MARS: Mass Accuracy Recalibration System

Mass recalibration tool for DIA mass spectrometry data from the ThermoFisher Stellar.

Overview

Mars learns m/z calibration corrections from spectral library fragment matches. The XGBoost model accounts for:

• Fragment m/z: Mass-dependent calibration bias
• Peak intensity: Higher intensity peaks provide more reliable calibration
• Absolute time: Calibration drift over the acquisition run
• Spectrum TIC: Space charge effects from high ion current
• Ion injection time: Signal accumulation duration effects
• Precursor m/z: DIA isolation window-specific effects
• RF temperatures: Thermal effects from RF amplifier (RFA2) and electronics (RFC2)

How It Works

1. Fragment matching: For each DIA MS2 spectrum, Mars finds library peptides where:

   • The precursor m/z falls within the DIA isolation window
   • The spectrum RT is within the peptide's elution window

2. Peak selection: For each expected fragment, Mars selects the most intense peak within the m/z tolerance (not the closest), filtering for minimum intensity

3. Model training: Each matched fragment becomes a training point with up to 16 features (see Model Features) and target: delta_mz

4. Calibration: The trained model predicts m/z corrections for all peaks in the mzML

Installation

From PyPI (recommended)

pip install mars-ms

From source

git clone https://github.com/maccoss/mars.git
cd mars
pip install -e .

Requirements: Python 3.10+, pyteomics, xgboost, numpy, pandas, matplotlib, seaborn, click

Usage

With PRISM CSV (Recommended)

Use a CSV file created using this Skyline report for accurate RT windows:

mars calibrate \
  --mzML data.mzML \
  --prism-csv prism_report.csv \
  --tolerance 0.2 \
  --min-intensity 500 \
  --max-isolation-window 5.0 \
  --output-dir output/

Note: Both --mzml and --mzML are accepted.

With DIA-NN Parquet Output

Use DIA-NN parquet files directly as a spectral library:

mars calibrate \
  --mzml data.mzML \
  --library report-lib.parquet \
  --output-dir output/

Mars automatically looks for report.parquet in the same directory to get RT windows. If the report file is in a different location:

mars calibrate \
  --mzml data.mzML \
  --library report-lib.parquet \
  --diann-report /path/to/report.parquet \
  --output-dir output/

Basic Usage (blib)

mars calibrate --mzml data.mzML --library library.blib --output-dir output/

Batch Processing

# Multiple files with wildcard (no quotes needed)
mars calibrate --mzml *.mzML --library library.blib --output-dir output/

# Positional arguments also work (no --mzml flag needed)
mars calibrate *.mzML --library library.blib --output-dir output/

# Specify files individually
mars calibrate --mzml a.mzML --mzml b.mzML --library library.blib --output-dir output/

# All files in directory
mars calibrate --mzml-dir /path/to/data/ --library library.blib --output-dir output/

Applying a Pre-Trained Model

If you've already trained a calibration model and want to apply it to new files without retraining:

# Apply existing model to new mzML files
mars apply --mzml new_data.mzML --model mars_model.pkl --output-dir output/

# Apply to multiple files (no quotes needed)
mars apply --mzml *.mzML --model mars_model.pkl --output-dir output/

# Or as positional arguments
mars apply *.mzML --model mars_model.pkl --output-dir output/

# Apply to all files in a directory
mars apply --mzml-dir /path/to/data/ --model mars_model.pkl --output-dir output/

This is useful when:

• You want to calibrate files from the same instrument/method without retraining
• You trained on a subset of files and want to apply to the rest
• You're reprocessing data with a validated model

Options

RT Window Behavior

• With --prism-csv: Uses exact Start Time and End Time from Skyline
• With DIA-NN parquet: Uses RT.Start and RT.Stop from report.parquet
• With blib only: Uses +/-5 seconds around the blib library RT

Isolation Window Filtering

Some DIA methods use wide isolation windows (e.g., 20-30 m/z) that may reduce calibration accuracy. Use --max-isolation-window to exclude these:

# Exclude windows wider than 5 m/z
mars calibrate --mzml data.mzML --prism-csv report.csv --max-isolation-window 5.0

This filters spectra during both model training and mzML recalibration. Typical narrow DIA windows (~1 m/z) are retained.

Output Files

Model Features

The XGBoost model uses up to 16 features to predict m/z corrections:

1. precursor_mz - DIA isolation window center
2. fragment_mz - Fragment m/z being calibrated
3. absolute_time - Time relative to first acquisition (seconds)
4. log_tic - Log10 of spectrum total ion current
5. log_intensity - Log10 of peak intensity
6. injection_time - Ion injection time (seconds)
7. tic_injection_time - TIC × injection time product
8. fragment_ions - Fragment intensity × injection time (total ions, not rate)
9. ions_above_0_1 - Total ions in (X+0.5, X+1.5] Th range above fragment m/z
10. ions_above_1_2 - Total ions in (X+1.5, X+2.5] Th range above fragment m/z
11. ions_above_2_3 - Total ions in (X+2.5, X+3.5] Th range above fragment m/z
12. ions_below_0_1 - Total ions in (X-1.5, X-0.5] Th range below fragment m/z
13. ions_below_1_2 - Total ions in (X-2.5, X-1.5] Th range below fragment m/z
14. ions_below_2_3 - Total ions in (X-3.5, X-2.5] Th range below fragment m/z
15. adjacent_ratio_0_1 - ions_above_0_1 / fragment_ions (relative adjacent density)
16. adjacent_ratio_1_2 - ions_above_1_2 / fragment_ions
17. adjacent_ratio_2_3 - ions_above_2_3 / fragment_ions
18. adjacent_ratio_below_0_1 - ions_below_0_1 / fragment_ions
19. adjacent_ratio_below_1_2 - ions_below_1_2 / fragment_ions
20. adjacent_ratio_below_2_3 - ions_below_2_3 / fragment_ions
21. rfa2_temp - RF amplifier temperature (°C)
22. rfc2_temp - RF electronics temperature (°C)

Note: Features 6-20 are only included if injection time data is available in the mzML files. Features 21-22 are only included if temperature CSV files are provided. Features with universally missing data are automatically excluded.

RF Temperature Data

Mars can incorporate RF temperature data to model thermal effects on mass accuracy. Temperature data is loaded from CSV files exported from Thermo chromatogram exports.

Temperature File Format

Temperature CSV files should be in Thermo's chromatogram export format:

• 3 header lines (skipped)
• Columns: Time(min), temperature value

Example naming convention:

RFA2-Sample_Name.csv  # RF amplifier temperature
RFC2-Sample_Name.csv  # RF electronics temperature  

Usage with Temperature Data

mars calibrate \
  --mzml data.mzML \
  --prism-csv report.csv \
  --temperature-dir /path/to/temperature_csvs/ \
  --output-dir output/

Mars automatically finds temperature files matching each mzML filename and interpolates temperature values at each spectrum's retention time.

Python API

from mars import load_blib, read_dia_spectra, match_library_to_spectra, MzCalibrator

# Load library and match
library = load_blib("library.blib")
spectra = read_dia_spectra("data.mzML")
matches = match_library_to_spectra(library, spectra, mz_tolerance=0.2, min_intensity=1500)

# Train and save model
calibrator = MzCalibrator()
calibrator.fit(matches)
calibrator.save("model.pkl")

Using DIA-NN Parquet

from mars import load_diann_library, read_dia_spectra, match_library_to_spectra, MzCalibrator

# Load DIA-NN library (auto-finds report.parquet in same directory)
library = load_diann_library("report-lib.parquet")

# Or specify report.parquet explicitly
library = load_diann_library("report-lib.parquet", report_parquet="/path/to/report.parquet")

# Filter to specific mzML file(s)
library = load_diann_library("report-lib.parquet", mzml_filename=["sample1.mzML", "sample2.mzML"])

spectra = read_dia_spectra("data.mzML")
matches = match_library_to_spectra(library, spectra, mz_tolerance=0.2, min_intensity=1500)

Requirements

• Spectral library: One of the following formats:
  • blib format from Skyline with fragment annotations
  • DIA-NN parquet output (report-lib.parquet + report.parquet)
• mzML files: DIA data from Thermo Stellar (or similar unit resolution instrument)
• PRISM CSV (optional): Skyline report with Start Time, End Time, Replicate Name columns

License

MIT