Mobile phases pure polar organic

Successful enantioseparations using polysaccharide CSPs in combination with pure polar organic mobile phases have been achieved... 

Some very hydrophobic samples, e.g., lipids, are strongly retained and not eluted in an acceptable time even with pure methanol or acetonitrile as the mobile phase. Such samples are usually adequately resolved by normal-phase chromatography, but they can be often equally well or even better separated by non-aqueous reversed-phase (NARP) chromatography in mixed mobile phases containing a more polar (e.g.. acetonitrile or methanol) and a less polar (e.g., tetrahydrofuran. dichloromethane. methyl-r-butyI ether) organic solvent. Ternary non-aqueous mobile phases may contain even hexane or heptane. The retention decreases with increasing concentration of the less-polar... 

The conductivity of the mobile phase is a major factor in the electrostatic disruption of the liquid surface during ESI nebulization. Stable ESI conditions can only be achieved with semiconducting liquids (conductivity 10 -10 Q m ). Pure organic solvents like dichloromethane, benzene, and hexane are only suitable for ESI after mixing with >10% polar solvent (Ch. 6.3). 

Because methanol is not very polar, the elution of strong organic acids and bases requires a mobile phase with even greater polarity. This has normally been done by adding a very low concentration of acids or bases into methanol, and then the modified methanol is mixed with CO2 for separation. Citric, acetic, and chlorinated acetic acids have been used as acidic secondary modifiers, whereas isopropylamine, triethy-lamine, and tetrabutylammonium hydroxide have been served as basic secondary modifiers. A good example with ternary systems is the separation of benzy-lamines, as shown in Fig. 1. None of the three tested amines were effectively eluted by pure CO2 (Fig. la) however, some of these amines were eluted with very poor peak shapes when methanol was added to the CO2 (Fig. lb). However, the addition of isopropylamine to the methanol-modified mobile phase effectively eluted all of the three benzylamines and dramatically improved the peak shapes, as shown in Fig. Ic. 

In NP gradient LC on polar adsorbents, the concentration of one (or more) polar solvent(s) in a nonpolar solvent increases. A simple equation (Eq. 5) can often adequately describe the experimental dependencies of the retention factors k of sample compounds on the volume fraction q> of a polar solvent B in a binary mobile phase comprised of two organic solvents with different polarities, if the sample solute is very strongly retained in the pure, less polar solvent ... 

Peaks are usually broadened more by the use of larger volumes of injection solvent than by excessive solute concentration. Analytical-scale achiral columns tend to have less capacity for polar sample solvents than a similar-sized column in rHPLC, because SLC samples are usually dissolved in pure methanol and then injected into a somewhat weaker mobile phase. In HPLC, samples tend to be dissolved in water, an aqueous buffer, or a mixture low in organic modifier which in reversed phase has less elution strength than a solvent like pure acetonitrile. 

The interaction of analytes with the residual silanols can be a nuisance to the user of reversed-phase packings. They make the retention behavior of basic analytes more difficult to interpret and less predictable. In some extreme cases, especially for basic peptides, plots of the retention factor versus the percent organic modifier exhibit a minimum while for pure hydrophobic interaction, a monotonous decline with increasing organic content of the mobile phase is expected. The increased tailing caused by the interaction of silanols with polar, but especially basic, compounds makes peak integration more difficult and reduces column performance. 

SFC is normal phase chromatography Perhaps 80-90% of HPLC is reversed phase , using aqueous based polar mobile phases and nonpolar stationary phases such as Cig. The fluid composition is programmed from polar (i.e., pure water) to less polar (more organic). Initially, HPLC was a normal phase technique in which the fluid composition was programmed from less polar (hexane) to more polar (ethanol) during the course of a separation and the stationary phase (silica) was more polar than the mobile phase. Normal phase HPLC has been largely replaced by reversed phase because it is slow, has very slow reequilibration, and tends to have poor reproducibility. 

Separation of polar solutes generally requires polar modifiers like methanol. When the pressure is reduced at the end of the column, the mobile phase breaks down into two phases one nearly pure carbon dioxide, the other nearly pure methanol. The carbon dioxide phase will be a low density, nonsolvating gas, the methanol phase will be a solvating liquid. The polar organic solutes will remain dissolved in the small volume of the liquid phase. Thus, the task of removing the solvent from the collected fractions is greatly reduced. 

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