Version Compatibility
Reference examples tested with: xcms 4.x+ (MsExperiment/XcmsExperiment), pmp 1.14+, ropls 1.34+, MetaboAnalystR 4.0+
Before using code patterns, verify installed versions match. If versions differ:
- R:
packageVersion('<pkg>') then ?function_name to verify parameters
This pipeline is only as honest as its weakest stage: a flawless feature table fed to a too-flexible drift model, or a clean OPLS-DA plot fed to background-free enrichment, produces confident wrong biology. Validate each stage against its own held-out check (QCs, permutation null, assay-coverage background), not against the next stage looking nice.
If code throws ImportError, AttributeError, or TypeError, introspect the installed
package and adapt the example to match the actual API rather than retrying.
Metabolomics Pipeline
"Process my LC-MS metabolomics data end-to-end" -> Chain xcms feature extraction, QC/normalization, confidence-stratified annotation, validated statistics, and background-aware pathway mapping, treating each stage's output as a hypothesis its component skill scrutinizes.
- R:
readMsExperiment() -> findChromPeaks() -> groupChromPeaks() -> fillChromPeaks() -> featureValues() (xcms), then QCRSC()/pqn_normalisation() (pmp), opls() (ropls), CalculateOraScore()/PerformPSEA() (MetaboAnalystR)
The governing principle
An untargeted metabolomics result is only as honest as its weakest seam: a feature table is a PARAMETERIZED HYPOTHESIS, not a measurement; injection-order drift is technical variance collinear with run order that masquerades as biology unless corrected BEFORE stats; and a database name is an MSI Level 4-5 guess until an authentic standard makes it Level 1. Three commitments are fixed at the bench and inherited by everything downstream:
- Ionization mode is a mode-lock. Positive and negative ESI have entirely different adduct chemistries ([M+H]+/[M+Na]+ vs [M-H]-/[M+Cl]-); the mode selects which mass-shift table is legal for every candidate, so annotation and pathway mapping must run against the SAME mode the feature was acquired in, and mixed-mode data must carry a per-feature mode column all the way through.
- The annotation-confidence contract gates entry into pathway mapping. MSI/Schymanski level is a made-once reporting decision: a bare DB hit with no orthogonal MS/MS or RT evidence is Level 4-5, NOT Level 2-3, and feeding tentative IDs into ORA as if confirmed launders uncertainty into a pathway p-value.
- The pooled-QC + blank + dilution baseline, fixed at the bench, makes every correction possible. The pooled QC is the substrate for BOTH drift correction and feature-quality filtering; biological samples MUST be block-randomized in run order (group confounded with order is the unwinnable "original sin"); conditioning injections are excluded from drift modeling.
What Each Stage Decides and Where the Traps Are
This skill is an orchestrator: it sequences the five component skills and enforces the honest handoffs between them. It does not re-teach each stage's parameters -- those live in the component SKILLs cited per row.
| Stage | The decision it owns | The trap it must not paper over | Defers to |
|---|
| 1. Feature extraction | centWave/grouping/alignment parameters that set the detection floor | A feature table is a parameterized hypothesis; fillChromPeaks fabricates intensities; 1 compound = 5-15 features | metabolomics/xcms-preprocessing |
| 2. QC + normalization | drift correction, RSD/D-ratio filtering, dilution normalization, mechanism-aware imputation | Over-correction is invisible to QC RSD; half-min-impute-then-test inflates significance; confounded design is unrescuable | metabolomics/normalization-qc |
| 3. Annotation | the MSI/Schymanski confidence level of every name | A database hit is Level 4-5, not an identification; ambiguous m/z inflates downstream pathways | metabolomics/metabolite-annotation |
| 4. Statistics | univariate FDR + permutation-validated multivariate, reconciled | A clean PLS-DA score plot is the generic output of p>>n; R2 is no evidence; scaling changes conclusions | metabolomics/statistical-analysis |
| 5. Pathway mapping | ORA on IDs vs mummichog on m/z, with an explicit background | The background IS the null; enrichment launders annotation uncertainty into confident biology | metabolomics/pathway-mapping |
Pipeline Flow
raw mzML (centroided)
| metabolomics/xcms-preprocessing
v readMsExperiment -> findChromPeaks -> adjustRtime -> groupChromPeaks -> fillChromPeaks
features x samples table (+ is_filled flags, mzmed/rtmed)
| metabolomics/normalization-qc
v blank/detection filter -> within-batch drift (QCRSC) -> RSD/D-ratio filter -> PQN -> mechanism-aware impute
QC-clean, dilution-normalized matrix
| split: statistics AND annotation (independent axes)
v
metabolomics/statistical-analysis metabolomics/metabolite-annotation
permutation-validated hits + univariate FDR confidence-stratified names (MSI level per feature)
| |
+-------------------- join on feature_id --------+
v metabolomics/pathway-mapping
identified compounds -> ORA/MSEA OR raw m/z (no IDs) -> mummichog/PSEA (background = FULL table)
v
pathways "consistent with perturbation", conditional on annotations + background
Stable-isotope tracing (flux) is a SEPARATE branch off labeled raw data, not a stage of this untargeted flow -- see metabolomics/isotope-tracing.
Stage 1 -- Feature Extraction (modern xcms 4.x)
Goal: Turn centroided mzML into a features-by-samples table, carrying the parameters as part of the result.
Approach: Use the MsExperiment/XcmsExperiment containers with *Param objects; align to pooled QC, group AFTER alignment (obiwarp aligns the raw profile directly, so no pre-grouping is needed; the PeakGroups method instead needs group -> align -> regroup because it uses grouped anchor peaks), and treat filled values as imputations. Full parameter rationale (ppm, peakwidth, bw, prefilter) lives in metabolomics/xcms-preprocessing.
library(xcms)
raw <- readMsExperiment(spectraFiles = mzml_files, sampleData = pd)
cwp <- CentWaveParam(ppm = 10, peakwidth = c(2, 20), snthresh = 10,
prefilter = c(3, 1000), noise = 1000)
xdata <- findChromPeaks(raw, param = cwp)
xdata <- adjustRtime(xdata, param = ObiwarpParam(binSize = 0.6,
subset = which(sampleData(xdata)sample_group subsetAdjust
pdp PeakDensityParamsampleGroups sampleDataxdatasample_group
bw minFraction binSize
xdata groupChromPeaksxdata param pdp
xdata fillChromPeaksxdata param ChromPeakAreaParam
feat featureValuesxdata value
defs featureDefinitionsxdata
Stage 2 -- QC, Drift, Normalization (not naive median + half-min)
Goal: Filter junk features, flatten injection-order drift, normalize per-sample dilution, and impute by mechanism -- before any test sees the data.
Approach: Follow the normalization-qc pipeline order: blank/detection filter -> within-batch drift correction (QCRSC) -> RSD/D-ratio filter -> PQN -> mechanism-aware imputation. Do NOT silently half-min-impute and feed limma; validate drift correction on held-out QCs, not on QC clustering.
library(pmp)
fm <- t(feat)
filtered <- filter_peaks_by_fraction(fm, classes = sample_class, min_frac = 0.5, qc_label = 'QC')
corrected <- QCRSC(df = filtered, order = injection_order, batch = batch_id,
classes = sample_class, spar = 0, minQC = 5, qc_label = 'QC')
rsd_filtered <- filter_peaks_by_rsd(corrected, max_rsd = 30, classes = sample_class, qc_label = 'QC')
normalized <- pqn_normalisation(rsd_filtered, classes = sample_class qc_label
Drift correction should lower QC RSD AND leave biological-sample RSD unchanged; if biological RSD rises, the spline absorbed signal. Mechanism-aware imputation (QRILC/GSimp for left-censored zeros) replaces the old half-min step, which collapses imputed-subset variance and inflates false significance.
Stage 3 -- Annotation Before Claiming IDs
Goal: Attach an MSI/Schymanski confidence level to each feature so the pathway stage knows what it is allowed to claim.
Approach: Match MS/MS to a library (Level 2a) or run SIRIUS/CSI:FingerID (formula Level 4, structure Level 2b/3); a bare m/z is Level 5. Collapse ion families (CAMERA) first so adducts of one compound are not counted as separate metabolites. Mechanics and thresholds live in metabolomics/metabolite-annotation. Annotation and statistics are independent axes -- run them in parallel and join on feature_id.
Stage 4 -- Statistics (univariate FDR + validated multivariate)
Goal: Decide which metabolites genuinely differ, with neither a score plot nor an unadjusted p-value standing alone.
Approach: Transform (if heteroscedastic), pick a scaling explicitly (run >=1 alternative and check the conclusion is not scaling-fragile), run a Welch/Mann-Whitney univariate test with BH FDR, AND a permutation-validated OPLS-DA (permI >= 1000), then reconcile the two. Full validation checklist in metabolomics/statistical-analysis.
library(ropls)
group <- factor(sample_info$group[sample_info$group != 'QC'])
oplsda <- opls(t(normalized)[study_samples, ], group, predI = 1, orthoI = NA,
scaleC = 'pareto', permI = 1000, crossvalI = 7,
fig.pdfC = 'none', info.txtC = 'none')
summ <- getSummaryDF(oplsda)
Univariate Welch + BH (p.adjust(method='BH') in R, multipletests(method='fdr_bh') in Python -- neither default is BH) gives the per-feature answer with effect sizes. Features are correlated (adducts, pathways), so collapse to compounds before counting "how many metabolites changed."
Stage 5 -- Pathway Mapping (the background is the null)
Goal: Interpret the differential result in pathway context without laundering annotation uncertainty into confident biology.
Approach: Two disjoint entry points. Confidently identified compounds -> ORA/MSEA with an assay-coverage background (NOT all of KEGG). Raw m/z with no IDs -> mummichog/PSEA whose permutation null is sampled from the FULL feature table. Either way, report mapping coverage and the MSI levels of the driving compounds; downgrade claims to "consistent with perturbation." Full method choice and background construction in metabolomics/pathway-mapping.
library(MetaboAnalystR)
mSet <- InitDataObjects('conc', 'pathora', FALSE)
mSet <- SetOrganism(mSet, 'hsa')
mSet <- Setup.MapData(mSet, identified_compounds)
mSet <- CrossReferencing(mSet, 'name')
mSet <- CreateMappingResultTable(mSet)
mSet <- SetKEGG.PathLib(mSet, 'hsa', 'current')
mSet <- SetMetabolomeFilter(mSet, TRUE)
mSet <- CalculateOraScore(mSet, 'rbc', 'hyperg')
Alternative Front End -- MS-DIAL
When peak detection happens in the MS-DIAL GUI/console (MS2Dec deconvolution, GC-EI, DIA/SWATH), import the alignment-result table and enter the pipeline at Stage 2. The framing is unchanged: the imported table is still a parameterized hypothesis. See metabolomics/msdial-preprocessing for the export-parsing details, then continue with normalization-qc onward.
QC Checkpoints
Each gate hands off to its component skill when it fails; "refresh" means re-run the upstream stage with revised parameters, not patch the symptom downstream.
| Stage | Keep (pass) | Refresh (fail) -> where |
|---|
| Feature extraction | EIC + alignment of top hits inspect cleanly; feature count plausible after redundancy collapse | Tune centWave/bw against EIC FWHM -> xcms-preprocessing |
| Drift correction | QC RSD dropped AND biological-sample RSD unchanged; dilution-QC linearity holds | Back off spline span / exclude weak-in-QC features -> normalization-qc |
| QC quality | QC RSD <= 20-30%, D-ratio <= 0.5, blank ratio >= 3-5x | Drop failing features; check instrument/injection -> normalization-qc |
| Missingness | impute only sparse residual holes, by mechanism (no half-min-then-test) | Detection-rate filter before imputing -> normalization-qc |
| PCA / QC clustering | pooled QCs cluster tightly at center; no batch-driven separation | Revisit batch correction / design -> normalization-qc, experimental-design/batch-design |
| Multivariate | Q2 high AND pQ2 small (permI >= 1000); PCA shows the same structure | Do not report a noise-separated score plot -> statistical-analysis |
| Annotation | each reported name carries an MSI level; ion families collapsed | Downgrade Level 3-5 names; do not promote a DB hit -> metabolite-annotation |
| Pathway background | ORA uses assay-coverage background; mummichog uses the FULL table | Set SetMetabolomeFilter(TRUE) / supply R_all -> pathway-mapping |
Common Errors
| Error / symptom | Cause | Solution |
|---|
could not find function "readMSData" | Legacy xcms <3 API | Use readMsExperiment() + *Param verbs (xcms 4.x) |
unused argument (ppm = ...) | Loose args to findChromPeaks | Wrap in CentWaveParam(...), pass via param = |
| Features on uncorrected RT | Grouped before alignment and never grouped after | Group AFTER adjustRtime (obiwarp needs no pre-grouping; PeakGroups alignment needs group -> align -> regroup) |
| Significance explodes after imputation | Half-min impute then test | Mechanism-aware QRILC/GSimp on sparse holes only |
| Clean OPLS-DA plot but it is noise | permI = 20 (ropls default) | permI >= 1000; read pQ2 from getSummaryDF |
| FDR is actually Holm/Holm-Sidak | R p.adjust default 'holm'; statsmodels 'hs' | Pass BH explicitly |
| Every pathway is significant | ORA on all-of-KEGG / mummichog on significant-only | Assay-coverage background; supply the FULL feature table |
References
- Smith CA, Want EJ, O'Maille G, Abagyan R, Siuzdak G. 2006. XCMS: processing mass spectrometry data for metabolite profiling using nonlinear peak alignment, matching, and identification. Anal Chem 78:779-787.
- Broadhurst D, Goodacre R, Reinke SN, Kuligowski J, Wilson ID, Lewis MR, Dunn WB. 2018. Guidelines and considerations for the use of system suitability and quality control samples in mass spectrometry assays applied in untargeted clinical metabolomic studies. Metabolomics 14:72.
- Westerhuis JA, Hoefsloot HCJ, Smit S, Vis DJ, Smilde AK, et al. 2008. Assessment of PLSDA cross validation. Metabolomics 4:81-89.
- Schymanski EL, Jeon J, Gulde R, Fenner K, Ruff M, Singer HP, Hollender J. 2014. Identifying small molecules via high resolution mass spectrometry: communicating confidence. Environ Sci Technol 48:2097-2098.
- Wieder C, Frainay C, Poupin N, Rodriguez-Mier P, Vinson F, Cooke J, Lai RPJ, Bundy JG, Jourdan F, Ebbels T. 2021. Pathway analysis in metabolomics: recommendations for the use of over-representation analysis. PLOS Comput Biol 17(9):e1009105.
Related Skills
- metabolomics/xcms-preprocessing - Stage 1 feature extraction parameters and the feature-table-as-artifact framing
- metabolomics/normalization-qc - Stage 2 drift correction, RSD/D-ratio filtering, PQN, mechanism-aware imputation
- metabolomics/metabolite-annotation - Stage 3 MSI/Schymanski confidence levels
- metabolomics/statistical-analysis - Stage 4 permutation-validated multivariate and dependence-aware FDR
- metabolomics/pathway-mapping - Stage 5 ORA vs mummichog and background construction
- metabolomics/msdial-preprocessing - Alternative front end entering at Stage 2
- metabolomics/lipidomics - Lipid-specific peak widths and annotation
- metabolomics/targeted-analysis - Absolute quantification branch
- metabolomics/isotope-tracing - Separate stable-isotope flux branch, not a stage of this untargeted pipeline
- multi-omics-integration/mofa-integration - Integrating the feature table with other omics layers
