Biomedical subjects

Mohamed Adahchour

Publications and source records attributed to Mohamed Adahchour.

8 recordsLinked to original sources

PubMed2006

Comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry in the identification of organic compounds in atmospheric aerosols from coniferous forest.

Comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC x GC-TOF-MS) was applied in the identification of organic compounds in atmospheric aerosols from coniferous forest. The samples were collected at Hyytiälä, Finland, as part of the QUEST campaign, in Spring 2003. Manual and automated search procedures were compared in the identification. An automated procedure is preferable when a large number of data files need to be processed; but manual search was more accurate with the present samples, where the number of compounds was large and most of the compounds of interest were present at trace level. Altogether, about 50 compounds were identified on the basis of mass spectra and linear retention indices. The identified compounds included oxidised monoterpenes, acyclic alkanes, alkenes, ketones and aldehydes, as well as a few alcohols, acids, and aromatic compounds.

Aerosols↗

PubMed2005

Twin comprehensive two-dimensional gas chromatographic system: concept and applications.

A twin GC x GC system has been designed which enables the analysis of a sample by means of two different and independent column combinations simultaneously. Both combinations are incorporated in the same oven, using the same temperature programme, and are fed using a 50:50 column-entrance-split. It is demonstrated that, employing combinations of a conventional non-polar x polar and a reversed-type polar x non-polar column set, the information content is as high, and the analytical performance is as good as when using two separate GC x GC systems. That is, there is an appreciable gain of time and a reduction of costs without any loss of quality. The general usefulness of performing, and comparing two mutually different GC x GC runs is further illustrated with FAMEs in olive oil, and pollutants in a sediment sample.

Chromatography, Gas↗

PubMed2005

Flow regime at ambient outlet pressure and its influence in comprehensive two-dimensional gas chromatography.

With method development in one-dimensional GC already being a tedious task, developing GC x GC methods is even more laborious. The majority of the present GC x GC applications are derived from previously optimised 1D-GC methods, from which especially the carrier gas flow settings are copied. However, in view of the high pressure inside the first-dimension column (high flow resistance of the narrow-bore second-dimension column), diffusion in the first column is much slower than in 1D-GC. Proper optimisation of the column combination and the carrier gas flow can considerably improve separations in GC x GC. To assist in the process of selecting column dimensions and flow rate optimization, we have developed a computer programme, based on Excel, that enables quick and simple calculation for all types of column combinations. The programme merely needs column dimensions and carrier gas type as input parameters and calculates all resolution and velocity parameters of the GC x GC separation by using flow rate and plate height equations. From the calculations a number of interesting conclusions can be drawn. As an example, the calculations clearly show that the majority of column combinations reported up till now have been operated at a far from optimal flow -- and, consequently, a far from optimal resolution. Probably even more important is the conclusion that the majority of column combinations used so far, i.e. those with 100 microm I.D. second-dimension columns, are not necessarily the best choice for GC x GC.

Chromatography, Gas↗

PubMed2005

Characterization of lipids in complex samples using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry.

Most lipids are a complex mixture of classes of compounds such as fatty acids, fatty alcohols, diols, sterols and hydroxy acids. In this study, the suitability of comprehensive two-dimensional gas chromatography coupled to a time-of-light mass spectrometer is studied for lipid characterization in complex samples. With lanolin, a refined wool wax, as test sample, it is demonstrated that combined methylation plus silylation is the preferred derivatization procedure to achieve (i) high-quality GC x GC separation and (ii) easily recognizable ordered structures in lipid analysis. Optimization of the GC x GC column combination, the influence of the temperature programme on the quality of the separation, and the potential and limitations of automated TOF-MS-based identification are discussed. The combined power of a 2D separation, ordered structures and MS detection is illustrated by the identification of several minor sample constituents.

Gas Chromatography-Mass Spectrometry↗

PubMed2005

Improved determination of flavour compounds in butter by solid-phase (micro)extraction and comprehensive two-dimensional gas chromatography.

The practicability and potential of comprehensive two-dimensional gas chromatography (GC x GC) coupled to both conventional flame ionisation (FID) and time-of-flight mass spectrometric (TOF-MS) detection, were compared with those of conventional one-dimensional (1D) GC, with the determination of flavour compounds in butter as an application. For polar flavour compounds, which were collected from the aqueous fraction of butter by means of solid-phase extraction (SPE), it was found that GC x GC dramatically improves the overall separation. Consequently, quantification and preliminary identification based on the use of ordered structures, can be performed more reliably. The improvement effected by replacing 1D-GC by GC x GC is considerable also in the case of TOF-MS detection, as illustrated by the high match factors generally obtained during identification. GC x GC was also used successfully for the characterisation of volatile flavour compounds in the headspace of butter collected by solid-phase microextraction (SPME) and to study the effect of heat treatment on the composition of butter samples in more detail.

Butter↗

PubMed2005

Comprehensive two-dimensional gas chromatography coupled to a rapid-scanning quadrupole mass spectrometer: principles and applications.

The principles, practicability and potential of comprehensive two-dimensional (2D) gas chromatography coupled to a rapid-scanning quadrupole mass spectrometer (GC x GC-qMS) for the analysis of complex flavour mixtures in food, allergens in fragrances and polychlorinated biphenyls (PCBs) were studied. With a scan speed of 10,000 amu/s, monitoring over a mass range of up to 200 atomic mass unit (amu) can be achieved at an acquisition frequency of 33 Hz. Extending this mass range and/or increasing the data acquisition frequency results in a loss of spectral quality. Optimal parameter settings allow, next to unambiguous identification/confirmation of target compounds on the basis of high-quality mass spectra, fully satisfactory quantification (three to four modulations per peak) with linear calibration plots and detection limits in the low-pg level. The potential of time-scheduled data acquisition to increase the effective mass range within one GC x GC run was also explored. The analyses, with baseline separation of the flavours, allergens and PCB target compounds, took less than 30 min.

Allergens↗

PubMed2004

Comprehensive two-dimensional gas chromatography of complex samples by using a 'reversed-type' column combination: application to food analysis.

The practicability and potential of a non-orthogonal approach in comprehensive two-dimensional gas chromatography (GC x GC) were studied and compared to those of the orthogonal approach for two different complex matrices, and using conventional flame ionisation (FID) and time-of-flight mass spectrometry (ToF MS) detection. The separation of a diesel oil showed that the non-orthogonal approach also provides interesting, but completely reversed, ordered structures. For the more extensively studied flavour analysis in food samples, improved peak shapes and, also, different types of ordered structures and retention behaviour, and improved detectability for polar compounds make the two approaches complementary to each other. As a consequence, identification and/or determination of targets and/or unknowns can be performed more reliably. Analytical performance (close to three-order linearity; LODs, 2-30 pg injected in most cases; R.S.D.s, 1-6% (n = 6)) was fully satisfactory.

Chromatography, Gas↗

PubMed2003

Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometric detection for the trace analysis of flavour compounds in food.

The practicability and potential of comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC x GC-TOF-MS) for the analysis of complex flavour mixtures in food were studied. With the determination of key flavour targets in dairy samples as an example, it was demonstrated that GC x GC dramatically improves the separation. As a consequence, identification and, more importantly, quantification down to the ng/g level can be performed more reliably: background interferences largely disappear. Next to the peak table generated from the GC-TOF-MS software after data processing, the additional use of well-ordered patterns in the 2D-plane and information from second-dimension retention times can substantially help the identification of unknowns. The technique was successfully used for an evaluation of extraction techniques and the characterisation of different types of samples.

Flavoring Agents↗