Applicability of surface-enhanced resonance Raman scattering for the direct discrimination of ballpoint pen inks

In situ surface-enhanced resonance Raman spectroscopy (SERRS) with excitati on at 685 nm is suitable for the direct discrimination of blue and black ba llpoint pen inks on paper. For black inks, shorter excitation wavelengths c an also be used. For blue inks, SERRS at 514.5 and 457.9 nm does not provid e adequate discriminative power. At these excitation wavelengths. the SERRS signals of the Methyl Violet derivatives present in inks easily dominate t he overall spectrum because of resonance enhancement and preferential inter action with silver sol particles. At 685 rim, this problem is not encounter ed as the Methyl Violet derivatives do not show resonance enhancement, whil e other components may still exhibit resonance. Thirteen blue and thirteen black ink lines were examined. For the blue and black inks, on the basis of the 685 nm SERR spectra, eight and six groups of spectra, respectively, co uld be distinguished. This discrimination largely agrees with information f rom thin layer chromatography (TLC) experiments, although some differences in group compositions are found. The in situ SERR spectra show good repeata bility with regard to the Raman frequencies, band shapes and relative inten sities of the spectral bands. However, absolute intensities cannot be used for discrimination purposes.