Evaluating Carryover During Method Development

Before discussing steps to address carryover, it is appropriate to mention how the extent of the problem is evaluated relative to the requirements of the analytical method. For a quantitative assay the problem is obviously going to be most important with respect to percentage error for a sample at the LLOQ that is injected immediately after an injection of a sample with a concentration at the ULOQ (or greater). With this in mind, one possible experimental measure of carryover would be (PAt, /PAuLOQ) 100%, where PA denotes peak area and the subscript bll denotes the peak observed on injection of the first blank sample (could be a solvent blank or a matrix blank) injected immediately following injection of a sample (e.g. a calibrator or a QC sample) at the ULOQ. Then the contribution of such a carryover to a sample at the LLOQ can be 

Carryover is one of the major problems faced by trace quantitative analysts and shonld be checked regularly throughout the entire process of method development it must also be evaluated and assessed during method validation (Section 10.2.9b) and sample analysis (see Section 10.5.5, where a more detailed method of evaluation of the effect of carryover on the accnracy of an assay (Zeng 2006) is described). 

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Even if a reconstitution solvent blank is normally used to evaluate carryover during validation and sample analysis, it is suggested to use all three types of blank as mentioned earlier for carryover evaluation during the method development. As considered previously (see Section 8.3.2) in the quantitative evaluation of carryover, linearity of detection together with sufficient accuracy and precision below LLOQ should be assumed. This assumption is questionable, and for this reason some bioanalytical laboratories have chosen different criteria to evaluate the carryover. 

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