Measurement Principle - Oxygen/Nitrogen/Hydrogen Analysis
For the analysis of oxygen, nitrogen, and hydrogen in different materials, two measurement principles are commonly employed: inert gas fusion-infrared absorption and thermal conductivity methods. These methods utilize different techniques to accurately measure the concentrations of these gases.
In the inert gas fusion-infrared absorption method, a sample is fused with an inert gas, typically helium (He) or argon (Ar), in a controlled environment. The fused sample is then subjected to infrared absorption analysis, where specific wavelengths absorbed by the gas molecules of interest are measured.
On the other hand, the thermal conductivity method relies on the principle that gases with different thermal conductivities will have distinct effects on heat transfer. By measuring the changes in thermal conductivity caused by the presence of oxygen, nitrogen, or hydrogen, their concentrations can be determined.
Both methods provide reliable and accurate measurements for oxygen, nitrogen, and hydrogen analysis in a wide range of materials, allowing for precise characterization and quality control in various industries.
The Gas Flow
In either measurement method, a sample is placed in a crucible and subjected to electrical heating to melt and gasify the elements present in the sample. Helium (He), an inert gas, is used as a carrier gas to transport the gasified elements to the detector. Depending on the element to be measured, detection is carried out using either an NDIR (Non-Dispersive Infrared) detector or a TCD (Thermal Conductivity) detector.
Both detectors have been carefully designed by HORIBA to ensure exceptional analytical accuracy and stability.
Key Component 1: Detector
Non-dispersive infrared detector (NDIR)
The main components of an NDIR sensor are an infrared light source, a sample chamber (cell), an optical filter and an infrared detector.
According to the Beer-Lambert law, the gas present in the sample chamber absorbs specific wavelengths of light. The detector measures the attenuation of these wavelengths' intensity, allowing for the determination of the gas concentration.
Carbon Monoxide Infrared Spectrum (NIST Chemistry WebBook https://webbook.nist.gov/chemistry/)
An optical filter is utilized to isolate the specific wavelength absorbed by the gas molecule of interest. The signal from the source undergoes chopping or modulation to offset thermal background signals from the desired signal. NDIR detectors are employed for the measurement of CO2, CO, SO2, and H2O (when H2 is to be measured by NDIR).
To measure CO2, two detectors are utilized along with two optimized filters, enabling accurate measurement of both low and high concentrations.
HORIBA, a pioneer in NDIR analyzers, has developed NDIR as one of its core technologies, offering market-leading instruments in many fields. It is currently used in not only the elemental analyzers but also multiple instruments covering a wide range of applications: Motor Exhaust Gas Analyzers, Ambient NOx Monitors and Stack Gas Analyzers.
For more detailed technical information about the NDIR gas detection method, please refer to What is Non-Dispersive Infrared Absorption Method (NDIR)?
Thermal Conductivity Detector (TCD)
A TCD is using a Wheatstone bridge circuit.
A schematic of Wheatstone bridge.
The reference cell is filled solely with the carrier gas, while the sample cell undergoes a change when combustion occurs and introduces the measured gas into it.
TCD (Thermal Conductivity Detector) does not exhibit selectivity; it solely measures changes in resistivity. If multiple gases are introduced into the cell, the measurement will reflect the combined resistivity changes from all the gases. To ensure accurate and reliable results, any gas that is not of interest must undergo careful filtration before detection.
TCD (Thermal Conductivity Detector) is commonly employed for the measurement of N2 and can also be utilized for H2, particularly in applications that demand high sensitivity.
The operation principle relies on the distinct conductivities between the measured gas and the carrier gas to achieve optimal sensitivity. In the case of N2 analysis, He gas is required as its thermal conductivity differs significantly from that of N2, whereas argon (Ar) gas has a thermal conductivity closer to N2. However, for dedicated H2 analysis, Ar gas is employed as the difference in thermal conductivity between H2 and Ar is greater than with He. The use of TCD for H2 detection offers a sensitivity that is at least 10 times better, making it the preferred technique for precise measurement of low H2 levels in highly demanding applications.
| Gas | Thermal conductivity (k/10-4Wm-1K-1) |
|---|---|
| He (as carrier) | 1649 |
| N2 (target) | 294 |
| Ar (as carrier) | 201 |
| H2 (target) | 2033 |
HORIBA Oxygen/Nitrogen/Hydrogen Analyzers Lineup
- EMGA-Expert - the Flagship High-Accuracy Model enables higher accuracy and wider range, with selectable options
- EMGA-Pro - the Entry Model can be equipped with automated mechanisms to improve analytical efficiency.
- EMGA-921 - the Hydrogen Dedicated Model enables ultrace trace Hydrogen analysis.
Products
Oxygen/Nitrogen/Hydrogen Analyzer
(Flagship High-Accuracy Model)
Oxygen/Nitrogen Analyzer (Entry Model)
Hydrogen Analyzer
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