There are many kinds of C18 columns in the market, some are 100% water compatible, some are not. Why can they be used at 100% water? How to distinguish? What is the principle of that? Now follow us and you will have more understanding of these problems.

Low bonding density of hydrophobic group

As we all know, the proportion of aqueous phase is recommended to be less than 90% on a common reversed-phase C18 column. But retention of polar compounds on a common C18 column is weak, sometimes retention is not good even if the proportion of organic phase reduced to 5%. However, too high proportion of aqueous phase will easily lead to the phenomenon that C18 chain curled up and failed to get adequate wetting and stretching, thus it will greatly reduce the retention ability of C18 column. Ultisil AQ-C18 column minimizes the collapse of the C18 chain by reducing the bonding density of the hydrophobic group (C18) in order to maximize the retention of polar compounds. Besides, it has a good retention ability for organic acids andnucleosides in the mobile phase with high water content.

Fig 1. Separation of 6 organic acids on Ultisil AQ-C18 column

Fig 2. Separation of 5 nucleotides on Ultisil AQ-C18 column

Steric hindrance of side chain group

The intolerance of common C18 chain to aqueous phase is also reflected in the phenomenon that the bonding phase is susceptible to the effect of water or salt molecules, resulting in the hydrolysis and loss of the bonding phase in the high proportion of aqueous phase or salt phase. 

Ultisil LP-C18 perfectly solves the problem. By introducing a larger functional group into the side chain of C18 alkyl, its stereoscopic effect hinders the effect of mobile phase molecules to the silicon-oxygen bond of bonding phase. At the same time, owning to the feature of no end-capping of Ultisil LP-C18, this series of columns still have excellent resistant ability under acidic conditions (even pH is as low as 0.5).

Fig 3.  Schematic diagram of Ultisil LP series column bonding phase

End-capping technique

Trimethylchlorosilane is usually used as the end-capping reagent in common C18 columns. But if we use the end-capping reagent with polar functional groups or hydrophilic part, column’s compatibility with 100% aqueous phase can be well improved. At the same time, the polar hydrophilic groups could substitute a large number of silanol groups loci, thus the interaction between alkaline compounds and silanol can be effectively avoided and peak shape is improved. Welch Ultisil ODS-3 column and Ultisil ALK-C18 all use this bonding technique.

Fig 4.  End-capping technique of polar hydrophilic group reagent

The embedding of polar functional groups

The principle of embedding polar groups into the bonding phase of the column is similar to end-capping of polar groups, the difference is that the position where the polar groups are embedded is in the C18 chain. That polar embedded groups are conducive to increasing the concentration of water molecules on the surface of the alkyl chain and improving the retention of polar compounds, and the resistant ability of aqueous phase, such as Welch Ultisil Polar-RP column.

Fig 5. Polar embedded group on C18 column

The embedding of polar functional groups can also shield the silicon hydroxyl group or dissociated silicon oxygen anion by the effect of hydrogen bond(as Fig 6), thus ensuring the peak shape of alkaline compounds.

Fig 6. The shielding effect of embedded polar functional groups of Ultisil Polar-RP column to silicon oxygen anions

In summary, the column that can be used with pure water has obvious advantages when analyzing polar hydrophilic compounds. In addition, sometimes it can provide different retention characteristics in the development of methods due to the differences in the above techniques, which makes the problem of separating complex polar samples readily solved.

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