precursor-mass-difference-calculation
Summary
Calculate the mass offset between precursor ions of two MS/MS spectra to enable alignment of peaks under the modified cosine similarity measure. This step is essential for detecting structurally related molecules that differ in mass.
When to use
When comparing two MS/MS spectra using modified cosine similarity and the precursor m/z values differ, indicating potential neutral losses, adduct variations, or analogs with different substituents. You need this calculation before attempting to align peaks across spectra with non-zero mass offsets.
When NOT to use
- Input spectra have identical or negligibly different precursor m/z values (use standard cosine similarity instead)
- Comparing spectra where mass differences are artifacts of measurement error rather than chemical modification (apply mass calibration first)
- Neutral loss similarity is being used instead of modified cosine similarity, as it does not require this offset calculation
Inputs
- MS/MS spectrum (query) with precursor_mz field
- MS/MS spectrum (reference) with precursor_mz field
Outputs
- precursor mass difference (float, in Da)
- mass offset parameter for peak alignment
How to apply
Extract the precursor m/z value from both the query spectrum and the reference spectrum. Subtract the reference precursor m/z from the query precursor m/z to obtain the mass difference (in Da). This offset is then used during peak matching: peaks are considered aligned if m/z_query matches m/z_reference + mass_difference within the specified tolerance. The mass difference accommodates the shift in the m/z axis caused by different precursor masses, allowing the modified cosine algorithm to score aligned peak intensities correctly. Typically this tolerance is 0.1 Da or specified by the instrument calibration.
Related tools
- spectrum_utils (Load and parse MS/MS spectra with precursor_mz annotation; extract precursor charge and m/z fields)
- cosine_neutral_loss repository (Reference implementation of modified cosine similarity scoring that uses the precursor mass difference for peak alignment) — https://github.com/bittremieux/cosine_neutral_loss
Examples
import spectrum_utils.spectrum as sus; spectrum1 = sus.MsmsSpectrum.from_usi(usi1); spectrum2 = sus.MsmsSpectrum.from_usi(usi2); mass_diff = spectrum1.precursor_mz - spectrum2.precursor_mz; print(f'Precursor mass difference: {mass_diff} Da')
Evaluation signals
- Precursor mass difference is a scalar float, typically in range ±500 Da for MS/MS comparisons
- When mass difference is applied as offset during peak matching, the number of matched peaks should increase relative to standard (non-offset) cosine similarity for true analogs
- Peak alignment consistency: verify that m/z_query == m/z_reference + mass_difference holds for matched peak pairs within specified tolerance
- Modified cosine score should be higher for mass-offset analogs compared to standard cosine score computed without offset
- Mass difference should be reproducible and stable across repeated calculations for the same spectrum pair
Limitations
- Precursor mass difference assumes both spectra originate from the same ionization method and charge state; different charge states require normalization first
- Very large mass differences (>1000 Da) may indicate data quality issues or misidentified precursor ions rather than true analogs
- The calculation does not account for measurement uncertainty; alignment tolerance must be set appropriately for instrument resolution
- Modified cosine similarity with mass offset is designed for discovery of structurally related molecules; it may produce spuriously high scores for unrelated spectra with coincidental mass differences
Evidence
- [other] Extract precursor m/z values from both spectra. 3. Calculate the precursor mass difference between the two spectra. 4. Match peaks between spectra allowing for the precursor mass offset (modified cosine: peaks at m/z_query and m/z_reference + mass_difference are considered aligned).: "Extract precursor m/z values from both spectra. 3. Calculate the precursor mass difference between the two spectra. 4. Match peaks between spectra allowing for the precursor mass offset"
- [other] Modified cosine similarity is a spectrum similarity measure implemented in the repository alongside cosine similarity and neutral loss similarity for comparing MS/MS spectra in the discovery of structurally related molecules.: "Modified cosine similarity is a spectrum similarity measure implemented in the repository alongside cosine similarity and neutral loss similarity for comparing MS/MS spectra in the discovery of"
- [readme] Modified cosine similarity Watrous, J. et al. Mass spectral molecular networking of living microbial colonies. Proceedings of the National Academy of Sciences 109, E1743–E1752 (2012).: "Modified cosine similarity ... Mass spectral molecular networking of living microbial colonies"
- [readme] spectrum1.precursor_charge = 1 spectrum1 = spectrum1.set_mz_range(0, spectrum1.precursor_mz): "spectrum1.precursor_charge = 1; spectrum1 = spectrum1.set_mz_range(0, spectrum1.precursor_mz)"