Ms1 Spectrum Subsetting

MS1-level spectrum subsetting

Summary

Filter a scan index data.frame to retain only MS1-level mass spectrometry scans, enabling targeted analysis of precursor ion spectra. This skill is essential when downstream analysis (base-peak extraction, retention time alignment, chromatogram construction) requires isolation of survey scans from a mixed MS1/MS2 dataset.

When to use

After generating a scan index from a Thermo Fisher Orbitrap raw file using readIndex(), apply this skill when your analysis goal requires working exclusively with MS1 (precursor) scans rather than tandem MS (MS2/MS3) spectra. Common triggers include: constructing base-peak m/z and intensity tables, aligning retention times, or generating extracted ion chromatograms where only survey-scan data is relevant.

When NOT to use

• Input is already restricted to MS1 scans (e.g., pre-filtered raw file or targeted MS1-only acquisition method).
• Analysis goal requires MS2 fragmentation spectra or depends on tandem MS data for peptide identification.
• Scan index does not contain an 'ms' column (malformed or non-standard index structure).

Inputs

• data.frame scan index (output of rawrr::readIndex())
• Thermo Fisher Orbitrap raw file (already indexed)

Outputs

• data.frame: subset scan index containing only MS1-level rows
• integer vector: scan numbers for downstream readSpectrum() calls

How to apply

After calling readIndex() to generate a data.frame indexing all scans in the raw file, subset the data.frame using the 'ms' column to retain only rows where ms == 1. This column indicates the tandem MS level of each scan. The filtering operation is a logical subsetting step (e.g., index[index$ms == 1, ]) that removes all MS2, MS3, and higher-order fragmentation scans. Verify the filter by checking that the resulting data.frame contains only rows with ms=1 and that scan counts are reasonable relative to total runtime (e.g., a 55-minute LC-MS run yielding ~24,000 total scans typically contains several thousand MS1 scans). This step precedes per-spectrum operations like readSpectrum() calls on the filtered scan numbers.

Related tools

• rawrr (Provides readIndex() function to generate scan index data.frame and readSpectrum() to retrieve individual MS1 spectra after subsetting) — https://github.com/fgcz/rawrr
• RawFileReader (Underlying .NET assembly wrapped by rawrr; enables binary raw file access from which scan metadata (including ms level) is extracted) — https://github.com/thermofisherlsms/RawFileReader
• Spectra (Bioconductor package that can consume MS1-filtered scan indices via MsBackendRawFileReader for downstream statistical and visualization workflows)

Examples

index_df <- rawrr::readIndex(raw_file_path); ms1_index <- index_df[index_df$ms == 1, ]; ms1_scan_numbers <- ms1_index$scan

Evaluation signals

• All rows in output data.frame have ms == 1 (invariant check).
• Output row count is substantially less than input row count (typical ratio ~10–50% depending on acquisition method), reflecting removal of MS2 and higher scans.
• Scan numbers in the MS1-only index are contiguous or sparse integers within the range [1, total_scans_in_file] (data integrity check).
• Retention time values in the filtered index span the expected LC runtime (e.g., 0 to 55 minutes for a 55-minute gradient).
• Subsequent readSpectrum() calls on filtered scan numbers succeed and return rawrrSpectrum objects with non-empty m/z and intensity arrays (functional downstream check).

Limitations

• Filtering by ms == 1 assumes the scan index column is correctly populated by the RawFileReader API; malformed or corrupted raw files may yield unexpected or missing ms values.
• This skill operates only on the in-memory data.frame; it does not re-parse or validate the binary raw file itself, so systematic errors in raw file metadata may not be caught.
• High-resolution Orbitrap data with data-dependent acquisition (DDA) typically generate MS1/MS2 ratios of 1:10 or higher; extremely low or zero MS1 counts may indicate instrument configuration issues or acquisition failures not detectable by this filter alone.
• The skill assumes the 'ms' column naming convention used by rawrr; alternative or custom scan indices may use different column names or encodings.

Evidence

• [other] Generate the scan index as a data.frame using readIndex() on the raw file.: "Generate the scan index as a data.frame using readIndex() on the raw file."
• [other] Subset the index to retain only MS1-level scans (ms == 1).: "Subset the index to retain only MS1-level scans (ms == 1)."
• [results] By using the readIndex() function a data.frame that indexes all scans found in a raw file is returned: "By using the readIndex() function a data.frame that indexes all scans found in a raw file is returned"
• [results] using only MS1-level scans: "Filter MS scans using ms filter; using only MS1-level scans"
• [readme] rawrr wraps the functionality of the RawFileReader .NET assembly: "rawrr wraps the functionality of the RawFileReader .NET assembly"