Metabolomics data pre-processing using xcms (original) (raw)
This workflow describes the basic data handling (I/O) of mass spectrometry data using the MSnbase package, and the LC/GC-MS data preprocessing using xcms. It showcases the new functionality and user interface functions of xcms, that re-use functionality from the MSnbase package. The first part of the workflow is focused on data import, access and visualization which is followed by the description of a simple data centroiding approach and finally the xcms-based LC-MS data preprocessing that comprises chromatographic peak detection, alignment and correspondence. The workflow does not cover data normalization procedures, compound identification and differential abundance analysis.
Data import and representation
The example data set of this workflow consists of two files in mzML format with signals from pooled human serum samples measured with a ultra high performance liquid chromatography (UHPLC) system (Agilent 1290) coupled with a Q-TOF MS (TripleTOF 5600+ AB Sciex) instrument. Chromatographic separation was based on hydrophilic interaction liquid chromatography (HILIC) separating metabolites depending on their polarity. The setup thus allows to measure small polar compounds and hence metabolites from the main metabolic pathways. The input files contain all signals measured by the MS instrument (so called profile mode data). To reduce file sizes, the data set was restricted to an m/z range from 105 to 134 and retention times from 0 to 260 seconds.
In the code block below we first load all required libraries and define the location of the mzML files, which are part of the msdata package. We also define a data.frame describing the samples/experiment and pass this to thereadMSData function which imports the data. The option mode = "onDisk" tells the function to read only general metadata into memory. The m/z and intensity values are not imported but retrieved from the original files on demand, which enables also analyses of very large experiments.
library(xcms)
library(magrittr)
## Define the file names.
fls <- dir(system.file("sciex", package = "msdata"), full.names = TRUE)
## Define a data.frame with additional information on the files.
pd <- data.frame(file = basename(fls),
injection_idx = c(1, 19),
sample = c("POOL_1", "POOL_2"),
group = "POOL")
data <- readMSData(fls, pdata = new("NAnnotatedDataFrame", pd),
mode = "onDisk") Next we set up parallel processing. This ensures that all required cores are registered and available from the beginning of the analysis. All data access and analysis functions of xcms and MSnbase are parallelized on a per-file basis and will use this setup by default.
#' Set up parallel processing using 2 cores
if (.Platform$OS.type == "unix") {
register(bpstart(MulticoreParam(2)))
} else {
register(bpstart(SnowParam(2)))
}The MS experiment data is now represented as an OnDiskMSnExp object. Phenotype information can be retrieved with the pData function, single columns in the phenotype table using $. Below we access sample descriptions.
#' Access phenotype information
pData(data)## file injection_idx sample group
## 1 20171016_POOL_POS_1_105-134.mzML 1 POOL_1 POOL
## 2 20171016_POOL_POS_3_105-134.mzML 19 POOL_2 POOL#' Or individual columns directly using the $ operator
data$injection_idx ## [1] 1 19General information on each spectrum in the experiment can be accessed with thefData function, that returns a data.frame each row with metadata information for one spectrum.
#' Access spectrum header information
head(fData(data), n = 3) ## fileIdx spIdx smoothed seqNum acquisitionNum msLevel polarity
## F1.S001 1 1 NA 1 1 1 1
## F1.S002 1 2 NA 2 2 1 1
## F1.S003 1 3 NA 3 3 1 1
## originalPeaksCount totIonCurrent retentionTime basePeakMZ
## F1.S001 578 898185 0.280 124.0860
## F1.S002 1529 1037012 0.559 124.0859
## F1.S003 1600 1094971 0.838 124.0859
## basePeakIntensity collisionEnergy ionisationEnergy lowMZ highMZ
## F1.S001 154089 NA 0 105.0435 133.9837
## F1.S002 182690 NA 0 105.0275 133.9836
## F1.S003 196650 NA 0 105.0376 133.9902
## precursorScanNum precursorMZ precursorCharge precursorIntensity
## F1.S001 NA NA NA NA
## F1.S002 NA NA NA NA
## F1.S003 NA NA NA NA
## mergedScan mergedResultScanNum mergedResultStartScanNum
## F1.S001 NA NA NA
## F1.S002 NA NA NA
## F1.S003 NA NA NA
## mergedResultEndScanNum injectionTime filterString
## F1.S001 NA 0 <NA>
## F1.S002 NA 0 <NA>
## F1.S003 NA 0 <NA>
## spectrumId centroided ionMobilityDriftTime
## F1.S001 sample=1 period=1 cycle=1 experiment=1 FALSE NA
## F1.S002 sample=1 period=1 cycle=2 experiment=1 FALSE NA
## F1.S003 sample=1 period=1 cycle=3 experiment=1 FALSE NA
## isolationWindowTargetMZ isolationWindowLowerOffset
## F1.S001 NA NA
## F1.S002 NA NA
## F1.S003 NA NA
## isolationWindowUpperOffset scanWindowLowerLimit scanWindowUpperLimit
## F1.S001 NA NA NA
## F1.S002 NA NA NA
## F1.S003 NA NA NA
## spectrum
## F1.S001 1
## F1.S002 2
## F1.S003 3Basic data access and visualization
The MS data in an OnDiskMSnExp object is organized by spectrum (similar to_mzML_ files), with Spectrum objects used as containers for the m/z and intensity values. General spectrum information can be retrieved using themsLevel, centroided, rtime or polarity functions that return the respective value for all spectra from all files. Here, the fromFile function can be helpful which returns for each spectrum the index of the file in which it was measured. This is shown in the code block below.
#' Get the retention time
head(rtime(data))## F1.S001 F1.S002 F1.S003 F1.S004 F1.S005 F1.S006
## 0.280 0.559 0.838 1.117 1.396 1.675#' Get the retention times splitted by file.
rts <- split(rtime(data), fromFile(data))
#' The result is a list of length 2. The number of spectra per file can
#' then be determined with
lengths(rts) ## 1 2
## 931 931The spectra function can be used to retrieve the list of all spectra (from all files). This will load the full data from all raw files, which can take, depending on the size of the files and number of spectra, relatively long time and requires, depending on the experiment, a considerable amount of memory. In most cases we will however work with sub-sets of the data, and retrieving that can, in the case of indexed mzML, mzXML and CDF files, be very fast. Data objects can be subsetted using the filter functions: filterFile,filterRtime, filterMz or filterMsLevel that filter the data by file, retention time range, m/z range or MS level. To illustrate this we retrieve below all spectra measured between 180 and 181 seconds. These contain the signal from all compounds that eluted from the LC in that time window. Note that we use the pipe operator %>% from the magrittr package for better readability.
#' Get all spectra measured between 180 and 181 seconds
#' Use %>% to avoid nested function calls
sps <- data %>%
filterRt(rt = c(180, 181)) %>%
spectra The result is a list of Spectrum objects. Below we determine the number of spectra we have got.
#' How many spectra?
length(sps) ## [1] 6We can use the fromFile function to determine from which file/sample each spectrum is.
#' From which file?
sapply(sps, fromFile) ## F1.S646 F1.S647 F1.S648 F2.S646 F2.S647 F2.S648
## 1 1 1 2 2 2We have thus 3 spectra per file. Next we plot the data from the last spectrum (i.e. the 3rd spectrum in the present retention time window from the second file).
plot(sps[[6]]) Figure 1: Spectrum at a retention time of about 180 seconds
We can immediately spot several mass peaks in the spectrum, with the largest one at a m/z of about 130 and the second largest at about 106, which matches the expected mass to charge ratio for the [M+H]+ ion (adduct) ofSerine. Serine in its natural state is not charged and can therefore not be measured directly with a MS instrument. Uncharged compounds, such as Serine, have thus to be first ionized to create charged molecules (e.g. using electrospray-ionization as in the present data set). Such ionization would create [M+H]+ ions of Serine, i.e. molecules that consist of Serine plus a hydrogen resulting in single charged ions (with a mass equal to sum of the masses of Serine and hydrogen).
MS data is generally organized by spectrum, but in LC-MS experiments we analyze the data along the retention time axis and hence orthogonally to the spectral data representation. To extract data along the retention time dimension we can use the chromatogram function. This function aggregates intensities for each scan/retention time along the m/z axis (i.e. within each spectrum) and returns the retention time - intensity duplets in a Chromatogram object, one per file. The Chromatogram object supports, similar to the Spectrum object, thertime and intensity functions to access the respective data. Below we use the chromatogram function to extract the total ion chromatogram (TIC) for each file and plot it. The TIC represents the sum of all measured signals per spectrum (i.e. per discrete time point) and provides thus a general information about compound separation by liquid chromatography.
#' Get chromatographic data (TIC) for an m/z slice
chr <- chromatogram(data)
chr## Chromatograms with 1 row and 2 columns
## 1 2
## <Chromatogram> <Chromatogram>
## [1,] length: 931 length: 931
## phenoData with 4 variables
## featureData with 1 variables#' Plot the tic
plot(chr) Figure 2: Total ion chromatogram
The object returned by the chromatogram function arranges the individualChromatogram objects in a two-dimensional array, columns being samples (files) and rows data slices. Below we extract the (total ion) intensities from the TIC of the first file.
ints <- intensity(chr[1, 1])
head(ints) ## F1.S001 F1.S002 F1.S003 F1.S004 F1.S005 F1.S006
## 898185 1037012 1094971 1135015 1106233 1181489The object contains also all phenotype information from the original datavariable. This can be accessed in the same way than for OnDiskMSnExp objects (or most other data objects in Bioconductor).
#' Access the full phenotype data
pData(chr)## file injection_idx sample group
## 1 20171016_POOL_POS_1_105-134.mzML 1 POOL_1 POOL
## 2 20171016_POOL_POS_3_105-134.mzML 19 POOL_2 POOLDepending on the parameter aggregationFun, the function can produce total ion chromatograms (TIC, sum of the signal within a spectrum) with aggregationFun = "sum", or base peak chromatograms (BPC, maximum signal per spectrum) withaggregationFun = "max". The chromatogram function can also be used to generate extracted ion chromatograms (EIC), which contain the signal from a specific m/z range and retention time window, presumably representing the signal of a single ion species. Below we extract and plot the ion chromatogram for Serine after first filtering the data object to the retention time window and m/z range containing signal for this compound.
#' Extract and plot the XIC for Serine
data %>%
filterRt(rt = c(175, 189)) %>%
filterMz(mz = c(106.02, 106.07)) %>%
chromatogram(aggregationFun = "max") %>%
plot() Figure 3: Extracted ion chromatogram for the Serine [M+H]+ ion in both files
The area of such a chromatographic peak is supposed to be proportional to the amount of the corresponding ion in the respective sample and identification and quantification of such peaks is one of the goals of the GC/LC-MS data preprocessing.
Centroiding of profile MS data
MS instruments allow to export data in profile or centroid mode. Profile data contains the signal for all discrete m/z values (and retention times) for which the instrument collected data (Smith et al. 2014). MS instruments continuously sample and record signals and a mass peak for a single ion in one spectrum will thus consist of a multiple intensities at discrete m/z values. Centroiding is the process to reduce these mass peaks to a single representative signal, the centroid. This results in much smaller file sizes, without loosing too much information. xcms, specifically the centWave chromatographic peak detection algorithm, was designed for centroided data, thus, prior to data analysis, profile data, such as the example data used here, should be centroided. TheMSnbase package provides all tools to perform this centroiding (and data smoothing) in R: pickPeaks and smooth.
Below we inspect the profile data for the [M+H]+ ion adduct of Serine. We again subset the data to the m/z and retention time range containing signal from Serine and plot the data with the option type = "XIC", that generates a combined chromatographic and map visualization of the data (i.e. a plot of the individual m/z, rt and intensity data tuples with data points colored by their intensity in the m/z - retention time space).
#' Filter the MS data to the signal from the Serine ion and plot it using
#' type = "XIC"
data %>%
filterRt(rt = c(175, 189)) %>%
filterMz(mz = c(106.02, 106.07)) %>%
plot(type = "XIC") Figure 4: Profile data for Serine
The plot shows all data points measured by the instrument. Each column of data points in the lower panel represents the signal measured at one discrete time point, stored in one spectrum. We can see a distribution of the signal for serine in both retention time and also in m/z dimension.
Next we smooth the data in each spectrum using a Savitzky-Golay filter, which usually improves data quality by reducing noise. Subsequently we perform the centroiding based on a simple peak-picking strategy that reports the maximum signal for each mass peak in each spectrum.
#' Smooth the signal, then do a simple peak picking.
data_cent <- data %>%
smooth(method = "SavitzkyGolay", halfWindowSize = 6) %>%
pickPeaks()
#' Plot the centroided data for Serine
data_cent %>%
filterRt(rt = c(175, 189)) %>%
filterMz(mz = c(106.02, 106.07)) %>%
plot(type = "XIC") Figure 5: Centroided data for Serine
The centroiding reduced the data to a single data point for an ion in each spectrum. For more advanced centroiding options that can also fine-tune the m/z value of the reported centroid see the pickPeaks help or the centroiding vignette in MSnbase.
The raw data was imported using the _onDisk_-mode that does not read the full MS data into memory. Any data manipulation (such as the data smoothing or peak picking above) has thus to be applied on-the-fly to the data each time m/z or intensity values are retrieved. To make any data manipulation on anOnDiskMSnExp object persistent we need to export the data to mzML files and re-read the data again. Below we thus save the centroided data as mzML files and import it again.
#' Write the centroided data to files with the same names in the current
#' directory
fls_new <- basename(fileNames(data))
writeMSData(data_cent, file = fls_new)
#' Read the centroided data.
data_cent <- readMSData(fls_new, pdata = new("NAnnotatedDataFrame", pd),
mode = "onDisk") Preprocessing of LC-MS data
Preprocessing of GC/LC-MS data in untargeted metabolomics experiments aims at quantifying the signal from individual ion species in a data set and consists of the 3 steps chromatographic peak detection, alignment (also called retention time correction) and correspondence (also called peak grouping). The resulting matrix of feature abundances can then be used as an input in downstream analyses including data normalization, identification of features of interest and annotation of features to metabolites.
Chromatographic peak detection
Chromatographic peak detection aims to identify peaks along the retention time axis that represent the signal from individual compounds’ ions. This can be performed with the findChromPeaks function and one of the available algorithms that can be configures with the respective parameter object: passing aMatchedFilterParam to findChromPeaks performs peak detection as described in the original xcms article (Smith et al. 2006). With CentWaveParam a continuous wavelet transformation (CWT)-based peak detection is performed that can detect close-by and partially overlapping peaks with different (retention time) widths(Tautenhahn, Böttcher, and Neumann 2008). With MassifquantParam a Kalman filter-based peak detection can be performed (Conley et al. 2014). Additional peak detection algorithms for direct injection data are also available, but not discussed here.
We use the centWave algorithm that performs peak detection in two steps: first it identifies regions of interest in the m/z - retention time space and subsequently detects peaks in these regions using a continuous wavelet transform (see the original publication for more details). The algorithm can be configured with several parameters (see ?CentWaveParam), the most important ones beingpeakwidth and ppm. peakwidth defines the minimal and maximal expected width of the peak in retention time dimension and depends thus on the setting of the employed LC-MS system making this parameter highly data set dependent. Appropriate values can be estimated based on extracted ion chromatograms of e.g. internal standards or known compounds in the data. Below we extract the chromatographic data for Serine and perform a peak detection on the Chromatogram object using the default parameters for centWave.
#' Get the XIC for serine in all files
srn_chr <- chromatogram(data_cent, rt = c(164, 200),
mz = c(106.03, 106.06),
aggregationFun = "max")
#' Plot the data
par(mfrow = c(1, 1), mar = c(4, 4.5, 1, 1))
plot(srn_chr)Figure 6: XIC for Serine
#' Get default centWave parameters
cwp <- CentWaveParam()
#' "dry-run" peak detection on the XIC.
res <- findChromPeaks(srn_chr, param = cwp)
chromPeaks(res)No peaks were identified by the call above. Looking at the default values for the centWave parameters helps understanding why peak detection failed:
cwp ## Object of class: CentWaveParam
## Parameters:
## ppm: 25
## peakwidth: 20, 50
## snthresh: 10
## prefilter: 3, 100
## mzCenterFun: wMean
## integrate: 1
## mzdiff: -0.001
## fitgauss: FALSE
## noise: 0
## verboseColumns: FALSE
## roiList length: 0
## firstBaselineCheck TRUE
## roiScales length: 0The default settings for peakwidth are 20 to 50 seconds, while from the plot above it is apparent that the chromatographic peak for Serine is about 4 seconds wide. Below we adapt the settings to accommodate peaks ranging from 2 to 10 seconds and re-run the peak detection. In general, it is advised to investigate peak widths for several ions in the data set to determine the most appropriatepeakwidth setting. Note that we use also integrate = 2 which uses a different way to define the peak borders that can (as in the present case) result in better peak boundary estimates (try with the default integrate = 1to compare the difference).
cwp <- CentWaveParam(peakwidth = c(2, 10), integrate = 2)
srn_chr <- findChromPeaks(srn_chr, param = cwp)
#' Plot the data and higlight identified peak area
plot(srn_chr)Figure 7: XIC for Serine with detected chromatographic peak
With our data set-specific peakwidth we were able to detect the peak for Serine. The identified chromatographic peaks have been added to the result object srn_chr and can be extracted/inspected with the chromPeaks function.
chromPeaks(srn_chr)## rt rtmin rtmax into intb maxo sn row column
## [1,] 181.356 178.566 187.215 73995.94 71599.56 37664.94 63 1 1
## [2,] 181.072 178.282 187.210 70373.61 70023.38 38517.76 606 1 2The matrix returned by chromPeaks contains the retention time and m/z range of the peak ("rtmin", "rtmax", "mzmin" and "mzmax" as well as the integrated peak area ("into"), the maximal signal ("maxo") and the signal to noise ratio ("sn").
Another important parameter for centWave is ppm which is used in the initial identification of the regions of interest. In contrast to random noise, the_real_ signal from an ion is expected to yield stable m/z values in consecutive scans (the scattering of the m/z values around the real m/z value of the ion is supposed to be inversely related with its intensity). In centWave, all data points that differ by less than ppm in consecutive spectra are combined into a region of interest that is then subject to the CWT-based peak detection (same as performed above on the XIC). To illustrate this, we plot the data for Serine with the option type = "XIC".
#' Restrict the data to signal from Serine
srn <- data_cent %>%
filterRt(rt = c(179, 186)) %>%
filterMz(mz = c(106.04, 106.06))
#' Plot the data
plot(srn, type = "XIC") We can observe some scattering of the data points in m/z dimension (lower panel in the plot above), that decreases with increasing intensity of the signal.
Below we identify the sample with the highest signal (sum of all intensities) for the selected m/z - rt range. We thus first extract the intensities for all spectra and split these by file. Then we sum all intensities reported for a file.
#' Extract intensities and split them by file. This will return
#' a list of lists.
ints_per_file <- split(intensity(srn), fromFile(srn))
#' For each file, sum up the intensities.
ints_sum <- lapply(ints_per_file, function(x) sum(unlist(x)))
ints_sum## $`1`
## [1] 263581.5
##
## $`2`
## [1] 250987We next calculate the differences in m/z values between consecutive scans in this data subset. We do this for one representative file, selecting the one with the highest total sum of intensities in the selected region.
#' Extract the Serine data for one file as a data.frame
srn_df <- as(filterFile(srn, file = which.max(ints_sum)), "data.frame")
#' The difference between m/z values from consecutive scans expressed
#' in ppm
diff(srn_df$mz) * 1e6 / mean(srn_df$mz) ## [1] 13.695973646 -27.391665930 1.112565444 13.695804399 0.000000000
## [6] -0.158840806 0.000000000 0.000000000 -0.682098923 0.000000000
## [11] 0.000000000 0.007189239 -13.695795336 13.695795336 -12.807200180
## [16] 0.000000000 0.000000000 13.443799681 0.000000000 -13.695795190
## [21] 13.957010392 0.000000000 0.000000000 -14.085629933The difference in m/z values for the Serine data is thus between 0 and 27 ppm. This should ideally be evaluated for several compounds and should be set to a value that allows to capture the full chromatographic peaks for most of the tested compounds. We can next perform the peak detection using our settings for the ppm and peakwidth parameters.
#' Perform peak detection
cwp <- CentWaveParam(peakwidth = c(2, 10), ppm = 30, integrate = 2)
data_cent <- findChromPeaks(data_cent, param = cwp) The result from the findChromPeaks call is an XCMSnExp object which contains all preprocessing results and, by extending the OnDiskMSnExp object, inherits all of its functionality that has been described so far. The results from the peak detection analysis can be accessed with the chromPeaks function, that, with the optional rt and mz parameters, allows to extract identified chromatographic peaks from specific areas in the data. Below we extract all identified peaks for a certain m/z - rt area.
#' Access the peak detection results from a specific m/z - rt area
chromPeaks(data_cent, mz = c(106, 107), rt = c(150, 190)) ## mz mzmin mzmax rt rtmin rtmax into intb
## CP125 106.0625 106.0606 106.0636 173.264 171.869 174.380 516.3588 513.2557
## CP155 106.0506 106.0505 106.0506 181.356 178.845 187.215 73974.5478 73808.6415
## CP454 106.0633 106.0609 106.0652 172.701 170.748 174.375 547.0585 542.8553
## CP495 106.0496 106.0494 106.0508 181.072 178.003 187.210 70373.6099 70153.0356
## maxo sn sample
## CP125 426.6084 46 1
## CP155 37664.9371 750 1
## CP454 381.6084 58 2
## CP495 38517.7622 897 2For each identified peak the m/z and rt value of the apex is reported (columns"mz" and "rt") as well as their ranges ("mzmin", "mzmax", "rtmin","rtmax"), the integrated signal of the peak (i.e. the peak area "into"), the maximal signal of the peak ("maxo"), the signal to noise ratio ("sn") and the index of the sample in which the peak was detected ("sample").
Note that after peak detection, extracted ion chromatogram data will also contain identified chromatographic peaks. Below we extract the EIC for Serine and display the identified peaks for that compound.
eic_serine <- chromatogram(data_cent, mz = c(106.04, 106.06),
rt = c(179, 186))
chromPeaks(eic_serine)## mz mzmin mzmax rt rtmin rtmax into intb
## CP155 106.0506 106.0505 106.0506 181.356 178.845 187.215 73974.55 73808.64
## CP495 106.0496 106.0494 106.0508 181.072 178.003 187.210 70373.61 70153.04
## maxo sn sample row column
## CP155 37664.94 750 1 1 1
## CP495 38517.76 897 2 1 2And plotting this extracted ion chromatogram will also show the identified chromatographic peaks.
plot(eic_serine)A plot of type "XIC" after peak detection will also indicate the rt-m/z region from which signal is integrated (the red rectangle in the plot below).
srn <- data_cent %>%
filterRt(rt = c(175, 188)) %>%
filterMz(mz = c(106.04, 106.06))
plot(srn, type = "XIC")Peak detection will not always work perfectly leading to peak detection artifacts, such as overlapping peaks or artificially split peaks. TherefineChromPeaks function allows to refine peak detection results by either or removing identified peaks exceeding a maximal allower rt width or by merging artificially split chromatographic peaks. For the former a CleanPeaksParamparameter object can be submitted to the function while the latter can be configured with the MergeNeighboringPeaksParam object. Below we post-process the peak detection results merging peaks overlapping in a 4 second window per file if the signal between in between them is lower than 75% of the smaller peak’s maximal intensity. See the MergeNeighboringPeaksParam help page for a detailed description of the settings and the approach.
mpp <- MergeNeighboringPeaksParam(expandRt = 4)
data_cent_pp <- refineChromPeaks(data_cent, param = mpp)Merged peaks can be identified with a TRUE in the "merged" chromatographic peak metadata column (see below).
chromPeakData(data_cent_pp)## DataFrame with 592 rows and 3 columns
## ms_level is_filled merged
## <integer> <logical> <logical>
## CP001 1 FALSE FALSE
## CP002 1 FALSE FALSE
## ... ... ... ...
## CPM663 1 FALSE TRUE
## CPM664 1 FALSE TRUEAn example for a joined (merged) peak is given below
mzr <- c(124.084, 124.088)
rtr <- c(150, 170)
chr_1 <- chromatogram(filterFile(data_cent, 2), mz = mzr, rt = rtr)
chr_2 <- chromatogram(filterFile(data_cent_pp, 2), mz = mzr, rt = rtr)
par(mfrow = c(1, 2))
plot(chr_1)
plot(chr_2)Figure 8: Result from the peak refinement
Left: peaks before merging, right after merging.
At last we replace the data_cent variable with the object containing also the peak refinement results.
data_cent <- data_cent_ppFor quality assessment we could now calculate summary statistics on the identified peaks to e.g. identify samples with much less detected peaks. Also, we can use the plotChromPeaks function to provide some general information on the location of the identified chromatographic peaks in the m/z - rt space.
par(mfrow = c(1, 2))
plotChromPeaks(data_cent, 1)
plotChromPeaks(data_cent, 2) Figure 9: Location of the identified chromatographic peaks in the m/z - rt space
Alignment
While chromatography helps to discriminate better between analytes it is also affected by variances that can lead to shifts in retention times between measurement runs. The alignment step aims to adjust these retention time differences between samples within an experiment. Below we plot the base peak chromatograms of both files of our toy data set to visualize these differences. Note that with peakType = "none" we disable plotting of identified chromatographic peaks that would be drawn by default on chromatograms extracted from an object containing peak detection results.
#' Extract base peak chromatograms
bpc_raw <- chromatogram(data_cent, aggregationFun = "max")
plot(bpc_raw, peakType = "none")Figure 10: BPC of all files
While both samples were measured with the same setup on the same day the two chromatograms are slightly shifted.
Alignment can be performed with xcms using the adjustRtime function that supports the peakGroups (Smith et al. 2006) and the obiwarp (Prince and Marcotte 2006)method. The settings for the algorithms can be defined with thePeakGroupsParam and the ObiwarpParam parameter objects, respectively. Note also that xcms supports now a subset-based alignment which would allow to align a whole data set based on retention time shift adjustments estimated on e.g. QC samples. See the xcms vignettefor more information.
For our example we use the peakGroups method that aligns samples based on the retention times of hook peaks, which should be present in most samples and which, because they are supposed to represent signal from the same ion species, can be used to estimate retention time shifts between samples. Prior to the alignment we thus have to group peaks across samples, which is accomplished by the peakDensity correspondence analysis method. Details about this method and explanations on the choices of its parameters are provided in the next section. After having performed this initial correspondence analysis, we perform the alignment using settings minFraction = 1 and span = 0.6. minFractiondefines the proportion of samples in which a candidate hook peak has to be detected/present. A value of 0.9 would e.g. require for a hook peak to be detected in in 90% of all samples of the experiment. Our data represents replicated measurements of the same sample pool and we can therefore require hook peaks to be present in each file. The parameter span defines the degree of smoothing of the loess function that is used to allow different regions along the retention time axis to be adjusted by a different factor. A value of 0 will most likely cause overfitting, while 1 would perform a constant, linear shift. Values between 0.4 and 0.6 seem to be reasonable for most experiments.
#' Define the settings for the initial peak grouping - details for
#' choices in the next section.
pdp <- PeakDensityParam(sampleGroups = data_cent$group, bw = 1.8,
minFraction = 1, binSize = 0.02)
data_cent <- groupChromPeaks(data_cent, pdp)
#' Define settings for the alignment
pgp <- PeakGroupsParam(minFraction = 1, span = 0.6)
data_cent <- adjustRtime(data_cent, param = pgp) Adjusted retention times are stored, along with the raw retention times, within the result object. Any function accessing retention times (such as rtime) will by default return adjusted retention times from an XCMSnExp object, if present. Note that also the retention times of the identified chromatographic peaks were adjusted by the adjustRtime call. After alignment it is suggested to evaluate alignment results e.g. by inspecting differences between raw and adjusted retention times.
#' Plot the difference between raw and adjusted retention times
plotAdjustedRtime(data_cent) Figure 11: Alignment results
Shown is the difference between raw and adjusted retention times and the hook peaks that were used for the alignment (shown as points).
The difference between raw and adjusted retention time should be reasonable. In our example it is mostly below one second, which is OK since the samples were measured within a short time period and differences are thus expected to be small. Also, hook peaks should ideally be present along the full retention time range. Next we plot the base peak chromatograms before and after alignment. Note that a chromatogram call will always include all detected chromatographic peaks in the requested rt range but for a base peak chromatogram we don’t need this information. With include = "none" in the chromatogram call we tell the function to not include any identified chromatographic peaks in the returned chromatographic data.
par(mfrow = c(2, 1))
#' Plot the raw base peak chromatogram
plot(bpc_raw, peakType = "none")
#' Plot the BPC after alignment
plot(chromatogram(data_cent, aggregationFun = "max", include = "none"))Figure 12: BPC before (top) and after (bottom) alignment
The base peak chromatograms are nicely aligned after retention time adjustment. The impact of the alignment should also be evaluated on known compounds or internal standards. We thus plot below the XIC for Serine before and after alignment.
#' Use adjustedRtime parameter to access raw/adjusted retention times
par(mfrow = c(1, 2), mar = c(4, 4.5, 1, 0.5))
plot(chromatogram(data_cent, mz = c(106.04, 106.06),
rt = c(179, 186), adjustedRtime = FALSE))
plot(chromatogram(data_cent, mz = c(106.04, 106.06),
rt = c(179, 186))) Figure 13: XIC for Serine before (left) and after (right) alignment
The Serine peaks are also nicely aligned after adjustment. Note that if we were not happy with the alignment results we could simply retry with different settings after removing old results with the dropAdjustedRtime function. This function restores also the original retention times of the identified chromatographic peaks.
Correspondence
The final step of the LC-MS preprocessing with xcms is the correspondence analysis, in which chromatographic peaks from the same ion are grouped across samples to form a feature. xcms implements two methods for this purpose:peak density (Smith et al. 2006) and nearest (Katajamaa, Miettinen, and Oresic 2006) that can be configured by passing either a PeakDensityParam or a NearestPeaksParamobject to the groupChromPeaks function. For our example we use the peak density method that iterates through m/z slices in the data and groups chromatographic peaks to features in each slice (within the same or across samples) depending on their retention time and the distribution of chromatographic peaks along the retention time axis. Peaks representing signal from the same ion are expected to have a similar retention time and, if found in many samples, this should also be reflected by a higher peak density at the respective retention time. To illustrate this we extract below an m/z slice containing the Serine peak and use the plotChromPeakDensity function to visualize the distribution of peaks along the retention time axis and to_simulate_ a correspondence analysis based on the provided settings.
#' Get default parameters for the grouping
pdp <- PeakDensityParam(sampleGroups = data_cent$group)
#' Extract a BPC for the m/z slice containing serine
bpc_serine <- chromatogram(data_cent, mz = c(106.04, 106.06),
aggregationFun = "max")
#' Dry-run correspondence and show the results.
plotChromPeakDensity(bpc_serine, param = pdp)Figure 14: BPC for a m/z slice and defined features within this slice based on default settings
The upper panel in the plot above shows the chromatographic data with the identified peaks. The lower panel shows the retention time of identified peaks (x-axis) per sample (y-axis) with the black solid line representing their distribution along the x-axis. Peak groups (features) are indicated with grey rectangles. The peak density correspondence method groups all chromatographic peaks under the same density peak into a feature. With the default settings we were able to group the Serine peak of each sample into a feature. The parameters for the peak density correspondence analysis are:
binSize: m/z width of the bin/slice of data in which peaks are grouped.bwdefines the smoothness of the density function.maxFeatures: maximum number of features to be defined in one bin.minFraction: minimum proportion of samples (of one group!) for which a peak has to be present.minSamples: minimum number of samples a peak has to be present.
The parameters minFraction and minSamples depend on the experimental layout and should be set accordingly. binSize should be set to a small enough value to avoid peaks from different ions, but with similar m/z and retention time, being grouped together. The most important parameter however is bw and, while its default value of 30 was able to correctly group the Serine peaks, it should always be evaluated on other, more complicated, signals too. Below we evaluate the performance of the default parameters on an m/z slice that contains signal from multiple ions with the same m/z, including isomers Betaine and Valine ([M+H]+ m/z 118.08625).
#' Plot the chromatogram for an m/z slice containing Betaine and Valine
mzr <- 118.08625 + c(-0.01, 0.01)
chr <- chromatogram(data_cent, mz = mzr, aggregationFun = "max")
#' Correspondence in that slice using default settings
pdp <- PeakDensityParam(sampleGroups = data_cent$group)
plotChromPeakDensity(chr, param = pdp)Figure 15: Correspondence analysis with default settings on an m/z slice containing signal from multiple ions
With default settings all chromatographic peaks present in the m/z slice were grouped into the same feature. Signal from different ions would thus be treated as a single entity. Below we repeat the analysis with a strongly reduced value for bw.
#' Reducing the bandwidth
pdp <- PeakDensityParam(sampleGroups = data_cent$group, bw = 1.8)
plotChromPeakDensity(chr, param = pdp)Figure 16: Correspondence analysis with reduced bw setting on a m/z slice containing signal from multiple ions
With a bw of 1.8 we successfully grouped the peaks into different features. We can now use these settings for the correspondence analysis on the full data set.
pdp <- PeakDensityParam(sampleGroups = data_cent$group, bw = 1.8,
minFraction = 0.4, binSize = 0.02)
#' Perform the correspondence analysis
data_cent <- groupChromPeaks(data_cent, param = pdp) Next we evaluate the results from the correspondence analysis on a different m/z slice containing isomers Leucine and Isoleucine ([M+H]+ m/z 132.10191). Settingsimulate = FALSE in plotChromPeakDensity will show the actual results from the correspondence analysis.
#' Plot the results for an m/z slice containing Leucine and Isoleucine
mzr <- 132.10191 + c(-0.01, 0.01)
chr <- chromatogram(data_cent, mz = mzr, aggregationFun = "max")
plotChromPeakDensity(chr, simulate = FALSE)Figure 17: Result of correspondence on a slice containing the isomers Leucine and Isoleucine
Despite being very close, chromatographic peaks of isomers were successfully grouped into separate features.
Results from the correspondence analysis can be accessed with thefeatureDefinition function. This function returns a data frame with the retention time and m/z ranges of the apex positions from the peaks assigned to the feature and their respective indices in the chromPeaks matrix.
#' Definition of the features
featureDefinitions(data_cent) ## DataFrame with 355 rows and 10 columns
## mzmed mzmin mzmax rtmed rtmin rtmax npeaks
## <numeric> <numeric> <numeric> <numeric> <numeric> <numeric> <numeric>
## FT001 105.042 105.042 105.042 167.687 167.453 167.920 2
## FT002 105.042 105.042 105.042 157.684 157.684 157.684 1
## ... ... ... ... ... ... ... ...
## FT354 133.973 133.973 133.973 206.864 206.319 207.410 2
## FT355 133.973 133.973 133.973 200.234 200.234 200.234 1
## POOL peakidx ms_level
## <numeric> <list> <integer>
## FT001 2 116,391 1
## FT002 1 115 1
## ... ... ... ...
## FT354 2 269,592 1
## FT355 1 270 1Note that the EIC chr above would also contain the correspondence results for the selected m/z range which could also be accessed withfeatureDefinitions(chr).
Also, we can calculate simple per-feature summary statistic with thefeatureSummary function. This function reports for each feature the total number and the percentage of samples in which a peak was detected and the total numbers and percentage of these samples in which more than one peak was assigned to the feature.
#' Per-feature summary.
head(featureSummary(data_cent)) ## count perc multi_count multi_perc rsd
## FT001 2 100 0 0 0.23642129
## FT002 1 50 0 0 NA
## FT003 2 100 0 0 0.22934418
## FT004 2 100 0 0 0.04687951
## FT005 2 100 0 0 0.22330558
## FT006 2 100 0 0 0.06138922The final result from the LC-MS data preprocessing is a matrix with feature abundances, rows being features, columns samples. Such a matrix can be extracted with the featureValues function from the result object (see further below for an alternative, preferred way to extract preprocessing results with thequantify method). The function takes two additional parameters value andmethod: value defines the column in the chromPeaks table that should be reported in the matrix, and method the approach to handle cases in which more than one peak in a sample is assigned to the feature.
Below we set value = "into" (the default) to extract the total integrated peak area and method = "maxint" to report the peak area of the peak with the largest intensity for features with multiple peaks in a sample.
#' feature intensity matrix
fmat <- featureValues(data_cent, value = "into", method = "maxint")
head(fmat)## 20171016_POOL_POS_1_105-134.mzML 20171016_POOL_POS_3_105-134.mzML
## FT001 3202.7445 2285.2830
## FT002 3605.3915 NA
## FT003 744.8752 1033.2323
## FT004 18126.4603 19369.4039
## FT005 23243.6129 31960.3709
## FT006 673.8201 617.7545While we do have abundances reported for most features, we might also have missing values for some, like for feature FT002 in the second sample above. Such NAs occur if no chromatographic peak was assigned to a feature, either because peak detection failed, or because the corresponding ion is absent in the respective sample. One possibility to deal with such missing values is data imputation. With the fillChromPeaks function, xcms provides however an alternative approach that integrates the signal measured at the m/z - retention time region of the feature in the original files of samples for which an NAwas reported hence filling-in missing peak data. Two different approaches are available to define the m/z - retention time region from which signal should be integrated: ChromPeakAreaParam defines the region based on the m/z and rt range of the identified chromatographic peaks of the feature andFillChromPeaksParam uses columns "mzmin", "mzmax", "rtmin" and "rtmax"in the featureDefinitions data frame. While the FillChromPeaksParamresembles the peak filling-in of the original xcms implementation, it is suggested to use the ChromPeakAreaParam approach instead.
Below we perform the gap-filling with the ChromPeakAreaParam approach.
#' Number of missing values
sum(is.na(fmat))## [1] 130data_cent <- fillChromPeaks(data_cent, param = ChromPeakAreaParam())
#' How many missing values after
sum(is.na(featureValues(data_cent)))## [1] 23fmat_fld <- featureValues(data_cent, value = "into", method = "maxint")
head(fmat_fld) ## 20171016_POOL_POS_1_105-134.mzML 20171016_POOL_POS_3_105-134.mzML
## FT001 3202.7445 2285.2830
## FT002 3605.3915 3184.7652
## FT003 744.8752 1033.2323
## FT004 18126.4603 19369.4039
## FT005 23243.6129 31960.3709
## FT006 673.8201 617.7545With fillChromPeaks we could rescue signal for all but 23 features with missing values. Note that filled-in peak information can also be removed any time with the dropFilledChromPeaks function. Also, setting filled = FALSE in the featureValues function would return only data from detected peaks.
The data analysis would now continue with the feature matrix and could comprise normalization of the abundances, identification of the compounds and differential abundance analysis.
Note also that, in addition to the featureValues method that simply extracts the feature matrix, the pre-processing results can also be converted to aSummarizedExperiment that, besides containing the feature quantification, stores also the pheno data (sample descriptions) and feature definitions. Below we use the quantify method to extract the pre-processing results as a SummarizedExperiment. The function takes any parameter of thefeatureValues function as additional arguments, thus, with the call below we extract only integrated signal from detected peaks and sum up the signals of all chromatographic peaks assigned to a feature in a sample.
library(SummarizedExperiment)
res <- quantify(data_cent, filled = FALSE, method = "sum", value = "into")The SummarizedExperiment is the main data object in Bioconductor to store quantified omics data and hence an ideal container for the LC-MS data pre-processing results. The column annotations can be accessed with colData, the feature definitions (row annotations) with rowData:
colData(res)## DataFrame with 2 rows and 4 columns
## file injection_idx
## <character> <numeric>
## 20171016_POOL_POS_1_105-134.mzML 20171016_POOL_POS_1_.. 1
## 20171016_POOL_POS_3_105-134.mzML 20171016_POOL_POS_3_.. 19
## sample group
## <character> <character>
## 20171016_POOL_POS_1_105-134.mzML POOL_1 POOL
## 20171016_POOL_POS_3_105-134.mzML POOL_2 POOLrowData(res)## DataFrame with 355 rows and 10 columns
## mzmed mzmin mzmax rtmed rtmin rtmax npeaks
## <numeric> <numeric> <numeric> <numeric> <numeric> <numeric> <numeric>
## FT001 105.042 105.042 105.042 167.687 167.453 167.920 2
## FT002 105.042 105.042 105.042 157.684 157.684 157.684 1
## ... ... ... ... ... ... ... ...
## FT354 133.973 133.973 133.973 206.864 206.319 207.410 2
## FT355 133.973 133.973 133.973 200.234 200.234 200.234 1
## POOL peakidx ms_level
## <numeric> <list> <integer>
## FT001 2 116,391 1
## FT002 1 115,654 1
## ... ... ... ...
## FT354 2 269,592 1
## FT355 1 270,699 1The quantified feature abundances can be accessed with the assay method providing with the second argument the name of the assay matrix to extract:
head(assay(res, "raw"))## 20171016_POOL_POS_1_105-134.mzML 20171016_POOL_POS_3_105-134.mzML
## FT001 3202.7445 2285.2830
## FT002 3605.3915 NA
## FT003 744.8752 1033.2323
## FT004 18126.4603 19369.4039
## FT005 23243.6129 31960.3709
## FT006 673.8201 617.7545The SummarizedExperiment supports several such assay matrices (as long as they have the same dimensions). We can thus add for example the feature quantification including also the filled-in signal as an additional assay matrix:
assays(res)$raw_filled <- featureValues(data_cent, method = "sum",
value = "into", filled = TRUE)With that we have now two assay matrices in our result object:
assayNames(res)## [1] "raw" "raw_filled"Analogously we could add normalized data to this object or any other processed values. The advantage of having all the data within one object is obvious: any subsetting of the data ensures that all assays and annotations are subsetted correctly.
One final thing worth mentioning is that XCMSnExp objects keep, next to the preprocessing results, also a history of all processing steps and all parameter objects used during the analysis. The process history can be accessed with theprocessHistory function.
#' Overview of the performed processings
processHistory(data_cent)## [[1]]
## Object of class "XProcessHistory"
## type: Peak detection
## date: Mon Jun 15 11:09:46 2020
## info:
## fileIndex: 1,2
## Parameter class: CentWaveParam
## MS level(s) 1
##
## [[2]]
## Object of class "XProcessHistory"
## type: Peak refinement
## date: Mon Jun 15 11:09:51 2020
## info:
## fileIndex: 1,2
## Parameter class: MergeNeighboringPeaksParam
## MS level(s) 1
##
## [[3]]
## Object of class "XProcessHistory"
## type: Peak grouping
## date: Mon Jun 15 11:09:54 2020
## info:
## fileIndex: 1,2
## Parameter class: PeakDensityParam
## MS level(s) 1
##
## [[4]]
## Object of class "XProcessHistory"
## type: Retention time correction
## date: Mon Jun 15 11:09:54 2020
## info:
## fileIndex: 1,2
## Parameter class: PeakGroupsParam
## MS level(s) 1
##
## [[5]]
## Object of class "XProcessHistory"
## type: Peak grouping
## date: Mon Jun 15 11:09:59 2020
## info:
## fileIndex: 1,2
## Parameter class: PeakDensityParam
## MS level(s) 1
##
## [[6]]
## Object of class "XProcessHistory"
## type: Missing peak filling
## date: Mon Jun 15 11:10:01 2020
## info:
## fileIndex: 1,2
## Parameter class: ChromPeakAreaParam
## MS level(s) 1The parameter object for one analysis step can be accessed with processParam:
#' Access the parameter class for a processing step
processParam(processHistory(data_cent)[[1]])## Object of class: CentWaveParam
## Parameters:
## ppm: 30
## peakwidth: 2, 10
## snthresh: 10
## prefilter: 3, 100
## mzCenterFun: wMean
## integrate: 2
## mzdiff: -0.001
## fitgauss: FALSE
## noise: 0
## verboseColumns: FALSE
## roiList length: 0
## firstBaselineCheck TRUE
## roiScales length: 0All this information is also stored in the metadata slot of theSummarizedExperiment:
metadata(res)## [[1]]
## Object of class "XProcessHistory"
## type: Peak detection
## date: Mon Jun 15 11:09:46 2020
## info:
## fileIndex: 1,2
## Parameter class: CentWaveParam
## MS level(s) 1
##
## [[2]]
## Object of class "XProcessHistory"
## type: Peak refinement
## date: Mon Jun 15 11:09:51 2020
## info:
## fileIndex: 1,2
## Parameter class: MergeNeighboringPeaksParam
## MS level(s) 1
##
## [[3]]
## Object of class "XProcessHistory"
## type: Peak grouping
## date: Mon Jun 15 11:09:54 2020
## info:
## fileIndex: 1,2
## Parameter class: PeakDensityParam
## MS level(s) 1
##
## [[4]]
## Object of class "XProcessHistory"
## type: Retention time correction
## date: Mon Jun 15 11:09:54 2020
## info:
## fileIndex: 1,2
## Parameter class: PeakGroupsParam
## MS level(s) 1
##
## [[5]]
## Object of class "XProcessHistory"
## type: Peak grouping
## date: Mon Jun 15 11:09:59 2020
## info:
## fileIndex: 1,2
## Parameter class: PeakDensityParam
## MS level(s) 1
##
## [[6]]
## Object of class "XProcessHistory"
## type: Missing peak filling
## date: Mon Jun 15 11:10:01 2020
## info:
## fileIndex: 1,2
## Parameter class: ChromPeakAreaParam
## MS level(s) 1