Paramagnetism gas analysis: identify the gas for which the paramagnetic method is not appropriate when determining composition in gas mixtures.

Introduction / Context:
Paramagnetic gas analyzers exploit the attraction of paramagnetic molecules to magnetic fields. The technique is widely used for oxygen measurement and, with specialized designs, for certain other paramagnetic species. Diamagnetic gases, however, are poorly suited to this method.

Given Data / Assumptions:

• Paramagnetism implies unpaired electrons and positive magnetic susceptibility.
• Oxygen (O2) is strongly paramagnetic.
• Some nitrogen oxides (e.g., NO) exhibit paramagnetism.
• Carbon dioxide (CO2) is diamagnetic.

Concept / Approach:
In a paramagnetic analyzer, the force on paramagnetic molecules in a non-uniform magnetic field leads to measurable effects (e.g., pressure or thermal changes) proportional to concentration. Oxygen’s strong paramagnetism makes it ideal. Nitric oxide also has unpaired electrons and can be detected with certain designs. Carbon dioxide lacks unpaired electrons and is diamagnetic, yielding negligible response, so paramagnetism is not used for CO2 composition measurements.

Step-by-Step Solution:

Verification / Alternative check:
Instrument catalogs list O2 paramagnetic analyzers; CO2 is measured by NDIR (non-dispersive infrared) or other optical methods, not by paramagnetism.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming “all gases” can be measured by one principle; analyzer selection depends on molecular properties such as magnetism or IR absorption.

Final Answer:
Carbon dioxide