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XCMS Extensions and Fixes

Ginkgo Bioworks' version of XCMS. Originally forked from version 1.39.2 from Bioconductor.

Main changes

1. Improved centWave peak detection

With this change, XCMS now picks the best scale for a peak using the CWT coefficient, rather than fixed area integration. Using fixed area integration tends to favor narrower peak shapes, even though a broader peak shape is better. To fine tune a broad peak, XCMS now narrows down the RT range of a peak by skipping regions less than 1% of the max intensity, rather than skipping regions with 0 intensities.

See commit 7a12243565

2. Improved centWave peak integration

Added Tony Larson's integration improvement to handle non-uniform scan densities, as discussed in the developer forum.

See commit 1980288a05

3. Added targeted peak detection to centWave

Prior to this change, XCMS's centWave did not pick up weak peaks consistently, sometimes missing them entirely. We believe XCMS failed to pick up these peaks for three reasons:

  1. XCMS tries to identify "regions of interest" (ROI) prior to peak detection and integration. It may fail to pick up a weak peak with only a few spikes spanning several scans as an ROI.
  2. XCMS can only pick up ROIs with contiguous scans. This was mentioned in the centWave peak detection paper, and is consistent with the behavior of the code. To handle weak peaks with scan gaps, XCMS extends each ROI laterally (in time domain) by a multiple of the specified minimum peak width prior to running peak detection on the ROI. But if the specified minimum peak width is small, this lateral extension may not pick up all the scans that make up the real peak.
  3. XCMS throws away an ROI if it believes there aren't enough above-noise signals in the ROI. XCMS determines the noise level locally, from an average of the signals in the ROI. For weak peaks from equipment with low real detector noise, real signals can be considered as noise, causing an ROI with weak peak to be thrown away before peak detection.

Targeted centWave works around all three of these problems:

  1. The user defines each target as a triple (m/_z_range, scan range, peak width range) and provides a target list to centWave.
  2. For each target, targeted centWave constructs an ROI that covers the m/z and scan ranges of the target, and runs CWT peak detection on that ROI, using the peak width range of the target. CWT peak detection works over large scan ranges and can pick up multiple peaks within the scan range.
  3. For each detected peak, instead of walking down from the detected peak center to find RT boundaries and integrating between the boundaries, targeted centWave uses the peak shape with the best CWT coefficient to compute the RT boundaries, removing regions less than 10% of max intensity at each end to remove trailing noise. This last integration techique (integrate=3 option to centWave) also works when running centWave in untargeted mode.
  4. Finally, in targeted mode, the user can set snthresh=1 for peak detection, bypassing centWave's noise estimation code. Because the user also specifies the optimal peak width for the target, any and all peaks matching those optimal peak width are returned, regardless of the computed signal to noise ratio.

See commit 8ca593a551

Installation and Use

To install, run install_xcms. Make sure you have all of the required dependencies installed; Usually, the easiest way to do this is to install xcms from Bioconductor first, and then install this version over it.

You can set the install directory as the first argument, or using the XCMS_INSTALL_DIR environment variable. It will default to whatever R CMD INSTALL would use, if not provided.

Alternatively, if you're using Python and setuptools, you can run python setup.py install, or include this repository in your Python project's requirements.txt file.

If you installed this XCMS in a custom directory, to use this it XCMS library, run the following in R (lib.loc specifies where package is installed).

  library(xcms, lib.loc='[wherever you installed it]')

or run R with the following environment variable: R_LIBS_USER=[wherever you installed it]

Acknowledgment

We at Ginkgo Bioworks are indebted to the XCMS team for creating and improving XCMS over the years.