Answer to Question #5525 Submitted to "Ask the Experts"

I carry out primary production measurements. Recently, our instrument developed a snag and since then we are able only to measure the ¹⁴C-activity in terms of cpm and not in dpm as desired. I want to know if there is any method to estimate dpm from cpm. I understand that factors like quench correction, which varies with samples, may influence the conversion. Also, the background cpm needs to be included. Is there any mathematical equation by which this conversion can be carried out? Also, how reliable will the results be?

While you do not say specifically what type of counting system you are using, I gather that you are doing liquid scintillation counting for the ¹⁴C. It is possible to carry out activity determinations based on count rate data, although, as you have noted or inferred, you do require other information.

The activity, in dpm of ¹⁴C, is given by the net sample count rate, in cpm, divided by the counting efficiency. The net count rate is easy enough to obtain by subtracting the background rate for a blank sample from the observed sample count rate. The counting efficiency is more difficult since it does depend on the degree of chemical and/or color quenching that applies to a given sample. There are various ways that you might determine the counting efficiency, taking account of quench effects.

One way is to use what is called an internal standardization technique. In this method the sample would be counted under usual conditions, and then a known activity of ¹⁴C standard (in as small a volume as can be achieved) would be added to the sample and the sample counted again. The net count rate due to the added standard would be determined and divided by the added standard activity to obtain the counting efficiency. This method is good if you have only a few samples to analyze or if all of the samples you are analyzing are known to have pretty much the same degree of quenching.

In another technique you can prepare a set of quenched standards using varying amounts of quenching agent (such as nitromethane) and develop appropriate quench correction curves. This method often involves the use of spectral shifts in the pulse height distribution to develop the required curves to interpret properly the counting efficiency.

A third possible technique uses an external gamma-emitting standard that is positioned in a reproducible spot close to the counting vial and counted for a short time. The standard usually produces a much higher count rate than would be expected from most samples. The extent of quench correction is obtained from the sample count rate with the external standard compared to the count rate from an unquenched pure cocktail mix (no added activity) with the external standard. The ratio of these two count rates will be a number less than or equal to unity and, when multiplied by the counting efficiency for an unquenched ¹⁴C standard, would yield the estimated counting efficiency for the sample. Naturally, your instrument must be equipped with the external source capability to use this method.

The methods above have individual advantages and disadvantages that we will not discuss here. There is a variety of information available on the Internet regarding many aspects of liquid scintillation counting, including quench corrections and counting efficiency, that you might want to investigate through the common search engines. Good luck.

George Chabot, PhD, CHP