Coupling of column liquid chromatography and surface-enhanced resonance Raman spectroscopy via a thin-layer chromatographic plate

Surface-enhanced resonance Raman (SERR) spectroscopy was used to characterize compounds separated by column liquid chromatography (LC). Three percent of the effluent from a conventional-size LC column were immobilized on a moving thin-layer chromatography (TLC) plate using a spray-jet solvent-elimination interface. Next, colloidal silver was applied to the analyte spots and in situ SERR spectra were recorded with a multichannel micro-Raman spectrometer. Storage of the LC effluent avoids the need to use a continuous flow of colloidal silver and, in principle allows compounds to be detected independently of LC conditions like eluent composition and flow rate. Using dyes as test compounds, the method was optimized and aspects were studied such as type of TLC plate, LC separation of the dyes, preservation of LC integrity during immobilization, and SERR analysis of the deposited compounds. With a silica TLC plate as deposition substrate, good-quality and characteristic SERR spectra were obtained for the dyes which were separated on a cyanoproyl-modified silica LC column with methanol-water containing the volatile additives ammonium acetate and triethylamine as eluent. The minimum identifiable concentration of the dyes was about 250 ng ml−1 (750 pg applied on the plate). For some strong Raman scatterers such as nile blue and pararosaniline, the major spectral peaks could be observed down to concentrations of 25 ng ml−1.