Chromatographic methods extrapolation

The versatility of chromatographic methods of hydrophobicity parameterization can be attributed to the use of organic modifiers of aqueous eluents. Normally, the retention parameters determined at various organic modifier-water (buffer) compositions are extrapolated to the water-alone eluent. The extrapolated parameters (log/ from HPLC and from TLC) depend on the oiganic modifier u.sed 2,17. ... 

Infinite dilution activity coefficients can be measured accurately by chromatographic methods. There are studies claiming that the extrapolation of data is reliable for homogeneous mixtures over the whole range of composition. 

The determination of diffusion coefficients by the gas chromatographic methods is convenient especially for molten polymers and small amounts of diffusing substances. The method offers the advantage that the values thus determined correspond to infinite dilution of the substance injected in the polymer, thus making possible to avoid extrapolation. 

The different organic modifiers used to derive the most suitable mobile phases lead to different parameters namely isocratic logfe and extrapolated logkw. The extrapolation method has no reality in terms of chromatographic behavior of solutes. However, mainly by correlation with log Pod (Eqs. 2 and 3) several studies have demonstrated the interest of these extrapolated retention factors as predictors of the lipophilicity of solutes. 

Experimentally there are two methods of determining the ] extracolumn band broadening of a chromatographic instrument. The linear extrapolation method, discussed above, is relatively straightforward to perform and interpret but rests on the validity.. of equation (5.1) and (5.3). The assu itlon that the individual contributions to the extracolumn variance are independent, may not be true in practice, and it may be necessary to couple some of the individual contributions to obtain the most accurate values for the extracolumn variance [20]. It is assumed in equation (5.3) ... 

The standard addition method is often used in cases when it is not possible to obtain suitable blank matrices one example is the analysis of endogenous compounds in body fluids. The approach is to add different weights of analyte to the unknown sample, which initially contains an unknown concentration of the analyte. After the chromatographic analysis, peak areas (or heights) are plotted versus the added concentration. Extrapolation of the calibration plot provides the original unknown concentration of the analyte. A standard addition method that possesses even greater accuracy and precision is obtained if one incorporates an internal standard [48],... 

There are two general experimental methods for estimating the extracolumn band broadening of a chromatographic instrament. The linear extrapolation method is relatively straightforward to perform and interpret but rests on the validity of Eq. (1.34) and the model used to calculate the contribution for the column variance. A plot of a T against tR, Vr or (1 + k) for a series of homologous compounds will be linear. The true column efficiency can be obtained from the slope of the line and o ext from the intercept on the vertical axis [162,167,168]. The assumption that the individual... 

Spectral, chromatographic, and chemical analysis methods have been used to reduce the duration of the experiment. Extrapolation of the distribution curves of plasticizer concentration is used to calculate the change in plasticizer concentration at the contact boundary and (if the diffusion coefficient is known) to calculate a distribution coefficient. However, these methods are difficult and labor-consuming because extraction of plasticizer from sections of polymer samples is needed. The resulting extract contains a plasticizer, a polymer, and impurities and this complicates analysis. It is frequently necessary to use a special procedure for each plasticizer and calibration curve. The interference method is useful with transparent samples having sufficient difference between the refractive indices of the components (polymer and plasticizer). All methods require perfect contact between the materials being investigated. 

Ashworth and Everett [1] were the first to determine the activity coefficients of hydrocarbons in squalane and dinonylphthalate by a static method. Unfortunately the errors resulting from extrapolation at infinite dilution are too large to allow comparison of these data with those obtained from gas-liquid chromatography. The study of these systems was resumed through static methods by other researchers [8, 55, 56] and a comparison of results with gas-liquid chromatographic values is shown in Table 4.4 [3]. 

---