Spectra: The Spectra class to manage and access MS data

In Spectra: Spectra Infrastructure for Mass Spectrometry Data

Description

The Spectra class encapsules spectral mass spectrometry data and related metadata.

It supports multiple data backends, e.g. in-memory (MsBackendDataFrame()), on-disk as mzML (MsBackendMzR()) or HDF5 (MsBackendHdf5Peaks()).

Usage

addProcessing(object, FUN, ...)

applyProcessing(object, f = dataStorage(object), BPPARAM = bpparam(), ...)

combineSpectra(
  x,
  f = x$dataStorage,
  p = x$dataStorage,
  FUN = combinePeaks,
  ...,
  BPPARAM = bpparam()
)

## S4 method for signature 'missing'
Spectra(
  object,
  processingQueue = list(),
  metadata = list(),
  ...,
  backend = MsBackendDataFrame(),
  BPPARAM = bpparam()
)

## S4 method for signature 'MsBackend'
Spectra(
  object,
  processingQueue = list(),
  metadata = list(),
  ...,
  BPPARAM = bpparam()
)

## S4 method for signature 'character'
Spectra(
  object,
  processingQueue = list(),
  metadata = list(),
  source = MsBackendMzR(),
  backend = source,
  ...,
  BPPARAM = bpparam()
)

## S4 method for signature 'ANY'
Spectra(
  object,
  processingQueue = list(),
  metadata = list(),
  source = MsBackendDataFrame(),
  backend = source,
  ...,
  BPPARAM = bpparam()
)

## S4 method for signature 'Spectra,MsBackend'
setBackend(object, backend, f = dataStorage(object), ..., BPPARAM = bpparam())

## S4 method for signature 'Spectra'
c(x, ...)

## S4 method for signature 'Spectra,ANY'
split(x, f, drop = FALSE, ...)

## S4 method for signature 'Spectra'
export(object, backend, ...)

## S4 method for signature 'Spectra'
acquisitionNum(object)

## S4 method for signature 'Spectra'
peaksData(object, ...)

## S4 method for signature 'Spectra'
centroided(object)

## S4 replacement method for signature 'Spectra'
centroided(object) <- value

## S4 method for signature 'Spectra'
collisionEnergy(object)

## S4 replacement method for signature 'Spectra'
collisionEnergy(object) <- value

## S4 method for signature 'Spectra'
dataOrigin(object)

## S4 replacement method for signature 'Spectra'
dataOrigin(object) <- value

## S4 method for signature 'Spectra'
dataStorage(object)

## S4 method for signature 'Spectra'
dropNaSpectraVariables(object)

## S4 method for signature 'Spectra'
intensity(object, ...)

## S4 method for signature 'Spectra'
ionCount(object)

## S4 method for signature 'Spectra'
isCentroided(object, ...)

## S4 method for signature 'Spectra'
isEmpty(x)

## S4 method for signature 'Spectra'
isolationWindowLowerMz(object)

## S4 replacement method for signature 'Spectra'
isolationWindowLowerMz(object) <- value

## S4 method for signature 'Spectra'
isolationWindowTargetMz(object)

## S4 replacement method for signature 'Spectra'
isolationWindowTargetMz(object) <- value

## S4 method for signature 'Spectra'
isolationWindowUpperMz(object)

## S4 replacement method for signature 'Spectra'
isolationWindowUpperMz(object) <- value

## S4 method for signature 'Spectra'
containsMz(
  object,
  mz = numeric(),
  tolerance = 0,
  ppm = 20,
  which = c("any", "all"),
  BPPARAM = bpparam()
)

## S4 method for signature 'Spectra'
containsNeutralLoss(
  object,
  neutralLoss = 0,
  tolerance = 0,
  ppm = 20,
  BPPARAM = bpparam()
)

## S4 method for signature 'Spectra'
spectrapply(
  object,
  FUN,
  f = as.factor(seq_along(object)),
  ...,
  BPPARAM = SerialParam()
)

## S4 method for signature 'Spectra'
length(x)

## S4 method for signature 'Spectra'
msLevel(object)

## S4 method for signature 'Spectra'
mz(object, ...)

## S4 method for signature 'Spectra'
lengths(x, use.names = FALSE)

## S4 method for signature 'Spectra'
polarity(object)

## S4 replacement method for signature 'Spectra'
polarity(object) <- value

## S4 method for signature 'Spectra'
precScanNum(object)

## S4 method for signature 'Spectra'
precursorCharge(object)

## S4 method for signature 'Spectra'
precursorIntensity(object)

## S4 method for signature 'Spectra'
precursorMz(object)

## S4 method for signature 'Spectra'
rtime(object)

## S4 replacement method for signature 'Spectra'
rtime(object) <- value

## S4 method for signature 'Spectra'
scanIndex(object)

## S4 method for signature 'Spectra'
selectSpectraVariables(object, spectraVariables = spectraVariables(object))

## S4 method for signature 'Spectra'
smoothed(object)

## S4 replacement method for signature 'Spectra'
smoothed(object) <- value

## S4 method for signature 'Spectra'
spectraData(object, columns = spectraVariables(object))

## S4 replacement method for signature 'Spectra'
spectraData(object) <- value

## S4 method for signature 'Spectra'
spectraNames(object)

## S4 replacement method for signature 'Spectra'
spectraNames(object) <- value

## S4 method for signature 'Spectra'
spectraVariables(object)

## S4 method for signature 'Spectra'
tic(object, initial = TRUE)

## S4 method for signature 'Spectra'
x$name

## S4 replacement method for signature 'Spectra'
x$name <- value

## S4 method for signature 'Spectra'
x[i, j, ..., drop = FALSE]

## S4 method for signature 'Spectra'
filterAcquisitionNum(
  object,
  n = integer(),
  dataStorage = character(),
  dataOrigin = character()
)

## S4 method for signature 'Spectra'
filterEmptySpectra(object)

## S4 method for signature 'Spectra'
filterDataOrigin(object, dataOrigin = character())

## S4 method for signature 'Spectra'
filterDataStorage(object, dataStorage = character())

## S4 method for signature 'Spectra'
filterIntensity(
  object,
  intensity = c(0, Inf),
  msLevel. = unique(msLevel(object)),
  ...
)

## S4 method for signature 'Spectra'
filterIsolationWindow(object, mz = numeric())

## S4 method for signature 'Spectra'
filterMsLevel(object, msLevel. = integer())

## S4 method for signature 'Spectra'
filterMzRange(object, mz = numeric(), msLevel. = unique(msLevel(object)))

## S4 method for signature 'Spectra'
filterMzValues(
  object,
  mz = numeric(),
  tolerance = 0,
  ppm = 20,
  msLevel. = unique(msLevel(object))
)

## S4 method for signature 'Spectra'
filterPolarity(object, polarity = integer())

## S4 method for signature 'Spectra'
filterPrecursorMz(object, mz = numeric())

## S4 method for signature 'Spectra'
filterPrecursorScan(object, acquisitionNum = integer())

## S4 method for signature 'Spectra'
filterRt(object, rt = numeric(), msLevel. = unique(msLevel(object)))

## S4 method for signature 'Spectra'
reset(object, ...)

## S4 method for signature 'Spectra'
bin(x, binSize = 1L, breaks = NULL, msLevel. = unique(msLevel(x)))

## 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
)

## S4 method for signature 'Spectra'
pickPeaks(
  object,
  halfWindowSize = 2L,
  method = c("MAD", "SuperSmoother"),
  snr = 0,
  k = 0L,
  descending = FALSE,
  threshold = 0,
  msLevel. = unique(msLevel(object))
)

## S4 method for signature 'Spectra'
replaceIntensitiesBelow(
  object,
  threshold = min,
  value = 0,
  msLevel. = unique(msLevel(object))
)

## S4 method for signature 'Spectra'
smooth(
  x,
  halfWindowSize = 2L,
  method = c("MovingAverage", "WeightedMovingAverage", "SavitzkyGolay"),
  ...,
  msLevel. = unique(msLevel(x))
)

Arguments

object	For Spectra: either a DataFrame or missing. See section on creation of Spectra objects for details. For all other methods a Spectra object.
FUN	For addProcessing: function to be applied to the peak matrix of each spectrum in object. For compareSpectra: function to compare intensities of peaks between two spectra with each other. For combineSpectra: function to combine the (peak matrices) of the spectra. See section Data manipulations and examples below for more details.
...	Additional arguments.
f	For split: factor defining how to split x. See base::split() for details. For setBackend: factor defining how to split the data for parallelized copying of the spectra data to the new backend. For some backends changing this parameter can lead to errors. For combineSpectra: factor defining the grouping of the spectra that should be combined. For spectrapply: factor how object should be splitted.
BPPARAM	Parallel setup configuration. See bpparam() for more information. This is passed directly to the backendInitialize() method of the MsBackend.
x	A Spectra object.
p	For combineSpectra: factor defining how to split the input Spectra for parallel processing. Defaults to x$dataStorage, i.e., depending on the used backend, per-file parallel processing will be performed.
processingQueue	For Spectra: optional list of ProcessingStep objects.
metadata	For Spectra: optional list with metadata information.
backend	For Spectra: MsBackend to be used as backend. See section on creation of Spectra objects for details. For setBackend: instance of MsBackend. See section on creation of Spectra objects for details. For export: MsBackend to be used to export the data.
source	For Spectra: instance of MsBackend that can be used to import spectrum data from the provided files. See section Creation of objects, conversion and changing the backend for more details.
drop	For [, split: not considered.
value	replacement value for <- methods. See individual method description or expected data type.
mz	For filterIsolationWindow: numeric(1) with the m/z value to filter the object. For filterPrecursorMz and filterMzRange: numeric(2) defining the lower and upper m/z boundary. For filterMzValues: numeric with the m/z values to match peaks against.
tolerance	For compareSpectra, containsMz: numeric(1) allowing to define a constant maximal accepted difference between m/z values for peaks to be matched. For containsMz and filterMzValues it can also be of length equal mz to specify a different tolerance for each m/z value.
ppm	For compareSpectra, containsMz, filterMzValues: numeric(1) defining a relative, m/z-dependent, maximal accepted difference between m/z values for peaks to be matched.
which	for containsMz: either "any" or "all" defining whether any (the default) or all provided mz have to be present in the spectrum.
neutralLoss	for containsNeutralLoss: numeric(1) defining the value which should be subtracted from the spectrum's precursor m/z.
use.names	For lengths: ignored.
spectraVariables	For selectSpectraVariables: character with the names of the spectra variables to which the backend should be subsetted.
columns	For spectraData accessor: optional character with column names (spectra variables) that should be included in the returned DataFrame. By default, all columns are returned.
initial	For tic: logical(1) whether the initially reported total ion current should be reported, or whether the total ion current should be (re)calculated on the actual data (initial = FALSE, same as ionCount).
name	For $ and $<-: the name of the spectra variable to return or set.
i	For [: integer, logical or character to subset the object.
j	For [: not supported.
n	for filterAcquisitionNum: integer with the acquisition numbers to filter for.
dataStorage	For filterDataStorage: character to define which spectra to keep. For filterAcquisitionNum: optionally specify if filtering should occur only for spectra of selected dataStorage.
dataOrigin	For filterDataOrigin: character to define which spectra to keep. For filterAcquisitionNum: optionally specify if filtering should occurr only for spectra of selected dataOrigin.
intensity	For filterIntensity: numeric of length 1 or 2 defining either the lower or the lower and upper intensity limit for the filtering, or a function that takes the intensities as input and returns a logical (same length then peaks in the spectrum) whether the peak should be retained or not. Defaults to intensity = c(0, Inf) thus only peaks with NA intensity are removed.
msLevel.	integer defining the MS level(s) of the spectra to which the function should be applied (defaults to all MS levels of object. For filterMsLevel: the MS level to which object should be subsetted.
polarity	for filterPolarity: integer specifying the polarity to to subset object.
acquisitionNum	for filterPrecursorScan: integer with the acquisition number of the spectra to which the object should be subsetted.
rt	for filterRt: numeric(2) defining the retention time range to be used to subset/filter object.
binSize	For bin: numeric(1) defining the size for the m/z bins. Defaults to binSize = 1.
breaks	For bin: numeric defining the m/z breakpoints between bins.
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.
SIMPLIFY	For compareSpectra whether the result matrix should be simplified to a numeric if possible (i.e. if either x or y is of length 1).
halfWindowSize	For pickPeaks: integer(1), used in the identification of the mass peaks: a local maximum has to be the maximum in the window from (i - halfWindowSize):(i + halfWindowSize). For smooth: integer(1), used in the smoothing algorithm, the window reaches from (i - halfWindowSize):(i + halfWindowSize).
method	For pickPeaks: character(1), the noise estimators that should be used, currently the the Median Absolute Deviation (method = "MAD") and Friedman's Super Smoother (method = "SuperSmoother") are supported. For smooth: character(1), the smoothing function that should be used, currently, the Moving-Average- (method = "MovingAverage"), Weighted-Moving-Average- (method = "WeightedMovingAverage"), Savitzky-Golay-Smoothing (method = "SavitzkyGolay") are supported.
snr	For pickPeaks: double(1) defining the Signal-to-Noise-Ratio. The intensity of a local maximum has to be higher than snr * noise to be considered as peak.
k	For pickPeaks: integer(1), number of values left and right of the peak that should be considered in the weighted mean calculation.
descending	For pickPeaks: logical, if TRUE just values between the nearest valleys around the peak centroids are used.
threshold	For pickPeaks: a double(1) defining the proportion of the maximal peak intensity. Just values above are used for the weighted mean calculation. For replaceIntensitiesBelow: a numeric(1) defining the threshold or a function to calculate the threshold for each spectrum on its intensity values. Defaults to threshold = min.

Details

The Spectra class uses by default a lazy data manipulation strategy, i.e. data manipulations such as performed with replaceIntensitiesBelow are not applied immediately to the data, but applied on-the-fly to the spectrum data once it is retrieved. For some backends that allow to write data back to the data storage (such as the MsBackendDataFrame() and MsBackendHdf5Peaks()) it is possible to apply to queue with the applyProcessing function. See the Data manipulation and analysis methods section below for more details.

For details on plotting spectra, see plotSpectra().

Value

See individual method description for the return value.

Creation of objects, conversion, changing the backend and export

Spectra classes can be created with the Spectra constructor function which supports the following formats:

• parameter object is a DataFrame containing the spectrum data. The provided backend (by default a MsBackendDataFrame) will be initialized with that data.

• parameter object is a MsBackend (assumed to be already initialized).

• parameter object is missing, in which case it is supposed that the data is provided by the MsBackend class passed along with the backend argument.

• parameter object is of type character and is expected to be the file names(s) from which spectra should be imported. Parameter source allows to define a MsBackend that is able to import the data from the provided source files. The default value for source is MsBackendMzR() which allows to import spectra data from mzML, mzXML or CDF files.

With ... additional arguments can be passed to the backend's backendInitialize() method. Parameter backend allows to specify which MsBackend should be used for data storage.

The backend of a Spectra object can be changed with the setBackend method that takes an instance of the new backend as second parameter backend. A call to setBackend(sps, backend = MsBackendDataFrame()) would for example change the backend or sps to the in-memory MsBackendDataFrame. Note that it is not possible to change the backend to a read-only backend (such as the MsBackendMzR() backend). setBackend changes the "dataOrigin" variable of the resulting Spectra object to the "dataStorage" variable of the backend before the switch.

The definition of the function is: setBackend(object, backend, ..., f = dataStorage(object), BPPARAM = bpparam()) and its parameters are:

• parameter object: the Spectra object.

• parameter backend: an instance of the new backend, e.g. MsBackendDataFrame().

• parameter f: factor allowing to parallelize the change of the backends. By default the process of copying the spectra data from the original to the new backend is performed separately (and in parallel) for each file. Users are advised to use the default setting.

• parameter ...: optional additional arguments passed to the backendInitialize() method of the new backend.

• parameter BPPARAM: setup for the parallel processing. See bpparam() for details.

Data from a Spectra object can be exported to a file with the export function. The actual export of the data has to be performed by the export method of the MsBackend class defined with the mandatory parameter backend. Note however that not all backend classes support export of data. From the MsBackend classes in the Spectra package currently only the MsBackendMzR backend supports data export (to mzML/mzXML file(s)); see the help page of the MsBackend for information on its arguments or the examples below or the vignette for examples.

The definition of the function is export(object, backend, ...) and its parameters are:

• object: the Spectra object to be exported.

• backend: instance of a class extending MsBackend which supports export of the data (i.e. which has a defined export method).

• ...: additional parameters specific for the MsBackend passed with parameter backend.

Accessing spectra data

• $, $<-: gets (or sets) a spectra variable for all spectra in object. See examples for details.

• acquisitionNum: returns the acquisition number of each spectrum. Returns an integer of length equal to the number of spectra (with NA_integer_ if not available).

• peaksData: gets the peaks matrices for all spectra in object. The function returns a SimpleList() of matrices, each matrix with columns "mz" and "intensity" with the m/z and intensity values for all peaks of a spectrum. Note that it is also possible to extract the peaks matrices with as(x, "list") and as(x, "SimpleList") as a list and SimpleList, respectively.

• centroided, centroided<-: gets or sets the centroiding information of the spectra. centroided returns a logical vector of length equal to the number of spectra with TRUE if a spectrum is centroided, FALSE if it is in profile mode and NA if it is undefined. See also isCentroided for estimating from the spectrum data whether the spectrum is centroided. value for centroided<- is either a single logical or a logical of length equal to the number of spectra in object.

• collisionEnergy, collisionEnergy<-: gets or sets the collision energy for all spectra in object. collisionEnergy returns a numeric with length equal to the number of spectra (NA_real_ if not present/defined), collisionEnergy<- takes a numeric of length equal to the number of spectra in object.

• dataOrigin, dataOrigin<-: gets or sets the data origin for each spectrum. dataOrigin returns a character vector (same length than object) with the origin of the spectra. dataOrigin<- expects a character vector (same length than object) with the replacement values for the data origin of each spectrum.

• dataStorage: returns a character vector (same length than object) with the data storage location of each spectrum.

• intensity: gets the intensity values from the spectra. Returns a NumericList() of numeric vectors (intensity values for each spectrum). The length of the list is equal to the number of spectra in object.

• ionCount: returns a numeric with the sum of intensities for each spectrum. If the spectrum is empty (see isEmpty), NA_real_ is returned.

• isCentroided: a heuristic approach assessing if the spectra in object are in profile or centroided mode. The function takes the qtlth quantile top peaks, then calculates the difference between adjacent m/z value and returns TRUE if the first quartile is greater than k. (See Spectra:::.isCentroided for the code.)

• isEmpty: checks whether a spectrum in object is empty (i.e. does not contain any peaks). Returns a logical vector of length equal number of spectra.

• isolationWindowLowerMz, isolationWindowLowerMz<-: gets or sets the lower m/z boundary of the isolation window.

• isolationWindowTargetMz, isolationWindowTargetMz<-: gets or sets the target m/z of the isolation window.

• isolationWindowUpperMz, isolationWindowUpperMz<-: gets or sets the upper m/z boundary of the isolation window.

• containsMz: checks for each of the spectra whether they contain mass peaks with an m/z equal to mz (given acceptable difference as defined by parameters tolerance and ppm - see common() for details). Parameter which allows to define whether any (which = "any", the default) or all (which = "all") of the mz have to match. The function returns NA if mz is of length 0 or is NA.

• containsNeutralLoss: checks for each spectrum in object if it has a peak with an m/z value equal to its precursor m/z - neutralLoss (given acceptable difference as defined by parameters tolerance and ppm). Returns NA for MS1 spectra (or spectra without a precursor m/z).

• length: gets the number of spectra in the object.

• lengths: gets the number of peaks (m/z-intensity values) per spectrum. Returns an integer vector (length equal to the number of spectra). For empty spectra, 0 is returned.

• msLevel: gets the spectra's MS level. Returns an integer vector (names being spectrum names, length equal to the number of spectra) with the MS level for each spectrum.

• mz: gets the mass-to-charge ratios (m/z) from the spectra. Returns a NumericList() or length equal to the number of spectra, each element a numeric vector with the m/z values of one spectrum.

• polarity, polarity<-: gets or sets the polarity for each spectrum. polarity returns an integer vector (length equal to the number of spectra), with 0 and 1 representing negative and positive polarities, respectively. polarity<- expects an integer vector of length 1 or equal to the number of spectra.

• precursorCharge, precursorIntensity, precursorMz, precScanNum, precAcquisitionNum: gets the charge (integer), intensity (numeric), m/z (numeric), scan index (integer) and acquisition number (interger) of the precursor for MS level > 2 spectra from the object. Returns a vector of length equal to the number of spectra in object. NA are reported for MS1 spectra of if no precursor information is available.

• rtime, rtime<-: gets or sets the retention times (in seconds) for each spectrum. rtime returns a numeric vector (length equal to the number of spectra) with the retention time for each spectrum. rtime<- expects a numeric vector with length equal to the number of spectra.

• scanIndex: returns an integer vector with the scan index for each spectrum. This represents the relative index of the spectrum within each file. Note that this can be different to the acquisitionNum of the spectrum which represents the index of the spectrum during acquisition/measurement (as reported in the mzML file).

• smoothed,smoothed<-: gets or sets whether a spectrum is smoothed. smoothed returns a logical vector of length equal to the number of spectra. smoothed<- takes a logical vector of length 1 or equal to the number of spectra in object.

• spectraData: gets general spectrum metadata (annotation, also called header). spectraData returns a DataFrame. Note that this method does by default not return m/z or intensity values.

• spectraData<-: replaces the full spectra data of the Spectra object with the one provided with value. The use of this function is disencouraged, as replacing spectra data with values that are in a different can break the linkage with the associated m/z and intensity values. If possible, spectra variables (i.e. columns of the Spectra) should be replaced individually. The spectraData<- function expects a DataFrame to be passed as value.

• spectraNames, spectraNames<-: gets or sets the spectra names.

• spectraVariables: returns a character vector with the available spectra variables (columns, fields or attributes) available in object.

• tic: gets the total ion current/count (sum of signal of a spectrum) for all spectra in object. By default, the value reported in the original raw data file is returned. For an empty spectrum, 0 is returned.

Data subsetting, filtering and merging

Subsetting and filtering of Spectra objects can be performed with the below listed methods.

• [: subsets the spectra keeping only selected elements (i). The method always returns a Spectra object.

• dropNaSpectraVariables: removes spectra variables (i.e. columns in the object's spectraData that contain only missing values (NA). Note that while columns with only NAs are removed, a spectraData call after dropNaSpectraVariables might still show columns containing NA values for core spectra variables.

• filterAcquisitionNum: filters the object keeping only spectra matching the provided acquisition numbers (argument n). If dataOrigin or dataStorage is also provided, object is subsetted to the spectra with an acquisition number equal to n in spectra with matching dataOrigin or dataStorage values retaining all other spectra. Returns the filtered Spectra.

• filterDataOrigin: filters the object retaining spectra matching the provided dataOrigin. Parameter dataOrigin has to be of type character and needs to match exactly the data origin value of the spectra to subset. Returns the filtered Spectra object (with spectra ordered according to the provided dataOrigin parameter).

• filterDataStorage: filters the object retaining spectra stored in the specified dataStorage. Parameter dataStorage has to be of type character and needs to match exactly the data storage value of the spectra to subset. Returns the filtered Spectra object (with spectra ordered according to the provided dataStorage parameter).

• filterEmptySpectra: removes empty spectra (i.e. spectra without peaks). Returns the filtered Spectra object (with spectra in their original order).

• filterIsolationWindow: retains spectra that contain mz in their isolation window m/z range (i.e. with an isolationWindowLowerMz <= mz and isolationWindowUpperMz >= mz. Returns the filtered Spectra object (with spectra in their original order).

• filterMsLevel: filters object by MS level keeping only spectra matching the MS level specified with argument msLevel. Returns the filtered Spectra (with spectra in their original order).

• filterMzRange: filters the object keeping only peaks in each spectrum that are within the provided m/z range.

• filterMzValues: filters the object keeping only peaks in each spectrum that match the provided m/z value(s) considering also the absolute tolerance and m/z-relative ppm (tolerance and ppm can be either of length 1 or equal to the length of mz to define a different tolerance for each m/z).

• filterPolarity: filters the object keeping only spectra matching the provided polarity. Returns the filtered Spectra (with spectra in their original order).

• filterPrecursorMz: retains spectra with a precursor m/z within the provided m/z range. See examples for details on selecting spectra with a precursor m/z for a target m/z accepting a small difference in ppm.

• filterPrecursorScan: retains parent (e.g. MS1) and children scans (e.g. MS2) of acquisition number acquisitionNum. Returns the filtered Spectra (with spectra in their original order).

• filterRt: retains spectra of MS level msLevel with retention times (in seconds) within (>=) rt[1] and (<=) rt[2]. Returns the filtered Spectra (with spectra in their original order).

• reset: restores the data to its original state (as much as possible): removes any processing steps from the lazy processing queue and calls reset on the backend which, depending on the backend, can also undo e.g. data filtering operations. Note that a reset call after applyProcessing will not have any effect. See examples below for more information.

• selectSpectraVariables: reduces the information within the object to the selected spectra variables: all data for variables not specified will be dropped. For mandatory columns (such as msLevel, rtime ...) only the values will be dropped, while additional (user defined) spectra variables will be completely removed. Returns the filtered Spectra.

• split: splits the Spectra object based on parameter f into a list of Spectra objects.

Several Spectra objects can be concatenated into a single object with the c function. Concatenation will fail if the processing queue of any of the Spectra objects is not empty or if different backends are used in the Spectra objects. The spectra variables of the resulting Spectra object is the union of the spectra variables of the individual Spectra objects.

Data manipulation and analysis methods

Many data manipulation operations, such as those listed in this section, are not applied immediately to the spectra, but added to a lazy processing/manipulation queue. Operations stored in this queue are applied on-the-fly to spectra data each time it is accessed. This lazy execution guarantees the same functionality for Spectra objects with any backend, i.e. backends supporting to save changes to spectrum data (MsBackendDataFrame() or MsBackendHdf5Peaks()) as well as read-only backends (such as the MsBackendMzR()). Note that for the former it is possible to apply the processing queue and write the modified peak data back to the data storage with the applyProcessing function.

• addProcessing: adds an arbitrary function that should be applied to the peaks matrix of every spectrum in object. The function (can be passed with parameter FUN) is expected to take a peaks matrix as input and to return a peaks matrix. A peaks matrix is a numeric matrix with two columns, the first containing the m/z values of the peaks and the second the corresponding intensities. The function has to have ... in its definition. Additional arguments can be passed with .... Examples are provided in the package vignette.

• applyProcessing: for Spectra objects that use a writeable backend only: apply all steps from the lazy processing queue to the peak data and write it back to the data storage. Parameter f allows to specify how object should be split for parallel processing. This should either be equal to the dataStorage, or f = rep(1, length(object)) to disable parallel processing alltogether. Other partitionings might result in errors (especially if a MsBackendHdf5Peaks backend is used).

• bin: aggregates individual spectra into discrete (m/z) bins. All intensity values for peaks falling into the same bin are summed.

• combineSpectra: combine sets of spectra into a single spectrum per set. For each spectrum group (set), spectra variables from the first spectrum are used and the peak matrices are combined using the function specified with FUN, which defaults to combinePeaks(). The sets of spectra can be specified with parameter f. In addition it is possible to define, with parameter p if and how to split the input data for parallel processing. This defaults to p = x$dataStorage and hence a per-file parallel processing is applied for Spectra with file-based backends (such as the MsBackendMzR()). Prior combination of the spectra all processings queued in the lazy evaluation queue are applied. Be aware that calling combineSpectra on a Spectra object with certain backends that allow modifications might overwrite the original data. This does not happen with a MsBackendDataFrame backend, but with a MsBackendHdf5Peaks backend the m/z and intensity values in the original hdf5 file(s) will be overwritten. The function returns a Spectra of length equal to the unique levels of f.

• compareSpectra: compare 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). 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. Additional parameters to functions FUN and MAPFUN can be passed with .... 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.

• filterIntensity: filters each spectrum keeping only peaks with intensities that are within the provided range or match the criteria of the provided function. For the former, parameter intensity has to be a numeric defining the intensity range, for the latter a function that takes the intensity values of the spectrum and returns a logical whether the peak should be retained or not (see examples below for details) - additional parameters to the function can be passed with .... To remove only peaks with intensities below a certain threshold, say 100, use intensity = c(100, Inf). Note: also a single value can be passed with the intensity parameter in which case an upper limit of Inf is used. Note that this function removes also peaks with missing intensities (i.e. an intensity of NA). Parameter msLevel. allows to restrict the filtering to spectra of the specified MS level(s).

• spectrapply: apply a given function to each spectrum in a Spectra object. The Spectra is splitted into individual spectra and on each of them (i.e. Spectra of length 1) the function FUN is applied. Additional parameters to FUN can be passed with the ... argument. Parameter BPPARAM allows to enable parallel processing, which however makes only sense if FUN is computational intense. spectrapply returns a list (same length than object) with the result from FUN. See examples for more details. Note that the result and its order depends on the factor f used for splitting object with split, i.e. no re-ordering or unsplit is performed on the result.

• smooth: smooth individual spectra using a moving window-based approach (window size = 2 * halfWindowSize). Currently, the Moving-Average- (method = "MovingAverage"), Weighted-Moving-Average- (method = "WeightedMovingAverage"), weights depending on the distance of the center and calculated 1/2^(-halfWindowSize:halfWindowSize)) and Savitzky-Golay-Smoothing (method = "SavitzkyGolay") are supported. For details how to choose the correct halfWindowSize please see MsCoreUtils::smooth().

• pickPeaks: picks peaks on individual spectra using a moving window-based approach (window size = 2 * halfWindowSize). For noisy spectra there are currently two different noise estimators available, the Median Absolute Deviation (method = "MAD") and Friedman's Super Smoother (method = "SuperSmoother"), as implemented in the MsCoreUtils::noise(). The method supports also to optionally refine the m/z value of the identified centroids by considering data points that belong (most likely) to the same mass peak. Therefore the m/z value is calculated as an intensity weighted average of the m/z values within the peak region. The peak region is defined as the m/z values (and their respective intensities) of the 2 * k closest signals to the centroid or the closest valleys (descending = TRUE) in the 2 * k region. For the latter the k has to be chosen general larger. See MsCoreUtils::refineCentroids() for details. If the ratio of the signal to the highest intensity of the peak is below threshold it will be ignored for the weighted average.

• replaceIntensitiesBelow: replaces intensities below a specified threshold with the provided value. Parameter threshold can be either a single numeric value or a function which is applied to all non-NA intensities of each spectrum to determine a threshold value for each spectrum. The default is threshold = min which replaces all values which are <= the minimum intensity in a spectrum with value (the default for value is 0). Note that the function specified with threshold is expected to have a parameter na.rm since na.rm = TRUE will be passed to the function. If the spectrum is in profile mode, ranges of successive non-0 peaks <= threshold are set to 0. Parameter msLevel. allows to apply this to only spectra of certain MS level(s).

Author(s)

Sebastian Gibb, Johannes Rainer

Examples

## Create a Spectra providing a `DataFrame` containing the spectrum data.

spd <- DataFrame(msLevel = c(1L, 2L), rtime = c(1.1, 1.2))
spd$mz <- list(c(100, 103.2, 104.3, 106.5), c(45.6, 120.4, 190.2))
spd$intensity <- list(c(200, 400, 34.2, 17), c(12.3, 15.2, 6.8))

data <- Spectra(spd)
data

## Get the number of spectra
length(data)

## Get the number of peaks per spectrum
lengths(data)

## Create a Spectra from mzML files and use the `MsBackendMzR` on-disk
## backend.
sciex_file <- dir(system.file("sciex", package = "msdata"),
    full.names = TRUE)
sciex <- Spectra(sciex_file, backend = MsBackendMzR())
sciex

## The MS data is on disk and will be read into memory on-demand. We can
## however change the backend to a MsBackendDataFrame backend which will
## keep all of the data in memory.
sciex_im <- setBackend(sciex, MsBackendDataFrame())
sciex_im

## The on-disk object `sciex` is light-weight, because it does not keep the
## MS peak data in memory. The `sciex_im` object in contrast keeps all the
## data in memory and its size is thus much larger.
object.size(sciex)
object.size(sciex_im)

## The spectra variable `dataStorage` returns for each spectrum the location
## where the data is stored. For in-memory objects:
head(dataStorage(sciex_im))

## While objects that use an on-disk backend will list the files where the
## data is stored.
head(dataStorage(sciex))

## The spectra variable `dataOrigin` returns for each spectrum the *origin*
## of the data. If the data is read from e.g. mzML files, this will be the
## original mzML file name:
head(dataOrigin(sciex))
head(dataOrigin(sciex_im))

## ---- ACCESSING AND ADDING DATA ----

## Get the MS level for each spectrum.
msLevel(data)

## Alternatively, we could also use $ to access a specific spectra variable.
## This could also be used to add additional spectra variables to the
## object (see further below).
data$msLevel

## Get the intensity and m/z values.
intensity(data)
mz(data)

## Determine whether one of the spectra has a specific m/z value
containsMz(data, mz = 120.4)

## Accessing spectra variables works for all backends:
intensity(sciex)
intensity(sciex_im)

## Get the m/z for the first spectrum.
mz(data)[[1]]

## Get the peak data (m/z and intensity values).
pks <- peaksData(data)
pks
pks[[1]]
pks[[2]]

## Note that we could get the same resulb by coercing the `Spectra` to
## a `list` or `SimpleList`:
as(data, "list")
as(data, "SimpleList")

## List all available spectra variables (i.e. spectrum data and metadata).
spectraVariables(data)

## For all *core* spectrum variables accessor functions are available. These
## return NA if the variable was not set.
centroided(data)
dataStorage(data)
rtime(data)
precursorMz(data)

## Add an additional metadata column.
data$spectrum_id <- c("sp_1", "sp_2")

## List spectra variables, "spectrum_id" is now also listed
spectraVariables(data)

## Get the values for the new spectra variable
data$spectrum_id

## Extract specific spectra variables.
spectraData(data, columns = c("spectrum_id", "msLevel"))

## Drop spectra variable data and/or columns.
res <- selectSpectraVariables(data, c("mz", "intensity"))

## This removed the additional columns "spectrum_id" and deleted all values
## for all spectra variables, except "mz" and "intensity".
spectraData(res)

## Compared to the data before selectSpectraVariables.
spectraData(data)


## ---- SUBSETTING, FILTERING AND COMBINING

## Subset to all MS2 spectra.
data[msLevel(data) == 2]

## Same with the filterMsLevel function
filterMsLevel(data, 2)

## Below we combine the `data` and `sciex_im` objects into a single one.
data_comb <- c(data, sciex_im)

## The combined Spectra contains a union of all spectra variables:
head(data_comb$spectrum_id)
head(data_comb$rtime)
head(data_comb$dataStorage)
head(data_comb$dataOrigin)

## Filter a Spectra for a target precursor m/z with a tolerance of 10ppm
spd$precursorMz <- c(323.4, 543.2302)
data_filt <- Spectra(spd)
filterPrecursorMz(data_filt, mz = 543.23 + ppm(c(-543.23, 543.23), 10))

## Filter a Spectra keeping only peaks matching certain m/z values
sps_sub <- filterMzValues(data, mz = c(103, 104), tolerance = 0.3)
mz(sps_sub)

## Filter a Spectra keeping only peaks within a m/z range
sps_sub <- filterMzRange(data, mz = c(100, 300))
mz(sps_sub)

## Remove empty spectra variables
sciex_noNA <- dropNaSpectraVariables(sciex)

## Available spectra variables before and after dropNaSpectraVariables
spectraVariables(sciex)
spectraVariables(sciex_noNA)


## ---- DATA MANIPULATIONS AND OTHER OPERATIONS ----

## Set the data to be centroided
centroided(data) <- TRUE

## Replace peak intensities below 40 with 3.
res <- replaceIntensitiesBelow(data, threshold = 40, value = 3)
res

## Get the intensities of the first and second spectrum.
intensity(res)[[1]]
intensity(res)[[2]]

## Remove all peaks with an intensity below 40.
res <- filterIntensity(res, intensity = c(40, Inf))

## Get the intensities of the first and second spectrum.
intensity(res)[[1]]
intensity(res)[[2]]

## Lengths of spectra is now different
lengths(mz(res))
lengths(mz(data))

## In addition it is possible to pass a function to `filterIntensity`: in
## the example below we want to keep only peaks that have an intensity which
## is larger than one third of the maximal peak intensity in that spectrum.
keep_peaks <- function(x, prop = 3) {
    x > max(x, na.rm = TRUE) / prop
}
res2 <- filterIntensity(data, intensity = keep_peaks)
intensity(res2)[[1L]]
intensity(data)[[1L]]

## We can also change the proportion by simply passing the `prop` parameter
## to the function. To keep only peaks that have an intensity which is
## larger than half of the maximum intensity:
res2 <- filterIntensity(data, intensity = keep_peaks, prop = 2)
intensity(res2)[[1L]]
intensity(data)[[1L]]

## Since data manipulation operations are by default not directly applied to
## the data but only added to the internal lazy evaluation queue, it is also
## possible to remove these data manipulations with the `reset` function:
res_rest <- reset(res)
res_rest
lengths(mz(res_rest))
lengths(mz(res))
lengths(mz(data))

## `reset` after a `applyProcessing` can not restore the data, because the
## data in the backend was changed. Similarly, `reset` after any filter
## operations can not restore data for a `Spectra` with a
## `MsBackendDataFrame`.
res_2 <- applyProcessing(res)
res_rest <- reset(res_2)
lengths(mz(res))
lengths(mz(res_rest))


## Compare spectra: comparing spectra 2 and 3 against spectra 10:20 using
## the normalized dotproduct method.
res <- compareSpectra(sciex_im[2:3], sciex_im[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

## To use a simple Pearson correlation instead we can define a function
## that takes the two peak matrices and calculates the correlation for
## their second columns (containing the intensity values).
correlateSpectra <- function(x, y, use = "pairwise.complete.obs", ...) {
    cor(x[, 2], y[, 2], use = use, ...)
}
res <- compareSpectra(sciex_im[2:3], sciex_im[10:20],
    FUN = correlateSpectra)
res

## 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(sciex_im[2:3], sciex_im[10:20], ppm = 10, type = "inner",
    FUN = function(x, y, ...) nrow(x))

## Apply an arbitrary function to each spectrum in a Spectra.
## In the example below we calculate the mean intensity for each spectrum
## in a subset of the sciex_im data. Note that we can access all variables
## of each individual spectrum either with the `$` operator or the
## corresponding method.
res <- spectrapply(sciex_im[1:20], FUN = function(x) mean(x$intensity[[1]]))
head(res)

## It is however important to note that dedicated methods to access the
## data (such as `intensity`) are much more efficient than using `lapply`:
res <- lapply(intensity(sciex_im[1:20]), mean)
head(res)


## ---- DATA EXPORT ----

## Some `MsBackend` classes provide an `export` method to export the data to
## the file format supported by the backend. The `MsBackendMzR` for example
## allows to export MS data to mzML or mzXML file(s), the `MsBackendMgf`
## (defined in the MsBackendMgf R package) would allow to export the data
## in mgf file format. Below we export the MS data in `data`. We
## call the `export` method on this object, specify the backend that should
## be used to export the data (and which also defines the output format) and
## provide a file name.
fl <- tempfile()
export(data, MsBackendMzR(), file = fl)

## This exported our data in mzML format. Below we read the first 6 lines
## from that file.
readLines(fl, n = 6)

## If only a single file name is provided, all spectra are exported to that
## file. To export data with the `MsBackendMzR` backend to different files, a
## file name for each individual spectrum has to be provided.
## Below we export each spectrum to its own file.
fls <- c(tempfile(), tempfile())
export(data, MsBackendMzR(), file = fls)

## Reading the data from the first file
res <- Spectra(backendInitialize(MsBackendMzR(), fls[1]))

mz(res)
mz(data)

Spectra documentation built on Nov. 27, 2020, 2 a.m.