Classification and Individualisation of Black Ballpoint Pen Inks Using Principal Component Analysis of UV--vis Absorption Spectra

• NCJ Number: 221727
• Journal: Forensic Science International Volume: 174 Issue: 1 Dated: January 2008 Pages: 16-25
• Author(s): Craig D. Adam; Sarah L. Sherratt; Vladimir L. Zholobenko
• Date Published: January 2008
• Length: 10 pages
• Sponsoring Agency: Nuffield Foundation
• Publication Format: Article
• Publication Type: Report (Study/Research)
• Language: English
• Country: United States of America

Annotation

This study evaluated the potential for the use of principle component analysis (PCA) in the forensic analysis of black ballpoint pen inks, using UV-vis spectrophotometry data.

Abstract

A forensic examination of inks is principally concerned with classification and comparison of chemically complex mixtures of dyes, pigments, and other materials usually present as writing on a document. The aim of most analysis is to determine whether two pieces of written texts originated from the same pen. The technique of principal component analysis has been applied to the UV-vis spectra of inks obtained from a wide range of black ballpoint pens available in the UK market. The loadings for the first three or four principal components were shown to characterize individual inks, both taken directly from the pen and when extracted from an ink line on paper using ethanol. Systematic classification of inks is possible using the signs and magnitudes of these loadings. Multiple measurements on the same type of pens showed that the statistical spread of the data could be small, though this might depend on the consistency of sample preparation. This approach may be used to classify and individualize a questioned ink from the database through matching the loadings for these principal components, and this approach may be facilitated by a two-stage principal component calculation based on first classifying then individualizing the sample. Analysis of the spectrum from multiple measurements on the same brand of pen showed excellent reproducibility and clear discrimination between inks that was supported by statistical analysis. It was possible to discriminate between the pen and the ink line from all brands examined in this way, suggesting that the solvent extraction process may influence the results. For the complete set of 25 pens, interpretation of the loadings for the first few principal components shows that both the pen inks and the extracted ink lines may be classified in an objective manner and in agreement with the results of parallel thin layer chromatography studies. Tables, figures, references