In suspensions and solutions, the dyes used are water soluble and include natural pigments such as chlorophylls, carotenoids and anthocyanins, and coal tar-based dyes. More effort may be required to remove interference by these materials. Solid phase extraction with ion exchange resins may be useful for removing anionic or cationic dyes, although simple extraction of the drug into organic solvent of moderate polarity may leave such dyes in the aqueous phase. Solutions tend to contain anti-microbial preservatives and anti-oxidants. These are usually either phenols or quaternary amines such as benzalkonium chloride and have strong enough chromophores to interfere in the analysis of a drug. These compounds have to be removed prior to analysis by extraction procedures. Suspensions also contain surfactant materials such as the polyethylene glycol-based detergents but these compounds do not have appreciable UV absorbance and thus have little potential for interference.
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