Answer to Question #4882 Submitted to "Ask the Experts"

Q

We have Beckman's LS 6500 multipurpose scintillation counter and we are facing the following problems: (a) when we count our sample there is a great variation in counts per minute (cpm) (like 900 cpm the first time counting and 1,900 in the recount of the same sample on the same day), and (b) we observe that in the sample of the same batch there is a variation in the #H value which should be within the recommended range, i.e., +/-10, but the #H value we are observing is +/-70. We think that the variation in the cpm of the sample in recounting is related to the #H value variation. We faced the above problems during the last few months, although we didn't change the protocol for sample preparation, the chemicals and the consumable that we were using previously. We are also facing some voltage fluctuation problem in the power supply system of our laboratory. Would that be a reason for our problem? Also, (c) the halt function of our system is not working and after counting the sample, the system doesn't detect the halt ID card and then we have to stop the counting manually. Kindly guide us how we can overcome our problems.

A

There is obviously a problem with your liquid scintillation counter. With a measurement of 900 counts in a minute, you would expect a standard deviation of more than 30 counts, thus a range of counts from about 810 to 990. A measurement of 1,900 counts is too far out of line. Similarly, the variation in (quench correction) H# value appears too large for the system specifications. If your sample preparation and counting cocktails are the same, then there is another problem. Perhaps it is the line voltage fluctuation, but you need to contact the manufacturer (Beckman Coulter) of the system.

I wish I could be more help, but you may need a service call.

David J. Allard, CHP