8.4 Aspects to be considered if analytes are unstable

Measure

Challenge

pH control

Sample extract pH may be crucial and therefore has to be monitored carefully.

A possible problem may be esterification of alcohols. If a low pH is used for sample preservation, then the alcohols in the sample may partially be esterified if there are also carboxylic acids present. This can be a problem if some of the analytes are alcohols.

Addition of stabilizers

Some additives themselves are not stable and therefore their concentration and thus also stabilizing efficiency changes in time.

The precise quantity of stabilizers in the sample is required to enable the optimal stabilizing effect [ref 62].

Protecting samples and standards from the light

Keeping samples and standards on the bench for minimum time possible. Working in the dark or under the yellow/red light.

Reducing sample processing time

Time-critical processes have to be managed – e.g. optimization of the time during each sample preparation step. 

Cold storage and handling of samples and standards

If analyte requires cooling, then all steps in the processing have to be carried out under cooling.

Long-term storage in cold (e.g. -70 °C; -80 °C)

Availability of -70 °C or -80 °C freezers.

Derivatization in an as early as possible stage (e.g. already at sample collection)

Time-critical and time-consuming.

Achieving completeness of reaction.

Different matrix effects when derivatization reagents are used compared to the initial method. Reproducibility issues during the derivatization that can be taken into account with usage of IS or ILIS. Derivatization often increases s_r and s_RW. IS and ILIS help reducing s_r and s_RW.