Chemical ionization mass spectrometry CI-MS

Figure 6.16. Mass fragment ions obtained by chemical ionization-mass spectrometry (CI-MS) analysis of 9- and 13-hydroperoxides from oxidized methyl linoleate, from Plattner and Gardner (1985) (note both hydroperoxide isomers produced intense ions at m/z 309 and 311 in the isobutane spectrum), and 9-hydroperoxy epidioxide and 9,16-dihydroperoxide from oxidized methyl linolenate, fromFrankel etal (1986).

As with analytical OPLC, off-line and on-line methods can be distinguished in preparative OPLC applications. In the off-line OPLC method, the steps of preparation after development are similar to conventional TLC methods drying, scraping of the sorbent layer, elution, and crystallization. Phorbol diester constituents of croton oil were identified by off-line OPLC separation followed by extraction and chemical ionization mass spectrometry (CI-MS) (90). The on-line method is more effective for preparative applications because time-consuming scraping and elution can be eliminated. 

Methods other than electron bombardment (EI-MS) can be used to obtain mass spectral data. In chemical ionization-mass spectrometry (CI-MS), the sample is sprayed with a pre-ionized gas such as methane or ammonia that causes the sample to ionize by electron transfer or proton transfer from the gas to the sample. Because the molecular ions produced by this technique are less apt to undergo fragmentation, the ability to obtain the molecular mass (and therefore the molecular formula) of the sample is enhanced. 

In chemical ionization-mass spectrometry (CI-MS), the sample molecules are combined with a stream of ionized reagent gas that is present in great excess relative to the sample. When the sample molecules collide with the preionized reagent gas, some of the sample molecules are ionized by various mechanisms, including proton transfer, electron nansfer, and adduct formation. Almost any readily available gas or highly volatile liquid can be used as a reagent gas for CI-MS. 

Chemical ionization mass spectrometry (CI-MS) is a mature technique for chemical analysis that was first developed in the mid-1960s [1,2], Since the ionization process in PTR-MS is a form of Cl, it is useful to first consider some general aspects of CI-MS before considering the thermodynamics and kinetics of proton transfer. 

Stereochemical studies based on C-nuclear magnetic resonance spectroscopy ( C-NMR) showed the presence of eight cis and trans allylic hydroperoxides (Table 2.1). To determine the isomeric distribution of allylic hydroxyooctadecenoate derivatives, cis and trans fractions were separated by silver nitrate-thin layer chromatography (TLC), a procedure that separates according to the number, position and geometry of double bonds, and they were hydrogenated prior to GC-MS analyses of the TMS ether derivatives. More recently, the six major hydroperoxide isomers of methyl oleate were partially separated by silica HPLC, and identified by chemical-ionization mass spectrometry and IH NMR (Table 2.1). These hydroperoxide isomers were better separated as the hydroxy octadecenoate derivatives by the same silica HPLC method and re-analysed by GC-MS. 

Atmospheric pressure laser-induced acoustic desorption chemical ionization/mass spectrometry (AP-LIAD/CI/MS) for crude oil analysis... 

The resolution of this latter problem passed through the development of the so-called soft ionization methods, in which ions are directly produced from the solid or liquid state. Field ionization (FI) and field desorption (FD) were two of the first alternative ionization methods. A few years later, other techniques were developed. Examples of these include desorption/chemical ionization (D/CI), secondary ion mass spectrometry (SI-MS), and fast atom bombardment (FAB). 

Biological specimen extraction can be accomplished by liquid-liquid, solid-phase or solid-phase microextraction with subsequent detection of GHB or GBL by gas chromatography-mass spectrometry (GC-MS) using electron ionization (El), positive or negative chemical ionization (CI) or gas chromatography with flame ionization detection (GC-FID). LeBeau et al. (1999) describes a method that employs two ahquots of specimen. The first is converted to GBL with concentrated sulfuric acid while the second is extracted without conversion. A simple liquid-liquid methylene chloride extraction was utilized, and the ahquots were then screened by GC-FID without derivatization. Specimens that screened positive by this method were then re-aliquoted and subjected to the same extraction with the addition of the deuterated analog of GBL. The extract was then analyzed by headspace GC-MS in the full-scan mode. Quantitation was performed by comparison of the area of the... 

Cl = chemical ionization CI-NI = monitoring negative ions in the chemical ionization mode GC = gas chromatogmphy HPLC = high performance hquid chromatography MS = mass spectrometry pmol = picomole SPE = solid-phase extraction TSD = thermionic specific detection... 

SC, HPLC) and finally analyzed by gas chromatography in combination with mass spectrometry, usually in the chemical ionization mode (MS/CI). The labelled standard and the odorant are separately quantified by using traces of their protonated molecular ions or main fragments as exemplified for 2-phenylethylthiol in Figure 8. From the amount of the added standard and by using calibration factors obtained with definite mixtures of standard and analyte [11] the concentration of the odorants in the food can be exactly determined. 

This review will first concentrate on the unimolecular gas-phase chemistry of diene and polyene ions, mainly cationic but also anionic species, including some of their alicyclic and triply unsaturated isomers, where appropriate. Well-established methodology, such as electron ionization (El) and chemical ionization (CI), combined with MS/MS techniques in particular cases will be discussed, but also some special techniques which offer further potential to distinguish isomers will be mentioned. On this basis, selected examples on the bimolecular gas-phase ion chemistry of dienes and polyenes will be presented in order to illustrate the great potential of this field for further fundamental and applied research. A special section of this chapter will be devoted to shed some light on the present knowledge concerning the gas-phase derivatization of dienes and polyenes. A further section compiles some selected aspects of mass spectrometry of terpenoids and carotenoids. 

Mass spectrometry (MS) has not been applied extensively to the study of naturally occurring xanthones, but the mass spectral data provide valuable information about the structure elucidation of xanthones. As well as electron impact MS, which is a routine technique for the structure elucidation of xanthones, recently developed soft ionization techniques, such as desorption-chemical ionization MS (D/CI-MS) and fast atom bombardment MS (FAB-MS), are of great interest for the analysis of glycosides. Molecular ion peaks can be observed without derivatization. Tandem MS/MS can be extensively employed in directly characterizing constituents of complex mixtures. Recently, xanthone profiles of H. perforatum cell cultures were identified by HPLC-MS/MS analysis [106]. 

Other Species Observed by El and PICI GC-MS In addition to TATP, DADP, the cyclic tetra-mer tetraacetone tetraperoxide (TrATrP), and the oli-goperoxides H[OOC(CH3)2] OOH (n = 2,3) have been observed by GC-EI-MS and gas chromatography-mass spectrometry using chemical ionization (GC-CI-MS) [33]. The El spectrum of DADP contains ions at m/z 43 (base peak), 58, 59, and 101, but does not exhibit a molecular ion (Figure 16.5A). The first El mass spectrum of DADP was reported by Bertrand et al. [43]. The... 

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