Systematic evaluation of the root cause of non‐linearity in liquid chromatography/tandem mass spectrometry bioanalytical assays and strategy to predict and extend the linear standard curve range

RATIONALE The linear range of a liquid chromatography/tandem mass spectrometry (LC/MS/MS) bioanalytical assay is typically about three orders of magnitude. A broader standard curve range is favored since it can significantly reduce the time, labor and potential errors related to sample dilution – one of the bottlenecks in sample analysis. Using quadratic regression to fit the standard curve can, to a certain degree, extend the dynamic range. However, the use of a quadratic regression is controversial, particularly in regulated bioanalysis. METHODS A number of compounds, with different physicochemical properties and ionization efficiencies, were evaluated to understand the cause of the non‐linear behavior of the standard curve. RESULTS The standard curve behavior is primarily associated with the absolute analyte response but not the analyte concentration, the properties of the analyte, or the nature of the matrix when a stable‐isotope‐labeled internal standard (SIL‐IS) is used. For all the test compounds, a non‐linear curve was observed when signals exceeded a certain response, which depends on the detector used in the mass spectrometer. With typical API4000 instruments used for the experiments, this critical response level was determined to be ~1 E+6 counts per second (cps) and it was successfully used to predict the linear ranges for the test compounds. By simultaneously monitoring two selective reaction monitoring (SRM) channels of different intensity and using SIL‐IS, a linear range of five orders of magnitude was achieved. CONCLUSIONS In this work, the root cause of the non‐linear behavior of the standard curve when using a SIL‐IS was investigated and identified. Based on the findings, an improved multiple SRM channels approach was proposed and successfully applied to obtain a linear dynamic range of five orders of magnitude for one test compound. This approach may work particularly well for LC/MS/MS bioanalytical assay of dried blood spot (DBS) samples, for which a direct dilution is cumbersome. Copyright © 2012 John Wiley & Sons, Ltd.