Solvent in mobile phase

Salt precipitation (especially in reversed-phase chromatography with high concentration of organic solvent in mobile phase) Ensure mobile phase compatihility with buffer concentration decrease ionic strength and water-organic solvent ratio premix mobile phase. 

When solvents are used in liquid chromatography, it is useful to know their polarity in relation to each other so that an appropriate choice of mobile phase or mobile phase combination might be chosen. Table 2.3 shows a list of commonly used solvents in mobile phases in order of increasing polarity. 

In liquid-solid adsorption chromatography (LSC) the column packing also serves as the stationary phase. In Tswett s original work the stationary phase was finely divided CaCOa, but modern columns employ porous 3-10-)J,m particles of silica or alumina. Since the stationary phase is polar, the mobile phase is usually a nonpolar or moderately polar solvent. Typical mobile phases include hexane, isooctane, and methylene chloride. The usual order of elution, from shorter to longer retention times, is... 

The development of micellar liquid chromatography and accumulation of numerous experimental data have given rise to the theory of chromatographic retention and optimization methods of mobile phase composition. This task has had some problems because the presence of micelles in mobile phase and its modification by organic solvent provides a great variety of solutes interactions. 

It should be first noted that the curves relating the concentration of ethyl acetate in the solvent mixture and on the stationary phase are straight and horizontal. As the initial concentration of ethyl acetate in mobile phase was 0.35 %w/v, the volume of mobile phase was 100 ml and the mass of silica was 10 g. It follows that, although a total of about 1.2 g of solute was added to the system, about a third of which resided on the silica surface, neither anisole nor nitrobenzene displaced any ethyl acetate from the silica gel. 

It is seen that the value of (H) is completely dependent on the diffusivity of the solute in the mobile phase, the column radius and the linear velocity of the mobile phase. The simple uncoated open tube can clearly be used to determine the diffusivity of any solute in any given solvent (the mobile phase). This technique for measuring diffusivities will be discussed in a later chapter. 

Figure 8.12 The universal elution gradient during the stages of AMD separation (a) change in mobile phase composition (b) change of solvent strength (c) change of selectivity value.

In the pneumatic pumping system, the pressure (and not the flow rate) is maintained constant as variations in chromatographic conditions occur. Thus, a change in mobile phase viscosity (e.g. gradient elution) or column back pressure will result in a change in flow rate for these types of pumps. The gas displacement pump in which a solvent is delivered to the column by gas pressure is an example of such a pneumatic pump. The gas displacement system is among the least expensive pumps available and is found in several low cost instruments. While the pump is nonpulsating and hence, produces low noise levels with the detectors in current use, its flow stability and reproducibility are only adequate. In addition, its upper pressure limit is only 2000 psi which may be too low in certain applications. 

HSCCC is attracting attention based on its high separation scale, 100% recovery of sample, and mild operating conditions. It is a chromatographic separation process based on the partition coefficients of different analytes in two immiscible solvent systems (mobile phase and stationary phase) subjected to a centrifugal acceleration field. 

The column needs to be reconditioned after about 12h of use or whenever the RPA 202248 peak has shifted to a retention time greater than about 6 min. To recondition, the column should be flushed with 100% acetonitrile for 15 min and then stored in that solvent for about 8 h before re-use. Storing columns in mobile phase will result in an extremely long retention time and a tailing peak for RPA 202248. Just before use the columns will have to be conditioned with the mobile phase for 5-30 min or until the RPA 202248 peak is fully separated from the isoxaflutole peak. 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

The use of solvents above their normal conditions of temperature and pressure, up to and including the supercritical state, expands the range of analytical methods exploiting an overall spectrum of solubility, polarity and volatility properties of solvents and mobile phases. The fundamentals and applications of SCFs have been reviewed [243] and described in numerous books [248,251-256],... 

A 5 to 10 ng amount of substance, dissolved in mobile phase (to avoid sample solvent interference with analyte signal) is injected with the detector potential set at + 1.0 volt and glassy carbon as the electrode material. 

HPLC Solid Solvent, aqueous, aqueous solution, or mixture of solvents Compounds or ions must be soluble in mobile phase and have appreciable attraction to the solid phase. 

Flexible and versatile dissolving solvent and mobile phase The choice of the sample solvent is not as critical in TLC as in HPLC because it is removed by evaporation before development of the plate. On the contrary, in HPLC the dissolving solvent chosen must be compatible in terms of composition and strength with the column and mobile phase. The same logic applies to the TLC mobile phase that is completely evaporated before detection. Therefore, the UV-absorbing properties, purity, or acid and base properties of the mobile phase are not as crucial as with HPLC. In addition, there is less solvent waste in TLC than in HPLC. 

The difference in movement rates of various compounds through a column is attributed to differential migration in HPLC. This can be related to the equilibrium distribution of different compounds such as X, Y, and Z between the stationary phase and the flowing solvent(s), or mobile phase. The speed with which each compound moves through the column (ux) is determined by the number of molecules of that compound in the moving phase, at any moment, since sample molecules do not move through the column while they are in the stationary phase. The molecules of the solvent or mobile phase move at the fastest possible rate except in size exclusion chromatography, where molecular... 

For HPLC, some fairly broad generalizations can be made about the selection of certain preferred solvents from the large number available. A suitable solvent will preferably have low viscosity, be compatible with the detection system, be readily available in pure form, and if possible have low flammability and toxicity. In selecting organic solvents for use in mobile phases, several physical and chemical properties of the solvent should be considered. From the standpoint of detection, the refractive index or UV cutoff values are also important. 

In all chromatographic techniques, the sample to be separated passes through the system carried by a solvent (the mobile phase). The rate at which... 

One of the main problems to be solved in the analysis of cationic surfactants is the strong adsorption of the surfactant to glassware, tubing and apparatus. To avoid losses, the solvent system used should contain a substantial percentage of organic solvent. Additionally, mobile phases containing more than 20-25% methanol will help to inhibit micelle formation [46]. 

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