Substrates for the at-line coupling of capillary electrophoresis and surface-enhanced Raman spectroscopy

In this preliminary study, we evaluated four different types of substrate for the at-line coupling of capillary electrophoresis and surface-enhanced (resonance) Raman spectroscopy, CE–SER(R)S, with emphasis on spectral repeatability. We tested Sub1: etched silver foil, Sub2: a vapour-deposited silver film, Sub3: a silver oxalateprecoated silica TLC plate and Sub4: a silica TLC plate on which colloid and poly(L-lysine) were manually added to the analyte spots, used earlier in at-line CE– SE(R)RS. All substrates were first tested by manual spotting using trans-1,2-bis(4-pyridyl)ethylene (BPE) as a model compound for SERS and crystal violet (CV) as a model compound for SE(R)RS. The spectral features of the SE(R)RS spectra of BPE and CV showed a most satisfactory repeatability on all substrates. As expected, the signal intensities varied considerably between runs; this implies that quantification in at-line CE–SE(R) RS should rather be done by means of an on-line absorbance detector. In addition, the suitability of Sub1, Sub2 and Sub4 as deposition substrates after CE was explored using two cationic dyes: CV and basic fuchsin (BF). Good-quality SERRS spectra could be recorded on all three substrates. Although Sub1 and Sub2 have a poor water-sorptivity, they were found to be good substrates for at-line CE–SERRS. They do not require any post-deposition addition of silver colloid and could therefore become attractive alternatives for Sub4.