Characterization of Ballpoint Pen Inks by Thermal Desorption and Gas Chromatography-Mass Spectrometry

Abstract

The characterization of ink on paper is of importance for dating and comparing questioned ink entries in forensic document examination. Inks are commonly characterized by their colorant profile that is identified by well-established analytical methods. Numerous ink formulations show identical colorant profiles, though. In order to differentiate inks that are not distinguishable by colorant analysis, a method for the characterization of colorless ink ingredients, namely binders, solvents and additives is necessary. In this paper, we propose a technique for the analysis of colorless compounds in ballpoint inks using direct thermal desorption of the ink on paper followed by chemical analysis of the desorbed volatile compounds by gas chromatography-mass spectrometry. As compared to liquid extraction and subsequent analysis of the extracts, the technique avoids possible contamination risks. Sensitivity is very high due to the enrichment of volatile components by thermal desorption. Even from old samples, the chromatograms obtained by the method enable the determination of binder polymers, solvents and additives. Pure binders as used by ink manufacturers were analyzed for unambiguous assignment of analytical results to specific polymers. To prove the practical applicability, we analyzed 121 ballpoint pens, not all having the same colorant profile, and grouped the pens into resin and solvent categories.

A study of the evaporation of a solvent from a solution—Application to writing ink aging

“…Definitions involving vapor pressure p solution ðtÞ ¼ vapor pressure of solution at time t and at a given temperature (9) p solvent ¼ vapor pressure of solvent at a given temperature (10) Raoult's law, which relates p solution (t) and p solvent , is given by:…”

“…Currently the major approach for estimating the age of an ink is to look at its volatile solvent components [8][9][10][11]. In some cases one can determine how much of its volatile solvents it has before and after inducing aging (e.g., with heat).…”

Organic and inorganic discrimination of ballpoint pen inks by ToF-SIMS and multivariate statistics

“…Other weight [9]. The remainder is a range of additives that may include resins, viscosity adjusters, antioxidants, surfactants, softeners for ink flow ability and fatty acids for ball lubrication.…”

“…These are added to form salts with the acidic dyes and raise the pH of the ink [4]. Softening agents, such as phthalates; corrosion inhibitors, such as organophosphates; antioxidants, such as butylated phenols; and surfactants, such as alkylamines, are other common additives [9].…”

New perspectives in the use of ink evidence in forensic science: Part I. Development of a quality assurance process for forensic ink analysis by HPTLC

“…Several other analytical techniques were proposed for the forensic analysis of ink [29][30][31][32][33][34]. Even for HPTLC, several different solvent systems were developed and reported [23,24,29,30,[35][36][37][38].…”