Extracting chromatograms

chromatogram: extract chromatographic data (such as an extracted ion chromatogram, a base peak chromatogram or total ion chromatogram) from an OnDiskMSnExp or XCMSnExp objects. See also the help page of the chromatogram function in the MSnbase package.

Usage

# S4 method for class 'XCMSnExp'
chromatogram(
  object,
  rt,
  mz,
  aggregationFun = "sum",
  missing = NA_real_,
  msLevel = 1L,
  BPPARAM = bpparam(),
  adjustedRtime = hasAdjustedRtime(object),
  filled = FALSE,
  include = c("apex_within", "any", "none"),
  ...
)

Arguments

object

Either a OnDiskMSnExp or XCMSnExp object from which the chromatograms should be extracted.

rt

numeric(2) or two-column matrix defining the lower and upper boundary for the retention time range(s). If not specified, the full retention time range of the original data will be used.

mz

numeric(2) or two-column matrix defining the lower and upper mz value for the MS data slice(s). If not specified, the chromatograms will be calculated on the full mz range.

aggregationFun

character(1) specifying the function to be used to aggregate intensity values across the mz value range for the same retention time. Allowed values are "sum" (the default), "max", "mean" and "min".

missing

numeric(1) allowing to specify the intensity value to be used if for a given retention time no signal was measured within the mz range of the corresponding scan. Defaults to NA_real_ (see also Details and Notes sections below). Use missing = 0 to resemble the behaviour of the getEIC from the old user interface.

msLevel

integer(1) specifying the MS level from which the chromatogram should be extracted. Defaults to msLevel = 1L.

BPPARAM

Parallelisation backend to be used, which will depend on the architecture. Default is BiocParallel::bparam().

adjustedRtime

For chromatogram,XCMSnExp: whether the adjusted (adjustedRtime = TRUE) or raw retention times (adjustedRtime = FALSE) should be used for filtering and returned in the resulting MChromatograms object. Adjusted retention times are used by default if available.

filled

logical(1) whether filled-in peaks should also be returned. Defaults to filled = FALSE, i.e. returns only detected chromatographic peaks in the result object.

include

character(1) defining which chromatographic peaks should be returned. Supported are include = "apex_within" (the default) which returns chromatographic peaks that have their apex within the mz rt range, include = "any" to return all chromatographic peaks which m/z and rt ranges overlap the mz and rt or include = "none" to not include any chromatographic peaks.

...

optional parameters - currently ignored.

Value

chromatogram returns a XChromatograms object with the number of columns corresponding to the number of files in object and number of rows the number of specified ranges (i.e. number of rows of matrices provided with arguments mz and/or rt). All chromatographic peaks with their apex position within the m/z and retention time range are also retained as well as all feature definitions for these peaks.

Details

Arguments rt and mz allow to specify the MS data slice (i.e. the m/z range and retention time window) from which the chromatogram should be extracted. These parameters can be either a numeric of length 2 with the lower and upper limit, or a matrix with two columns with the lower and upper limits to extract multiple EICs at once. The parameter aggregationSum allows to specify the function to be used to aggregate the intensities across the m/z range for the same retention time. Setting aggregationFun = "sum" would e.g. allow to calculate the total ion chromatogram (TIC), aggregationFun = "max" the base peak chromatogram (BPC).

If for a given retention time no intensity is measured in that spectrum a NA intensity value is returned by default. This can be changed with the parameter missing, setting missing = 0 would result in a 0 intensity being returned in these cases.

Note

For XCMSnExp objects, if adjusted retention times are available, the chromatogram method will by default report and use these (for the subsetting based on the provided parameter rt). This can be changed by setting adjustedRtime = FALSE.

See also

XCMSnExp for the data object. Chromatogram for the object representing chromatographic data.

[XChromatograms] for the object allowing to arrange
multiple [XChromatogram] objects.

[plot] to plot a [XChromatogram] or [MChromatograms] objects.

`as` (`as(x, "data.frame")`) in `MSnbase` for a method to extract
the MS data as `data.frame`.

Author

Johannes Rainer

Examples

## Load a test data set with identified chromatographic peaks
library(MSnbase)
data(faahko_sub)
## Update the path to the files for the local system
dirname(faahko_sub) <- system.file("cdf/KO", package = "faahKO")

## Disable parallel processing for this example
register(SerialParam())

## Extract the ion chromatogram for one chromatographic peak in the data.
chrs <- chromatogram(faahko_sub, rt = c(2700, 2900), mz = 335)

chrs
#> XChromatograms with 1 row and 3 columns
#>             ko15.CDF        ko16.CDF        ko18.CDF
#>      <XChromatogram> <XChromatogram> <XChromatogram>
#> [1,]        peaks: 0        peaks: 1        peaks: 0
#> phenoData with 1 variables
#> featureData with 5 variables
#> - - - xcms preprocessing - - -
#> Chromatographic peak detection:
#>  method: centWave 

## Identified chromatographic peaks
chromPeaks(chrs)
#>        mz mzmin mzmax       rt   rtmin    rtmax    into    intb  maxo sn sample
#> CP095 335   335   335 2786.199 2764.29 2812.803 1496244 1451652 58736 55      2
#>       row column
#> CP095   1      2

## Plot the chromatogram
plot(chrs)

## Extract chromatograms for multiple ranges.
mzr <- matrix(c(335, 335, 344, 344), ncol = 2, byrow = TRUE)
rtr <- matrix(c(2700, 2900, 2600, 2750), ncol = 2, byrow = TRUE)
chrs <- chromatogram(faahko_sub, mz = mzr, rt = rtr)

chromPeaks(chrs)
#>        mz mzmin mzmax       rt    rtmin    rtmax    into    intb   maxo sn
#> CP095 335   335   335 2786.199 2764.290 2812.803 1496244 1451652  58736 55
#> CP003 344   344   344 2679.783 2646.919 2709.517 5210016 5135917 152320 68
#> CP090 344   344   344 2686.042 2648.483 2714.211 5700652 5574745 151744 87
#> CP193 344   344   344 2682.914 2643.790 2731.427 5255689 4946143 125632 41
#>       sample row column
#> CP095      2   1      2
#> CP003      1   2      1
#> CP090      2   2      2
#> CP193      3   2      3

plot(chrs)


## Get access to all chromatograms for the second mz/rt range
chrs[1, ]
#> XChromatograms with 1 row and 3 columns
#>             ko15.CDF        ko16.CDF        ko18.CDF
#>      <XChromatogram> <XChromatogram> <XChromatogram>
#> [1,]        peaks: 0        peaks: 1        peaks: 0
#> phenoData with 1 variables
#> featureData with 5 variables
#> - - - xcms preprocessing - - -
#> Chromatographic peak detection:
#>  method: centWave 

## Plot just that one
plot(chrs[1, , drop = FALSE])