The challenge of quantifying two compounds in a single assay with drastic dynamic ranges is to obtain linearity without source or detector saturation at the mass spectrometer.Results:
In positive-ionization mode, the nonlinear relationships for Desmethyl Mebeverine Acid (DMAC) were demonstrated using three common strategies to overcome this issue: using offset voltage parameters, less-sensitive product ion or 13C mass SRM transitions. On the contrary, nonlinear relationships for DMAC were overcome if negative-ionization mode was used. Due to Mebeverine analytical LLOQ, dilution was not suitable for a single assay of Mebeverine and DMAC. However, polarity switching in negative mode for DMAC was successfully found to compensate for the nonlinearity at the mass spectrometer while preserving Mebeverine linear regression model in positive mode.Conclusion:
The polarity switching strategy has demonstrated the advantage of improving linearity for analytes having different ionization polarities and three orders of magnitude difference in concentration.