Mass spectrometry measures data in so called profile mode, were the
signal corresponding to a specific ion is distributed around the ion's
actual m/z value [@Smith:2014di]. The accuracy of that signal depends
on the resolution and settings of the instrument. Profile mode data
can be processed into centroid data by retaining only a single,
representative value, typically the local maximum of the distribution
of data points. This centroiding substantially reduces the amount of
data without much loss of information. Certain algorithms, such as the
centWave method in the
r Biocpkg("xcms") package for
chromatographic peak detection in LC-MS experiments or proteomics
search engines that match MS2 spectra to peptides, require the data to
be in centroid mode. In this vignette, we will focus on metabolomics
Many manufacturers apply centroiding of the profile data, either
directly during the acquisition or immediately thereafter so that the
user immediately receives processed data. Alternatively, third party
software, such as
msconvert from the
[@Chambers:2012] allow to apply various data centroiding algorithms,
including vendor methods. In some cases however, the software provided
by some vendors generate centroided data of poor quality.
also provides some functionality to perform centroiding of profile MS
data. These processed data can then be further quantified or analysed
within R or serialised to mzML files, and used as input for other
In this vignette we use a subset of a metabolomics profile-mode LC-MS
data of pooled human serum samples measured on a AB Sciex TripleTOF
5600+ mass spectrometer (the employed chromatography was a hydrophilic
interaction high-performance liquid chromatography (HILIC HPLC)). The
mzML file contains profile mode data for an m/z range from 105 to 130
and a retention time from 0 to 240 seconds. For more details on the
?msdata::sciexdata. Below we load the required packages
and read the MS data.
library("MSnbase") library("msdata") library("magrittr") fl <- dir(system.file("sciex", package = "msdata"), full.names = TRUE) basename(fl) data_prof <- readMSData(fl, mode = "onDisk", centroided = FALSE)
We next extract the profile MS data for the [M+H]+ adduct of serine
with the expected m/z of 106.049871. We thus filter the
object using an m/z range containing the signal for the metabolite and
a retention time window from 175 to 187 seconds corresponding to the
time when the analyte elutes from the LC.
## Define the mz and retention time ranges serine_mz <- 106.049871 mzr <- c(serine_mz - 0.01, serine_mz + 0.01) rtr <- c(175, 187) ## Filtering the object serine <- data_prof %>% filterRt(rtr) %>% filterMz(mzr)
We can now plot the profile MS data for serine.
plot(serine, type = "XIC") abline(h = serine_mz, col = "red", lty = 2)
The lower panel in the plot above shows all the individual signal intensities measured by the mass spectrometer over the retention time and the m/z ranges of interest. The upper panel displays the base peak chromatogram (BPC), which represents the maximum signal (across the range of m/z values) for each discrete retention time. The rows of points in this lower panel indicate the resolution of the mass spectrometer while the columns of data points (i.e. the data collected for a discrete retention time point) represents the signal for the ion in one spectrum.
Below we plot the signal for one of of the
r length(serine) spectra
containing signal for serine, the one at retention time
r round(rtime(serine[]), 2)
The MS instrument recorded a signal along the m/z range in discrete intervals (which depend on the resolution of the instrument). The profile-mode signal of the serine ion at the respective retention time (the mass peak) consists therefore of multiple intensities that follow approximately a gaussian distribution.
As described in the introduction, centroiding aims to reduce this
signal distribution to a single representative intensity, a single
data point, for the ion in a spectrum. The simplest approach selects
the largest intensity for each mass peak and report its intensity and
m/z value. This can be done using the
pickPeaks method with default
parameters as shown below.
data_cent <- data_prof %>% pickPeaks() serine_cent <- data_cent %>% filterRt(rtr) %>% filterMz(mzr) ## Plot the centroided data for serine plot(serine_cent, type = "XIC") abline(h = serine_mz, col = "red", lty = 2)
After centroiding the data consists of a single intensity for each mass peak. In the example above the centroids from consecutive scans do however not have the same m/z value, but they fluctuate between the discrete m/z values defined by the instruments's resolution. For lower intensity signals this variation can be substantial.
To further illustrate this, we plot below the centroided signal for the [M+H]+ ion of proline.
prol_mz <- 116.070608 prol_mzr <- c(prol_mz - 0.01, prol_mz + 0.01) prol_rtr <- c(165, 175) proline <- data_prof %>% pickPeaks() %>% filterRt(prol_rtr) %>% filterMz(prol_mzr) plot(proline, type = "XIC") abline(h = prol_mz, col = "red", lty = 2)
For proline, the centroids jump between 3 bins of m/z values in consecutive scans and the chromatographic data does not show a nice, regular peak shape. Additional data processing, such as data smoothing prior to centroiding and/or refining the centroid's m/z can reduce these effects and improve overall data quality as we will see in the next section.
While the simple centroiding using
pickPeaks as described in the
previous section might be sufficient for many experiments and setups,
MS data smoothing and refinement of the identified centroids' m/z
values can improve data quality.
Raw mass spectrometry data is usually smoothed in m/z dimension by applying
e.g. a Savitzky-Golay filter [@Savitzky:1964bn] which reduces the noise and
hence improves data quality. Below we use the
smooth method to apply a
Savitzky-Golay filter with a half-window size of 4 to the data within each
?smooth for more details on the parameters).
data_sg <- data_prof %>% smooth(method = "SavitzkyGolay", halfWindowSize = 4L)
We next apply the simple peak picking on the smoothed data, filter the desired retention time and m/z ranges, and subsequently plot the such centroided data for serine.
data_sg_cent <- data_sg %>% pickPeaks %>% filterRt(rtr) %>% filterMz(mzr) ## Plot the centroided data for serine plot(data_sg_cent, type = "XIC") abline(h = serine_mz, col = "red", lty = 2)
Smoothing the raw data prior to peak picking improved the quality of the centroided data of serine as well as proline as can be seen below.
prol_sg_cent <- data_sg %>% pickPeaks %>% filterRt(prol_rtr) %>% filterMz(prol_mzr) plot(prol_sg_cent, type = "XIC") abline(h = prol_mz, col = "red", lty = 2)
The smoothed centroided data for proline still show a systematic deviation of m/z values as well as poor chromatographic data.
In addition to smoothing the signal in m/z dimension, we can also
smooth the signal along the retention time dimension using the
combineSpectraMovingWindow function. This function aggregates signal
for the same m/z value from neighbouring spectra in a moving window
approach, thus smoothing the chromatographic data (by replacing the
intensity in the middle spectrum by the average signal of all
intensities for the m/z in the spectra within the defined window). To
reduce the run-time of the example we apply the smoothing only to the
profile-mode data for a retention time window containing proline (in a
real data analysis this should be performed on the full data).
## Subset to the data for proline, smooth it in rt dimension and ## perform the centroiding proline_c_cent <- data_prof %>% filterRt(prol_rtr) %>% combineSpectraMovingWindow() %>% pickPeaks() %>% filterMz(prol_mzr)
plot(proline_c_cent, type = "XIC") abline(h = prol_mz, col = "red", lty = 2)
As can be seen above, smoothing in retention time dimension improves the chromatographic peak shape of proline.
Note however that, to combine data from multiple spectra, the
combineSpectraMovingWindow function has to first load the full data
into memory (i.e. it converts the
OnDiskMSnExp object into a
MSnExp object) and that it returns also a
MSnExp object. In a real
use case it is thus advisable to apply
separately on each file of an experiment and to export the results as
an mzML file using the
Thus far we applied only a simple peak picking strategy, but the
pickPeaks method allows also to refine the identified centroid's
m/z value by considering also the signal from the full, or parts of
the, mass peak. Currently two methods are implemented, descendPeak
and kNeighbors that can be selected by passing either
refineMz = "kNeighbors" to the
method. The m/z value of the reported centroid is calculated using an
intensity-weighted mean of m/z-intensity values from the mass
peak. This can improve the accuracy of the reported m/z values. The
two methods differ only in the way in which the peak area for the
final calculation is defined:
kNeighbors takes the
k = 2) left and right of the centroid and
descendPeak walks, on both sides from the centroid, down until the
signal is below
signalPercentage% of the centroid's intensity (by
default 33%), or until the signal increases again. All m/z intensity
pairs within this range are used for the weighted average calculation
of the centroid's m/z value.
## Use pickPeaks with descendPeak m/z refinement data_sg_cent_mz <- data_sg %>% pickPeaks(refineMz = "descendPeak")
Below we first extract the data for serine and then plot the smoothed and centroided data without and with m/z refinement.
## Extract the data for serine serine_sg_cent <- data_sg_cent %>% filterRt(rtr) %>% filterMz(mzr) serine_sg_cent_mz <- data_sg_cent_mz %>% filterRt(rtr) %>% filterMz(mzr) ## Plot the data layout(matrix(1:4, ncol = 2)) ## No m/z refinement plot(serine_sg_cent, type = "XIC", layout = NULL) abline(h = serine_mz, col = "red", lty = 2) abline(v = rtime(serine_sg_cent), col = "red", lty = 3) ## With m/z refinement plot(serine_sg_cent_mz, type = "XIC", layout = NULL) abline(h = serine_mz, col = "red", lty = 2) abline(v = rtime(serine_sg_cent_mz), col = "red", lty = 3)
As shown above (right), the accuracy of the centroided data with m/z refinement was improved, where the difference between the largest signal centroids' m/z value and the theoretical m/z value for the [M+H]+ ion of serine is reduced.
For the simple peak picking on raw data the difference is:
## only centroided mz(filterMz(filterRt(data_cent, rtr), mzr))[] - serine_mz
Smoothing already improves the accuracy:
## smoothed and centroided mz(serine_sg_cent)[] - serine_mz
And refining the m/z value during the centroiding can improve accuracy even more:
## smoothed and centroided with m/z refinement mz(serine_sg_cent_mz)[] - serine_mz
Similarly, the m/z refinement also improved the accuracy for proline.
proline_sg_cent <- data_prof %>% smooth(method = "SavitzkyGolay", halfWindowSize = 4L) %>% pickPeaks() %>% filterRt(prol_rtr) %>% filterMz(prol_mzr) proline_sg_cent_mz <- data_prof %>% smooth(method = "SavitzkyGolay", halfWindowSize = 4L) %>% pickPeaks(refineMz = "descendPeak") %>% filterRt(prol_rtr) %>% filterMz(prol_mzr) layout(matrix(1:4, ncol = 2)) plot(proline_sg_cent, type = "XIC", layout = NULL) abline(h = prol_mz, col = "red", lty = 2) abline(v = rtime(proline_sg_cent_mz), col = "red", lty = 3) plot(proline_sg_cent_mz, type = "XIC", layout = NULL) abline(h = prol_mz, col = "red", lty = 2) abline(v = rtime(proline_sg_cent_mz), col = "red", lty = 3)
The difference between the m/z of the centroid with the largest signal and the theoretical m/z for the [M+H]+ ion of proline is shown below.
## only centroiding mz(filterMz(filterRt(data_cent, prol_rtr), prol_mzr))[] - prol_mz ## smoothed and centroided mz(proline_sg_cent)[] - prol_mz ## smoothed and centroided with m/z refinement mz(proline_sg_cent_mz)[] - prol_mz
Summarizing, smoothing raw profile MS data, e.g. by applying a Savitzky-Golay filter, improves quality considerably. Additional smoothing in retention time dimension can be advantageous too, specifically for the chromatographic peak shape. Accuracy can be further improved for smoothed profile MS data by refining the m/z value of the identified centroids.
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