. Djerassi, J . Am. Chem. Soc., 96, 6168 (1976). A. Lavanchy, T. Varkony, D. H. Smith, N. A. B. Gray, W. C. White, R. E. Carhart, 6. Buchanan, and C. Djerassi, submitted for publication. I. Matsumoto, T. Kuhara. M. Yoshino and M. Tetsuo. in "Mass Spectrometry in Drug Metabolism", A. Frigerio and E. L. Chisalberti, Eds., Plenum Press, New York, 1977, p 73. J. E. Patterson, Biomed. Mass Spectrom.,5 , 488 (1978). W. T. Wipke, and T. M. Dyott, J . Chem. I n f . Comput. Sci., 15, 140 (1975). N. E. Gibbs, J . Assoc. Comput. Mach., 16, 564 (1969). E. J. Corey, and G. A. Petterson, J . Am. Chem. Soc.,94, 460 (1972). M. Bersohn. J . Chem. Soc., Perkin Trans. 1 , 1239 (1973).
LITERATURE CITED (1) Part XXXI in our series "Applications of Artificial Intelligence to Chemical Inference"; for part XXX, see T. H. Varkony, Y. Shiloach, and D. H. Smith., J . Chem. Inf. Comput. Sci., 19. 104 (1979). (2) P. C. Jurs and T. L. Isenhour, "Chemical Applications of Pattern Recognition", Wiiey-Interscience. New York, 1975. (3) L. R. Crawford, and J. D. Morrison, Anal. Chem., 43, 1790 (1971). (4) D. H . Smith, Anal. Chem., 44, 536 (1972). (5) N. A. B. Gray, and T.0. Gronneberg, Anal. Chem., 47, 419 (1975). (6) K. S. Kwok, R. Venkataraghavan, and F. W. McLafferty, J . Am. Chem. Soc., 95, 4185 (1973).
RECEIVED for review August 28, 1979. Accepted February 28, 1980. This work was supported by the United Kingdom Science Research Council (B/RF/4955) and the National Institutes for Health (RR-00612). Computer resources were provided by the SUMEX facility at Stanford [Jniversity under National Institutes of Health grant RR-0785.
Direct Mass Spectrometric Determination of Silicone Membrane Permeable Electrochemical Products William J. Pinnick, Barry K. Lavine, Clemens R. Weisenberger, and Larry B. Anderson* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
A thln silicone rubber membrane in intimate contact with a goid-minigrid electrode Is used to separate electroactive materlals from the solvent and background electrolyte. The membrane-permeable products are accumulated in the evacuated Inlet system of a mass spectrometer. Mass spectra of the reactants and products of an electrochemical reaction can be obtained on microgram quantities of electrolysis products.
Mass spectrometry (MS) is useful in unequivocally identifying t h e products of a n electrochemical reaction (1-8). Three problems are often encountered in such analyses: the products must be volatile; they must be separated from a large excess of "matrix" consisting mainly of solvent and background electrolyte; a n d a high concentrational sensitivity is M, typically. required, where analytes will be to Product volatility generally limits mass spectrometry to analysis of uncharged species of moderate molecular weight. 0003-2700/80/0352-1102$01 .OO/O
In electrochemical solutions, the method is therefore insensitive to salts (such as the background electrolyte) and to polymeric products. The principal problem is to separate the great excess of electrochemical solvent from the reactants and products of interest. Gas and liquid chromatography have been used for this separation, but they both have the tendency to dilute the sample prior to mass spectrometric analysis (6-9). We wish to report here the use of permselective silicone membrane to sample electroactive species in the diffusion layer. T h e membrane is situated in contact with the electrochemical solution on one side and the evacuated inlet system of a mass spectrometer on the other. Silicone rubber acts as a nearly immiscible phase in contact with solvents such as water and hexamethylphosphorictriamide (HMPA), but apolar organics dissolve readily in it, and are free to diffuse to the vacuum side of the membrane. If a working electrode is in contact with the silicone surface, some electroactive reactants and products may be partitioned into and diffuse through the membrane to be detected in the mass spectrometer. 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
-
1103
Table I. Permeation Rates of Air and Various Solvents through Silicone Rubber Membrane
Torr Gold W i r e Contact
L \
fluid
~
D'
I
Med. Glass ,-'F r i t
50 - 200 Wrn 7 5 0 Line/,,,h Gold M e s h
Silicone Membrane
'0
END VIEW
Figure 1. Details of construction of the gold-grid/silicone-membrane
electrode Bruckenstein and co-workers (1-5) have described electrochemical mass spectrometry using a porous Teflon membrane between the solution and the MS inlet. Small molecules such as CO,, NO, and methane permeate the membrane more rapidly than larger molecules such as the solvent and larger organics. T h u s enrichment of lower molecular weight components is achieved. Tou and co-workers ( 1 0 , I I ) and Mason a n d co-workers (12) have described t h e use of thin silicone membranes t o directly sample various trace pollutants from water and identify them mass spectrometrically. Silicone membranes have also been successfully applied to separating t h e permeable organics from t h e impermeable carrier gas in a gas chromatography/mass spectrometry separator (13). T h e silicone membrane electrode is used here t o study the reduction of dibromocyclohexane t o cyclohexene. EXPERIMENTAL Membrane Composition and Properties. The membranes used in this work were fabricated from clear Silicone Rubber Sealant (Dow Corning Corporation, Midland, Mich. 48640). Membranes from 30 to 200 pm thick were tested for leakage in air, leakage in electrochemical solvents and their vapors, and permeability to some compounds of interest. General properties of commercially prepared silicone membranes of nominal 1-mil thickness are summarized in manufacturer's literature ( 1 4 ) . Construction of the membrane-mounted electrochemical probe is shown in Figure 1. The membrane was attached using silicone rubber sealant around the perimeter of the glass frit. The probe was attached by a ground-glass joint to an evacuated inlet on the MS-9 mass spectrometer. When this probe was immersed in a polar solvent (e.g., water or HMPA), there was a very low background leak rate, corresponding principally to leakage of permanent gases dissolved in the solvent. For example, after pumping down the inlet (volume approximately 140 mL) and source to 1.4 X Torr, leakage gases from a membrane electrode (98 gm thick) immersed in water were collected in the inlet for 3 min. The pressure in the source rose to 1.6 X lo-' Torr, and a mass spectrum taken of this sample showed principally masses 28 (NZ, CO), 32 (02),and 44 (C02). The peak a t mass 18 (H20)was an order of magnitude smaller than the peak a t mass 28. When the probe was exposed to air at atmospheric pressure, the leakage rate was higher, but the composition of the gas accumulated was similar (pressure after 3 min, 1.6 X 10-7Torr). When the probe was exposed to vapor in the headspace above neat acetone, the leak rate increased dramatically (pressure after 3 min, 1.6 X lo4 Torr), and prominent acetone peaks were observed in the spectrum of the accumulated gas.
air water HMPA DMF DMSO acetic acid acetonitrile tetrahydrofuran dichloromethane ace tone methanol a
rnem brane thickness,
w 45 45 45 45 45 290 56
permeation rate. torrs ' cm-' x IO4 29 9.1 4.4 13
3.9 79 2.7 x 103
56
9 x io3
56
a
45 45
i5x
permeability coefficient, cm-' s - ' x 10' 0.17 1.7 71: IOZ 1.6 X l o 2 40 1.6 X l o 2 1.7 X l o 2 2.8 x
lo2
a
io3
2.3 X 10'
5.8 3.0
X X
lo2 lo2
Permeation t o o rapid to measure by this method.
The membrane thickness was measured by weighing a sample of measured area, assuming a density of 1.047 g ~ m - ~ . Permeation rates of various materials including common electrochemical solvents, were measured by exposing the silicone membrane electrode to equilibrium vapor pressure in the headspace above the neat liquid. The permeation rate shown in Table I was calculated from the observed rate of increase of pressure in the MS inlet system (140 mL) divided by the projected area of the membrane. The permeability coefficient in Table 1 was calculated as the permeation rate divided by the vapor pressure of the liquid times the membrane thickness. The permeation rate (which is a flux) is particularly useful in distinguishing which solvents can be used with the probe. The solvents water, HMPA, dimethylformamide (DMF) and dimethylsulfoxide (DMSO) have satisfactorily low permeation rates to be used with the silicone probe. We have obtained useful data on the Kolbe' electrolysis in glacial acetic acid ( 1 5 ) , though this :solvent is on the borderline of being too permeable for good sensitivity. We feel the membrane electrode cannot be used in the solvents following acetonitrile in Table I. Mass spectra were taken on an AEI Model MS-9 instrument using a source temperature of 125 "C. The standard cold sample inlet system was employed at ambient temperature. An ionizing voltage of 70 V, ionizing current of 100 PA, and accelerating voltage of 8 kV were used.
RESULTS A N D D I S C U S S I O N M e m b r a n e - P e r m e a b l e Reactant. When the probe illustrated in Figure l is immersed in a well-stirred solution containing a membrane-soluble species, that species is partitioned into the silicone and diffuses t o t h e evacuated face. An ion current is then observed in the mass spectrometer in direct proportion t o t h e partial pressure of the permeable species in the source. This process is schematically illustrated in Figure 2. T h e flux of material emerging frorn t h e membrane at x = 1 is related t o t h e concentration a t t h e surface a t x = 0 by t h e following expression (15): (flux),,l (moles cm-2 s-l) =
where C" is t h e concentration at t h e interior surface of t h e membrane, 1 is t h e membrane thickness, D is t h e diffusion coefficient, and t is the time since contact between the membrane and solution. T h e ratio of concentrations of material in t h e solution and membrane at x = 0 are assumed t o be equilibrium values specified by the partition coefficient, KD = C"/CS.
1104
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
a
0
x-
-
I
D a
CL
e
0
xFigure 2. Schematic concentration profiles in a silicone membrane separator: (A) a membrane soluble species dissolved in the electrochemical solution; (6)a membrane soluble product of an electrochemical reaction at the membrane surface
w
5
z L
0
T 30
60 90 120 150 180 IMMERSION T I M E , SECONDS
210
Figure 3. Source pressure as a function of time after immersion of the membrane electrode in different solutions: acetone in water, dibromocyclohexane in HMPA, and the reduction product, cyclohexene After a period of time, t,, long compared to 12/6D,the flux described by Equation 1 becomes constant: steady state flux = DK1,@/l
(2)
We have tested the behavior of our membrane by measuring the pressure in the source of an MS-9 mass spectrometer as a function of time after the membrane was immersed in water containing acetone. T h e vapor which permeates the membrane accumulates with time as shown in Figure 3. The amount of diffusant which passes through the membrane, Q, is proportional to the source pressure, P, and is given by the expression (15):
which at steady state becomes:
Q ( t )=
C”D I (t
-
&)
(4)
C,H,d3r2
+ 2e-
~t
C6HIo+ 2Br-
and this cyclohexene readily permeates the silicone membrane. A schematic concentration profile for a membrane-soluble electrochemical product in HMPA is shown in Figure 2 and a typical pressure vs. time profile for the cyclohexene is shown in Figure 3. T h e mass spectrum of the accumulated membrane-permeable reduction product (Figure 4) is unequivocally that of cyclohexene. There is no evidence of brominated products, dimers or higher oligomers. Monitoring the mass spectrum of the membrane-permeable species as a function of the gold electrode potential provides more information on the DBCH reduction reaction. Such a mass-voltammogram is shown in Figure 5. Masses 67,54, and 82 are important fragment ions of cyclohexene. The potential dependence of these ion currents is plotted, and the electrochemical current is shown for comparison. These three ion currents show that cyclohexene is produced as a product of an electrolysis process occurring in the vicinity of -1.1 V. T h e electrochemical current shows a similar potential dependence, though a 200-mV discrepancy is observed in the “half-wave’’ potentials of electrochemical and mass
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7 , JUNE 1980
120-
-120
fn
k z
-
3
&
I
-
/
/
100-
-100
a
LL
k m a
I
-
0
ao-
-a0
n
-
t-
t f
-
5 I]
m
z
Z W
$
-I
60-
-60
Y
1
a
I
-
-
LT W
a
co
40-
~
5
r r D
-40
-
-
v)
m
-06
-08 -10 VOLTS v s
Hg
-12 POOL
-14
Figure 5. Mass spectral ion currents at various masses after controlled potential electrolysis of DBCH for 5 min at each potential. Steady-state current shown for comparison ( 0 ) .Mass 67 (A), mass 54 (O), mass 82 ( 0 )
spectral curves. There is substantial reduction current at 4 . 9 V, where the mass spectrum shows very little cyclohexene produced. At least two explanations can be considered for this halfwave potential discrepancy. Possibly, a membrane-impermeable product, such as monobromocyclohexane or bis(bromocyclohexane), is produced at potentials corresponding to the rising portion of the electrochemical current ( E positive relative to -1.1 V). Alternatively, the principal product produced a t the base of the wave reacts with the silicone membrane material and cannot be transported freely through the membrane. In either case, electrolysis a t potentials negative of -1.1 V produces cyclohexene directly at the electrode and this material diffuses through to be detected. A third alternative which cannot be discounted from these experiments, is that this is an unidentified artifact of the mini-grid membrane electrode system. However, it should be noted that no such discrepancy in electrochemical and MS half-wave potentials occurred when cyclooctatetraene was reduced and protonated to form cyclooctatriene using the same solvent/electrolyte a n d membrane/electrode system (16). T h e relatively high sensitivity of this method is probably the most important analytical point to be made about the detection of the cyclohexene product. On the current plateau (-1.2 V), approximately 0.1 Kmol of DBCH was electrolyzed
1105
during the 5 min taken to collect the sample of membranepermeable products, and this gave ample material for a thorough mass spectral study. In part, this sensitivity is t,he result of the high selectivity of the silicone membrane for the hydrocarbon over the solvent, background electrolyte and permanent gases (principally Ar). T h e impermeability toward the electrochemical reactant (DBCH) is convenient, but certainly not a necessary condition for product analysis. The mass spectrum could easily be corrected for a second membrane-permeable species by subtracting the known spectrum weighted by the height of a characteristic peak in the spectrum of the mixture. Correction for multiple species obviously becomes more complex and less likely to succeed ( 1 7 ) . T h e analysis of product distribution from complex electrochemical reactions can be a challenging and time-consuming task. I t becomes even more difficult when the distribution is strongly dependent on electrode potential. But quantitative information on product yield is essential for optimization of any electrosynthetic procedure. T h e technique of electrochemical mass spectrometry described here may be a rapid and versatile tool for positive identification of a large class of electroactive species.
ACKNOWLEDGMENT We thank J. Tou for discussions of the silicone rubber membrane materials.
LITERATURE CITED (1) S. Bruckenstein and R . R. Gadde, J . Am. Chem. Soc., 93, 793 (1971). (2) S. Bruckenstein and J. Comeau, J . Chem. Soc., Faraday Discuss., 58, 285 (1973). (3) M. Petek and S.Bruckenstein, J . Electroanal. Chem., 42, 397 (1973). (4) M. Petek, S. Bruckenstein, B. Feinberg, and R . N. Adams, J. Electroanal. Chem., 47, 329 (1973). (5) L. Grambow and S. Bruckenstein, Nectrochim. Acta, 22, 377 (1977). (6) G. M. McNamee, B. C. Willett, D. M. LaPerriere, and D. G. Peters, J . Am. Chem. SOC., 99, 1831 (1977). (7) B. C. Wiliett, W. M. Moore, A. Salajegheh, and D. G. Peters, J . Am. Chem. SOC., 101, 1162 (1979). (8) D. L. Taggart, Ph.D. Dissertation, The Ohio State University, Columbus, Ohio, 1974. (9) H. Y. Cheng, P. H. Sackett, and R. L. McCreery, J , Am. Chem. Soc., 100, 962 (1978). IO) L. B. Westover, J. C. Tou, and J. H. Mark, Anal. Chem. 46, 568 (1974). 11) J. C. Tou, L. B. Westover, and L. F. Sonnaben, Am. Ind. Hyg.,36, 374 (1975). 12) J. C. Weaver, M. K. Mason, J. A. Jarrel, and J. W. Petorson, Biochem. Siophys. Acta, 438, 296 (1976). 13) W. H. McFadden, "Techniques Combined G a s Chromatography/Mass Spectrometry". John Wiley and Sons, New York, 1973, pp 188 ff. 14) General Electric, "Permselective Membranes", Brochure GEA-8685-A, Medical Systems Division, 1 River Road, Schenectady, N.Y. 12345. 15) J. Crank and G. S. Park, "Diffusion in Polymers", Academic Press, New York, 1968, p 6. (16) W. J. Pinnick, M.S. Thesis, The Ohio State University, Columbus, Ohio 43210. 1979. (17) J. Comeau, Ph.D. Thesis, State University of New York at Buffalo, Buffalo, N.Y., 1975.
RECEIVED for review September 6, 1979. Accepted March 10, 1980.
