Pyrolysis-gas chromatographic analysis of alkylbenzenes

Summary The identification of alkylbenzenes by pyrolysis-gas chromatography was carried out through the analysis of the light pyrolysis products. In the range of C₇ to C₁₂ a number of 43 alkylbenzenes was investigated. All these compounds were unambiguously identified as well as the isomers which do not give significant differences in the mass spectra. A possible degradation mechanism is discussed.

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Pyrolyse-gas-chromatographische Analyse von Alkylbenzolen

Zusammenfassung Die Identifizierung von Alkylbenzolen durch Pyrolyse-Gas-Chromatographie wurde durch die Analyse der leichten Pyrolyseprodukte vorgenommen. Im Bereich von C₇ bis C₁₂ wurden 43 Alkylbenzole untersucht. Alle diese Verbindungen wurden zweifelsfrei identifiziert, ebenso wie auch die Isomeren, die im Massenspektrum keine signifikanten Unterschiede ergeben. Ein möglicher Abbaumechanismus wird diskutiert.

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Analyse d'alkylbenzènes par chromatographie en phase gazeuse de leurs produits de pyrolyse

Résumé L'identification d'alkylbenzènes par chromatographie en phase gazeuse a été effectuée par l'analyse des produits légers de la pyrolyse. Dans le cas des composés en C₇ à C₁₂, 43 alkyl-benzènes ont été étudiés. Tous ces composés ont été identifiés sans ambiguité, ainsi que les isomères auxquels ne correspondent pas de différences significatives des spectres de masse. Un mécanisme possible de dégradation est étudié.

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This work was supported by grants from the Scientific Exchange Agreement SEA     

Mobile and stationary phases for SFC: Effects of using modifiers

The effects of modifiers on the mobile-phase and the stationary-phase properties in packed-column supercritical-fluid chromatography were studied. Modifiers, may affect retention and improve peak shapes and efficiencies, because of (i) increased mobile-phase polarity, (ii) increased mobile-phase density, (iii) stationary-phase deactivation, and (iv) increased solvation or swelling of the stationary phase. In this paper these four parameters are evaluated and evidence to establish their relative importance is considered.The introduction of a modifier can lead to a substantial increase in the density of the mobile phase. The reliability of several methods for calculating the critical properties and densities of binary fluids is investigated. The Chueh and Prausnitz method gives the most accurate results for the calculation of the critical properties. The Lee and Kesler equation of state yields accurate density estimates.Adsorption isotherms have been measured on several stationary phases for packed-column SFC. These data suggest that stationary-phase deactivation is the most important effect of adding modifiers. Adsorption data on different stationary phases can also be used to judge their applicability. Relevant indicators are the maximum amount of modifier that can be adsorbed on the surface (saturation level) and the initial steepness of the isotherm (surface activity). The latter parameter can be related more closely to the peak shape observed in the absence of modifiers.All silica-based materials studied so far give rise to broad and highly asymmetrical peaks for certain classes of polar solutes. Different silica-based materials appear to differ in degree of activity, but the kind of interactions are the same. On a polystyrene-divinylbenzene stationary phase the mobile-phase modication effect could be studied without the interference of active sites.     

Endgroup-based separation and quantitation of polyamide-6,6 by means of critical chromatography

Polyamide-6,6 is a polycondensation product from the two monomers adipic acid and 1,6-hexamethylenediamine. Depending on the reacted amount of these monomers, different ratios of amine and carboxylic acid endgroups can be formed. Besides linear chains, cyclic polyamides will also be formed. Using critical chromatography, polyamide-6,6 can be separated independently of molar mass. Retention is based solely on endgroup functionality. It is demonstrated that high-molecular-mass polyamide-6,6 (Mw approximately 20,000-30,000) can be separated using this approach. The separation was optimized by using different parameters, such as percentage modifier, temperature and pressure. The concentration of phosphoric acid was used for selective retention of the different endgroup functionalities. Using this property, a new method called critical gradient chromatography was performed where the mobile phase changes from a weak to a strong solvent with respect to the endgroup functionality, while retaining the critical conditions of the backbone unit. Quantification using UV detection is discussed.     

Profiling of uremic serum by high-resolution gas chromatography-electron-impact, chemical ionization mass spectrometry

A fast and reliable procedure for gas chromatographic profiling of components in ultrafiltrated uremic serum has been developed, using glass capillary columns. Sample pretreatment consists of ultrafiltration, evaporation and silylation. Some twenty components are identified by electron-impact and chemical ionization mass spectrometry. A comparison is made between profiles of sera from a series of uremic patients, before and after hemodialysis, and from non-uremic sera. Significant differences are found between these profiles. A "dialysis ratio" is introduced as a parameter for the removal of retained components by hemodialysis treatment.     

Use of a temperature programmed injector with a packed liner for direct water analysis and on-line reversed phase LC–GC

A system is described that allows the introduction of large volumes of water samples in capillary GC. Water elimination is carried out in the solvent split mode in a PTV injector with a packed liner. Two ways of separating water and analytes, i.e. evaporative and non-evaporative (solid-phase extraction), are compared. Sampling in the solid-phase extraction mode is favorable both in terms of recovery as well as with regard to sampling time. Quantitative recovery is obtained for priority pollutants ranging in volatility from dimethyl-phenol to phenanthrene. Losses occur for more volatile compounds, but even for these compounds the repeatability of the recoveries remains acceptable. With the system described here, water samples up to at least 1 ml of water can be directly analyzed. The detection limits are in the sub-ppb range.     

Minimum analysis time in capillary gas chromatography vacuum- versus atmospheric-outlet column operation

Previous studies on open tubular column operation at vacuum outlet vs. atmospheric outlet pressures focused on comparisons of given columns, or comparisons of columns with the same inner diameters. It was demonstrated that, for a given separation problem, vacuum outlet operation of columns with a constant i. d. always yields the shortest analysis times (under minimum plate height conditions). In this paper, the comparison of vacuum vs. atmospheric outlet operation is broadened to columns with different dimensions. A general equation for the gain in speed of analysis by vacuum outlet operation of any column, as compared to atmospheric outlet operation of all possible open tubular columns with the same maximum plate number is presented. The resulting equation is further evaluated for thin film columns of different dimensions. It appears that vacuum outlet operation is beneficial only, in terms of speed of analysis, if low maximum plate numbers are required. The gain in speed of analysis is more pronounced for wide-bore than for narrow-bore columns.     

Ageing processes of alkyl bonded phases in HPLC; a chromatographic and spectroscopic approach

Summary Laboratory use of HPLC columns packed with C₈ and C₁₈ bonded phases leads to changes in selectivities and retention volumes. FTIR,¹H NMR of hydrolysed bonded phases and solid state¹³C- and²⁹Si NMR were applied to characterize the materials. The results of the various techniques are in fair agreement except solid state NMR. Loss of silane and hydrolysis of surface siloxane groups have been observed for the C₈ bondes phase, while for the C₁₈ material the latter process seems to dominate. The solid state NMR results have been tentatively explained in terms of changing chain arrangements and mobilities.Dedicated to Professor J. F. K. Huber on the occasion of his 60th birthday.     

Electron caputure detection in high-speed narrow-bore capillary gas chromatography: Fast and sensitive analysis of PCBs and pesticides

In this work the combination of high-speed narrow-bore capillary GC with electron capture detection is evaluated. The make-up gas flow rate is a key parameter in the successful coupling of narrowbore columns and ECD detection. The make-up flow has to be as high as possible to eliminate peak tailing caused by the large detection cell volume. The sensitivities at these elevated make-up flow rates (400 to 1000 ml/min), measured for some pesticides like HCB and dieldrin, were very good. Detection limits for these compounds of 0.1 pg were obtained, resulting in minimum detectable concentrations of approximately 0.2 ppb. The performance of the system is illustrated by several high-speed analyses of environmentally relevant samples of PCBs and pesticides.     

Detection limits of thermal conductivity and photoionization detectors in high speed narrow bore CGC

Not only the required input band width and the availability of compatible instrumentation limit the reduction of column diameter but so do the column sample capacity and detector characteristics, such as volume and sensitivity. In this paper the scope and limitations of thermal conductivity and photoionization detection at atmospheric and reduced pressures for capillary gas chromatography are discussed at length. It is shown that the sensitivity of a thermal conductivity detector is inversely proportional to the pressure. Reduction of cell volume and decrease of pressure appear equivalent with respect to minimum detectable amount and peak broadening. This results in femtogram detection limits for column diameters as low as 10 μm. For photoionization detectors the sensitivity did not improve at reduced pressures, so that the effect of reduced pressure and addition of make-up gas on the detection limits is the same. Nevertheless, the applicability of a low volume photoionization detector (40 μl) for capillary columns with a diameter as low as 50 μm will be demonstrated by series coupled PID and FID detectors and illustrated with various applications.     

Preparation of thermally stable phenylpolysiloxane fused silica capillary columns. Optimization and evaluation of the deactivation by capillary GC and solid state 29Si NMR

The deactivation of fused silica capillary columns with a laboratory-made poly-diphenylvinylmethylhydrosiloxane copolymer has been investigated. The deactivation obtained at different temperatures and reaction times is characterized with a dual column capillary GC system [1]. In parallel, the effect of the silylation temperatures and reaction times on the nature, the structure, and the chemical properties of the deactivation layer has also been studied by solid-state ²⁹Si NMR spoctroscopy. A fumed silica, Cab-O-Sil M5, was used as a model substrate for these spectroscopic studies. The deactivated fused silica capillaries show an excellent thermal stability (up to 400°C), a high resistance to solvolysis, and a minimal interaction to various critical test components. A good wettability of the fused silica capillary columns deactivated with this reagent was confirmed by successful subsequent coating with polysiloxanes with different phenyl contents.