Mass spectrometry electron ionisation

M.P. Colombini, F. Modugno, E. Ribechini, Chemical study of triterpenoid resinous materials in archaeological findings by means of direct exposure electron ionisation mass spectrometry and gas chromatography/mass spectrometry, Rapid Communications in Mass Spectrometry, 20, 1787 1800 (2006). 

Mass spectrometry is used to identify unknown compounds by means of their fragmentation pattern after electron impact. This pattern provides structural information. Mixtures of compounds must be separated by chromatography beforehand, e.g. gas chromatography/mass spectrometry (GC-MS) because fragments of different compounds may be superposed, thus making spectral interpretation complicated or impossible. To obtain complementary information about complex mixtures as a whole, it may be advantageous to have only one peak for each compound that corresponds to its molecular mass ([M]+). Even for thermally labile, nonvolatile compounds, this can be achieved by so-called soft desorption/ionisation techniques that evaporate and ionise the analytes without fragmentation, e.g. matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). 

Microprobe laser desorption laser ionisation mass spectrometry (/xL2MS) is used to provide spatial resolution and identification of organic molecules across a meteorite sample. Tracking the chemical composition across the surface of the meteorite requires a full mass spectrum to be measured every 10 p,m across the surface. The molecules must be desorbed from the surface with minimal disruption to their chemical structure to prevent fragmentation so that the mass spectrum consists principally of parent ions. Ideally, the conventional electron bombardment ionisation technique can be replaced with an ionisation that is selective to the carbonaceous species of interest to simplify the mass spectrum. Most information will be obtained if small samples are used so that sensitivity levels should be lower than attomolar (10—18 M) fewer than 1000 molecules can be detected and above all it must be certain that the molecules came from the sample and are not introduced by the instrument itself. 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

Gas Chromatography Coupled with Electron Impact Ionisation Mass Spectrometry... 

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]. 

Stephanou [38] identified non ionic surfactants in non saline water by gas chromatography coupled with chemical ionisation mass spectrometry. Tertiary octylphenol and lauryl alcohol ethoxylates were qualitatively detected using their chemical ionisation mass spectra and the results used to compare the chemical ionisation and electron impact mass spectrometry techniques. 

ESCA = electron spectrometry for chemical analysis IC = ion chromatography SIE = selective ion electrode TIMS = thermal ionisation mass spectrometry. 

The value of chemical ionisation mass spectrometry as a complementary technique to conventional electron impact mass spectrometry has been illustrated by Fales et al., ° who investigated the behaviour of representatives of nine of the major alkaloid families, including homatropine. In C.I. mass spectrometry the quasi-molecular ion [QM i.e. (M -I- 1) ] is invariably more abundant than is the molecular ion in E.I. mass spectrometry. Thus, with methane as reactant gas, homatropine shows a moderately strong quasi-molecular ion, and an ion at mje 258 owing to QM — HjO. In the E.I. mass spectrum, homatropine shows a very small molecular ion, and no ion at mje 258 thus the presence of a hydroxy-group passes unnoticed. ... 

Ir (CH2CN)(Me3tpa)] + in acetonitrile and [Ir" (CH2COMe)(Me3tpa)] + in acetone has been detected with ESI-MS (ESI-MS = electron-spray ionisation mass spectrometry). 

The stmctures of HA and FA are not yet determined. A combination of many techniques is required to determine the structure of HSs. Schulten et al. have employed p)>Tolysis-gas chromatography with electron impact and field ionisation mass spectrometry (Py-GC/MS), in-source pyrolysis-field ionisation mass spectrometry (Py-FIMS), CP/MS NMR, oxidative and reductive degradation, colloid chemical methods, and electron microscopy to develop a carbon network structure for soil HS (Schulten (1994), Schulten and Schnitzer (1993)). The elemental composition of FLA was C3()8H32s09oN5 fot MW of 5540 Da. This indicates of the complexity of such compounds and the extensive techniques required. If carbohydrates or proteinaceous materials are bonded covalently with HA, %C content decreases and %0 content increases. 

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. 

Most identified compounds gave observable molecular ions in electron impact mass spectrometry. Chemical ionisation mass spectrometry (CIMS) with isobutane was carried out to determine the relative molecular masses. In all instances the quasi-molecular ion was M + 1 with a relative abundance of 10% - no M + 57 was observed. Some compounds were identified on the basis of their mass spectra only because no reference compounds were obtainable. 

Moldovan, Z., lover, E., Bayona, J. M. (2002). Systematic characterisation of long-chain aliphatic esters of wool wax by gas chromatography-electron impact ionisation mass spectrometry. Journal of Chromatography A, 952(1-2), 193-204. 

Resonance (NMR) spectroscopy, InfraRed (IR) absorption, Gas Chromatography / Electron Ionisation - Mass Spectometry (GC/EI-MS) and by Gas Chromatography / Chemical Ionisation - Mass Spectrometry (GC/CI-MS)(see below for fiirther details of the analyses). 

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