Capillaries

What is a capillary?

Capillaries are narrow-bore tubes used to transport very small volumes of liquid or gas through analytical instruments.

They are critical components of LC–MS, GC–MS and many ion sources.

Why dimensions matter

A capillary is characterised by its:

ID — internal diameter

OD — outer diameter

length

The internal diameter strongly affects flow resistance and internal volume.

For a cylindrical capillary:

\[ V = \pi r^2L \]

where \(r\) is the internal radius and \(L\) is the capillary length.

Small changes in internal diameter can therefore have a large effect on volume and fluid behaviour.

Materials

Common materials include:

The appropriate material depends on pressure, solvent compatibility, electrical requirements and application.

Dead volume

Connections between capillaries should minimise unswept or dead volume.

Excessive dead volume can cause:

band broadening → wider chromatographic peaks → reduced peak height and resolution

This becomes especially important in UHPLC and nanoLC.

Blockages

Because the internal diameters can be extremely small, capillaries are susceptible to blockage by:

Sample filtration and correct solvent preparation can help prevent these problems.

Key takeaway

Capillaries may look simple, but their dimensions, material and connections can strongly influence chromatography, flow and MS sensitivity.