The MS analysis of colorants in pen ink described in the preceding section is relatively straightforward in that dyes are relatively small organic molecules and strong UV chromophores, two features which allow them to be easily observed in LDI mass spectra. Examination of documents produced with inkjet printers is more complicated, as the colorants used in inkjet inks are pigments, which are insoluble sub-micron particles that often require dipersants (surfactants and polymers) to obtain stable formulations and prevent the fine pigment particles from settling. Although most pigments, just like dyes, are strong UV chromophores, their large size (commercial inkjet inks have pigment particle diameters that range from 0.1 to 0.2 µm ) frequently prevents generation of analytically useful ionic signal. Instead, pigments in LDI MS measurements play the same role that matrices do in MALDI MS, i.e. they convert the energy of laser radiation to other forms of energy, leading to ionization and ejection of other ink components from the surface. LDI MS analyses of samples cut off from documents printed with four different inkjet printers generate very distinct mass spectral patterns (Figure ). The main feature of each spectrum is the presence of wide distribution(s) of peaks spaced by 44 Da, which indicate the presence of polyethylene glycol (PEG) in each ink sample. This is hardly surprising, since PEG is a common component in many printing ink formulations, which prevents ink drying and enhances jetting performance. What is interesting (and was certainly unexpected), is the fact that all four inks whose LDI mass spectra are shown in Figure appear to contain different PEG blends. The distinction among these inks can be made based upon average molecular weight of PEG blends (e.g., compare panels C and D in Figure ), or their composition (e.g., compare binary blend in panel E to single-mode PEG distributions in panels B-D).