Precursor Mass Calculation

precursor-mass-calculation

Summary

Calculate the theoretical precursor m/z value for a compound given its molecular structure (SMILES or formula) and ionization adduct type. This step is essential for anchoring MS/MS spectral analysis, noise filtering, and library searching workflows.

When to use

When you have a compound's SMILES string or molecular formula and need to determine the expected precursor ion m/z for comparison against observed spectra, particularly before applying formula-based denoising, entropy similarity scoring, or denoising search against reference libraries.

When NOT to use

• Input compound structure is already an observed precursor m/z from raw data (use directly instead)
• Adduct type is unknown or ambiguous; resolve ionization mode before calling this function
• Working with fragment ions or neutral losses rather than intact precursor ions

Inputs

• SMILES string (compound molecular structure)
• molecular formula string (e.g., 'C5H4N4O')
• adduct type string (e.g., '[M+H]+', '[M+Na]+', '[M-H]-')

Outputs

• precursor m/z (float): theoretical precursor ion mass-to-charge ratio

How to apply

Call calculate_precursormz with the adduct type (e.g., '[M+H]+', '[M+Na]+', '[M-H]-') and either a SMILES string or molecular formula as input. The function computes the monoisotopic mass of the neutral molecule using the provided structure, applies the mass delta corresponding to the adduct (e.g., +1.00783 for [M+H]+, +22.98977 for [M+Na]+), and returns the theoretical precursor m/z. This value is then used downstream as a reference point for mass tolerance windows, precursor ion region retention, and entropy similarity calculations between observed and reference spectra.

Related tools

• RDkit (parses SMILES strings and computes molecular masses)
• molmass (calculates atomic and molecular masses from chemical formulas)
• chemparse (parses and manipulates chemical formula strings)
• spectral_denoising.chem_utils.calculate_precursormz (main function implementing precursor m/z calculation) — https://github.com/FanzhouKong/spectral_denoising

Examples

pmz = calculate_precursormz('[M+Na]+', 'O=c1nc[nH]c2nc[nH]c12')

Evaluation signals

• Returned m/z value matches literature or database values for the same compound and adduct within <5 ppm mass error
• Precursor m/z lies within expected range for the molecular weight and charge state (e.g., m/z > 50 for singly charged ions of typical metabolites)
• Value is consistent when recalculated using both SMILES and derived molecular formula inputs
• Downstream entropy similarity scores improve when denoised spectra are compared using this precursor m/z as the mass reference point
• Precursor ion is correctly retained and not removed by electronic or formula-based denoising filters because its m/z matches the calculated value

Limitations

• Assumes the adduct is correctly specified; incorrect adduct selection will produce an incorrect m/z and downstream filtering will fail
• Does not account for isotopic peaks ([M+1], [M+2]) or multiply-charged ions unless explicitly modeled
• SMILES parsing depends on RDkit correctness; malformed or non-standard SMILES will produce incorrect masses
• Adduct mass deltas are fixed based on standard ionization modes; unusual or custom adducts not in the adduct list will not be handled correctly

Evidence

• [other] Extract the precursor m/z from the SMILES and adduct using calculate_precursormz: "Extract the precursor m/z from the SMILES and adduct using calculate_precursormz."
• [other] precursor m/z calculation is a foundational step in denoising and searching workflows: "pmz = calculate_precursormz(adduct,smiles)"
• [other] precursor m/z is used as a mass reference for entropy similarity scoring: "entropy_similairty(peak_with_noise,peak, pmz = pmz)"
• [other] Precursor m/z is retained after denoising as part of precursor ion region: "Retain only ions tagged as valid, then add back the precursor ion region using add_spectra."
• [readme] Function accepts SMILES or formula as alternative inputs: "Note: Even all functions have a default 'smiles' information column, the function would also accept formula as input."