Retention behaviour

The retention behaviour of oligomers in RPLC/-NPLC-APCI-MS (35 to 1500 Da) has been described [637]. In isocratic chromatography only a limited number n of oligomers can be separated, as the retention usually increases excessively at high n, so that gradient elution is necessary for successful separation of samples with broader molar mass distribution. RPLC-APCI-UV/QITMS was used for the analysis of accelerators (CBS, MBT, MBTS, BT) extracted... 

Burke, T.W., Mant, C.T., Black, J.A., Hodges, R.S. (1989). Strong cation-exchange high-performance liquid chromatography of peptides. Effect of non-specific hydrophobic interactions and linearization of peptide retention behaviour. J. Chromatogr. 476, 377-389. 

Steiner V, Schar M, Bomsen KO, Mutter M (1991) Retention behaviour of a template-assembled synthetic protein and its amphiphilic building blocks on reversed-phase columns. J Chromatogr 586 43-50... 

Jamberg, U., Asplund, L., Jakobsson, E. (1994) Gas chromatographic retention behaviour of polychlorinated naphthalenes on nonpolar, polarizable, polar and smectic capillary columns. J. Chromatog. A 683, 385-396. 

Many separation and detection methods applied in combination with liquid chromatography (LC) that are described in the literature for the determination of surfactants are not specific to the detection of these compounds at trace levels. Even ultraviolet (UV) spectra obtained from diode array detectors often give only limited information. Furthermore, non-reproducible retention behaviour as well as coelution interference effects are frequently observed during the separation of surfactant-containing extracts. This is recognised, however, only in those cases where specific detection methods such as mass spectrometry (MS) are applied. 

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) ... 

Unfortunately, mixtures of natural pigments generally contain one or more unidentified pigment components making impossible the prediction of their retention behaviour in any RP-HPLC system. 

The retention behaviour of the same set of coumarins and flavonoids was also investigated on polyamide and alumina layers using an aqueous mobile phase with various organic modifiers. The retention of solutes showed high variations on polyamide plates according... 

The retention behaviour of flavonoids has also been extensively studied on silica stationary phases using heptane, benzene or dichloromethane as weaker components of the binary mobile phase and ethyl acetate and methyl ethyl ketone as modifier. Flavones (3-hydroxy, 5-hydroxy and 7-hydroxyflavone, tectochrysin, chrysin, apigenin, genkwanin, baicalein), flavonols (galangin, pilloin, kaempferol, rhamnetin, quercetin, robinetin,... 

The RP-TLC behaviour of some common food dyes was investigated in detail. The chemical structure of dyes are listed in Fig. 3.2. Measurements were carried out on RP-18 silica plates using aqueous ammonium sulphate (0.1 0.5 1.0 M), ethanol and acetone in various volume ratios. Developments were performed at room temperature (22 2°C) in chambers previously saturated with the vapours of the mobile phase. It was found that the presence of dissociable anorganic salt modifies markedly the RP retention behaviour of dyes. The retention of dyes generally decreases with increasing concentration of the organic modifier in the mobile phase. It was further concluded that RP-TLC can be successfully used for the separation of this class of synthetic food dyes [81]. 

There is everlasting controversy and everlasting cooperation between analytical chemists dealing with chromatography. Academic research is generally not interested in the solution of practical problems, only with the theory of separation, with the development of new separation processes and with the mathematically based explanation of retention behaviour. 

The pH dependence of the tailing of Dil was investigated in separate experiments. The experimental conditions were the same but the pH of the mobile phase was adjusted to different values by HC1. The effect of pH on the retention behaviour of the dye is illustrated by chromatograms in Fig. 3.113. The pH dependence of tailing was tentatively explained by the marked contribution of free silanol groups to the reversed-phase retention of the dye... 

T. Cserhati and G. Oros, Impact of molecular surface characteristics on the reversed-phase retention behaviour of synthetic dyes. Biomed. Chromatogr., 13 (1999) 525-530. 

Comparison of the retention behaviour in 50% aqueous DMF and acetonitrile (Figure 4.3D) indicated that DMF is a selective modifier for the separation of alkanols ( , 1-6) and PAHs (11-16) and of aromatic and aliphatic compounds. Unfortunately, DMF is too aggressive as a solvent and attacks organic polymer piston seals in the pump and, therefore, highly concentrated solutions cannot be used as an eluent. 

The qualitative analysis of retention behaviour in liquid chromatography has now become possible. Quantitative retention-prediction is, however, still difficult the prediction of retention time and the optimization of separation conditions based on physicochemical properties have not yet been completely successful. One reason is the lack of an ideal stationary phase material. The stationary phase material has to be stable as part of an instrument, and this is very difficult to achieve in normal-phase liquid chromatography because the moisture in organic solvents ages the silica gel. 

Jandera, R et al.. Effect of the mobile phase on the retention behaviour of optical isomers of carboxylic acids and amino acids in liquid chromatography on bonded teicoplanin columns, J. Chromatogr. A, 917, 123, 2001. 

Garrigues, P, Radke, M., Druez, O., Willsch, H., and Bellocq, J., Reversed-phase liquid chromatographic retention behaviour of dimethylphenanthrene isomers, J. Chromatogn, 473, 207, 1989. 

KayiUo, S., Deimis, G.R., and ShaUiker, R.A., An assessment of the retention behaviour of polycyclic aromatic hydrocarbons on reversed phase stationary phases selectivity and retention on C18 and phenyl-type surfaces, J. Chromatogr. A, 1126, 283, 2006. 

Jinno, K., Ibuki, T., Tanaka, N., Okamoto, M., Fetzer, J.C., Biggs, W.R., Griffiths, P.R., and Olinger, J.M., Retention behaviour of large polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography on a polymeric octadecylsilica stationary phase, J. Chromatogn, 461,209, 1989. 

Luthe, G., Ariese, F., and Brinkman, U.A.T., Retention behaviour of higher fluor-inated polycyclic aromatic hydrocarbons in reversed-phase hquid chromatography, Chromatographia, 59, 37, 2004. 

Progent, R, and Taverna, M. (2004). Retention behaviour of peptides in capillary electrochromatography using an embedded ammonium in dodecacyl stationary phase.. Chromatogr. A 1052, 181-189. 

Specificity Peak Purity Degradation Products Retention Behaviour Diode array detection test Accelerated degradation Chromatogram... 

Polyakova, Y. and Row, K. H., Retention behaviour of N-CBZ-D-phenylalanine and D-tryptophan Effect of ionic liquid as mobile-phase modifier, Acta Chromatogr., 17, 210-221,2006. 

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. 

Surface complex models (SCMs) are now finding widespread application in the fields of pollutant retention behaviour (Zachara etal., 1989,1992), the soil chemistry of plant nutrient retention (Goldberg and Sposito, 1984 Goldberg and Glaubig, 1986 Goldberg and Traina, 1987) and the retention of radionuclides by sediments and transport of pollutants by colloids (Davis and Kent, 1990 Dzombakand Morel, 1990 Goldberg, 1992). 

Koschuh, W., V.H. Thang, S. Krasteva, S. Novalin and K.D. Kulbe, Flux and Retention Behaviour of Nano filtration and Fine Ultrafiltration Membranes in Filtrating Juice from a Green Biorefinery A Membrane Sreening, J. Membr. Sci., 261, 121-128 (2005). 

We can describe the retention behaviour in GSC in similar terms as we did for GLC in the previous section. However, we have to reconsider our definitions of the concentration in the stationary phase and of the distribution coefficient. It is common practice to express the concentration of the solute adsorbed onto the stationary phase in terms of moles per gram of adsorbent, i.e. 

Even more than in other LC techniques, the exact mechanism of retention-in RPLC is unclear. Certainly, a simple picture that would enable us to derive unambiguous equations for the variation of retention with the various parameters of interest cannot yet be drawn. Unfortunately, there has been too much speculation in the literature throughout the last decade, often accompanied with insufficient experimental data to justify the contusions drawn. Therefore, it is not surprising that there are many different propositions for expressions to describe the retention behaviour in RPLC. 

Obviously, eqn.(3.43) will not be able to yield a successful description of the retention behaviour in the range of low

[Pg.61]

Since then, however, it has been shown that the value of S does vary systematically with the retention behaviour of the solute [322,333]. If binary mixtures of water and methanol are used as the mobile phase, S tends to increase with an increase in the absolute retention. This is illustrated by the diverging set of lines in figure 3.14. ... 

Three experiments are in principle sufficient to establish the three coefficients in eqn.(3.70) for a given solute. In practice this is only true if the three experiments are taken at such values of the pH (relative to pKJ that a sensible estimate of all three coefficients can be made. This implies one experiment within one pH unit of the pKa value, one experiment at a higher and one at a lower pH. If the pKa value of a solute is known, then the retention behaviour can be estimated from a minimum of two experiments. Another way to reduce the minimum number of required experiments is to assume a negligible capacity factor for the charged species. Of course, once more experimental data points become available initial assumptions about the value of any of the coefficients in eqn.(3.70) can be abandoned. 

This section deals with interpretive optimization methods. In these. methods, the extent of chromatographic separation is predicted indirectly from the retention behaviour of the individual solutes. The data are interpreted to locate the optimum in terms of the complete chromatogram. The interpretive methods may involve a limited number of experiments according to a pre-planned experimental design (section 5.5.1) or may start with a minimum number of experiments in order to try and locate the optimum by an iterative process (section 5.5.2). 

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