What is zero-grade air?

To analyze trace amounts of carbon monoxide, carbon dioxide, formaldehyde, etc., TCD’s insufficient sensitivity leads to the use of FID for detection; however, FID responds well only to organic compounds, especially hydrocarbons. By passing them through a nickel-catalyzed hydrogenation column to convert these small molecular carbon oxides into formaldehyde, the sensitivity is increased by hundreds of times, allowing for the accurate detection of these carbon oxides. However, the FID requires hydrogen and air as auxiliary combustion gases; if the concentration of organic substances in the air exceeds a certain level, it will increase the noise level of the FID and reduce its detection sensitivity. That’s how people came up with a method to remove organic compounds from the air; using zero-level air generators (also known as zero-point air generators, etc.), these devices remove organic compounds from the air through catalytic combustion. Regarding your question about whether carbon dioxide in the air could affect the detection, think about where the combustion air mixes with hydrogen to undergo combustion: it is after the components have been separated by the chromatography column and subjected to a conversion reaction in a nickel catalyst column, at which point there is no impact. Just look at a gas circuit diagram of a FID with a methane converter to find out.

Thank you, teacher, for the explanation. It’s really me who didn’t learn it clearly enough. CO2 in the air has no effect. That student would like to ask another question. Gas chromatographs generally use ordinary air generators, or zero-grade air generators. I took a look at our air generator: it uses activated carbon to absorb hydrocarbon compounds in the air, and silica gel and molecular sieves to absorb moisture, after which the air goes straight into the chromatograph. Of course, it is also called an oil-free air generator. As I understand it, the difference between the two lies in the fact that zero-stage air generators remove organic substances more thoroughly, resulting in very low levels of organic matter in the output; from what I’ve found online, this level can generally be as low as 0.05–0.1 ppm. But I’m not sure about air generators. Under normal circumstances, how should one choose between a regular air generator and a grade-zero air generator? After all, the latter is much more expensive than the former.

Carbon dioxide in the air does not affect the FID detector, but carbon oxides in the air, once converted into methane, will affect the FID’s response. Air generators can only adsorb and remove organic compounds with higher molecular weights as well as moisture from the air; they are not very effective at removing trace amounts of methane and small-molecule alkanes, so they cannot be used as grade-zero air. The zero-stage air generator is designed for the removal of trace organic compounds from the air; the purposes of the two devices are different, as are the types of impurities they remove from the air. Therefore, their applications vary, and their prices differ as well due to the costs and complexity associated with each type of device.