Obtaining a Suitable Control Matrix

The availability of a true control (blank) matrix, particularly of multiple lots and/or sources, varies widely and this represents a major consideration in method development, validation and sample analysis. This concern is even greater when there is a possibility of significant matrix effects (suppression/enhancement of ionization efficiency), in which case relative matrix effects (Sections 5.3.6a and 9.6) must be tested for by comparing nominally similar control matrices from different lots and/or suppliers. 

In some cases a suitable control matrix may not be available, e.g., soils from different sources around the world (and thus widely variable). In some cases it may be necessary to resort to use of a matrix containing known analyte concentrations that lie well below the LLOQ required for the particular analysis at hand, or analyte-stripped or surrogate matrices (Section 9.4.7c), but all of these approaches are problematic and may require additional vahdation procedures. Unavailability of a truly blank matrix was until very recently a major intrinsic problem in quantitation of endogenous analytes but novel new procedures have been recently developed for this particular application (Section 11.7). 

A control matrix, once obtained, is just as important as the analytical or internal standard and should be stored under appropriate conditions to maintain stability, such that any effects on extraction efficiency or ionization efficiency of the analyte do not change with time. 

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