Black Toner Comparison using Solubility Tests: A Case Report

This paper describes an actual case examination in which optical methods, solubility tests, and thinlayer chromatography (TLC) were used to determine whether two questioned documents, printed/ copied on an office machine(s) using black toner, were produced on or around their purported dates (several years apart), or whether the documents were produced contemporaneously (in close proximity in time, i.e. not several years apart, as dated). In this civil case, based on the results of the optical and TLC examinations of the toner on both questioned documents, the Defendant’s expert found and testified that the black toners in these documents had a connection and were consistent with each other, and this finding was used as a chemical basis for the expert’s opinion that the two documents were most probably produced contemporaneously—not several years apart, as dated. This author, retained as a rebuttal expert, repeated the above examinations conducted by the Defendant’s expert, and determined that the only chemical connection and consistency between the two toners was that they both contained a common pigment—carbon black. While optical and TLC examinations could not distinguish between the black toners on the two documents (these results were in agreement with the finding of the Defendant’s expert), the solubility tests (chemical extraction of toner samples in chloroform to prepare extracts to be applied on TLC plate for chromatographic analysis) showed that the two toners had significantly different extractability in chloroform; evidence that the toners were of differing chemical composition.
Two common parameters of solubility (extractability) of a solid substance in a solvent are the speed at which the solid dissolves (rate of dissolution/extraction) and the completeness of the dissolution (extent of dissolution/extraction). In this case, the difference in the solubility of the two toners in chloroform was clearly evident and very significant (as in, for example, the drastic difference between the solubility of instant coffee and ground coffee in hot water): one toner was extracted in chloroform almost instantly (within seconds) and completely, while the other toner dissolved only partially (not completely) and the process of dissolution (formation of a suspension of carbon black in chloroform) was slow. Thus, in this comparative examination of two documents produced by electrophotography using black toners, the solubility tests proved to be more discriminating (provided more discriminating power) than the optical and TLC methods.