Electron-capture negative ionisation

Flow limitations restrict application of the DFI interface for pSFC-MS coupling. pSFC-DFI-MS with electron-capture negative ionisation (ECNI) has been reported [421], The flow-rate of eluent associated with pSFC (either analytical scale - 4.6 mm i.d. - or microbore scale 1-2 mm, i.d.) renders this technique more compatible with other LC-MS interfaces, notably TSP and PB. There are few reports on workable pSFC-TSP-MS couplings that have solved real analytical problems. Two interfaces have been used for pSFC-EI-MS the moving-belt (MB) [422] and particle-beam (PB) interfaces [408]. pSFC-MB-MS suffers from mechanical complexity of the interface decomposition of thermally labile analytes problems with quantitative transfer of nonvolatile analytes and poor sensitivity (low ng range). The PB interface is mechanically simpler but requires complex optimisation and poor mass transfer to the ion source results in a limited sensitivity. Table 7.39 lists the main characteristics of pSFC-PB-MS. Jedrzejewski... 

ECNI Electron capture negative ionisation ESIMS Electrospray ionisation mass... 

Korytar, P., Parera, j., Leonards, P.E.G., Santos, E j., de Boer, j., Brinkman, U.A.T. (2005) Characterization of polychlorinated n-alkanes using comprehensive two-dimensional gas chromatography-electron-capture negative ionisation time-of-flight mass spectrometry. J. Chrom-atrogr. A 1086 71-82. 

A recent method of detection is electron capture negative ionisation (ECNI) as ionization technique in combination with GC-MS analysis. This method is advantageous because it offers a high sensitivity for compounds with four or more bromine atoms [36]. The sensitivity of ECNI for these compounds is approximately 10 times higher than with the use of an electron capture detector (ECD) [5]. In the analytical method which was developed to quantitate PCBs and PBBs in human serum, GC/ECD was used [30]. Because the response, and therefore the sensitivity, of the ECD depends on the position of the halogen on the biphenyl nucleus as well as the number of halogen atoms, it is necessary to run a standard for each compound to be determined [2], The use of narrow bore (0.15 mmi.d.) capillary columns is advised to obtain the required resolution [5]. 

Sin DWM, Chung LPK, Lai MMC, et al.. Determination of quinoxaline-2-carboxylic acid, the major metabolite of carbadox, in porcine liver by isotope dilution gas chromatography-electron capture negative ionisation mass spectrometry. Area/. Chim. Acta 2004 508 147-158. 

Yu Ma and Bayne [198] differentiated different aroclors in soil using linear discrimination and analyses by electron-capture negative-ion chemical ionisation mass spectrometry. 

Examples of this type of herbicide are imazapyr, m-imazamethabenz, p-imazamethabenz, m,p-imazamethabenzmethyl, imazethapyr and imazaquin. Imazapyr has been determined at the xg/kg level in 0.1 M ammonium acetate extracts of soil by microwave-assisted extraction using electron capture negative chemical ionisation mass spectrometry [432]. High-performance liquid chromatography with UV detection at 250 nm has been used to determine imazapyr in methanol extracts of soil [433]. 

A commonly used method for interpreting IP energies from Photo-Electron Spectroscopy (PES) is to employ Koopmans Theorem [25] which states that the IP (or EA) is equal to the negative of the MO energy from which the electron is ejected (or the energy of the acceptor MO for EAs). Loosely speaking, one can extend the analogy to excitations from one MO to another in as much as the combination of ionisation and electron capture is equivalent to an excitation. However, there are several caveats to bear in mind. 

Barcelo [13] characterised selected pesticides by negative ion chemical ionisation thermospray high performance liquid chromatography-mass spectrometry. Ions observed in the negative ion chemical ionisation spectra corresponded to mechanisms of anion attachment ([M + acetate], electron capture ([M] ) and dissociative electron capture ([M-R] ). Sensitivity was lower in the negative ion chemical ionisation mode than in the positive ion mode. 

Ar. Ns This detector employs a source of P radiation (usually 6JNi-cf. helium ionisation detector). The 1000-10.000 velocities of the high energy electrons are reduced to thermal velocities by coOisioo with atoms of inert gas with which the detector chamber is purged. When a sample of a gas with a greater election affinity than the inert gas is introduced into the ccC some of the electrons are captured and form negative ioas. 

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