Ruggedness testing of quantitative atmospheric pressure ionization mass spectrometry methods: the effect of co-injected matrix on matrix effects

A number of techniques have been suggested to date for assessing matrix effects on quantitative atmospheric pressure ionization mass spectrometry (API-LC/MS) methods. A newly designed experiment has the aim of efficiently simulating the quantitative behavior of an LC/MS method as a function of the amount of co-injected matrix extract. Two sets of mixtures were prepared in different formats to study matrix effects as a function of analyte or matrix amount. Chromatographic conditions were varied as well, to alter the separation between analyte and co-extractants, and thereby provide different matrix effect conditions for testing the same mixtures. Graphical presentation of the results was used to gain insight into the matrix effect phenomenon. The results suggest that ruggedness for API-LC/MS methods may be defined as the absence of significant variation in results as a function of the amount of co-injected matrix. That is, a non-rugged API-LC/MS method may give consistent results only if a fixed amount of matrix is co-injected on a specific instrument. The results also point to the existence of a specific matrix concentration for the onset of matrix effects, below which these effects are not significant. These issues are important to the US FDA Center for Veterinary Medicine, which has regulatory authority for methods used to monitor for drug residues in food tissues from animals. The ruggedness testing technique suggested here may be an important factor in determining that a method is ready for multi-laboratory testing on multiple instruments.