Patent GB-1487775-A (GB)
Title: Apparatus for measuring changes in absorbance per unit time
IPC: G01N21/27
Filed: 1974-12-23
Abstract: 1487775 Absorbance change measurement MICROMEDIC SYSTEMS Inc 23 Dec 1974 [19 Feb 1974] 55541/74 Heading G1A The change in absorbance per unit time of sample is measured by periodically developing digital signals, each comprising a pulse train having a length proportional to the logarithm of the ratio of the intensities of radiation passed through the sample and a standard, respectively, and utilizing counting apparatus to compare the numbers of pulses generated in two consecutive time intervals forming two sequential digital signals. Photo-electric absorption measurement system, Fig. 1. A wide band optical source 104 illuminates a mono-chromator section 200, including inlet and outlet apertures 215, 216 and a diffraction grating 201. The selected wavelength is displayed on monitor 800. When red, infrared or ultraviolet light is used, an extra filter appropriate to the radiation is introduced by slide 205, so as to prevent stray or second order diffraction light reaching the sample. The sample and reference cuvette section 300 may be supplied intermittently or continuously with fluids. The optical beam is switched between illumination of the sample and the reference by a pair of quartz, plastics or glass cross-mounted elongated prismatic elements 401, 402. A synchronization 600 coupled to sense which of the sample and reference is being illuminated controls delivery of the signal developed by photo-multiplier 501 sensing transmitted beam to the evaluation circuit 700. A prism 52 directs the alternate beams on to the detector 501. Detector and counting circuit, Fig. 2. A clock 806 periodically stimulates generation of an exponentially decaying signal from circuit 808. Applied to inputs 810, 812 are voltages representing the detector output for the reference and sample transmissions respectively. As the exponential signal falls below these negative voltage levels, as sensed by comparators 802, 804, a constant height pulse of duration logarithmically proportional to the difference between the voltages is produced from Exclusive -OR gate 818. This is converted into a digital signal i.e. a pulse train by oscillator 902 and gate 904. Depending on which is greater of the sample and reference voltages, up or down counting is initiated in counters 912, 914. Except for the first timing interval, the counters operate as follows:- counter 912 counts the currently supplied pulses in a digital signal and at the end of the interval transfers this number to counter 914, after which it is cleared; counter 914 counts in the opposite direction to counter 912 and so effectively subtracts the current member from the number for the previous digital signal, timing interval; thereby at the end of the timing interval, having accumulated therein an absorbance difference representative number. If there is a difference, this is printed out at 916. Other parts of Fig. 2 circuit are for scaling so that the printer provides the required units. Variations on the repetition rate and length of timing intervals are possible. Counters 912, 914 may be arranged to subtract alternately, the other counter storing the number for the current digital signal.