Application of Raman Spectroscopy and Surface-Enhanced Raman Scattering to the Analysis of Synthetic Dyes Found in Ballpoint Pen Inks*

Raman spectroscopy can be useful for characterizing and discriminating inks based on their composition. It has been applied extensively to the analysis of a variety of inks including iron gall ink (1), lithographic ink (2), gel pen ink (3), and ballpoint ink (4,5). A variant of the technique, surface-enhanced Raman scattering (SERS) has also recently found its niche in ink analysis and has been used for ballpoint ink (6,7) and inkjet dye evaluations (8).

The major advantages of Raman include small sample size requirements, minimal sample preparation, and the lack of chemical or mechanical pretreatments (9). Raman spectroscopy has been found to be applicable for the analysis of several dye classes including azo (10) and arylmethane (11). Additionally, studies of various dyes have been conducted and the results have revealed excellent reproducibility for the technique (12,13). However, fluorescence interference and low sensitivity are a common problem, and they can often result in poor analytical performance.

SERS is becoming increasingly popular for the identification of organic dyes as it can quench fluorescence and provide enhanced signal, all of which ultimately lead to improved detection limits (14). Due to increased sensitivity and consequent ability to use only minimal samples, it can be considered a minimally destructive technique. The possibility of using SERS directly in situ on artwork and on thin layer chromatography (TLC) plates has been demonstrated in a few selected cases (15,16). SERS has been successfully used to obtain the spectra of natural and synthetic dyes of several classes and colors (17–20). Additionally, a study that focused on the separation of dyes found surface-enhanced resonance Raman scattering to be an effective technique in differentiating analytes with similar structures (21). The aim of this study was to compare normal Raman (NR) and SERS techniques for the analysis of dyes commonly found in ballpoint inks (22) to determine if the techniques are feasible for the forensic analysis of writing inks.

Materials and Methods

Results and Discussion

Normal Raman spectra were obtained for all the dyes with three different laser wavelengths. Based on the evaluation of all of the results, it was determined that only the FT system with the 1064 nm Nd/YAG laser performed consistently. All of the 1064 nm spectra had excellent signal intensity and signal to noise ratios. Spectra are displayed in 1-10 as follows: Acid Blue 1 (Fig. 1), Acid Orange 10 (Fig. 2), Acid Red 52 (Fig. 3), Aniline Blue (Fig. 4), Crystal Violet (Fig. 5), Methyl Violet (Fig. 6), Pararosaniline (Fig. 7), Rhodamine B (Fig. 8), Sudan Black B (Fig. 9), and Victoria Blue B (Fig. 10). All spectra were manually normalized for ease of comparison. No background subtraction or other spectral manipulations were performed.

The spectra obtained with the 633 and 785 nm lasers differed dramatically in their signal clarity for each of the dyes. Additionally, high levels of fluorescence were observed in all of the 633 nm spectra. Only Acid Orange 10 (Fig. 2) responded well to both laser wavelengths showing consistent peaks of high intensity. The 633 nm laser proved superior in performance over the 785 nm wavelength for Aniline Blue (Fig. 4), Sudan Black B (Fig. 9), and Victoria Blue (Fig. 10). These results could be explained by a resonance Raman enhancement (25), as the three dyes were similar in their deep blue color. The 785 nm laser provided clear spectra for the rest of the analytes including both blue-colored dyes, Acid Blue 1 (Fig. 1), and red-colored dyes, Acid Red 52 (Fig. 3) and Rhodamine B (Fig. 8). Overall, the results contained enough individualizing peaks to easily differentiate the dyes. The exceptions were the spectra of Crystal Violet (Fig. 5) and Methyl Violet (Fig. 6) which were extremely similar but so were the molecular structures of the compounds differing only in the number of methyl groups.

With the use of the 633 and 785 nm lasers, excellent SERS were obtained for all the dyes. There was c. a 4-fold increase in signal intensity observed with the 633 nm laser; however, Pararosaniline (Fig. 7) presented higher intensity spectra with the 785 nm laser. The NR and SERS spectra were found to be different in the intensities of individual peaks (e.g., Fig. 4, 917 cm⁻¹ peak), and the appearance of certain peaks in only one type of the spectrum (e.g., Fig. 2, 1618 cm⁻¹ peak). Additionally, some peak shifts between NR and SERS were observed (e.g., Fig. 8, 1280–1284 cm⁻¹ peak). These peak variations are explained by selection rules dictating which molecular bonds are Raman and SERS active (25). It must be noted that the SERS spectra were obtained from dilute solutions of the dyes as opposed to the solid samples used for NR measurements. The fact that good quality Raman spectra were obtained in these conditions is an indication of the signal enhancement obtained with SERS.

The TLC analysis showed that the Methyl Violet standard separated into three spots on the plate while the ink extraction presented only two spots corresponding to Methyl Violet in color, Rf values, and SERS spectra. Such results were probably due to different combinations of the demethylated pentamethyl Pararosaniline found in both the standard and the ink. The SERS of the ink dyes obtained from the TLC plate (Fig. 11) contained high intensity peaks and were consistent with the SERS spectra of the Methyl Violet standard (6, 11).

Conclusions

This study has demonstrated that high quality dye spectra can be obtained from pure dye samples with NR spectroscopy by carefully selecting the excitation frequency, but SERS consistently provided significant signal enhancement and fluorescent quenching for ballpoint ink dyes. Three laser wavelengths were evaluated, and for NR, the best results were observed when using the 1064 nm Nd/YAG laser. SERS spectra of equally high quality were obtained with the 633 and 785 nm lasers. Although fluorescence was a factor in the 633 nm NR spectra, it was mitigated with the use of SERS ultimately allowing for successful data collection.

The SERS spectra obtained after the ballpoint ink dyes and the reference dyes were developed on a TLC plate showed a high level of consistency with the standard dye spectra obtained with the drop method and on the TLC plate. Overall, the study successfully showed the applicability of NR spectroscopy and SERS to the analysis of synthetic dyes found in ballpoint inks. As only a single pen was used for this study, evaluation of dye components of an array of inks from different manufacturers would be valuable in establishing dye variations between manufacturers and batch-to-batch. Further studies should investigate additional dyes and pigments (e.g., phthalocyanines) and consider other ink components (e.g., vehicles, lubricants).