Electron capture ionization

Wei, J. Liu, S. Fedoreyev, S.A. Vionov, V.G. A Study of Resonance Electron Capture Ionization on a Quadrupole Tandem Mass Spectrometer. Rapid Commrni. Mass Spectrom. 2000, 74,1689-1694. 

The combination of electron capture ionization with liquid chromatography/mass spectrometry produces a valuable technique for metabolism studies of fluorinated compounds. Selective ionization of halogena-ted components simplifies sample purification since most biological constituents are transparent to detection. This report details the application of this technique to in vitro and in vivo metabolism studies of fluorinated herbicides. 

Sensitivity of Electron Capture Ionization. We have found that negative ion LC/MS provides substantial increases in sensitivity as compared with positive ion LC/MS during analysis of fluorinated pyridines such as dithiopyr 1. An example of such sensitivity enhancement is shown in Figure 2. The top panel illustrates the total ion current trace obtained from injection of increasing amounts of dithiopyr in the positive ion mode with discharge ionization. 

The selectivity of electron capture ionization for fluorinated compounds provides a convenient means to locate and identify fecal metabolites of dithiopyr. 

Chemical ionization (positive and negative) and electron capture ionization 332... 

The use of negative ions produced by the electron capture ionization may constitute another alternative for increasing sensitivity. Stapleton and Bowie [226] applied this method to the study of the structures of the nitrobenzoyl derivatives of various natural amino acids. All the species studied led to clear-cut and interpretable fragmentations, enabling the structure of the amino acid in question to be determined. 

A. Cappiello, G. Famiglini, A. Lombardozzi, A. Massari, and G. G. Vadala, Electron capture ionization of explosives with a microflow rate particle beam interface, J. Am. Soc. Mass Spectrom. 7, 753-758 (1996). 

For these classes of substances, other methods have been developed, such as the coupling of ESI with an electrochemical cell [20-31], the coordination ion-spray [31-46], or the dissociative electron-capture ionization [37-41]. The APPI or the dopant-assisted (DA) APPI presented by Syage et al. [42, 43] and Robb et al. [44,45], respectively, are relatively new methods for photoionization (PI) of nonpolar substances by means of vacuum ultraviolet (VUV) radiation. Both techniques are based on photoionization, which is also used in ion mobility mass spectrometry [46-49] and in the photoionization detector (PID) [50- 52]. 

In negative ion mode a range of ion-molecule reactions can be used (Harrison 1992) to provide true Cl (involving chemical reactions). However, for trace quantitative analysis, electron capture ionization is particularly important in the case of highly electronegative... 

Upon ionization, the number of electrons in a molecule may change. For example, El causes the loss of a valence electron from a neutral molecule (see Chapter 2). Because neutral molecules contain even numbers of electrons, El produces odd-electron ions from neutral molecules. The odd-electron ions, denoted by M+, are radical cations. The dot symbol indicates the presence of an unpaired electron. In contrast to the radical cations, electron capture ionization produces radical anions, denoted by M . M+ and M are generally known as molecular ions. 

A wide variety of other MS techniques are used to detect explosives. Two notable techniques are Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and CE-MS. FT-ICR-MS is used to probe pseudomolecular ion formation of RDX, PETN, and TNT using several ionization sources including EDI, El, electron capture ionization (EC), and chemical ionization (Cl). Analyses are performed both in the positive and negative ionization mode, and identities are assigned to the major pseudomolecular ion peaks seen in the spectra from each explosive [198]. TTie composition of several explosive compounds from postblast residue is assessed with FT-ICR-MS by identifying the explosive and inactive ingredients in a smokeless powder, TNT,... 

GC-gas chromatography, HPLC-high performance liquid chromatography, SFC-supercritical fluid chromatography, TLC-thin layer chromatography, MS-mass spectrometry, MF-mass fragmentography, Cl-chemical ionization, El-electron ionization, ECI-electron capture ionization, IS-ionspray, FAB-fast atom bombardment, ESI-electrospray, SIMS-secondary ion mass spectrometry. 

Lim, H.K., Andrenyak, D., Francom, P. and Foltz, R.L. (1988) Quantification of LSD and N-demethyl-LSD in urine by gas chromatography/resonance electron capture ionization mass spectrometry. Anal. Chem., 60, 1420-1425. 

Transition to the final product, the cyclobutane LV, can be greatly facilitated by the use of electron-donating substituents R. In this case, the electron transfer from substituted ethylene to the orbital of the cation-radical LIV occupied by one electron switches on the chain mechanism represented on the above scheme, similar to that for the reactions of the SrnI type (Sect. 9.1). Such mechanisms are a characteristic feature for the class of reactions, catalyzed by electron transfer, in which the sequence 1) ionization (electron capture), 2) reaction in the ion-radical, 3) electron capture (ionization) - is a very convenient route for effecting the needed transformation [96]. 

Cappiello, A Pamiglini, G. Lombar-dozzi. A Massari, A Vadala, G.G. Electron Capture Ionization of Explosives With a Microllow Rate Particle Beam Interface. J. Am. Soc. Mass Spectrom. 1996, 7,753-758. 

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