Choosing NP Systems

In addition, more product can be loaded on a silica gel than on reversed-phase materials. Furthermore the separated products are readily recovered by simple evaporation procedures, while the isolation of products from an RP separation process often requires additional work-up steps. These steps include the removal of salts or other aqueous-phase additives and extraction of the target product after evaporation of the organic phase (Section 3.2.5.5). 

The combination of water and silica is not recommended due to the strong interaction between water and adsorbent. Furthermore silica is slightly soluble in water (strongly depending on the pH value ), which results in a shortened lifetime of the adsorbent. Normal phase systems are limited to organic solvents and thus the solutes have to be soluble in these solvents. 

Aqueous mobile phases are not used because the interaction between water and the polar stationary phase would be too strong. However, a minimal amount of water is needed to saturate the above-mentioned hot spots of polarity due to metal atoms. For these two reasons the water content of the solvents should be controlled very carefully (Section 3.2.4.2). Changes in the range of a few ppm, resulting for 

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