Retention organic solvent effects

Naphthalenedisulfonate-acetonitrile as the only mobile phase with a silica column coated with a crosslinked aminofluorocarbon polymer has proven to be an effective combination for the separation of aliphatic anionic surfactants. Indirect conductivity and photometric detection modes are used to monitor these analytes. The retention of these surfactants is found to depend on both the ionic strength and the organic solvent content of the mobile phase. The mechanism of retention is considered to be a combination of both reverse phase and ion exchange processes. Selective separation of both alkanesulfonates and... 

This technique is based on the same separation mechanisms as found in liquid chromatography (LC). In LC, the solubility and the functional group interaction of sample, sorbent, and solvent are optimized to effect separation. In SPE, these interactions are optimized to effect retention or elution. Polar stationary phases, such as silica gel, Florisil and alumina, retain compounds with polar functional group (e.g., phenols, humic acids, and amines). A nonpolar organic solvent (e.g. hexane, dichloromethane) is used to remove nonpolar inferences where the target analyte is a polar compound. Conversely, the same nonpolar solvent may be used to elute a nonpolar analyte, leaving polar inferences adsorbed on the column. 

Ion exchange equilibria are usually established or altered much faster at higher temperatures. Increasing the temperature will improve efficiency, decrease retention and may alter the selectivity of the separation. The use of organic solvents in the mobile phase will also cause retention to decrease, but because the use of solvents will change many of the variables in an ion-exchange separation, their other effects are not easy to predict. 

Effective and simple immobilization of enzymes can be obtained by the cross-linking of enzyme aggregates, so-called CLEAs [55]. In this way, essentially any enzyme, including crude preparations, can be transformed into a heterogeneous type of material, insoluble in both water and organic solvents, that is stable and recyclable with high retention of the enzyme s original activity [56], These enzyme preparations are, therefore, of special value for both bio-bio and bio-chemo cascade processes. 

E-(P-Alkylvinyl)phenyliodonium salts react with tetra-n-butylammonium halides to yield the correspondingly substituted Z-haloethenes (80-100% for chloro-, bromo- and iodo-derivatives) [41], In contrast, in the corresponding reaction with Z-(2-benzenesulphonyl-ethenyl)phenyliodonium salts, nucleophilic substitution occurs with retention of configuration to yield the Z-2-benzenesulphonyl-l-haloethenes [42], The ammonium fluorides fail to yield the fluoroethenes, but produce the ethynes by simple elimination [41]. Where carboxylic acids have low solubility in organic solvents, their conversion into the acid chlorides is frequently difficult. Phase-transfer catalysis not only allows the conversion to be effected rapidly, it also results in high yields of a wide range of acid chlorides [43]. 

The effect of the moisture level in the eluent on the retention behavior is most pronounced with the least polar solvents, such as the hydrocarbons, in which water has a very limited solubility, usually less than 100 ppm. Conversely, the greater the solubility of water in the organic solvent is, the smaller the effect. For example, the water content of methylene chloride may vary by several parts per million without appreciably changing the k values as the solubility of water in CHtCIt is about 0.2%. 

The theoretical treatment outlined above does not involve any attractive interaction biptween stationary phase and eluite besides that caused by van der Waals>force. Consequently, the above obscrvniion is not predicted by the ther y. Nevertheless, surface silanols can have profound effect on the selectivity of the stationary phase in contact with eluents rich in organic solvent. The phenomenon and its practical importance in RPC of polar substances, particularly those carrying positive charge, have long been appreciated, Yet, only recently has retention behavior analyzed... 

This crude analysis is based on the behavior postulated by the Born equation. However, ion-pair formation equilibrium constants have been observed to deviate ma edly from that behavior (22/ -222)1 Oakenful, and Fenwick (222) found a maximum in the ion-pair formation constants of several alkylamines with carboxylic acids when determined at various methanol-water solvent compositions as shown by their data in Fig. 54. The results demonstrate that in this system the stability constant decreases with increasing organic solvent concentration above a.critical value which yields maximum stability. The authors suggested that this was due to a weakening of hydrophobic interactions between the ion-pair forming species by increased alcohol concentrations. In practice the effect of added organic solvent has been either to decrease the retention factor or to have virtually no effect. 

In spite of the preceding observation that eluite retention in RPC with hydrocarbonaceous bonded phases may not occur by partitio ng of the eluite between two liquid phases, theoretical considerations based on the solvophobic treatment of solvent effects shows that it might be possible to relate the observed retention factors to partition coefficients between water and an organic solverit. Such a relationship would be quite useful in light of the scale developed by Hansch and his co-workers (2/12, 283) to characterize hydrophobic properties of drugs and other biologically active... 

RPC with mobile phases containing relatively high concentrations of salts (0.1-0.5M) is occasionally employed for separations of organic acids or bases [67-69], Eqnation 5.7 can often describe the effect of the concentration of organic solvents on the retention in these phase systems, so that Eqnations 5.8 and 5.9 can be principally used for calculation of the retention data in the LC with organic solvent gradients. 

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