Electron ionization chemical derivatization

Electron capture is a highly selective ionization method, as only a limited number of analytes are prone to efficient electron capture. Fluorinated compounds are extremely sensitive to electron capture. Chemical derivatization is often applied to label analytes with, for example, pentafluorobenzyl groups, as is frequently done in gas chromatography with electron-capture detection (see section 4.3). 

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 (Cl), 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. 

At that time the mass spectrometric ionization techniques of electron ionization (El) [1] and chemical ionization (Cl) [2] required the analyte molecules to be present in the gas phase and were thus suitable only for volatile compounds or for samples subjected to derivatization to make them volatile. Moreover, the field desorption (FD) ionization method [3], which allows the ionization of non-volatile molecules with masses up to 5000 Da, was a delicate technique that required an experienced operator [4], This limited considerably the field of application of mass spectrometry of large non-volatile biological molecules that are often thermolabile. 

Parameters such as solvent, basic medium and reaction time, affecting the derivatization of alcohols and phenols with benzoyl chloride, were investigated. End analysis was by GC with UVD . a sensitive method proposed for trace determination of phenols in water consists of preconcentration by SPE with a commercial styrene-divinylbenzene copolymer, acylation with pentafluorobenzoyl chloride in the presence of tetrabutylammonium bromide and end analysis by GC with either ECD or ITD-MS. LOD was 3 to 20 ngL for ECD and 10 to 60 ngL for ITD-MS, with 500 mL samples . Acylation with the fluorinated glutaric acid derivative 43 was proposed for determination of urinary phenols, as indicative of exposure to benzene and other aromatic hydrocarbons. End analysis by GC-MS shows strong molecular ions of the derivatives by electron ionization. The proto-nated ions are the base peaks obtained by chemical ionization. LOD was 0.5 mgL and the linearity range 0-100 mg L for phenol . 

Historically mass spectrometric analysis has required that samples be volatile. This has limited the application of this important analytical technique in structure studies of conjugated metabolites to volatile derivatives. The widely realized potential of electron impact ionization, chemical ionization, and gas chromatography mass spectrometry for the analysis of glucuronldes derivatized as acetates, methyl ethers and trimethylsilyl ethers has been reviewed... 

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... 

These methods usually require chemical derivatization to produce volatile species with sufficient vapour pressure to form a gas in the MS sample compartment. The sample may be introduced either by itself, for electron impact (El) methods, or mixed with a large excess of another gas for chemical ionization (Cl). The volatile sample mixture is bombarded with a beam of electrons with energies (typically up to 100 eV) which may be captured to produce negatively charged species (M ), or where electron impact is sufficient to displace electrons and produce positive ions (M" ). This may also result in fragmentation of the unstable radicals produced from the sample molecules, and the characteristic fragmentation patterns are frequently useful in structural identification. 

Until the 1980s the only ionization techniques used for trace analysis on-line with chromatography were electron ionization (El, formerly known as electron impact ionization) and chemical ionization (Cl). These closely related ion sources are discussed below but for now it is sufficient to emphasize that both sources require introduction of analytes in the gas phase, and that the sources are located within the high vacuum chamber of the mass spectrometer thus it is essential to severely limit the amount of mobile phase that can enter the mass spectrometer in order to maintain the necessary vacuum conditions. As a result, for quantitative analyses it is necessary to either restrict the flow rate of the mobile phase or to develop external on-Une devices that remove the bulk of the mobile phase prior to introduction into an El or Cl source. This is fairly easy to achieve for GC, but remains difficult for HPLC (see below). The restriction to vapor phase analytes implies that El and Cl are applicable to only thermally stable and volatile compounds, and sometimes chemical derivatization is necessary to achieve this (Section 5.2.1b). 

Capillary GC-MS is an extremely powerful approach, combining the high separation efficiency of the capillary GC column with the identification power of the MS in electron-ionization mode. However the applicability range of GC is limited to relatively volatile compounds. In order to widen the applicability range, precolumn analyte derivatization strategies are often applied to enhance the volatility of the analytes. Methylation, silylation and acetylation reactions are most often applied for the analysis of compounds with amine, (poly-) hydroxy and/or carboxylic acid functional groups. Furthermore, derivatization to pentafluorobenzyl derivatives is applied to enhance the sensitivity of analytes in electron-capture negative-ion chemical ionization. 

Following extraction, the analyte is derivatized, if necessary, and analysed by GC-MS. As discussed previously, electron ionization continues to be favoured. However, use of chemical ionization is increasing because it often provides better sensitivity. Also, a chemical ionization mass spectrum will frequently provide valuable complementary information. For example. Figure 2 shows electron ionization and chemical ionization mass spectra for... 

Sulfur Mustard stability in nonpolar solvents has been determined by GC and GC-MS methods. The situation is more complex for Lewisite because GC methods generally involve derivatization with thiols, and are also complicated by the fact that Lewisite and its hydrolysis products give the same compound after derivatization. The solution to this problem can be found by measuring Lewisite without derivatization. In a study reported by Down in 2005 [63], toluene was selected as the extraction solvent because Lewisite slowly decomposed in other organic solvents, such as acetone and hexane. Thermal oligomerization in the injection port was prevented by on-column injection and a deactivated guard column was used to prevent the well-known problems of memory effects and column deterioration that occur with Lewisite. The extracts were analysed by GC-AED and GC-MS with both electron and chemical ionization [63]. More GC-MS techniques will be described in Chapter 4 with respect to the numerous degradation products of Sulfur Mustard. 

After the labeling experiment, the biomass is harvested, chemically fractionated, derivatized, and, finally, in most cases analyzed by GC-MS (see below and Section IV.C). Electron impact ionization is used, which leads to fragmentation of the metabolite, so that not only the molecular ion is measured but also several fragments, a circumstance that makes the measured information more valuable for subsequent data evaluation. 

Simpson JT, Torok DS, Girard JE, Markey SP. 1996. Analysis of amino acids in biological fluids by pentafluorobenzyl chloroformate derivatization and detection by electron capture negative chemical ionization mass spectrometry. Anal Biochem 233 58. 

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