xcms result object for very large data sets

The xcms result objects XcmsExperiment() and XCMSnExp() keep all preprocessing results in memory and can thus (depending on the size of the data set) require a large amount of memory. In contrast, the XcmsExperimentHdf5 class, by using an on-disk data storage mechanism, has a much lower memory footprint allowing also to analyze very large data sets on regular computer systems such as desktop or laptop computers. With some exceptions, including additional parameters, the functionality and usability of this object is identical to the default XcmsExperiment object.

This help page lists functions that have additional or different parameters or properties than the respective methods for XcmsExperiment() objects. For all other functions not listed here the usability is identical to those for the XcmsExperiment() object (see the respective help page for information).

Usage

toXcmsExperimentHdf5(object, hdf5File = tempfile())

toXcmsExperiment(object, ...)

# S4 method for class 'XcmsExperimentHdf5'
chromPeakData(
  object,
  msLevel = integer(),
  peaks = character(),
  return.type = c("DataFrame", "data.frame"),
  bySample = FALSE
)

# S4 method for class 'XcmsExperimentHdf5'
filterChromPeaks(
  object,
  keep = rep(TRUE, nrow(chromPeaks(object))),
  method = "keep",
  ...
)

# S4 method for class 'XcmsExperimentHdf5,PeakGroupsParam'
adjustRtimePeakGroups(object, param = PeakGroupsParam(), msLevel = 1L)

# S4 method for class 'XcmsExperimentHdf5'
filterFeatureDefinitions(object, features = integer())

Arguments

object: XcmsExperimentHdf5 object.
hdf5File: For toXcmsExperimentHdf5(): character(1) with the path and name of the (not yet existing) file where the preprocessing results should be stored to.
...: additional parameters eventually passed to downstream functions.
msLevel: For chromPeaks() and chromPeakData(): optional integer with the MS level(s) from which the data should be returned. By default msLevel = integer() results from all MS levels are returned (if present). For refineChromPeaks(): integer(1) with the MS level from which chromatographic peaks should be refined.
peaks: For chromPeakData(): optional character with the ID of chromatographic peaks (row name in chromPeaks()) for which the data should be returned. By default (peaks = character()) the data for all chromatographic peaks is returned.
return.type: For chromPeakData(): character(1) specifying the type of object that should be returned. Can be either return.type = "DataFrame" (the default) to return a DataFrame, or return.type = "data.frame" to return the results as a data.frame.
bySample: For chromPeaks() and chromPeakData(): logical(1) whether the data should be returned by sample, i.e. as a list of matrix or data.frame objects, one for each sample.
keep: For filterChromPeaks(): defining the chromatographic peaks to keep: either a logical with the same length than the number of chromatographic peaks, an integer with the indices or a character with the IDs of the chromatographic peaks to keep.
method: For filterChromPeaks(): character(1); currently only method = "keep" is supported.
param: parameter object defining and configuring the algorithm to be used.
features: For filterFeatureDefinitions(): defining the features to keep: either a logical with the same length than the number of features, an integer with the indices or a character with the ID of the features to keep.

Value

See description of the individual methods for information.

Details

The XcmsExperimentHdf5 object stores all preprocessing results (except adjusted retention times, which are stored as an additional spectra variable in the object's Spectra::Spectra() object), in a file in HDF5 format.

XcmsExperimentHdf5 uses a different naming scheme for chromatographic peaks: for efficiency reasons, chromatographic peak data is organized by sample and MS level. The chrom peak IDs are hence in the format CPS with being the MS level in which the chromatographic peaks were detected and the ID of the sample (usually related to the index in the original MsExperiment object) and the the index of the chromatographic peak in the chrom peak matrix of that sample and MS level.

HDF5 files do not support parallel processing, thus preprocessing results need to be stored or loaded sequentially.

All functionality for XcmsExperimentHdf5 objects is optimized to reduce memory demand at the cost of eventually lower performance.

Conversion between `XcmsExperiment` and `XcmsExperimentHdf5`

To use the XcmsExperimentHdf5 class for preprocessing results, the hdf5File parameter of the findChromPeaks() function needs to be defined, specifying the path and name of the HDF5 file to store the results. In addition it is possible to convert a XcmsExperiment object to a XcmsExperimentHdf5 object with the toXcmsExperimentHdf5() function. All present preprocessing results will be stored to the specified HDF5 file. To load all preprocessing results into memory and hence change from a XcmsExperimentHdf5 to a XcmsExperiment object, the toXcmsExperument() function can be used.

Using the HDF5 file-based on-disk data storage

Calling findChromPeaks() on an MsExperiment using the parameter hdf5File will return an instance of the XcmsExperimentHdf5 class and hence use the on-disk data storage mode described on this page. The results are stored in the file specified with parameter hdf5File.

Subset

[: subset the XcmsExperimentHdf5 object to the specified samples. Parameters keepChromPeaks (default TRUE), keepAdjustedRtime (default FALSE) and keepFeatures (default FALSE) allow to configure whether present chromatographic peaks, alignment or correspondence results should be retained. This will only change information in the object (i.e., the reference to the respective entries in the HDF5 file), but will not change the content of the HDF5 file. Note that with keepChromPeaks = FALSE also keepFeatures is set to FALSE.
filterChromPeaks() and filterFeatureDefinitions() to filter the chromatographic peak and correspondence results, respectively. See documentation below for details. Subset using unsorted or duplicated indices is not supported.

chromPeaks() gains parameter bySample = FALSE that, if set to TRUE returns a list of chromPeaks matrices, one for each sample. Due to the way data is organized in XcmsExperimentHdf5 objects this is more efficient than bySample = FALSE. Thus, in cases where chrom peak data is subsequently evaluated or processed by sample, it is suggested to use bySample = TRUE.
chromPeakData() gains a new parameter peaks = character() which allows to specify from which chromatographic peaks data should be returned. For these chromatographic peaks the ID (row name in chromPeaks()) should be provided with the peaks parameter. This can reduce the memory requirement for cases in which only data of some selected chromatographic peaks needs to be extracted. Also, chromPeakData() supports the bySample parameter described for chromPeaks() above.
filterChromPeaks() allows to filter the chromatographic peaks specifying which should be retainend using the keep parameter. This can be either a logical, character or integer vector. Duplicated or unsorted indices are not supported. Eventually present feature definitions will be updated as well. The function returns the object with the filtered chromatographic peaks.

Retention time alignment

adjustRtimePeakGroups() and adjustRtime() with PeakGroupsParam: parameter extraPeaks of PeakGroupsParam is ignored. Anchor peaks are thus only defined using the minFraction and the optional subset parameter.

Correspondence analysis results

featureDefinitions(): similarly to featureDefinitions() for XcmsExperiment objects, this method returns a data.frame with the characteristics for the defined LC-MS features. The function for XcmsExperimentHdf5 does however not return the "peakidx" column with the indices of the chromatographic peaks per feature. Also, the columns are returned in alphabetic order.
featureValues(): for parameter value, the option value = "index" (i.e. returning the index of the chromatographic peaks within the chromPeaks() matrix per feature) is not supported.
filterFeatureDefinitions(): filter the feature definitions keeping only the specified features. Parameter features can be used to define the features to retain. It supports a logical, integer indices or character with the IDs of the features (i.e., their row names in featureDefinitions()). The function returns the input XcmsExperimentHdf5 with the filtered content.

Author

Johannes Rainerr, Philippine Louail

Examples


## Create a MsExperiment object representing the data from an LC-MS
## experiment.
library(MsExperiment)

## Define the raw data files
fls <- c(system.file('cdf/KO/ko15.CDF', package = "faahKO"),
         system.file('cdf/KO/ko16.CDF', package = "faahKO"),
         system.file('cdf/KO/ko18.CDF', package = "faahKO"))

## Define a data frame with the sample characterization
df <- data.frame(mzML_file = basename(fls),
                sample = c("ko15", "ko16", "ko18"))
## Importe the data. This will initialize a `Spectra` object representing
## the raw data and assign these to the individual samples.
mse <- readMsExperiment(spectraFiles = fls, sampleData = df)

## Perform chromatographic peak detection storing the data in an HDF5 file
## Parameter `hdf5File` has to be provided and needs to be the path and
## name of a (not yet existing) file to which results are going to be
## stored. For the example below we use a temporary file.
xmse <- findChromPeaks(mse, param = CentWaveParam(prefilter = c(4, 100000)),
    hdf5File = tempfile())
xmse
#> Object of class XcmsExperimentHdf5 
#>  Spectra: MS1 (3834) 
#>  Experiment data: 3 sample(s)
#>  Sample data links:
#>   - spectra: 3 sample(s) to 3834 element(s).
#>  xcms results:
#>   - chromatographic peaks in MS level(s): 1 
#>  results storage file:
#>    /tmp/RtmpoSkzhK/file2c47282dfc4

## Extract selected columnds from the chromatographic peak detection
## results
chromPeaks(xmse, columns = c("rt", "mz", "into")) |> head()
#>                   rt  mz     into sample
#> CP1S1000001 2682.913 360  5641322      1
#> CP1S1000002 2679.783 344  5210016      1
#> CP1S1000003 2678.218 343 24147443      1
#> CP1S1000004 2679.783 365 14975761      1
#> CP1S1000005 2659.438 365  3520591      1
#> CP1S1000006 2784.635 280  2537599      1

## Extract the results per sample
res <- chromPeaks(xmse, columns = c("rt", "mz", "into"), bySample = TRUE)

## The chromatographic peaks of the second sample:
res[[2]] |> head()
#>                   rt  mz     into
#> CP1S2000001 2686.042 360 10248211
#> CP1S2000002 2686.042 344  5700652
#> CP1S2000003 2686.042 343 26229546
#> CP1S2000004 2596.840 365  2358688
#> CP1S2000005 2686.042 365 15565868
#> CP1S2000006 2797.154 279 10916521

## Convert the result object to the in-memory representation:
xmse_mem <- toXcmsExperiment(xmse)
xmse_mem
#> Object of class XcmsExperiment 
#>  Spectra: MS1 (3834) 
#>  Experiment data: 3 sample(s)
#>  Sample data links:
#>   - spectra: 3 sample(s) to 3834 element(s).
#>  xcms results:
#>   - chromatographic peaks: 181 in MS level(s): 1