Chromatography retention parameters

Retention in SFC is a complex function of the experimental parameters and is not as easily rationalized as in the case of gas and liquid chromatography. Retention in SFC is dependent upon temperature, pressure, density, sample concentration, composition of the mobile phase and the composition of the stationary phase. Many of these variables are interactive nd do not change in a... 

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. 

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]. 

In a chromatographic separation procedure the parameters of the chromatographic system (stationary phase, flow, temperature, etc.) have to be selected respectively optimized with respect to some criterion (resolution, time, etc.). In gas chromatography retention data series are published and used for the sttidy of solvent/solute interaction, prediction of the retention behaviour, activity coefficients, and other relevant information usable for optimization and classification. Several clKmometrk techniques of data anal s have been employed, e.g. PCA, numerical taxonomic methods, information theory, and j ttern recognition. 

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. 

Rohrschneider [205,210] has developed a scheme for the characterization of stationary phases for gas chromatography. The scheme is based on the retention index (/). The retention index is a dimensionless retention parameter, designed to be independent of flow rate, column dimensions and phase ratio. The retention index of a solute is defined as 100 times the number of carbon atoms in a hypothetical n-alkane, which shows the same net retention time as that solute. This definition is illustrated in figure 2.2. By plotting the logarithm of the net retention time against the number of carbon atoms in n-alkanes, a straight line is obtained. The net retention time for a solute may then be located on the vertical axis, and the retention index found on a horizontal scale, which represents 100 times the scale for na... 

Instrumental methods in chemistry make it possible to characterize any chemical compound by a very large number of different kind of measurements. Such data can be called observables. Examples are provided by Spectroscopy (absorbtions in IR, NMR, UV, ESCA. ..) chromatography (retentions in TLC, HPLC, GLC. ..) thermodynamics (heat capacity, standard Gibbs energy of formation, heat of vaporization. ..) physical propery measures (refractive index, boiling point, dielectric constant, dipole moment, solubility. ..) chemical properties (protolytic constants, ionzation potential, lipophilicity (log P)...) structural data (bond lengths, bond angles, van der Waals radii...) empirical structural parameters (Es, [Pg.34]

It has been shown that gas-Hquid chromatographic methods are particularly suitable for a quantitative characterization of the polarity of solvents. In gas-liquid chromatography it is possible to determine the solvent power of the stationary liquid phase very accurately for a large number of substances [98, 99, 259, 260]. Many groups of substances exhibit a certain dependence of their relative retention parameters on the solvation characteristics of the stationary phase or of the separable components. In determining universal gas-chromatographic characteristics, the so-called retention index, I, introduced by Kovats [100], is frequently used. The elution maxima of individual members of the homologous series of n-alkanes (C H2 +2) form the fixed points of the system of retention indices. The retention index is defined by means of Eq. (7-41),... 

All the choices the chromatographer has in terms of bonded phase, aqueous phase modifler, and organic modifler can have synergistic effects on the analyte retention and selectivity in reversed-phase chromatography. These parameters will be discussed in this chapter, with specific examples illustrating the power of the selection of the most suitable parameters for control of the analyte retention and selectivity. 

The most commonly used retention parameter in gas chromatography is the Kovats index. When the adjusted retention times are used to calculate Kovats indices, retention parameters are obtained which depend only on the column temperature and the stationaiy phase used. Kovats indices are highly reproducible, and with a well designed experimental technique and an accurate timing mechanism, an inter-laboratory reproducibility of one unit for larger values of Kovats indices and two units for indices below 400 is possible [14]. Instead of Kovats indices, sometimes in QSRR studies the logarithms of retention volumes of solutes are used. 

The LFER-based retention parameter in high-performance liquid chromatography (HPLC) is the logarithm of the phase capacity ratio or retention factor k. The capacity... 

The above-given Martin equation form the basis for the Kovats retention index system in gas chromatography as well as for several HPLC retention prediction schemes. It must be noted here that the relationships between retention parameters and carbon numbers are usually linear at some limited range of the aliphatic chain length up to 6-8 carbon atoms in reversed-phase HPLC [491. 

From the literature there is evidence that in GC on polar phases and in normal-phase (adsorption) liquid chromatography (HPLC and TLC) the chemically specific, molecular size-independent intermolecular interactions play the main retention-determining role. For example, the HPLC retention parameters determined for substituted benzenes on porous graphite are described by QSRR equations comprising polarity descriptors but containing no bulk descriptors [93-95]. Because, in general, it is difficult to quantify the polarity properties precisely, the QSRR for GC on polar phases and for normal-phase HPLC are usually of lower quality than in the case of GC on non-polar phases and in the case of reversed-phase liquid chromatography. 

The driving force in chromatography for the. separation of an analyte is the equilibrium between the stationary and the mobile phases. As it was di.scus.sed in Chapter 11 in more detail, the chromatographic equilibrium can be related to the chemical potential of the compound. Unfortunately, the relationship between retention parameters and the quantities related to the chemical structure cannot be solved in. strictly thermodynamic terms. Therefore, the extra-thermodynamic approach is applied to reveal the relationships. During chromatography we do not achieve a proper equilibrium, the separation is still a result of the difference of equilibrium constants for the compounds in the stationaiy and the mobile phases. The.se equilibrium con.stants can be related to measured retention data as was discussed in the previous chapter. So whenever our chromatographic system (the stationary and the mobile phase) can be considered as two immiscible phases the retention data (equilibrium data) will provide a partition coefficient. 

Although the retention volume is independent of flow rate, relative retention parameters are preferred because they utilize dimensionless parameters as well as providing additional information about the chromatographic process. The relative chromatographic mobility (Re) in liquid chromatography is defined by... 

Benzoic acid derivatives often contain amino, hydroxy, carboxy, and nitro groups. Analysis of substimted benzoic acids by thin layer chromatography was performed on silica gel, polyamide, and cellulose containing UF254 fluorescent indicator. For the mobile phase, different mixtures were used hexane-acetic acid hexane-ethyl acetate-formic acid chloroform-methanol-phosphoric acid cyclohexane-acetic acid benzene-ethanol etc. Because benzoic acid derivatives have similar retention parameters, their separation requires a thorough optimization of conditions (the nature of the stationary phase, the composition of the mobile phase, and the pH of the solutions). 

In the theory and practice of chromatography, another parameter of solute retention is also employed, the so-called R value. This quantity was defined by Bate-Smith and Westall [1] as... 

Xiang, Y.H., Liu, M., Zhang, X., Zhang, R. and Hu, Z. (2002) Quantitative prediction of liqitid chromatography retention of N-benzylideneanilines based on quantum chemical parameters and radial basis function neural network./. Chem. Inf. Comput. Sci., 42, 592-597. 

There have been numerous attempts to determine HLB numbers from other fundamental properties of surfactants, e.g., from cloud points of nonionics (Schott, 1969), from CMCs (Lin, 1973), from gas chromatography retention times (Becher, 1964 Petrowski, 1973), from NMR spectra of nonionics (Ben-et, 1972), from partial molal volumes (Marszall, 1973), and from solubility parameters (Hayashi, 1967 McDonald, 1970 Beerbower, 1971). Although relations have been developed between many of these quantities and HLB values calculated from structural groups in the molecule, particularly in the case of nonionic surfactants, there are few or no data showing that the HLB values calculated in these fashions are indicative of actual emulsion behavior. 

Fig. 4 A, Comparison of the separations of polystyrene standards carried out by micro-TFFF at constant temperature drop AT and programed decrease in AT with the chromatograms from high-performance size-exclusion chromatography. B, Dependence of the product of retention parameter A and of the temperature drop AT on the molar mass M of polystyrene standards.

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