The challenge that chlorine presented during the development of a benzodiazepine assay.

During the routine validation of a benzodiazepine method (performed on a Liquid Chromatography - Tandem Mass Spectrometer), it was noted that lorazepam, triazolam, and α-hydroxytriazolam showed a quadratic shift/bias in the calibration curve, particularly at high concentrations. The ultimate cause of this bias was determined to be due to the natural presence of chlorine (Cl) isotopes (35Cl and 37Cl) in these benzodiazepines. The presence of the heavy (37Cl) isoforms of Cl resulted in the analyte's mass being the same as the internal standard which, in turn, caused the internal standard to appear "falsely increased", thus skewing the calibration curve. One solution to this potential issue was to take advantage of this natural phenomenon and use the Cl heavy isoforms of the respectively labeled internal standards.