Mass Spectrometry Precursor Identification

Mass Spectrometry Precursor Identification

Summary

Identify and extract precursor m/z values and retention times for peptide standards from Thermo Orbitrap raw files using extracted ion chromatograms (XIC) with tight mass tolerance. This skill enables validation of LC-MS method linearity and peptide behavior through comparison of experimental retention times against known reference standards.

When to use

When you need to locate and extract quantitative retention time and intensity data for known peptide standards (e.g., iRT peptides) from a Thermo .raw file to validate LC-MS retention time linearity, assess method reproducibility, or establish retention time calibration curves. Apply this skill when standard peptides are available with known m/z values and reference retention time scores (iRTscore).

When NOT to use

• Input is already a processed feature table or aligned retention time matrix — use this skill only when working directly with raw binary .raw files that require chromatogram extraction.
• Working with non-Thermo instrument formats (e.g., Bruker .d, Waters .raw, mzML) — rawrr and RawFileReader are specific to Thermo Orbitrap instruments.
• No known peptide standards or reference retention time scores are available — the skill requires ground truth m/z and iRT scores to construct the validation regression.

Inputs

• Thermo Fisher Scientific .raw file (e.g., 20181113_010_autoQC01.raw from Orbitrap Q Exactive HF)
• List of precursor m/z values for peptide standards
• Reference retention time scores or iRT scores for standards
• Mass tolerance parameter (typically 10 ppm)
• MS-level filter specification (e.g., 'ms' for MS1 scans)

Outputs

• Extracted ion chromatogram (XIC) objects (rawrrChromatogram)
• Fitted retention times at maximum intensity (rtFittedAPEX) for each precursor
• Linear regression model object with R-squared value
• Quantified R-squared metric indicating retention time linearity

How to apply

First, obtain the target precursor m/z values and reference retention time scores (e.g., iRTscore) for your peptide standards. Load the raw file using rawrr::readFileHeader() to confirm instrument configuration (e.g., Q Exactive HF, Orbitrap). Extract extracted ion chromatograms (XIC) for each precursor using rawrr::readChromatogram() with type='xic', specifying the precursor mass list, a tight mass tolerance (e.g., 10 ppm), and filter='ms' to isolate MS1-level precursor signals. Fit the intensity traces stored in the returned rawrrChromatogram objects to identify the retention time at maximum intensity (rtFittedAPEX) for each peptide. Finally, construct a linear regression model (e.g., lm(rtFittedAPEX ~ iRTscore)) to evaluate the linearity of retention time behavior across standards; an R-squared value near 1.0 (e.g., 0.9999) indicates highly linear RT behavior and validates method performance.

Related tools

• rawrr (Primary R package for reading Thermo Orbitrap .raw file chromatogram data and extracting precursor ion intensities and retention times via readChromatogram() and readFileHeader() functions) — https://github.com/fgcz/rawrr
• RawFileReader (Underlying .NET assembly (C# wrapper) that rawrr invokes via system calls to access binary Thermo .raw file structure) — https://github.com/thermofisherlsms/RawFileReader
• Spectra (Optional Bioconductor backend via MsBackendRawFileReader for high-level spectral data access and manipulation in R) — https://bioconductor.org/packages/Spectra/
• MsBackendRawFileReader (Bioconductor-compatible backend that bridges rawrr functionality into the Spectra package ecosystem for standardized spectral accessor functions) — https://github.com/cpanse/MsBackendRawFileReader

Examples

C <- rawrr::readChromatogram(rawfile, mass = c(450.21, 481.24, 502.26), tol = 10, type = "xic", filter = "ms"); rtFittedAPEX <- sapply(C, function(x) x$mz[which.max(x$intensity)]); fit <- lm(rtFittedAPEX ~ iRTscore); summary(fit)$r.squared

Evaluation signals

• Extracted retention times (rtFittedAPEX) for all precursor standards are successfully returned from the rawrrChromatogram object and match the expected peptide elution window in the LC method.
• Linear regression R-squared value is ≥ 0.99 or higher, confirming highly linear retention time behavior across the standard peptides; values significantly lower (e.g., < 0.95) indicate potential instrument drift, calibration failure, or incorrect precursor identification.
• Fitted retention times exhibit low residual error and no systematic deviation across the iRTscore range, indicating unbiased and stable retention time prediction.
• All expected peptide standards (e.g., 11 iRT peptides) are detected and extracted without missing peaks; absence or very low intensity for any standard suggests chromatographic or ionization problems.
• Mass accuracy of extracted precursor m/z values remains within the specified tolerance (e.g., 10 ppm); larger deviations indicate potential mass calibration drift or incorrect filter specification.

Limitations

• rawrr and RawFileReader are exclusive to Thermo Fisher Scientific Orbitrap instruments; other mass spectrometer formats (Bruker, Waters, AB Sciex) require alternative reading libraries.
• Windows systems require explicit decimal symbol configuration as '.' for proper data extraction via the RawFileReader .NET assembly; this platform dependency may complicate automated workflows across OS.
• The skill depends on access to compiled C# RawFileReader assemblies; availability and licensing of these binaries may restrict deployment in some environments.
• Performance and accuracy of retention time fitting depend on adequate XIC signal intensity and peak shape; low-abundance precursors or poor chromatographic resolution may lead to unreliable rtFittedAPEX estimates.
• The linear regression model assumes a simple linear relationship between iRTscore and observed RT; non-linear retention time behavior or systematic drift across the gradient may not be captured by this univariate approach.

Evidence

• [other] Extract precursor m/z values and retention times for known iRT peptide standards using rawrr::readChromatogram() with mass tolerance 10 ppm and XIC filter.: "Extract precursor m/z values and retention times for known iRT peptide standards using rawrr::readChromatogram() with mass tolerance 10 ppm and XIC filter."
• [other] Fit intensity traces stored in rawrrChromatogram objects to extract retention times at maximum intensity (rtFittedAPEX).: "Fit intensity traces stored in rawrrChromatogram objects to extract retention times at maximum intensity (rtFittedAPEX)."
• [other] Construct a linear regression model lm(rtFittedAPEX ~ iRTscore) using annotated iRT reference scores and report R-squared value.: "Construct a linear regression model lm(rtFittedAPEX ~ iRTscore) using annotated iRT reference scores and report R-squared value."
• [methods] The example file 20181113_010_autoQC01.raw used throughout this manuscript contains Fourier-transformed Orbitrap spectra (FTMS) recorded on a Thermo Fisher Scientific Q Exactive HF: "The example file 20181113_010_autoQC01.raw used throughout this manuscript contains Fourier-transformed Orbitrap spectra (FTMS) recorded on a Thermo Fisher Scientific Q Exactive HF"
• [methods] R functions requesting access to data stored in binary raw files (reader family functions listed in Table 1) invoke compiled C# wrapper methods using a system call: "R functions requesting access to data stored in binary raw files (reader family functions listed in Table 1) invoke compiled C# wrapper methods using a system call"
• [results] we extract the RTs at the maximum of the fitted intensity traces stored in the rawrrChromatogram object and fit a linear model: "we extract the RTs at the maximum of the fitted intensity traces stored in the rawrrChromatogram object and fit a linear model"
• [results] The corresponding R-squared indicates that the RTs behave highly linear: "The corresponding R-squared indicates that the RTs behave highly linear"
• [readme] rawrr wraps the functionality of the RawFileReader .NET assembly: "rawrr wraps the functionality of the RawFileReader .NET assembly"
• [discussion] On Windows, the decimal symbol has to be configured as a '.'!: "On Windows, the decimal symbol has to be configured as a '.'!"