moving resistors R2, R9, R10, and R4 and diodes D1, D2, and D3. The circuit as shown in Figure 1 is designed to operate with signals of the polarity shown at the input terminals. If operation with signals of the opposite polarity are to be handled, the following changes must be made: Reverse diodes D1, D2, and D3; and connect R9 (27 K) to ( f l 5 V) instead of (- 15 V). Operation. Before an analysis is started, the zero switch should be closed and the output set to the desired zero level by manually adjusting the zero reference level control, R l l . After the initial zero adjustment, the drift should be set by adjusting R10 to provide a rate just sufficient to overcome any upward base-line drift produced by the chromatograph.
If a downward drift in the chromatograph signal is to be corrected for, the signal itself will provide the necessary negative drift, and potentiometer R10 may be set at the lower end of its range. Zeroing during an analysis may be accomplished by closing S1 which bypasses diode D1 and causes the output to move immediately to the reference level set by potentiometer R11. Because of this, the electrometer or chromatographic detector signal zero level is no longer critical and need not be adjusted unless zero offset is enough to cause nonlinearity. RECEIVED for review October 19, 1970. Accepted February 12, 1971.
An Improved Kel-F Vacuum Valve T. A. O’Donnell Department of’Inorganic Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia
WHILETHE STRUCTURAL material polychlorotrifluoroethylene (Kel-F) may be regarded as useful in the manipulation of many reactive and corrosive liquids and gases, its use is almost indispensable when quantitative studies are being made involving fluorides such as hydrogen fluoride, the halogen fluorides, and most of the volatile fluorides of metals and nonmetals. Typical procedures, reviewed recently ( I ) , include measurement of conductance and of absorption and resonance spectra. Recently, simple Kel-F sample holders have been used for Laser Raman spectrometry (2). Halogen fluorides have been used for determination of oxygen in inorganic ( 3 ) and organic (4) compounds. Canterford and O’Donnell ( I ) have described the several components of a typical vacuum system constructed almost entirely from Kel-F, the only other material exposed to reactive liquids or vapors being the softer polymer Teflon (Du Pont) used in 0 rings and sealing glands. An essential feature of such a system is the Kel-F vacuum valve which appears to have been developed in its earliest form by Kilpatrick and coworkers at Illinois Institute of Technology. In the valve as originally reported (5), the Kel-F needle moved on a thread machined internally in the Kel-F body of the valve. The external vacuum seal was made by a circular Teflon gland, triangular in section, compressed on to the flat upper surface of the body and on to the needle by a brass nut which turned on the threaded outer surface of the upper section of the Kel-F body. Vacuum connections to the valve were made by brass nuts which compressed flared Kel-F tubing on to male tapers, machined in the Kel-F at either end of the valve. These nuts turned on Kel-F threads behind the male tapers. All of these features can be seen in Figure 2 of the original paper (5). In service, this design suffered from two major defects. The three brass nuts turned on soft Kel-F threads which were easily stripped. Also, scoring and de(1) J. H. Canterford and T. A. O’Donnell, in “Technique Of Inorganic Chemistry,” H. B. Jonassen and A. Weissberger, Ed., Vol. VII, Wiley, New York, N. Y . , 1968, pp 273-306. (2) R. J. Gillespie and M. J. Morton, Znorg. Chem., 9,616 (1970). (3) H. R. Hoekstra and J. J. Katz, ANAL. CHEM., 25, 1608 (1953). (4) I. Sheft and J. J. Katz, ibid., 29,1322 (1957). (5) M. E. Runner and G. Balog, ibid., 28,1180 (1956).
formation of the needle and radial splitting of the circular valve seat occurred, especially with aging, as the needle was forced into position. To a great extent, each of these defects was reduced or eliminated by modifications of the basic design made at the Argonne National Laboratory, Ill. Hyman and Katz, from that laboratory, described the application of the valve to apparatus for handling anhydrous hydrogen fluoride and gave photographs of the valve (6). A detailed drawing of the valve at that stage of development is given as Figure 10 in the review by Canterford and O’Donnell ( I ) . Brass (or other metal) split collars were used to “back up” the two male tapers by which the valve was connected to flared Kel-F tubing. As a result, the tightening nuts turned on durable brass threads rather than on soft, easily damaged Kel-F threads. Also, the tip of the Kel-F needle was fitted with a Teflon ring. Contact for the on-off seal was made between soft Teflon and Kel-F rather than between two hard Kel-F surfaces, as in the earlier design. Each of these modifications materially lengthened the usage time for the valves. Subsequently, two further major modifications were made at the University of Melbourne. The Teflon sealing gland, which had been retained in the Argonne design was replaced by an 0 ring, with two direct advantages. First, an 0 ring requires less compression for a reliable vacuum seal than a massive gland. Second, the last load-bearing Kel-F thread, that for the nut which had been used to compress the gland, was removed. The two brass collars for the flare connections were completely re-designed so that one of them now provided a brass thread for the compression nut for the sealing 0 ring. Under these conditions, the Kel-F becomes simply a lining material for the mechanically-strong structure, In addition, an important usage modification was made. In all earlier designs, the external seal was located above the threaded section of the needle. Therefore, it was possible that, in handling liquids, material could be trapped above the (6) H. H. Hyman and J. J. Katz in “Non-Aqueous Solvent Systems,’’ T. C. Waddington, Ed., Academic Press, London, 1965, pp 47-8 1.
ANALYTICAL CHEMISTRY, VOLi 43, NO. 7, JUNE 1971
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n I
-KNOB
n
>-Kel-F
Kel - F
NEEDLE
TEFLON
RING
m-
TEFLON '0' RI
BRASS NUT
F.7
J-BRASS m-'0'
NEEDLE
BRASS SLEEVE
SLEEVE
TEFLON
RING
RING
BRASS JACKET
i upper1
a -Kel-F
BODY
Figure 1. Exploded view of valve components
thread and could contaminate solutions passing through the valve subsequently. In the Melbourne modification, the 0 ring was placed below the thread on the needle. A brass sleeve, which is not in contact with corrosive liquids or vapors, then fulfills two roles. First, pressure from the tightening nut is transmitted through it to the 0 ring seal. Additionally, being threaded internally, it carries the needle. A pin in the brass sleeve fits into a slot in the Kel-F valve body and prevents rotation of the sleeve as the needle is raised or lowered. All of these features are shown in Figures 1 and 2. Many variations of this basic design are possible. For example, if connections other than flared fittings are required, the Kel-F body of the valve can be machined with Swageloktype female connections so that tubing of any material with l/Anch 0.d. (or any other pre-determined 0.d.) can be vacuumsealed directly to the valve. Of course, the metal split collars need to be modified accordingly. It will be appreciated that, if a Kel-F valve (as shown in Figures 1 and 2) is to be connected to a larger or fixed com-
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Assembled valve without, brass jacket, and knob
nut
Figure 2. Section of assembled valve without ancillary fittings ponent of a vacuum system, the valve must be rotated relative to that component in order to seat the male taper in the corresponding female taper in the other component. If the valve must be connected without change in orientation relative to the other component, a freely-rotating split nut can be fitted to the split collar so that the male taper can be drawn into position without rotating the whole valve. Each of these last two modifications was made while the author was on leave at McMaster University, Hamilton, Canada. ACKNOWLEDGMENT
The author wishes to record gratefully the value of many discussions and of information freely supplied by Dr. M. Kilpatrick, formerly of the Illinois Institute of Technology and by Dr. H. H. Hyman and colleagues of the Argonne National Laboratory. Also Mr. E. Taylor and his staff of the Chemistry Department workshop at the University of Melbourne and their counterparts at McMaster University, Mr. S. Kocsis and staff, contributed to the present designs more than can be recalled.
RECEIVED for review November 20, 1970. Accepted January 14, 1971.
