Multidimensional Feature Annotation

Multidimensional Feature Annotation

Summary

Annotate detected mass spectrometry features with isotopologue labels and membership metadata by identifying C13 isotopic substitution patterns across m/z, drift time, and retention time dimensions. This enriches feature tables with monoisotopic designation, isotope count, and offset mass information for downstream filtering and interpretation.

When to use

Apply this skill when you have a peak-picked feature table (HDF5 format) from high-dimensional MS data (m/z, drift_time, retention_time, intensity) and need to identify and label isotopic signatures to distinguish monoisotopic peaks from isotopologues, reduce feature redundancy, and support multi-dimensional alignment confidence.

When NOT to use

• Input is already a deconvoluted or charge-state-assigned feature table; isotope detection assumes raw peak-picked data.
• Analysis targets multiply-charged species (z > 1) without explicit charge-state constraints; DEIMoS isotope detection parameters are optimized for singly charged ions.
• Retention time or drift time dimensions are missing or unreliable; multi-dimensional tolerance constraints require all three dimensions for robust matching.

Inputs

• Peak-picked feature table in HDF5 format with columns: m/z, drift_time, retention_time, intensity
• Charge state constraints (e.g., +1 for singly charged species)

Outputs

• Annotated feature table (HDF5) with added columns: isotopologue label, monoisotopic designation, isotope count, offset mass
• Filtered isotopic signature membership (≥3 members per signature)

How to apply

Load the peak-picked feature table from HDF5 using deimos.load() with explicit columns for m/z, drift_time, retention_time, and intensity. Apply deimos.isotopes() to enumerate C13 isotopologue patterns by computing mass offsets (1.003355 Da per substitution, up to 5 substitutions for singly charged species) and constraining matches within m/z tolerance, drift time tolerance, and retention time tolerance before applying a maximum m/z error of 50 ppm for final downselection. Filter detected isotopic signatures to retain only those with at least 3 members using a membership threshold criterion to reduce false positives. Annotate the feature table with new columns encoding the monoisotopic designation, isotope count (0 = monoisotopic, 1+ = isotopologue member), and offset mass, then export the enriched table as HDF5 with isotope metadata columns preserved for subsequent analysis.

Related tools

• deimos (Python API and command-line tool for isotope detection and feature annotation via deimos.isotopes() and deimos.load()) — http://github.com/pnnl/deimos
• Python (Scripting language for loading, filtering, and annotating feature tables)
• HDF5 (Data format for storing and retrieving peak-picked features and annotated results)

Examples

ms1_peaks = deimos.load('example_data_peaks.h5', key='ms1', columns=['mz', 'drift_time', 'retention_time', 'intensity']); ms1_peaks = deimos.isotopes(ms1_peaks); ms1_peaks = ms1_peaks[ms1_peaks['isotope_count'] >= 0]; ms1_peaks.to_hdf5('annotated_features.h5', key='ms1')

Evaluation signals

• All detected isotopic signatures have ≥ 3 member peaks; no signatures with 1–2 members remain in the output.
• Monoisotopic designation is consistent: exactly one peak per isotopic signature is labeled as monoisotopic (isotope count = 0); all others have isotope count ≥ 1.
• Mass offsets match expected C13 substitution pattern: each member peak differs from monoisotopic by n × 1.003355 Da (n = isotope count) within 50 ppm m/z error.
• Annotated feature table retains all original columns (m/z, drift_time, retention_time, intensity) plus new isotope metadata columns without data loss.
• Feature count increases or remains stable after annotation (no false positive isotope linkages should cause loss of valid monoisotopic peaks).

Limitations

• Isotope detection is optimized for singly charged species (z = +1); extension to multiply-charged ions requires explicit charge-state constraints and mass offset recalculation.
• Detection sensitivity depends on intensity threshold applied before isotope detection; weak isotopologue members may be missed if below the threshold (e.g., threshold=1000 is more stringent than threshold=100).
• M/z tolerance (50 ppm maximum), drift time tolerance, and retention time tolerance must be tuned for the specific mass spectrometry platform and acquisition method; mismatch will cause false negatives (missed isotopologues) or false positives (erroneous linkages).
• Multi-dimensional approach assumes drift time and retention time are reliable and well-calibrated; poor drift time resolution or time-alignment artifacts will degrade isotope pattern coherence.

Evidence

• [other] DEIMoS isotope detection enumerates C13 isotopologue offsets by specifying an m/z delta of 1.003355 Da with up to 5 isotopic substitutions for singly charged species, using m/z tolerance, drift time tolerance, and retention time tolerance to constrain the search before applying a maximum m/z error of 50 ppm for final downselection.: "DEIMoS isotope detection enumerates C13 isotopologue offsets by specifying an m/z delta of 1.003355 Da with up to 5 isotopic substitutions for singly charged species, using m/z tolerance, drift time"
• [other] Load peak-picked feature table from HDF5 file using deimos.load() with columns for m/z, drift_time, retention_time, and intensity. Apply deimos.isotopes to detect C13 isotopologue patterns, computing mass offsets between monoisotopic and isotopically substituted peaks for charge state +1.: "Load peak-picked feature table from HDF5 file using deimos.load() with columns for m/z, drift_time, retention_time, and intensity. Apply deimos.isotopes to detect C13 isotopologue patterns, computing"
• [other] Filter detected isotopic signatures to retain only those with at least 3 members using a membership threshold criterion.: "Filter detected isotopic signatures to retain only those with at least 3 members using a membership threshold criterion."
• [other] Annotate the feature table with isotopologue labels (monoisotopic designation, isotope count, offset mass) and export as HDF5 with isotope metadata columns.: "Annotate the feature table with isotopologue labels (monoisotopic designation, isotope count, offset mass) and export as HDF5 with isotope metadata columns."
• [readme] Functionality includes feature detection, feature alignment, collision cross section (CCS) calibration, isotope detection, and MS/MS spectral deconvolution: "Functionality includes feature detection, feature alignment, collision cross section (CCS) calibration, isotope detection, and MS/MS spectral deconvolution"