Analysis of Black Writing Ink by Electrospray Ionization Mass Spectrometry

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Emily C. Lennert

Category: Chemistry

Keywords: ink, pens, black ink, ballpoint, gel, rollerball, paper, electrospray ionization, mass spectrometry, ESI-MS, documents

Article to be reviewed:
1. Williams, M. R.; Moody, C.; Arceneaux, L.-A.; Rinke, C.; White, K.; Sigman, M. E. “Analysis of black writing inks by electrospray ionization mass spectrometry.” Forensic Science International. 2009, 191, 97–103.

Disclaimer: The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.

Summary:

Ink analysis can serve a number of purposes in forensic casework, such as to determine if two or more samples share a common source or to date a document, e.g. to identify how long ago a document was signed. Ballpoint, gel, and rollerball pens are frequently used writing instruments. The ink required for each type of pen will vary, having different properties due to the difference in the mechanism by which the ink is deposited on the writing surface. Regardless of the difference in formula, all writing inks share two main components, a colorant and a vehicle. The colorant is either the dye or pigment, which gives the ink its color. Dyes are colorants that solubilize, or dissolve, within the vehicle, whereas pigments are finely ground colorants that are mixed into the vehicle. The vehicle entails the other components of the ink, including solvents, lubricants, resins, biocides, corrosion inhibitors, and more, which moves the colorant from the pen to the paper. Variation in the formulas of inks can allow for differentiation of inks; consequently, writing instruments may be linked to a crime based on the ink formulation.

Electrospray ionization-mass spectrometry (ESI-MS) was chosen to perform ink analysis in this study. The authors state that ESI-MS has previously been investigated for ink analysis, is a well established technology, and is widely available in forensic laboratories. This research was performed on black pen inks, which were to be extracted from various papers. The authors state that this research presents an analytical scheme that is minimally destructive to the sample source and provides a method for dye and vehicle extraction as well as ESI-MS analysis.

A total of 18 black ink pens were analyzed in this experiment: 6 rollerball, 6 gel, and 6 ballpoint. Each pen was tested on 11 different types of paper, including receipt paper, lined legal pad paper, cotton resume paper, a colored Post-it note, and a floral collage letterhead paper. Each pen was used to write on each paper, samples were collected within an hour of writing, once the ink was dry. Tweezers were used to remove a small amount of ink-coated fibers from the paper, as seen in Fig. 1 within the paper. Fibers with no ink were also collected from each paper to use as a control; analysis of the paper alone allows the analyst to remove the profile of paper from the results of the ink-coated fibers, leaving only the data from the ink. After fibers were removed, the fibers were placed in small glass tube and 20μL of solvent was added for ink extraction. Three different extraction solvents were examined: methyl alcohol (methanol), ethyl alcohol (ethanol), and butyl alcohol. After the addition of

solvent, each tube was sealed and a 30 minute extraction was carried out at room temperature. Of the solvents tested, methanol was the most successful in extraction of inks from paper fibers. Ten of the 18 pens showed visible dye extraction by methanol: 2 rollerball, 2 gel, and all 6 ballpoint. The authors state that many gel and rollerball pens employ insoluble pigments as colorants, thus no dye was extracted from those pens. Although no dyes were extracted in those samples, the vehicle was extracted from the paper fibers, allowing for ESI-MS analysis.

After extraction, each sample was analyzed by three different ESI-MS methods, to determine what method would be most successful for ink analysis. The method that successfully analyzed all of the compounds in the inks was selected. The ESI-MS methods shared similar parameters, with variations in a mobile phase additive, ionization mode, and voltage. The spectra obtained from the clean paper fibers were subtracted from the spectra of its respective ink-coated fibers, producing very similar spectra for each pen, regardless of which paper it was extracted from. Standards, or solutions containing components that may be in the inks (e.g. dyes, lubricants, resins, etc.), were also analyzed by ESI-MS to aid in identification of compounds within the ink spectra.

Fig. 3, within the study, displays the ions observed in the mass spectra for each pen, by each ESI-MS method, on one type of paper. Dyes and vehicles within the inks were identified based on correspondence with ions observed in the spectra obtained from analysis of the standards, as seen in Fig. 4 within the study.

Ballpoint pen inks: Two dyes were identified: basic violet and acid yellow 36. The vehicle 2- phenoxyethanol was also identified, and was absent from most gel and rollerball inks. The authors state that 2-phenoxyethanol is a common vehicle used in ballpoint pen inks.

Rollerball and gel pen inks: Similar compounds were identified in rollerball and gel inks. Benzotriazole, a rust inhibitor, was identified in all gel inks and four of the six rollerball inks. Triethanolamine, a pH buffer, was identified in five gel inks and one rollerball ink.

Visual comparison of the data available in Fig. 3 shows that, for all samples, no pens shared the same spectra. Not only are the classes of pen (ballpoint, gel, rollerball) easily distinguished, but pens within the same class do not share the same spectra, for example, ballpoint pen 1 does not correspond to ballpoint pen 4. Spectra obtained from rollerball pens were observed to contain significantly more ions than those obtained from gel pens, making the two classes distinguishable despite some shared compounds.

The last consideration in this study is the volatile components that may be found in inks. The authors cite a previous study that brought volatile, or easily evaporated, components of inks into question. Volatile compounds may be lost as the ink dries or ages. In an effort to examine this, the authors conducted a small-scale experiment to determine the effect of time on volatile ink components. Three pens were used, one from each class, with each pen being selected since 3 volatile compounds were present in its initial spectral, obtained from the earlier experiments. The pens were used to mark paper, and samples were taken and extracted at 4 separate intervals: immediately after drying (approximately 1 minute), 1 hour, 1 day, and 1 week. All samples were analyzed by ESI- MS method. The authors reported that ions associated with the 3 volatile compounds were present in samples extracted at times up to 1 hour, but no longer present in samples extracted at 1 day or at 1 week. When ions associated with those 3 volatile compounds were removed from the data of the earlier experiment, Fig. 3 within the study, several ballpoint pens were no longer differentiable. However, differentiation of all rollerball and gel pens remained unchanged.

This study demonstrated a minimally destructive technique for ink analysis from documents; the removal of fibers was either visually insignificant or not noticeable at all. Additionally, an efficient extraction method was discovered, and the study presented several methods for ESI-MS that are suitable for ink analysis.

Scientific Highlights:

  • The method put forth requires a small sample quantity and is minimally damaging to the source.
  • Extraction of a small quantity of fibers in 20μL of methanol provides sufficient amounts of sample for analysis by ESI-MS.
  • The extraction solvent, methanol, allowed for extraction of colorants from inks containing dyes, and of vehicles within all inks.
  • Fresh ink samples appear to be differentiable, regardless of ink type.
  • Dried or aged ballpoint pen inks may become indistinguishable as volatile components are lost. However, rollerball and gel pens do not.

Relevance: Ink analysis can provide valuable evidence. Inks that are distinguishable may provide a link between a crime and suspect.

Potential conclusions:

  • ESI-MS is a useful tool for ink analysis; it is a well tested and commonly accessible instrument.
  • Pen inks can be differentiated by analysis of compounds that serve as colorants and vehicles within the inks.
  • Inks, particularly ballpoint pen inks, may become less differentiable as the time between writing and extraction increases.