| Direct Analysis in Real Time (DART) is an ionization method for mass spectrometry (MS) that was introduced by Cody and coworkers in 2005. 1 This source operates by passing helium or nitrogen between a high voltage needle and a counter electrode to produce an electrical discharge. Additional electrodes are present to remove all but the excited-state gas atoms/molecules. The excited species produced by the DART source may either ionize analytes directly or may interact with ambient components to create reagent ions that may ionize analyte molecules. Analytes are inserted directly into the stream of excited gas atoms or molecules that exit the DART source for desorption and ionization. Because little or no sample preparation is required and analyses are performed in ambient conditions, DART-MS offers rapid analyses of a variety of compound classes with minimal effort.;For the current work, a DART source was fabricated and interfaced to several different mass spectrometers. The source was optimized and its analytical figures of merit were determined with analytes representative of illicit drugs, explosives, pesticides, and chemical warfare agents. Sensitivity and signal stability of the mass spectrometers were improved with the addition of a passively sampling flared inlet modification. A commercial actively sampling inlet modification was added and compared to the passive design.;The utility of the custom-built DART source was demonstrated with a number of different applications with little or no sample preparation. Analytes were sampled from a wide variety of surface types including glass, metal, soil, sand, money, food products, and more. Matrixes such as ocean water, wine, and dimethyl sulfoxide caused suppression of analyte signal, while salt content and the presence of other solvents showed little or no effect. The addition of dopants to the sampling region was explored as a method of modifying ionization characteristics and expanding DART capabilities.;The mechanistic factors governing DART sensitivity were systematically studied. With volatile compounds, proton affinity was found to be the most important determinant of sensitivity. In addition to the mechanisms previously proposed in the literature, evidence of a self-protonation mechanism was observed. A spatial variation in the mechanism of ion formation was demonstrated with polycyclic aromatic hydrocarbons. Generally, the pathways of DART ionization appeared to fall into the class of mechanisms operative in atmospheric pressure chemical ionization (charge exchange, proton transfer, etc.).;DART was coupled to several FT-ICR MS instruments. The highest resolving powers ever reported with DART-MS analyses were demonstrated. The capability of the FT-ICR MS to do extended ion trapping was exploited to conduct infrared multiple photon dissociation experiments for spectroscopic structural determination of DART-ionized molecules. |
