Using a MS library to differentiate black ballpoint pen inks by Py-GC/MS

Importance of the topic

Black ballpoint ink analysis is crucial in forensic investigations because these pens are widely used and their inks often appear in questioned documents. Differentiating inks helps establish authorship, detect forgeries, and support legal evidence.

Objectives and overview of the study

The study aimed to develop and validate a mass spectral library of oil-based black ballpoint pen inks using pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). By compiling pyrograms from 35 commercial inks, the researchers sought to identify unknown ink samples rapidly and accurately against the library.

Methodology and instrumentation

Sample preparation:

• Ink was applied to copy paper and a 5 mm square punch was taken.
• 50 µL dichloromethane was used to extract ink components; solvent was evaporated under N₂.
• Residue was pyrolyzed at 550 °C.

GC/MS conditions:

• Pyrolysis temperature: 550 °C.
• GC oven: 40 °C to 300 °C at 20 °C/min.
• Column: Ultra ALLOY+-5 (5 % diphenyl/95 % dimethylpolysiloxane, 30 m × 0.25 mm i.d., 0.25 µm film).
• Carrier gas: helium, 1 mL/min; split ratio 1:50.
• Sample load: ~200 µg.

Instrument used

The analysis utilized a multi-functional pyrolyzer coupled to a vent-free GC/MS adapter, with spectral matching performed via F-Search software.

Results and discussion

Pyrolysis yielded two types of compounds:

• Major solvents (phenoxyethanol, dimethylaniline) eluting before 7 min, common to many inks.
• Dyes and specialized additives eluting after 7 min, unique to each ink.

Mass spectra generated for the entire run provided limited discrimination due to common solvent ions (m/z 77, 94, 138). By constructing a second spectral library using only peaks after 7 min, the match quality improved markedly and allowed unambiguous identification of an unknown sample as Ink C.

Benefits and practical applications

The described Py-GC/MS approach:

• Enables rapid, reproducible differentiation of black ballpoint inks.
• Reduces misidentification by excluding common solvent signals.
• Supports forensic laboratories in document examination and forgery detection.

Future trends and potential uses

Advancements may include:

• Expansion of the library with more ink formulations and colors.
• Automated data processing and machine-learning algorithms for pattern recognition.
• Integration with other spectroscopic methods (e.g., Raman, infrared) for comprehensive ink profiling.

Conclusion

The study demonstrates that selective mass spectral library construction focusing on minor components enhances the forensic discrimination of black ballpoint pen inks. Py-GC/MS proves to be a robust, sensitive tool for establishing ink identity in legal contexts.

Reference

Watanabe Y., et al. Proceedings of the 8th Annual Meeting of the Japan Association of Forensic Science and Technology (2002)