R: Merge neighboring and overlapping chromatographic peaks

MergeNeighboringPeaksParam {xcms}

R Documentation

Merge neighboring and overlapping chromatographic peaks

Description

Peak detection sometimes fails to identify a chromatographic peak correctly, especially for broad peaks and if the peak shape is irregular (mostly for HILIC data). In such cases several smaller peaks are reported. This function tries to combine such peaks again if they are overlappoing on m/z dimension and considering their distance in retention time dimension and the measured intensity between them.

Chromatographic peaks are first expanded in m/z and retention time dimension by expandMz and expandRt and subsequently grouped into candidates for merging if they are (after expansion) overlapping in both m/z and rt (within the same sample). Peaks are merged if the intensity at the position at half way between them (i.e. at half the distance between "rtmax" of the first and "rtmin" of the second peak) is larger than a certain proportion (minProp) of the smaller maximal intensity ("maxo") of both peaks. The joined peaks get the "mz", "rt", "sn" and "maxo" values from the peak with the largest signal ("maxo") as well as its row in the metadata data frame of the peak (chromPeakData). The "rtmin", "rtmax" of the merged peaks are updated and "into" is recalculated based on all the signal between "rtmin" and "rtmax" of the new merged peak.

Usage

MergeNeighboringPeaksParam(expandRt = 2, expandMz = 0, ppm = 10,
  minProp = 0.75)

## S4 method for signature 'XCMSnExp,MergeNeighboringPeaksParam'
refineChromPeaks(object,
  param = MergeNeighboringPeaksParam(), msLevel = 1L,
  BPPARAM = bpparam())

Arguments

`expandRt`	`numeric(1)` defining by how many seconds the retention time window is expanded on both sides to check for overlapping peaks.
`expandMz`	`numeric(1)` constant value by which the m/z range of each chromatographic peak is expanded (on both sides!) to check for overlapping peaks.
`ppm`	`numeric(1)` defining a m/z relative value (in parts per million) by which the m/z range of each chromatographic peak is expanded to check for overlapping peaks.
`minProp`	`numeric(1)` between `0` and `1` representing the proporion of intensity to be required for peaks to be joined. See description for more details. The default (`minProp = 0.75`) means that peaks are only joined if the signal half way between then is larger 75 of the two peak's `"maxo"` (maximal intensity at peak apex).
`object`	XCMSnExp object with identified chromatographic peaks.
`param`	`MergeNeighboringPeaksParam` object defining the settings for the method.
`msLevel`	`integer` defining for which MS level(s) the chromatographic peaks should be merged.
`BPPARAM`	parameter object to set up parallel processing. Uses the default parallel processing setup returned by `bpparam()`. See `bpparam()` for details and examples.

Value

XCMSnExp object with chromatographic peaks matching the defined conditions being merged.

Note

Note that each peak gets expanded by expandMz and expandRt, thus peaks differing by 2 * expandMz (or expandRt) will be identified as overlapping. As an example: m/z max of one peak is 12.2, m/z min of another one is 12.4, if expandMz = 0.1 the m/z max of the first peak will be 12.3 and the m/z min of the second one 12.3, thus both are considered overlapping.

refineChromPeaks methods will always remove feature definitions, because a call to this method can change or remove identified chromatographic peaks, which may be part of features.

Author(s)

Johannes Rainer

Examples


xd <- readMSData(system.file('cdf/KO/ko15.CDF', package = "faahKO"),
    mode = "onDisk")
xd <- findChromPeaks(xd, param = CentWaveParam(noise = 5000))

## Example of a split peak that will be merged
mzr <- 305.1 + c(-0.01, 0.01)
chr <- chromatogram(xd, mz = mzr)
plot(chr)

## Combine the peaks
res <- refineChromPeaks(xd, param = MergeNeighboringPeaksParam(expandRt = 4))
chr_res <- chromatogram(res, mz = mzr)
plot(chr_res)

## Example of a peak that was not merged, because the signal between them
## is lower than the cut-off minProp
mzr <- 496.2 + c(-0.01, 0.01)
chr <- chromatogram(xd, mz = mzr)
plot(chr)
chr_res <- chromatogram(res, mz = mzr)
plot(chr_res)

[Package xcms version 3.9.1 Index]