A 30 meter column is suitable for most applications.
Use a 15 meter column for simple samples (less than 10 components) or for sample screening purposes.
Use a 60 meter or longer column for very complex samples or for situations requiring the highest possible number of theoretical plates. These long columns are usually limited to use for the analysis of complex samples such as petroleum products, PCB congeners, dioxins, etc.
Use a 0.25mm I.D. column for split and splitless injections when sampling overloading is not a problem. High column efficiencies are realized with these small diameter columns.
Use 0.32mm I.D. columns for splitless and on-column injections, especially when injecting large amounts of sample.
Use 0.53mm I.D. (Megabore) as replacements for packed columns or for many purge and trap applications.
Use 0.45mm I.D. columns when ease of use like Megabore is desired, but greater column efficiency is needed.
Use 0.18mm I.D. columns for GC/MS systems with low pumping capacities or when very high column efficiencies are needed.
The capacity of a column is defined as the maximum amount of sample that can be injected into a coolumn before significant peak distortion occurs.
Capacity is directly related to film thickness, column diameter and stationary phase polarity. Increased capacity results as film thickness and column diameter increase.
The more soluble a solute is in the stationary phase, the greater the column capacity for the solute. For example, a polar stationary phase will have a higher capacity for a polar solute (e.g., methanol) than a nonpolar solute (e.g., hexane).
Exceeding the column capacity or "overloading" is indicated by peak broadening or asymmetry. Usually, overloading is evident as a peak with a leading edge (fronting or sharkfin shaped). For gas-solid phases, an overloaded peak will appear to be tailing. Injector problems can also give peak shapes similar to those of overloaded peaks.
Film thickness will primarily affect the retentive character and capacity of a column. Increasing film thickness will cause a substantial increase in the retention of a solute.
Use a standard film thickness column for most applications (Film Thickness Table) .
Use thin film columns for high boiling solutes such as petroleum waxes, triglycerides, steroids, etc.
Use thick film columns for very volatile solutes such as gases, low boiling solvents and purgeables.
Within a constant set of operating conditions, it is the structure of the stationary phase that determines the relative retention (elution order) of the compounds. Focusing only on the column, the stationary phase determines the relative amount of time required for two compounds to travel through the column. The stationary phase "retards" the progress of the compounds moving through the column. If any two compounds take the same amount of time to migrate through the column, these two compounds will not be separated (i.e., they co-elute).
Columns are often selected on the basis of their polarity. Polarity is a bulk property of the stationary phase and is determined by the structure of the polymer. Stationary phase polarity does not have a direct influence on the separations obtained. Polarity will have an effect on a variety of column characteristics like column lifetime, temerature limits, bleed levels and sample capacity.
Selectivity can be thought of as the ability of the stationary phase to differentiate between two compounds by virtue of a difference in their chemical and/or physical properties.
Stationary phase and solute factors such as polarizability, solubility, magnitude of dipoles and hydrogen bonding behavior will influence selectivity. In many ways, more than one factor will be significant, thus there will be multiple selectivity influences. Unfortunately, most compound characteristics, such as the strength of hydrogen bonding or dipoles, are not readily available or easily determined. This makes it very difficult to accurately predict and explain the separations obtained for a column and set of compounds. However, some generalizations can be made. All stationary phases will have polarizability related interactions. Increased retention occurs for solutes that are more polarizable. For methyl and phenyl substituted polysiloxanes, it will be the only significant interaction. Solubility of the solute in the stationary phase will impact retention. The more soluble a solute is in the stationary phase, the greater its retention. Polyethylene glycols and cyanopropyl substituted polysiloxanes have strong dipole and hydrogen bonding characteristics. Trifluoropropyl substituted polysiloxanes will have a moderate dipole characteristic. Due to the inexactness of these characteristics, predictions and precise explanations of solute separations are very difficult.
