QUALITATIVE AND SEMIQUANTITATIVE TRACE ANALYSIS OF ACIDIC MONOAZO DYES BY SURFACE-ENHANCED RESONANCE RAMAN-SCATTERING

Resonance Raman (RR) and surface enhanced resonance Raman scattering ( SERRS) procedures are described for the analysis of acidic monoazo dye s. By the comparison of the RR spectra, discrimination is achieved bet ween 20 acidic monoazo dyes, including structural isomers. However, di fficulties are experienced due to fluorescence, to the narrow concentr ation range over which scattering is observed (10(-3)-10(-4) mol l(-1) ) and to the relatively high detection limit (approximately 3-5 mu g). These difficulties were overcome by the development of a robust, sens itive and selective SERRS procedure. Controlled aggregation of a citra te-reduced silver colloid and strong SERRS of the adsorbed dyes can be obtained if a 0.01% aqueous solution of poly(L-lysine) is added to an aliquot of colloid followed by aqueous solutions of the dye and ascor bic acid. The enhancement in scattering intensity compared to solution resonance is approximately 10(5)-10(6) and strong SERRS is observed f or sub-nanogram amounts of dye. In addition, the fluorescence backgrou nd is quenched and a wide concentration range can be examined. Models are proposed for the bonding of poly(L-lysine) with o-hydroxy-, p-hydr oxy- and o-dihydroxyarylazo dyes and a single model of interaction is proposed for the adsorption of the full set of dyes on the silver surf ace in the presence of ascorbic acid. The results of a blind trial con firm the usefulness of SERRS for qualitative analysis and highlight th e importance of sample purity. Linearity in a plot of concentration ve rsus scattering intensity was observed at low solution concentrations (<3 x 10(-6) mol l(-1)), supporting the application of SERRS for both qualitative and semi-quantitative analysis of trace amounts (greater t han or equal to 300-500 mu g) of acidic monoazo dyes.