Solvents decreasing solvent strength

The mobile phases employed in reversed-phase chromatography usually consist of water to which an organic solvent - miscible with water (mostly methanol or acetonitrile) - is added. Generally an increased polarity of the mobile phase gives a decreased solvent strength, i.e. an increased k. For alkaloids - analyzed by means of water - methanol mixtures - systematical studies have been made . 

Each selectivity point can be characterized by a coordinate, defined by the volume ratio of the three solvents. The ratio is written in the order of decreasing solvent strength of the undiluted solvent (A, B, C). 100% solvent A with the highest solvent strength is represented by Ps = 10-0-0 and the solvent with the lowest strength (C) by Ps = 0-0-10. 

Both tR and k increase exponentially with decreasing percentage of organic solvents (or solvent strength) in the mobile phase. 

Figure 6.10. Comparison of normal development and incremental multiple development with a decreasing solvent strength gradient (AMD) for the separation of poly(ethylene glycol) 400 as its 3,5-dinitrobenzoate ester. The AMD separation employed a 15- step gradient with methanol, acetonitrile and dichloromethane as...

Figure 8.12(b) shows that if dichloromethane (5j = 3.1, Sy = 1.61) or diethyl ether (5-p = 2.8, Sy = 4.08) are used, then the change in solvent strength is not very large. If, however, selectivity values are considered (Figure 8.12(c)) it is apparent that the selectivity value decreases when dichloromethane is used and increases when diethyl ether is used. It can be concluded that selectivity values must also be considered in the search for suitable solvents for "D. 

The GC-MS data of fraction 1 revealed a strong peak of verticilla-4(20),7,ll-triene (compound 1) accompanied by small amounts of cembrane A and cembrane C. To purify the violet spot and isolate compound 1, it was necessary to reduce the solvent strength. In the mobile phase dichloromethane-hexane (9 + 1 v/v), the development time decreases, which leads to minor diffusion of the zone. The zone of (compound 1) was marked by X = 254 nm UV light. To exclude the impurities, the separation process had to be repeated several times. The zone was removed from the glass plate and eluted from die adsorbent with dichloromethane. The concentrated solution achieved was applied onto a TLC plate as well as injected onto a GC column the... 

Type of organic modifier Retention decreases with increasing solvent strength. 

The term polarity refers to the ability of a sample or solvent molecule to interact by combination of dispersion, dipole, hydrogen bonding, and dielectric interactions (see Chapter 2 in reference 5). The combination of these four intermolecular attractive forces constitutes the solvent polarity, which is a measure of the strength of the solvent. Solvent strength increases with polarity in normal phase, and adsorption HPLC decreases with polarity in reversed-phase HPLC. Thus, polar solvents preferentially attract and dissolve polar solute molecules. 

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