A comparative study

It is clear that the timing of the doctoral thesis of this young chemist could not be better. More concretely, the “Molecular analysis of printing inks for forensic purposes” assesses three analytical techniques with regard to their application to the analysis of ‘inkjets’ directly on the paper. Three techniques which, in other words, make possible both a surface analysis of paper samples and an examination of the dyestuffs contained in these inks: Raman Spectroscopy and Mass Spectrometry equipped with ionization sources DESI (Desorption ElectroSpray Ionisation) and MALDI ([Matrix-Assisted] Laser Desorption Ionization. The originality of the approach chosen by Laetitia Heudt lies, as we will see, in “considering these different methods as being potentially complementary and not in competition with each other”. Following the preliminary results obtained with the DESI source, such as was available at the laboratory whilst Laetitia Heudt worked on her thesis, this method was rapidly eliminated from the forensic methods potentially worth considering for analysing ink on suspect documents. That is why this method will not be explained in detail in the rest of this article.

What is involved exactly? Firstly, the perfection of a technique right from the start: “An analysis by Raman Spectroscopy involves the illumination of a sample with a laser beam, that is to say, a source of light. The sample, in this case, will be a piece of paper. When the laser beam enters into contact with the sample, a small fraction of the light diffused is characteristic of the molecules that the sample is composed of. Thanks to a software program, the technique makes it possible to visualize a spectrum that characterizes the constituent dyes and pigments of the ink present on the piece of paper. Two distinct molecules are characterized by a spectrum that is specific to each one of them”. Raman Spectroscopy, on the condition that the power of the laser is properly controlled, is a non-destructive method for characterizing the molecular composition of a sample: it gives a sort of digital imprint to each molecule. Due to their chemical structure, the dyes contained in the inks easily lend themselves to analysis by Raman Spectroscopy. Over the last few years, Raman Spectroscopy has really taken off in the area of forensic science analysis and particularly for analysis of hand-written inks. Therefore it seemed ‘legitimate’ to want to also assess this method for the analysis of ‘inkjet’ inks. However, “the Raman spectrum of an ink is often dominated by the Raman signal of the main dye. If this is identical in the two inks, they will be difficult to differentiate by Raman Spectroscopy”. It is necessary, in this case, to use other methods of analysis.

So what about Mass Spectrometry equipped with the ionization source known as Laser Desorption Ionisation? It is an analysis technique which, in order to identify the molecules of a sample (such as an ink), measures its mass. All the masses of all the molecules of the sample constitute the mass spectrum. Concretely, in the case of the LDI source, a laser beam is directed onto the sample to “desorb” , meaningviolently extract molecules, from it. How does it do this? It does so by means of the energy that the laser supplies to the sample. “The dyes, in particular, which are part of the composition of the inks, will absorb this energy more easily than their chemical structure would normally allow”. These molecules are vaporized and ionized, in such a way that a device known as an ‘analyzer’, will then classify these ions according to their mass. “However, some compounds require the intervention of a ‘matrix’ in order to be desorbed and ionized by this method. This method is known as MALDI ionization (Matrix Assisted Laser Desorption Ionization) which relies on the intervention of a ‘matrix’. It is a small organic molecule, a kind of intermediary which is capable of absorbing the energy of the laser instead of that of the sample before transferring the energy to it. This also facilitates vaporization and ionization of the molecules of the sample, while protecting them from destruction by a direct laser. “The MALDI method is semi-destructive for two reasons: firstly because the molecules are extracted from the sample, but also because the sample must be placed in a void. However, it is difficult to place an entire sheet of paper in a void. It is therefore necessary to extract a much smaller piece”.