An Auto-Zero and Base-Line Drift Corrector for Gas Chromatography W. A. Riggs Shell Oil Co., Houston Research Laboratory, Box 100, Deer Park, Texas 77536
INTHE OPERATION of a gas chromatograph, adjustment of the zero or base line must be made at least once during an analysis. If temperature programming is used, an upward drift in the base line occurs during the analysis. Precise manual adjustment of the base line in this case is very difficult. Although most commercial integrator-digitizers provide auto-zero and base-line drift correction functions, many gas chromatographs use nothing more than recorders, possibly with mechanical integrators, for readout. Operation of these simple systems can be much more convenient if an auto-zero system is included. Base-line drift correction is especially useful with temperature programming and may be necessary when the output signal is to be integrated. The instrument described here provides an auto-zero function and correction of base-line drift during isothermal or programmed temperature gas chromatograph operation. The device described was designed especially for use with the 0-10 volt output of an electrometer used with a flame ionization detector in a capillary gas chromatograph, but it can be applied to other detector systems if the signal levels are above 1 volt. Circuit Description. Figure 1 shows a complete schematic diagram of the auto-zero and base-line drift corrector. The input signal (the chromatograph detector signal) is passed through a follower amplifier (Al) to the output, Capacitor C2 is placed in series with the signal input. The output voltage is therefore the sum of the signal input voltage and any voltage across C2. This permits an arbitrary zero level to be established at the output by adjusting the charge on capacitor C2. Amplifier A2 continuously compares the output signal with a manually adjustable zero reference voltage which appears across resistoi R6. The reference voltage is adjustable through a range of +300 mV by potentiometer R11. The output of amplifier A2 will become positive when the output of A1 (the drift corrector output) becomes negative relative to the reference voltage. As the output of amplifier A2 becomes positive, current Aows through resistor R3 and diode D1. This current changes the charge on capacitor C2 and thus prevents the drift corrector output from going further in the negative direction. The drift corrector output is permitted to become positive with respect to the reference voltage because, at that time, the output of amplifier A2 will be negative and diode D1 prevents current flow from the output of amplifier A2 to capacitor C2. The action of A2 prevents the output from drifting below the reference level set by R11. As long as the downward drift rate exceeds any upward drift present in the input signal, the output signal will be held at the reference level when at base-line. The downward drift rate can be adjusted by means of potentiometer R10 and should be set to a value which is slightly greater than the maximum positive drift that is anticipated during the analysis. The downward drift necessary to hold the signal at the base line between signal peaks also causes a small loss in peak area, The advantage gained in using a base-line correcting device of this type lies in the fact that the error introduced in the peak integrals is small compared to the error due to base-line drift accumulated during an analysis. It is assumed that the 976
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ANALYTICAL CHEMISTRY, VOL. 43,
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Figure 1. Circuit diagram, auto-zero and base-line drift corrector a. A1 and A2 are Analog Devices Model 40K b. Power supply is Analog Devices Model 904 or equal C. Diodes D1 and D2 are Fairchild d. Diode D3 is Texas Instruments e. Resistor R2 is Victoreen, f 5 f. Except as noted, resistors are W, f 5 %, carbon g. Potentiometers are 10-turn wirewound h. Capacitors are f 5 %, 50 W VDC, Mylar
signal returns to base line many times during an analysis. The downward drift which is introduced during each peak is completely removed each time the signal returns to base line. Application. The base-line drift corrector may be applied to any chromatograph having a high level (0-10 V range), low impedance output signal-e.g., an electrometer output. Installation consists simply of breaking the signal line and inserting the drift corrector between the chromatograph and the recorder or integrator. If it is desired to use the baseline corrector with low signal levels, for example those from a thermal conductivity detector, the signal must first be amplified. In order to function correctly, the signals must be free of noise. Since the drift corrector automatically adjusts the base line so that no signal can cause the output to go below the point to which the base line was adjusted by the zero balance control, the presence of noise in the signal will cause the base line to appear to be elevated from the desired value. A low-pass signal filter with a time constant of approximately 0.25 second is built into the input circuit to help eliminate high-frequency noise in the signal. If this time constant is objectionable, the value of capacitor C1 may be changed. Should the circuit be desired only as an auto-zero, the drift correcting function can be completely removed by re-
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). CHEM., 25, 1608 (1953). (3) H. R. Hoekstra and J. J. Katz, ANAL. (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.
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