Desorption chemical ionization

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100] 

Note Although desorption chemical ionization being the correct term, [92] DCI is sometimes called direct Cl, direct exposure Cl, in-beam Cl, or even surface ionization in the literature. 

DCI spectrum of mannitol, a non-volatile compound, with H20 as an reagent gas. Note that water yields radical cation adducts (M +HiO) +. 

The observed spectrum probably results from the superposition of several phenomena evaporation of the sample with rapid ionization, direct ionization on the surface of the filament, direct ion desorption and, at higher temperature, pyrolysis followed by ionization. 

Generally, the molecular species ion is clearly detected in the case of non-volatile compounds. The method can be useful, for example, for tetrasaccharides, small peptides, nucleic acids and other organic salts, which can be detected in either the positive or negative ion mode. 

Note DCI requires fast scanning of the mass analyzer because of the rather sudden evaporation of the analyte. 

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For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

One of the reasons for lack offlterature was probably because environmental analysis depends heavily on gas chromatography/mass spectrometry, which is not suitable for most dyes because of their lack of volatility (254). However, significant progress is being made in analyzing nonvolatile dyes by newer mass spectral methods such as fast atom bombardment (EAB), desorption chemical ionization, thermospray ionization, etc. 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

Ayanoglu, E. Wegmann, A. Pilet, O. Marbury, G. D. Hass, J. R. Djerassi, C. Mass spectrometry of phospholipids—some applications of desorption chemical ionization and fast atom bombardment. /. Am. Chem. Soc. 1984, 106, 5246-5251. 

V.N. Reinhold and S.A. Carr, Desorption chemical ionization with poly-imide coated wires, Anal. Chem., 54 (1982) 499-503. 

If the analyte is exposed to energetic electrons the method is called direct electron ionization (DEI) or desorption electron ionization (DEI), and accordingly it is termed direct or desorption chemical ionization (DCI) if the analyte is immersed into the reagent gas under conditions of chemical ionization (Chap. 7). 

Prokai, L. Hsu, B.-H. Farag, H. Bodor, N. Desorption Chemical Ionization, Thermospray, and FAB-MS of Dihydropyridine - Pyridinium Salt-Type Redox Systems. Anal. Chem. 1989, 61, 1723-1728. 

Scheme 12.1 Synthesis of epimino[60 fullerene 5 and formation of nitrogen heterofullerenes in the gas phase by desorptive chemical ionization (DCl) mass spectrometry.

Sakushima, A., Nishibe, S., and Brandenberger, H., Negative ion desorption chemical ionization mass spectrometry of flavonoid glycosides, Biomed. Environ. Mass Spectrom., 18, 809, 1989. 

Ionization methods such as electron impact, chemical ionization, desorption chemical ionization, and negative-ion chemical ionization are all based on ionization of gas-phase samples and, thus, fall within the first category of gas-phase ionization. 

Desorption chemical ionization (DCI) is a variation on chemical ionization in which the analyte is placed on a filament that is rapidly heated in the CI plasma. The direct exposure to the CI reagent ions, combined with the rapid heating, acts to reduce fragmentation. Some samples that cannot be thermally desorbed without decomposition can be characterized by the fragments produced by pyrolysis desorption chemical ionization. 

DCI. see Desorption chemical ionization Debye-Huckel model in interface studies, 625-626... 

Troendle F, Reddick C, Yost R (1999) Detection of pharmaceutical compounds in tissue by matrix-assisted laser desorption/ionization and laser desorption/chemical ionization tandem mass spectrometry with a quadrupole ion trap. J Am Soc Mass Spectrom 10 1315-1321... 

GC-MS and Electron and Chemical Ionization (EI/CD-MS rely on the ability of organic species to survive volatilization prior to ionization. In many cases, this requires a degree of heating which often leads to decomposition. In desorption chemical ionization (DCI), field ionization (FI), thermospray (TSP) or fast atom bombardment (FAB) ionization occurs before volatilization, and measurement by the mass spectrometer often occurs before decomposition can result. These techniques have allowed determination of many high molecular weight and polar species, which could not previously be analyzed. 

Figure 1. Desorption chemical ionization (DCI) mass spectrum of aspartylphenylalanine methyl ester.

Spanos, G.A., Schwartz, S.J., van Breemen, R.B. and Huang, C.-H. (1995) High performance liquid chromatography with light scattering detection and desorption chemical-ionization tandem mass spectrometry of milk fat triacylglycerols. Lipids, 30, 85-90. 

This probe carries the sample within a hollow or contains the end part of a capillary coming from a chromatograph or carries a filament on which the sample was deposited. In the last case, we talk about desorption chemical ionization (DCI). The pumping speed is sufficient to maintain a 60 Pa pressure within the box. Outside, the usual pressure in a source, about 10-3 Pa, will be maintained. 

Radical anions are not often observed. Fullerene [20] has no hydrogen atom in it, and thus ionization can only occur through electron capture or anion attachment. Figure 7.14 displays its spectrum obtained under negative ion desorption chemical ionization conditions with a CH4-N2O mixture as the ionizing gas [21],... 

Desorption chemical ionization (DCI) mass spectrum of natural cocoa butter and collision-induced dissociation DCI/MS/MS traces of deprotonated molecular ions of 887 Th (A), 859 Th (B) and 831 Th (C). Reproduced (modified) from Stroobant V., Rozenberg R., Bouabsa E.M., Deffense E. and de Hoffmann E., J. Am. Soc. Mass Spectrom., 6, 498-506, 1995, with permission. 

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