findChromPeaks-matchedFilter {xcms} | R Documentation

## Peak detection in the chromatographic time domain

### Description

The matchedFilter algorithm identifies peaks in the chromatographic time domain as described in [Smith 2006]. The intensity values are binned by cutting The LC/MS data into slices (bins) of a mass unit (`binSize` m/z) wide. Within each bin the maximal intensity is selected. The chromatographic peak detection is then performed in each bin by extending it based on the `steps` parameter to generate slices comprising bins `current_bin - steps +1` to `current_bin + steps - 1`. Each of these slices is then filtered with matched filtration using a second-derative Gaussian as the model peak shape. After filtration peaks are detected using a signal-to-ratio cut-off. For more details and illustrations see [Smith 2006].

The `MatchedFilterParam` class allows to specify all settings for a chromatographic peak detection using the matchedFilter method. Instances should be created with the `MatchedFilterParam` constructor.

The `findChromPeaks,OnDiskMSnExp,MatchedFilterParam` method performs peak detection using the matchedFilter algorithm on all samples from an `OnDiskMSnExp` object. `OnDiskMSnExp` objects encapsule all experiment specific data and load the spectra data (mz and intensity values) on the fly from the original files applying also all eventual data manipulations.

`binSize`,`binSize<-`: getter and setter for the `binSize` slot of the object.

`impute`,`impute<-`: getter and setter for the `impute` slot of the object.

`baseValue`,`baseValue<-`: getter and setter for the `baseValue` slot of the object.

`distance`,`distance<-`: getter and setter for the `distance` slot of the object.

`fwhm`,`fwhm<-`: getter and setter for the `fwhm` slot of the object.

`sigma`,`sigma<-`: getter and setter for the `sigma` slot of the object.

`max`,`max<-`: getter and setter for the `max` slot of the object.

`snthresh`,`snthresh<-`: getter and setter for the `snthresh` slot of the object.

`steps`,`steps<-`: getter and setter for the `steps` slot of the object.

`mzdiff`,`mzdiff<-`: getter and setter for the `mzdiff` slot of the object.

`index`,`index<-`: getter and setter for the `index` slot of the object.

### Usage

    MatchedFilterParam(binSize = 0.1, impute = "none", baseValue = numeric(),
      distance = numeric(), fwhm = 30, sigma = fwhm/2.3548, max = 5,
      snthresh = 10, steps = 2, mzdiff = 0.8 - binSize * steps,
      index = FALSE)

    ## S4 method for signature 'OnDiskMSnExp,MatchedFilterParam'
    findChromPeaks(object, param,
      BPPARAM = bpparam(), return.type = "XCMSnExp", msLevel = 1L)

    ## S4 method for signature 'MatchedFilterParam'
    show(object)

    ## S4 method for signature 'MatchedFilterParam'
    binSize(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    binSize(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    impute(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    impute(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    baseValue(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    baseValue(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    distance(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    distance(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    fwhm(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    fwhm(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    sigma(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    sigma(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    max(x)

    ## S4 replacement method for signature 'MatchedFilterParam'
    max(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    snthresh(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    snthresh(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    steps(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    steps(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    mzdiff(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    mzdiff(object) <- value

    ## S4 method for signature 'MatchedFilterParam'
    index(object)

    ## S4 replacement method for signature 'MatchedFilterParam'
    index(object) <- value

### Arguments

`binSize` | `numeric(1)` specifying the width of the bins/slices in m/z dimension.
`impute` | Character string specifying the method to be used for missing value imputation. Allowed values are `"none"` (no linear interpolation), `"lin"` (linear interpolation), `"linbase"` (linear interpolation within a certain bin-neighborhood) and `"intlin"`. See `imputeLinInterpol` for more details.
`baseValue` | The base value to which empty elements should be set. This is only considered for `method = "linbase"` and corresponds to the `profBinLinBase`'s `baselevel` argument.
`distance` | For `method = "linbase"`: number of non-empty neighboring element of an empty element that should be considered for linear interpolation. See details section for more information.
`fwhm` | `numeric(1)` specifying the full width at half maximum of matched filtration gaussian model peak. Only used to calculate the actual sigma, see below.
`sigma` | `numeric(1)` specifying the standard deviation (width) of the matched filtration model peak.
`max` | `numeric(1)` representing the maximum number of peaks that are expected/will be identified per slice.
`snthresh` | `numeric(1)` defining the signal to noise cutoff to be used in the chromatographic peak detection step.
`steps` | `numeric(1)` defining the number of bins to be merged before filtration (i.e. the number of neighboring bins that will be joined to the slice in which filtration and peak detection will be performed).
`mzdiff` | `numeric(1)` defining the minimum difference in m/z for peaks with overlapping retention times
`index` | `logical(1)` specifying whether indicies should be returned instead of values for m/z and retention times.
`object` | For `findChromPeaks`: an `OnDiskMSnExp` object containing the MS- and all other experiment-relevant data. For all other methods: a parameter object.
`param` | An `MatchedFilterParam` object containing all settings for the matchedFilter algorithm.
`BPPARAM` | A parameter class specifying if and how parallel processing should be performed. It defaults to `bpparam`. See documentation of the `BiocParallel` for more details. If parallel processing is enabled, peak detection is performed in parallel on several of the input samples.
`return.type` | Character specifying what type of object the method should return. Can be either `"XCMSnExp"` (default), `"list"` or `"xcmsSet"`.
`msLevel` | `integer(1)` defining the MS level on which the peak detection should be performed. Defaults to `msLevel = 1`.
`value` | The value for the slot.
`x` | For `max`: a `MatchedFilterParam` object.

### Details

The intensities are binned by the provided m/z values within each spectrum (scan). Binning is performed such that the bins are centered around the m/z values (i.e. the first bin includes all m/z values between `min(mz) - bin_size/2` and `min(mz) + bin_size/2`).

For more details on binning and missing value imputation see `binYonX` and `imputeLinInterpol` methods.

Parallel processing (one process per sample) is supported and can be configured either by the `BPPARAM` parameter or by globally defining the parallel processing mode using the `register` method from the `BiocParallel` package.

### Value

The `MatchedFilterParam` function returns a `MatchedFilterParam` class instance with all of the settings specified for chromatographic detection by the matchedFilter method.

For `findChromPeaks`: if `return.type = "XCMSnExp"` an `XCMSnExp` object with the results of the peak detection. If `return.type = "list"` a list of length equal to the number of samples with matrices specifying the identified peaks. If `return.type = "xcmsSet"` an `xcmsSet` object with the results of the peak detection.

### Slots

`.__classVersion__,binSize,impute,baseValue,distance,fwhm,sigma,max,snthresh,steps,mzdiff,index`

  See corresponding parameter above. `.__classVersion__` stores the version from the class. Slots values should exclusively be accessed via the corresponding getter and setter methods listed above.

### Note

These methods and classes are part of the updated and modernized `xcms` user interface which will eventually replace the `findPeaks` methods. It supports chromatographic peak detection on `MSnExp` and `OnDiskMSnExp` objects (both defined in the `MSnbase` package). All of the settings to the matchedFilter algorithm can be passed with a `MatchedFilterParam` object.

### Author(s)

Colin A Smith, Johannes Rainer

### References

Colin A. Smith, Elizabeth J. Want, Grace O'Maille, Ruben Abagyan and Gary Siuzdak. "XCMS: Processing Mass Spectrometry Data for Metabolite Profiling Using Nonlinear Peak Alignment, Matching, and Identification" Anal. Chem. 2006, 78:779-787.

### See Also

The `do_findChromPeaks_matchedFilter` core API function and `findPeaks.matchedFilter` for the old user interface.

`XCMSnExp` for the object containing the results of the chromatographic peak detection.

Other peak detection methods: `chromatographic-peak-detection`, `findChromPeaks-centWaveWithPredIsoROIs`, `findChromPeaks-centWave`, `findChromPeaks-massifquant`, `findPeaks-MSW`

### Examples

    ## Create a MatchedFilterParam object. Note that we use a unnecessarily large
    ## binSize parameter to reduce the run-time of the example.
    mfp <- MatchedFilterParam(binSize = 5)
    ## Change snthresh parameter
    snthresh(mfp) <- 15
    mfp

    ## Perform the peak detection using matchecFilter on the files from the
    ## faahKO package. Files are read using the readMSData from the MSnbase
    ## package
    library(faahKO)
    library(MSnbase)
    fls <- dir(system.file("cdf/KO", package = "faahKO"), recursive = TRUE,
               full.names = TRUE)
    raw_data <- readMSData(fls[1:2], mode = "onDisk")
    ## Perform the chromatographic peak detection using the settings defined
    ## above. Note that we are also disabling parallel processing in this
    ## example by registering a "SerialParam"
    register(SerialParam())
    res <- findChromPeaks(raw_data, param = mfp)
    head(chromPeaks(res))

[Package xcms version 3.2.0 Index]