Direct sample analysis-mass spectrometry vs separation mass spectrometry techniques for the analysis of writing inks

Abstract

Direct sample analysis (DSA) coupled with high resolution, high mass accuracy time of flight mass spectrometry (TOF-MS) is an emerging technique for the in situ analysis of various substances. DSA is similar to desorption electrospray ionization (DESI) and direct analysis in real time (DART), however, it uses an atmospheric pressure chemical ionization source for ionization of organic compounds. In this study, DSA was applied to the identification of 80 black and blue writing inks from both ballpoint and non-ballpoint pens. Approximately 3mm sections of each writing was placed on the DSA sample holder and analyzed in an automated fashion. The holder can analyze 13 samples in a serial manner. Utilizing a high mass accuracy TOF-MS and using internal calibration, mass accuracy of <6ppm was achieved. Moreover, the results obtained from DSA analysis were compared with the extraction of the ink from the paper and its analysis by gas chromatography–mass spectrometry (GC–MS) and nano ultra-high performance liquid chromatography–Orbitrap mass spectrometry (nLC–MS). Analysis by DSA and LC-MS resulted in the identification of colorants as well as vehicles and additives while analysis by GC–MS mainly resulted in the identification of the non-colorant ingredients. Of the three techniques utilized, DSA provided the greatest number of ink compound identifications and in more samples. However, no single method detected all the components of the ink formulations. For profiling purposes, it is therefore necessary to utilize more than one method, such as the combination of DSA, LC-MS, and GC–MS presented in this study.