Peak Table Filtering By Mz And Retention Time

peak-table-filtering-by-mz-and-retention-time

Summary

Filter LC-MS chromatographic peaks by m/z and retention time windows to isolate target analytes or remove interfering signals before downstream analysis. This skill selects a subset of detected peaks matching exact mass and temporal criteria, reducing computational load and background noise in spectral matching and feature grouping workflows.

When to use

Apply this skill when you have a table of detected chromatographic peaks (e.g., from CentWave peak detection in xcms) and need to isolate a single target m/z (e.g., m/z 304.1131 for a pesticide) or a narrow m/z range, or when you must restrict analysis to a known retention time window (e.g., 230–610 seconds) to focus on a region of interest or remove co-eluting interference.

When NOT to use

• Input is already a feature table or MS2 spectrum—filtering is most useful on raw chromatographic peak tables, not on consensus spectra or post-grouping feature summaries.
• Your analysis requires untargeted discovery across the full m/z–rt plane; restrictive filtering may eliminate unexpected or unexpected compounds of interest.
• Retention time or m/z range is unknown or very broad (>100 ppm or >60 seconds); filtering becomes ineffective and you should use subsequent annotation steps instead.

Inputs

• XcmsExperiment or OnDiskMSnExp object with detected chromatographic peaks (from findChromPeaks)
• target m/z value (numeric, e.g., 304.1131)
• ppm tolerance (numeric, e.g., 10–20)
• optional: retention time range [rt_min, rt_max] in seconds

Outputs

• filtered XcmsExperiment or OnDiskMSnExp object containing only peaks within the specified m/z and/or retention time window
• chromPeaks table (matrix or data.frame) with columns: mz, mzmin, mzmax, rt, rtmin, rtmax, into, intb, maxo, sn

How to apply

Use chromPeaks() with mz and ppm parameters to filter peaks by mass-to-charge ratio with a specified mass accuracy tolerance (e.g., 20 ppm), or use filterRt() to restrict the data object to a retention time range before peak extraction. The ppm tolerance should be set based on your instrument's mass accuracy—10–20 ppm is typical for high-resolution instruments. For isolation of a single peak, calculate the target m/z range as [mz - (mz × ppm / 1e6), mz + (mz × ppm / 1e6)]. Combine m/z and retention time filters sequentially if both constraints apply. Validate that the filtered result contains the expected number of peaks and that signal intensity is retained at the target m/z-rt location.

Related tools

• xcms (provides chromPeaks() and filterRt() methods for mass-accuracy and retention-time filtering of chromatographic peak tables) — https://github.com/sneumann/xcms
• Spectra (filters extracted MS2 spectra by retention time and MS level after peak detection)

Examples

chromPeaks(dda_data, mz = 304.1131, ppm = 20); dda_data_filtered <- filterRt(dda_data, rt = c(230, 610))

Evaluation signals

• Filtered peak table contains only peaks with m/z within [target_mz - (target_mz × ppm / 1e6), target_mz + (target_mz × ppm / 1e6)]
• All peaks in filtered result have rt within the specified [rt_min, rt_max] window (if retention time filter applied)
• Number of filtered peaks is ≤ number of input peaks; verify no unexpected peaks are retained due to rounding or tolerance miscalculation
• Chromatographic peak signal (intensity, 'into' column) is preserved and non-zero for the target peak
• Downstream spectral matching against reference compounds (Flumazenil, Fenamiphos) shows expected similarity scores and correct compound identification

Limitations

• Mass accuracy tolerance (ppm) must be set appropriately for your instrument; setting ppm too large may capture off-target peaks, too small may exclude true signals due to calibration drift.
• Retention time filtering assumes reproducible chromatography; large shifts between samples or runs may cause correct peaks to fall outside the specified window.
• Filtering does not perform gap-filling; samples in which no chromatographic peak was detected at the target m/z–rt location will not be recovered by this step alone.
• m/z filtering is sensitive to isotope and adduct form; if your target is [M+H]+ but signal appears as [M+Na]+, filters on nominal m/z will miss the peak.

Evidence

• [methods] Filter by m/z: "Filter chromatographic peaks by m/z using chromPeaks() with mz and ppm parameters"
• [methods] Filter by retention time: "Filter data to retention time range using filterRt()"
• [intro] Example m/z filtering in context: "chromPeaks(dda_data, mz = ex_mz, ppm = 20)"
• [intro] Example retention time filtering in context: "We thus filter the DDA data to this retention time range. dda_data <- filterRt(dda_data, rt = c(230, 610))"
• [intro] Purpose in workflow context: "Spectra for identified chromatographic peaks can be extracted with the chromPeakSpectra() method."