Determination of double bond positions in polyunsaturated fatty acid methyl esters by combined gas chromatography-chemical ionization mass spectrometry

MASS SPECTROMETRIC FRAGMENTATION OF TRIMETHYLSILYL AND RELATED ALKYLSILYL DERIVATIVES

This review describes the mass spectral fragmentation of trimethylsilyl (TMS) and related alkylsilyl derivatives used for preparing samples for analysis, mainly by combined gas chromatography and mass spectrometry (GC/MS). The review is divided into three sections. The first section is concerned with the TMS derivatives themselves and describes fragmentation of derivatized alcohols, thiols, amines, ketones, carboxylic acids and bifunctional compounds such as hydroxy- and amino-acids, halo acids and hydroxy ethers. More complex compounds such as glycerides, sphingolipids, carbohydrates, organic phosphates, phosphonates, steroids, vitamin D, cannabinoids, and prostaglandins are discussed next. The second section describes intermolecular reactions of siliconium ions such as the TMS cation and the third section discusses other alkylsilyl derivatives. Among these latter compounds are di- and trialkyl-silyl derivatives, various substituted-alkyldimethylsilyl derivatives such as the tert-butyldimethylsilyl ethers, cyclic silyl derivatives, alkoxysilyl derivatives, and 3-pyridylmethyldimethylsilyl esters used for double bond location in fatty acid spectra. © 2019 Wiley Periodicals, Inc. Mass Spec Rev 0000:1-107, 2019.

Probing Interfacial Chemistry of Single Droplets with Field-Induced Droplet Ionization Mass Spectrometry: Physical Adsorption of Polycyclic Aromatic Hydrocarbons and Ozonolysis of Oleic Acid and Related Compounds

The recently developed technique of field-induced droplet ionization (FIDI) is applied to study interfacial chemistry of a single droplet. In a new variation of the FIDI method, 1-2-mm-diameter droplets hang from a capillary and undergo heterogeneous reactions between solution-phase analytes and gas-phase species. Following a specified reaction time, the application of a high electric field induces FIDI in the droplet, generating fine jets of highly charged progeny droplets that are characterized by mass spectrometry. Sampling over a range of delay times following exposure of the droplet to gas-phase reactants, the spectra yield the temporal variation of reactant and product concentrations. We illustrate the technique with three examples: the adsorption of the polycyclic aromatic hydrocarbon naphthalene into a water-methanol droplet, the ozonolysis of oleic acid, and localization of the carbon-carbon double bond within a lysophosphatidic acid. Gas-phase naphthalene reacts with 80% methanol-20% water droplets containing 100 microM silver nitrate. Positive ion mass spectra show increasing concentrations of silver ion-naphthalene adducts as exposure times increase. To examine the ozonolysis of organic molecules, gas-phase ozone generated by a mercury pencil-style lamp reacts with either 10 microM oleic acid or 100 microM oleoyl-L-alpha-lysophosphatidic acid (LPA; 18:1). Negative ion spectra from the ozonolysis of oleic acid show azelaic acid and 9-oxononanoic acid as the principle reaction products. Ozonolysis products from LPA (18:1) unambiguously demonstrate the double bond position in the original phospholipid.

Identification of fatty acids in fishes collected from the Ohio River using gas chromatography-mass spectrometry in chemical ionization and electron impact modes

Analyses of fatty acid (FA) composition in freshwater fishes promote understanding of the potential relationship between fish health or human nutrition and specific FAs. Therefore, the chemical identity of FAs in endemic fishes must be established. Paddlefish, sauger, and white bass were collected from the Ohio River. The structural identification of esterified FAs from fish-fillet lipids was conducted using gas chromatography-mass spectrometry (GC-MS). The same 13 FAs, composing more than 90% of the mass of FAs extracted by techniques used in this research, were found in all three species examined. Carbon chain length and degree and position of unsaturation were determined from the characteristic ionization and fragmentation of FA methyl esters (FAMEs) resulting from GC-MS electron impact (EI) and chemical ionization (CI) modes. Assignment of structure to the extracted FAs required complementary interpretation of both EI and CI MS. The EI spectra observed substantiate findings reported in the literature. The novelty of this research is in the thorough interpretation of CI spectra for which less data are available. The observations reported for analyses of fishes will be useful to all researchers studying FAs regardless of sample media.

Review: derivatization in mass spectrometry--5. Specific derivatization of monofunctional compounds

The present paper is complementary to the foregoing reviews and describes some additional methods of the derivatization of particular functional groups mainly to enhance the structural information content of electron ionization and chemical ionization mass spectra. Derivatization approaches for the modification of unsaturated compounds, alcoholic, carboxylic, carbonyl, amine and other functional groups, are discussed. Derivatization for separation and quantitative determination of chiral enantiomeric compounds is also considered. Preliminary chemical and physicalchemical degradation for structure elucidation of high molecular weight compounds (biopolymers, synthetic polymers) is mentioned. Chemical aspects of derivatizations and characteristic mass spectral features of derivatives are described briefly. Some particular applications of chemical modification, in conjunction with mass spectral measurements for the analysis of various important bioorganic compounds and compounds in biological fluids, air, environmental etc., are considered.

Analysis of alkenes by copper ion chemical ionization gas chromatography/mass spectrometry and gas chromatography/tandem mass spectrometry

A novel chemical ionization/fast atom bombardment (CI/FAB) source was used to analyse alkenes by chemical ionization mass spectrometry (CI-MS) using copper ions as the ionizing agent. The Cu(+)-CI mass spectra showed abundant pseudomolecular adduct ions [alkene-Cu](+) and characteristic fragment ions. Mass-analysed ion kinetic energy spectroscopy was used to study the product ions resulting from the decomposition of adduct ions and to eliminate background interferences derived from the copper ions. The major fragmentations permitted the localization of double bonds and minor fragments allowed the differentiation of alkene isomers. The CI/FAB source was coupled to a gas chromatograph and simple and complex mixtures of octene isomers were analysed by gas chromatography (GC)/Cu(+)-CI-MS and GC/Cu(+)-CI-MS/MS. Copyright 1999 John Wiley & Sons, Ltd.

Location of double bonds in two unsaturated forms of phytanic acid from Refsum disease as determined by mass spectrometry

A monounsaturated and a triunsaturated form of phytanic acid (3,7,11,15-tetramethylhexacosanoate) were isolated from plasma lipids of a patient with Refsum disease. Both were converted to their methyl esters, oxidized to polyhydroxy acids by treatment with OsO4 and converted to their vicinal trimethylsilyl ethers. These derivatives were analyzed by gas chromatography-mass spectrometry using both electron impact ionization (at 21 and 70 eV) and chemical ionization conditions to obtain clear evidence to establish the structure of the monounsaturated form of phytanic acid as 3,7,11,15-tetramethylhexadec-15-monoenoic acid and that of the triunsaturated form of phytanic acid as 3,7,11,15-tetramethylhexadec-6,10,14-trienoic acid. The possible metabolic and dietary sources for these novel fatty acids are discussed.

Radiolysis of alpha, alpha-trehalose in concentrated aqueous solution; the effect of co-irradiated proteins and lipids

gamma-Radiolysis (dose-rate: 0 X 89 Gy/s) or electron (e)-radiolysis (dose-rate: 5 X 10(7) Gy/s) of unbuffered aqueous solutions of alpha, alpha-trehalose (concentration: 60 mg/ml, radiation dose: 20 kGy) at 0 degree C yielded glucose (Ggamma = 1 X 7; Ge = 0 X 63) and 5-deoxyxylohexodialdose (Ggamma = 0 X 21; Ge = 0 X 05). Buffering at pH-values of 5 X 0 or 5 X 5 and irradiation caused increased formation of these monomeric products, particularly of the deoxy-compound. On addition of increasing amounts of bovine serum albumin or ovalbumin (10-30 mg/ml) and irradiation the yields of products were markedly reduced. The decrease in glucose formation was less pronounced when sperm whale myoglobin was present during gamma- or electron-irradiation. The G-values of 5-deoxyxylohexodialdose, however, were increased by 45 per cent (gamma-irradiation) and 70 per cent (electron-irradiation) at approximately 10 mg/ml of admixed myoglobin. Further increase in myoglobin concentration led to a gradual decrease in the yields of the deoxy-product. The observed effects are explained by scavenging of water radicals and by interactions of the added substrates with sugar radicals. Emulsified lipids (palmitic acid methylester or trilinoleic glycerol) did not affect the radiation-induced formation of products from trehalose.

Topographical distribution of gamma-aminobutyric acid within the cat thalamus in relation to the basal ganglia, as determined by mass fragmentometry

Chemical ionization mass spectrometry was introduced for the assay of GABA in the cat brain. The method is quite simple, sensitive, and specific for quantitative analysis. Study of the regional distribution of the GABA content within the thalamus disclosed that the ventromedial nucleus (VM) of the thalamus had a high concentration of GABA. The VM receives the afferent projection from the zona reticulata of the substantia nigra. The result, together with the results obtained by physiological as well as pharmacological studies, supports the hypothesis that the transmitter substance of the nigrothalamic pathway is GABA.

Selective ion monitoring in clinical chemistry

The principles of selective ion monitoring are described. Choice of instrumentations, derivatives, and internal standards is discussed. The most important factors influencing sensitivity, specificity, and precision are summarized. Applications of selective ion monitoring for quantitative assay of steroids, fat soluble vitamins, triglycerides, prostaglandins, biogenic amines, amino acids, carbohydrates, and several other organic compounds of clinical interest are critically reviewed. It is concluded that isotope dilution selective ion monitoring is one of the most sensitive and accurate techniques presently available for quantitation of a large number of endogenous compounds of clinical interest. In view of the high accuracy, the technique is useful not only for the clinical chemists dealing with determination of specific compounds which are difficult to analyze by other methods, but also for those dealing with quality control of routine analyses of simple organic compounds.