Light absorption in air
Air does not transmit wavelengths below about 190 nm, because oxygen and water vapour are highly absorbent at these short wavelengths. Some of the strongest emission lines for some very important elements are in this region, notably H I 121.5 nm, O I 130.2 nm, Cl I 133.5 nm & 134.7 nm, N I 149.2 nm, C I 156.1 nm & 165.7 nm, P I 177.4 nm & 178.2 nm and S I 180.7 nm, where I means the neutral atom.
To use these lines we need to eliminate oxygen and water vapour from both the spectrometer and the optical path to the emission source. This can be achieved either by evacuating the optical system or by purging the system with a gas not containing oxygen or water vapour, the obvious choice is nitrogen, and today about 50% of all new spectrometers working below 200 nm are evacuated and about 50% nitrogen-purged.
Nitrogen-purging eliminates contamination from vacuum pumps and simplifies alignment of the spectrometer above 200 nm.
People have wondered how it is possible to detect
nitrogen signals when there is nitrogen purge-gas in the spectrometer.
The answer is that the nitrogen signals comes from nitrogen atoms
in the source while the purge gas in the spectrometer is molecular
N2 and the energy levels in atomic nitrogen and molecular
nitrogen are sufficiently different to prevent nitrogen molecules
absorbing emission lines from nitrogen atoms.
First published on the web: 15 December 1999.
Author: Richard Payling