Chemical ionization (NO) spectra of n-alkenoic acids and their esters

It is shown that the position of one double bond in long-chain unsaturated fatty acids and their esters can be determined by chemical ionization (NO) mass spectra. Homoconjugated dienoic acids (as linoleic acid) do not give a characteristic framentation pattern, in contrast to those which contain three or more homoconjugated double bonds.     

Resonance electron capture for determining double bond and hydroxy group locations in fatty acids

The nature and location of modifications of fatty acids are determined by resonance electron capture (REC) ionization of free acids and their methyl esters and pyrrolidides. The molecular negative ions (MNI) formed in the high resonance region undergo both charge-remote and charge-driven decomposition. The spectra of fragments arising from dissociation of these high-energy MNI contain decisive information on the original structure of the neutrals. The pyrrolidides of fatty acids result in simpler spectra on the one hand, and on the other hand these spectra give complete structure information.     

质谱特征结合等效链长定性分析植物油中的脂肪酸

建立了质谱特征结合等效链长快速定性植物油中脂肪酸的方法。首先根据质谱特征判断脂肪酸的类型并鉴定出其中的饱和脂肪酸甲酯,然后利用它们的保留时间信息计算得到不饱和脂肪酸甲酯的等效链长值,与已建立的脂肪酸甲酯数据库对照实现不饱和脂肪酸甲酯的结构鉴定。用NaOH-甲醇对5种常见植物油(花生调和油、茶籽调和油、菜籽油、葵花籽油、大豆油)中的脂肪酸进行衍生和提取,采用DB-23石英毛细管柱(30m×0.25mm×0.25μm)分离脂肪酸甲酯的同系物和异构体,气相色谱质谱法(GC/MS)测定,结果表明,5种样品油中所含不饱和脂肪酸的组成和含量上均存在明显差异。本方法无需标准品即可快速定性检测脂肪酸同系物及异构体,适用于油脂、食品中脂肪酸的成分分析。     

Determination of particulate fatty acids as P-bromophenacyl- or phenylphenacylesters using HPLC

Summary A method for the determination of fatty acids of the molecular weight range of propionic to erucic acid is reported. The acids are converted to either p-bromophenacyl-or p-phenylphenacylesters and the esters are separated by HPLC and detected by UV-absorption. Twentytwo esters, including saturated, monoolefinic and polyunsaturated ones were separated within one HPLC-run. The detection limit is about 5 pmol per fatty acid. The application of the method to particulate material is described.     

Mass spectral studies of alkylbenzenesulphonic acids through their S-benzylisothiouronium salts

The mass spectra of alkylbenzenesulphonic acids in the form of their S-benzylisothiouronium salts have been studied. These S-benzylisothiouronium salts dissociated into two parent reactant ions: (i) alkylbenzenesulphonic acid and (ii) S-benzylisothiourea. The characteristic fragmentation patterns of alkylbenzenesulphonic acids (0-5 substituted alkyls) were studied and compared with those of the parent hydrocarbons. The intensity of the molecular ion peak decreased with the increase in the molecular weight of the sulphonic acids. Desulphonation as well as loss of the alkyl group was observed in all the spectra. Migration of the alkyl group from S to O, followed by degradation, was also observed in all the spectra studied.     

Capillary electrochromatographic enantioseparations using a packed capillary with a 3 microm OD-type chiral packing

A technique for separating methyl esters of monounsaturated fatty acids by argentation chromatography using silver nitrate-impregnated TLC plates is described. Monounsaturated fatty acid methyl esters are separated from polyunsaturated and saturated fatty acid methyl esters and the monounsaturated fatty methyl esters are resolved according to chain length. cis isomers are well resolved from the corresponding trans isomers. R(F) values for individual monounsaturated fatty acids are very reproducible. The potential of the technique in metabolic studies is demonstrated in the chain elongation of [14C]-18:1(n-9) and delta-9 desaturation of [14C]-18:0 by human skin fibroblasts. Recoveries of individual [14C]-fatty acids for scintillation counting exceed 94%.     

脂肪酸的色谱保留时间规律与质谱特征研究及其在食品分析中的应用

用石油醚提取食品中的脂肪,经甲酯化反应后,采用HP-88(100m×0.25mm,0.33μm)弹性石英毛细管柱分离脂肪酸甲酯的同系物及异构体,GC/MS法测定。研究了不同链长脂肪酸的同系物及异构体的气相色谱出峰顺序,得到其保留时间规律;研究了不同脂肪酸的质谱断裂规律,选择3个特征离子来鉴定脂肪酸成分。建立了3个特征离子确定脂肪酸碳数及双键数目,色谱保留时间规律确定脂肪酸顺反异构体及双键位置异构体的方法。本法无需标准品即可快速测定脂肪酸同系物及异构体的含量,适用于脂肪酸组成的研究;及油脂、食品中脂肪酸,特别是反式脂肪酸的测定。     

An improved derivatization method for sensitive determination of fatty acids by high-performance liquid chromatography using 9-(2-hydroxylethyl)-carbazole as derivatization reagent with fluorescence detection

Summary Tagging techniques with reagents used for fluorescent detection for short and long-chain fatty acids using high-performance liquid chromatography are evaluated in terms of the tagging reactions, handing, flexibility, stability of the reagents. Emphasis is given to the applications of the tagging techniques to relatively high molecular mass fatty acids. The fatty acids or carboxylic compounds were derivatized to their corresponding esters with 9-(2-hydroxy ethyl)-carbazole (HEC) in acetonitrile at 60°C with N, N′-carbonyldiimidazole (CDI) as a coupling agent in the presence of 4-dimethylaminopyridine (DMAP). A mixture of esters of C₁−C₂₀ fatty acids was completely separated with 45 min using gradient elution on a reversed-phase C₁₈ column. The maximum fluorescence emission for the derivatized fatty acids is at 365 nm (λ_ex 293 nm). Studies on derivatization conditions indicated that fatty acids react rapidly and smoothly with HEC in the presence of CDI and DMAP in acetonitrile to give the corresponding sensitively fluorescent derivatives. The application of this method to the analysis of long chain fatty acids in plasma is also investigated. The LC separation shows good selectivity and reproducibility for fatty acids derivatives. The relative standard deviations (n=6) for each fatty acid derivative are <5.0%. The detection limits are at 38–57 fmol levels for C₁₄−C₂₀ fatty acids and lower levels for <C₁₄ fatty acids.     

HPLC of fatty acid esters of mono- and polyhydric alcohols. Part 1: Analytical separation

This paper shows how simply and yet very rapidly fatty acid esters of monohydric alcohols, but particularly partial and full fatty acid esters of fully hydric alcohols can be separated and determined by means of high-pressure liquid chromatography on LiChrosorb RP-8 with methanol/water. We have separated quantitatively the methylesters of the fatty acids C_8:0 to C_22:0 and C_24:0, the i-propyl-, i-butyl-, n-hexyl- and i-octyl-esters of the even-numbered fatty acids C_8:0 to C_18:0, mono- and difatty acid esters of the 1,3-bis-(2-hydroxyethyl)-5,5-dimethyl-hydantoin, mono-, di- and triesters of the trimethylolpropane as well as the tetraesters of the penta-erythrite.     

Exploring the fatty acids of vernix caseosa in form of their methyl esters by off-line coupling of non-aqueous reversed phase high performance liquid chromatography and gas chromatography coupled to mass spectrometry

Vernix caseosa is a greasy biofilm formed on the skin of the human fetus in the last period of pregnancy. This matrix is known to contain a range of uncommon branched chain fatty acids. In this study, we studied the fatty acid composition of vernix caseosa by non-aqueous reversed phase high performance liquid chromatography (RP-HPLC) fractionation followed by gas chromatography-electron ionization mass spectrometry (GC/EI-MS) of the fractions. For this purpose the fatty acids from vernix caseosa were converted into the corresponding methyl esters. These were fractionated by non-aqueous RP-HPLC using three serially connected C(18)-columns and pure methanol as the eluent. Aliquots of the HPLC fractions were directly analyzed by GC/EI-MS in the selected ion monitoring mode. Data analysis and visualization were performed by the creation of a two dimensional (2D) contour plot, in which GC retention times were plotted against the HPLC fractions. Inspection of the plot resulted in the detection of 133 different fatty acids but only 16 of them contributed more than 1% to the total fatty acids detected. Identification was based on HPLC and GC retention data, GC/MS-SIM and full scan data, as well as plotting the logarithmic retention times against the longest straight carbon chain. In selected cases, aliquots of the HPLC fractions were hydrogenated or studied by means of the picolinyl esters. Using these techniques, the number of double bonds could be unequivocally assigned to all fatty acids. Moreover, the number of methyl branches, and in many cases the positions of methyl branches could be determined. The enantioselective analysis of chiral anteiso-fatty acids resulted in the dominance of the S-enantiomers. However, high proportions of R-a13:0, R-a15:0, and R-a17:1 were also detected while a17:0 was virtually S-enantiopure.     

Perfluoroalkyl ketones: novel derivatization products for the sensitive determination of fatty acids by gas chromatography/mass spectrometry in electron impact and negative chemical ionization modes

Analytically useful pentafluoro ketone derivatives of fatty acids are described. The gas chromatographic/mass spectrometric characteristics of these new derivatives are compared with those of methyl, trimethylsilyl and pentafluorobenzyl esters. Pentafluoro ketones exhibit excellent chromatographic properties and significantly shorter chromatographic retention times than these other esters. The electron impact mass spectra of these new compounds show informative acylium ions, whose intensity decreases with the degree of unsaturation of the parent fatty acid. The formation of strong and informative fragment ions in negative chemical ionization (CH(4)) mass spectra of pentafluoro ketone derivatives allows the detection and the characterization (length of the chain and number of double bonds) of fatty acids at trace levels (femtomole), even in the case of polyunsaturated compounds. The scope and limitations of this new derivatization technique are also discussed.     

Rapid identification of fatty acid methyl esters using a multidimensional gas chromatography-mass spectrometry database

A multidimensional approach for the identification of fatty acid methyl esters (FAME) based on GC/MS analysis is described. Mass spectra and retention data of more than 130 FAME from various sources (chain lengths in the range from 4 to 24 carbon atoms) were collected in a database. Hints for the interpretation of FAME mass spectra are given and relevant diagnostic marker ions are deduced indicating specific groups of fatty acids. To verify the identity of single species and to ensure an optimized chromatographic resolution, the database was compiled with retention data libraries acquired on columns of different polarity (HP-5, DB-23, and HP-88). For a combined use of mass spectra and retention data standardized methods of measurement for each of these columns are required. Such master methods were developed and always applied under the conditions of retention time locking (RTL) which allowed an excellent reproducibility and comparability of absolute retention times. Moreover, as a relative retention index system, equivalent chain lengths (ECL) of FAME were determined by linear interpolation. To compare and to predict ECL values by means of structural features, fractional chain lengths (FCL) were calculated and fitted as well. As shown in an example, the use of retention data and mass spectral information together in a database search leads to an improved and reliable identification of FAME (including positional and geometrical isomers) without further derivatizations.     

Phase Behavior and Polymorphism of Saturated and Unsaturated Phytosterol Esters

This study investigated how the physicochemical characteristics of phytosterol esters are influenced by the chain length and degree of unsaturation of the fatty acid ester moiety. Saturated and unsaturated phytosterol esters (PEs) were synthesized by the esterification of different types of fatty acids (stearic, palmitic, lauric, oleic, and linoleic acid) to β-sitosterol. The non-isothermal crystallization and melting behavior of the pure PEs were analyzed. It was proven by X-ray diffraction that saturated β-sitosteryl esters and β-sitosteryl oleate formed a bilayer crystal structure. The lamellar spacings of the bilayer structure decreased with decreasing fatty acid chain length and with an increasing degree in unsaturation. The degree of unsaturation of the fatty acid chain of the β-sitosteryl esters also influenced the type of subcell packing of the fatty acid moieties in the bilayer structure, whether or not a metastable or stable liquid crystalline phase was formed during cooling. Furthermore, it was found that the melting temperature and enthalpy of the β-sitosteryl esters increased with an increasing fatty acid chain length while they decreased with an increasing degree of unsaturation. The microscopic analyses demonstrated that β-sitosteryl oleate formed much smaller spherulites than their saturated β-sitosteryl analogues.     

Gas chromatography of fatty acids.

Lipids in foods contain a wide variety of fatty acids differing in chain length, degree of unsaturation, position and configuration of double bonds and the presence of special functional groups. Modern capillary gas chromatography offers excellent separation of fatty acids. Fused-silica capillary columns with stationary phases of medium polarity and non-polar methylsilicone stationary phases successfully separate most of the natural fatty acids. Special applications, such as the separation of complex cis-trans fatty acid mixtures and cyclic fatty acids, required particular chromatographic conditions, including the use of very long capillary columns or more polar stationary phases. The derivatization methods for the preparation of fatty acid esters also need to be optimized to obtain accurate quantitative results. This paper reviews the derivatization techniques, capillary columns and stationary phases commonly used in the gas chromatography of fatty acids in food.     

Applications of high-pressure liquid chromatography and field desorption mass spectrometry in studies of natural porphyrins and chlorophyll derivatives.

Mixtures of porphyrins derived from natural sources can be readily separated by high-pressure liquid chromatography both analytically and on a preparative scale. A variety of procedures have been developed not only for the esters but also for free acids, and on the analytical scale quantitation is easily achieved by visible absorption. The retention times are largely characteristic of the number of carboxylic acid side chains (or other polar groups) but further information can be obtained by mass spectrometric studies of the various fractions. Field desorption mass spectrometry is particularly useful for this purpose because the emitter wire can be dipped directly into the eluates. The field desorption spectra of porphyrin free acids and esters as well as their metal complexes give essentially molecular ions with little or no fragmentation in most cases, whereas electron-impact mass spectrometry, particularly of free acids, is impeded by the low volatility of porphyrins. Mixtures can also be analysed by field desorption mass spectrometry, and this provides not only a rapid qualitative assessment of the components of a mixture, but also a check on the subsequent chromatographic separations.     

Tandem mass spectrometry for identifying fatty acid derivatives that undergo charge-remote fragmentations

A variety of fatty acid functional derivatives were desorbed by fast atom bombardment and collisionally activated. Derivatives having a high proton affinity such as fatty amides, pyrrolidides and picolinyl esters fragment to give decompositions that originate remote from the charge site. In contrast, derivatives having a low proton affinity such as fatty acids and fatty esters fragment to give charge-initiated decompositions. Therefore, the choice of the fatty acid functional derivative is important in effecting charge-remote decompositions of the [M + H]⁺ ions. The collisional activation spectra of the [M + H]⁺ ions of fatty alcohols and fatty amines were also compared. Based on this comparison, the internal energy required in order for charge-remote fragmentations to occur is estimated to be between 1.4 and 2.9 eV. This work is a guide in designing functional derivatives of fatty acids that undergo charge-remote reactions.     

气相色谱/质谱法分析孔石莼中的脂肪酸

建立了孔石莼脂肪酸的气相色谱/质谱(GC/MS)测定方法。使用Folch法提取了孔石莼中的总脂,经过2 mol/L HCl-甲醇溶液的甲酯化处理后,采用GC/MS法对其脂肪酸组成进行了分离分析,同时结合有机质谱学规律,分别对饱和脂肪酸甲酯、单不饱和脂肪酸甲酯和多不饱和脂肪酸甲酯的裂解规律和质谱特征进行了分析归纳。通过质谱数据库检索和标准品对照,鉴定出孔石莼中的24种脂肪酸,其中9,12,15-十八碳三烯酸、4,7,10,13-十六碳四烯酸和6,9,2,15-十八碳四烯酸3种主要多不饱和脂肪酸占总脂肪酸含量的45.14%。通过对孔石莼中脂肪酸的分析,表明特征离子在脂肪酸甲酯尤其是多不饱和脂肪酸甲酯的定性方面具有很好的应用价值。     

Study of fatty acid profiles in cancer cells grown in culture using gas chromatography--mass spectrometry

The determination of long-chain fatty acids in the phospholipid, triglyceride and free fatty acid fractions of HT29/219 colon cancer cells grown in a medium containing either foetal calf serum or horse serum, was carried out using gas chromatography - mass spectrometry. Several bonded-phase capillary columns were tested for the separation of the fatty acid methyl esters, and a 30-m poly(ethylene glycol) column was found to give optimum separation. The mass spectrometer was set to the multiple ion detection mode to increase the sensitivity of the recording of the characteristic ions, consisting of the molecular ion and the base peak. The phospholipid and triglyceride compositions of the cells were different when the cells were grown in media containing different sera. Differences were also found in the turnover of the acids in the different lipid fractions, the phospholipids being the most important, when the cells were grown in different media. The cellular metabolism and turnover of certain fatty acids differed from others in the same cell. These differences emphasise the importance of a precise knowledge of the lipid composition of the culture medium in in vitro studies of cancer cells.     

Preparation and characterization of cellulose β-ketoesters prepared by homogeneous reaction with alkylketene dimers: comparison with cellulose/fatty acid esters

Cellulose was dissolved in lithium chloride/1,3-dimethyl-2-imidazolidinone (LiCl/DMI), and reacted with alkylketene dimers (AKDs) under non-aqueous and homogeneous conditions to prepare cellulose/AKD β-ketoesters with high degrees of substitution (DS). Six AKDs synthesized from octanoic, decanoic, dodecanoic, tetradecanoic, hexadecanoic and octadecanoic acids via their fatty acid chlorides were used in this study. The cellulose/AKD β-ketoesters obtained were gummy solid at room temperature, and had DS values ranging from 1.9 to 2.9. Cellulose/fatty acid esters with DS 2.5–2.9 were also prepared as references. ¹³C-NMR spectra of the cellulose/AKD β-ketoesters showed that cellulose carbons and substituent carbons close to cellulose chains were restricted in motion and behaved like solid in solutions. In contrast, the cellulose/fatty acid esters did not demonstrate such anomalous ¹³C-NMR spectra. The unique ¹³C-NMR patterns are characteristic for the cellulose/AKD β-ketoesters, which have long and branched alkyl substituents in each anhydroglucose unit. Size-exclusion chromatography furnished with multi-angle laser light scattering (SEC-MALLS) revealed, on the other hand, that all cellulose/AKD β-ketoesters and cellulose/fatty acid esters prepared had flexible or random-coil conformations in tetrahydrofuran (THF). There were no clear differences in conformation or stiffness of cellulose chains between cellulose/AKD β-ketoesters and cellulose/fatty acid esters.     

Structural characterization of 5,6-dichlorotetradecanoic acid, an isolated metabolite of 9,10-dichlorooctadecanoic acid, by studying picolinyl esters, pyrrolidides and methyl esters with electron ionization mass spectrometry

A method is presented for identification of positional isomers of dichlorinated fatty acids, based on derivatization to picolinyl esters prior to gas chromatographic/mass spectrometric analysis in the electron ionization mode. The mass spectra of the picolinyl esters showed structure-specific fragmentation patterns. By using the picolinyl ester, 5,6-dichlorotetradecanoic acid was identified as a metabolite from a cell-culture medium obtained by culturing human cell lines in media supplemented with threo-9,10-dichlorooctadecanoic acid. This indicates that dichlorinated fatty acids are degraded by beta-oxidation. It is also possible to locate tentatively the position of chlorine atoms in 5,6-dichlorotetradecanoic acid as its methyl ester or pyrrolidide.