Therefore, it appears that for many CECs, processing onto SPE cartridges after sample collection is best practice. This indicates that CECs in general appear more stable on sorbent then in aqueous form. This is in agreement with Singh et al., who reported that shipping internationally on SPE cartridges (kept cold) provided the best analyte stability (R. R. . Whilst deuterated analogue internal standards account for degradation in transport and storage, it is not practical in large multi-residue methods for every CEC to have its own deuterated internal standard. Whilst internal standards tend to be assigned across class, assuming similar chemical properties, results from this study do indicate that stability can vary across compounds of a class. Therefore, spiking of internal standards and processing onto cartridge as soon as samples are collected is recommended as greater stability has been observed when CECs are immobilized onto sorbent.