Direct probes, mass spectrometry

Volatile lipids or lipid derivatives are conveniently introduced into the ion source of the mass spectrometer by a gas chromatograph (Kuksis and Myher, 1989). The modem capillary columns can be inserted directly into the ion source thus avoiding the use of an interface. This method of sample introduction is widely employed for fatty acid and diacylglycerol (DG) 

This review is restricted largely to discussion of LC/MS and LC/MS/MS combinations for glycerolipid, glycerophospholipid and sphingolipid analyses. Recent applications to complex fatty acids and other non-glycerolipids are discussed as required for structural analyses of glycerolipids and sphin-golipids. 

Fatty acids are readily separated and identified by GC/MS following addition of methyl (Me), pentafluorobenzyl (PFB), or other alkyl groups to the carboxyl moieties, and trimethylsilyl (TMS) groups to any hydroxyl functions, and methoxime groups to carbonyl functions prior to analysis by El or Cl (Murphy, 1993 Murphy and Zirrolli, 1994 Couderc, 1995). 

Cyclopentyl and cyclohexyl fatty acid monomers are present in heated vegetable oils. Sebedio et al. (1989) have used GC/MS to identify the major products from heated linseed oil as a mixture of 1 1 cis and trans cyclopentyl and cyclohexyl isomers the products of heated sunflower oil were found to be mostly cyclopentyl isomers. The major cyclopentyl isomers were the cis and trans isomers of methyl 7-(2 -hexyl cyclopentyl)-heptanoates and methyl 10-(2 -propylcyclopentyl)-decanoates. The major cyclohexyl isomers were trans and cis methyl 9-(2 -propylcyclohexyl)-nonanoates and made up 50% of the cyclic fatty acid isomers of linseed oil. Dobson et al (1995) have used AgNOs and GC/MS for the structural analysis of cyclic diene fatty acids, and Mossoba et al (1996) have presented confirmatory MS data for cyclic fatty acid monomers by using deuteration. 

Ford et al. (1996) have applied ES/MS/MS to demonstrate that unsaturated acyl carnitines are the predominant molecular species of acyl-carnitine which accumulate during myocardial ischemia (5-20 min) 18 2 (w/z424) 18 1 (m/z 426) 16 0 (m/z400) 18 0 (w/z428) carnitines. On prolonged global ischemia (20 min) substantial amounts of 20 1 (m/z 454) and 20 2 (m/z 452) acylcarnitine species represented 15% of the total increase in accumulation. The identity of the acylcarnitine species was confirmed by ES/MS/MS which demonstrated the production of ions at m/z 85 (carnitine) and the respective fatty acid in each case. 

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Direct probe mass spectrometry. Glycosphingolipids (30-100 pg) were permethylated as described (12). The samples (less than 5 p g) were subjected to electron impact/desorption analysis with a Varian MAT CH-5 DF mass spectrometer under the following conditions emission current, 300pA electron energy, 70 eV acceleration voltage, 3KV ion source temperature, 160° C emitter wire current, programed from 0 to 35mA. 

This information was obtained from samples of less than 5 pg by electron impact/desorption direct probe mass spectrometry. On the basis of complete structural analyles, to be presented elsewhere, we have been able to determine that the four fractions isolated thus far actually contain six different glycosphingolipids with the following structures ... 

Frequently industrial hygiene analyses require the identification of unknown sample components. One of the most widely employed methods for this purpose is coupled gas chromatography/ mass spectrometry (GC/MS). With respect to interface with mass spectrometry, HPLC presently suffers a disadvantage in comparison to GC because instrumentation for routine application of HPLC/MS techniques is not available in many analytical chemistry laboratories (3). It is, however, anticipated that HPLC/MS systems will be more readily available in the future ( 5, 6, 1, 8). HPLC will then become an even more powerful analytical tool for use in occupational health chemistry. It is also important to note that conventional HPLC is presently adaptable to effective compound identification procedures other than direct mass spectrometry interface. These include relatively simple procedures for the recovery of sample components from column eluate as well as stop-flow techniques. Following recovery, a separated sample component may be subjected to, for example, direct probe mass spectrometry infra-red (IR), ultraviolet (UV), and visible spectrophotometry and fluorescence spectroscopy. The stopped flow technique may be used to obtain a fluorescence or a UV absorbance spectrum of a particular component as it elutes from the column. Such spectra can frequently be used to determine specific properties of the component for assistance in compound identification (9). 

The use of preparative TLC is, moreover, a way of obtaining sufficiently large amounts of degradation products for the structure elucidation direct probe mass spectrometry or nuclear magnetic resonance spectrometry. 

Herod AA, Kandiyoti R. 1995. Fractionation by planar chromatography of a coal tar pitch for characterization by size exclusion chromatography, UV fluorescence and direct probe mass spectrometry. J Chromatogr 708 143-160. 

Figure 19 Mass spectrum of 25-OH-D3 purified from Hep 3B cells (upper panel) compared to the mass spectrum of synthetic 25-OH-D3 (lower panel). Hep 3B cells were incubated with vitamin D3 (50 pM) for 48 h. Flasks were then extracted and the lipid extract dried under nitrogen and purified on HPLC (conditions Zorbax SIL [6.2 mm X 25 cm], solvent HIM 96/3/3, flow rate 2 mL/min). A metabolite peak possessing the vitamin D chromophore and comigrating with synthetic 25-OH-D3 was collected, purified further on a different HPLC system (conditions Zorbax CN [4.6 mm X 25 cm], solvent HIM 94/ 5/1, flow rate 1 mL/min), and then dried under nitrogen and subjected to direct probe mass spectrometry using electron impact EI(+) ionization. The putative 25-OH-D3 gave the expected molecular ion with m/z 400 the other ions observed were consistent with the molecule being 25-hydroxylated (see inset fragmentation pattern). (From Ref. 207.)...

Where the antidegradant is not volatile enough to be identified using a GC based technique, direct probe mass spectrometry or LC-MS can be used and the... 

Direct insertion probe mass spectrometry (DIP-MAS) analyses of poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), and their coalesced and precipitated blends were performed [51] (see Fig. 21). The fact that the pyrolysis mass... 

Radicals can also be studied directly by mass spectrometry using a molecular beam inlet. It is difficult to design a satisfactory molecular beam probe and there are problems connected with the calibration of the spectrometer but a number of successful studies have been made (Foner and Hudson, 1954 Fristrom and Westenberg, 1965 Homann et al, 1963). 

One of the first reports of a forensic analysis of TATP in the United States came following an accidental detonation of a homemade TATP explosives device [41]. The TATP was analyzed by direct insertion probe-mass spectrometry (DIP-MS) with El, as well as methane and... 

Two papers describe in detail methods used to transfer material separated by TLC into sample holders suitable for direct insertion probe mass spectrometry. Rix et al. (26) transfer the scraped sample spot into a drawn-out elution column, and then elute the sample through a plug into a separate part of the column (Figure 2). The concentrated sample solution is then evaporated onto the tip of a standard direct insertion probe. Kohler (27) describes a similar method that can also be used for the collection of samples for a subsequent GC/MS analysis. 

J.A. Hiltz and T. Foster, Direct Exposure Probe/Mass Spectrometry and Pyrolysis-Gas Chromatography/Mass Spectrometry Study of the Effect of Gamma Radiation Exposure Thermal Degradation Products of Natural Rubber Polycaprolactone Mixtures, Report No. DREA-TM-95/214, Defence R D, Canada, 1995. 

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