Patent GB-1435382-A (GB)
Title: Apparatus for compensating for drift in spectrophotometric measurements
IPC: G01N21/27
Filed: 1973-04-25
Abstract: 1435382 Spectrophotometers MICROMEDIC SYSTEMS Inc 25 April 1973 [28 April 1972] 19566/73 Heading G1A [Also in Division G4] The output of a spectrophotometer is multiplied by a factor to correct for any changes in the test sample which may have occurred from the time the sample was taken, the factor being initially set by inserting a standard sample in the spectrophotometer, comparing the multiplied output signal with a pre-selected standard value and changing the multiplying factor in accordance therewith until the two signals are equal. Spectrophotometer, Fig. 1 A beam of white light 101 from lamp 104 is reflected by spherical mirror 108 on to a grating 201, the angle of which is adjustable about vertical axis 209 by micrometer screw 212 to select the required wavelength passing through aperture 202. The monochromatic beam 102 after passing through one of the pre-selected ultraviolet 203, infrared 204 or white light 206 filters is scanned to and fro across the ends of reference liquid (blank) cuvette 301 and sample liquid cuvette 302 by scanner 400. Scanner 400 comprises a pair of plates 401, 402 of rectangular cross-section arranged in overlapping cross-wise superposition, Fig. 2 (not shown) and held at one end in support 407 connected to lever 408 pivotable about horizontal axis 409 by the action of eccentric 410 driven by motor 411. Thus optical plates 401, 402 describe an up and down movement so that they are inserted alternately in path 102 and the light beam is shifted alternately between one position in alignment with cuvette 301 and another position in alignment with cuvette 302. After passing through the cuvettes the light beams are refracted by prism 502 on to the same spot of photo-multiplier 501, the output of which is connected to processing circuit 700. Processing circuit, Fig. 3 The photo-multiplier 501 output is amplified at 701, provided with a feedback circuit 702 for dark current compensation, and applied to one or other of capacitors 712, 713 dependent upon the position of switches 798, 799 which are controlled by synchronization pulses from pulse generator 706 which is under the control of pulses from coils 605, 606 of synchronization generating means 600, Fig. 1 indicating which of the two paths the light beam 102 lies. The output current from 501 charges capacitor 712 when the light passes through the reference cuvette and charges capacitor 713 when it passes through the sample cuvette. The capacitor voltages are compared in respective comparators 716, 717 with an exponential waveform from generator 722 to provide rectangular pulses having a time duration proportional to the log of the ratio between the voltages on lines 714, 715 and the peak voltage from generator 722. Gate 725 acts as a subtractor and provides a rectangular pulse the duration of which is proportional to the log of the ratio between the signals on 714, 715 i.e. the attenuation of the light beam in the sample compared with the reference. The pulse is digitized at 726 and applied via drift correcting circuit 729 to counter 801. Drift correcting circuit 729 Fig. 4 (not shown) The pulse train generated by the digitizer is applied to a multiplier which multiplies the pulses by a factor less than one to compensate for changes in the sample. The factor is set by first filling the sample cuvette with a standard test sample solution i.e. one which may have changed due to chemical action (urea, blood) and applying the digitized output to the multiplier, the output from which is then compared in a digital comparator with a pre-selected number e.g. the value of the attenuation or concentration of the standard used for initial calibration of the spectrophotometer. An up/down converter determining the multiplying factors counts up, does not count, or counts down according to whether the digital number inputed to the comparator is greater than, equal to, or lower than the predetermined value. The value of the multiplying factor is changed until equality exists between the multiplied factor and the pre-set value when the factor is fixed for subsequent measurements of samples in the sample cuvette. In order to decrease the time to bring about equality the up/down counter has its contents modified in greater steps at the beginning and then in steps of successively reduced magnitude as the correction proceeds. The output of the up/down counter is also connected to a threshold detector which monitors the amount of correction needed and sounds an alarm if it exceeds predetermined upper and lower limits Fig. 6 (not shown). The standard sample may be periodically placed in the sample cuvette to determine the amount of correction needed.