Determination of Chlorides, Sulfates, Bromides and Iodides in Food Coal-Tar Dyes by Ion Chromatography

Abstract

Soluble chlorides, sulfates, bromides and iodides in food coal-tar dyes can be determined by suppressed ion chromatography with a conductivity monitor.
Each food coal-tar dye was dissolved in water and ion-chromatographed directly with eluent containing 20% acetonitrile, 2mM TBAOH (tetrabutylammonium hydroxide) and 0.8mM sodium carbonate. IonPac NG1 was used as the guard column, and IonPac NS1 as the separating column. These columns are made from neutral polystyrene resin. The dyes are adsorbed on the guard column in the eluent containing the ion-pair reagent TBAOH, but four anions are separated by the separating column. After separation of the four anions, the columns were switched and the dyes were eluted from the guard column with 50% acetonitrile.
The recoveries of sodium chloride, sodium sulfate, sodium bromide and sodium iodide (each 10μg/ml) added to 1000μg/ml dye solution were in the range of 89.6-106.9%. The determination limits were 0.1μg/ml (NaCl), 0.2μg/ml (Na₂SO₄), 0.2μg/ml (NaBr) and 0.5μg/ml (NaI).
Thirty-one commercial samples were analyzed. Chlorides (as sodium chloride) and sulfates (as sodium sulfate) of all samples were within the limits of specifications. The commercial Food Red No. 104 contained 0.86-1.16% bromides as sodium bromide. The commercial Food Red No. 3 and the commercial Food Red No. 105 contained 0.07-0.22% iodides as sodium iodide.