Several studies (Jones, Spencer, & Truesdale, 1982; Stolc &
Nemeth, 1961; Truesdale & Smith, 1975) have postulated that
not only iodine can catalyse the Sandell and Kolthoff reaction,
but sodium and iron can accelerate this reaction, raising the positive
bias of the catalytic spectrophotometric methods compared
with ICP-MS. Food samples (Table 3) with high levels of sodium
and iron, especially fermented fish and shrimp paste, had a very
high relative bias. Iodine content in iodine-enriched hen egg was
higher than in fermented fish but the relative bias for
iodine-enriched hen egg was lower than that for fermented fish.
This may be due to the lower level of sodium in iodine-enriched
hen egg (134 mg/100 g) compared with the levels in fermented fish
and shrimp paste (7180 and 6000 mg/100 g, respectively). Sodium
in the studied milk powder was lower (average of 220 mg/100 g)
than that in the SRM milk samples (average 426 mg/100 g), but
the iron content was much higher (6.2 mg/100 g compared with
0.2 mg/100 g). The relative positive bias of the iodine concentrations
in the studied milk powder determined by spectrophotometry
was 38–70%. This observation may provide additional evidence
for a catalytic effect of iron on iodine measurements based on
Sandell and Kolthoff reaction