Packings underivatized silica

As mentioned above, silica is the most common stationary phase, and as such, it is available in a host of shapes and sizes. Underivatized silica is most commonly seen in one of three sizes low-performance, flash, or HPLC. Low-performance silica stationary phases have large diameters (and often with a large particle-size distribution) and are packed into low-pressure glass or stainless steel columns. HPLC silica is typically 10 pm particles that are packed in finely polished stainless steel tubes. Flash silica particles are 30-70 pm and have low enough pressure drop that laboratory nitrogen... 

Micropacked columns are available with most of the bonded-phase packings used in HPLC. Porous and non-porous silica particles are optionally functionalized with covalently ound silanes or other strongly adsorbed materials. ALkyl-bonded silicas produce separations, generally based on solute volatility, but with the potential for selectivity differences based on interaction with silanol groups. Underivatized silica is popular for petroleum separations of aliphatic and aromatic hydrocarbons. Silver-ion-containing silica columns are selective for olefin separations. Huoroalkyl-bonded... 

Underivatized fatty acids and mediyl esters of fatty acids are easily eluted using a bonded silica phase packed column and pure carbon dioxide... 

The separation of underivatized VFA is now commonly accomplished on specialty capillary GC columns. These have replaced packed GC columns which were frequently used in the Capillary fused silica columns designed for organic acid separation are of... 

TEA and diethylamine are very strong bases. Therefore, they will aggressively dissolve silica-based support materials. A precolumn should be used to protect the analytical colunm from a r id and irreversible loss of bonded phase. A precolunui. which is positioned after the pump head and before the injector, contains packing material that matches the analytical column in functionality (e.g., octyl, octadecyl, underivatized base silica) but is made from large-particle (40-60 mesh) material. These columns are effectively and efficiently dry-packed in the laboratory but need frequent regeneration (i.e., all packing material is completely removed from flie precolunui and replaced). 

There may be times when it is necessary to analyse fatty acids by GC in the free (underivatized) form, and procedures for achieving this have been reviewed in some detail by Kuksis [505], While this is not easily accomplished on the more conventional polyester phases, it is not at all difficult If specially made packings are used, such as the FFAP and Carbowax 20M -terephthalic acid phases discussed briefly in Section B.1 above, in glass or fused silica WCOT columns (see for example [211,731]). Fused silica columns containing a chemically bonded phase of the FFAP type are now available commercially specifically for this type of analysis. It is also possible to achieve acceptable separations on fused silica columns coated with methylsiloxane polymers, provided that the column Is not overloaded [131],... 

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