Retention index analysis

Musumarra et al. [44] also identified miconazole and other drugs by principal components analysis of standardized thin-layer chromatographic data in four eluent systems and of retention indexes on SE 30. The principal component analysis of standardized R values in four eluents systems ethylacetate-methanol-30% ammonia (85 10 15), cyclohexane-toluene-diethylamine (65 25 10), ethylacetate-chloroform (50 50), and acetone with plates dipped in potassium hydroxide solution, and of gas chromatographic retention indexes in SE 30 for 277 compounds provided a two principal components model that explains 82% of the total variance. The scores plot allowed identification of unknowns or restriction of the range of inquiry to very few candidates. Comparison of these candidates with those selected from another principal components model derived from thin-layer chromatographic data only allowed identification of the drug in all the examined cases. 

The concentration data obtained from each sample analysis were expressed as fractional parts and normalized to sum to 100. The normalized data were statistically analyzed, and three principal components (A=3, Equation [1]) were calculated. The PCB constituents (varlbles) are numbered sequentially and correspond to peak 1, peak 2,. .. to peak 105. The structure and retention index of each constituent in the mixture were reported by Schwartz et al. (9). The tabular listing of the data is availedile from the present authors. 

Fig. 15.17 Simultaneous EI-MS (top trace) and PTR-MS (bottom trace) total ion coimt analysis of coffee headspace. Identification was based on MS spectra obtained at 70 eV and the retention index of the reference compounds. (Adapted from [199])...

Fig. 16.2 Flavour dilution (FD) chromatogram obtained by application of aroma extract dilution analysis on an extract prepared from parsley leaves. The odorants were identified as 1 methyl 2-methylbutanoate, 2 myrcene, 3 l-octen-3-one, 4 (2)-l,5-octadien-3-one, 5 2-isopropyl-3-me-thoxypyrazine, 6p-mentha-l,3,8-triene, 71inalool, 8 2-sec-butyl-3-methoxypyrazine, 9 (.Z)-6-dece-nal, 10 / -citronellol, 11 ( , )-2,4-decadienal, 12 / -ionone, 13 myristicin, 14 unknown. RI retention index. [30, 31]...

W. Jennings and T. ShSoaxnoto, Qualitative Analysis of Flavor andFragrance Volatiles by Glass Capillary Gas Chromatography, Academic Press, Inc., New York, 1980 also includes retention indexes and mass spectral data. 

A great number of stationary phases are listed in catalogues and it is sometimes difficult to choose the best column for a particular analysis. The chemical nature of the phases and their polarities do not always allow one to predict which column will be optimal for a given separation. Therefore, a technique called the retention index system has been developed with the use of reference compounds whose retention factors differ with different stationary phases. Using retention indices obtained on columns of different stationary phases, it is possible to characterise a compound and facilitate its identification. 

There are many samples that can be injected into a gas chromatograph in order to determine what aroma active volatile is present or to quantify a particular one (Maarse and van der Heij, 1994). Unfortunately, many of the protocols that were followed in the past did not rely on meaningful standards or retention indexing, or failed to use GC sniffing or the more formal GC/O procedures to establish that the peaks detected had odor activity however, if the object of the analysis is to simply monitor one or two odorants known to contribute positively or... 

Retention index of odorants on the stationary phase used in the chiral analysis (single oven) or on the precolumn (MDGC), and on more than one stationary phase if found to be necessary for the identification of odorants, determined by using a hydrocarbon standard, described above and in unitgu. 

The VOCs in the inside air are enriched by means of active sampling on Tenax TA tubes, which are thermodesorbed. After internal standards have been added, analysis is carried out by capillary GC—MS. The hundred compounds on the chromatogram which have the most intense signals are identified by retention index and mass spectrum (Wensing, Schulze and Salthammer, 2002), and are then semi-quantitatively evaluated with toluene as the reference substance. The toluene equivalents are summed, and this result serves as a semi-quantitative estimation of the total VOC concentration (TVOC). 

A. Manninen, M.-L. Kuitunen and L. Julin. Gas chromatographic properties of the M-series of universal retention index standards and their application to pesticide analysis, J. Chromatogr., 394, 465-471 (1979). 

For LC a similar relationship should apply if the retention mechanism shows the expected theoretical dependence on carbon number. The situation is more complex since the partition coefficient is a function of many intermolecular forces. Several papers have been published showing a homologous series retention like that described for GC.5 In principle then, the retention index concept should also apply in those cases. However, little interest has been shown in developing an index for LC, probably because the paraffins are not usually run by LC and the modes of analysis by LC are much more variable and complex, so that the data are not as widely usable. 

It has been shown from an analysis of the thermodynamic significance of retention index [29] that the retention index of a compound i = CH3(CH2) X (cf., eqn. 5) can be expressed as... 

Donnelly JR, Sovocool GW (1992), Chemosphere 25 1299-1304. Gas chromatographic elution order and elution shift modelling for isomer specific analysis of halogenated dioxins" Donnelly JR, Munslow WD, Grange AH Pettit TL, Simmons RD, Sovocool GW (1991a), J. Chromat. 540 293-310. Correlation of structure with linear retention index for bromo- and bromochlorodibenzo-p-dioxins and bromodibenzofurans"... 

Using a different set of standard substances, i.e. substituting 1-butanol, pentan-2-one, and 1-nitropropane for the rather volatile ethanol, butan-2-one, and nitromethane, McReynolds developed an analogous approach [103]. Altogether, he characterized over 200 liquid stationary phases using a total of 10 probes. A statistical analysis of the McReynolds retention index matrix using the principal component analysis method has shown that only three components are necessary to reproduce the experimental data matrix [262]. The first component is related to the polarity of the liquid phase, the second depends almost solely on the solute, and the third is related to specific interactions with solute hydroxy groups [262]. 

A typical chromatographic separation is presented in Figure 5.1. The compounds are identified in GC-MS analysis on the basis of two pieces of information, namely the retention time (or relative retention index) and mass spectral data. 

It is often the case that a peak in a gas chromatogram can be identified from its relative retention index (or time). This provisional identification can be confirmed by spiking the unknown sample with an authentic sample which should co-chromatograph with the unknown. However, unambiguous identification of an unknown compound demands structural analysis, which can either be done on-line (e.g., using coupled GC-MS equipment) or off-line. 

The introduction of GC as an analytical technique has had a profound impact on both qualitative and quantitative analysis of organic compounds. Identification of compounds by GC can be accomplished by their retention times on the column as compared to known reference standards, by inference from sample treatment prior to chromatography, " or by the concept of retention index. " The latter method and tables of retention indices " with associated conditions have been reported. " Although qualitative data and analytical techniques for identification of compounds are well-established " and relative retention data for over 600 substances also have been published, " the main utility of GC undoubtedly lies in its powerful combination of separation and quantitative capabilities. Use in quantitative analysis involves the implementation of two techniques being performed concurrently, i.e., separation of components and subsequent quantitative measurement. 

Headspace peaks from the wastewaters were correlated with one another and identified using the reference compounds by a statistical comparison of their retention indices, using an in-house computer program, Chromscan. Peak retention indices were calculated using two NHCs, pyridine and quinoline, as reference points with index values of 100 and 200, respectively [8 ]. The peak comparison procedure was based on repetitive calculation of analysis of variance (anova) in which the retention index of each peak in triplicate chromatograms of one of the samples was compared with the 3 nearest peaks in each of the other samples. Peaks were considered correlated, and hence probably identical, if the 95% F-statistic appropriate to the number of samples analyzed was not exceeded. For most correlations, the F values were much smaller... 

Extensions of the definition of a retention index were done by choosing gradient temperature conditions [42] or methyl esters as standards instead of normal hydrocarbons. Tables for retention indexes of certain substances have been published [43]. However, the retention index system has limited utility for the GC analysis of pyrolysates due to the complexity of such samples. 

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