Automatic Range Changer for Beckman GC-2 Gas Chromatographs D. J. Darling, F. D. Miller, R. C. Bartsch, and F. M. Trent, Research Division, N ROUTINE
chromatographic analyses
Iin this laboratory, man-hours were wasted and accuracy was sacrificed
through inability to predict the proper attenuation to produce a maximum, usable signal for each peak on the chromatogram. An automatic range changer was constructed TI hich would continuously monitor the signal so that a maximum on-scale peak height would always occur. With minor modifications this attenuator should be applicable to any gas chromatograph. A circuit diagram and a typical chromatogram are show in Figures 1 and 2. When the recorder pen is driven up scale b y the incoming signal, it actuates an Acro-switch ( a type of microswitch) upon reaching 95y0 of the full scale range. The Acro-switch places voltage on the stepping coil of the stepper switch. This in turn causes the slider arm of the switch to move to the second position, tapping less voltage off the voltage divider network, which decreases the signal input to the recorder. Because of the fast response of the recorder, this causes the pen to drive down scale until the differential voltage is overcome. If the signal to the recorder is still increasing and reaches 95% of the second scale range, it trips
U. S. Industrial Chemicals Co., Cincinnati, Ohio
the Acro-switch again and follows the same range-changing sequence. This sequence continues until the proper attenuation is reached. The attenuated signal appears a t l/*, l / b , I/lo,. l/20, I/EO, 1 / 1 ~ ~.or , l/2o0 of the full signal. After passing through the peak signal, the pen travels down scale with the decreasing signal. When the recorder pen moves to within 5 scale divisions of the base line, or zero position, a second Acro-switch is tripped, causing a, voltage to be placed on the reset coil of the stepper switch returning the slider arm to the home position. -4 thermal delay tube in this circuit disconnects the power from the reset coil TI hile the pen of the recorder is in the zero position, to avoid undue loading of the reset coil. A manual bypass push button for operating the stepper and reset is included. The second ITafer on the stepper switch denotes the range being used a t that time by snitching in pilot lamps on the control panel. This system can also be used a t a fixed attenuation, The recorder pen is moved off the clearing Acro-switch and the Acro-switch is locked closed. The desired attenuation is then set by the manual push button, and the recorder pen reset to zero. The attenuator can now either be left on, in which case the P,B 2
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Acro-switch must be kept locked shut, or turned off, in which case the Acrosnitch can be released. The resistor values given here apply only to a Beckman GC-2 with a Brown recorder. For other gas chroniatographs other resistors may be required, as it is necessary to match impedance between the detector and recorder. The attenuator supplied with the GC-2 must be disconnected for installation of this attenuator. The benefits of such a range changer are threefold. The unit is alway run a t a maximum sensitivity, which increases accuracy. Once the sample is introduced, the operator need not stand by the instrument to keep the pen on scale. Prior knonledge of the sensitivity requirements of any particular sample is not necessary. Results in this laboratory have shon n that such attenuated signals do not affect the accuracy of peak height measurements. although some difficulty may be encountered in area measurements. This latter difficulty can be overcome by running a t a fixed attenuation. It is not necessary to observe the range changer pilot lamps to determine the attenuation. With a knowledge of the attenuation factors, one needs only count the number of times the recorder changes scale to determine the range in nrhich the peak occurs. 2 x-
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Figure 1. Tube 1. sw 1,2. sw 3. P B 1, 2. RI, Rz. Ra. 1, 1s. Coil 1.
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Coil 2.
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Circuit diagram of automatic range changer
Amperite delay relay 1 15C2 30-ohm, 1 -watt, 1 % RI. Acro-switch ICM D1 2AXXA24 RE. 50-ohm, 1 -watt, 1% Toggle switch DPST R6. 100-ohm, 1-watt, 1% Push buttons, normally closed R,. 300-ohm, 1-watt, 1 % 5-ohm, 1-watt, 1% R8. 500-ohm, 1 -watt, 1 % 1 0-ohm, 1-watt, 1 % 6-8 VAC No. 40 pilot lamps Stepper coil, stepper-switch relay, and Reset Guardian 1 1 5VAC MER series Reset coil, stepper-switch relay, and Reset Guardian 1 1 W A C MER series ANALYTICAL CHEMISTRY
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Figure 2.
10 MINUTES
Typical
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attenuated chromatogram
