Resolution of vitamin A cis/trans isomers resolves proficiency test assessments

Proficiency tests run by FAPAS for vitamin A analysis in cod liver oil were observed to give rise to multi-modal distributions of data. For this reason, setting the assigned value for the tests was difficult or even impossible, without too high uncertainty values. The methods used by participants could broadly be categorised into normal phase or reversed-phase high performance liquid chromatography. In a recent test, participants were asked if their method could separate the cis and trans isomers of vitamin A. Results from participants who could separate the isomers gave rise to a unimodal distribution of data. This was synonymous with participants using the normal phase chromatography method. Conversely, results from participants who were unable to separate the isomers, or who were using reversed-phase chromatography, gave rise to multi-modal distributions of data. The recommendation for future tests is to set the assigned value on the basis of the ability to separate the isomers.