Lockmass

Lock mass refers to a calibration strategy used in mass spectrometry to maintain accurate mass measurements during an analysis. By continuously compensating for instrument drift, the measured m/z values stay aligned with their true masses, improving data reliability in high-resolution instruments such as Orbitraps, FT-ICRs, and Q-TOFs.

In practice, a reference mass with a precisely known m/z value is monitored during acquisition. This lock mass can be provided by an internal calibrant added to the sample, or it can arise from persistent background ions present in the system. When the observed lock mass deviates from its known value, the instrument applies a correction to the entire mass axis in real time.

Corrections may be additive (offset) or, in some systems, include a small slope adjustment to account for mass-dependent drift. The aim is to minimize mass errors (ppm) across the relevant mass range, enabling more confident assignment of peaks and routine identification of small mass differences.

Practical considerations include selecting a lock mass that remains stable across solvent composition and temperature changes, ensuring the signal does not interfere with analytes, and noting that lock mass performance depends on instrument firmware and calibration status. Lock-mass methods typically reduce calibration downtime between runs.

Related concepts include internal calibration, external calibration, and reference mass strategies used to achieve high mass accuracy in modern mass spectrometry.