Applications of comprehensive two-dimensional gas chromatography to the characterization of petrochemical and related samples

This article discusses the application of comprehensive two-dimensional gas chromatography (GC x GC) to samples derived from petrochemicals. The use of GC x GC for characterization of petroleum and petroleum derivatives, through group type analysis, such as benzene, toluene, ethylbenzene, xylenes (BTEX), total aromatic hydrocarbons, polyaromatic hydrocarbons, and heteroatomic sulfur-, oxygen-, and nitrogen-containing compounds is presented. The capability of GC x GC to provide additional chemical-specific information regarding petroleum-processing steps, such as linear alkanes dehydrogenation, Fischer-Tropsch process, hydrogenation and oligomerization, is also described. In addition, GC x GC analyses of petrochemical biomarkers and environmental petrochemical-derived pollutants are reported. The role of comparison of samples through use of simple fingerprint approaches is highlighted.     

ESI-FTICR mass spectrometric study of alcohol complexation properties of mono- and diphosphonate-bridged cavitands

Alcohol complexation properties of eight mono- and diphenyl phosphonate-bridged cavitands were studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR) and theoretical calculations. The cavitands varied in number and position of phenyl phosphonate bridges and their orientation with respect to the cavity, length of the lower rim alkyl chains, and substituents at apical positions of the resorcarene skeleton. The specificities of the different cavitands toward primary, secondary, and tertiary alcohols varying long of the alkyl chain were investigated, together with the stabilities of the formed complexes. The number, position, and orientation of the P = O moieties affected the complex formation of the cavitands and stability of the complexes dramatically. Methyl groups at apical positions of the resorcarene skeleton also affected the complexation properties. Although length and branching of the alkyl chain of the alcohol influenced the complex formation, the effect on stability of the complexes was negligible.     

High efficiency liquid chromatography techniques coupled to mass spectrometry for the characterization of mate extracts

There is growing interest related to rapid screening and full characterization of the constituents of plants with medicinal properties; among these, “Mate” or Yerba Maté is a tea-like beverage widely consumed in South America, obtained from the dried leaves of Ilex paraguariensis. The high content in polyphenols accounts for in vitro and in vivo antioxidant activity of the extracts obtained from this plant; on the other hand, the high complexity of the samples extracted, depending on the method employed, may preclude complete resolution by conventional HPLC techniques. For this purpose, a comprehensive two-dimensional liquid chromatography (LC × LC) system, comprised of an RP-Amide first dimension and a partially porous octadecylsilica column in the second dimension, has been compared with a one-dimensional system. The latter was operated using a partially porous octadecylsilica column, with diode array (DAD) and electrospray/ion trap-time of flight (ESI/IT-TOF) detection for the most complex extracts. The employment of the hybrid mass spectrometer allowed unequivocal identification of several compounds in the mate extracts. Using LC × LC–MS³, it was possible to discriminate between congeners of chlorogenic acids, along with monoacyl- and diacylchlorogenic acid esters.     

Advantages of polarized two-beam second-harmonic generation in precise characterization of thin films

Polarized second-harmonic generation using two fundamental beams, instead of one, offers significant advantages for characterizing nonlinear optical thin films. The technique is more precise and allows the internal consistency of the results to be verified. The superiority of the two-beam arrangement over the traditional single-beam arrangement is demonstrated by determining the susceptibility tensors of Langmuir-Blodgett films. We show that, for a well-understood reference sample, the results obtained using two fundamental beams agree qualitatively with those obtained with a single fundamental beam, but are more precise. In a more complicated situation, however, the single-beam technique appears to work well but yields results that are, in fact, incorrect. The two-beam technique, instead, yields clearly inconsistent results, thereby highlighting systematic errors in the experimental arrangement or in the theoretical model used to interpret the results.     

Halogenated-2-pyrones in Sonogashira cross-coupling: limitations, optimisation and consequences for GC analysis of Pd-mediated reactions

The Sonogashira couplings of 4-bromo-6-methyl-2-pyrone (5) with phenylacetylene, mediated by Pd(PPh₃)₂Cl₂ in the presence of a CuI co-catalyst, have been investigated in detail. The concentration of Pd dramatically influences the product yield, with lower Pd-loadings favouring higher conversions and purer cross-coupled product. A post reaction time-dependence in product conversion is seen in samples quenched solely on silica-gel (eluted with CH₂Cl₂). The effect is mirrored in reactions employing 4-nitro-bromobenzene (14) and to a lesser extent (E) and (Z)-ethyl 3-iodo-2-propenonate (16) under similar conditions. A more efficient quenching system (using excess dppe) has been developed to enable accurate determinations in product conversions. Alternatively, solvent and base (Et₃N) removal in vacuo, or quench with saturated aqueous ammonium chloride, prevents further turnover in Sonogashira coupling. An ESI-MS study on samples eluted through silica was undertaken to probe the nature of the soluble Pd/Cu species. The Sonogashira cross-coupling of 4-chloro- and 6-chloro-2-pyrone (18 and 20, respectively) has further been investigated. The former undergoes successful coupling, however the latter decomposes in polar aprotic and protic solvents under standard conditions, through a chlorine substitution process, making Pd-mediated reactions problematic.     

N-oxides of the quinoxaline series

A further study of the chemical behavior of quinoxaline and its -methyl and hydrdxy derivatives and their N-oxides in oxidation reactions and reactions with acetic anhydride has been carried out. The features of these reactions connected with the structure of the reacting N-oxides are discussed.For part XIV, see [10].     

N-oxides of the quinoxaline series

The reaction of quinoxaline N-oxides with benzenesulfonyl chloride and benzoyl chloride is investigated, and its mechanism discussed. The reaction products are investigated.     

Study of the protonation of 1-methylimidazo (4,5-B)pyrazines by1H and13C NMR methods

Conclusions A study of the¹H and¹³C NMR spectra of the neutral molecules, monocations, and dications of 1-methyl- and 1,5,6-trlmethylimidazo(4,5-b)pyrazLnes has shown that the first protonation of these compounds occurs at the 3-N atom of the imidazole fragment and the second at the 4-N atom of the pyrazine ring.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1060–1068, May, 1977.     

Synthesis, 1 H, 13 C, 15 N, and 113 Cd NMR, ESI-TOF MS, Semiempirical MO (PM3), ab initio/HF and Cation/Anion Binding Studies of N-deoxycholyl-l-tryptophan

The synthesis, structural characterization, and cation/anion binding properties ofa new bile acid-amino acid conjugate, N-deoxycholyl-l-tryptophan, aredescribed. The structures of the ligand and its cadmium adduct at different pHconditions and various cadmium perchlorate concentrations were determined bymodern multinuclear magnetic resonance spectroscopic as well as ESI-TOF MStechniques. Also semiempirical PM3 and ab initio/HF molecular modellingstudies were performed. Based on ¹H,¹H NOESY measurementsN-deoxycholyl-l-tryptophan in alkaline conditions was found to appearin a bent conformation which was clearly different from the conformations in neutraland acidic solutions. According to molecular modelling in its minimum energy structurethe tryptophan backbone of the ligand was folded close to the deoxycholic acid skeletonand the structure was stabilized by an intramolecular hydrogen bond. The multinuclearmagnetic resonance experiments indicated that Cd²⁺-cation was bound with theligand in neutral and alkaline conditions while in acidic conditions protons block thebinding site. ESI-TOF MS revealed clearly a competition between sodium and cadmiumions, the ligand having a stronger affinity for sodium. Cadmium binding occurred onlywhen excess of cadmium was used. Further, ESI-TOF MS spectra showed that variouschlorine oxyanions originated from perchlorate anion formed together with cationsdifferent adducts with the ligand.