Division 3.1.1.4 Stability

It has been observed that insufficient stability of the analyte or matrix constituents in the sample during storage or analysis may give rise to significant deviations in the outcome of the result of analysis. Furthermore, the stability of the calibration standard in solution should be checked. Usually analyte stability is well characterised under various storage conditions. Monitoring of the storage condition will form part of the normal laboratory accreditation system. When this is not known, examples are given below on how the stability can be determined.

Stability of the analyte in solution:

• 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 (around the minimum required performance limit for substances for which no permitted limit has been established or around the permitted limit for other substances. Prepare both solutions of the analyte used for fortification and used in the final analysis solution, and any other solution that is of interest (e.g. derivatised standards).

• Measure the analyte content in the freshly prepared solution according to the test instructions.

• Dispense appropriate volumes into suitable containers, label and store according to the scheme:

  Table 12

  Scheme for the determination of analyte stability in solution

  	– 20 °C	+ 4 °C	+ 20 °C
  Dark	10 aliquots	10 aliquots	10 aliquots
  Light			10 aliquots

• The storing time could be selected as one, two, three and four weeks or longer if necessary, e.g. until the first degradation phenomena are observable during identification and/or quantification. The maximum storing time and the optimum storing conditions have to be recorded.

• The calculation of the concentration of the analyte(s) in each aliquot should be carried out by using the solution of the analyte freshly prepared at the time of analysis as 100 %.

  • Analyte Remaining (%) = C_i × 100/C_fresh

  • C_i= concentration at time point

  • C_fresh= concentration of fresh solution

Stability of analyte(s) in matrix

• Whenever possible, incurred samples should be used. When no incurred material is available, matrix fortified with the analyte should be used.

• When incurred material is available, the concentration in the material should be determined while the material is still fresh. Further aliquots of material could be taken after one, two, four and 20 weeks and the concentrations should be determined. The tissue should be stored at least minus 20 °C or lower if required.

• If no incurred material is available, take some blank material and homogenise it. Divide the material into five aliquots. Fortify each aliquot with the analyte, which should preferably be prepared in a small quantity of aqueous solution. Analyse one aliquot immediately. Store the remaining aliquots at least minus 20 °C or lower if required and analyse them after one, two, four and 20 weeks.