Patent GB-731942-A (GB)
Title: Improvements in methods of and systems for spectrochemical analysis
IPC: G01J3/32
Filed: 1952-07-30
Abstract: 731,942. Photo-electric spectrometer. LEEDS & NORTHROP CO. July 30, 1952 [Feb. 4, 1952], No. 19276/52. Class 40 (3). In a recording spectrometer of the type in which the light source is an electric spark between electrodes consisting of the material under examination, the detector is made sensitive to the radiation for a period in the interval between the initiation of successive sparks. This is said to enable the apparatus to be so adjusted as to eliminate air lines and background " noise " and to enable the " arc " lines to be distinguished from " spark " lines. Fig. 4A shows the output of a spectrometer of which the intensity is uniform over the period between sparks. BL represents the background level, I the arc lines and X spark lines. If the sensitivity of the system is high only for the first few seconds of the spark period the arc lines become negligible whereas if the initiation of detector response is delayed for 15 microseconds (Fig. 4E) the arc lines become prominent, spark lines fall and background level falls. Further gain of sensitivity may be achieved by both delaying the initiation of detector sensitive period and by shortening it. As shown (Fig. 8) the output of photo multiplier tube 17A receiving light from the spectrometer is applied to an amplifying triode 31A direct coupled to a cathode follower valve 31B providing 100 per cent negative feedback to the valve 31A thus enabling a larger input resistor 36 to be used while maintaining high frequency response. The output of cathode follower 31B is applied to an electric switch comprising a cathode follower valve 46 the normal single cathode resistor being replaced by a pair 47, 48 connected by a germanium diode 49. The signal output to the main amplifier and recorder 18B is taken across resistor 48. A further diode 51 connects the upper end of diode 49 to a source of negative potential. Thus when the switch is open no current of the cathode follower passes through resistor 48 and in consequence-no signal is passed to the amplifier. The switch is closed by a positive pulse produced by a pair of oneshot multivibrators 37A, 37B which fire in succession. 37A is initiated by a signal produced in an antenna 38 by the spectrometer spark. The length of the pulse produced is adjustable by condensers 44 and resistors 60 and since its trailing edge is used after differentiation to trigger the second multivibrator, determines the delay after the beginning of the spark before the photo-multiplier signal is passed to the recorder. The length of time for which the multiplier is connected to the recorder depends upon the length of pulse delivered by multivibrator 37B and this is adjustable by condenser 68 and resistor 41. When the positive pulse produced by the multivibrators is applied through condenser 75 to the side of diode 51 remote from resistor 47, the flow of current through diode 51 is prevented, the cathode follower current passes through resistor 48 and the signal voltage developed across this resistor passes to the output. The signal, however, is superimposed on the gating pulse and the pulse is cancelled from the output by reversing its polarity in a tube 55 and applying it to cathode resistor 48. In a modification (Fig. 15, not shown) cathode follower 45 is omitted, the switching pulse being taken direct from a cathode of one of the valves of the second multivibrator while the pulse which closes the switches is prevented from reaching the output by a neutralising pulse produced at the anode of the same valve which is D.C. restored by a condenser and germanium diode. In a still further embodiment (Fig. 5, not shown) a photo-cell responsive to the commencement of the spark triggers a pulse generator which produces a pulse the trailing edge of which unblocks a valve connected in series with two dynodes of the photo-multiplier tube, so that the tube then operates with full gain. In this embodiment the length of the pulse determines the delay before the photo-multiplier is made sensitive. There is no control of the length of the sensitizing period except the decay of potential from a condenser in the grid circuit of the valve. Specification 732,058 is referred to.