Chromatographic retentions

Comparison of the mass spectrum from a target compound (top), with the three best fits from the library of standard spectra (lower three traces). The closeness of fit of the mass spectra and the chromatographic retention time lead to a positive identification of 2, 6-dimethylheptane. 

The amino group is readily dia2oti2ed in aqueous solution, and this reaction forms a basis for the assay of sulfas. Aldehydes also react to form anils, and the yellow product formed with 4-(dimethylamino)hen2a1dehyde can be used for detection in thiu-layer and paper chromatography. Chromatographic retention values have been deterrnined in a number of thiu layer systems, and have been used as an expression of the lipophilic character of sulfonamides (23). These values have corresponded well with Hansch lipophilic parameters determined in an isobutyl alcohol—water system. 

The development of micellar liquid chromatography and accumulation of numerous experimental data have given rise to the theory of chromatographic retention and optimization methods of mobile phase composition. This task has had some problems because the presence of micelles in mobile phase and its modification by organic solvent provides a great variety of solutes interactions. 

There are two fundamental chromatography theories that deal with solute retention and solute dispersion and these are the Plate Theory and the Rate Theory, respectively. It is essential to be familiar with both these theories in order to understand the chromatographic process, the function of the column, and column design. The first effective theory to be developed was the plate theory, which revealed those factors that controlled chromatographic retention and allowed the... 

The purity of the product is greater than 99% as determined by gas chromatographic analysis using a 6-m. column of 30% Carbowax 20M on 60-80 Chromosorb W. The major impurity (<1%) was shown to be 3-heptanol by comparison of gas chromatographic retention times and mass spectral fragmentation patterns with those of an authentic sample. 

Simplistically, chromatography can be regarded as the separation of the components of a mixture to allow the identification and/or quantitation of some or all of them. Identification is initially carried out on the basis of the chromatographic retention characteristic. This is not sufficient to allow unequivocal identification because of the possibility of more than one analyte having virtually identical retentions. Further information is usually required from an auxiliary technique - often some form of spectroscopy. 

Chlorambucil - there is no problem with the quantitation ion (at m/z 254), although the second ion proves to be a little difficult. While the ion at m/z 303 is the obvious choice, this is not very intense and therefore for samples containing small amounts of analyte the precision of measurement of this ion will be reduced and it may not be detectable at all levels at which the quantitation ion is observed. We could possibly consider the (M- -2) ion, as the combination o/m/z 254 (high mass, and therefore reasonable specificity), the presence of one chlorine, and the chromatographic retention time could be considered sufficient for definitive identification in those cases in which the intensity o/m/z 303 is insufficient. 

Hall GL, Whitehead WE, Mourer CR, et al. 1986. A new gas chromatographic retention index for pesticides and related compounds. J High Resolut Chromatogr Commun 9 266-271. 

Vol. 93. Quantitative Structure-Chromatographic Retention Relationships. By Roman Kaliszan... 

Nasal, A., Siluk, D., Kaliszan, A. Chromatographic retention parameters in medicinal chemistry and molecular pharmacology. Curr. Med. Chem. 2003, 10, 381 26. 

S. W., Melton, C. M. Estimation of gas-liquid chromatographic retention times from molecular structure.. Chromatogr. A 1994, 662, 269-280. 

By way of graphical example of the various algebraic and geometrical concepts that are introduced in this chapter, we will make use of a measurement table adapted from Walczak etal.[ ]. Table 31.2 describes 23 substituted chalcones in terms of eight chromatographic retention times. Chalcone molecules are constituted of two phenyl rings joined by a chain of three-carbon atoms which carries a double bond and a ketone function. Substitutions have been made on each of the phenyl rings at the para-positions with respect to the chain. The substituents are CFj, F, H, methyl, ethyl, i-propyl, t-butyl, methoxy, dimethylamine, phenyl and NO2. Not all combinations two-by-two of these substituents are represented in the... 

Chromatographic retention times of 23 doubly substituted chalcones, as determined by 8 HPLC chromatographic methods using heptane as the mobile phase. The methods differ by addition of 0.5 percent of a chemical modifier. 

Fig. 31.6. Biplot of chromatographic retention times in Table 31.2, after column-centering of the data. Two unipolar axes and one bipolar axis have been drawn through the representations of the methods DMSO and methylenedichloride (CH2CI2). The projections of three selected compounds are indicated by dashed lines. TTie values read off from the unipolar axes reproduce the retention times in the corresponding columns. The values on the bipolar axis reproduce the differences between retention times.

Fig. 31.8. Biplot of chromatographic retention times in Table 31.2, after log column-centering of the data. The values on the bipolar axis reproduce the (log) ratios between retention times in the two corresponding columns.

Fig. 31.10. Same biplot of chromatographic retention times as in Fig. 31.9. The line segments connect compounds that share a common substituent. The horizontal contrast reflects the presence or absence of a NO2 substituent. The vertical contrast expresses the electronegativity of the substituents.

The rank of the transformed table of chromatographic retention times Z is equal to seven. 

It is assumed that the structural eigenvectors explain successively less variance in the data. The error eigenvalues, however, when they account for random errors in the data, should be equal. In practice, one expects that the curve on the Scree-plot levels off at a point r when the structural information in the data is nearly exhausted. This point determines the number of structural eigenvectors. In Fig. 31.15 we present the Scree-plot for the 23x8 table of transformed chromatographic retention times. From the plot we observe that the residual variance levels off after the second eigenvector. Hence, we conclude from this evidence that the structural pattern in the data is two-dimensional and that the five residual dimensions contribute mostly noise. 

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