Printing inks analysis and discrimination abilities using laser induced breakdown spectroscopy and energy dispersive x-ray fluorescence

Abstract

Establishing the authenticity of a questioned document remains one of the pertinent aspects in criminal as well as civil investigations. Because different manufacturers may use varying compositions of inorganic materials for producing inks, its quantitation may lead to the possibility of identifying the unique chemical fingerprint for each manufacturer. Hence, this present research that reported the use of in-house developed laser-induced breakdown spectroscopy (LIBS) and energy dispersive x-ray fluorescence spectrometry (EDXRF) to determine the inorganic composition of several different black printing inks, coupled with principal component analysis (PCA), acquires forensic significant. The samples analysed were from three types of printers viz. inkjet, laser, and photocopier (three different brands for each type) and one control blank white A4 paper. The optimum laser energy for enabling LIBS analysis was found at 900 mJ. While LIBS was found suitable for detecting several elements, especially those with low atomic numbers (Z<30), detection of higher atomic number (Z>30) elements was evidently suitable following the use of EDXRF. PCA-LIBS was found to sufficiently discriminate all the different printing inks based on qualitative elemental differences. Lower discriminative abilities were observed for PCA-EDXRF and PCA-LIBS-EDXRF combinations. Therefore, considering its time and cost effectiveness as well as requiring only minute amount of sample with no sample pre-treatment steps, combination of PCA-LIBS evaluated here, may prove useful for forensic questioned document practical caseworks.