Usually the analyte stability is well characterised under various storage conditions. The experiments carried out for monitoring the storage conditions of standards and samples, which are carried out as part of the normal laboratory accreditation and quality control system, can provide the required information. If stability data for analytes in the matrix are available (e.g. on the basis of information from the EURLs, published data, etc.), these data do not need to be determined by each laboratory. However, referring to available stability data of analytes in solution and in matrix is only acceptable if identical conditions are applied.
In case the required stability data are not available, the following approaches should be used.
2.5.1.
Determination of the stability of the analyte in solution
1.
Prepare fresh stock solutions of the analyte(s) and dilute as specified in the test instructions to yield sufficient aliquots (e.g. 40) of each selected concentration. Samples shall be prepared of:
(a) Solutions of the analyte, which are used for fortification;
(b) Analyte solutions, used for the final analysis;
(c) Any other solution that is of interest (e.g. derivatised standards).
2.
Measure the analyte content in the freshly prepared solution according to the test instructions.
3.
Dispense appropriate volumes into suitable containers, label and store according to the light and temperature conditions of the scheme included in Table 7. The storage time shall be chosen taking into account the applied analytical practice, ideally until the first degradation phenomena are observable during identification and/or quantification. If no degradation is observed during the stability study, the storage duration of the stability study shall be equal to the duration of the maximum storage period of the solution.
4.
Calculate the concentration of the analyte(s) in each aliquot compared to the concentration of the analyte in the freshly prepared solution, following the formula below:
Analyte Remaining (%) = C
i
× 100/C
fresh
C
i
= concentration at time point i
C
fresh
= concentration of fresh solution
The mean value of five replicate solutions, which were stored, shall not differ by more than 15 % from the mean value of five freshly prepared replicate solutions. The mean value of the five freshly prepared solutions shall be used as the basis for calculating the percentage difference.
Table 7
Scheme for determination of analyte stability in solution
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