interpret-chromatogram

name	interpret-chromatogram
description	Interpret GC or HPLC chromatogram data to verify system suitability, identify peaks by retention time and spectral matching, perform accurate peak integration, calculate chromatographic figures of merit, and assess peak quality for reliable quantitation. Triggers on chromatogram data files, integration reports, or requests to analyze chromatographic results.
license	MIT
allowed-tools	Read Grep Glob WebFetch WebSearch
metadata	{"author":"Philipp Thoss","version":"1.1","domain":"chromatography","complexity":"intermediate","language":"natural","tags":"chromatography, peak-analysis, resolution, integration, system-suitability"}

Interpret a Chromatogram

Systematic interpretation of gas chromatography (GC) and high-performance liquid chromatography (HPLC) data for analytical chemistry workflows.

Purpose

Enable confident qualitative and quantitative analysis of chromatographic data by guiding analysts through system suitability verification, peak identification, integration, parameter calculation, and quality assessment. Ensures analytical results meet method specifications and regulatory requirements.

When to use

• Reviewing chromatographic data before reporting analytical results
• Verifying that a system suitability test passes before running a sample sequence
• Identifying unknown peaks or confirming known analytes by retention time or spectral data
• Troubleshooting unexpected peaks, baseline anomalies, or integration artifacts
• Training analysts on chromatographic data interpretation
• Calculating chromatographic figures of merit (resolution, tailing factor, theoretical plates)
• Validating peak integration for regulatory compliance

Required inputs

• Chromatogram data: Digital or printed chromatogram with time axis and detector response axis
• Reference standard data: Retention times and responses of known analytes under the same method conditions
• Method parameters: Column specifications, mobile phase/carrier gas composition, temperature/gradient program, detector settings

Optional inputs

• Spectral data: UV-Vis spectra (DAD), mass spectra (MS), or other spectral information for peak confirmation
• Previous chromatograms: Historical data from the same method for trend comparison
• System suitability criteria: Acceptance limits from the method or regulatory standard
• Sample preparation details: Dilution factors, extraction recovery, internal standard concentration

When NOT to use

• Developing a new chromatographic method — use develop-gc-method or develop-hplc-method instead
• Troubleshooting instrument hardware or separation problems — use troubleshoot-separation instead
• Interpreting raw mass spectrometry data without chromatographic context — use interpret-mass-spectrum instead
• Performing formal method validation — use validate-analytical-method instead
• Routine data entry or LIMS upload without interpretation — this skill focuses on analytical decision-making, not data transfer

Procedure

Step 1: Verify System Suitability

Confirm that the chromatographic system is performing within specification before interpreting sample data.

1. Locate the system suitability injections (typically 5-6 replicates of a reference standard at the start of the sequence).
2. Calculate each parameter from the following table:

Parameter	Typical Specification	Calculation
Retention time RSD	<= 1.0%	RSD of tR over n >= 5 injections
Peak area RSD	<= 2.0% (assay), <= 5.0% (impurity)	RSD of area over n >= 5 injections
Tailing factor (T)	0.8-2.0 (USP), ideally 0.9-1.2	T = W0.05 / (2 * f) where W0.05 = width at 5% height, f = front half-width
Resolution (Rs)	>= 1.5 (baseline), >= 2.0 (regulated)	Rs = 2(tR2 - tR1) / (w1 + w2)
Theoretical plates (N)	Per column spec (e.g., >= 2000)	N = 16(tR / w)^2 or N = 5.54(tR / w0.5)^2
Capacity factor (k')	2.0-10.0 for primary analyte	k' = (tR - t0) / t0

3. Compare calculated values against the method's acceptance criteria.
4. Document all system suitability results in the batch record.

Step 2: Identify Peaks

1. Compare each peak's retention time (tR) against the reference standard chromatogram.
   • Acceptable retention time match: within +/- 2% of the reference tR (or +/- 0.1 min for short runs).
2. For ambiguous identifications, use co-injection (spiking): add reference standard to the sample and re-inject. The target peak should increase without broadening or shouldering.
3. For DAD-equipped HPLC: compare the UV-Vis spectrum of each peak against a spectral library.
   • Spectral match index >= 990 (out of 1000) for positive identification.
   • Check spectral purity across the peak (front, apex, tail spectra should overlay).
4. For MS-equipped systems: confirm molecular ion (m/z) and key fragment ions against reference spectra.
5. Flag any peak that cannot be identified -- report it as "unknown" with its retention time and relative response.

Step 3: Perform Peak Integration

1. Select integration mode:
   • Automatic integration with data system defaults as a starting point
   • Manual adjustment only when automatic integration demonstrably misplaces baseline or peak boundaries
2. Set integration parameters:
   • Baseline detection sensitivity (slope sensitivity / threshold)
   • Minimum peak area or height to reject noise
   • Peak width parameter matching the narrowest expected peak
3. Verify baseline placement:
   • Baseline should connect the start and end of each peak at the true chromatographic baseline
   • For overlapping peaks, use valley-to-valley or perpendicular drop methods as specified by the method
   • For gradient methods, baseline may rise -- use a tangent skim or exponential skim for peaks on a rising baseline
4. Check for integration errors:
   • Split peaks integrated as two when they should be one
   • Shoulder peaks merged into the main peak when they should be separate
   • Noise spikes integrated as peaks
   • Baseline drawn through a peak (negative peak clipping)
5. Record the final integration parameters and any manual adjustments with justification in an audit trail.

Step 4: Calculate Chromatographic Parameters

Calculate the following for all reported peaks:

1. Resolution (Rs) between adjacent peaks:
   • Rs = 2(tR2 - tR1) / (w1 + w2)
   • Rs >= 1.5 indicates baseline separation; Rs >= 2.0 provides margin for routine use
2. Tailing factor (T) at 5% peak height:
   • T = W0.05 / (2f)
   • T = 1.0 is perfectly symmetric; T > 2.0 indicates significant tailing
3. Theoretical plates (N):
   • N = 16(tR / w)^2 using baseline width, or N = 5.54(tR / w0.5)^2 using half-height width
   • Higher N means better column efficiency
4. Capacity factor (k'):
   • k' = (tR - t0) / t0 where t0 is the dead time (void volume / flow rate)
   • Ideal range: 2-10 for good separation with reasonable run time
5. Selectivity factor (alpha) between critical pair:
   • alpha = k'2 / k'1
   • alpha > 1.05 is generally needed for adequate separation
6. Tabulate results for all analytes and compare against method specifications.

Step 5: Assess Peak Quality

1. Symmetry: Peaks should be Gaussian or near-Gaussian. Significant fronting (T < 0.8) suggests column overload; tailing (T > 1.5) suggests secondary interactions or dead volume.
2. Baseline separation: For quantitative work, critical pairs must be baseline-resolved. If valley between peaks does not return to baseline, note the percentage valley and assess impact on accuracy.
3. Peak width consistency: Peaks that are significantly broader than expected (compared to the standard) may indicate on-column degradation, extra-column band broadening, or injection issues.
4. Spectral purity (DAD/MS): If the purity index indicates spectral inhomogeneity across the peak, a co-eluting impurity is likely present. Consider adjusting the method for better resolution.
5. Negative peaks or baseline disturbances: Negative peaks in UV indicate the sample solvent absorbs more than the mobile phase at the detection wavelength -- this is normal for the solvent front but abnormal elsewhere.
6. Ghost peaks: Peaks present in the blank injection indicate carryover, contaminated mobile phase, or column bleed. Identify the source before reporting sample results.
7. Summarize overall chromatographic quality and note any limitations on the reported results.

Output contract

Successful interpretation produces:

• System suitability report: All calculated parameters with pass/fail status against acceptance criteria
• Peak identification table: Each peak labeled with compound name or "unknown", retention time, match confidence, and spectral confirmation status where available
• Integration audit trail: Integration parameters used, any manual adjustments with scientific justification
• Chromatographic parameters table: Resolution, tailing factor, theoretical plates, capacity factor, and selectivity for all reported peaks
• Peak quality assessment: Notation of any asymmetry, co-elution, ghost peaks, or other quality issues with impact on quantitation
• Recommendation: Whether the data is suitable for reporting, requires re-analysis, or needs method adjustment

Failure handling

System suitability failure

If any system suitability parameter fails, the system is not suitable -- do not proceed to sample interpretation until the issue is resolved.

• Retention time RSD fails: Check for temperature instability, mobile phase preparation errors, or column degradation. Re-equilibrate system and re-inject standards.
• Tailing factor fails: Inspect the inlet liner (GC) or column frit (HPLC). Replace if contaminated or degraded.
• Resolution fails: Check column performance with a dedicated test mix and replace if necessary. Verify mobile phase composition and flow rate.
• Peak area RSD fails: Investigate injection precision, autosampler performance, and sample stability.

Peak identification failure

• Retention time shift (uniform): All peaks moving together indicates systematic change (column aging, temperature drift, mobile phase error). Re-inject the reference standard to establish current retention times before re-evaluating.
• Ambiguous identification: Use co-injection or spectral confirmation. If still ambiguous, flag as "tentative" and note the uncertainty.
• Unknown peaks: Report retention time, area, and relative response. Do not assign arbitrary identities without confirmation.

Integration failure

• Automatic integration mishandles peaks: Create a timed-events integration method with custom parameters for that retention window. Document the change.
• Baseline anomalies: For gradient baselines or complex peak shapes, consult the instrument software manual for advanced integration options.
• Never manually adjust integration to achieve a desired result: All adjustments must be scientifically justified and audit-trailed. Adjusting to make a result pass specification is data falsification.

Peak quality failure

• Co-elution confirmed by spectral impurity: The data may not be reportable for the affected peaks. Flag the results, investigate root cause, and re-run after corrective action or method adjustment.
• Ghost peaks at analyte retention times: Indicates carryover or contamination. Re-run blanks until clean before reporting sample results.
• Significant tailing or fronting: Assess impact on quantitation accuracy. If quantitation is affected, the data may need to be rejected.

Common pitfalls to avoid

• Accepting automatic integration without review: Data systems can misplace baselines, especially for shoulders, small peaks near large ones, and gradient baselines. Every chromatogram must be visually reviewed.
• Confusing retention time shift with a new peak: Uniform retention time shifts (all peaks move together) indicate a systematic change, not new compounds. Re-inject the standard to recalibrate before making identification calls.
• Reporting peaks below the noise level: Peaks with signal-to-noise ratio below 3 (detection) or 10 (quantitation) should not be identified or quantitated. Calculate S/N explicitly for trace-level peaks.
• Ignoring the solvent front: The void volume peak is not an analyte. Ensure t0 is correctly identified and excluded from analyte reporting.
• Manual integration to achieve a target result: Adjusting integration to make a result pass specification is data falsification. All integration changes must be scientifically justified and audit-trailed.
• Neglecting spectral purity checks: A clean-looking peak can hide a co-eluting impurity. Always check peak purity when DAD or MS data is available.

Next steps

• If method development is needed: Proceed to develop-gc-method or develop-hplc-method
• If separation problems persist: Proceed to troubleshoot-separation
• If formal method validation is required: Proceed to validate-analytical-method
• If MS data needs detailed interpretation: Proceed to interpret-mass-spectrum

References

• USP <621> Chromatography — general chapter on chromatographic methods
• ICH Q2(R1) Validation of Analytical Procedures — guidance on method validation
• Snyder, L.R., Kirkland, J.J., & Dolan, J.W. "Introduction to Modern Liquid Chromatography" (3rd ed.) — comprehensive HPLC reference
• Grob, R.L. & Barry, E.F. "Modern Practice of Gas Chromatography" (4th ed.) — comprehensive GC reference
• International Council for Harmonisation (ICH) guidelines on analytical method validation

Related skills

• develop-gc-method — method development for the GC technique producing the chromatogram
• develop-hplc-method — method development for the HPLC technique producing the chromatogram
• troubleshoot-separation — diagnosing problems identified during chromatogram interpretation
• validate-analytical-method — formal validation of the method generating the chromatographic data
• interpret-mass-spectrum — detailed interpretation of MS data for GC-MS and LC-MS peak confirmation