• Select the least polar phase that will perform the separation you require.
• Non-polar stationary phases separate analytes predominantly by order of boiling point. Increase the amount of phenyl and/or cyanopropyl content in the phase, and the separation is then influenced more by differences in dipole moments or charge distributions (BP10, BPX35, BPX50, BP225, BPX70).

• To separate compounds that differ more in their hydrogen bonding capacities (for example aldehydes and alcohols), polyethylene glycol type phases are best suited – BP20 (Wax), BP21 (FFAP), SolGel-WAX.
• Wherever possible use published retention indices to assist in your selection. Retention indices are calculated for a range of probe compounds which can highlight specific selectivity characteristics of a stationary phase.

• The smaller the diameter the greater the efficiency and therefore the better the resolution. Reduce the diameter by half and the column efficiency doubles.

• As the diameter increases, the film thickness can increase to maintain the same phase ratio. The thicker the film, the greater the loading capacity. Overloading a column will always result in loss of resolution. If the column diameter is halved while maintaining the same film thickness, then the loading capacity will also be halved.

VOLATILITY OF SOLUTE:
The greater the film thickness the greater the retention of a solute, therefore the higher the elution temperature. As a rule, doubling the film thickness results in an increase in elution temperature of approximately 15-20°C, under isothermal conditions. Using a temperature program, the increase in elution temperature is slightly less.

As well as film thickness, changing the column internal diameter will also effect the elution temperature. To avoid using two parameters that can alter individually, phase ratio is often used as it takes both into account.

^{The chromatograms above demonstrate the effect on elution temperature for a mixture of compounds on a 0.25, 1.0 and 5.0 micron 0.32mm ID column. An increase in film thickness from 0.25 micron to 5.0 micron needed a change in analysis temperature of 80°C to maintain the same elution time.}

Phase Ratio encompasses both the film thickness and column internal diameter to give a value that can characterize all column internal diameters and film thickness combinations.
Calculate Phase Ratio using following formula:

ß = d/4df
where
ß = phase ratio
d = column internal diameter (in microns)
d_f = film thickness (in microns)

From the phase ratio value, a column can be categorized for the type of application it would best suit. The smaller the ß value, the greater the concentration of phase to the volume of the column, making it better suited for analyzing volatile compounds. Columns which have thin films, are generally better suited for high molecular weight compounds and are characterized by large ß values.

COLUMN TO COLUMN COMPATIBILITY:
Phase ratio also enables the transfer of an analysis from one column internal diameter to another, without having to change the method substantially.

Retention Indices for Nine SGE Cross-linked Stationary Phases
The use of retention indices is a valuable tool in assisting selection of the stationary phase, which will provide maximum resolution for the compounds to be analyzed.

The retention indices of the five test compounds indicate the differences and similarities of each stationary phase. The values are calculated in reference to a homologous series of n-alkane hydrocarbons plotted on a logarithmic scale. Each n-alkane has a retention index of 100 times the carbon number (ie. C6, RI=600). Therefore, the retention index for each of the test compounds illustrates the elution position in reference to this n-alkane series.

Retention Indices are calculated using the following formula:

IA = 100N+100n (log t'R(A) - log t'R(N) ) / (log t'R(N+n) - log t'R(N) )

IA is the retention index of compound A (from corrected retention times) which elutes between two n-paraffins separated by either one or two carbon numbers.