Coincidence corrections

An alternative approach is to model the probability of coincidence as a function of either sample concentration or the fraction of time the sensing zone is occupied by passing cells (dead time). The model produces a coincidence-correction factor which is applied to the count. Still another approach is to reduce the size of the sensing zone, which reduces the probability of coincidence. In practice, commercial counters combine the latter two approaches. 

The more complex machines (e.g. Coulter counter model ZM) have variable upper and lower thresholds and will measure cell number with automatic coincidence correction if required and average cell diameter or volume. When coupled with the Channelizer 256... 

A coincidence correction can be applied for primary coincidence but it is preferable to use a concentration so low that this effect is negligible. Under this condition, secondary coincidence errors are also reduced. 

The paralysis time of the counting set up was adjusted to 300 2 ft sec, thereby enabling accurate coincidence corrections to be made. 

Starting with an initial cell concentration of 2.5 ( 0.5) X 10 cells/mL, two serial dilutions of 10 to 1 resulted in 2.5 X 10 cells/mL, or 1.25 X 10 cells/50 ixL. At this concentration a Coulter Counter ll-fim aperture will give the true count of cell concentration without necessitating coincidence corrections (26). As aggregation occurred with the addition of PEI, the particle number concentration decreased and fewer dilutions were required to achieve the same significance of count. Apertures of 30 and 70 /xm were used to size the larger aggregates as they formed in the stirrer-reactor assembly. 

In Chapter 6,1 mentioned the moves being made in the direction of providing detector calibrations based upon the dimensions and physical parameters of the detector and its encapsulation. Once this happy state of affairs has been attained, then the extension of that to include true coincidence corrections becomes possible. Again, one can envisage a situation where a new detector is delivered with a CD-ROM holding the theoretical calibration curve and the latest nuclear data for the nuclides of most common interest that would be accessed by spectrum-analysis programs for automatic summing corrections. For that, we must wait, but help does now appear to be on the horizon. 

E. J. Wynn, M. J. Hounslow, Coincidence Correction for Electrical-Zone (Coulter Counter) Particle Size Analyzers , Powder TechnoL, 93 (1997) 163—175 Information on the Coulter Counter is available from Coulter Particle Characterization Electronics Inc., R O. Box 169015, M/C 195-10, Miami, Florida, USA, 33116-9015. Information on Celloscope and Electrozone resistazone counters can be obtained from Particle Data, Inc., P. O. Box 265. Elmhurst, Illinois 60126. 

The analysis was performed, according to the British Standard recommendations, in the single threshold mode, with coincidence correction for, at least, 15 size cI asses. 

Distributional Coincidence Correction for Enhanced Electrozone PSA Precision... 

For phytoplankton cultures it may be necessary to dilute the sample in order to avoid coincidence corrections. From a preliminary count of the sample, and with reference to Table XVII, it may be determined if the count exceeds an acceptable level of coincidence. Generally, counts which have a coincidence correction of greater than 5% should be diluted. Dilution to a known volume should be made with membrane-filtered (0.22 p) sea water. 

Providing the coincidence correction is less than 5% see Table XVII), the precision of counts made with the Coulter Counter is a function of the number of particles counted. The precision decreases with the total number of particles counted but is independent of the size of the aperture tube employed. The following data will serve as a guide to the precision of count data. (Confidence limits given are two standard deviations.)... 

The precision of counts and coincidence corrections are the same as described previously in Section IV.12.IV.1 and in Table XVII. 

Find the radioactive count of the sample, using precisely the same assembly as that used for phytoplankton samples. Evaluate the counting rate in counts per minute, having counted a total of at least 10,000—preferably 50,000 — counts. Correct this rate for any coincidence correction and for the natural background rate if this is significant. 

VI.17. COINCIDENCE CORRECTION CHART FOR DIFFERENT SIZED APERTURES... 

Table XVII. Coincidence correction chart for different sized apertures (taken from Instruction and Service Manual for the Model B Coulter Counter ).

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