Optimized gradients for separation of dyes are achieved
considering the formation of neutral species from the ionized
analytes. A gradient elution prepared by a mixture of sodium acetate
buffer (0.1 M and pH 7) with acetonitrile was used for the
successful separation of 14 synthetic food colorants in standard
solutions [26]. At pH 7 all dyes are suitable for reverse phase
chromatography since they are neutral. The colorants were eluted
from the column according to increasing polarity. The first colorant
eluted from the column was the more polar dye tartrazine, while
the last was metanil yellow which presents a single sulfonated
group and no alcoholic substituents. Quinoline yellow (E104) consists
essentially of sodium salts of a mixture of disulfonates (principally),
monosulfonates and trisulfonates [34]. It shows three
peaks corresponding to relative isomers. The sum of the three
peaks was used for determination of recoveries and precision
validation data. Figs. 1e3 show chromatograms related to three
samples spiked with red (group AeB together), yellow (group C)
and blue dyes (group D).