Glossary
Mass feature
The mass feature is the working unit of untargeted LC/MS. Before a compound has a name, it exists in your data as a feature, and the quality of every result depends on how well those features are detected and aligned.
Definition
A mass feature is a signal in LC/MS data localized by two coordinates, a mass to charge ratio and a retention time, with an associated intensity. It represents an ion observed at a particular mass eluting at a particular time. A single compound can give rise to several features, because it may form multiple adducts, carry isotopes, and appear in more than one charge state. Untargeted analysis works at the feature layer because most detected ions are not yet identified, so the feature, not the named compound, is what gets detected, aligned, and quantified across a study.
Worked example
In practice
A metabolite eluting at ninety seconds might appear as a protonated feature and a sodium adduct feature, each with its own isotope pattern, so one compound contributes several rows to a feature table before any annotation is attempted.
What matters
Where this makes a difference
Why the feature is the unit
In untargeted work most ions are unidentified, so the analysis has to operate on detected signals rather than named compounds. The feature is the object that can be measured consistently before identity is known.
One compound, several features
Adducts, isotopes, and charge states mean a single molecule can generate multiple features. Grouping them correctly is part of turning a feature table into a compound level result.
Where quality is won or lost
Detection and alignment at the feature layer set the ceiling for everything downstream. Errors here propagate into every statistic, so this is where Metablify concentrates.
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