AIDS FOR ANALYTICAL CHEMISTS
I
I
I Two-Stage Molecular Separator for Connecting a Gas Chromatograph to a Mass Spectrometer Michael A. Grayson and Clarence J. Wolf McDonneN Research Laboratories, McDonnell Douglas Corp., St. Louis, M o . 63166
THE COMBINATION of a gas chromatograph with a mass spectrometer (GC/MS) is an extremely powerful instrumental technique for the analysis of complex organic mixtures. In the first GClMS systems, the G C was connected directly to the MS with a restrictor and a portion of the effluent stream analyzed ( 1 , 2). If large samples are used and the MS has sufficient pumping capacity, this approach can be used. However, in most experiments it is desirable to remove as much carrier gas as possible from the effluent stream while retaining the material to be analyzed. In recent years, a number of techniques have been introduced which interface the two instruments with a molecular separator. The interface has a twofold task: to transmit the majority of the G C eluate from the outlet of the chromatograph to the ion source of the mass spectrometer and to reduce the carrier gas pressure from one atmosphere at the G C outlet to less than 10-5 Torr in the ion source of the MS. In performing this twofold task, the molecular separator enriches the ratio of eluate to eluent from the G C to the MS; consequently, such interfaces are also referred to as enrichers or molecular enrichers. The ability of an interface to perform this twofold task can be measured by determining its yield Y and enrichment N , defined as (3):
Y =
QMS ~
x
100
QGC
where QMs = amount of eluate entering MS QGC = amount of eluate leaving G C HeMs= flow of carrier entering MS HeGc = flow of carrier leaving G C
It is important to determine both Y and N so that an accurate basis can exist for the comparison of various molecular separators. With the single exception of the jet molecular separator described by Ryhage (4, all separators contain a physical barrier which acts in one of two ways: the barrier permits the carrier gas to pass through while the organic goes into the MS, or the barrier permits the organic to pass through into the MS while the carrier gas is vented. GC/MS interfaces of the first type use barriers of fritted glass (5), sintered stainless (1) J. C. Holmes and F. A. Merrell, Appl. Spectrosc., 11,86 (1957). (2) R. S. Gohlke, ANAL.CHEM.,31, 535 (1959). (3) M. A. Grayson and C. J. Wolf, ibid., 39, 1438 (1967). (4) R. Ryhage, ibid., 36, 759 (1964). (5) J. T. Watson and K. Biemann, ibid., p 1135; 37,8446 (1965). 426
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, M A R C H 1970
steel (6), porous silver (7, 8), thin wall Teflon (DuPont) (9),
a palladium alloy (IO), or an adjustable annular slit (11). Thus far, those barriers which permit the organic to pass have been limited to a thin film of polymeric silicone (12-14). With the exception of the variable conductance device described by BrunCe, Biiltemann, and Kappus ( I I ) , the performance of most separators deteriorates rapidly at high flow rates even though the separators may have limited use in the high flow region. Other separators (8, 9, 13) are specifically designed for flow rates encountered in capillary GC. The two-stage separator described in this report utilizes both types of separator action and is efficient for flow rates as high as 60 cm3/min. The operation of the G C with a wide flow rate range permits the use of flow programmed GC/MS (15): this technique helps reduce one of the major experimental difficulties in analyzing complex mixtures with GC/MS, namely, the “column bleed” which occurs in temperature programming, In addition, the G C outlet is maintained at atmospheric pressure. This is a major advantage for those laboratories that require separate G C studies prior to complete analysis with the GC/MS. EXPERIMENTAL
Separator Design. Details of the construction of the twostage molecular separator are shown in Figure 1. The first stage contains a thin membrane of dimethylsilicone polymer, and the second stage uses a silver frit. The chromatograph is connected to one of the tubes. The effluent traverses the spiral groove machined in the gas manifold and is vented to the atmosphere through the other tube. The spiral V-shaped groove has a total length of 1 m, a base width of 0.15 mm and a depth of 0.5 mm. The membrane, 0,001 in. in thickness with a surface area of approximately 20 cm* is supported on a circular piece of sintered stainless steel welded to a stainless steel ring. The outlet manifold collimates the material transmitted by the membrane and also provides mechanical backing for the sintered stainless support. (6) P. M. Krueger and J. A. McCloskey, ibid., 41, 1930 (1969). (7) R. F. Cree, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1967. (8) M. Blumer, ANAL.CHEM., 40,1590 (1968). (9) S. R. Lipsky, C. G. Horvath, and W. J. McMurray, ibid., 38, 1585 (1966). (10) D. P. Lucero and F. C. Haley, J. Gas Chromatogr., 6, 477 (1 968). (11) C. Brunte, H. J. Bultemann, and G. Kappus, 17th Annual Conference on Mass Spectrometry and Allied Topics, May 1969, Dallas, Texas, paper 46. (12) P. M. Llewellyn and D. P. Littlejohn, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 19G6. (13) D. R. Black, R. A. Flath, and R. Teranishi, J . Chrorncttogr. Sci., 7,203 (1969). (14) J. E. Hawes, R. Mallaby, and V. P. Williams, ibid., p 690. (15) M. A. Grayson, R. L. Levy, and C. J. Wolf, unpublished data.
"'
4
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BRIDGE 1
TOF.
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-
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++]
ION/
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PUMP
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Z CHANNEL AMPLIFIER
Figure 1. Block diagram (exploded view) of two-stage molecular separator The silver frit in the second separator stage is installed in the leg of a tee between the first stage and the variable leak to the MS. The frit has a surface area of 3 cm2 and pore s u e 50.2 pm. The variable leak consists of a needle valve which controls the flow into the mass spectrometer as well as the pressure differential across the silver frit. The forepump removes the carrier gas which passes through the silver frit. A vacuum seal is provided by a silicone rubber O-ring between the membrane support and outlet manifold. The seal between the inlet manifold and the membrane support is provided by the membrane. The need for two separator stages is best illustrated by consideration of the operation of the silicone membrane alone. Organics dissolve in the G C side of the membrane, diffuse through it, and volatilize on the MS side. This process is quite efficient for most organic compounds and relatively inefficient for permanent gases such as are likely to be used as a G C carrier gas. Thus, the organic is enriched relative to the carrier gas. However, since the carrier gas is in large excess, the total amount of it passing through the membrane may be relatively latge unless the total surface area of the membrane is small. If the membrane area is small, the separator is inefficient at high flow rates because the organic compounds stream by the membrane too rapidly to establish equilibrium between solution and vapor. The residence time that the organic is in contact with the membrane is one of the primary factors determining the efficiency of the first stage. For a given membrane area, the residence time will decrease with increased flow rate. For the first stage to operate efficiently at flow rates of 60 cma/min,the membrane area must be larger than that required for efficient operation at 20 cm3/min. Increasing the surface area also increases the amount of helium transmitted by the first stage. Thus, if high flow rates are to be used, a second separator stage must be added to dispose of the carrier gas passing through the first stage. One of the major problems associated with the design of a silicone separator is concerned with minimizing the distortion in the shape of the chromatographed peak as it passes through the separator. The peak can be distorted prior to entering the ion source by a combination of four different mechanisms: long connecting lines and/or cold spots in the lines; leakage across the walls of the spiral groove; long residence time in the spiral groove; or slow diffusion through the silicone membrane. However, if the system is carefully constructed, peak broadening can be held to a minimum. The connecting lines should be as short as possible and heated. Special care must be taken to ensure that the spiral groove face of the inlet manifold and the membrane support are flat so that the gas stream cannot spread across the groove face. The transit time through the spiral groove should be less than 10
Figure 2. Block diagram of experimental system used to calibrate and test molecular separator
sec at the lowest flow rate one expects to use (in our design this corresponds to 5 cm3/min). The membrane temperature should be maintained between 50 and 75 "C below the boiling point of the material passing through. This temperature is chosen as a compromise value between maximizing the solubility of the organic in the membrane (low temperature) and minimizing the time of diffusion through it (high temperature). Procedure. The arrangement for testing the separator is shown in Figure 2. Two microthermal conductivity detectors (Carle Instruments, Inc., Fullerton, Calif.) were housed in a single oven. The chromatographic column was 6 ft x in. i.d. copper packed with 1 % SE-30 on 60/80 mesh Chromasorb-P. The restrictor was used to split the G C effluent stream for calibration of the mass spectrometer. The first detector (TCJ measures the concentration and peak shape of the compounds before they enter the first stage of the separator; the second detector (TC2) determines these parameters after the compound passes the membrane separator. Data from these detectors were amplified and recorded on FM magnetic tape (Honeywell 7600) for subsequent analysis with a digital integrator (Infotronics CRS100). A time-of-flight mass spectrometer (Bendix Model 12-101) equipped with a total ion monitor was used to detect material passing through the separator. Data from the mass spectrometer were recorded, and peak areas were measured with the integrator. The flow rate into the MS is equal to the difference in flow rates at TC1 with valve V, closed and open. The mass spectrometer was calibrated for the flow (0.7 cm3/min in our experiments) by determining the ion current at m/e 4 with one of the analog gates. Blends of test compounds were injected (2 p1) with a microsyringe and both T C detectors and the mass spectrometer were calibrated. Column flow rate was chosen during calibration so that 2 z of the injected material entered the ion source of the mass spectrometer. After calibration V1 was closed and VZ (a needle valve) was adjusted to give an ion current at m/e 4 equal to that measured during calibration, thus assuring 0.7 cm3/min He flow into the ion source of the mass spectrometer during test. The mixture (0.2 pl) under investigation was then injected. The carrier gas flow rates both entering and exiting the membrane separator were measured with a soap bubble meter. The response of the two TC detectors was the same within experimental error, i.e., k5z. Therefore, the yield of the first stage is determined directly from areas measured by the two TC detectors; Yl
=
TC1 - TC2 TCi
x
100
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
(3) 427
Table I. Yields and Enrichments for Operation of the Membrane, Silver Frit, and Two-Stage Separator at 100 "C as a Function of GC Flow Rate for Different Compounds Flow Rate B.P., 10 crn3/min 30 cma/min 60 cm3/min Compound "C NI Yl NZ Yz NT YT N I YI NZ YZ N T YT Ni Yi N2 Y2 N T YT 9 7.7 5.3 35 1 . 7 60 9 . 0 21 5 . 1 17 1.5 52 Hexane 69 2.9 57 1.8 64 5 . 3 37 9 7.7 5.4 36 1 . 5 52 8 . 1 19 3.9 13 2.0 69 Hexene-1 64 2.8 55 1.7 60 4 . 7 33 Decane 174 5 . 0 99 1 . 4 49 7 . 0 49 14.1 94 1.3 45 18.4 43 27.3 79 1.1 40 27.4 32 Decene-1 171 4 . 9 98 1.5 52 7.3 51 14.1 94 1.3 47 19.3 45 22.5 75 1.4 49 31.7 37 Dodecene 214 5 . 0 99 1.5 54 7 . 7 54 14.9 99 1.4 48 20.6 48 28.2 94 1.2 42 34.3 40 Tridecane 234 5 . 0 100 1 . 7 58 8.3 58 14.9 99 1 . 5 53 22.7 53 29.4 98 1.2 41 35.1 41 1-Pentanol 138 4 . 5 89 1.5 54 6.9 46 10.4 69 1.2 42 12.4 29 12.6 42 1 . 1 40 14.6 17 1-Octanol 195 5.0 99 1 . 5 54 7 . 7 54 14.9 99 1 . 1 38 16.3 38 26.7 89 1.1 40 30.9 36 Methylheptanoate 172 4 . 9 97 1 . 4 50 7 . 0 47 12.9 86 1.4 50 18.4 43 16.8 56 1 . 5 53 25.7 30 Methyldecanoate 224 5.0 99 1.6 50 8 . 0 49 14.9 99 1 . 5 53 22.7 53 29.1 97 1.3 47 39.4 46 Carbon tetra9 . 0 60 1.2 41 10.7 25 23.7 22 1 . 1 59 27.4 13 chloride 77 3 . 5 70 1.5 51 5 . 1 36 Trichloro9 . 6 64 1 . 6 56 15.4 36 11.4 38 1.3 47 15.4 18 ethylene 87 4 . 3 85 1.5 54 6.6 46
I
3.01
p 5.0 =5 4.5n.
The enrichment of the first stage is determined according t o Equation 2.
FLOW CC PER MINUTE
4.0-
where
-z5 3.5-
H e c = flow rate out of the G C Hethru= flow rate through the membrane into the second stage
z$ 3.1)2.5 4
2 2.0e 1.5 -
/*
B
Similarly, the enrichment of both stages operating together is determined by :
1.0 0.5O L
(7) where H e c = ffow rate out of the GC HeMs = flow rate of helium to the mass spectrometer From Ni and NT, the enrichment of the second stage alone can be calculated :
where
TCI = area of GC peak prior to entering membrane separator TG = area of GC peak after exiting membrane separator The yield of the two stages operating together is determined by the relation:
(4) where Qc
= sample size during calibration =
The yield of the second stage can be calculated since YI and YT are determined experimentally,
Yz
YT
= Yl
428
NT
= -
N,
The yield and enrichment of the two-stage separator was measured at 100, 150, and 200 "C with chromatographic flow rates of 10, 30, and 60 cm3/min. Compounds including hydrocarbons, ketones, alcohols, methyl esters, amines, and some halogenated materials were used to measure efficiency and enrichment. RESULTS AND DISCUSSION
sample size during test s = split ratio during calibration At = area of peak recorded by ion monitor during test A , = area of peak recorded by ion monitor during calibration Qt
Nz
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
The total amount of helium carrier gas transmitted through the silicone membrane is essentially independent of flow rate. This observation is illustrated in Figure 3A; a t 100 OC, 2.3 cm3/min of helium passed through a 20 cm2 membrane with carrier gas flow rates varying between 10 and 60 cm3!min. However, the helium flow through the membrane varies markedly with temperature, increasing to approximately 4 cm3/min at 200 O C . The flow through the first stage as a function of temperature is shown in Figure 3B. Unless the pumping capacity of the mass spectrometer is sufficient to reduce the pressure in the ion source of the MS to less than
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HEXANE TRICHLOROETHYLENE OCTANOL C10 METHYL ESTER FLOW RATE 10 cm3,'MIN FLOW RATE 60 cm3,' MIN
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10-5 Torr with these helium flow rates, a second stage of carrier gas removal is needed. The yields and enrichments of the silicone membrane, the porous silver frit stage, and the combined two-stage separator at 100 OC for He flow rates of 10, 30, and 60 cm3/min for 12 different organic compounds are given in Table I. The enrichments and yields of the membrane separator are listed as Nl and Y1; N2and YZare the enrichments and yields of the silver frit; and NT and YT are the values for the two stages connected into one device. Although the data reported are specific for the design and materials used, the general trends and conclusions are valid particularly with respect to the silicone membrane separator. The enrichment with the silicone membrane ( N l ) increased with carrier gas flow rate for all compounds investigated except for the two low boiling point compounds, hexane and hexene. With a flow rate of 10 cm3/min, the yield ( Y l ) is greater than 90% for those compounds with boiling point greater than 140 "C. At a flow rate of 30 cm3/min, 90% of those compounds with boiling point greater than 170 "C passed through the membrane, i.e., Yl > 90. At a flow rate of 60 cm3/min, those compounds with boiling points higher than 195 "C had yields ( Y I )of 90 % or larger. The flow rate into the second separator stage, i.e., the silver frit, is independent of the G C flow rate and is less than 4 cm3/min even when the first stage is heated to 200 OC (see Figures 3 A and 3B). The silver frit separator is designed according to Blumer (8) to accommodate flow rates less than 5 cm3/min. Accordingly, both the enrichment (Nz)and yield ( Yz)of the second stage are essentially independent of flow rate when used in the two-stage device. For all the compounds investigated, the enrichments are between 1 and 2, and the yields are between 40 and 60 %. The yield ( Y T )as a function of temperature at two flow rates (10 and 60 cm3/min) for a few typical compounds is shown in Figure 4. Four different classes of compounds are represented: hydrocarbons, halogenated hydrocarbons, alcohols, and esters. The yields decrease linearly with temperature for all compounds studied except the methyl esters. The decrease in yield with increasing temperature is a result of the decrease in solubility of the organic compound in the membrane at elevated temperatures.
I
I
100
125
I
I
150 175 TEMPERATURE (OC)
I
200
i 5
Figure 5. Total enrichment of two-stage separator ( N T )as a function of the temperature of the first stage for helium flow rates of 10 and 60 cm3/min The enrichments ( N T )for the same four classes of compounds : hydrocarbons, halogenated hydrocarbons, alcohols, and methyl esters as a function of temperature at two flow rates are shown in Figure 5 . With the exception of hexane, the temperature variation is greater at the higher flow rate. The enrichment decreases rapidly with temperature ; the decrease arises from a combination of two effects: a decreased solubility of the organic in the membrane with increased temperature and increased transmission of helium through the membrane at elevated temperatures. As a general operating procedure to ensure maximum yield and enrichment, the separator should be operated at a temperature approximately 50 "C below the boiling point of the material to be transmitted. Therefore, it is desirable to temperature program the separator when working with mixtures containing compounds with a wide range of boiling points. The chromatographed peak must retain its shape as it passes through the separator to accurately correlate the MS with GC. In addition, since the MS sensitivity is concentration dependent, it is desirable to inject the chromatographed unknown into the ion source of the MS in as sharp a peak as possible. Thus, the GC/MS separator should neither distort nor spread the G C peak. The distortion in the-GC peak as a result of the two-stage separator was determined by comparing the peak width in TC1(Le., at the G C outlet) with that recorded with the total ion monitor of the MS. The peak width measured in the ion source is a result of 2 independent factors: peak distortion in the separator and distortion produced within the ion source. Therefore, the measured values represent an upper limit to the distortion produced by the separator itself. The shapes of the peaks were not altered by passing through the separator and, for most compounds, the peaks were only 10 to 20% wider in the MS than in the GC; although for a few materials, such as high molecular weight amines, the peak width may be increased as much as 50 %.
Several other membranes were tested in the first stage: 0.002-in. thick silicone, 0.005-in. thick silicone, 0.001-in. silicone irradiated with cobalt-60 y-rays to a total dose of 108 ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
429
rads, and a 0.001-in1 thick polycarbonate. The 0.002- and 0.005-in. silicone membranes were unsatisfactory because they produced more peak distortion while transmitting the same amount of helium carrier gas. The irradiated membrane was unsatisfactory because it was mechanically unstable and tore readily. The polycarbonate membrane was also unsatisfactory because it developed pin hole leaks when heated above 100 "C. There are several advantages inherent t o the two-stage separator containing a silicone membrane and a silver frit. First, the outlet of the chromatograph is at atmospheric pressure, thereby permitting a one-to-one correlation between GC alone and GCiMS studies. Second, the silicone membrane can be designed to transmit more than 90% of most organic compounds by proper control of its surface area and operating temperature. Third, the separator can be used with widely varying carrier gas flow rates permitting GC/MS operation with flow programming (15).
The yield of the first stage ranges from 90 t o 100% at flow rates from 5 t o 60 cma/min provided that the temperature of the separator is 50 t o 75 "C below the boiling point of the material t o be transmitted. The yields of the second stage are fixed between 40 and 60%. The yields of the two stages opperating together vary from 40 to 60 %. The enrichments of the first stage are between 2 and 20 depending upon the flow rate of the chromatograph. Enrichments of second stage are between 1 and 2. Enrichments of the two stages operating together are between 2 and 40. Peak broadening is not a major problem as the width of most chromatographed peaks only increases 10 t o 20z upon passage through the dual-stage separator.
RECEIVED for review November 3, 1969. Accepted January 12, 1970. Research conducted under the McDonnell Douglas Independent Research and Development Program.
Simple Electrode for Thin-Layer Electrochemistry James C. Sheaffer and Dennis G . Peters Department of Chemistry, Indiana Unioersity, Bloomington, Ind.
IN RECENT YEARS many papers have discussed the great versatility and wide applicability of thin-layer electrochemistry (1-9). A variety of electrode designs have appeared, including micrometer electrodes ( 2 , 3), thin-metal-film sandwich electrodes ( 4 , 6), a gold electron-microscope-grid electrode (7), and metal cylinder capillary electrodes ( I , 2,8). While the advantages of each type of electrode have been well documented, most designs are either difficult to construct or require unusual manipulative techniques. One finds that vapordeposited films are fragile and tend to peel, whereas liquiddeposited films may contain significant impurities (IO). Furthermore, the thin-layer cavity may be quite inaccessible for removal of contaminants from the electrode surface, for examination of electrolysis products, or for changing electrodes after construction. Since no commercial thin-layer electrodes are presently available and since each worker must devise his own, it is desirable to construct the simplest possible electrode consistent with thin-layer behavior and quantitative measurements. We describe a thin-layer electrode which may be easily assembled from readily available materials and used compatibly with ordinary electrochemical apparatus, but which requires only a few microliters of sample solution. (1) C. R. Christensen and F. C. Anson, ANAL.CHEM.,35, 205 (1963). (2) A. T. Hubbard and F. C. Anson, ibid., 36, 723 (1964). (3) J. E. McClure and D. L. Maricle, ibid., 39, 236 (1967). (4) L. B. Anderson and C . N. Reilley, J. Electroanal. Chem., 10, 295 (1965). ( 5 ) A. T. Hubbard and F. C. Anson, ANAL.CHEM., 38, 58 (1966). (6) A. Yildiz, P. T. Kissinger, and C. N. Reilley, ibid., 40, 1018 (1968). (7) W. R. Heineman, J. N. Burnett, and R. W. Murray, ibid., p 1970. (8) L. P. Zajicek, J. Electrochem. SOC.,116, 80C (1969). (9) C. N. Reilley, Rev. Pure Appl. Chem., 18, 137 (1968). (10) B. S. Pons, J. S. Mattson, L. 0. Winstrom, and H, B. Mark, ANAL.CHEM., 39, 685 (1967). 430
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
EXPERIMENTAL
Construction and Use of Thin-Layer Electrode. As shown in Figure 1, the thin-layer electrode is fabricated from a glass tube into which is inserted a short length of Teflon (DuPont) needle (Hamilton Co., Whittier, Calif.), the latter being flush with or protruding slightly beyond the end of the tube. The glass-Teflon interface must either be a solutiontight pressure fit or be sealed with a suitable material such as epoxy resin. An interchangeable r-shaped wire electrode extends the entire length of the glass tube (and slightly below the bottom of the Teflon needle) and is held in a reproducible position by a notch in the top of the tube. Thus, the thin-layer cavity is the space between the wire and the inner surface of the Teflon needle. It must be recognized that the thickness of the thin-layer cavity is not necessarily uniform. Although this latter characteristic does not preclude the use of the thin-layer electrode for a wide variety of conventional applications, specialized studies of electrodeposition phenomena and of electrode kinetics require invariant solution thickness. In use, the electrode is positioned so that approximately one fourth of the thin-layer region is immersed into pure supporting electrolyte solution contained in a conventional electrochemical cell ( I I ) with a platinum auxiliary electrode and a saturated calomel reference electrode (SCE). To remove and exclude oxygen from the system, the thin-layer electrode may be purged with nitrogen before each filling, and nitrogen can be passed continuously through a plastic tent built around the cell. To clean the thin-layer cavity, one withdraws the entire thin-layer electrode from the electrochemical cell, removes solution from the cavity by using a water aspirator, and rinses the cavity with water, supporting electrolyte solution, and the sample solution. Final filling is accomplished in one to two seconds by capillary action from a separate airfree solution containing the electroactive species. Although the depth of immersion of the electrode into the sample solution does not affect the filling, the top of the Teflon (11) J. J. Lingane, J. Electroanal. Chem., 1, 379 (1960).
