GC068 Guidelines for Selecting Capillary Columns

Rule respect length: Use the shortest useful column. This saves time, it is cheaper. If more resolution is required, consider to reduce the film thickness or the internal diameter.

Rules respect internal diameter (i.d.):

A)      Megabore (0.53 mm i.d.) are preferred for high carrier flow rates. It allows for simple and direct injection techniques.

B)      Medium size columns (0.25-0.35 mm i.d.) are used for good compromise.

C)      Narrow columns (0.10 mmd i.d.). Produced increased speed and separation efficiency. Shorter columns are possible, and hence, reduced times.
Some limitations: high split ratios needed (500:1). Trace analysis is difficult. Higher carrier gas pressures required. Manipulation and equipment become critical.

Rules respect to film thickness:

Thick films produce higher retention and is frequently used for analysis of volatiles. Increased capacity; important for GC-MS or FTIR.

Thin films produce maximum separation efficiencies, faster analysis and lower temperatures are required to produce the separation.

Rules respect to stationary phases:

Nonpolar phases like DB-1 or DB-5 are more efficient, more inert and generally useful for most sample types. Polar compounds are less soluble in them, which allows lower temperatures to be used. This translates to better stability for thermolabile compounds. They are the primary choice.

Polar phases like OV-1701 or some version of Carbowax® can be used if greater selectivity is needed.

Rules respect carrier gases:

H2 or He are much faster than N2. They should be preferred. At the same time, H2 posses some advantages over He:

Higher separation efficiency, faster analysis, better sensitivity (sharper peaks) and lower temperatures required to produce the separation, which lead to better resolution and longer column life. The main drawback of using H2 is its potential explosion hazard if the proportion in air reaches more than 5%. Not recommended, especially not for GC-MS.

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