Capillary column selection method - Database & Sql Blog Articles

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1.1 Stationary Phase Selection

When dealing with an unknown sample, it's often best to start with a known stationary phase. In gas chromatography, if the separation isn't ideal, consider the chemical properties of your sample. The basic principle is that analytes interact with the stationary phase when they have similar characteristics. Therefore, the more you know about your sample, the easier it becomes to choose the right phase.

Non-polar molecules —— These are typically composed of carbon and hydrogen only, with no dipole moment. Examples include n-alkanes.

Polar molecules —— These contain carbon and hydrogen, but also other atoms such as nitrogen, oxygen, phosphorus, sulfur, or halogens. Common examples include alcohols, amines, ketones, and organic halides.

Polarizable substances —— These usually contain unsaturated bonds, like alkyne or aromatic compounds.

If your sample contains non-polar components with similar properties, such as hydrocarbons in most petroleum fractions, try using an OV-1 capillary column for separation based on boiling point. If you suspect the presence of aromatic compounds, consider a phenyl-based column like SE-52 or SE-54.

For polar or polarizable samples, use a medium-polarity stationary phase or one that can accommodate such interactions, such as phenyl groups found in OV-17 or OV-225 columns. For higher polarity, polyethylene glycol (PEG), commonly called WAX, is an effective choice.

1.2 Film Thickness Selection

Thinner films allow for faster elution and better peak resolution at lower temperatures. Generally, film thickness ranges from 0.25 to 0.5 µm. For samples that elute below 300°C, such as waxes, triglycerides, and steroids, this range works well. For higher temperatures, a 0.1 µm film is suitable. Thicker films are better for low-boiling compounds and work well between 100°C and 200°C with 1–1.5 µm films. Ultra-thick films (3–5 µm) are used for gases, solvents, and volatile materials to increase interaction with the stationary phase. Thick films also help maintain resolution when using large-diameter columns.

1.3 Column Length Selection

Short columns (15m) are useful for screening simple mixtures or high molecular weight compounds. The most common length is 30m. For very complex samples, longer columns (50m, 60m, 100m, or even 150m) may be necessary. However, doubling the column length increases analysis time by about double, but peak resolution improves by only 40%. Other methods like thinner films, optimized carrier gas flow, or temperature programming are often more effective than simply increasing column length.

Special cases involve highly active components. If the sample interacts strongly with the column material, peaks may become severely tailing. Using thicker films and shorter columns can reduce surface exposure and minimize interactions.

1.4 Inner Diameter Selection

Increasing the inner diameter increases the stationary phase capacity and allows for larger sample volumes, but reduces separation efficiency and increases losses. Small-bore columns are ideal for complex samples, though they often require split injection due to limited capacity. Large-diameter columns may be preferred if some loss of resolution is acceptable. For samples requiring high capacity, such as gases or volatile compounds, large internal diameters or PLOT columns may be appropriate.

Finally, consider the instrument’s limitations when choosing the inner diameter. Large-bore columns (e.g., 0.53mm) can be used for packed column inlets, while smaller ones may not fit. Most GC/MS systems require small-bore columns due to vacuum pump limitations. Always check your system compatibility before selecting a column size.