Modifier concentration

The main equation of the model describes the dependence of retention factor, k, from surfactant concentration, c and modifier concentration, c ... 

The nature of the modifier and the modifier concentration impact both retention and selectivity in packed column SFC. SFC offers considerable flexibility in modifier selection because nearly all commonly used organic modifiers, including methanol and acetonitrile, are miscible with CO,. In contrast, methanol and acetonitrile are rarely used as modifiers in normal phase LC because they are immiscible with hexane [68]. 

In general, retention decreases as the modifier concentration increases because the modifier competes with the analytes for sites on the stationary phase. The effect on retention of changes in modifier concentration seems to be more pronounced for CSPs than for achiral stationary phases in SFC, and peak shapes are apt to degrade rapidly at low modifier concentrations [12]. Efficiency tends to decrease as the modifier concentration increases because analyte diffusion is slowed by the increased viscosity of the eluent [39]. 

To introduce some interfacial physico-chemical linkage between EVA and PRP, blends were made by adding different quantities of M AH-PP. Some results are demonstrated in Table 12, The physico-mechanical properties of the PRP-EVA compositions modified with MAH-PP showed that properties are influenced by MAH-PP concentration. Compositions with better impact strength and improved brittleness can be prepared by varying the modifier concentration. Tensile strength and elongation are not significantly influenced by the addition of a modifier. An increase in the modifier con-... 

FIGURE 6.14 (a) Principle of SBCD, elution with five interstitial volumes on 4-cm distance (5x4 cm) is faster than single development on 20-cm distance (thick line), (b) Rp values of sample components plotted as a function of modifier concentration. Optimal concentration (Y) for SBCD (5x4 cm) is lower than for development on the full distance of 20 cm (X). (Modified from Soczewinski, E., Chromatographic Methods Planar Chromatography, Vol. 1, Ed., Kaiser R.E., Dr. Alfred Huetig Verlag, Heidelberg, Basel, New York, 1986, pp. 79-117.)... 

The values ( )o for the calibration standards are estimated by plotting log fe values as a function of organic modifier concentrations assessed from at least three iso-cratic experiments and obtained with [39] ... 

Antia, F. D. and Horvath, Cs., Dependence of retention of the organic modifier concentration and multicomponent adsorptive behavior in reversed-phase chromatography, /. Chromatogr., 550, 411, 1991. 

Applications Off-line SFE-HPLC appears to be applicable and quantitative for a variety of samples in many real -world matrices. The main challenge lies in the use of this technique for the more polar compounds. Quantitative off-line SFE-SFC-UV analysis of HDPE/Ethanox 330 was described after extensive method development (varying modifiers, modifier concentration, temperature) [129]. Soxhlet extraction and SFE-RPLC-UV of PE samples were compared [127]. A sample size (inhomogeneity) problem was pointed out when a SFE reproducibility study was performed on five 3-mg samples of PE. This points to limits... 

This approach is not only very cost-effective, since it adopts the normal commercial processing operation as the reaction step at no additional cost, but has the added advantage also of preparing modified concentrates which are fully compatible with the unmodified polymer. 

Pt/Al2C>3-cinchona alkaloid catalyst system is widely used for enantioselective hydrogenation of different prochiral substrates, such as a-ketoesters [1-2], a,p-diketones, etc. [3-5], It has been shown that in the enantioselective hydrogenation of ethyl pyruvate (Etpy) under certain reaction conditions (low cinchonidine concentration, using toluene as a solvent) achiral tertiary amines (ATAs triethylamine, quinuclidine (Q) and DABCO) as additives increase not only the reaction rate, but the enantioselectivity [6], This observation has been explained by a virtual increase of chiral modifier concentration as a result of the shift in cinchonidine monomer - dimer equilibrium by ATAs [7],... 

As shown in Figure 14.5 adapted from Geng and Regnier,11 an increase in retention of small biphenyl molecules will be detected when the percent of organic modifier concentration is reduced. However, for a polypeptide biological sample, the lysozyme is not retained when the organic solvent composition exceeds 45%. At 35%, organic, lysozyme is completely retained. 

The narrow range of organic modifiers required to elute and desorb polypeptides from the reversed-phase column packing material accounts for the separation of polypeptides from a short C18 capillary column. Because polypeptide elution and separation depend on the accuracy of solvent composition in gradient nano LC, it is very important to use a system that can precisely control the LC modifier concentration even at low percents of organic modifiers in chromatographic elution compositions. 

The maximum retention factor (kQ) is related to the log P value and k and k are the retention factors of the cationic and anionic forms, respectively. The pKa values are known, and the retention factor in a given eluent can therefore be predicted in reversed-phase liquid chromatography using an alkyl-bonded silica gel or polystyrene gel column. The separation conditions can be adjusted according to their logP and pKa values by the selection of a suitable organic modifier concentration and the pH of the eluent.3,4... 

Further, an optimization of the organic modifier concentration for the separation of known compounds was proposed. When the value of the actual column plate number, the resolution, and log P of pairs of compounds a and b were known, the percentage concentration (x) required for their separation was calculated from Equations 6.4 to 6.6 19,20... 

Figure 4.7 Effect of high organic modifier concentration on peptide retention. Data obtained using an octadecyl column and 20 mM ammonium acetate acetonitrile mobile phases. (Reproduced from C.T. Wehr and L. Correia, LC at Work LC-121, Varian. With permission from Varian Associates.)...

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