Mass Spectrum Adduct Assignment

mass-spectrum-adduct-assignment

Summary

Rank and assign multiple ionization adduct types (e.g., [M+H]+, [M+Na]+, [M+K]+, [M+NH4]+) to tandem mass spectra using energy-based neural scoring, enabling chemical formula assignment without relying on fragmentation trees or spectrum databases.

When to use

When analyzing tandem mass spectra with unknown precursor adduct identity, especially for positive-mode data containing non-protonated adducts ([M+Na]+, [M+K]+, [M+NH4]+). Adduct assignment is critical when spectra originate from diverse sample preparation or ionization conditions where [M+H]+ cannot be assumed.

When NOT to use

• Input is negative-mode mass spectrometry data; MIST-CF currently supports only positive-mode ionization.
• Precursor m/z is unknown or not provided; adduct assignment requires a measured parent ion mass.
• Sample is already annotated with correct adduct identity; ranking and assignment are redundant.

Inputs

• tandem mass spectrum (MGF format) with known precursor m/z
• trained MIST-CF formula transformer model checkpoint
• list of candidate positive-mode adduct types to consider (e.g., [M+H]+, [M+Na]+, [M+K]+, [M+NH4]+)

Outputs

• ranked list of (adduct_type, chemical_formula) pairs with scores
• top-ranked adduct assignment and corresponding neutral formula
• ranking metrics (ground-truth adduct rank, recall@k for evaluation datasets)

How to apply

Load a pre-trained MIST-CF formula transformer model and provide a tandem mass spectrum in MGF format with known precursor m/z. Use the internal chemical subformula assignment module (rather than SIRIUS fragmentation trees) to enumerate candidate adduct-formula pairs for multiple positive-mode adduct types. For each candidate pair, the formula transformer scores agreement between the precursor formula and observed MS/MS peaks using an end-to-end energy-based model trained on fragmentation patterns. Rank candidates by score and extract the top-ranked adduct assignment. Compute ranking metrics (e.g., rank of ground-truth adduct, recall@k) to assess whether multi-adduct consideration improves over single-adduct baselines.

Related tools

• SIRIUS (Dynamic programming algorithm for enumerating candidate chemical formulae from observed m/z; used for formula candidate generation (though MIST-CF uses internal subformula assignment for adduct scoring rather than SIRIUS fragmentation trees).) — https://bio.informatik.uni-jena.de/software/sirius/
• MIST-CF (End-to-end formula transformer neural network that scores and ranks adduct-formula pairs for tandem mass spectra; primary implementation for this skill.) — https://github.com/samgoldman97/mist-cf

Examples

python src/mist_cf/mist_cf_score/predict_mgf.py --model_checkpoint quickstart/mist_cf_model.pt --input_spectra data/demo_specs.mgf --adduct_types '[M+H]+,[M+Na]+,[M+K]+,[M+NH4]+' --output_dir quickstart/mist_cf_out/

Evaluation signals

• Ground-truth adduct appears in top-k ranks (recall@k metric) on evaluation dataset; higher recall indicates effective multi-adduct ranking.
• Ranking accuracy on spectra with non-protonated adducts ([M+Na]+, [M+K]+, [M+NH4]+) exceeds single-[M+H]+ baseline, confirming multi-adduct consideration improves performance.
• Score distribution is unimodal and concentrated on top-ranked pair; bimodal or diffuse distributions suggest model uncertainty or calibration issues.
• Precursor formula mass computed from top-ranked adduct matches observed m/z within instrument mass accuracy tolerance (typically <5 ppm for high-resolution data).
• Output adduct-formula pairs satisfy chemical validity constraints (e.g., positive charge, non-negative atom counts).

Limitations

• MIST-CF currently supports positive-mode ionization only; negative-mode adducts ([M−H]−, [M+Cl]−, etc.) are not yet implemented.
• Model performance is calibrated on NPLIB1 (public natural products) and NIST20 (proprietary) spectral libraries; transfer to novel chemical classes or unusual ionization conditions may degrade ranking accuracy.
• Requires sufficient MS/MS fragment peaks (formula transformer shown to be robust to <10 peaks, but very sparse spectra may limit discrimination between candidate adducts).
• Internal chemical subformula assignment protocol is data-dependent; adduct ranking inherits any systematic biases in the training dataset or model parameterization.

Evidence

• [readme] MIST-CF ranks chemical formula and adduct assignments for an unknown mass spectrum using an end-to-end energy based modeling approach, without referencing any spectrum databases.: "MIST-CF ranks chemical formula and adduct assignments for an unknown mass spectrum using an end-to-end energy based modeling approach, without referencing any spectrum databases."
• [readme] Utilizing an internal chemical subformula assignment protocol rather than SIRIUS fragmentation trees for adduct assignment.: "Utilizing an internal chemical subformula assignment protocol (rather than SIRIUS fragmentation trees)"
• [readme] Multi-adduct support in positive mode beyond [M+H]+ adduct type.: "Considering multiple adduct types beyond [M+H]+ (still only positive mode)"
• [other] Does extending MIST-CF to consider multiple positive-mode adduct types beyond [M+H]+ improve ranking accuracy for chemical formula and adduct assignments on spectra with non-protonated adducts?: "Does extending MIST-CF to consider multiple positive-mode adduct types beyond [M+H]+ improve ranking accuracy for chemical formula and adduct assignments on spectra with non-protonated adducts?"
• [other] For each spectrum, run the extended adduct-assignment protocol using the internal chemical subformula assignment method to score and rank candidate adduct-formula pairs.: "For each spectrum, run the extended adduct-assignment protocol using the internal chemical subformula assignment method to score and rank candidate adduct-formula pairs."