Review of sodium analysis proficiency test results

Proficiency testing results for sodium analysis in foods raised concerns over the proportion of laboratories achieving satisfactory z-scores. Test materials for sodium analysis include fruit juice, canned meat meal, tomato sauce, cheese and pasta meal, and snack food. Fruit juice and tomato sauce data sets are the most problematic in deriving the assigned value with sufficiently low uncertainty to provide evaluative z-scores. The standard deviation for proficiency is derived from Horwitz, with a lack of collaborative trial data for these matrices to provide other guidance. The status of accreditation for the method/matrix does not appear to influence the observed variation in results. Microwave digestion is much less commonly used than simple acid digestion. The choice of determination method appears to be entirely matrix dependent (whether flame atomic absorption spectroscopy, flame photometry or inductively coupled plasma spectroscopy). Results for juice matrix may be overestimated if flame photometry is used, due to either potassium interference or careless reporting where potassium is also determined.