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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

(m1/2)

Probable Error (0.67 s)

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

Confidence Level

(% at 2 s)

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