Patent GB-1405783-A (GB)
Title: Method of and apparatus for determining the concentration of a substance in a solution
IPC: G01N33/483
Filed: 1972-08-08
Abstract: 1405783 Photo-electric measurement of concentration AMERICAN MONITOR CORP 8 Sept 1972 [9 Sept 1971] 41912/72 Heading G1A [Also in Division H3] The concentration of a substance in a test solution is determined by determining the value of a parameter characteristic of the concentration of the solution, determining the value of said parameter for a standard solution having a known concentration of the substance therein, automatically calculating a value of a standardizing scale factor from said parameter value for said standard solution, and computing the value of the percentage concentration of the substance in the test solution from the standardizing scale factor and the value of said parameter for said test solution. In the embodiment shown in Fig. 3 the concentration of a substance affecting the optical density of a solution is found by digital analysis of the outputs of a spectrophotometer when light is first transmitted through the solution and then along the same path with the solution removed, as follows: the through-the-solution signal is integrated at 74 for a first time interval and the peak value stored at 86; the air path reference signal is then integrated at 74 for an identical time interval and then allowed to decay exponentially until it equals the value stored at 86 as detected by comparator 90, clock pulses being counted for the duration of the decay period and added to a number pre-set in counter 108 during calibration (described below) thereby obtaining a number representing the difference in optical density of the solution and a blank; and the concentration is then derived by counting clock pulses at 112 for the time it takes to fill counter 110 (starting from a number set during calibration) with clock pulses of the same frequency multiplied by the number stored in counter 108, the thus derived result being printed out at 134. Calibration. An automatic gain control loop 76, 78, 80 (details), Fig. 4 and 5 (not shown), is switched on to adjust the H.V. supply to the photo-multiplier 66 while the concentration of a standard solution to be used for calibration is set into counter 112 and its 9's complement transferred to counter 110. The optical density of a reagent blank or serum blank is then measured using the spectrophotometer, the result being transferred to store 102 and its 9's complement set in counter 108. The optical density of the standard solution is then measured whereby counter 108 then contains the difference in optical density of the blank and standard. Clock pulses are then counted in counter 112 until clock pulses of the same frequency multiplied by the number stored in counter 108 fill counter 110. The 9's complement of the contents of counter 112 (which may be regarded as a standardizing scale factor) are then set in counter 110 and the 9's complement of the blank optical density is reset in counter 108. Programmed control. All the steps involved in calibration, measurements on test solutions and print out of results are controlled automatically by logic in response to a program card. As well as end point of reaction tests, rate of reaction test may be carried out by making two or more measurements on the same or identically prepared solution(s).