Synthetic colorants quantification

HPLC is often reported to be the technique of best choice for the quantification of food colorants. According to European Directive 94/36/EC, the quantities of synthetic colorants to be added to foods are restricted and thus reliable methods for their quantification must be established. Approved colorants, defined by E-coded numbers (Table 6.6.2), are permitted for non-alcoholic beverages, confectionery products, and even for caviar (dying fish roe). For example, a specific HPLC chromatographic method for the quantization of 14 synthetic food colorants belonging to azo dye, triphenyhnethane, or quinophthalone classes (E 102,104, 110, 122,123, 124, 127, 128, 129, 131, 132, 133, 142, 151) was reported to check their contents in caviar. ... 

Kirschbanm, 1., Kranse, C., and Brnckner, H., Liquid chromatographic quantification of synthetic colorants in fish roe and caviar, Eur. Food Res. TechnoL, 222, 572, 2006. 

Reverse-phase and ion-pair chromatography have been widely used for the separation and quantification of synthetic colorants. Because of the large number of dyes that may be present in food, it is preferable to use at least two quite different HPLC systems to ensure the identification of such compounds (168). 

Only for dietary supplements or processed food samples, extraction of anthoeyanins followed by solid phase purification with subsequent analysis by HPLC with UVA/ IS detection is performed as first level analysis. The matrix in those samples is complex and may include synthetic colorants in accordance with applicable food legislation, hence simple UVA IS analysis would yield most certainly erroneousness results. At this stage of analysis a decision is necessary on whether or not HPLC/MS analysis has to be performed. HPLC/ MS is powerful for the confirmation of anthocyanin structures but seldom useful for quantification as the calibration is complicated and robustness is low. 

To avoid quantification errors caused by the multiple manipulations of the sample during the various steps of extraction and preparation, the use of an internal standard (IS) in combination with the external calibration is advisable. The IS must be chosen carefully, as it has to meet a series of minimum requirements. It must be a carotenoid pigment not present in the sample to be analyzed, it must be chromato-graphically separable from the others under the analytical conditions used, it must have a A ax absorption as close as possible to the A of detection employed, and it must be as stable as possible. Various ISs have been proposed (3-apo-8 -carotenal and canthaxanthin are commonly used in the analysis of vegetable foods.The use of artificial colorants, such as Congo red and Sudan 1, and of synthetic carotenoids not present in natural samples, such as C45-(3-carotene, has also been proposed. ... 

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