Does sodium lauryl sulfate concentration vary with time?

When performing skin irritation tests with sodium lauryl sulfate (SLS), the quality of the test solution is of major importance for the reproducibility and comparability of the results. The influence of 4 different storage parameters (concentration, duration, temperature, material of the storage vials) on the stability of aqueous SLS solutions was investigated under non-sterile conditions. SLS solutions at 5 different concentrations (0.001%, 0.01%, 0.1%, 0.5%, 1%) were analysed by mobile phase ion chromatography. Analyses were performed after a storage time of 3 h, 1 week, and 4 weeks. Storage was carried out at different temperatures: -18 degrees C, 6 degrees C, and 23 degrees C. The storage containers were of 2 different materials (glass and polypropylene). After a storage of 1 week, no decrease in SLS concentration was observed, regardless of the chosen conditions. After 4 weeks at 6 degrees C and 23 degrees C, the SLS concentration was found to be significantly decreased for the 2 lowest concentrations (0.001% and 0.01%). At higher concentrations and lower temperatures, no decrease in SLS concentration had occurred. In parallel to the loss of SLS, contamination with bacteria was found in the solutions, especially at the 2 lowest concentrations. Bacterial growth was not observed at higher concentrations. The possibility of biodegradation of SLS has to be taken into account, especially when dealing with low concentrations of SLS.