where [P]t is the amount of product P formed by time t, [P]total is the total combined amount of reactants S and P (representing Stotal t=0 added at time t =

net — (cpm S* cpmbackgroun d). The right-hand quotient is the fractional conversion of substrate to product at time t, as corrected for any background in a sample containing no radioactive substances.

As would be expected on theoretical grounds, the measurement error depends on the total number of disintegrations detected (Table 4.15). A recommended practice is to count samples for a period that is sufficient to obtain at least 5,000 accumulated counts. Therefore, with 20-min counting periods per sample, 250 cpm represents an effective lower limit for an accurate radioactivity determination.

Finally, while radioactive decay is itself a random process, the Gaussian distribution function fails to account for probability relationships describing rates of radioactive

TABLE 4.15 Statistics of Measurement Error in Radioactivity Measurement

Total Count


Probable Error (0.67 s)

Probable % Error ((P/m) x 100)

Confidence Level

(% at 2 s)

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