Ionization source, mass spectral

Representative mass spectral conditions (negative chemical ionization) ion source temperature, 150°C ionizing current, 0.20 mamp electron energy, 70 eV methane reagent gas (source pressure 0.5 to 1 torr). 

A common method for identification of organic compounds is mass spectrometry (MS) in combination with GC. After separation of the component by GC the mass spectrometer transform the analyte into gaseous ions in vacuum in the ion source. For electron impact ionization this results in different mass fragmentation patterns with different mass-to-charge ratios (m/z). From this fragmentation pattern it will be possible to identify the compound by comparison with commercial mass spectral libraries. Identification of unknown compounds can be facilitated by... 

All major mass spectral data collections consist of El mass spectra, mostly recorded under accepted standardized conditions such as an ionization voltage of 70 eV, an emission current of 100-200 xA, and an ion source temperature of 150-200°C. Several types of GC/MS systems may be applied, for instance, magnetic sector, quadrupole, or ion trap analyzers. Ion trap systems are considered less applicable, when data comparison is required with spectra from a reference library. In particular, basic compounds related to VX or the three nitrogen mustards tend to produce protonated molecular ions by self-protonation. Magnetic sector and quadrupole mass spectrometers suffer less from interference of self-protonation, and spectra produced with these types of instruments are generally reproducible. 

Mass spectrometry can be used to determine the precise mass of a protein. Because of their large size and low volatility, proteins require specialized mass spectral techniques, such as electrospray ionization spraying a charged, heated stream of droplets into the evacuated source chamber. The solvent evaporates to leave ions of the compound to be analyzed. 

In addition, quantitative and qualitative elemental analysis of inorganic compounds with high accuracy and high sensitivity can be effected by mass spectrometry. For elemental analysis, atomization of the analysed sample that corresponds to the transformation of solid matter in atomic vapour and ionization of these atoms occur in the source. These atoms are then sorted and counted with the help of mass spectrometry. The complete decomposition of the sample in the ionization source into its constituent atoms is necessary because incomplete decomposition results in complex mass spectra in which isobaric overlap might cause unsuspected spectral interferences. Furthermore, the distribution of any element in different species leads to a decrease in sensitivity for this element. 

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) is a direct mass spectral technique that has found application for the analysis of various phytochemicals. MALDI represents the ion source, whereas TOF is the mass analyzer. The two techniques may be coupled with other ion sources or mass analyzers, but are most commonly used in tandem. In MALDI, the analytes are mixed with a matrix, which is usually an aromatic organic acid, which aids in... 

Mass spectral data were obtained for the various oil and asphaltene aromatic fractions using a CEC 21-110B mass spectrometer equipped with a modified combination field-ionization/electron-impact (FI/EI) ion source and for the asphaltene fractions using a KRATOS MS-50 mass spectrometer equipped with a low-voltage electron-impact (LV/EI) ion source. With the former instrument, 70-eV EI/MS were recorded at a static resolution of approximately 21,000 on Ilford Q2 photographic plates, and FI/MS were produced with an emitter potential of 5.8 kV and a counterelectrode potential of +800 V to —800 V. Low-resolution FI/MS were... 

The samples were analyzed on the VG TofSpec-SE MALDI TOP mass spectrometer in the reflectron mode with positive ion detection. The samples were spotted on the sample plate in acetonitrile water (60 40) or chloroform methanohTFA (1 1 0.1) mixture plus ammonium suliate on alpha-C (a-cyano-4-hydroxycinnamic acid) matrix. The ionization of the samples was carried out with Nd YAG laser at 355 nm or nitrogen laser at 337 nm. Some of the fractions were analyzed by SIMS on a Kratos 890 mass spectrometer equipped with a Phrasor Scientific SIMS source. The mass spectral data were analyzed by the MSFIT program at the University of California, San Francisco. 

The second problem is that ion-ion or ion-neutral reactions can occur. Reactions (e.g., proton transfer) result in high abundance of protonated molecular ions in the mass spectrum. Thus QIT can be disadvantageous for determining chemical composition in manual spectral interpretation, because the presence of the (M + l)" ion tends to confuse the interpretation. Library spectral matching, however, is not affected if the spectral matching algorithm reflects the unique features of the QIT spectrum. An external ionization source with ion injection into the QIT is an alternative solution, because only ions are present in the trap (i.e., neutral analyte molecules that could participate in ion-molecule reactions are not present). The Thermo Finnigan PolarisQ GC/MS is an example of such an instrument. 

The displaced electron is generally assumed to be the electron with the lowest ionization energy. In order of probability, this will be a nonbonding electron followed by a 7t bond electron and then a a bond electron. Thus El yields, in the first instance, a molecular ion which is a radical cation with an unpaired electron. In principle, any remaining energy will then be dissipated by bond cleavages that result in the formation of the most stable cation with a paired electron (even-electron ion). These even-electron ions may be formed by homolytic or heterolytic cleavages. This whole process happens very rapidly (<10-8s) and is the reason for the close similarity of El spectra produced across all different instruments. It is important to remember that mass spectral reactions in the El source are unimolecular. This is because the pressure in the El source is too low for bimolecular (ion-molecule) reactions to occur. 

At some point, however, assay recovery for an LC/MS method will be estimated and when this is done several important points are worth keeping in mind. Over the instrument linear range, mass spectral response is proportional to the number of ions which have entered the high vacuum region of the instrument. This number of ions is, in turn, dependent on how efficiently an analyte is ionized in the source. For a given analyte, the extent of ionization dictates its ability to compete with the matrix components present. As the amount of ionizable matrix components increases, the ability of an analyte to ionize, hence its apparent concentration, is diminished. This so-called ionisation suppression, or ion suppression, can lead to nonquantitative behavior in LC/MS [51,52],... 

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