Internal standard feature extraction

internal-standard-feature-extraction

Summary

Extract and isolate internal standard peaks from processed LC-MS feature tables by matching known m/z and retention time windows, enabling quality control assessment of instrument performance across samples. This skill is essential for verifying consistency of internal standard signals in untargeted metabolomics workflows.

When to use

Apply this skill when you have processed LC-MS run data (feature table or peak detection output) containing internal standard identifications and need to monitor internal standard retention time, m/z, and intensity variation across samples as part of automated or user-defined QC checks during instrument runs.

When NOT to use

• Input data is raw vendor format (.raw, .d, .yep files)—convert to mzML using MSConvert first.
• No internal standards were spiked into the samples or their m/z and retention time windows are not known.
• Goal is to identify unknown metabolites; internal standard extraction is a QC-specific task.

Inputs

• processed LC-MS feature table or peak detection output (tabular format with columns: m/z, retention time, intensity, sample identifier)
• internal standard reference list (known m/z values, retention time ranges, and identifiers for each internal standard compound)

Outputs

• filtered feature table containing only internal standard peaks across all samples
• internal standard intensity matrix (samples × internal standards) with retention times and m/z values

How to apply

Load processed LC-MS data (feature table or peak detection output) containing internal standard identifications, retention times, m/z values, and intensity measurements. Define or retrieve known m/z and retention time windows for your internal standards (these are typically experiment-specific reference values established during method validation). Filter or subset the feature table to isolate peaks matching these m/z and retention time windows—this reduces false matches by excluding off-target peaks. Retain all sample identities and their associated intensity measurements for the matched internal standard peaks. The rationale is that internal standards should appear at consistent retention times and m/z values across all samples; deviations indicate instrument drift, column degradation, or data acquisition problems that warrant investigation or automated alerts.

Related tools

• MSConvert (vendor format data conversion prior to feature extraction; enables uniform mzML input) — https://proteowizard.sourceforge.io/tools/msconvert.html
• MS-DIAL (data processing and peak detection to generate feature table input) — http://prime.psc.riken.jp/compms/msdial/main.html
• Pandas (tabular data filtering and subsetting to isolate internal standard peaks)

Evaluation signals

• All internal standard peaks appear at retention times within ±0.5–1.0 min of expected windows across samples (exact tolerance depends on method).
• Extracted internal standard m/z values match reference list within instrument mass accuracy (typically <5 ppm for high-resolution MS).
• Intensity coefficient of variation (CV) for each internal standard across samples is <20% (or experiment-specific threshold); high CV flags potential QC issues.
• No features outside the defined m/z and retention time windows are included in the filtered output (verify by spot-checking edge cases).
• Feature table row count after filtering equals (number of samples × number of internal standards), or fewer if some samples lack certain internal standards.

Limitations

• Extraction accuracy depends on accurate reference m/z and retention time windows; misalignment (e.g., due to column degradation or method drift) can cause internal standard peaks to fall outside windows and be missed.
• Does not account for isobaric or co-eluting compounds; if another metabolite has identical m/z and similar retention time to an internal standard, it may be incorrectly included.
• Requires prior knowledge of internal standard identities and their expected chromatographic properties; cannot be used for exploratory internal standard discovery.
• Rapid QC-MS has been tested extensively on Thermo Fisher mass spectrometers; results may vary with other vendor instruments.

Evidence

• [other] Load processed LC-MS run data (feature table or peak detection output) containing internal standard identifications, retention times, m/z values, and intensity measurements across samples.: "Load processed LC-MS run data (feature table or peak detection output) containing internal standard identifications, retention times, m/z values, and intensity measurements across samples."
• [other] Filter or subset data to isolate internal standard peaks by matching known m/z and retention time windows.: "Filter or subset data to isolate internal standard peaks by matching known m/z and retention time windows."
• [other] Rapid QC-MS provides interactive data visualization of internal standard retention time, m/z, and intensity across samples as a key feature for streamlining untargeted metabolomics research.: "Rapid QC-MS provides interactive data visualization of internal standard retention time, m/z, and intensity across samples"
• [readme] Automated and user-defined quality control checks during instrument runs.: "Automated and user-defined quality control checks during instrument runs"
• [readme] its dependency on MSConvert for vendor format data conversion and MS-DIAL for data processing and identification.: "its dependency on MSConvert for vendor format data conversion and MS-DIAL for data processing and identification"