The Molecular Anatomy (MAN) Program at the Holifield National Laboratory is involved in the development of clinical techniques for analyzing biomaterials on a molecular level. The GeMSAEC Fast Analyzer, a type of centrifuge, is one of the results of the program. The centrifuge is capable of analyzing the contents of minute samples placed in separate cuvettes on a rotor. The samples are spun around a light source and the analysis is made using one of four types of measurements, the amount of light the sample (1) absorbs, (2) fluorescences, (3) scatters, or (4) chemiluminescences. The measurements are made by a photomultiplier tube in the analyzer.
Two major problems exist with prior art circuits controlling the gain of the photomultiplier. One such prior circuit is described in U.S. Pat. No. 3,783,300, issued Jan. 1, 1974, to Wayne F. Johnson. One problem is the time required for the controller to set the gain on the P.M. tube. It is important to be able to begin the analysis as quickly as possible because reactions can begin to take place in the cuvettes in less than three tenths of a second. The prior method of setting the gain on the P.M. tube takes around 1.5 seconds. By that time, the reaction may have reached a steady state and valuable information is lost. The present invention, to be described hereinbelow, can begin analysis after 3 or 4 revolutions of the rotor. This is approximately 0.2 second at a rotor speed of 1000 rpm.
The other problem of prior art systems is that the reference cuvette has to be a water blank due to feedback complications. The prior P.M. tube controller can only use a water blank in the first cuvette as a reference. Other reference solutions tend to have a small degree of instability or noise during a run. A slight variation of the reference cuvette induces a large response in the feedback mechanism of the analyzer. For instance, assume the reference is a fluorescent solution that exhibited a small increase in fluorescence for a short time. The prior control system responds to this small increase by decreasing the results obtained in the cuvettes undergoing analysis. Thus, a small change in the reference solution greatly distorts the analysis results of the other cuvettes.
Thus, there exists a need for an improved system for regulating the gain of the photomultiplier tube of a photometric solution analyzer wherein the gain can be set at a desired or required level and in a shorter time interval, and the analyzer is not limited to a water blank cuvette as a reference. This need has been met in the present invention in a manner to be described hereinbelow.