All of the techniques differ in the sample processing (destructive versus nondestructive) and information obtained from ink analysis.
Some of these procedures are nondestructive (Harris 1991; Trzcinska 1993; Varlaskin and Low 1986), whereas others are more destructive than thin-layer chromatography extractions because they require that more samples be physically removed from the document (Merrill and Bartick 1992; Trzcinska 1990).
Brewer Chemist Federal Bureau of Investigation Laboratory Chemistry Unit Quantico, Virginia Abstract | Introduction | Experimental Samples | Capillary Electrophoresis| Cationic Dye | Anionic Dye | Thin-Layer Chromatography | Laser Desorption/Ionization Time-of-Flight Mass Spectrometry | Results and Discussion | Reliability and Validation | Ballpoint Pen Inks | Other Applications | Conclusion | References Capillary electrophoresis with ultraviolet-visible photodiode array detection (190–600 nm) was studied as an alternative separation and identification tool for forensic ink examination.
Two different buffer systems were designed to analyze dye compounds in various black ballpoint pen ink formulations.
A limited sample set of available inks allows infrared patterns to be distinguished, but as more pens are added to a database, statistical analysis methods will become less useful.
Laser-induced infrared luminescence has been investigated as another method to quickly distinguish two inks of the same color (Horton and Nelson 1991).
Results were compared to thin-layer chromatography experiments to evaluate the sensitivity and performance of capillary electrophesis.
A database of ballpoint pen ink analyses and common-dye reference standards has been constructed for future forensic use.
Typical forensic techniques on questioned documents involve ink extractions followed by thin-layer chromatography analysis (Standard Guide for Test Methods for Forensic Writing Ink Comparison 1996; Tebbett 1991).
Knowledge gained from this procedure is limited to colored spots that are correlated to a calculated retardation factor (R).
Densitometry also can be used to probe the concentration of dye on a specific area of the thin-layer chromatography slide in reflectance mode (Aginsky 1994).
Carefully prepared standards are required for quantitative analysis of an unknown sample.
Problems with thin-layer chromatography reproducibility can be related to the difficulty in spotting uniform samples and maintaining constant environmental influences, which lead to changes in R values.