Sub-Stocks and Spiking Solutions

Sub-stocks are also used for multi-analyte assays where a mixture of analytes in a single solution is prepared for subsequent preparation of QCs, calibration solutions or spiking solutions used for the preparation of matrix matched calibrators. In this case, individual stock solutions are combined to make one or more sub-stock solutions containing all of the analytes in one solution. This practice will reduce the number of steps that would be required versus making an individual spiking solution for each analyte. However, for methods that required a different LLOQ for each analyte, the sub-stocks and spiking solutions need to be prepared at the corresponding appropriate concentration for each analyte. 

At the onset of method development the appropriate concentrations of the sub-stock and spiking solutions may not be known, but estimates can be based upon the initial method development scheme and by considering what types of spiking solutions will be required for early sensitivity and recovery assessments. Also, to determine the required concentrations for these solutions, consideration must be given to the type of solvent and the volume of solution that will be used for any subsequent dilutions or for spiking the analyte into the control matrix (Section 9.5.6c). 

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The chemical stability of an analyte in a given solution that is stored under specific conditions for given time intervals is an important validation parameter that was discussed in general terms in Section 9.4.4f. An extreme form of instability is the propensity of the analyte to explode or be set afire, and such properties should be available in the MSDS information or, if the analyte is a new compound, the chemists associated with its synthesis or isolation will have discovered such properties before the analyst A crucial special case is the stability of the analyte (and thus of the analytical standard) in solution, as in stock, sub-stock and spiking solutions. The stabihty of analyte or of an analog internal standard in solution should be determined by comparing stored stock solution(s) to freshly made-up stock solution(s). For a stable isotope-labeled internal standard, stability data for the corresponding analyte are often used to estabhsh stahUity for such an SIS. 

Stability should be clearly established for the anticipated duration of storage and use by comparing peak areas for stored solutions against those for solutions made from fresh weighings of analytical standard at selected time intervals such as two weeks, one month, three months etc. As a practical consideration, it is often desirable to establish a full year of stock stability under appropriate storage conditions. If sub-stocks or spiking solutions are used and made in different solvents, then similar stability studies would be required for these but, for reasons of practicality, probably for shorter periods. 

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