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 commonly used stationary phase. In gas chromatography, if the column isn't separating the components well, you can choose a stationary phase that matches the chemical properties of your analyte. The more you know about your sample, the easier it becomes to select the right phase.

Non-polar molecules —— These are typically composed of carbon and hydrogen only, without any dipole moment. Examples include n-alkanes, which are non-polar in nature.

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

Polarizable substances —— These usually have unsaturated bonds, like alkenes or aromatic compounds. They can interact with the stationary phase through induced dipoles.

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

For polar or polarizable samples, medium polarity columns such as OV-17 or OV-225 are suitable. If higher polarity is required, a polyethylene glycol (PEG) stationary phase, commonly known as WAX, can be used for better separation.

1.2 Film Thickness Selection

The thinner the film, the faster the elution of the components, leading to better peak resolution and lower operating temperatures. Typically, film thickness ranges from 0.25 to 0.5 µm. For most samples that elute below 300°C, including waxes, triglycerides, and steroids, this range works well. For higher temperature applications, a 0.1 µm film may be more appropriate.

Thicker films are beneficial for low-boiling-point compounds and are more effective for materials between 100°C and 200°C with a film thickness of 1–1.5 µm. Ultra-thick films (3–5 µm) are used for gases, solvents, and volatile compounds to increase interaction with the stationary phase. Thick films are also useful for large-diameter columns to maintain resolution and retention time. However, thicker films lead to higher losses and reduced temperature limits.

1.3 Column Length Selection

Short columns (15 m) are ideal for quick screening of simple mixtures or high molecular weight compounds. The most commonly used length is 30 m. For very complex samples, longer columns (50 m, 60 m, 100 m, 150 m) may be necessary.

Column length is not the most critical factor in performance. Doubling the length doubles the analysis time but increases peak resolution by only around 40%. If the separation is not optimal, there are better alternatives than increasing column length, such as using thinner films, optimizing carrier gas flow, or employing programmed temperature gradients.

For highly active components, thick films and shorter columns can help reduce tailing by minimizing surface interactions. This is especially important when the sample interacts strongly with the column material.

1.4 Inner Diameter Selection

A larger inner diameter allows for more stationary phase and higher sample capacity, but it reduces separation efficiency and increases band broadening. Small-bore columns provide excellent resolution for complex samples but often require split injection due to limited capacity.

Larger diameter columns are suitable when sample capacity is a priority, such as in gas sampling, purge-and-trap, or headspace analysis. PLOT columns may also be used in such cases. However, the choice should also consider the instrument's limitations. Large-bore columns (e.g., 0.53 mm) can be used with packed column inlets, while smaller columns may not fit certain systems.

Capillary columns with various inner diameters (0.1 mm, 0.25 mm, 0.32 mm, 0.53 mm) are generally compatible with GC/MSD systems. However, small-bore columns are often required for mass spectrometry due to vacuum pump limitations. Always check your system’s specifications to ensure compatibility with the selected column.