"Nitrogen 5.0" – seeing this notation for the first time, you might take it for a version number. In fact there is an elegant system behind it: the first digit states the number of nines, the second the final digit after them. 5.0 means 99.999% purity, 4.6 means 99.996%, 6.0 accordingly 99.9999%.
The overview table
| Grade | Purity | Max. impurity | Typical application |
|---|---|---|---|
| 2.8 | 99.8% | 2000 ppm | Technical applications, tyre filling |
| 4.6 | 99.996% | 40 ppm | Welding shielding gas, general inerting |
| 4.8 | 99.998% | 20 ppm | Food, simple laboratory applications |
| 5.0 | 99.999% | 10 ppm | GC carrier gas, analytics, laser cutting |
| 5.5 | 99.9995% | 5 ppm | Demanding analytics, ICP |
| 6.0 | 99.9999% | 1 ppm | Semiconductor manufacturing, trace analytics (ECD, GC-MS) |
What the residual impurities consist of
The total figure splits into individual components stated on the certificate of analysis. Typical for nitrogen 5.0: O₂ ≤ 3 ppm, H₂O ≤ 5 ppm, hydrocarbons ≤ 0.2 ppm. At 6.0 the limits drop to O₂ ≤ 0.5 ppm and H₂O ≤ 0.5 ppm – the latter corresponding to a dew point of about −80 °C. Important: what matters is often not the total purity but the one critical component. An ECD detector chokes on oxygen traces, a process furnace on moisture – reading the certificate closely pays off more than buying one grade higher. How dew point and ppm relate is shown by the Dew Point to ppm Calculator (Moisture in Gases).
Every nine costs exponentially
From 4.6 to 5.0 the surcharge is moderate; from 5.0 to 6.0 the price multiplies – the last nine is the most expensive to produce. So the honest question pays: what purity does the process really need at the point of use? For welding shielding gas 4.6 is entirely sufficient, for GC carrier gas usually 5.0; 6.0 is for those measuring in the ppb range or coating wafers.
The cylinder is only the beginning
The specified purity applies at the cylinder valve – what arrives at the point of use is decided by the piping network. Elastomer seals let moisture permeate, unpurged branches introduce air, and every cylinder change without a purge procedure contaminates the system. More in our article The 7 Most Common Mistakes in Handling High-Purity Gases; the matching tools are the Purge Time Calculator for Gas Lines and the Leak Rate Converter (Leak Testing). Rule of thumb: better 5.0 through clean piping than 6.0 through a rubber hose.
