GB1395223A - Method and apparatus for chemical analysis - Google Patents

Abstract

1395223 Biological measurements BECKMAN INSTRUMENTS Inc 3 Aug 1972 [6 Aug 1971] 36271/72 Headings G1A and G1N In an arrangement for determining the concentration of a component in a sample, e.g. urea in blood, the output of a sensor monitoring a characteristic e.g. conductivity of a solution of the sample when added to a reagent and representative of the component is differentiated and the differentiated value obtained as a measure of the concentration after a predetermined time from the introduction of the sample into the reagent. In the arrangement of Fig. 7 the A.C. conductivity of the fluids in sample cup 21 is monitored using a pair of A.C. energized electrodes and the amplitude modulated output signal is demodulated at 25 to provide a D.C. output signal which is differentiated at 31. In order to obviate the effects due to the jump in conductivity when the ionic blood serum sample is injected into the sample cup 21 the differentiator 31 is inhibited for a time long enough to permit the transient jump to disappear and permit thorough mixing of the sample. At the end of this period the inhibition signal is removed and the output of the differentiator 31 rises to the actual signal level of the differentiated A.C. conductivity due to ionic ammonium carbonate formed by urea in the blood sample reacting with the enzyme urease in the sample cup 21, and then falls with the reaction rate. The signal peak obtained is thus proportional to the urea concentration in the sample and is sensed and held in rate measuring circuit 40. The inhibition of the differentiator 31 may be achieved as shown by applying the output of the demodulator to a rate sensing circuit 35 which senses the jump in conductivity and supplies a signal to a time delay circuit 37, the latter generating a control signal a characteristic of which changes after a predetermined time to operate the differentiator. Alternatively the output on line 26 may be applied to a conductivity level sensing circuit which senses the jump in conductivity and generates a signal to be applied to the time delay means. The rate measuring circuit 40 may be a sample and hold circuit, Figs. 5 and 8 (not shown) which is operated by a signal from the time delay 37 after the differentiator has been operated. In another arrangement the rate measuring circuit 40 samples the differentiator output at a time later than the rate peak, Fig. 6 (not shown). Sample chamber, Fig. 9 The conducting reagent is supplied through one side of passage 63, and passageway 62 into the chamber 59 provided with a magnetic stirrer 70. The sensor 22 is in the form of a cylinder which screws into aperture 75 and carries two planar electrodes. The end of the cylinder may be spherical in which case the electrodes are D-shaped on the surface with the straight portions parallel, Figs. 10, 11 (not shown). The sensor may be a spectrophotometer.