Mass spectrometry structure analysis, isomers

The urine samples were analyzed using a modified version of a published method.8 The method involved fortification of the urine samples with an internal standard 3,4,5-trichloro-2-pyridinyl, which is a structural isomer of the 3,5,6-TCP metabolite of chlorpyrifos hydrolysis of labile acid conjugates to 3,5,6-TCP solvent extraction derivitization to the f-butyl-dimethylsilyl ester of 3,5,6-TCP and subsequent negative-ion chemical ionization gas chromatography/mass spectrometry (GC/MS) analysis. Creatinine was determined in urine using a modification of a method of Fabiny and Erting-shausen.9... 

Fig. 3.2.4 Structures of the isomers 2-methylbutyryl- and isovalerylcarnitine. These isomers can not be differentiated by acylcarnitine analysis as described here because both appear as butylated Cs-acylcarnitine esters at m/z 302. Differentiation would require an additional chromatography step prior to tandem mass spectrometry analysis...

Studies on the chemical structure of olive wax esters have shown that the homologues present in olive-pomace oil are almost entirely esters of oleic acid with long chain alkanol constituting the homologous series C40, C42, C44, C46. Odd-chain esters identified in the oil were esters of oleic acid with C23, C25, C27 alcohols. Gas chromatography and mass spectrometry analysis has shown that each carbon chain of the esters in made up of a single isomer in which the acyl moiety is that of oleic acid (Bianchi el al., 1994). Thus, for example, ester C44 was found to be made up of the couple acid-alcohol Cl8 1 and C26, whereas other possible isomers such as C16 l-C28 were not detected. This is unusual if it is compared with the composition of epicuticular ester fractions of oil seeds for which, in cases studied, each ester chain was composed of several positional isomers of the ester group. 

The characteristic H(4)-H(5) H NMR coupling constants of the 3-oxabicyclo[3.2.0]heptane derivatives anti-% (V//,//=1.3 Hz) and ry -51 Jh,h = 5.6 Hz) have been used to establish the structural assignments for these isomers <20060L491>. The structure of the rearranged cembrane derivative ciereszkolide 52 has been elucidated by onedimensional (TD) and 2-D NMR spectra, high-resolution fast atom bombardment mass spectrometry (HRFAB-MS), infrared (IR), and ultraviolet (UV) as well as by single crystal X-ray analysis <2004EJ03909>. 

One of the major challenges inthe structural analysis of flavonoids using mass spectrometry is the difficulty of differentiating between isomers. Rutinosides and neohesperidosides are the two most common flavonoid diglycosides. These isomers differ only by the... 

The metabolic and/or hydrolytic products of parathion encountered as residues in the urine include both diethyl phosphoric acid and diethyl phosphorothioic acid, most probably as their salts (potassium or sodium). Derivatization of these residues with diazomethane would result in the formation of three trialkyl phosphate compounds, namely, 0,0-diethyl O-methyl phosphate (DEMMP), 0,0-diethyl 0-methyl phosphoro-thionate (DEMMTP), and 0,0-diethyl S-methyl phosphorothiolate (DEMMPTh). Earlier (15), it had been shown by combined gas chromatography-mass spectrometry and other analytical data that a later-eluting major product ca. 85%) of the methylation of diethyl phosphorothioic acid formed under the conditions of the analytical method was DEMMPTh, and the minor product formed (ca. 15%) was DEMMTP. Accordingly, all three trialkyl phosphates were observed and confirmed by mass spectrometry in the analysis of the human urine extract. Sufficient internal bond energy differences are associated with the isomeric structures DEMMPTh and DEMMTP that qualitatively and quantitatively dissimilar fragmentation patterns are observed for both isomers as can be seen from the mass spectra of these compounds shown in Figure 4. 

The structure of 33 has been supported by using elemental analysis, mass spectrometry and NMR spectroscopy [119]. However, the NMR spectra provided a more complex depiction. They indicated the presence of five species in a CDCI3 solution of sanguinarine fi ee base (I) a major bimole-cular stereoisomer, the racemate 6R,6"R + 6S,6 S, vsdiich is thermodynamically voured according to AMI calculations [120] (II) a minor isomer, a... 

Often there is a need for structural identification of unknowns without available reference compounds and the identification can be done in connection with the chromatographic separation. One approach is to run measurements directly on-line using HPLC as the separation technique with UV-detection and monitoring at several wavelengths, but this is often not enough for safety identification. The last 15 years have seen a rapid development of combined liquid chromatography-mass spectrometry instrumentation, and this technique is the most valuable tool in qualitative analysis today (se below). In the absence of a reference compound some unknown substances e.g., isomers of the desired compound may require NMR for their definitive identification. 

Gas chromatography-mass spectrometry analysis of the dimer and trimer fractions of the polymerisation products of propene and but-l-ene also suggested intramolecular hydride and methanide shifts as the source of the isomers formed. A mechanistic scheme for the oligomerisation of propene has been proposed, Reaction sequence (2.4) [31]. In most of the studies described above, the products have been analysed after hydrogenation. However, a study analysed the Cy product of a propene/but-l-ene copolymerisation before hydrogenation and confirmed the role of hydride and methanide shifts in determining the structures of the products [32],... 

A second fluorescent Chl-catabolite, Ca-FCC-2, was isolated from another in vitro system, based on enzymatic activity obtained from ripe (red) sweet pepper (Capsicum annuurri) and its structure was analyzed (67). The new fluorescent catabolite could be shown by mass spectrometry to be an isomer of 10 Further NMR-spectroscopic analysis revealed Ca-FCC-2 to have the same constitution and to differ from pFCC (10) only in the absolute configuration at C(l). Ca-FCC-2 was thus assigned as the epimeric primary I -epz-pFCC (epi-10) (67). 

Mass spectrometry is an excellent technique for both qualitative and quantitative analysis. On the basis of molecular weight alone, many compounds can be identified (with the exception of isomers). Using a combination of molecular weight and fragmentation patterns, unambiguous identification is possible as well as structural elucidation of unknown compounds. MS can also be used for quantitative work using either internal or external standards for reference. Limits of detection are very low so sensitivity is normally excellent. 

Interest in alkaloids of the nicotine group appears to be on the increase. A review has appeared covering tobacco-specific nitrosamines, which may be causative factors in tobacco-related cancers. The solution conformation, and proton, deuterium, carbon-13, and nitrogen-15 n.m.r. spectra of nicotine and its 2- and 4-isomers, have been studied. A new synthesis of nornicotine and nicotine has been described, and a quantitative carbon-13 n.m.r. spectral analysis of nicotine that is labelled at positions 1, 2, and 3 with carbon-13 been presented. The synthesis and mass spectrometry of several structurally related nicotinoids have been reported. Nicotine is dehydrogenated on irradiation in benzene solution in the presence of benzophenone to l -methyl-2 -(3-pyri-dyl)pyrrole. ° Nornicotine has been synthesized in four steps from 3-bromo-pyridine and N-3-butenyl-phthalimide, using a palladium-catalysed vinylic... 

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