Carbon Monoxide Detection in Hydrogen Gas: Methods, Standards, and Applications

Carbon monoxide (CO) detection in hydrogen gas is a critical trace-impurity analysis that safeguards fuel cell performance, product quality, and process safety across the hydrogen value chain. Because even sub-ppm levels of CO can permanently poison proton exchange membrane (PEM) fuel cell catalysts, standards such as ISO 14687, ISO 21087, ASTM D7652, ASTM D7653, and GB/T 37244 mandate stringent detection limits and validated analytical methods. This article provides a comprehensive technical overview of CO testing in hydrogen, covering sample types, testing items, analytical methods, applications, and frequently asked questions.

Technical Overview

Carbon monoxide is one of the most consequential trace impurities in hydrogen gas. Although hydrogen produced by water electrolysis is inherently CO-free, hydrogen generated by steam methane reforming, methanol reforming, partial oxidation, or recovered as a byproduct stream from petrochemical and chlor-alkali processes can contain carbon monoxide at concentrations ranging from ppb to percent levels. In fuel cell applications, CO molecules adsorb strongly onto platinum-group catalyst surfaces at the anode, blocking active sites and causing severe, partially irreversible performance degradation even at exposures below 1 µmol/mol.

International and national standards therefore impose very tight limits on CO in hydrogen fuel. ISO 14687-2 specifies a maximum CO concentration of 0.2 µmol/mol (ppm by volume) in hydrogen for PEM fuel cell road vehicles, a limit also adopted in GB/T 37244. ISO 14687-3 governs hydrogen for stationary and other fuel cell applications, while ISO 21087 defines the analytical validation requirements for methods used to verify hydrogen fuel quality. ISO 19880-1 addresses fuel quality control at hydrogen refueling stations. For industrial hydrogen grades, specifications such as GB/T 3634.1 apply, and semiconductor-grade hydrogen imposes its own stringent impurity requirements.

Verifying compliance at the 0.2 µmol/mol level demands analytical techniques with detection limits well below the specification limit, typically at or below 0.05 µmol/mol, supported by traceable calibration gas mixtures prepared according to ISO 6142 and calibrated in accordance with ISO 6143. Accredited third-party laboratories routinely perform this testing using gas chromatography, infrared spectroscopy, and laser absorption spectroscopy platforms.

Testing Samples

Hydrogen Fuel for Fuel Cell Vehicles

Samples are collected from hydrogen refueling stations, pre-cooling units, tube trailers, cascade storage banks, and dispenser nozzles in accordance with hydrogen fuel sampling guidance such as ISO 19880-8. These samples are tested against the full ISO 14687-2 impurity suite, in which CO is a key limiting parameter at 0.2 µmol/mol.

Industrial and Byproduct Hydrogen

Hydrogen produced by steam methane reforming, coal gasification, or recovered as a byproduct of chlor-alkali, methanol, ethylene, or coke-oven operations is sampled at production outlets, purification units, PSA (pressure swing adsorption) downstreams, and pipeline delivery points. CO content may range from trace levels after purification up to percent levels in crude reformate streams.

Electrolytic Hydrogen

Hydrogen from alkaline, PEM, or solid-oxide electrolyzers is sampled to confirm the absence of carbon-bearing impurities and to verify that ambient CO ingress, lubricant carryover, or contamination during compression and storage has not degraded fuel quality.

High-Purity Hydrogen for Electronics and Specialty Applications

Semiconductor manufacturing, optical fiber production, and metallurgical processing require hydrogen with ultra-low carbon compound content. Samples are typically taken from point-of-use lines in passivated stainless steel cylinders to preserve trace-level integrity.

Common sample containers and considerations include:

• Passivated or electropolished stainless steel sampling cylinders with inert valve surfaces
• Clean, hydrogen-compatible pressure regulators free of hydrocarbon and CO background
• Multiple purge-and-fill cycles to eliminate air contamination and adsorption artifacts
• Minimal hold time between sampling and analysis to prevent surface adsorption or desorption effects
• Chain-of-custody documentation with pressure, temperature, and sampling point records

Testing Items

The primary testing item is the quantitative determination of carbon monoxide concentration in hydrogen, expressed in µmol/mol (ppmv). Testing is frequently performed alongside related carbonaceous impurities to provide a complete impurity profile, including:

• Carbon monoxide (CO), typically reporting range 0.02–100 µmol/mol with detection limits at or below 0.05 µmol/mol
• Carbon dioxide (CO2), often measured simultaneously by the same method
• Methane and total hydrocarbons (THC), which may be co-reported with CO and CO2 under methods such as GB/T 8984
• Supporting impurities of the ISO 14687-2 suite, including oxygen, nitrogen, argon, water, total sulfur compounds, formaldehyde, formic acid, ammonia, and halogenated compounds, when full fuel quality certification is required

Method performance criteria follow ISO 21087 validation principles: the detection limit should be no greater than one third of the applicable specification limit, and the expanded measurement uncertainty should generally not exceed 30% of the limit value for conformity assessment decisions.

Testing Methods

Gas Chromatography with Methanizer and Flame Ionization Detection (GC-FID)

Described in ASTM D7652 and the GB/T 8984 series, this technique separates CO on a gas chromatographic column, converts it catalytically to methane over a heated nickel methanizer, and detects it with a flame ionization detector. The methanization step provides high sensitivity, with typical detection limits of 10–50 nmol/mol. This method is widely regarded as a reference approach for sub-ppm CO in hydrogen and simultaneously quantifies CO2.

Gas Chromatography with Helium Ionization Detection (GC-PDHID)

Pulsed discharge helium ionization detectors respond to virtually all compounds, including permanent gases, with a virtually universal response at low ppb levels. GC-PDHID permits direct determination of CO, CO2, nitrogen, oxygen, methane, and argon in a single run without derivatization, making it valuable for high-purity hydrogen characterization.

Fourier Transform Infrared Spectroscopy (FTIR)

ASTM D7653 defines the use of multi-pass FTIR gas cells to determine CO and numerous other trace impurities in hydrogen fuel, including CO2, methane, formaldehyde, formic acid, ammonia, and water. FTIR offers rapid multi-component analysis with minimal sample consumption and detection limits for CO approaching 0.02–0.1 µmol/mol depending on path length and cell volume.

Laser Absorption Spectroscopy (CRDS and TDLAS)

Cavity ring-down spectroscopy and tunable diode laser absorption spectroscopy exploit characteristic mid-infrared or near-infrared absorption lines of CO, delivering ppb-level sensitivity, fast response, and field deployability. These optical techniques are increasingly used for online monitoring at hydrogen refueling stations and rapid quality screening during fuel station conformity checks.

Electrochemical and Portable Sensor Screening

Electrochemical CO sensors and portable gas analyzers provide cost-effective screening at ppm levels for workplace safety and process monitoring. While unsuitable on their own for certifying the 0.2 µmol/mol fuel cell specification, they serve as useful preliminary indicators that trigger confirmatory laboratory analysis.

All quantitative methods are calibrated using certified reference gas mixtures traceable to national metrology institutes, prepared gravimetrically per ISO 6142, with certificate contents reported according to ISO 6141 and calibration curves evaluated per ISO 6143. Typical quality control measures include:

• Multi-point calibration bracketing the specification limit
• Zero hydrogen and blank checks to control carrier gas and system background
• Duplicate injections and control samples to verify repeatability
• Participation in interlaboratory comparisons for hydrogen purity analysis

Applications

Carbon monoxide detection in hydrogen supports quality control, safety, and regulatory compliance across diverse sectors, including:

• Fuel cell electric vehicle programs and hydrogen refueling station commissioning, periodic verification, and conformity assessment against ISO 14687-2 and GB/T 37244
• Hydrogen production plants using steam methane reforming, methanol reforming, or electrolysis, for process optimization and purification unit performance monitoring
• PSA and deoxo unit acceptance testing, where residual CO must be verified after purification stages
• Backup power and telecom fuel cell systems requiring fuel quality assurance before deployment
• Semiconductor and electronics manufacturing, where carbon-bearing impurities affect wafer processing and epitaxial film quality
• Research and development of catalysts, membranes, electrolyzers, and hydrogen purification materials
• Supplier qualification and batch release testing of cylinder hydrogen and tube trailer deliveries

FAQ

Why is the CO limit for fuel cell hydrogen so low? Carbon monoxide chemisorbs strongly on platinum anode catalysts in PEM fuel cells. Exposure to even fractions of a ppm progressively blocks active sites, reducing cell voltage and efficiency, and prolonged exposure can cause partially irreversible degradation. This is why ISO 14687-2 and GB/T 37244 cap CO at 0.2 µmol/mol.

What detection limit must a laboratory achieve? Following ISO 21087 validation guidance, the method detection limit should be no greater than one third of the specification limit, meaning approximately 0.06 µmol/mol or lower, with many laboratories achieving 0.02 µmol/mol using GC-FID with methanization or laser spectroscopic techniques.

Which method is most appropriate for my sample? GC-FID with methanizer (ASTM D7652) and GC-PDHID offer the highest sensitivity for certification testing; FTIR (ASTM D7653) provides fast multi-component profiling; CRDS/TDLAS instruments suit online and field monitoring; electrochemical sensors are suitable only for ppm-level screening.

Can CO be measured together with other impurities? Yes. CO, CO2, and methane are frequently quantified in a single determination under methods such as GB/T 8984, and FTIR can report a broad impurity suite in one analysis, supporting complete ISO 14687 conformity assessment.

How should hydrogen samples be collected and shipped? Samples should be filled into passivated stainless steel cylinders after thorough purging, using hydrogen-cleaned regulators, and analyzed promptly by an accredited third-party laboratory to minimize adsorption-related biases and maintain sample integrity.

Does industrial hydrogen need CO testing? CO testing is recommended whenever hydrogen is derived from reforming or byproduct sources, feeds catalytic or thermal processes, or enters applications where carbon contamination affects downstream product quality or catalyst life.