Publication: Chemical profiling of erasable and non-erasable black gel pen inks using atr-ftir spectroscopy and chemometrics techniques
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Date
2026-02
Authors
Inn, Chee Jia
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Abstract
Ink analysis is a key component of forensic document examination, aiding forensic document experts in detecting physical or chemical alterations, fabrication, or tampering of documents, particularly in cases involving forgery, authentication, or other offenses related to written and printed evidence. Gel pens are commonly encountered in questioned documents, and differentiating erasable and non-erasable inks is forensically important. The aim of this study is to chemically profile and discriminate 10 different brands of erasable and 10 different brands of non-erasable black gel pen inks using ATR-FTIR spectroscopy combined with chemometric analysis. Pen ink samples from erasable and non-erasable gel pens were firstly analysed using ATR-FTIR at wavelength of 4000 cm-1 to 600 cm-1. Fingerprint region of spectral data which between 1500 cm-1 to 600 cm-1 were then processed using Principal Component Analysis (PCA) for pattern recognition. Linear Discriminant Analysis (LDA) was applied to classify the inks into their respective groups. ATR-FTIR spectroscopy provided information on the functional groups present in black pen inks but demonstrated limited discriminatory capability when used alone to distinguish between erasable and non-erasable samples. PCA showed clear clustering between erasable and non-erasable inks. The first three principal components (PCs) of PCA achieved 98.4% of total variance data of pen ink samples (PC1 89.2%, PC2 8.2% and PC3 1.0%). This percentage indicates an excellent data representation, showing that PCA successfully retained almost all meaningful variation in the pen ink samples with minimal information loss. LDA showed strong discriminative performance, with 95.0% correct classification in the original data and 90.0% accuracy under leave-one-out cross-validation, indicating good model stability and predictive ability. The coupling of ATR-FTIR spectroscopy with chemometric techniques of PCA and LDA markedly improves the discrimination and classification of gel pen inks by extracting meaningful chemical patterns from complex spectral data, thereby providing a robust, non-destructive, and forensically reliable approach for questioned document examination.
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