How Does ONH Analysis Work?

Materials used in industries such as aerospace, automotive, and steel manufacturing often contain small amounts of oxygen, nitrogen, and hydrogen (ONH) within their crystal structure. While these interstitial gases are invisible to the eye, they can significantly affect material performance. ONH analysis measures them by extracting the gases from the sample and analyzing each one separately, providing the accurate compositional data needed for informed manufacturing and quality decisions.

The Science Behind ONH Analysis

Oxygen, nitrogen, and hydrogen exist as interstitial elements, meaning they occupy microscopic spaces within a material’s crystal lattice. Since they are embedded throughout the bulk material, conventional surface analysis cannot accurately determine their concentration. Reliable measurement requires a technique capable of releasing the gases completely before they are quantified. ONH analysis achieves this using Inert Gas Fusion (IGF). During the analytical process, a solid sample is heated until it melts completely, allowing the trapped gases to escape into an inert carrier gas stream. Once liberated, oxygen, nitrogen, and hydrogen follow separate detection pathways so their concentrations can be measured independently with high accuracy.

How ONH Analysis Works

The Melt Phase

Before heating takes place, the sample is placed inside a high-purity graphite crucible. The analysis chamber is then purged using high-purity helium or argon to remove atmospheric oxygen, nitrogen, moisture, and other contaminants. Eliminating residual gases ensures the analytical results represent only the sample being tested.

Next, a high electrical current passes through the graphite crucible, rapidly increasing the furnace temperature up to 3,000°C, depending on the material. Within seconds, the sample melts completely, allowing the interstitial gases to separate from the metal matrix.

Once the material reaches the required temperature, the three target elements respond differently.

• Oxygen reacts with the carbon in the graphite crucible to produce carbon monoxide (CO)
• Nitrogen is released as elemental nitrogen gas (N₂)
• Hydrogen is released as elemental hydrogen gas (H₂).

Following extraction, the inert carrier gas transports the mixture from the furnace into the analytical system, where the gases are separated and measured.

The Detection Phase

After leaving the furnace, the extracted gases travel through a controlled analytical pathway designed to isolate oxygen, hydrogen, and nitrogen prior to measurement. Dedicated detectors convert the gas signals into precise concentration values expressed as parts per million or weight percentage.

Measuring Oxygen with Infrared Detection

The carbon monoxide generated during fusion passes through a copper oxide catalyst, converting it into carbon dioxide (CO₂), which next enters a Non-Dispersive Infrared (NDIR) detector. CO₂ absorbs infrared light at a specific wavelength. By measuring the reduction in infrared intensity, the ONH analyzer determines how much carbon dioxide is present, allowing the original oxygen concentration in the sample to be calculated accurately.

Measuring Hydrogen

Hydrogen is analyzed separately following extraction from the furnace. Depending on instrument configuration, it is either measured after conversion to water vapor using an NDIR detector or analyzed directly with a highly sensitive Thermal Conductivity Detector (TCD). Both approaches provide precise hydrogen measurements across a wide analytical range.

Measuring Nitrogen

Prior to nitrogen measurement taking place, chemical scrubbers remove carbon dioxide and moisture from the carrier gas stream. Once those gases have been removed, only the inert carrier gas and elemental nitrogen remain.

Finally, the purified gas mixture enters a Thermal Conductivity Detector. The detector compares the thermal conductivity of the sample gas against a reference stream containing pure carrier gas. Any measurable difference allows the instrument to calculate the nitrogen concentration with excellent precision.

The Reporting Phase

After each gas has been measured, the software of the ONH analyzer processes detector responses generated throughout the analysis. Recorded signals are compared against certified calibration standards to calculate the final oxygen, nitrogen, and hydrogen concentrations. The ONH analyzer then reports the results as either parts per million (ppm) or weight percentage, providing a completed analytical record for the sample.

Made for Reliable ONH Testing

XRF Scientific offers dedicated instruments designed to carry out every stage of ONH analysis with precision and repeatability. The G6 LEONARDO delivers efficient routine testing for industrial laboratories, whereas the G8 GALILEO adds advanced automation and enhanced analytical performance for high-throughput laboratories. Meanwhile, the G4 PHOENIX DH focuses on diffusible hydrogen measurement, making it particularly well suited to weld inspection and high-strength steel testing. Engineered for accuracy and efficiency, our ONH analyzers support quality control, metallurgical testing, and advanced materials research applications alike, accommodating diverse materials and evolving testing requirements. Explore our range of ONH analyzers in more detail to locate the perfect solution that can keep pace with your laboratory’s changing demands.