... all silica based HPLC stationary phases had polar, acidic, silanol (Si-OH) functional groups on the surface. Even with end-capping technology after exhaustive bonding, as much as 30%-50% of the silanols remain un-bonded and can contribute to unwanted separation results due to electrostatic interaction with solutes. To minimize type-B silica silanols it may be desirous to use small end capping groups such as C1 to cover these sites. The disadvantage of this concept is that these small groups are readily hydrolyzed away in reverse phase solvents below pH 3. Therefore this column technology is suited to a higher pH range; 6 to 9. Since many of the silanols sites are not fully ionized in the mid pH range, using this it will cause a lack of precision in the method. Silanols may be fully ionized (therefore do not contribute to lack of precision) at the higher pH but higher pH causes dissolution of silica; end capping is effective at retarding this dissolution (which begins to occur at and above pH 8). Most loss of retention at high pH is not due to loss of bonded phase and end capping groups (lost to hydrolysis) as it is at low pH but is due to dissolution of the underlying silica bed that results in the production of newly formed silanol sites causing a change of carbon load and instability of the packed column.

...to perform Organic-Normal Phase HPLC you were limited to un-bonded silica or specialty bonded phases such as Cyano or Amino. Silica supports were hydroscopic in nature and quite strongly retained water, the most polar common solvent used and adsorbed by organic solvents variably depending on atmospheric conditions and the type of solvent used. The stationary phase (Type B silica) then adsorbs the water from the mobile phase due to free silanols and as the water content on the support begins to increase, the retention of the analytes begin to change. Long tedious steps had to be taken to tightly control the water content in the mobile phase solvents.