compounds. This is attributed to the low sample pressure, the high flow rate, and the very small surface area of the membrane. CONCLUSIONS
The system permits G C operation independently or simultaneously with MS scanning at maximum chromatographic efficiency. The elution of a G C peak can be observed both on the hydrogen flame recorder and the total ion current monitor of the MS; the latter is disabled during an MS scan but the flame signal can still be monitored. The silver membrane carrier gas separator is easily constructed from standard commercial parts. The rigid metal construction requires no glass blowing or soldered or brazed joints; all parts are easily exchanged within minutes and contaminated flow lines may be disassembled and cleaned ultrasonically in a solvent bath. Compared to previous designs, the active area of this exchanger has been reduced by more than two orders of magnitude. The thickness of the silver membrane and therefore the surface area of its internal pores is another two orders of magnitude less than that of the glass frits previously used. As a consequence, this design has led to a striking reduction in adsorptive losses, in dead volume, in peak delay and peak broadening. The design is flexible and should permit operation outside the conditions described here. Parts List 202-1-316 Nuts, 16 reqd. 402-1-316 Nuts, 10 reqd. 812-1-316 Nuts, 1 reqd.
200-6-316 Unions, 2 reqd. 400-6-316 Unions, 2 reqd. 200-R-4-316Reducers, 8 reqd. 810-6-4-316Reducing union, 1 reqd. 200-3-316 Union Tee, 1 reqd. 400-3-316 Union Tee, 1 reqd. All fittings Swagelok, Crawford Fitting Co., Solon, Ohio Silicone Rubber 0-Ring, ‘/*-inch i.d. and brass backup ring (Varian Aerograph, Walnut Creek, Calif.) Teflon washers, 2 reqd., see text Silver membrane, 1 reqd. see text TI = 11-cm Tubing, l/s-inch o.d., 0.049-inch wall, packed with 100-140 mesh glass beads, siliconized, see text Tz = 3-cm Tubing, l/e-inch o.d., 0.049-inch wall, packed with 1OC-140 mesh glass beads, siliconized, see text T3 = Tubing, l/s-inch o.d., 0.049-inch wall Tubing, ’/*-incho.d., 0.015-inch wall Tubing, l/a-incho.d., 0.065-inch wall All tubing 304 stainless, Tube Sales, Englewood, N. J. VI = Shutoff valve, No. 413 HT, Hoke, Inc., Cresskill, N. J. V z = Bellows metering valve, SS-4BMW, Nupro Co., Cleveland, Ohio Gas chromatograph oven, e.g., Aerograph Model 550-B (Varian Aerograph, Walnut Creek, Calif.) with flame detector, separate amplifier, and recorder. Temperature programmer optional Enricher Oven, heatable to 300 “C, forced circulation. Vacuum pump, e.g. Welch Model 1400, Duo Seal, Welch Scientific Co., Skokie, Ill. RECEIVED for review February 26, 1968. Accepted May 8, 1968. Supported by ONR (N0014-66-contract CO-241), by NSF (GA-539) and by API (85 A). Contribution No. 2080 of the Woods Hole Oceanographic Institution.
Induction Heating in Zone Melting of Organic Compounds Henry Plancher, J. C. Morris, and W. E. Haines Laramie Petroleum Research Center, Bureau of Mines, U.S . Department of Interior, Laramie, Wyo. INDUCTIONGENERATOR.The induction generator, deAN INVESTIGATION was undertaken to study the feasibility of signed for continuous use, has an output of 2.5 kW, and the using induction heating in the zone melting of organic compower output frequency is a nominal 450 kHz. The capacity pounds. In using induction heating in the processing of of the generator is sufficient to produce simultaneously molten organic materials, a conductor is employed as an intermediate zones in 10 samples with melting points as high as 440 “C. heating device or susceptor. Pfann (I) suggested that the LOADCOILS. The pancake, or plate-concentrator, load susceptor might be imbedded in the sample when using this coil with two loops is shown in Figure 2. Plate coils were heating technique for zone melting. used because they concentrate the radio frequency field in a This investigation was part of a search for a means of proshallow horizontal plane, conducive to flat, low-volume molten zones. Coils were made from 3/16-inchcopper tubing ducing and maintaining low-volume molten zones necessary in which the inner turn of the planar spiral was silver-soldered for using zone melting as a separation tool for complex mixto the l/s-inch copper sheet that serves as the secondary tures such as petroleum fractions and residues. A theoretical inductor or base. A radial slot was cut through the secondary discussion of such a process has been published (2), and the inductor to prevent electrical continuity about the periphery. most difficult of the prescribed conditions is maintaining lowThe coil assembly was insulated with Glyptal, and the genvolume molten zones. Attempts to reduce zone lengths in erator leads were further protected with asbestos or rubber conventional, externally heated systems have been reported tubing. ( 3 , 4 ) ;however, molten zones with lengths less than the radius For the work reported here, various sizes of coils were used of the container have been difficult to maintain. to accommodate tubes of different diameters, and as many as A technique and an apparatus for using induction heating 10 coils were connected horizontally in series, permitting up to 10 samples to be processed simultaneously. with an imbedded susceptor to produce and maintain lowTUB=. Glass tubes ranging from 1 to 10 cm in diameter volume molten zones in the zone melting of organic materials and of various lengths were used. Precision-bore glass tubes have been devised. were used to permit closer tolerances between the susceptor EXPERIMENTAL and the tube. For zone melting under vacuum, a side-arm Apparatus. Figure 1 is a schematic drawing of the ascontaining susceptors for the desired number of passes of a sembled apparatus. molten zone was attached near the top of the tube. Susceptors were disks of 1%mesh, galvanized SUSCEPTORS. (1) W. G. Pfann, “Zone Melting,” Wiley, New York, 1958, p 75. wire screen whose cross linkages were soldered to ensure (2) W. G. Pfann, ANAL.CHEM., 36,2231 (1964). rigidity. The disks were cut with a punch and die 0.39 mm (3) F. Ordway, ibid., 37,1178 (1965). smaller than the i.d. of the tube to be used. (4) W. G. Pfann, “Zone Melting,” 2nd ed., Wiley, New York, Each susceptor was balance-tested and matched to the tube. 1966, p 97. 1592
ANALYTICAL CHEMISTRY
R a d i a l slot i n secondary inductor \ I Dcum
Pulloy
I
Cable
as shown
7'p
be holder
I
I irG'asr lube
OD tubinq P r i m a r y inductor
Induclibn generalor
v
Secondary inductor/ W
Figure 1. Apparatus for zone melting using induction heating Only those susceptors that consistently remained horizontal as they sank through a viscous oil in the tube were used in this work. The selected susceptors were gold-plated to avoid contamination of the sample. TUBE-RAISING MECHANISM.Tubes were raised through the coils using the windlass and pulley system shown in Figure 1. The windlass consisted of a 2.54-cm drum attached to a Troemner 22-speed Instrument Drive. In the experimental runs, speeds ranging from 7.24 to 121.6 mm per hour were used. Flexible 3/64-in~h stainless steel cables connected the drum to brass weights which were threaded to accept the tube holders. TUBEHOLDERS.Tube holders were male Swagelok fittings with Teflon ferrules to hold the sample tubes. This device permitted rapid attachment or removal of the sample tubes. The weight of the fittings contributed to the stability of the tubes as they hung within the load coils. Procedure. A susceptor is placed on top of a solidified column of sample contained in a precision-bore glass tube closed at the bottom. The tube is suspended vertically through the load coil of the induction generator at a level that places the susceptor about 2.5 cm below the plane of the load coil. The tube is raised through the energized coil at a selected constant speed. As the susceptor, supported by the solidified sample, approaches the high-energy field of the coil, it becomes heated by the induced current and melts the top of the sample column. The susceptor sinks through the molten material, remaining below the coil at a distance at which the susceptor receives sufficient energy to melt the sample. The material above the susceptor cools and solidifies. As the tube is pulled upward through the coil, the susceptor and the associated molten zone pass through the column. When the susceptor reaches the bottom of the tube, the tube-raising mechanism is stopped to prevent the susceptor from being drawn into the high-energy field of the coil causing overheating. The tube is lowered, another susceptor is placed on top of the solidified column, and a new molten zone is started as before. In vacuum operation, the susceptor is transferred from the sidearm to the top of the column by means of a magnet.
I
Mater ial-copper
Figure 2. Plate-type load coil RESULTS AND DISCUSSION
The plate-type load coils used with the induction generator produced low-volume molten zones that were easily controlled and reproducible in tubes up to 5 cm in diameter. Coils designed for tubes greater than 5 cm in diameter produced a magnetic field of uneven intensity. The field intensity was lowest at the center of the susceptor and highest at the outer edges. Intense heating at the outer edges of the susceptor tended to produce high-volume molten zones that were difficult to control. A variety of susceptor designs and materials were tested. Susceptors made of 18-mesh, galvanized wire screen were most consistent in producing low-volume (zone length usually about 1 mm) molten zones which were controllable and reproducible. Susceptors made from 316 stainless steel screen were also fairly consistent in these respects. However, those made from 40 M nickel steel, 430 stainless steel, and Nichrome produced molten zones that were erratic in size and shape. Perforated disks and wire screen with openings smaller than 18-mesh (about 1-mm openings) were unsatisfactory because they produced high-volume molten zones. Planar spirals did not heat uniformly and, owing to an apparent lack of balance, produced molten zones tilted from the horizontal. With any given material and design of the susceptor, zone lengths were dependent upon tube speed and diameter; increasing either of these factors increased the zone length. Vibrations of the susceptor, caused by electromagnetic induction, produced mixing within the molten'zone and contributed to the efficiency of the technique. When a smalldiameter tube was used, vibration of the tube itself was evident. The important feature of the technique is the inherent constancy of the length of the molten zone produced. Energy programming is not required. With the generator vernier set for maximum power output, the susceptor remains below the plane of the coil at a distance where the energy received is just sufficient to melt the sample, allowing the susceptor to sink VOL. 40, NO. 10, AUGUST 1968
1593
through the molten material. The susceptor thus approaches or falls away from the plane of the coil as the melting point of the sample mixture changes. Therefore, the zone length remains constant at the selected speed. RECEIVED for review February 28, 1968. Accepted April 29, 1968. Presented in part, Division of Petroleum Chemistry,
American Chemical Society, March 1966, Pittsburgh, Pa, Work upon which this report is based was done under cooperative agreements between the Bureau of Mines, U. S. Department of the Interior; the American Petroleum Institute; and the University of Wyoming. Reference to specific brand names is made for identification only and does not imply endorsement by the Bureau of Mines.
Linearization of Electron Capture Detector Response by Analog Conversion D. C. Fenimore Texas Research Institute of Mental Sciences, Houston, Texas 77025
Albert Zlatkis and W. E. Wentworth Department of Chemistry, University of Houston, Houston, Texas 77004
THERELATIVELY NARROW linear response range of the electrpn capture detector is often a disadvantage when this device is used in quantitative gas chromatographic analyses. This problem is particularly acute in multicomponent analyses where the electron capture coefficients and concentrations of the individual compounds can differ by many orders of magnitude. To improve the accuracy and facility of electron capture quantitation, we have devised an analog converter by which the amplified detector signal may be recorded as a linear function of sample concentration over an appreciably extended range. The electron capture detector responds to the presence of species having an affinity for thermal electrons by exhibiting a decrease in detector current ( I ) . This is normally displayed by subtracting the detector current, I,, from a constant applied current, 1 0 , of a magnitude equal to 1 8 when only the carrier gas is flowing through the detector-ie., 10 - I, = 0 when no capturing species are present, and sample response is then recorded as a departure from zero base line. The response is quite linear with sample concentration if Ib - I , is very small with respect to l a , but when l a - l e exceeds approximately 1.0% of the value of 10, nonlinearity becomes apparent. As the response approaches the value of the total deRctor current, further increases in sample concentration produce undiscernible changes in response and the detector is said to be saturated. Unfortunately, this region of nonlinearity is the most useful portion of the response curve with regards to absence of detector and electronic noise. Originally the relationship between vapor concentration and the observed current change in the pulse sampled electron capture detector was believed to be similar to that of Beer’s law for light absorption (2). In studies of the kinetics of this mode of operation, however, Wentworth and his coworkers (3) derived the following expression : (Ib
- le)/Ie = Ka
(1)
where K is the electron capture coefficient of a given substance and is similar to the relative coefficient described by Lovelock, (1) J. E. Lovelock and S. R. Lipsky, J. Am. Chem. SOC.,82, 431 (1960). (2) J. E. Lovelock, ANAL.CHEM., 35, 474 (1963). (3) W. E. Wentworth, E. Chen, and J. E. Lovelock, J. Phys. Chem., 70,445 (1966). 1594 *
ANALYTICAL CHEMISTRY
8 .O
7.0 6.0
5.0
4.0
3.0
2.0 I .o
0
1.0
2.0
3.0
CONCENTRATION(ARBITRARY UNITS)
Figure 1. Fraction of standing current, (Ib - Is)/&, and linear response function, (I* - Ze)/Io us. concentration and a is the instantaneous sample concentration. A comparison of the above function with the normally displayed l a I , is shown in Figure 1. Here, for the purpose of illustration, the Wentworth function is assumed to be linear and the normal response is derived and shown as (IO - Ie)/Io, i.e., as a fraction of the total standing current. At small values of - Ze the two functions are seen to converge, but when Ib - Ie is 10% of the total standing current, this response is in error by 10% compared with the linear function. A chromatographic fraction is eluted in a finite time, and the vapor concentration varies during this interval. Conversion by means of the above expression should therefore be per-
