ISO 6975:1997

Overview

ISO 6975:1997 - Natural gas - Extended analysis - Gas‑chromatographic method specifies a gas‑chromatography (GC) based procedure for a complete, extended compositional analysis of natural gas. The standard defines what instrumentation, calibration materials and analytical performance are required to quantify main, associated and trace components using packed or capillary columns with thermal‑conductivity (TCD) and flame‑ionization detectors (FID). It also provides guidance on retention indices and response factors for components not present in calibration mixtures.

Key topics and requirements

• Scope of components: helium, hydrogen, argon, oxygen, nitrogen, carbon dioxide, saturated hydrocarbons C1–C5, hydrocarbon fractions C6+, and selected aromatics (e.g., benzene, toluene).
• Quantitative ranges: specified detection ranges for many components (examples: oxygen, helium, hydrogen, argon down to 0.001 % (n/n); methane 50–100 % (n/n); ethane and propane lower limits to 0.001 % (n/n)).
• Instrumentation: one or more gas chromatographs capable of isothermal and/or temperature‑programmed operation with TCD and/or FID, sample introduction and data integrator. Packed or open tubular (capillary) columns are used depending on component groups.
• Calibration and reference gases: use of Certified Reference Materials (CRMs) and Working Reference Mixtures (WRMs) prepared gravimetrically (ISO 6142) and traceable to primary standards; control gases for statistical control charts.
• Response and identification: FID responses assumed proportional to carbon number; components absent from calibrations are quantified via relative response factors or reference components. Linear retention index (RI) calculation is used for identification (temperature‑programmed analyses).
• Performance criteria: chromatographic resolution minima between adjacent peaks (e.g., R ≥ 2 for N2→2‑methylpropane, R ≥ 4 for 2‑methylpropane→n‑pentane; O2/N2 ≥ 1.5).
• Limitations: not intended for oxygenated compounds (water, methanol, glycols), sulfur compounds, dense phase gases, or samples containing hydrocarbon condensate. Hydrocarbons above C6 cannot always be unambiguously identified and are often reported by carbon number group.

Applications and users

ISO 6975:1997 is used by:

• analytical laboratories performing extended compositional analysis of natural gas;
• gas producers and processing plants for composition reporting and quality control;
• energy companies and engineers performing calorific value, density and vapour equilibrium calculations that require detailed C6+ information;
• downstream users needing aromatic or trace hydrocarbon data for chemical feedstock evaluation.

Practical benefits include accurate component breakdown for property calculations, process modelling and compliance with traceability and calibration best practices.

Related standards

• ISO 6142, ISO 6143 (calibration and validation of gas mixtures)
• ISO 6974 series (gas‑chromatographic composition with defined uncertainty)
• ISO 6570‑1, ISO 6326 series (hydrocarbon liquid content and sulfur compounds)
• ISO 10715, ISO 14111, ISO 5725‑1 (sampling, traceability and measurement accuracy)