Identification and Quantitation of Linear Alkanes in Lubricant Base Oils by Using GC×GC/EI TOF Mass Spectrometry

Linear alkanes are a class of compounds known to negatively affect the physical performance of lubricant base oils. The ability to rapidly identify and quantify linear alkanes in lubricant base oils would enable oil companies to more effectively evaluate their refinery methods for converting crude oil to lubricant base oils. While mass spectrometry is a powerful method for elucidation of the structures of compounds in complex mixtures, it is not innately quantitative. An approach is presented here for the identification and quantitation of linear alkanes in base oil samples by utilizing GC×GC/EI TOF MS. Identification of the linear alkanes in base oils was achieved based on their retention times in both GC columns as well as their EI mass spectra. Linear alkane model compound mixtures were used to generate calibration plots for quantitation of the linear alkanes in the base oils. The accuracy of this method was greater than 83.8%, within-day precision lower than 6.2%, between-day precision lower than 16.2%, and total precision lower than 17.2%. All noted figures of merit surpass the acceptable limits for a new validated quantitative method, where accuracy must be better than 80% and precision lower than 20% at the lower limit of quantitation. The n-alkane content in both base oil samples was further validated using a GC×GC/FID method (the gold standard for quantitation), which provided nearly identical results to those obtained using the GC×GC/EI TOF MS method. Therefore, GC×GC/EI TOF MS can be used to both identify and quantitate linear alkanes.

Naturally occurring ω-Hydroxyacids

ω-Hydroxyacids are fatty acids bearing a hydroxyl group on the terminal carbon. They are found in mammals and higher plants and are often involved in providing a permeability barrier, the primary purpose of which is to reduce water loss. Some ω-hydroxyacid derivatives may be involved in waterproofing and signalling. The purpose of this review was to survey the known natural sources of ω-hydroxyacids. ω-Hydroxyacids are produced by two different P450-dependent mechanisms. The longer (30-34 carbons) ω-hydroxyacids are produced by chain extension from palmitic acid until the chain extends across the membrane in which the extension is taking place, and then the terminal carbon is hydroxylated. Shorter fatty acids can be hydroxylated directly to produce C16 and C18 ω-hydroxyacids found in plants and 20-eicosatetraenoic acid (20-HETE) by a different P450. The C16 and C18 ω-hydroxyacids are components of polymers in plants. The long-chain ω-hydroxyacids are found in epidermal sphingolipids, in giant-ring lactones from the sebum of members of the equidae, as a component of meibum and in carnauba wax and wool wax.

Structures of the ceramides from porcine palatal stratum corneum

Ceramides are the major type of lipid found in stratum corneum from the skin, gingiva and hard palate. The present study examined the ceramides of the stratum corneum from the hard palate. Six fractions of ceramides were isolated by preparative thin-layer chromatography. The least polar fraction contained an unusual acyl ceramide (EOS) consisting of long omega-hydroxy acids amide-linked to sphingosine with mostly saturated fatty acids ester-linked to the omega-hydroxyl group. The second and third fractions contained normal fatty acids amide-linked to sphingosine (NS) and phytosphingosine (NP), respectively. In each of these ceramides, the fatty acids consisted of a mixture of saturated and monoenoic species. The three most polar fractions all contain amide-linked alpha-hydroxy acids. The fourth fraction contained long alpha-hydroxy acids amide-linked to sphingosine (ASl), while the fifth fraction contained short alpha-hydroxy acids amide-linked to sphingosine (ASs). The most polar ceramide contained alpha-hydroxy acids amide-linked to phytosphingosine (AP). EOS, NS and NP differed from their epidermal counterparts in terms of the compositions of the normal fatty acids. ASl, ASs and AP from palatal stratum corneum were essentially identical to their epidermal counterparts. The differences between palatal and epidermal EOS, NS and NP contribute to the differences in permeability of palate compared to skin.

Novel function of lipids as a pheromone from the Harderian gland of golden hamster

Sexual diversity of ADG in Harderian gland of golden hamster was demonstrated on TLC. Female ADG contained iso- and anteiso-branched acyl and alkyl components, but male ADG contained only straight chain ones, which suggested the hormonal control of the expression of acyl-CoA dehydrogenases in the catabolism of BCAA. Acyl-CoA dehydrogenases were not expressed in the absence of testosterone, and then isovaleryl-CoA, 2-methylbutyryl-CoA, and isobutyryl-CoA accumulated, and acted as primers for the synthesis of iso- and anteiso-branched fatty acids. The incorporation of [U-(14)C] leucine into lipids was monitored by TLC. The cholesterol fraction was labeled in males but not in female, which means that cholesterol was not produced from BCAA in female gland due to the lack of expression of acyl-CoA dehydrogenases. We monitored the behavior of male hamsters toward female gland lipids, and found slightly greater attractiveness in female ones than that in male ones although the difference was not significant. Considering the lifestyle of golden hamster in nature, we propose a hypothesis that the lipids from the Harderian gland of golden hamster serve as a pheromone to declare their territory and to seek the mate with good congeniality.

Photodegradation chemistry of the insecticide phosmet in lipid models and in the presence of wool wax, employing a 15N-labeled compound

The organophosphorus insecticide phosmet is commonly used for plant protection as well as against pests on animals. Phosmet features numerous degradation pathways induced by UV irradiation. In this study, we focused on the reaction possibilities of phosmet in the presence of lipophilic substances with olefinic structure elements, as they are frequently found in animal fur lipids, especially in wool wax. In the first step, simple models were employed to characterize the structural types of formed photoaddition products. On irradiation in the presence of cyclohexene, three photoaddition products were identified, a (4pi + 2pi) cycloaddition product of phosmet and cyclohexene and two diastereoisomer carbinols. Likewise, in more sophisticated models employing fatty acid methyl esters for irradiation experiments, phosmet was readily photodegraded. In the presence of methyl oleate, also 1:1 photoaddition products could easily be identified by means of liquid chromatography-mass spectrometry, using a stable isotope-labeled phosmet (15N-phosmet, 50 at. %) for the photolysis experiments. To converge from models to the more complex natural environment of animal skin lipids, irradiation experiments of phosmet in the presence of wool wax were performed. After 24 h of irradiation, only a remnant of 1.4% of the initial phosmet was detectable. Phosmet-oxon was detected in concentrations up to 27.3 mol %.

Novel long-chain anteiso-alkanes and anteiso-alkanoic acids in Antarctic rocks colonized by living and fossil cryptoendolithic microorganisms

Saponified extracts of rock samples colonized by cryptoendolithic microbial communities from the McMurdo Dry Valleys of Southern Victoria Land, Antarctica, were separated into hydrocarbon and fatty acid fractions by silica gel column chromatography. Hydrocarbons and methyl esters of fatty acids were analyzed by capillary gas chromatography-mass spectrometry. Unusually, a suite of long-chain anteiso-alkanes (a-C20 to a-C30) and anteiso-alkanoic acids (a-C20 to a-C30) were detected in many samples, together with straight-chain, branched and/or cyclic and acyclic isoprenoid compounds. These novel compounds are probably derived from unidentified heterotrophic bacteria or symbiotic processes in a unique microbial community in the Antarctic cold desert and suggest the occurrence of a special biosynthetic pathway. Long-chain anteiso-alkanes are probably formed through microbial decarboxylation of corresponding anteiso-alkanoic acids. They may serve as new biomarkers in environmental and geochemical studies.

The role of 18-methyleicosanoic acid in the structure and formation of mammalian hair fibres

Although branched chain fatty acids perform many functions in biological systems, the importance of the anteiso 18 methyleicosanoic acid (MEA) has only recently been recognized. In this first review on MEA its role and distribution is explored. MEA has been found in minor amounts in the fatty acid components of a wide range of biological materials, but the current interest results from it being the major covalently bound fatty acid in mammalian hair fibres, a finding which is unusual because protein-bound fatty acids are typically straight-chain, even-numbered acids (C14-C18). MEA is released by surface restricted reagents indicating that it is located exclusively in or on the surface of the cuticle cells, a conclusion that has been verified by analysis of isolated cuticle cells, X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectroscopy (SIMS) studies support these results in that they show the surface of the cuticle to be predominantly hydrocarbon. When either neutral hydroxylamine or acidic chlorine solutions are applied to hair and wool fibres fatty acids are liberated, indicating the presence of thioester bonds. Calculations, based on fatty acid and amino acid analysis, indicate that approximately one residue in 10 of the cuticular membrane protein is a fatty acid thioester of cysteine. Removal of this covalently linked fatty acid renders the fibre hydrophilic, thus offering a chemical explanation for many technological and cosmetic treatments of mammalian fibres. Examination of the fibre surface and that of isolated cuticle cells by transmission electron microscopy (TEM) confirms the presence of a thin non-staining continuous layer surrounding the cuticle cells. Alkaline treatments which remove the bound fatty acids were found to disrupt this layer. TEM examination of developing hair fibres has indicated that the fatty acid layer on the upper surface and scale edges of the cuticle cell differs from that of the underside of the cell. Similar structural studies of hair from patients with maple syrup urine disease (MSUD) support the findings that thioester-bound MEA is limited to the upper surface of fibre cuticle cells. The current model proposed for the boundary layer consists of crosslinked protein with surface thioester-linked fatty acids, forming a continuous hydrophobic layer on the upper surface and scale edges of the cells.

Quantification of n-alkanes in stratum corneum in the hereditary ichthyoses

Chromatographic assay of n-alkanes in skin showed detectable levels in normal controls and in patients with various forms of hereditary ichthyosis. Raised n-alkanes were found in some, but not all, patients with non-bullous and bullous ichthyosiform erythroderma and in individual patients with lamellar ichthyosis, ichthyosis vulgaris and Netherton's syndrome. The finding of elevated scale n-alkanes is neither consistent in ichthyosis, nor specific to any one type of ichthyosis, and n-alkane assay is not helpful in distinguishing one type of hereditary ichthyosis from another. The source of n-alkanes in ichthyotic scale and their role, if any, in the pathogenesis of ichthyosis remain obscure.

The origin of alkanes found in human skin surface lipids

Lipids extracted from human skin contain variable amounts of paraffin hydrocarbons. Although the composition of these alkanes strongly resembles petroleum waxes, it has been proposed that they are biosynthetic products of human skin. To investigate this question, skin surface lipids from 15 normal subjects were analyzed for the amount and composition of alkanes, using quantitative thin-layer chromatography and quartz capillary gas chromatography. The alkanes were found to constitute 0.5% to 1.7% of the skin lipids. Subjects differed greatly in the chain length distribution of their alkanes between 15 and 35 carbon atoms, and in the relative amounts of normal alkanes (like those in petroleum waxes) and branched chain alkanes (like those in petroleum lubricating oils). In 6 subjects, the alkane content of cerumen from each ear was examined to investigate whether alkanes arrive at the skin surface by a systemic route or by direct contact with environmental surfaces. No trace of alkanes was found in 11 of the 12 cerumen samples. Using a tandem accelerator mass spectrometer for carbon-14 dating, a combined sample of the skin surface alkanes was found to have a theoretical age of 30,950 years, similar to that of a sample of petrolatum. These analyses indicate that the alkanes found on the surface of human skin are mixtures of a variety of petroleum distillation fractions that are acquired by direct contamination from the environment.

Free sphingosines in porcine epidermis

Sphingosines and phytosphingosines serve as intermediates in the synthesis of ceramides and glucosylceramides, which are prominent components of mammalian epidermis. In the present study, we have investigated the possibility that free sphingoid bases also may be present in epidermal tissue. Samples of pig epidermis were trypsinized to separate the stratum corneum from the unkeratinized portion of the epidermis. After drying, the lipids were extracted and analyzed by thin-layer chromatography using ninhydrin to detect free amino groups. Both the stratum corneum and the unkeratinized epidermal material contained a ninhydrin-positive material with the same mobility as the sphingosine standard. Quantitation of the chromatograms by photodensitometry indicated that free sphingosine bases account for 0.44% by weight of the total stratum corneum lipid and 0.09% of the lipid in the viable portion of the epidermis. To further identify this material, it was treated with 1-fluoro-2,4-dinitrobenzene, which resulted in the production of an intensely yellow N-2,4-dinitrophenyl derivative with the same mobility as N-2,4-dinitrophenylsphingosine on thin-layer chromatography. Oxidation of the isolated dinitrophenyl derivative with lead tetraacetate produced a mixture of aldehydes which were analyzed by gas-liquid chromatography. This analysis indicates that the free sphingoid bases from the stratum corneum consist of a mixture of mainly 16- through 20-carbon sphingenines and sphinganines, the most abundant components being d17:0, d17:1, d18:1 and d20:1. The production of these free sphingosine bases may be significant in regulating epidermal differentiation.

Integral lipids of mammalian hair

1. It has been demonstrated that hair contains lipids which cannot be removed by extensive extraction with chloroform-methanol mixtures. These integral lipids can be extracted only after the hair has been subjected to alkaline hydrolysis. 2. Integral hair lipids include cholesterol sulfate (0.7-2.9 mg/g hair), ceramides (0.6-1.4 mg/g), cholesterol (0.3-1.4 mg/g), fatty alcohols (trace-0.2 mg/g) and fatty acids (2.3-4.0 mg/g). 3. One of the major integral hair lipids, representing 38.4-47.6% of the total fatty acids, is the anteisobranched 18-methyleicosanoic acid. 4. The species examined included human (Homo sapiens), pig (Sus scrofa), dog (Canis familiaris), sheep (Ovis ammon aries) and cow (Bos taurus).

Integral lipids of human hair

It has long been recognized that hair is coated with nonpolar lipids originating in the sebaceous glands, and recently it has been shown that hair also contains cholesterol sulfate and small amounts of ceramides, similar to those found in the keratinized portion of the epidermis. In the present study, it is demonstrated that significant amounts of several additional lipids are tightly associated with hair in such a way as to be highly resistant to solvent extraction. These integral hair lipids included cholesterol sulfate (3.3 mg/g of extracted hair), cholesterol (0.6 mg/g), fatty alcohols (0.2 mg/g) and free fatty acids (4.3 mg/g). The principal fatty acid, comprising 40% of the total fatty acids, was identified as 18-methyl-eicosanoic acid by cochromatography with authentic standard on gas-liquid chromatography (GLC) and by mass spectrometry (MS).

The time course of lipid biosynthesis in pig epidermis

In order to examine the time course of lipid metabolism in the epidermis, pairs of sites on each of two pigs were given seven intradermal injections of 10 microCi of [1-14C]-acetate at 1, 3, 6, 12, 24, 36, and 48 h before the animals were killed. A second time course was obtained with two other pigs at 1 h and 3.5, 7, 10.5, 14, 17.5, and 21 d after labeling. The epidermis over each injection site was recovered, and the lipids were extracted and analyzed by thin-layer chromatography and by autoradiography. Virtually all of the radioactivity in aliphatic chains was found first in phospholipids and was then transferred to glucosylceramides, and then into ceramides, between 1 and 7 d after labeling. Labeling of free fatty acids showed a similar time course. Most of the radioactivity acquired by acylglucosylceramide was transferred to hydroxyceramide, bound to the corneocyte envelope, between 2 and 10 d after labeling. The hydroxyacid and sphingosine moieties of the hydroxy-ceramide had equal specific activity. The results indicate that the aliphatic lipids of the stratum corneum are synthesized de novo in the epidermis via phospholipids. Cholesterol, however, was synthesized directly from acetate within hours of labeling, and its esters acquired label 3 to 7 d later.

Composition and morphology of epidermal cyst lipids

The contents of epidermal cysts were used as a source of desquamated human keratinocytes uncontaminated by sebaceous, subcutaneous, or bacterial lipids. Lipids extracted with chloroform:methanol mixtures included six series of ceramides (41% of the total extractable lipid), cholesterol (27%), cholesteryl esters (10%), fatty acids (9%), cholesteryl sulfate (1.9%), a novel class of ceramide esters (3.8%), and a sterol diester (0.9%). Electron microscopy revealed that the lipids in the cyst contents existed as multiple intercellular lamellae, as in stratum corneum. One lamella, adjacent to the horny cell protein envelope, was resistant to lipid extraction and is thought to represent covalently bound lipid on the outer surface of the keratinocyte. The results indicate that the degradation of intercellular lipid lamellae is not required for desquamation.

Possible genetic control of the proportions of branched-chain fatty acids in human sebaceous wax esters

The composition of human sebum varies among individuals with respect to the proportions of various types of fatty acids in the ester lipid classes. Iso-branched fatty acids with even numbers of carbons (iso-even) are particularly variable. However, in 13 identical twin pairs, intrapair differences in the proportions of iso-even fatty acids in sebaceous wax esters were found to be very small, while interpair differences were as large as in the nontwin population. Intrapair differences in iso-even fatty acids were also quite small in 4 of 8 fraternal twin pairs. Heritability estimates suggested that proportions of iso-even fatty acids in human sebum are controlled, at least in part, by genotype.

The composition of the ceramides from human stratum corneum and from comedones

Human epidermal surface lipids were collected by an ethanol wash and the ceramides were quantified by thin-layer chromatography-photodensitometry. Six ceramide fractions were isolated and the structural components of each were analyzed in detail. The most unusual of the epidermal ceramides contained a sphingosine base with amide-linked 30- and 32-carbon omega-hydroxyacids and an ester-linked nonhydroxyacid, 41% of which was linoleic acid. The proportion of linoleic acid in the analogous ceramide from comedones was 6%. This supports the hypothesis that a localized insufficiency of linoleic acid in the follicular epithelium is an etiologic factor in comedogenesis.

The composition of the skin surface lipids of the gerbil

The skin surface lipids of the gerbil were found to consist sterol esters (10%), wax diesters (36.3%), triacylglycerol (26.1%), free fatty alcohols (8.8%), free fatty acids (5.4%), cholesterol (8.4%) and polar lipids (5%). The wax diesters, identified as Type II, were made up of saturated 1,2-diols with odd carbon number, esterified with two molecules of unsubstituted fatty acids with even carbon number. Both the triacylglycerols and the free fatty acid fractions had saturated and unsaturated components. The free and esterified sterols were all cholesterol. The sterol esters contained saturated monoenoic and dienoic fatty acids, with both straight- and branched-chain components. The fatty alcohols were all straight-chain in structure, mostly of even carbon number. Comparison of these results with those previously reported for other species, indicates that the gerbil skin surface lipids are unique in that they contain diacyl alkane diols and fatty alcohols, both of which consist exclusively of saturated components.

Skin surface lipids of the horse

Skin surface lipids from the sides of male and female horses (Equus caballus) were collected in acetone and analyzed by thin layer chromatography and gas liquid chromatography. The sole components in both sexes were cholesterol, cholesteryl esters and the lactones of 32-, 32- and 36-carbon omega-hydroxy acids, each including a methyl group in the n-1 position. Most of the lactones were monounsaturated (either n-8 or n-10), but small amounts of saturated and dienoic species were present. A pooled sample of the skin surface lipids contained 14% cholesterol, 38% cholesteryl esters and 48% lactones.

15-Methyl-1,2-hexadecanediol, a major constituent of hamster surface wax

Long chain 1,2-alkanediol diesters comprise about 15-20% of the acetone soluble skin surface wax of golden Syrian hamsters. The constituent 1,2-alkanediols, obtained through acidic methanolysis, were fractionated by preparative gas liquid chromatography of their isopropylidene derivatives. The major component (57%) was identified by nuclear magnetic resonance and mass spectrometry as 15-methyl-1,2-hexadecanediol.

Skin surface lipids of the dog

The skin surface lipid of the dog has been reported to contain a high proportion of diol diesters having a lower mobility on thin layer chromatography than diesters from other species in spite of containing similar fatty acid and diol components. In the present study, dog skin surface lipid was separated by preparative thin layer chromatography into sterol esters (42%), wax diesters (32%), free sterols (9%), polar lipids (7%), and unidentified components (10%). The diesters contained 1,2-diols, each estrified with one long chain fatty acid and one isovaleric acid moiety. The diols were principally branched chain C21 and C22 compounds while the long chain fatty acids esterified with them were mainly C20 and C21 branched compounds. The fatty acids from the sterol esters were mostly saturated, branched chain C19 TO C23, together with 7% of straight chain monoenoic acids, principally C21 and C22. There were only trace amounts of free sterols other than cholesterol, while the esterified sterols contained 96% cholesterol and 4% lathosterol.

Skin surface lipids of the guinea pig

Skin surface lipids of the guinea pig were found to contain sterol esters (33%), wax diesters (diacyl alkanediols) (24%), glycerol ether diesters (28%), free fatty alcohols (6%) and free sterols (9%). The sterol esters and diacyl alkanediols contained saturated fatty acids (40 and 67%, respectively) having straight and singly-branched chains and mono-unsaturated acids (60 and 33%,respectively) derived predominantly by delta 9-desaturation of C15 and C16 straight-chain saturated fatty-acid precursors. The 1-O-alkylglycerols and fatty acids from the glycerol ether diesters were both entirely saturated series containing straight, branched and multi-branched chains. Both the free and the esterified sterols consisted principally of cholesterol with a small proportion of lathosterol.

Genetic control of β-diketone and hydroxy-β-diketone synthesis in epicuticular waxes of barley

Five eceriferum, (cer) mutants in barley which influence β-diketone and hydroxy-β-diketone synthesis in spike and internode epicuticular waxes have been characterized. The mutation cer-u (69) blocks the synthesis of hydroxy-β-diketones and leads to a compensatory increase in the amount of β-diketones, indicating that β-diketones are precursors of the hydroxy-β-diketones. Furthermore, highly lobed wax plates were observed for the first time on barley lemmas, in addition to the characteristic wax tubes. Both diketone classes are selectively and proportionally reduced in the spike wax of cer-i (16), which has shorter wax tubes. The three mutants cer-c (36), -q (42), and -c,u (108) synthesize neither diketone class and form no wax tubes. In contrast to the variable composition of most individual barley wax classes, only a single β-diketone was identified, namely hentriacontan-14,16-dione.