Patent GB-1230831-A (GB)
IPC: G01J1/16
Filed: 1968-04-17
Abstract: 1,230,831. Photo-electric measurement of samples; voltage measurement. COULTER ELECTRONICS Ltd. April 17, 1968 [April 17, 1967; March 18, 1968], No. 18227/68. Headings GIA and G1U. [Also in Division G4] In photo-electric absorption investigation of samples, a first voltage signal representative of the absorption of a reference sample is stored and a second voltage signal representing the absorption of a test sample is compared with the stored signal by emptying the store until the stored and the second signals balance, the time taken in emptying the store to reach this state of balance representing the voltage difference between the two signals and thus in absorption between the test and reference samples. The method can be applied to the determination of the haemoglobin content blood samples, diluted with saline solution and preservative, and reacted with haemoglobin reagent, quantities of such diluent being used as reference samples. Alternatively, air, clear water or other reference fluid may be used as the reference sample, or even a filter interposed in the light path may be used. In one embodiment, Fig.2, the light transmitted by a sample is detected by a photo-cell 40, the current through which is converted to a voltage by device 42. In operation of the apparatus, a programme control device (20) Fig. 1 (not shown) first causes a reference sample to be passed through the flow cell. The transducer circuit output is applied to transistor 44 which is rendered conductive, allowing capacitor 46 of the storage circuit 12 to charge and store the reference sample voltage signal. Next, the reference sample is replaced by the test sample. A test sample voltage signal is produced on line 14 at the same time as a triggering voltage on lines 22 switches on transistor 50 of receiving circuit 16, allowing capacitor 46 to discharge to earth. The test sample voltage signal is less than the original reference sample voltage signal and therefore does not at this time switch on transistor 44. The triggering signal on line 22 also enables a bi-stable circuit through transistor 52 of enabling circuit 26. Thus, as capacitor 46 starts to discharge, the bistable is switched on. The capacitor discharges until the voltages at the base and emitter of transistor 44 balance, when the transistor 44 is switched on and an output pulse on line 32 switches off the bi-stable circuit. The output pulse from the bi-stable thus has a duration representing the difference in the reference and test sample voltages and thus their transmissivities, from which the haemoglobin content is calculable. Analog recording of this pulse duration is possible (34) Fig. 1 (not shown), or the bi-stable output can gate pulses from a generator (36) to a digital recorder, e. g. a printer (38). After this measurement cycle, a feedback pulse from line 32, for example, can cause recycling and measurements on a fresh reference and test sample to commence. In a modification of the arrangement of Fig.2, a two-step reference sample is used, the first part being completely clear water or air. Enabling of transistor 50 occurs when the second part of the reference is measured, discharge of the capacitor 46 from the high voltage stored due to the first part of the reference, being initiated but enabling of the bi-stable circuit is delayed until the sample itself is introduced into the flow cell. In a second embodiment, Fig. 6, the voltage from photo-cell circuit 110 representing the reference sample is coupled through an operational amplifier 112 of the balanced differential type, a diode 116, a biasable switch 118, and a controlled switch 122 to a capacitor 128 which stores the voltage. At this time, output convertor circuit 132 is not operative due to the reverse bias effect of the reference sample voltage. The programme control unit for the circuit then causes switch 122 to open and isolate capacitor 128. The reference sample is replaced by the test sample, followed by closing of switch 122. Since the stored voltage is higher than the incoming test sample voltage, biasable switch 118 (diode 116) is reverse biased and the capacitor starts to discharge into circuit 126. A voltage representative of the reference is also fed back on line 150 to the input of differential amplifier 112 which becomes saturated, its output voltage being caused to go negative, thereby forward biasing diode 152 and switching on circuit 132. This circuit remains switched until the capacitor 128 has discharged sufficiently to balance the voltages at point C and then at the input of the amplifier. During the time that circuit 132 is switched on (representing the differences in the reference and test samples voltages) transistor 154 is off and uni-junction 156 is conductive and its output is integrated at 158. A test cell suitable for use with the apparatus of Fig. 2 or 6 is described, Fig. 4 (not shown) and has a square section test area and a green filter associated with the photo-cell. The cell has means for separating out the white blood cells to allow counting of these. Instead of a single flow cell and photo-cell arrangement, one each for the test and reference samples may be provided. However, the resulting voltage signals have to be fed sequentially to the circuit of Fig. 1 or Fig. 6, for comparison.