2 Types of Commonly Used Capillary Columns

Columns are a central part of chromatography. To the untrained eye, they might seem unvaried and interchangeable. However, there are actually various types of columns, including normal-phase, reverse-phase, ion-exchange, size-exclusion and capillary. In many cases, those columns can be further broken down. As a case in point, there are two different types of capillary columns to choose from.

In this post, we’ll explain what capillary columns are and what they do, along with the two types of capillary columns.

What are capillary columns?

While there are several different types of chromatography columns, the broadest distinction is between packed and capillary.

As the name suggests, packed columns are filled with small particles made from materials such as silica, amino, alumina or diol. These particles are typically smaller than 5µm, with many less than 2µm. The columns themselves generally have a diameter of around 4mm, measuring 150-300mm in length, with the packed materials acting as the stationary phase.

On the other hand, capillary columns are much thinner and don’t need to be packed with tiny particles. Why? The materials within the column are lined with the stationary phase.

Capillary refers to the tube, which has a very thin internal diameter. Capillary columns typically measure a few tenths of a millimetre. Combined, this allows the mobile phase to interact with the stationary phase as it passes through the thin column.

Generally speaking, capillary columns are seen as more efficient than their packed counterparts. They’re commonly used in pharmaceutical and petroleum industries when testing for impurities.

2 types of capillary column

Digging a little deeper, there are actually two distinct types of capillary column, which differ depending on the way the stationary phase is added:

• Wall-coated open tubular (WCOT) capillary columns
• Support-coated open tubular (SCOT) capillary columns

WCOT columns comprise a capillary tube which has walls coated with the stationary phase in liquid form. On the other hand, SCOT columns are made using a capillary tube where the inner wall is lined with support material. The stationary phase is adsorbed onto this thin layer of material, so it’s present for analysis. WCOT columns are typically the more efficient of the two.

It's also worth mentioning a sub-type of WCOT – a fused silica open tubular (FSOT) column. In this case, a fused silica tube has a stationary phase chemically bonded to its walls. That’s strengthened with a polyimide coating on the outside, granting it both strength and flexibility.

If you’d like to read more about capillary columns, check out the article ‘Unique Separation of Mint Essential Oils by Gas Chromatography Mass Spectrometry Using Two Different Capillary Phases: Bonded Polyethylene Glycol and a Novel Ionic Liquid Phase’.