Analysis of GC Data

Gas chromatography can be applied to both qualitative and quantitative analyses. There are two general methods for qualitative identification of eluted compounds. A compound may be identified by retention time or by peak enhancement. If you have some idea of the identity of an unknown peak or if you know it is one of three or four compounds, then you can inject the known compounds into the GC under conditions identical to those for the unknown sample. Compounds with the same retention times ( 2 or 3%) can generally be considered identical. Alternatively, you can add pure sample of a suspected component to the unknown sample and inject the mixture into the GC. If a recorder peak is increased in size, this also provides evidence for the identity of an unknown. The latter technique is called peak enhancement or spiking. 

Identification of unknown compounds by retention times or peak enhancement is not conclusive or absolute proof of identity. It is possible for two different substances to have identical retention times under the same experimental conditions. For positive identification, the sample must be collected at the exit port and characterized by mass spectrometry, infrared, nuclear magnetic resonance, or chemical analysis. 

The percent of x in the sample is equal to the area represented by component x divided by the sum of all peak areas. Areaa and Area/ refer to other components in the mixture. 

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The MAB ion source offers several advantages over El for PyMS. By eliminating excessive fragmentation, characteristic of electron ionisation, and by producing highly reproducible mass spectra MAB (Kr) greatly simplifies the analysis of pyrolysis data. Furthermore, MAB ionisation, when combined to MS/MS, provides a useful tool for structural elucidation of pyrolysis products. The ability for selective ionisation can be very useful to reduce the background combination in techniques such as GC-MS, LC-MS or SFC-MS. 

Marine lipids with their diversity of unsaturated and branched chain acid moieties are a difficult class of materials to analyze. Ruminants (sheep, goats, cows, etc.) have a bacterial "factory" in the rumen which is able to produce branched-chain partially-hydrogenated lipids from ingested plant lipids. These lipids are incorporated into the milk and meat of the animals and eventually into animals which feed upon the ruminants. As a rule animal lipids are highly complex in comparison to plant materials. Although the branched chain materials are usually present in low concentration when compared to the common fatty acid moieties, complete description of these fats requires more sophisticated GC and thus long open tubular columns in tandem with mass spectrometry and computer analysis of the data has become an important approach. Even with a 100-m column, subcutaneous lipids of barley-fed lambs were so complex that prior fractionation with urea adducts was necessary (17). 

Steiner, F.M., Schilck-Steiner, B.C., Nikiforov, A., Kalb, R. and Mistrik, R. (2002). Cuticular hydrocarbons of Tetramorium ants from Central Europe Analysis of GC-MS data with self-organizing maps (SOM) and implications for systematics. 

Kalahalide D (41) has been isolated from the marine mollusk Elysia rufescens and from the alga Bryopsis sp. and identified by analysis of spectroscopic data, chemical degradations, and chiral HPLC, GC, and TEC [65]. 

In a second paper on the factor analysis of GC-MS data ( ), Sharaf and Kowalski concentrated on determining the elution vectors of the individual components contributing to a fused peak. In favorable cases, where there were unique masses in the mass spectra of the overlapping con nents, the peak profiles could be determined with excellent certainty. Data was presented to show the possibility of detecting and identifying two components even if their retention times were identical - all that is needed is some difference in their peak profiles. 

Schlich et al. (1987) proposed a new approach to selecting variables in principal component analysis (PCA) and getting correlations between sensory and instrumental data. Among other studies, Wada et al. (1987a,b) evaluated 39 trade varieties of coffee by coupling gas chromatographic data with two kinds of multivariate analysis. The objective classification was compared with the sensory data (cup test), directly or after statistical treatment. The results were concordant. Murota (1993) used qualitative sensory data to interpret further the results of GC data and canonical discriminant analysis. He could thus suggest which were the components responsible for the flavor characteristics in different coffee cultivars. 

The analysis of sequence data has provided useful information about the genes encoded in the nuclear genome of P. falciparum. The most relevant features are the following (i) the coding strand is purine-rich (ii) A is predominant in all codon positions (iii) the third codon positions are extremely GC-poor and, as a consequence, codon usage is strongly biased (Weber, 1988 Hyde and Sims, 1987). 

This methylation/CpG transition deserves a more detailed discussion. An analysis of the data presented in Part 5 indicates that (i) two positive correlations hold between the 5mC and GC levels of the genome of fishes/amphibians and mammals/birds, respectively (see Fig. 5.13) (ii) the higher methylation of fishes and amphibians is not related to the higher amounts of repetitive DNA sequences (see Fig. 5.14) and (iii) the 5mC and CpG observed/ expected values show no overlap between the two groups of vertebrates and suggest the existence of two equilibria in 5mC and CpG levels (see Fig. 11.14). Several important questions then arise concerning (i) the two equilibria in methylation and CpG shortage (ii) the transition between the two equilibria and (iii) the causes of the methylation/CpG transition. 

ROM either with or without structures [8]. The current version is the 6th containing some 229,119 reference spectra from 200,500 compounds which can be combined with a version of the NIST database, increasing the total size to 275,821 spectra of 226,334 compounds. The version marketed by the Palisade Corporation is available with a useful data format conversion utility MASSTransit (http //www.wi-leyregistry.com). There is also the PC BenchTop/PBM search system described below which can be purchased for off-spectrometer data processing and analysis of GC/MS runs for example. 

NIST have produced a freely available package called AMDIS (automated mass spectrometry deconvolution and identification system) for the analysis of GC/MS data sets. Developed to assist in the task of verifying the international Chemical Weapons Convention (http //www.opcw.org/) financially supported by the US Defense Special Weapons Agency (DSWA, US Department of Defense) the AMDIS program is also distributed with the NIST 02 Mass Spectral Library (see above). 

It has been over 40 years since infrared spectroscopy was first applied to GC by identifying trapped effluents taken directly from the column. With the advances of FTIR spectrometers and computer techniques, these modern FTIR spectrometers and data systems provide rapid scanning, increased sensitivity, and endless disk space for data storage. All of these characteristics are necessary to merge FTIR with today s sensitive high-resolution capillary gas chromatography. Therefore, the reliability of qualitative analysis of GC/FTIR is greatly enhanced. 

A mass spectrometer produces an enormous amount of data, especially in combination with chromatographic sample inlets [42]. Over the years, many approaches for analysis of GC-MS data have been proposed using various algorithms, many of which are quite sophisticated, in efforts to detect, identify, and quantify all of the chromatographic peaks. Library search algorithms are com monly provided with mass spectrometer data systems with the purpose to assist in the identi cation of unknown compounds [43]. 

Automated Qualitative and Quantitative Analysis of GC/MS Data of Complex Mixtures from Physiological Fluids... 

The mechanism of this reaction was discussed based on the preliminary results of control experiments and analysis of GC-MS data (Scheme 4.35). The reaction is initiated by the addition of azido free radical and terminated with molecular oxygen to generate the peroxide intermediate 65 [136]. Then this species would undergo continuous multi step rearrangement to give product 56. The details of this rearrangement were disclosed in their ensuing research in which DFT calculations were carried out (Scheme 4.36). This research provides concise and practical access to synthetic important molecules, which may find broad applications in synthesis. 

Because of the large number of samples and repetitive nature of environmental analysis, automation is very important. Autosamplers are used for sample injection with gc and Ic systems, and data analysis is often handled automatically by user-defined macros in the data system. The high demand for the analysis of environmental samples has led to the estabUshment of contract laboratories which are supported purely by profits from the analysis. On-site monitoring of pollutants is also possible using small quadmpole ms systems fitted into mobile laboratories. 

Applications. The capabiHties of a gc/k/ms in separating and identifying components in complex mixtures is very high for a broad spectmm of analytical problems. One area where k information particularly complements ms data is in the differentiation of isomeric compounds. An example is in the analysis of tricresyl phosphates (TCPs) used as additives in a variety of products because of thek lubricating and antiwear characteristics (see Lubrication and lubricants). One important use of TCPs is in hydrauHc fluid where they tenaciously coat metal surfaces thereby reducing friction and wear. Tricresyl phosphate [1330-78-5] (7.2 21 exists in a variety of isomeric forms and the commercial product is a complex mixture of these isomers. 

In order to prove that intramolecular cyclization occurs before telechelic oligomer formation, an experiment similar to previous work by Calderonlf is performed using 14 in place of Calderon s classical catalyst system. Macrocyclic species are formed when a toluene solution of polybutadiene is exposed to this catalyst, supported by both NMR and GC data. The vinylic resonances are clearly shifted upfield from polybutadiene. GC analysis shows macrocyclic trimers and tetramer regioisomers. 

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