Mass spectrometry electron impact ionisation

Electron impact ionisation (El) stands for extensive fragmentation, but also produces molecular ions. The other ionisation methods shown in Table 6.10 mainly generate quasi-molecular ions for various compound classes. Protonation of organic compounds is one of the most fundamental processes of Cl, FAB and ESI mass spectrometry. Apart from electrospray (ESI), which... 

Supersonic molecular beam (SMB) mass spectrometry (SMB-MS) measures the mass spectrum of vibra-tionally cold molecules (cold El). Supersonic molecular beams [43] are formed by the co-expansion of an atmospheric pressure helium or hydrogen carrier gas, seeded with heavier sample organic molecules, through a simple pinhole (ca. 100 p,m i.d.) into a 10 5-mbar vacuum with flow-rates of 200 ml. rn in. In SMB, molecular ionisation is obtained either through improved electron impact ionisation, or through hyperthermal surface ionisation... 

Mass spectrometry (MS) has been shown as the most valuable tool for the identification and characterisation of AHLs with detection levels in the pi-comole range. The major fragmentation ions in the electron impact ionisation-MS (EI-MS) of the AT-acyl and AT-(3-oxoacyl) classes of AHLs are summarised in Fig. 2 [37]. 

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... 

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. 

Amu, atomic mass units BDMS, ferf-butyidimethylsilyl BHT, 2,6-di-fe/f-butyl-p-cresol Cl, chemical ionisation DNP, dinitrophenyl ECL, equivalent chain-length ECN, equivalent carbon number El, electron-impact ionisation FCL, fractional chain-length GC, gas chromatography GC, gas-liquid chromatography HPLC, high-performance liquid chromatography IR, infrared MS, mass spectrometry NMR, nuclear magnetic resonance ... 

This article describes the direct analysis of additives by pyrolysis-gas chromatography-mass spectrometry. Using this method it is possible to identify additives quickly and reliably without separation of the polymer matrix. A small specimen of the sample is pyrolysed, i.e. degraded under inert gas atmosphere. The pyrolysis products are separated by gas chromatography and ionised in the mass spectrometer by electron-impact ionisation. By comparing the results with library data it is possible to identify unknown additives unambiguously and with high accuracy. 10 refs. 

Principles and Characteristics Electron impact (El) ionisation is the original ionisation method (1918). Before 1980, mass spectrometry was merely restricted to electron impact (El), with chemical ionisation (Cl) being applied mainly for those samples which resist generation of satisfactory El data. Nowadays, El is still a widely used universal and nonselective ionisation method. In El, the sample is introduced as a vapour... 

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). 

NMR) [24], and Fourier transform-infrared (FT-IR) spectroscopy [25] are commonly applied methods. Analysis using mass spectrometric (MS) techniques has been achieved with gas chromatography-mass spectrometry (GC-MS), with chemical ionisation (Cl) often more informative than conventional electron impact (El) ionisation [26]. For the qualitative and quantitative characterisation of silicone polyether copolymers in particular, SEC, NMR, and FT-IR have also been demonstrated as useful and informative methods [22] and the application of high-temperature GC and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) is also described [5]. 

There are a number of other methods for ionising the sample in a mass spectrometer. The most important alternative ionisation method to electron impact is Chemical Ionisation (Cl). In Cl mass spectrometry, an intermediate substance (generally methane or ammonia) is introduced at a higher concentration than that of the substance being investigated. The carrier gas is ionised by electron impact and the substrate is then ionised by collisions with these ions. Cl is a milder ionisation method than El and leads to less fragmentation of the molecular ion. 

The only mass spectrometric methods available during the era of the first cascade synthesis in 1978 [30] were electron impact (El) and field desorption (FD) mass spectrometry [31]. Fast atom bombardment (FAB) mass spectrometry is limited to fairly low mass ranges and not very suitable for compounds of low polarity. It was not until the development of new and gentle ionisation methods such as MALDI (matrix-assisted laser desorption ionization) [32] and ESI (electrospray ionization) [33] that the conditions were fulfilled for the start of intense research in the field of dendrimer chemistry. The following section will present the special features of these mass-spectrometric methods and their importance in dendrimer analysis. 

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. 

HRTG-EI/SI QMSHigh resolution thermogravimetry-electron impact/soft ionisation quadrupole mass spectrometry... 

Mass Spectrometry. Mass spectrometry was conducted on the coke concentrates using a VG instrument in which the probe was heated from ambient to 5mass range 50-600 were recorded every 5 s. Spectra were recorded in both electron impact (El) and chemical ionisation (Cl, with ammonia) modes. Field ionisation (FI) spectra of some of the deactivated catalysts from the n-hexadecane MAT runs were obtained at the Stanford Research Institute as described elsewhere (16). 

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