A Selective Glow Discharge Detector for Gas Chromatography George L. Evans E. I . du Pont de Nemours & Co., Inc., Marshall Research Laboratory, 3500 Grays Ferry Aue., Philadelphia, Pa. 19146 A VARIETY OF DETECTORS which depend upon the ionization of gases have been reported, and have been evaluated for both qualitative and quantitative gas chromatographic applications ( I ) . Of these, however, the glow discharge detector has received relatively little attention, probably because of its reputation for high noise and erratic behavior. The use of a glow discharge tube for gas chromatographic detection was first reported by Harley and Pretorious in 1956 (2). Pitkethly (3) and Fisher and McCarty (4) later described glow discharge detectors having high output levels and extreme sensitivity to paraffin hydrocarbons and permanent gases. However, the response of the glow discharge to organic compounds other than hydrocarbons has apparently not been studied. The present study was prompted by interest in selective detectors for use in qualitative gas chromatographic analysis. A small brass glow discharge detector was constructed, and its response to a variety of organic compounds was evaluated. Using stainless steel cathode and anode, good stability was attained, and high sensitivity to organic compounds was observed. Moreover, it was found that the response of the detector varied markedly depending upon the type of compound introduced. Although detailed evaluation is incomplete, preliminary data show that the response characteristics of the glow discharge detector differ from those of other ionization type detectors, notably the electron capture detector ( I , 5). Used in an appropriate dual detector system, the glow discharge detector appears promising for use in qualitative gas chromatography. EXPERIMENTAL
Apparatus. Figure 1 shows a diagram of the gas chromatographic system. In many respects, the arrangement was similar t o that of Fisher and McCarty (4). Prepurified grade nitrogen, reduced to 20 psig, and passed through a Molecular Sieve 5A trap, was controlled by a needle valve to provide the desired gas flow through the column, measured at the exit. The gas chromatographic column was made of stainless steel 183 cm long and 3.2 mm o.d., packed with 10% Apiezon L on Gaschrom Q. Column and detector were enclosed in an oven maintained at 125 "C, and the injection port was maintained at 150 "C. Nitrogen flow through the column was 30.5 cc/minute, of which 18.5 cc/minute passed through the detector, and the remainder vented to the atmosphere. The vacuum required for operation of the glow discharge detector was provided by a Hyvac pump. A one-gallon glass surge tank, enclosed in a metal container, proved adequate to damp out pressure fluctuations caused by the pump. The needle valve off the line between the surge tank and pump served as bleed to allow pressure adjustment. A Nupro Type B-1M metering valve with micrometer handle was connected to the detector through a 30-cm length (1) J. E. Lovelock, ANAL.CHEM., 33, 162 (1962). (2) J. Harley and V. Pretorius, Nature, 178, 1244 (1956). (3) R. C. Pitkethly, ANAL.CHEM., 30, 1309 (1958). (4) E. R. Fisher and M. McCarty, ibid., 37, 1208 (1965). (5) J. E. Lovelock and S . R. Lipsky, J. Am. Chem. SOC.,82, 431 (1960),
1 142
ANALYTICAL CHEMISTRY
COLUMN r E F F L U E N T SPLITTER
y-TRAP
,,
i,
1
METERINGJ VALVE
PREPURIFIED NITROGEN
SURGE
/-BLEED
VALVE
VAC. PUMP
Figure 1. Gas chromatographic system of l/la-inch o.d., 0.010-inch i.d. stainless-steel capillary tubing. This valve, when opened 10-14 micrometer divisions, served to control the required air supply to the detector at a rate sufficient to maintain stable discharge. The air metered into the cell had a relative humidity of about 10% at 24 "C. Figure 2 shows a diagram of the glow discharge detector. The cell was constructed of brass, with insulators made of Teflon (Du Pont), and stainless steel electrode faces. The round cell housing was 22.2 mm in diameter, and 32.6 mm long. The cell cavity was 10.8 mm in diameter, and 10 mm long, and had a calculated volume of about 0.75 cc. The cathode insulator made of Teflon (Du Pont) contained a tight-fitting brass terminal, into the end of which was screwed a flat faced 316 stainless steel electrode having an exposed edge 1.42-mm thick and 4.65 mm in diameter. The exposed section of terminal between insulator and electrode was protected by a piece of 5-mm borosilicate glass tubing t o suppress spurious discharge and increase leakage path. The anode insulator made of Teflon (Du Pont) was 6.35 mm in diameter, and contained a tight-fitting 4-40 threaded brass terminal which allowed convenient adjustment of interelectrode spacing. The 1.58-mm diameter, 0.5-mm thick 303 stainless steel electrode face screwed into the end of the terminal.
VACUUM
-
LOD Figure 2. Glow discharge detector
HEATHKIT I P - 3 2 POWER SUPPLY
I35 VA
VOLTAGE TRANSFORMER
DETECTOR
E T H Y L PROPIONATE
1;
n-OCTANE
n-OCTANE
n-OCTANE
.1
i l /I I/
I watt
n-OCTANE
n- 0 C TA N E
CRYLO-
Y
1 *-
T O RECORDER
Figure 4. Gas chromatograms of binary mixtures
Figure 3. Electrical circuit
The anode insulator was machined with a groove around the circumference which connected via a small radial hole with an axial hole drilled t o clear the threaded terminal. This arrangement allowed air t o be fed into the interior of the cell while maintaining a n otherwise air tight system. The insulators were constructed with rims which could be compressed against narrow 0.3-mm high ridges machined into the cell housing by moderate tightening of the screws securing the compression rings. This arrangement provided a convenient and effective leak tight seal. The vacuum connection, made of 6.35-mm 0.d. thin-walled brass tubing, was soft soldered t o the cell. The sample inlet jet, directed between the electrodes, was made by flame-constricting a 1.5-mm 0.d. glass melting point capillary so that nitrogen at atmospheric pressure passed through the orifice a t a rate of 18.5 cc/minute when the cell pressure was 4.5-mm Hg. The capillary was cemented into its brass fitting with epoxy. Figure 3 shows a schematic of the electrical circuit used with the detector. The detector housing was well grounded to eliminate shock hazard caused by an occasional internal arc to the cell wall. The detector was operated in a bridge circuit containing 10-turn potentiometers in opposing arms. Potentiometer RI was used to sample the detector signal, and R2 t o balance out the quiescent signal and zero the recorder. A vacuum tube voltmeter (11 megohm input impedance) was connected across RI to monitor detector output. The bridge output was attenuated as required for presentation t o the 5-mV recorder. With the instrumentation described above, 26 binary mixtures were chromatographed. Each contained n-octane as one component, and was prepared with component weights such that a n approximately 50/50 w/w mixture resulted. I n each case, a 0.4-pl sample was chromatographed, and each peak recorded, under the following conditions: cathode voltage, -400 (relative to ground); cathode current, 0.5 mA (measured across RI); electrode spacing, 1.27 mm; detector pressure, 4.5 mm H g ; RI setting, 1000 ohms; and attenuator setting, 200 x, The area of each recorded peak was measured with a planimeter, and a detector response factor calculated for each test compound relative to n-octane reference using the expression Response factor
=
(Wt n-octane) x (area of component) (Wt component) x (area of n-octane)
RESULTS AND DISCUSSION
A tabulation of detector response factors of the compounds tested relative t o the n-octane reference is shown in Table I. The peak caused by n-octane in each chromatogram was slightly tailed, whereas the peaks of those test compounds which gave relatively large responses were sharp and well defined. Those compounds which showed negative or small positive responses gave imperfectly shaped peaks which were difficult t o measure, but still useful. Several of the chromatograms are shown in Figure 4. With the stainless steel cathode, a noise band of 10 mV was observed during a 10 minute period. The sensitivity of the detector to n-octane was calculated to be about 17 coulombs/g, (corresponding to an
Table I. Response Factors Relative to n-Octane
Compound Acetone Acrylonitrile Allyl acetate Allyl alcohol Allyl ether Benzene n-Butyl bromide n-Butyl chloride Carbon disulfide 1,l-Dimethoxyethane 1,2-Dimethoxyethane Ethanol Ethyl acetate Ethyl acrylate Ethyl ether Ethyl formate Ethyl propionate Methallyl acetate Methyl methacrylate rz-Propyl acetate n-Propylamine Propylene oxide Styrene Tetrahydrofuran Thiophene Vinyl acetate
Response factor Mole basis 0.43 0.22 0.13 0.06 0.54 0.47 0.53 0.27 0.48 0.41 0.12 0.08 0.16 0.19 -0.15 -0.14 -0.57 -0.39 0.62 0.49 0.75 0.59 0.55 0.28 0.57 0.44 0.73 0.64 0.38 0.35 0.41 0.27 0.76 0.68 0.59 0.59
Weight basis
0.53
0.73 0.66 0.44 0.81 0.58 -0.20 0.42
0.46 0.65 0.34 0.22
0.74 0.37
-0.14
VOL. 40, NO. 7, JUNE 1968
0.32
1143
Table 11. Response Factors of Homologous Alcohols Relative to Methanol Response factor Alcohol Weight basis Mole basis Methanol 1 .o I .o Ethanol 0.8 1.1 1-Propanol 1 .o 2.0 1-Butanol 1.3 3.1 1-Hexanol 2.7 8.6 1-Octanol 3.8 15.4 ionization efficiency of 2 7 3 , and the limit of detection, Qo, was found t o be 8 X 10-6 mg/ml(6). The response factors of the members of a n homologous series of alcohols relative t o methanol were also determined, and are shown in Table 11. Although the stainless steel anode was used throughout, several types of cathode materials and shapes were evaluated to determine their effects on glow discharge stability, sensitivity, noise, and ease of starting. Cathodes of high purity silver or aluminum gave frequent arcing, and a glow discharge was difficult t o obtain. After a glow discharge was finally obtained with such cathodes, it was stable only over a narrow voltage range. Considerable sputtering was also evident with the silver cathode. With various iron or stainless steel cathodes, cold starting of the glow discharge (for example, at the beginning of the day) was a random process, and frequently required 15-20 minutes. Once started, however, the discharge was stable, reasonably quiet, and operable over a range of 360-400 V, depending upon electrode configuration, spacing, and pressure. After running for some time, the discharge could be extinguished then restarted more readily. It was eventually found that the discharge could be started immediately with such cathodes, even from a cold ~~~~
~
(6) S . Dal Nogare, and R. S . Juvet, “Gas-Liquid Chromatography, Theory and Practice,” Interscience, New York, 1962, p 181.
start, by introducing a small amount of acetone or 1,l-dimethoxyethane vapor into the cell through the air bleed line. Efforts t o obtain a glow discharge with helium carrier gas at 400 V were unsuccessful. It should, however, be possible to use helium, despite its high ionization potential (24.6 eV, compared t o 14.5 eV for nitrogen) if voltages somewhat above 400 are used. When only pure nitrogen was used as carrier, no discharge could be obtained with the stainless steel cathode. Under such conditions, however, introduction of a small sample of n-butane produced a discharge which disappeared as soon as the butane peak had passed. Continuous introduction of a small flow of moist air into the cell with the stainless steel cathode gave a stable discharge which stopped when a CaClz drying tube was inserted in the air line. Air saturated with water at room temperature produced a strong discharge, but the cell then failed to respond t o n-butane. Apparently excess moisture saturated the detector and desensitized it. When a n iron cathode (made from a common nail) was substituted for the stainless one, introduction of dry air was sufficient and necessary to produce discharge. The discharge was somewhat sensitive to pressure and voltage fluctuations, but signal output variations caused by these factors were not troublesome. Although the detector was not continuously exposed to organic vapors over a long period of time, cathode fouling was not a problem during the period of study. The stainless steel cathode did acquire a bronzed appearance o n part of its surface, but the central area directly exposed to the full discharge remained clean and bright. The introduction of (moist) air into the detector may have served t o reduce cathode fouling, but did not prevent formation of a tenacious black deposit (possibly carbon) on the face of the anode. The effect of oxygen upon the response of the detector was not studied.
RECEIVED for review December 1, 1967. Accepted March 1 1 , 1968.
Separation of Resin from Fatty Acid Methyl Esters by Gel-Permeation Chromatography Duane F. Zinkel and Lester C. Zank Forest Product Laboratory, Forest Serrice, U. S . Department of Agriculture, Madison, Wis. 53705
ANALYSIS OF COMPLEX MIXTURES of resin and fatty acids for the individual constituents by gas-liquid chromatography (GLC) is complicated by the overlap of the two solute systems. A two column analysis on diethyleneglycol succinate (DEGS) and a modified SE-30 packing was suggested as a practical approach to the problem ( I ) . The object of this study was t o achieve the quantitative group separation of fatty acid methyl esters from resin acid methyl esters by gel-permeation chromatography (GPC); this separation would simplify and improve the subsequent gas chromatographic analysis. One of the earlier reports on the application of the GPC technique t o the separation of low molecular weight materials is the paper by Cortis-Jones (2). Bartosiewicz (3) has separated fatty acid polymers into monomer, dimer, and trimer fractions. Cazes and Gaskill ( 4 ) recently investigated the G P C separations of low molecular weight, oxygen-containing 1144
ANALYTICAL CHEMISTRY
substances, including a number of saturated fatty acids. Chang ( 5 ) has developed a GPC method for the group analysis of tall oil as total resin acids, fatty acids, fatty acid dimers, and resin acid dimers. EXPERIMENTAL
A jacketed glass column cooled with tapwater (13 “C) and fitted with a reservoir was used. For most of the work, a column was gsed which contained a bed of 1 X 200 cm of Styragel, 40 A porosity, particle size of 37-70 p (Waters As(1) F. H. M. Nestler and D. F. Zinkel, ANAL.CHEM.,39, 1118 (1967). (2) B. Cortis-Jones, Nature, 191,272 (1961). (3) R. Bartosiewicz, J. Paint Technol., 39, 28 (1967). (4) J. Cazes and R. Gaskill, Separation Sci., 2,421 (1967). (5) T. Chang, ANAL.CHEM.,40, 989 (1968).
