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The characterization of ink jet dyes and inks printed onto paper surfaces is of importance for the development of ink jet ink formulations and for use in forensic science. Raman spectroscopy is an effective and informative probe for this purpose if problems associated with fluorescence are overcome. A comparison is made here between two effective techniques, surface-enhanced resonance Raman scattering (SERRS) and near-infrared Fourier transform Raman scattering (NIR-FTR). SERRS provides a good method for obtaining in situ measurements using a visible laser system. It is fast (1–10 s accumulations) and requires very low laser powers (<1 mW). However, it requires the addition of a small dot of silver colloid to the paper surface in order to produce the enhancement. NIR-FTR is noninvasive and simpler to use. However, much higher laser powers are required (>200 mW), and the spectral accumulation time is significantly longer. Both methods overcome fluorescence effectively in most samples studied; however, each is more effective than the other with selected inks and paper substrates. SERRS involves resonant enhancement of the chromophore of the dye in contact with the silver surface, and consequently it is the dye chromophore which is uniquely identified. With NIR-FTR, signals from the paper and the filler are also observed. Comparable spectral patterns with clear, explicable differences are obtained from each method, indicating in particular that SERRS spectra can be interpreted without recourse to specific surface selection rules. The combination of the two techniques provides some information on the electronic as well as the vibrational properties of the dyes in situ.

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A study of the e †ect of concentration of dye and of the conditions of aggregation for surface enhanced resonance Raman scattering (SERRS) from rhodamine 3B and 3G adsorbed on silver colloid indicates that quantitative results can be obtained provided that the concentration of the dye on the colloid surface is kept well below monolayer coverage. There are no interferences due to other adsorbates because of the predominance of the scattering from the chromophore. In addition, Ñuorescence is quenched and low excitation powers are required. The signals are stable for at least 1 h and the RSDs are in the range 6-11% depending on the aggregation method. The preferred method of aggregation is to use as an aggregating agent poly(L-lysine) with ascorbic acid because poly(L-lysine) increases the control over the process. At concentrations equivalent to surface coverage above about one tenth of a monolayer, the dependence of signal on concentration is complex owing to changes in the packing and orientation of the dye at the surface and to multi-layer e †ects.