Direct analysis in real-time

The direct analysis in real time (DART) method has been described by Cody et al. [88] and commercialized by JEOL. This method allows direct detection of chemicals on surfaces, in liquids and in gases without the need for sample preparation. All of these analyses take place under ambient conditions in a space just in front of the inlet of the mass spectrometer. The sample is not altered because no exposure to high voltage or to vacuum is required. 

Several mechanisms involved in ion formation are possible, depending on the analysed molecule and the operating conditions like the polarity and the gas used. In positive ion 

Spectrum obtained by DESI exposure of the finger of a person after intake of 10 mg Loratadine. From Takats Z., Wiseman J.M., Golo-gan B. and Cooks R.G., Science, 306, 471 173, 2004. Reprinted, with permission. 

Cutaway view of a DART source. Discharge is produced in the first chamber and the gas then flows into a second chamber where the ions can be discharged. The gas flows through a tube that can optionally be heated and then flows out to the sample through a grid that allows removal of ions of opposite polarity. From Cody R.B., Laramee J.A. and Dupont Durst H., Anal. Chem., 77, 2297-2302, 2005. Reprinted, with permission. 

This section on direct analysis in real time (DART) [2] has not been placed at the end of this chapter because it would be inferior to the preceding techniques, but because DART relies on much different phenomena. In fact, DART delivers analytical results very similar to those of DESI, even more so to those of DAPCI or DAPPI, and thus presents an alternative concept of ambient MS. 

FIGURE 3.14 Cutaway view of a DART (direct analysis in real time) source. (From Cody et al., Versatile new ion source for the analysis of materials in open air under ambient conditions, Anal. Chem. 2005, 77, 2297-2302. With permission.) 

Metastable helium atoms or nitrogen molecules are extracted from the plasma and directed to a sample surface, where Penning ionization makes product ions that can be analyzed by mass spectrometry or IMS. DART sources have been used to ionize a number of classes of compounds as solids or adsorbed directly onto a variety of surfaces. 

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A new family of ionization techniques allows ions to be created under ambient conditions and then collected and analyzed by MS. They can be divided into two major classes desorption electrospray ionization (DESI) and direct analysis in real time (DART). 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

Direct analysis in real time DART Discharge Nonvolatile molecular ions Direct, preparation-free analysis of samples... 

Direct analysis in real time (DART) was introduced in 2005 by Cody et al. [109]. Similarly to DESI samples can be analyzed directly without preparation. While analytes in the small protein range can be analyzed by DESI, DART is in practice limited to analysis of ions in the region below 1 kDa or slightly above. However, as in SIMS, for example, fragment spectra of larger componds can be acquired, which can provide useful information, although the molecular ion cannot be observed. 

API offers unique opportunities for the implementation of new sources or to develop new applications. Atmospheric pressure matrix assisted laser desorption (AP-MALDI) [21] can be mounted on instruments such as ion traps which were originally designed only for electrospray and LC-MS. New API desorption techniques such as desorption electrospray (DESI) [22] or direct analysis in real time (DART) [23] have been described and offer unique opportunities for the analysis of surfaces or of solid samples. 

Desorption Electrospray and Direct Analysis in Real Time... 

DART Direct analysis in real time (DART) is an analogous technique to DESI that does not require the electrospray solvent (Cody et al., 2005 McEwen et al., 2005 Williams et al., 2006). 

For elucidation of biosynthesis and TA identification in plants mass spectromet-ric procedures including LC-MS [16, 17], LC-MS/MS [18] and DART MS (direct analysis in real time MS) [19] were successfully applied. 

Yew, J.Y., Cody, R.B., and Kravitz, E. A. (2008). Cuticular hydrocarbon analysis of an awake behaving fly using direct analysis in real-time time-of-flight mass spectrometry. Proc. Natl. Acad. Sci. USA, 105, 7135-7140. 

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.

Seemingly, the next frontier for fast bioanalysis is the removal of the column altogether. Techniques such as direct analysis in real time (DART) and desorption electrospray ionization (DESI) have shown great potential. These techniques are discussed in more detail in Chapter 13. 

One of the most significant developments in mass spectrometry in the recent years is the introduction of a new class of ionization methods where samples in either solid or liquid state can be directly ionized in their native environment under ambient conditions (rather than inside a mass spectrometer) without any sample preparation. This new class of ionization methods is often referred to as ambient ionization methods [1,2], Because these methods generally ionize analytes on the surface or near the surface of the samples at atmospheric pressure, they have also been called atmospheric pressure surface sampling/ionization methods or direct/open air ionization methods [3], Since the first reports on ambient ionization with desorption electrospray ionization (DESI) [4] and direct analysis in real time (DART) [5], numerous reports have been published on the applications of these new ionization methods as well as the introduction of many related ambient ionization methods such as desorption atmospheric pressure chemical ionization (DAPCI) [6], atmospheric solid analysis probe (ASAP) [7], and electrospray-assisted laser desorption/ionization (ELDI) [8], Recently, two reviews of the various established and emerging ambient ionization methods have been published [2,3],... 

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