HolobiomicsLab LC-MS feature grouping by retention time
Summary
Groups chromatographic peaks detected across LC-MS samples into feature groups based on similarity of retention time within a specified time window. This is the first dimensionality-reduction step in the xcms preprocessing workflow, consolidating ions that likely derive from the same compound eluting at similar times.
When to use
Apply this skill immediately after chromatographic peak detection (findChromPeaks) when you have detected peaks across multiple samples and need to identify which peaks represent the same feature across the sample cohort. Use it when you have a pre-processed XcmsExperiment or xcmsSet object with detected peaks but peaks have not yet been grouped or compared across samples.
When NOT to use
- Input is already feature-grouped by retention time or beyond (e.g., already passed through abundance-correlation grouping); apply only once per workflow
- Chromatographic peaks have not yet been detected in raw data; run findChromPeaks first
- Retention time calibration across samples is severely misaligned (>RT window); recalibrate retention times before grouping
Inputs
- XcmsExperiment or xcmsSet object with detected chromatographic peaks (output from findChromPeaks)
- SimilarRtimeParam object specifying retention time window threshold (e.g., 10 seconds)
Outputs
- XcmsExperiment or xcmsSet object with features grouped by retention time similarity
- Feature group membership table mapping peaks to group IDs
How to apply
Call groupFeatures() on your peak-detected xmse object with a SimilarRtimeParam parameter specifying a retention time window (typically 10 seconds based on the article workflow). The algorithm groups peaks whose retention times fall within this window across all samples, assuming features of the same compound co-elute. Choose the time window based on your chromatographic resolution and instrument stability; tighter windows require better RT reproducibility but reduce false grouping of co-eluting compounds. This output serves as input to downstream abundance-correlation or EIC-similarity refinement steps to progressively separate co-eluting ions of different compounds.
Related tools
- xcms (Provides groupFeatures() function and SimilarRtimeParam class; implements retention time-based grouping algorithm) — https://github.com/sneumann/xcms
- MsFeatures (Defines general MS feature grouping functionality; xcms extends with LC-MS-specific implementations)
Examples
groupFeatures(xmse, param = SimilarRtimeParam(10))
Evaluation signals
- Output object contains a valid featureDefinitions data frame with column 'npeaks' reflecting number of peaks per group and 'rtmed' showing median retention time per group
- Total number of groups is less than the initial number of detected peaks (dimensionality reduction achieved)
- Retention time range within each group does not exceed the specified window threshold (e.g., max RT – min RT ≤ 10 seconds)
- Peaks within a group span multiple samples (indicating cross-sample consolidation), not just single-sample replicates
- Downstream abundance-correlation or EIC-similarity grouping produces monotonically decreasing or stable feature group counts, not artificial inflation
Limitations
- Requires good retention time reproducibility across samples; poor calibration or batch effects can create false groups
- Does not use m/z or peak shape information; co-eluting ions of different compounds will be incorrectly grouped together unless refined by later steps (abundance correlation, EIC similarity)
- Fixed retention time window may be suboptimal for complex samples with variable ionization efficiency or for metabolites with very similar RT; manual tuning recommended
- Assumes linear or monotonic retention time behavior; non-linear RT drift across a large run batch may require per-sample or sliding-window approaches not captured by single global window
Evidence
- [intro] Features (ions) of the same compound should have similar retention time.: "Features (ions) of the same compound should have similar retention time."
- [intro] Group features by similar retention time within specified time window: "Group features by similar retention time within specified time window"
- [intro] groupFeatures(xmse, param = SimilarRtimeParam(10)): "groupFeatures(xmse, param = SimilarRtimeParam(10))"
- [intro] The feature grouping functions base on the following assumptions/properties of LC-MS data: - Features (ions) of the same compound should have similar retention time.: "The feature grouping functions base on the following assumptions/properties of LC-MS data: - Features (ions) of the same compound should have similar retention time."