LC Interface

What is the LC–MS interface?

The LC interface connects a liquid chromatograph to the mass spectrometer's ion source.

This presents a fundamental challenge: LC produces a continuous stream of liquid, while the mass analyser operates under high vacuum.

Modern LC–MS solves this by using atmospheric-pressure ionisation techniques, particularly:

ESI — Electrospray Ionisation

and

APCI — Atmospheric Pressure Chemical Ionisation

LC–ESI–MS pathway

A simplified system is:

Autosampler → LC pump → analytical column → capillary → ESI source → inlet → mass spectrometer

The LC separates compounds over time. The column effluent then enters the ion source.

In ESI, a high voltage applied to the liquid produces charged droplets. Solvent evaporates from these droplets and gas-phase ions are eventually generated.

Important parameters

The behaviour of the interface is affected by:

• LC flow rate

• mobile-phase composition

• capillary dimensions

• source voltage

• nebulising gas

• drying gas

• source temperature

• solvent volatility

Mobile-phase compatibility

LC solvents and additives must be compatible with MS detection.

Common volatile additives include:

• formic acid

• acetic acid

• ammonium formate

• ammonium acetate

Non-volatile salts and buffers can contaminate the source and suppress ionisation.

Flow-rate considerations

Different LC configurations produce very different flows.

Conventional LC may operate at hundreds of µL/min, whereas nanoLC may operate at hundreds of nL/min.

Lower flow rates can produce smaller droplets and can substantially improve electrospray efficiency.

Common problems

Loss of signal may result from:

poor LC flow → leak/blockage → unstable spray → reduced ion formation

Other issues include source contamination, ion suppression, incompatible mobile phases and poorly connected tubing.

Key takeaway

The LC interface converts a flowing chromatographic liquid into a form from which gas-phase ions can enter the mass spectrometer.