Gas Chromatography

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 l

 Is very important to avoid void volumes in the inlet or the detector, because their existence can result in extra broad peaks, reducing efficiency. To minimize this problem, installation of the columns must be performed closely following the manufacturer’s instructions.

 Column bleed usually occurs when columns are temperature programmed near the maximum permitted column temperature. It is the process of evaporation or thermal decompose of the stationary phase. Column bleed can be seen as an increase in the baseline as the column is heated and it becomes constant when it is held constant. The effect can become more pronounced as the column ages. Column bleed can be diminished by using thinner films, or by selecting a stationary phase with greater temperature resistance or that whose interactions with the analytes is smaller.

 Even though fused silica columns were thought to be inert, it has active “hot spots” of silanol groups, and polar compounds, particularly basic amines, can be adsorbed strongly to its surface, producing peak tailing and poor quantitative results. The Grob test can be used to evaluate the effectiveness of the deactivation process. The test proposes a mixture of six classes of compounds, that would probe for any unwanted column adsorption: 1)       Hydrocarbons. They are neutral compounds and always should produce sharp and symmetrical peaks. If not, they indicate a poorly installed column. 2)       Fatty acid methyl esters. Used to determine separation efficiency of the column. Small peak heights indicate adsorption losses in the system. 3)       Alcohols. They interact if there is presence of silanol in the injection port liner or the column, producing reduced peak heights, due to hydrogen bonding. 4)       Aldehydes. Reduced heights or symmetrical peaks imply adsorption of al

With packed columns and thick film megabore columns, nitrogen is the chosen carrier gas, since the B term in van Deemter equation (which corresponds to longitudinal diffusion in the gas phase) dominates. Since nitrogen is heavier than helium, it minimizes this term and more efficiency is attained. In capillary columns, particularly those with thin films, hydrogen is the best carrier gas. Since the separations are usually good enough, the emphasis is placed on speed. Hydrogen can be used at faster-than-optimal flow rates, with a minimal loss in efficiency. High-speed analysis is not possible with packed columns or with thick capillary columns.

Currently, most commercial columns have been conditioned in the factory, thus requiring only minimal conditioning after that. A good conditioning practice is as follows: Eliminate any air in the column by making the gas to flow for several minutes before heating the column. Then program the temperature increase at a rate of 3-5ºC/min to slightly above your intended operation temperature. Never surpass the manufacturer’s recommended maximum column temperature. When a stable baseline is obtained, the column can be used.