compareSpectra: Spectra similarity calculations

In rformassspectrometry/Spectra: Spectra Infrastructure for Mass Spectrometry Data

Spectra similarity calculations

Description

compareSpectra() compares each spectrum in x with each spectrum in y using the function provided with FUN (defaults to ndotproduct()). If y is missing, each spectrum in x is compared with each other spectrum in x. The matching/mapping of peaks between the compared spectra is done with the MAPFUN function. The default joinPeaks() matches peaks of both spectra and allows to keep all peaks from the first spectrum (type = "left"), from the second (type = "right"), from both (type = "outer") and to keep only matching peaks (type = "inner"); see joinPeaks() for more information and examples). The MAPFUN function should have parameters x, y, xPrecursorMz and yPrecursorMz as these values are passed to the function.

In addition to joinPeaks() also joinPeaksGnps() is supported for GNPS-like similarity score calculations. Note that joinPeaksGnps() should only be used in combination with FUN = MsCoreUtils::gnps (see joinPeaksGnps() for more information and details). Use MAPFUN = joinPeaksNone to disable internal peak matching/mapping if a similarity scoring function is used that performs the matching internally.

FUN is supposed to be a function to compare intensities of (matched) peaks of the two spectra that are compared. The function needs to take two matrices with columns "mz" and "intensity" as input and is supposed to return a single numeric as result. In addition to the two peak matrices the spectra's precursor m/z values are passed to the function as parameters xPrecursorMz (precursor m/z of the x peak matrix) and yPrecursorMz (precursor m/z of the y peak matrix). Additional parameters to functions FUN and MAPFUN can be passed with .... Parameters ppm and tolerance are passed to both MAPFUN and FUN. The function returns a matrix with the results of FUN for each comparison, number of rows equal to length(x) and number of columns equal length(y) (i.e. element in row 2 and column 3 is the result from the comparison of x[2] with y[3]). If SIMPLIFY = TRUE the matrix is simplified to a numeric if length of x or y is one. See also the vignette for additional examples, such as using spectral entropy similarity in the scoring.

Usage

## S4 method for signature 'Spectra,Spectra'
compareSpectra(
  x,
  y,
  MAPFUN = joinPeaks,
  tolerance = 0,
  ppm = 20,
  FUN = ndotproduct,
  ...,
  SIMPLIFY = TRUE
)

## S4 method for signature 'Spectra,missing'
compareSpectra(
  x,
  y = NULL,
  MAPFUN = joinPeaks,
  tolerance = 0,
  ppm = 20,
  FUN = ndotproduct,
  ...,
  SIMPLIFY = TRUE
)

Arguments

x	A Spectra object.
y	A Spectra object.
MAPFUN	For compareSpectra(): function to map/match peaks between the two compared spectra. See joinPeaks() for more information and possible functions. Defaults to joinPeaks().
tolerance	numeric(1) allowing to define a constant maximal accepted difference between m/z values for peaks to be matched. This parameter is directly passed to MAPFUN.
ppm	numeric(1) defining a relative, m/z-dependent, maximal accepted difference between m/z values for peaks to be matched. This parameter is directly passed to MAPFUN.
FUN	function to compare intensities of peaks between two spectra. Defaults to ndotproduct().
...	Additional arguments passed to the internal functions.
SIMPLIFY	logical(1) defining whether the result matrix should be simplified to a numeric if possible (i.e. if either x or y is of length 1).

Author(s)

Sebastian Gibb, Johannes Rainer, Laurent Gatto

Examples

## Load a `Spectra` object with LC-MS/MS data.
fl <- system.file("TripleTOF-SWATH", "PestMix1_DDA.mzML",
    package = "msdata")
sps_dda <- Spectra(fl)
sps_dda

## Restrict to MS2 (fragment) spectra:
sps_ms2 <- filterMsLevel(sps_dda, msLevel = 2L)

## Compare spectra: comparing spectra 2 and 3 against spectra 10:20 using
## the normalized dotproduct method.
res <- compareSpectra(sps_ms2[2:3], sps_ms2[10:20])
## first row contains comparisons of spectrum 2 with spectra 10 to 20 and
## the second row comparisons of spectrum 3 with spectra 10 to 20
res

## We next calculate the pairwise similarity for the first 10 spectra
compareSpectra(sps_ms2[1:10])

## Use compareSpectra to determine the number of common (matching) peaks
## with a ppm of 10:
## type = "inner" uses a *inner join* to match peaks, i.e. keeps only
## peaks that can be mapped betwen both spectra. The provided FUN returns
## simply the number of matching peaks.
compareSpectra(sps_ms2[2:3], sps_ms2[10:20], ppm = 10, type = "inner",
    FUN = function(x, y, ...) nrow(x))

## We repeat this calculation between all pairwise combinations
## of the first 20 spectra
compareSpectra(sps_ms2[1:20], ppm = 10, type = "inner",
    FUN = function(x, y, ...) nrow(x))

rformassspectrometry/Spectra documentation built on Oct. 30, 2024, 5:42 a.m.