ISO 6974-6:2002

Overview

ISO 6974-6:2002 specifies a laboratory gas chromatographic method for the quantitative determination of helium, hydrogen, oxygen, nitrogen, carbon dioxide and C1–C8 hydrocarbons in natural gas using three capillary columns. The method produces compositional data with defined uncertainty that are used to calculate calorific value, relative density and Wobbe index. ISO 6974-6:2002 is to be used in conjunction with ISO 6974‑1 (guidelines) and ISO 6974‑2 (measuring‑system characteristics and data processing).

Key topics and technical requirements

• Analytical scope and mole-fraction ranges: Component-specific mole-fraction ranges are defined (examples: helium 0.002–0.5%, hydrogen 0.001–0.5%, methane 40–100%, nitrogen up to 40%, CO2 up to 10%). These ranges indicate where the stated precision applies, not the absolute detection limits.
• Three-capillary-column setup: Uses a PLOT precolumn, a molecular‑sieve PLOT column for permanent gases and methane, and a WCOT (thick-film) apolar column for C2–C8 hydrocarbons. Columns are installed across one or two gas chromatographs (dual-oven or separate instruments).
• Detection and optional methanizer:
  • Permanent gases and methane: thermal conductivity detector (TCD).
  • C2–C8 hydrocarbons: flame ionization detector (FID).
  • For natural gas substitutes, a methanizer can be used to convert CO and CO2 to CH4 for FID detection at low mole fractions (recommended when CO/CO2 < ~0.02% or to detect down to ~0.001%).
• Sampling and instrumentation: Requires controlled sample loops or microvalves, column-switching for backflushing, flow regulators suited to capillary columns, and ovens capable of temperature programming (instrument oven example range 40–140 °C, ±0.1 °C stability).
• Reference and performance gases: Use working reference mixtures prepared/certified per ISO 6142/6143, and performance test gases (e.g., CO/CO2/CH4 mixes, benzene/cyclohexane for resolution checks).
• Precision and reporting: The standard defines performance requirements, calculation methods, precision estimates and required elements for the test report.

Practical applications and users

• Who uses it: Analytical laboratories, natural gas quality teams, energy companies, custody-transfer laboratories, gas trading verification labs, and equipment manufacturers for GC-based gas analyzers.
• Typical uses:
  • Accurate compositional analysis for billing, custody transfer and regulatory compliance.
  • Feed data for heating value, Wobbe index and relative density calculations.
  • Characterization of natural gas substitutes and upgraded gas streams (with methanizer when needed).
  • Method validation, instrument qualification and inter-laboratory comparisons.

Related standards

• ISO 6974‑1: Guidelines for tailored analysis
• ISO 6974‑2: Measuring-system characteristics and statistics for data processing
• ISO 6142 / ISO 6143: Preparation and certification of calibration gas mixtures
• ISO 7504: Gas analysis vocabulary

Keywords: ISO 6974-6, natural gas analysis, gas chromatography, capillary columns, hydrogen, helium, carbon dioxide, C1–C8 hydrocarbons, TCD, FID, methanizer, laboratory gas analysis.