Review Ho & Gao
Spike analyte
Extract Analysis (Exp 1) Rpre-extraction spiked
Spike analyte Sample† Extract Analysis (Exp 3)‡ Rnot spiked Analysis (Exp 2) Rpost-extraction spiked
Spike analyte Analysis (Exp 4) Solvent
Sample can be an incurred sample or control matrix with or with endogenous analyte. ‡If sample is blank matrix void of analyte, Exp 3 is not needed and Equation 2 becomes Equation 1. Exp: Experiment; R: Response, such as peak area.
Figure 2. Evaluation of recovery and matrix effects. †
Among the possible stability evaluations listed in
Table 3 for solid tissue, the long-term storage stability is the most relevant and, hence, most evaluated [5,16,22,27]. Both spiked tissue QC and real samples have been used for the evaluation. When a compound is shown to be not stable, the stability of analyte in solid tis- sue and in tissue homogenate can be interconnected and should be examined together to guide sample han- dling. Such interconnection can be better understood by examining the case of arachidonic acid (AA) and its metabolites stability in liver and liver homogenate. The analyte concentrations increase significantly in homogenate when liver was homogenized after keep- ing on ice for up to 60 min (bench-top stability) and after a freeze–thaw cycle [13]. Extract (processed sample) stability must be evalu-
ated for samples in both surrogate and in tissue. In gen- erally, only the autosampler stability is evaluated for the extract unless there is a need to store the extract in conditions that differ from the autosampler [13].
Application of a method for analysis When applying a surrogate matrix method for tissue sample analysis, it is important to remember that only selected method parameters are evaluated during the method validation since the method is validated based on the fit-for-purpose principle. For that reason, moni- toring method performance during tissue sample anal- ysis becomes more critical comparing to that of plasma sample analysis using a fully validated method. If pos- sible, we recommend including QC samples in both
2428 Bioanalysis (2015) 7(18)
surrogate matrix and matching tissue in each batch. For simultaneous quantification of multiple types of tissues, we recommend including QC samples in each matching tissue and in each batch. This allows moni- toring method performance for each type of issues dur- ing the sample analysis. It is possible that the results of the QC samples for one type of tissue passing the acceptance criteria but not for the other within a single analytical batch. In that situation, only the results for the passing tissue QC should be reported.
Conclusion Tissue analysis is complex and there are many difficul- ties that may affect the analysis. Authentic tissues can be difficult or impossible to obtain sufficient quantity for analysis. The endogenous level in tissue can pre- vent accurate determination of analyte at the desired LLOQ. In those cases, the surrogate matrix method is a suitable and practical approach. However, surrogate matrix is a double-edged sword. On one hand, it allows quantitation of analyte without authentic matrix and quantitation of endogenous analyte at more desired LLOQ than would be impossible with the authen- tic matrix. On the other hand, it introduces risks of data inaccuracy. The impact on quantitation must be assessed if the method performs differently between the surrogate matrix and tissue. Utilization of surrogate matrix methods for tissue analysis requires verification that the surrogate matrix is a proper substitute for the tissue. This is achieved by establishing the validity of surrogate matrix and by evaluating method perfor-
future science group Rneat solution
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