This invention is directed to particle counting methods and apparatuses which provide a statistic correction to a detected train of particle derived count pulses, such that effective random coincidence loss or gain of count does not induce ultimate counting error.
The particle counting methods and apparatuses concerned employ particle sensing zones in which more than one particle might reside at any one time and thereby randomly generate a coincidence condition. This invention particularly is directed to, but not limited to the determination of non-electrical properties, such as size and count of particles, by measuring electrical properties (Patent Office class 324-71NE).
Now well known in the art of electronic particle counting and analyzing is apparatus marketed primarily under the trademark "Coulter Counter". Such apparatus and portions thereof are disclosed in several U.S. Pats., for example Nos. 2,656,508; 2,985,830; and 3,259,842 (each in class 324-71). A significantly important portion of such Coulter type of apparatus is the minute scanning aperture or scanning ambit or sensing zone relative to or through which pass and are detected single particles at a rate often well in excess of one thousand per second. Because of the physical parameters of the scanning aperture and particle concentration, there frequently results the coincidence of two or more particles in the scanning ambit. As a result, there effectively is detected and counted only one particle, not two or more. Such loss of count condition is identified as primary coincidence.
On the other hand, two or more small particles, which individually would not be counted because of their small size, can coincide to produce a pulse which is counted as a single (non-existent) larger particle. Such gain of count condition is identified as secondary coincidence.
Although such coincidence count conditions are random in time, they follow a statistically ascertainable form, from which curves, tables, and formulae are obtainable. A relatively simple one of such formulae is: n = N - kN.sup.2 /2; in which
n = the detected number of particles, or raw count; PA1 N = the true, or corrected count; and PA1 k = a constant which relates to the physical parameters of the scanning elements of the analyzing apparatus.
Heretofore, the operator of a "Coulter Counter" would obtain the raw count by analysis of a suspension of particles and then would refer to a coincidence correction chart which presented the corrected count for a selection of conditions.
Although the use of charts often can provide an accurate result, it is both time consuming and prohibits the fully automatic recording and processing of the corrected counts. Also, of course, the accumulating count during analysis is uncorrected. Different charts must be used with apertures of different sizes, and different counting procedures, such as the position of the counting threshold level in relation to the particle size distribution.
The use of analog, non-linear meters and/or elements in the output stage of a "Coulter Counter" also has been accomplished with limited success; however, in many uses a direct reading digitalized output greatly is to be preferred.
A recently developed, automatic digitalized system and method for coincidence correction, that yields a continuously corrected true count N, is the subject of U.S. Pat. No. 3,626,164. According to the therein described invention, various amounts of the addends are periodically generated by a somewhat complex arrangement of counters and are periodically applied to the continuously accumulating augend count of the particles to yield the true count. Although such method and apparatus provide a distinct advantage over the prior art, they possess the basic limitation of being programmed to a specific correction factor constant k, which itself is tied to the physical parameters of the detecting apparatus, such as the size and volume of the detecting aperture. Hence, changes, such as in aperture size, require program changes in the correcting apparatus. Additionally, this somewhat complex arrangement presents a purchase and maintainence cost which must be considered from a commercial sense.
Considerable research effort has been devoted to the phenomena of particle coincidence in a "Coulter Counter". A few of the many publications resulting from such consideration are next listed: Wales, M. and Wilson, J. N., Theory of Coincidence in Particle Counters, Review of Scient. Instruments, Vol. 32, Nr. 10, pp. 1132-1136. Oct. 1961; Princen, L. H. and Kwolek, W. F., Coincidence Corrections for Particle Size Determination with the Coulter Counter, Review of Scientific Instruments, Vol. 36, Nr. 5, pp. 646-653, May 1965; Strackee, J., Coincidence Loss in Bloodcounters, Medical and Biological Engineering, Vol. 4, pp. 97-99, 1966; Princen, L. H., Improved Determination of Calibration and Coincidence Correction Constants for Coulter Counters, Review of Scient. Instruments, Vol. 37, Nr. 10, pp. 1416-1418, Oct. 1966; Edmundson, I. C., Coincidence Error in Coulter Counter Particle Size Analysis, Nature, Vol. 212, pp. 1450-1452; and Bennert, W. and Hilbig, G., The Theory of the Coincidence Error for Digital Particle Size Analysis, Staub-Reinholt, Luft, Vol. 27, Nr. 4, April 1967.
K is the critical volume (in terms of the unit volume of fluid passed through the aperture during a particle count, for instance 1 ml) defined by the scanning aperture of a Coulter type particle analyzer. Unfortunately, the critical volume is not a fixed amount for any one aperture, and certainly is not the same under varying input conditions. One of the variables that goes into the determination of the critical volume is the electric field in the immediate vicinity outside of the scanning aperture, which must be included in what has been termed the "scanning ambit" of the particle detector. Such varying electric field is discussed in U.S. Pat. No. 3,668,531.
Although the Coulter type of particle analyzer, with its aperture form of scanning or sensing zone, is specifically described herein, other forms of particle analyzers, such as those operating with light or acoustic energy and having optical or acoustic sensing zones are encompassed by the invention herein, to the extent that these other types of particle analyzers are subject to particle coincidence in their sensing zones.