huanlab / breathxplorer

An advanced instrument for breathomics examination

BreathXplorer

• breathXplorer
  • Introduction
  • Quick start
    • Installation
    • File format
      • Feature table CSV
      • Aligned feature table CSV
      • MS/MS spectra MGF
    • Feature extraction
    • Feature alignment
  • Utilities
    • MS/MS spectra export
    • Peak recognition
  • Citation

Introduction

BreathXplorer is a bioinformatic solution to process breath data generated from HRMS analysis. It contains a suite of functions, including feature extraction, feature alignment, and breath recognition.

Quick start

Installation

The package can be installed using pip, supported python version are 3.7, 3.8, 3.9 and 3.10. It's recommended to install the package in a virtual environment or a conda environment, after activating the environment, run the following command to install the package:

pip install breathXplorer

File format

The input file should be in mzML or mzXML format (Perhaps more in the future). For the output, the single or aligned feature table can be exported as csv file. The related MS/MS spectra can be exported in an mgf file.

Feature table CSV

ID	m/z	intensity	0.0072	0.0267	0.0438
0	70.0043	42831	90863	34955	0
1	70.0650	7697202	6714245	6476909	6479075
2	70.0730	18459	0	0	0
3	70.1257	65085	0	0	0

The index of the table is the m/z value of the features, and the 1st column is the total intensity of the feature. The other columns are the intensity of the feature over time, the time is the name of the corresponding column.

Aligned feature table CSV

ID	m/z	S01_Before	S02_Before	S03_Before
0	70.0652	8400258	3229242	8472742
1	71.0489	449896	11058	413906
2	71.0683	386030	12110	398033

The index of the table is the m/z value of the features, and each column is the total intensity of the feature in a sample ( experiment of a subject). The name of the column is the sample name.

MS/MS spectra MGF

BEGIN IONS
PEPMASS=70.0040
MSLEVEL=2
50.6306 1466
50.6316 2041
END IONS

BEGIN IONS
PEPMASS=70.0649
MSLEVEL=2
53.0012 1509
71.0627 7731
71.0650 870
END IONS

The file contains the MS/MS spectra of the features, each feature has a PEPMASS (precursor mass) and MSLEVEL field, and the following pairs are the m/z and intensity of the MS/MS spectra.

Feature extraction

Feature extraction is used to find the volatile organic compound (VOC) in the breath sample. The feature extraction is performed using the find_feature function. The function takes the path to an mzMl/mzXML file as input, and returns an object containing the extracted feature table:

from breathXplorer import find_feature

fs = find_feature("sample.mzML", False, .8, "Topological", 6)

The False indicates that the input file is not a line spectrum, and the .8 controls the quality of the extracted features, higher value means higher quality. The "Topological" indicates the algorithm used for feature extraction, the other option is "Gaussian". The 6 is the prior knowledge of the number of breath in an experiment (only used for Gaussian algorithm, and you don't need to impute it if using Topological).

The fs is a FeatureSet object, it contains the following information:

fs.mz  # m/z values of the extracted features
fs.scan_time  # scan time of the experiment
fs.intensity  # the total intensity of each feature (calculated by integrating the intensity over scan time)
len(fs)  # the number of extracted features
fs[96.7654]  # get the intensity with m/z value 96.7654 over scan time
fs.rsd  # the relative standard deviation of each feature for quality control

In practice, the relative standard deviation (RSD) can be used to filter out the noise which doesn't have a consistent intensity with breath peaks:

fs = fs.rsd_control(.1)  # use specific RSD value
fs = fs.rsd_control(fs.rsd.quantile(0.1))  # use the 10% quantile of the RSD

FeatureSet object can be exported as csv file using the to_csv method:

fs.to_csv("feature_table.csv")

BreathXplorer can infer the adducts and isotope of the features, to enable this function:

fs.to_csv("feature_table.csv", adduct=True, isotope=True)

Feature alignment

The merge_feature function takes as input a list of FeatureSet objects, and returns a Sample object. It aligns the features with the similar m/z value from different samples, and calculate the total intensity of each feature in each sample. To use the function, you can do the following:

from breathXplorer import merge_result, find_feature

fss = [find_feature(f, False, .8, "Gaussian", 6) for f in ["sample1.mzML", "sample2.mzXML", "sample3.mzML"]]
fss = [fs.rsd_control(fs.rsd.quantile(0.1)) for fs in fss]  # filter out the noise (optional)
sample = merge_result(fss, ["sample1", "sample2", "sample3"])

The first statement creates a list of FeatureSet objects. One thing very cool is the function can deal with different file formats in one line of code (though it's not recommended to do so, because we usually want to keep our experiment data in a more consistent way).

The second statement aligns those FeatureSet objects using merge_result. The fss is a list of FeatureSet objects, the ["sample1", "sample2", "sample3"] is the customizable names assigned to each sample.

Just like the FeatureSet object, the Sample object contains the following information:

sample.mz  # m/z values of the extracted features
sample.sample_name  # the name of each sample
len(sample)  # the number of extracted features
sample[96.7654]  # get the total intensity with m/z value 96.7654 of all samples
sample.to_csv("aligned_table.csv")  # export the feature table of all samples as csv file
sample.to_csv("aligned_table.csv", adduct=True)  # export the feature table of all samples as csv file with adducts

Utilities

MS/MS spectra export

If you're using tandem MS, you can also export the MS/MS spectra as mgf file using the to_mgf function:

from breathXplorer import retrieve_tandem

tandem = retrieve_tandem("sample.mzML", fs, 0.005)
tandem.to_mgf("ms2.mgf")

The file sample.mzML contains tandem MS data. The fs object represents either a FeatureSet or a Sample, from which the m/z values are extracted. These m/z values are then used to obtain the corresponding MS/MS spectra. A tolerance of 0.005is applied to the m/z values. Any MS/MS spectra with a difference smaller than 0.005 between the precursor m/z value and the feature's m/z value will be retrieved.

Peak recognition

The total ion current (TIC) of mass spectral data from breath analysis exhibits two key properties that preclude the use of existing peak detection methods but facilitate the development of specialized breath analysis algorithms:

1. Breath peaks are characteristically irregular and contain a substantial number of subsidiary peaks.
2. Prior knowledge about the expected number of breath peaks is typically available.

We developed 2 algorithms to detect breath peaks from the TIC of mass spectral data from breath analysis: Topological and Gaussian mixture model (GMM). Both the 2 methods share the similar interface, and can be imported as follows:

from breathXplorer.find_peak import find_peak, find_gaussian

Both function takes in two arrays, x representing the time points and y representing the corresponding values such as concentration or intensity. Besides, when using find_gaussian, the parameter n determines the number of peaks to be considered. The output is a list of tuples, where each tuple contains the start and end time point values of the identified ranges, along with the maximum intensity values of the respective peaks.

Citation

If you use this package in your research, please cite the following paper: