Partition coefficients retention time

The partition coefficient of a substance between several Immiscible solvent pairs can be combined with retention time data to confirm the identity of a substance when a pure standard is available [706]. Devised by Bowman and Beroza, the substance specific partition coefficient ("p-value") was defined as the fractional amount of substance partitioning into the less polar phase of an equal-volume, two-phase system. Only nanogram quantities of sample are required for the measurement and p-values are often sufficiently characteristic to distinguish between closely related substances. 

Note that from Eqs. (7.17) and (7.18), R EmCw(0/EpCp(O) for t > Tlag- The substitution of the apparent partition coefficient with the retention ratio allows us to state the mole balance at time t (provided t > Tlag) in a much simplified form ... 

Figure 3.1 shows the appearance of dihydromethysticin in the acceptor well as a function of time [15], The solid curve is a least-squares fit of the data points to Eq. (1), with the parameters Pe = 32 x 10-6 cm s 1, R = 0.42, and t s = 35 min. The membrane retention, R, is often stated as a mole percentage (%R) of the sample (rather than a fraction). Its value can at times be very high - up to 90% for chlor-promazine and 70% for phenazopyridine, when 2% wt/vol DOPC in dodecane is used. Figure 3.2 shows a plot of log %R versus log Ka(7.4), the octanol/water apparent partition coefficient. It appears that retention is due to the lipophilicity of molecules this may be a good predictor of the pharmacokinetic volume of distribution or of protein binding. 

Measurements of gas chromatographic retention time are often used as a fast and easy method of estimating vapor pressure. These estimated pressures are related to the gas/substrate partition coefficient, which can be regarded as a ratio of solubility of the substance in the gas to that in the substrate, both solubilities being of the substance in the liquid state. As a result the estimated vapor pressures are of the liquid state. To obtain the solid vapor pressure requires multiplication by the fugacity ratio. It is important to establish if the estimated and reported property is of the vapor or liquid. 

Su,Y., Lei, D.L. Daly, G.,Wania, F. (2002) Determination of octanol-air partition coefficient (KqA) values for chlorobenzenes and polychlorinated naphthalenes from gas chromatographic retention times. J. Chem. Eng. Data, 47, 449 455. 

Wania, F., Lei, Y.D., Hamer, T. (2002) Estimating octanol-air partition coefficients of nonpolar semivolatile organic compounds form gas chromatographic retention times. Anal. Chem. 74, 3478-3483. 

Polychlorinated biphenyls structure, retention times, response factors, and octanol/water partition coefficients (log Kow)... 

Using the retention data and the chromatogram shown in Fig. 14.8, tabulate the following for each peak retention time ( r), adjusted retention time (t K), retention factor (k), partition coefficient (Kc) and number of theoretical plates (N). The column phase ratio was 250 and the gas hold up time ( m) was 0.995 min. 

The studies with barbiturates revealed that the logarithm of the retention time is linearly related to the octanol/water partition coefficients [66,67]. It has been observed that the retention index of the drug is linearly related to the octanol/water partition coefficient (logP), and that results are very close to that of the 2-keto alkane standard (solid line in Fig. 15.9). 

HPLC retention times Partition coefficients can also be derived from retention times in high-pressure liquid chromatography (HPLC) analyses This approach provides some experimental advantages that simplify the analytical procedures and allow the handling of mixtures... 

The retention times of test solutes are correlated with reference compounds whose partition coefficients in octanol/water (Kow) are known The reliability of this technique depends on the extent to which the stationary and mobile phases simulate the octanol/water system... 

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. 

Attempts have also been made to determine octanol-water partition coefficients empirically using liquid chromatography by comparison of retention times with those of compounds of known logP.3-12... 

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. 

Using the expression introduced in Chapter 2 for the effect of pH on partition coefficient of an acid, it is possible to predict approximately the effect of pH on retention time since the effect of pH on partition coefficient will reflect its effects on capacity factor and in theory ... 

Table 24.2 Polychlorinated Biphenyls Structure, Retention Times, Response Factors, and Octanol/Water Partition Coefficients (log K J... 

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