Valves

What are valves?

Valves control the direction of fluid flow through an analytical system.

They are especially important in LC and LC–MS systems, where they allow streams to be switched between different pathways without manually reconnecting tubing.

Injection valves

A common design is a six-port rotary valve with two positions:

LOAD and INJECT.

During LOAD, sample fills a sample loop while the mobile phase continues flowing toward the column.

During INJECT, the valve changes position so that the mobile phase flows through the sample loop and carries the sample onto the column.

Other uses

Valves can also be used for:

• column switching

• diverting flow to waste

• trap-and-elute workflows

• online sample cleanup

• switching between solvents

• multidimensional chromatography

Divert valves

A divert valve is particularly useful in LC–MS.

For example:

LC → waste

during the early part of a run, followed by:

LC → MS

when analytes of interest begin eluting.

This can prevent salts and highly contaminating matrix components from entering the source.

Common problems

Valve problems include:

• leaks

• blocked ports

• worn rotor seals

• incorrect valve timing

• carryover

• incorrect plumbing

Incorrect valve configuration can result in no sample reaching the column or MS at all.

Key takeaway

Valves provide programmable control over where samples and solvents travel through the system.