Retention parameters

The compressibilities of solvents vary significantly from one solvent to another. The compressibility of cyclohexane is about 0.67% per thousand p.s.i. change in pressure [11] and, thus, for a column operated at 6,000 p.s.i. (mean pressure 3,000 p.s.i.), there will be an error in retention volume measurement of about 2%. In a similar manner, n-heptane has a compressibility of about 1.0% per 1,000 p.s.i. change in pressure [11] and, under similar circumstances, would give an error of about 3% in retention volume measurement. Fortunately, as already discussed in Part 1 of this book, there are other retention parameters that can be used for solute... 

A general approach to the problem of identification, should more definitive detectors not be available, is to change the chromatographic system , which in the case of HPLC is usually the mobile phase, and redetermine the retention parameter. The change obtained is often more characteristic of a single analyte than is the capacity factor with either of the mobile phases. 

The following Soezewinski equation is a simple linear relationship with respect to logA, linking the retention parameter (i.e., R,J) of a given solute with quantitative eomposition of the binary eluent applied ... 

Coupling Equation 2.16 and Equation 2.17, we can obtain the following relationship, which describes the dependence of the solute s retention parameter, R, on the quantitative composition of a given binary eluent ... 

Equation 4.32 expresses the difference between the retention parameters Ai p of two solutes as a chromatographic response function ... 

Relationships between lipophilicity and retention parameters obtained by RPLC methods using isocratic or gradient condition are reviewed. Advantages and limitations of the two approaches are also pointed out, and general guidelines to determine partition coefficients in 1-octanol-water are proposed. Finally, more recent literature data on Hpophilicity determination by capillary electrophoresis of neutral compounds and neutral forms of ionizable compounds are compiled. Quotation is restricted to key references for every method presented - an exhaustive listing is only given for the last few years. 

It has been shown for many RPLC methods that correlations between log Pod and retention parameters were improved by separating compounds in two classes, i.e. H-bond acceptor and donor compounds. Minick et al. [23] propose to add 0.25% (v/v) of 1-octanol in the organic porhon of the mobile phase (methanol was preferred in this study) and to prepare the aqueous portion with 1-octanol-saturated water to minimize this discriminahon regarding H-bond properties. For a set of heterogenous neutral compounds, the addition of 0.25% (v/v) of 1-octanol in methanol and the use of water-saturated 1-octanol to prepare mobile phase improve the correlahon between log few obtained on the LC-ABZ column and log Poc, [13]. 

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

A chromatographic separation step provides various advantages to the analytical procedure (i) each component is isolated from the others (which facilitates identification) (ii) minor components in mixtures may be detected more readily than by direct analysis techniques (iii) the chromatographic retention parameter provides additional confirmation that a particular component is present or absent and (iv) quantitative analysis. However, chromatography alone does not provide information on the identity of a totally unknown sample. 

HPLC log P techniques, first described by Mirrlees et al. [374] and Unger et al., [375], are probably the most frequently used methods for determining log/1. The directly measured retention parameters are hydrophobicity indices, and need to be converted to a log P scale through the use of standards. The newest variants, breadths of scope, and limitations have been described in the literature [292-298]. A commercial automated HPLC system based on an extension of the approach described by Slater et al. [150] has just introduced by Sirius (www. sirius-analytical. com). 

A very promising method, immobilized artificial membrane (IAM) chromatography, was developed by Pidgeon and co-workers [299-304,307], where silica resin was modified by covalent attachment of phospholipid-like groups to the surface. The retention parameters mimic the partitioning of drugs into phospholipid bilayers. The topic has been widely reviewed [47,298,307,309-311]. 

The negative-charge lipid content in the egg lecithins is not as high as that found in BBM and especially BBB lipids (Table 7.1). Furthermore, the negative-charge content in the egg lecithin is about one-fourth that in the soy lecithin. This is clearly evident in the membrane retention parameters for the bases at the 10% lecithin levels (models 12.0 or 14.0 in Table 7.8 vs. model 16.0 in Table 7.12), as they are 20-30% lower for the lipophilic bases in egg, compared to soy. 

The retention parameters in HPLC are similar to those discussed in Chapter 14 on GC. To recap for HPLC, the average velocity of the sample band of X molecules (ux) depends on M, the fraction of X molecules in the mobile phase, and the average velocity of solvent (u, cm/sec) ... 

It is known that the RPLC retention parameters are often strongly correlated to the analyte s distribution coefficient in organic solvent/ water. Generally, the relationship between liquid/liquid (LL) distribution and RPLC retention are of the form of the dimensionless Collander-type equations, e.g., see Eq. (15.21)... 

Much effort has been devoted to the development of reliable calculation methods for the prediction of the retention behaviour of analyses with well-known chemical structure and physicochemical parameters. Calculations can facilitate the rapid optimization of the separation process, reducing the number of preliminary experiments required for optimization. It has been earlier recognized that only one physicochemical parameter is not sufficient for the prediction of the retention of analyte in any RP-HPLC system. One of the most popular multivariate models for the calculation of the retention parameters of analyte is the linear solvation energy relationship (LSER) ... 

M.L. Bieganowska, A. Petrucznik and A.M. Zobel, Retention parameters of coumarins and flavonoids in binary reversed-phase HPTLC systems. J. Plan. Chromatogr.—Mod. TLC 9 (1996) 273-279. 

The chromatographic retention parameters used in correlation studies are normally assumed to be proportional to the free energy change. 

The above approach could also be applied to evaluate the orthogonality between CE systems and CSs using migration and retention parameters, respectively. 

All these test procedures cannot help in the selection of another colnmn with identical properties when the used one fails and a replacement is not available or a different colnmn has to be selected with similar selectivity. In these cases, the whole and often tedions procednre of method development with a new brand of colnmn has to be started. Recently, on the basis of LFER (linear free enthalpy relationship), retentive parameters were determined by Snyder et al. to describe the properties of commercially available colnmns. The principles were presented in a series of papers [65,66],... 

The mean thickness analyte layer (Equation 12.4) normalized vs. the channel width w (see Figure 12.5) defines the retention parameter X... 

FFF Technique Field Velocity Parameter Force Retention Parameter (X<0.01) Retention... 

The explicit expression for the retention parameter X (Eqnation 12.8) in SdFFF is given by [19]... 

Figure 7.2 shows that in CCC, the observed chromatogram significantly depends on the Us/Um phase ratio that is expressed using the stationary phase retention parameter Sf as... 

HPLC units have been interfaced with a wide range of detection techniques (e.g. spectrophotometry, fluorimetry, refractive index measurement, voltammetry and conductance) but most of them only provide elution rate information. As with other forms of chromatography, for component identification, the retention parameters have to be compared with the behaviour of known chemical species. For organo-metallic species element-specific detectors (such as spectrometers which measure atomic absorption, atomic emission and atomic fluorescence) have proved quite useful. The state-of-the-art HPLC detection system is an inductively coupled plasma/MS unit. HPLC applications (in speciation studies) include determination of metal alkyls and aryls in oils, separation of soluble species of higher molecular weight, and separation of As111, Asv, mono-, di- and trimethyl arsonic acids. There are also procedures for separating mixtures of oxyanions of N, S or P. 

Govers, H.A.J., van der Wielen, F.W.M., Olie, K. (1995) Derivation of solubility parameters of chlorinated dibenzofurans and dibenzo[p]dioxins from gas chromatographic retention parameters via SOFA. J. Chmmatogr. A, 715, 267-278. 

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