Normal-phase chromatography eluent strength

Adsorption chromatography on bare silica is an example of normal-phase chromatography, in which we use a polar stationary phase and a less polar solvent. A more polar solvent has a higher eluent strength. Reversed-phase chromatography is the more common... 

An alternative to normal phase chromatography is reversed phase LC (RPLC). This method is the most popular today owing to its unmatched simplicity, versatility and scope. In RPLC, the stationary phase is nonpolar, while the mobile phase is polar and usually contains water. The strength of the eluent increases as the polarity of the mobile phase decreases. This reversal of the properties of the stationary and the mobile phases compared to normal-phase chromatography... 

Why does eluent strength increase in normal-phase chromatography when a more polar solvent is added ... 

Retention volumes in normal-phase chromatography depend much on the polarity and polarizability of the analyte molecules, and little on their molecular masses [42]. Thus, all alkyl benzenes, or all alkyl esters, will be eluted in a narrow range of retention volumes. On the other hand, structural changes affecting the polarity of a molecule are easily seen. The amount of water adsorbed by the stationary phase controls its activity, hence the resolution between analytes, and this is in turn a sensitive function of water dissolved in the mobile phase. Thus, control of the water content of the eluent is a primary concern. The influence of the composition of the mobile phase on retention has been discussed by Snyder, who introduced the concept of eluotropic strength [42],... 

In extraction techniques (LLC, SPE, etc.), normal-phase liquid chromatography (NPLC), and thin-layer chromatography, aliphatic hydrocarbons (e.g., -hexane, -heptane) are usually used. The elution strength of these solvents is often modified by addition of more polar solvents. The fundamental problem with the eluents in NPLC is dissolved water and trace amounts of olefins. These contaminations can induce a change in the wavelength cut-off values (UV detection, spectrophotometry), baseline perturbation, and poor reproducibility of retention data. Halogenated solvents such as dichloromethane can react with some organic solvents (e.g., acetonitrile) to form crystalline products. 

The setnp for polymer HPLC is qnite similar to SEC systems with a few modifications. Liqnid Adsorption Chromatography (LAC) requires the adsorption and desorption on a stationary phase. Therefore, in most cases isocratic separation is not sufficient. Gradients with respect to pH valne, ionic strength, eluent composition, or temperature are applied. The most common approach is to use eluent composition gradients. In contrast to SEC where polymeric phases dominate, silica-based column packings are the most important stationary phase. Both normal phase and reversed phase separations have been described. A summary of different applications in copolymer separation is offered by Pasch [27]. Detectors used in gradient LAC are mainly UV/DAD detectors and ELSD. 

---