A new compact system encompassing in flow gas diffusion unit and a wall-jet amperometric FIA detector,
coated with a supramolecular porphyrin film, was specially designed as an alternative to the time consuming
Monier-Williams method, allowing fast, reproducible and accurate analyses of free sulphite
species in fruit juices. In fact, a linear response between 0.64 and 6.4 ppm of sodium sulphite,
LOD = 0.043 ppm, relative standard deviation of ±1.5% (n = 10) and analytical frequency of 85 analyses/h
were obtained utilising optimised conditions. That superior analytical performance allows the precise
evaluation of the amount of free sulphite present in foods, providing an important comparison between
the standard addition and the standard injection methods. Although the first one is most frequently used,
it was strongly influenced by matrix effects because of the unexpected reactivity of sulphite ions with the
juice matrixes, leading to its partial consumption soon after addition. In contrast, the last method was not
susceptible to matrix effects yielding accurate results, being more reliable for analytical purposes.