Mass chromatograms demonstrating analytical challenges. Representative mass chromatograms demonstrating improvement in signal to noise in mass transition (m/z 291→255) representing DHT recovered from normal male serum, following optimisation of the wash step. (A) Wash step of 5% methanol in water. (B) Wash step of 50% methanol in water. DHT, dihydrotestosterone; cps, counts per second. (C) Mass chromatogram (m/z 289→97) demonstrating separation of endogenous isomers of testosterone. cps, counts per second.