Selection, stationary-phase polyethylene glycol phases

For fused sihca columns, the majority of separations can be done with fewer than 10 bonded hquid stationary phases of varying polarity. This is because with their very high resolving power, and selectivity of the stationary phase is less critical. The stationary phases are high-molecular-weight, thermally stable polymers that are liquids or gums. The most common phases are polysiloxanes and polyethylene glycols (Carbowax), with the former the most widely used. The polysiloxanes have the backbone ... 

For instance, if solute 1 is a hydrocarbon and solute 2 an alcohol, a polar stationary phase such as the polyethylene glycols or the polar silicones (Table 22.1) will increase the retention of the alcohol because of the strong specific interactions involving the -OH group. This is an example of stationary-phase selectivity. 

Columns Of the GC methods for herbicides reported in literature, more than 90% use capillary columns (25-30 m length, 0.25-0.32 mm inner diameter, and 0.15-0.30 pm film thickness). Medium polarity stationary phases are widely used (SE 52, 54, OV17, DB 5, DB7, or equivalent). Triazines and some phenyl ureas are separated also on apolar PDMSs (OV 1 or equivalent). Organophosphorus herbicides and chloro-triazines are separated with increased selectivity on SPB-35 or polyethylene glycols (Carbowax 20 M, Supelcowax RSL 300, or equivalent). 

Stationary phases with chemically bonded branched hydrocarbons, perfluoroalkanes, polyethylene glycol, cholesterol, or alkylaryl groups show different separation selectivities which can be useful for specific separations. For example, chemically bonded phenyl groups show preferential retention of aromatic compounds and increased shape selectivity for planar and rigid rod-like molecules. 

Two types of stationary phases are most popular the polysiloxanes and polyethylene glycol phases. Both types of phases may be characterized as having the necessary high viscosity and the capability for crosslinking and/or chemical bonding with fused silica. One should note the presence of more recent additions to the capillary column family, namely, specialty columns designed for selected EPA methods, chiral separations, and gas-solid chromatographic separations. These specialty phases are considered in Section 3.11.6. 

---