fitpeaks {alsace} | R Documentation

## Fit chromatographic peaks with a gaussian profile

### Description

Find chromatographic peaks, and fit peak parameters using a gaussian profile. The algorithm is extremely simple and could be replaced by a more sophisticated algorithm. In particular one can expect bad fits if peaks are overlapping significantly.

### Usage

    findpeaks(y, span = NULL)
    fitpeaks(y, pos)

### Arguments

`y` | response (numerical vector)
`span` | number of points used in the definition of what constitutes a "local" maximum. If not given, a default value of 20 percent of the number of time points is used.
`pos` | locations of local maxima in vector y

### Details

Finding peaks with function `findpeaks` is based on the position of local maxima within a window of width `span`.

Peak parameters are calculated using `fitpeaks`, assuming a normal distribution. Peak width is given as a standard deviation, calculated from the full width at half maximum (FWHM); the peak area is given by the ratio of the peak height and the density.

### Value

Function `findpeaks` simply returns the locations of the local maxima, expressed as indices.

Function `fitpeaks` returns a matrix, whose columns contain the following information:

`rt` | location of the maximum of the peak (x)
`sd` | width of the peak (x)
`FWHM` | full width at half maximum (x)
`height` | height of the peak (y)
`area` | peak area

Again, the first three elements (rt, sd and FWHM) are expressed as indices, so not in terms of the real retention times. The transformation to "real" time is done in function `getAllPeaks`.

### Note

Function `findpeaks` was modelled after code suggested by Brian Ripley on the R help list.

### Author(s)

Ron Wehrens

### See Also

`getAllPeaks`

### Examples

    data(tea)
    new.lambdas <- seq(260, 500, by = 2)
    tea <- lapply(tea.raw, preprocess, dim2 = new.lambdas)
    tea.split <- splitTimeWindow(tea, c(12, 14), overlap = 10)

    Xl <- tea.split[[2]]
    Xl.opa <- opa(Xl, 4)

    Xl.als <- doALS(Xl, Xl.opa)

    tpoints <- getTime(Xl.als)
    plot(tpoints, Xl.als$CList[[2]][,2], type = "l", col = "gray")
    pk.pos <- findpeaks(Xl.als$CList[[2]][,2], span = 11)
    abline(v = tpoints[pk.pos], col = 4)

    pks <- fitpeaks(Xl.als$CList[[2]][,2], pk.pos)
    apply(pks, 1,
          function(pkmodel) {
            lines(tpoints,
                  dnorm(1:length(tpoints), pkmodel["rt"], pkmodel["sd"]) *
                  pkmodel["area"],
                  col = 2)
            invisible()
          })
    ## reasonably close fit, apart from the small peak in the middle...

[Package alsace version 1.22.0 Index]