DART Ion Source

Direct analysis in real time (DART) is another ambient mass spectrometry technique that has emerged recently [92]. Electronically excited helium or vibronic excited-state species of nitrogen (i.e., metastable helium atoms or nitrogen molecules) are, however, the primary beam particles in this approach. These species are produced by electrical discharge of the respective gases. The beam is directed toward the mass spectrometer orifice or reflected off a sample surface into the 

The technique has been used successfully in the analysis of organic compounds, biomolecules, pharmaceuticals, organometallics, drugs of abuse, explosives, and toxic chemicals on a variety of unusual surfaces, such as concrete, asphalt, human skin, currency, boarding passes, business cards, fruits, vegetables, body fluids, cocktail glasses, and clothing. 

Ionization is the first essential step in mass spectrometry. The most popular mode of ionization is electron ionization, in which a gas-phase sample is bombarded by a beam of energetic electrons. It is applicable to relatively volatile and thermally stable compounds. Thermally labile compounds are preferentially analyzed with chemical ionization, which makes use of acid-base ion-molecule reactions 

Ionization of condensed-phase analytes occurs by mixing a sample in a suitable matrix and bombarding the matrix-analyte mixture with an energetic beam made of either laser photons as in MALDI, high-energy fission particles as in Cf plasma desorption, or high-energy fast atoms or ions (FAB or liquid SIMS). When an analyte is present in a solution, such as an effluent from a separation device, it can be ionized via thermospray ionization, atmospheric-pressure chemical ionization, atmospheric-pressure photoionization, or electrospray ionization. Desorption electrospray ionization and direct analysis in real time are new modes of ionization that are accomplished in ambient air. 

Draw a schematic picture of an El source and describe the function of each component. 

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A new ion source has been developed for rapid, non-contact analysis of materials at ambient pressure and at ground potential [8,9], The new source, termed direct analysis in real time (DART), is based on the atmospheric pressure interactions of long-lived electronic excited-state atoms or vibronic excited-state molecules with the sample and atmospheric gases. Figure 5 shows a schematic diagram of the DART ion source. 

Fig. 5. Schematic diagram of direct analysis in real time (DART) ion source. Reproduced with permission from Cody et al. [9]. Copyright 2005 American Chemical Society.

Two new independently developed techniques called Dart ° (direct analysis in real time) and Desi (desorption electrospray ionisation) are making a huge impact on mass spectrometry. Together they remove the need for sample preparation and vacuum, speed up analysis time and can work in the open air. The sample is held in a gas or liquid stream at room temperature and the impact induces the surface desorption of ions. The ions then continue into the vacuum interface of the MS for analysis. Samples can be hard, soft or even liquid in nature. Ifa et al. have used Desi to image biological samples in two dimensions, recording images of tissue sections and the relative concentrations of molecules therein. Jeol have launched a commercial Dart ion source for non-contact analysis of materials in open air under ambient conditions. 

Figure 2.18 (a) Scheme of DART-ion source and (b) scheme of a gas-ion separator (Vapur interface) equipped with a vacuum pump [124], Reproduced with permission from Cody, R.B., Laramee, J.A., Durst, H.D. (2005) Versatile New Ion Source for the Analysis of Materials in Open Air under Ambient Conditions. Anal. Chem. 77 2297-2302. Copyright (2005) American Chemical Society. See colour plate section for colour figure... 

FIGURE 49.8 (A) Overlaid mass chromatograms of the adjacent zones isopropylthioxanthone (ITX) (16 ng/band, w/z 255 [M-hH] ) and a fluorescent product of synthesis called MP (5 ng/band, m/z 264 [M-tH] ) (B) decay of the ITX signal for repeated introduction of the same zone into the DART ion source. 

With the DART ion source with the desorption at an angle option, the scanning of surfaces by DART-MS was possible, and a simple gas beam focus visualization technique was suggested for any surface analysis using DART-MS [74], This visualization is based upon the use of different chemical reactions which lead to color formation upon local heating of the surface (e.g.,derivatiza-tion on a TLC/HPTLC plate). Such a visualization is important for selecting the optimal coordinates of the DART ion source with regard to the sample for an optimal detectability. 

TLC plate on the carrier was directly introduced into a DART ion source by physical impulsion from a syringe pump. This system was shown to be a powerful analytical tool for efficient quantitation of natural products from crude drugs. 

FIGURE 12.2 Experimental setup with the DART ion source with desorption at an angle (DART SVP-A). (From Chernetsova, E.S., Revelsky, A.I., and Morlock, G.E., Rapid Commun. Mass Spectrom., 25, 2275-82, 2011. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.)... 

Another ionization technique that can be carried out under atmospheric conditions and is utilized for direct analysis in real time is DART-MS. Similar to DESI-MS, samples do not require pretreatment, and thus, the time required for DART-MS analysis decreases. The DART ion source has been used to analyze a number of substances such as drugs in seized materials and biological fluids, dyes and inks, food, and environmental compounds. All of these analytes can be ionized directly on surfaces such as glass, TEC plates, concrete, and paper [11]. 

Several applications of forensic science were reported. Methilenedioxymetham-phetamine (MDMA) was detected by placing ecstasy pills in front of the DART ion source. Ropero Miller et al. characterized 25 cocaine samples acquired by local police. The use of DART-MS allowed for the detection of adulterants and other components, thus generating a complete profile of the samples [12,13]. 

The sequences of reactions that can follow have already been discussed in reactions (8.1) through (8.9) for APCI sources. The DART ion source may also be operated in the negative ion mode when the electrons generated in reaction (8.72) attach to molecular oxygen creating negative ions... 

Fig. 8.8 A schematic diagram of a DART ion source. (Used by permission from Jeol USA, Ine.)...

In the medical area, one study using DART has been applied to human breath. Diagnosis through online breath analysis was fonnd to be difficult due to the dispersion of the breath samples in open air. A confined DART ion source (cDART) was developed and this was tested on fonr standard compounds—ethanol, acetone, 2-hexanone, and limonene— with detection limits approaching 1 ppbv after calibration [188]. 

FIGURE 24 Hydrogen-deuterium exchange of melamine in a DART ion source. Reproduced from Ref. 156. 

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