Ionization in the mass spectrometer

A) the solvent is completely removed before ionization in the mass spectrometer source ... 

Only one major compound (SF-X) was present in the estrogenically active fractions, and this was isolated using semipreparative HPLC. The principles of separation are the same as for qualitative HPLC (see Section 25.4.1.2), with the difference that higher amounts of material can be loaded onto the ODS-column (Alltech, Econosil, Cig lOp, 250x22mm). For the identification of SF-x, a combination of spectroscopic techniques was used. Electrospray ionization in the mass spectrometer (HPllOO LC/MSD, Hewlett-Packard) with positive ionization mode gave a pseudo-molecular ion with m/z=439. H-NMR, C-NMR, DEPT, HMQC, and COSY spectra were recorded on a Varian-300 (300MHz) spectrometer. Analysis of the COSY spectram showed the presence of a lavandulyl (5-methyl-2-isopropenyl-hex-4-enyl) side chain. From the HMQC spectrum and a DEPT experiment, it appeared that SF-x possesses a disubstituted flavanone skeleton. 

The C2H5 radicals are ionized in the mass spectrometer to yield C2H5+ ions which are detected at a mass/charge ratio of 29. The moveable injector controls the relative concentration of ethyl radicals. 

While chromatographic separations and efficient ionization in the mass spectrometer do present difficulties for mixtures of widely different compound types, the most difficult problems in development of multiresidue methods usually concern the need for simultaneous extraction and clean-up of analytes with widely different polarities. With respect to multi-residue extraction and clean-up, SPE approaches (Section 4.3.3c) are the most popular these can use either a single SPE medium in a single extraction step or a combination of two SPE materials used either in series or in parallel, thus delivering the analytes as two or more groups defined by their physico-chemical properties relevant to the SPE process. 

Reactions in a mass spectrometer are divided into three classes, ionization, fragmentation, and reeirreuigement reactions. Ionization in the mass spectrometer converts a molecule without cheirge or unpaired electron into a positively charged ion with an unpaired electron, a cation radical. Ionization steps take place exclusively at the beginning of a fragmentation path. We use these ionization reactions ... 

Draw the molecular orbital diagram for ethylene and then decide which electron is most likely to be lost during ionization in the mass spectrometer. Draw a possible structure for the resulting radical cation. 

An important enhancement to the mass resolving and mass determining capabilities of mass spectrometry is using it in sequence with chromatographic separation techniques. Indeed, most aromatic compounds identification when MS is applied result from the combination with chromatographic methods. This allows the sample separation prior to ionization in the mass spectrometer and we can even refer this as a requirement for complex mixtures. Combined methods will be described below in this chapter. 

In this method, the effluent from the gas chromatograph enters a combustion unit, and the carbon dioxide that forms is ionized in the mass spectrometer. An on-line computer monitors the peaks at m/e=44 ( C02 ) and m/e=45 ( C02 ) alternately at intervals of 0.5 sec, and a line graph is plotted showing the isotope ratio as a function of time. As long as the ratio stays 1.11/98.89 the line is linear. When the ratio changes, a peak is drawn, indicating that a metabolite labeled with appeared. From the change in the ratio one can calculate the number of carbon atoms involved. 

A molecular sample can be ionized in the mass spectrometer via a gas phase ion-molecule reaction. This process is known as chemical ionization (Cl) and evolved as an alternative to the formation of molecular ions via direct EL Because the ionization process involves a chemical reaction, there are potentially many variations of the technique, employing different reagent ions adapted to the properties of the molecular sample. Consequently, a variety of Cl reagent gases have been developed for use with different natural product classes 4 — 6). 

Nevertheless, GC-MS still has a major role to play in VOC analysis because they can be used to classify chemical compounds within a complex aroma via separation in the GC and then by ionization in the mass spectrometer. Having achieved this identification, the advantages of the fast analytical capabilities of PTR-MS technology can then be mobilized to monitor temporal changes of the VOCs. 

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