Application of LC/MS to determine specific activity variation in a series of sulfonamide tracers from the [35S]methane sulfonate reagent

The specific activities for a series of S‐35 tracers were found to vary from the decay‐corrected specific activity of the labeled reagent. If not known before the stock solution preparation and binding assay, this variation would have resulted in performing the assay at approximately two to three times over the targeted concentration, thereby leading to considerable error in the calculated binding and related conclusions. Copyright © 2012 John Wiley & Sons, Ltd. The specific activities for a series of S‐35 labeled tracers were found to vary from the decay corrected specific activity (SA) of the labeled reagent with the average being only ~60% of the stated value. In two cases, with further reduced tracer SA, this reduction was at least partially attributed to the presence of minor amounts of unlabeled final product in the unlabeled precursor. It was also shown that the SA of one batch of the reagent, [35S]methane sulfonate, was only approximately 10% of the expected value.