Modus operandi

Based on several series of experiments, Laetita Heudt proposes to the reader, and in particular to the experts who are currently entrusted with conducting such analyses in a forensic context, a ‘flow chart’, an “analytical sequence which preserves to a maximum the integrity of the sample while offering a greater differentiation power”. This modus operandi, which represents an important part of the originality of this doctoral research, has been evaluated based on tests carried out on 10 samples: seven Hewlett-Packard, one Epson, one Lexmark, and one Canon. “These are the most representative brands of the Belgian market for ink cartridges”. Laetitia Heudt proposes the following procedure: if the samples are always identical after a preliminary optical analysis (luminescence, behavior in the infrared,…), it is preferable to change to analysis by Raman Spectroscopy, which is entirely non-destructive, easily carried out on the sheet of paper itself, and precious to the extent that it makes it possible to characterize the ink used according to the chemical nature of its dyes and/or principal pigments. Finally, if the samples remain indiscernible, it will be useful only in this case to use mass spectrometry. “I suggest the LDI method at first: take a sample of the sheet of paper, that is to say, a confetti-sized piece, and introduce it into the system to see what comes out. If the analysis yields good results, it is probably because we are faced with a pigment-based dye. In the opposite case, it is necessary to apply a matrix, and therefore to resort to a MALDI analysis. The thesis demonstrates the relevance of this latter kind of analysis: it is semi-destructive certainly, but it has a strong differentiation power and can be easily implemented, on condition that there is access to such an instrument as it is more expensive than the Raman analysis. In a general way, the thesis demonstrates that the methods which have shown themselves to be the most adapted were the Raman Spectroscopy, due to its non-destructive character, and the MALDI Mass Spectrometer for its versatility and its informative aspect”.

The fourth cartridge

A final original aspect of this doctoral research deserves to be mentioned. In a chapter entirely devoted to this subject, Laetitia Heudt focusses on the black ‘inkjet’ inks, which due to their specific nature require a different approach to colored inks. “Such inks had not yet been studied, to my knowledge” points out the researcher whose work on inks has been acclaimed by several experts. Here too, the Raman, LDI and MALDI techniques have been assessed based on ten printer ink samples. “We print in black and white most of the time. It is therefore the fourth cartridge of our printer which is most often used”. However, this poses a major question to the extent that the main pigment of these black inks — carbon black, a very common pigment also used in black pencils — invariably gives the same spectrum when it is subjected to Raman Spectroscopy. This method is therefore not at all a distinguishing feature from one ink to another. “On the other hand, at low masses, the LDI analysis makes it possible to detect a characteristic polymer in the six samples, whose mass is different each time. In addition, by comparing the spectra two by two, we notice  specific peaks in one ink and not in the other at low masses. The combination of these two types of information makes it possible to distinguish the inks of different brands, but also different models”. In collaboration with a North-American post-doctoral student, Laetitia Heudt has sought to systematize the comparison by developing a software program which makes it possible to compare a large number of spectra quickly and objectively. “This system allows us to make a preliminary scanning: to distinguish between the samples that are very different from each other, and those which become confused in a kind of buffer zone. The result is a similarity score between each ink of the samples in question."

All the light has certainly not yet been shed on the chemical composition of inks studied here, whether they are black or colored. “Series of inks have not been distinguished by means of the analytical sequence suggested, even though they come from different cartridges. Given the high cost of development when an ink of high quality is designed, the manufacturers use it in the different models of commercial cartridges. The differences are at an electronic or design level, the system of expulsion of the ink droplet etc.” There are many differences which are not detected by chemical analysis of the ink. “It therefore remains difficult to link an ink sample to a determined commercial cartridge, and therefore to an instrument model”.