Retention time calculation predicted values

A first prediction attempt of GCxGC retention using this strategy was carried out by Beens et al. [10]. Retention times in the D column were calculated from those of n-alkanes and the analyte retention index. For the analyte retention times in D, k was first obtained by interpolation for the n-alkane series at the elution temperatures, the retention times f of these compounds were calculated using the corresponding holdup times, and then t Ri for the analyte was calculated from its RI at the elution temperature using Equation (3). Differences between calculated and experimental values were observed for D retention times, although predicted elution profiles were similar to the experimental patterns. A similar approach [27] used k values measured at several temperatures to obtain a better interpolation. The accuracy of the prediction of retention times was... 

The retention time of phenolic compounds in reversed-phase liquid chromatography was predicted via molecular interaction energy values calculated using the MM2 program. The precision of the capacity ratios predicted by this new method was equivalent to a former method in which the retention time was predicted by log P calculated using the MOPAC program. Furthermore, the prediction of capacity ratios of phenolic compounds in reversed-phase... 

It will be seen that the chair transition state (9) is predicted to be more stable than the boat (8) by 6.5 kcal/mole the experimental evidence implies 29) that 9 is favoured by not less than 5.7 kcal/mole . The experimental value is based 29) on the presence of a very small amount of product (< 1%) that could have been formed via 8 since the amount was so small, this was identified only by its g.l.c. retention time. If the identification was correct, the ratio of products implies a difference in activation energy of 5.7 kcal/mole between 8 and 9 our calculations suggest that the identification was correct and that Doering and Roth were overcautious. 

A quantitative analysis of the structure-retention relationship can be derived by using the relative solubility of solutes in water. One parameter is the partition coefficient, log P, of the analyte measured as the octanol-water partition distribution. In early work, reversed-phase liquid chromatography was used to measure log P values for drug design. Log P values were later used to predict the retention times in reversed-phase liquid chromatography.The calculation of the molecular properties can be performed with the aid of computational chemical calculations. In this chapter, examples of these quantitative structure-retention relationships are described. 

The prediction of retention times in a given eluent from log P has been proposed for aromatic hydrocarbons.19 The log A values of phenols21 and nitrogen-containing compounds22 were also related to their logP, and the calculated log P was used for the qualitative analysis of urinary aromatic acids, i.e. for the identification of metabolites in urine from the differences of log P in reversed-phase liquid chromatography.23,24... 

The agreement between the observed and predicted k values of aromatic acids was within 10%. The correlation coefficient was 0.954 (n = 32). An error of greater than 10% for 3-hydroxy-2-naphthoic acid and 2-hydroxybenzoic acid was attributed mainly to an error in their K.A values.25 The partition coefficient, logP, and dissociation constant, pKA, of analytes can be obtained by simple calculations and by computational chemical calculations, and thus the retention time can be predicted in reversed-phase liquid chromatography. 

The retention times of peptides with fewer than 20 residues in reversed-phase chromatography can be predicted with a high degree of accuracy based on their amino acid composition and the characteristics of their N-terminal and C-terminal amino acids. A number of researchers (66 -75) have studied the role of amino acids in peptide retention and have established retention coefficients for the different amino acids. The retention coefficient value of each amino acid is normally calculated by regression analysis of the retention times for peptides of known composition. 

Table 3 (73) compares the retention coefficients for synthetic peptides from various sources. To ensure comparability, the data has been standardized with respect to lysine and assigned a value of 100. The table shows that there are discrepancies between the results obtained using different chromatographic systems. Predictions of retention times should therefore be made using chromatographic systems similar to those used to calculate the retention coefficients for the amino acids. Casal et al. (75a) have made a comparative study of the prediction of the retention behavior of small peptides in several columns by using partial least squares and multiple linear regression analysis. 

The physicochemical approach to retention time prediction has the advantage of accounting for the pH of the system by explicitly calculating p/C values... 

The apparent plate munber can be calculated from the experimental profiles [27]. However, this number depends on the fractional height at which the bandwidth is measured. The value of Nth is calculated from the profiles predicted, under the same experimental conditions, by the ideal model. Finally, Nion is derived from the band profiles recorded in linear chromatography, e.g., with a very small sample size, using the relationships valid for Gaussian profiles. From Eqs. 7.24 and 7.26, we can derive the band width at half height, Wi/2, and the retention time of the band profile, ty, obtained with an infinitely efficient column. In the case of a Langmuir isotherm, we obtain [31]... 

The model of [22] or others described in [17] could be used in the calculation of GCxGC retention times or other parameters (e.g., RI values) for both and columns. However, at the time of writing this chapter, these procedures have not yet been applied in GCxGC. A publication on GCxGC retention time prediction [23] uses the previously mentioned CODESSA program package and experimental retention times obtained from the column in GCxGC, but the data are not included in the calculation. 

Lu et al. [28] used a model based in Equation (6) to predict the retention times of a mixture of only 13 compounds which however included both nonpolar n-alkanes and polar p)Tidine derivatives. Agreement between calculated and experimental retention time values was good for different programming rates. 

The strategy of Seeley and Seeley [29] only requires as starting data ID GC retention times from temperature-programmed runs. After calculation of retention indices in both and columns, a transformation of these values was used to construct a plot that tries to reproduce the 2D experimental retention. The method was applied to 139 volatile organic compounds with good results in the reproduction of general patterns, although prediction of D retention shows some errors, especially for compounds whose RI values depend markedly on temperature. 

Prediction of resolution in GCxGC requires, besides calculation of retention times in both columns (which has been described in Section 3.2), estimation of peak widths (Equation (7)). Beens et al. [10] used input band width and column efficiency for column to estimate with good results. Lu et al. [28] used, as a basis for the estimation of peak width, experimental and values measured for the same columns. The van Deemter equation was used in [31]. 

Retention time in gas chromatography is related to a combination of retention and volatility, similar to solubility in liquid chromatography. Predicting volatility is as difficult as predicting solubility. Volatility has been explained as the enthalpy of vaporization (Avap f), and a method for predicting volatility has been proposed." If the A apH values are available, it may be possible to predict retention time. Unknown A apH values have been calculated from the relationship between the van der Waals volume and reference AyapH values. The values are summarized with the corresponding reference values in Table 1 of the Appendix (p. 278). Values of A apH have also been related to capacity ratios. The correlation coefficients were 0.896 and 0.852 ( = 48) for DBl and CPSilS columns, respectively, which appear to be acceptable correlation coefficients, except that the relationship for allq l alcohols deviated from those of other compounds as seen Figure 4.4. 

In general, boiling point and AyapH cannot be predicted. The prediction of retention time in chromatography requires the use of predictable properties. The retention time can be calculated in silica using a model phase, but vaporization has not been quantitatively analyzed. In gas chromatography, vaporization may be related to analyte volatility. The retention on methylsilicone phases was quantitatively analyzed in silica as the molecular interaction energy values. In this model system, retention time may correspond with the molecular interaction as described for retention on the methylsilicone phase. The smallest MIPS was obtained for PAHs, and the values... 

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