Pen ink analysis: detection of a personal check forgery. Dyes used in pen inks are small organic molecules that absorb light in the visible region; they also exhibit strong absorption in the near-UV region (this is due to the presence of multiple aromatic rings with significant degree of conjugation 17c, ). Because of the strong UV absorbance of dyes, their analysis does not require application of MALDI matrices, whose purpose is to absorb the laser radiation and use this energy to eject/ionize other molecules to the gas phase. This allows the dyes to be desorbed directly from the paper surface upon irradiation with a laser beam, and use the resulting mass spectra to identify specific dyes present in a particular ink preparation or blend. 16b-d LDI mass spectra of two ink samples from a mock forged check ("original" and "suspect" ink, Figure and, respectively) contain some common features (e.g., peaks 1, 2 and 4). Based on the measured ionic masses, these peaks were assigned as methyl violet B, Victoria pure blue BO, and copper (II) phthalocyanine, respectively. These dyes are commonly found in ball point and gel pen inks 17c, 28 (their chemical structures are shown in Figure ). Additional confirmation of the peak assignment comes from close examination of the isotopic distributions of ion peaks (see insets in Figures and). While isotopic distributions of small (below 1,000 Da) organic molecules containing carbon, hydrogen, nitrogen and oxygen atoms are expected to be monotonic, presence of elements such as chlorine, as well as some transition metals give rise to convoluted isotopic distributions. A close match between the calculated isotopic distribution of copper (II) phthalocyanine and the measured one for peak 4 is another identity proof for this ink component. The most significant difference between the two spectra is the cluster of peaks in the m/z range 470-500 (labeled 3 and 3'), which are present only in the mass spectrum of the original ink. The ionic mass of peak 3 allows it to be assigned as crystal violet, another common dye, while 3' is likely to represent a fully methylated version of this molecule (a common chemical modification in dyes that converts, inter alia, methyl violet B to Victoria pure blue BO, see Figure ). This ability of LDI MS to provide a clear distinction between the original and the suspect inks not only allows the document examiner to prove that two different inks were used in a single document, but also to establish which specific entries were made with a particular pen. Furthermore, an LDI mass spectrum collected from a document spot where the marks left by the original and the suspect inks overlap (Figure ), exhibits the entire collection of dyes 1, 2, 3, 3' and 4. However, the relative abundance of peaks 3 and 3' is very low compared to 4 (less than 1:30), while this ratio exceeds 1:1 in the mass spectrum of the original ink. This observation confirms that the writing done with the suspect ink was added to the check later, a feature that can be used for establishing chronological order of placing entries in disputed documents.