Patent GB-1025351-A (GB)
Title: Gas chromatography equipment
Inventor: ROBERTS GORDON THOMAS
IPC: G06G7/75
Filed: 1961-03-16
Abstract: 1,025,351. Cathode-ray tube Circuits. BRUCE PEEBLES & CO. Ltd. June 14, 1962 [March 16, 1961], Nos. 9699/61 and 9702/61. Heading H4T. [Also in Division B1] A selected part of a chromatogram is displayed on a cathode-ray tube. The selection of the signal to be displayed is controlled by a signal generator of adjustable repetition frequency which also controls, via a variable delay device, the scanning sweep generator the repetition frequency of which is also adjustable. The invention is described in connection with the display of signals from a gas chromatography detector 14 (Fig. 1) which receives from the column 13 samples of gas from a production plant 42 together with a carrier gas (argon) from bottle 11. Each sample is supplied via a valve 12 controlled via a signal generator 19 of adjustable repetition frequency which also controls the sweep generator 22 via an adjustable delay device 21. After the injection of the first sample a period of about eighteen seconds elapses before the first component is eluted from column 13 and after a further ten seconds all the components will have been eluted. For examination of the first sample the delay device 21 is adjusted such that the sweep generator 22 is triggered seventeen seconds after the occurrence of the sampling control pulse from generator 19 and the sweep speed control is adjusted to cause the tube beam to scan the width of screen 17 in about ten seconds the screen having a persistence of at least 20 seconds. Generator 19 is adjusted to produce a pulse every twelve seconds, i.e. a new sample is injected into column 13 before the preceding sample has been fully eluted so that successive samples are spaced apart by a time corresponding to a little more than the elution time of ten seconds and loss of time in examining the second and subsequent samples may therefore be avoided by adjusting delay 21 to trigger sweep generator five seconds after the occurrence of each pulse from generator 19. The output of detector 14 is supplied to the vertical deflection control device of the tube alternately over a direct path 26 and over a path containing an integrating amplifier 27 by means of a (electronic) switch 28 operating at a rate faster than the persistence of vision so that the direct chromatogram trace and the integral thereof may be displayed, effectively, simultaneously. The peak corresponding to each component is stated to be related to a Gaussian curve and the performance of the apparatus may be estimated by comparing a single peak with a reference Gaussian curve which may be marked on a screen placed in front of the display tube or be traced by an auxiliary beam in the tube. In addition to the tube display the instantaneous total integral of the detected signal is indicated on a meter 31 and by means of a gate 33 controlled by switches 35, 36 to operate for a desired part of the sweep period the corresponding portion of the detected signal may be supplied via an integrating amplifier 34 to a meter 32 and the corresponding portion of the tube trace identified by additional brightening of the trace under the control of a signal from gate 33. By differentiating the signals to be displayed a desired peak may be utilized to control gate 33. Alternatively, the opening and closing of the gate may be set on a magnetic or punched tape. By means of a ganged potentiometer 37 the indication on meter 31 may be made equal to full scale deflection so that the indication on meter 32 (quantity of the selected component) may be read directly as a proportion of the complete. A plurality of meters 32 indicating the quantities of different components may be utilized and the signals representative of which may be fed over 41 to mechanism of the gas production plant 42 for controlling the composition of the gas. The ratio of the selected integral of one peak to the total integral may be derived by means of a dividing circuit comprising a multiplier 43, sign-changing unit gain amplifier 44 and a highgain summing amplifier 45 and such derived signal employed to operate motor 47 of a pen recorder 48, the paper record associated with which is driven by a stepping motor 49 every time a gas sample is injected. The quotient signals may also be displayed on a cathode-ray tube 51 and may also be employed to control plant 42 instead of the signal fed over conductor 41. An alarm 52 may be operated if the value of the ratio passes a preset limit.