The spectral wavelength for nitrogen is 149.262nm, where the wavelength for iron, at 149.25nm, and the wavelength for chromium, at 149.298, are located nearby. The figure below shows a plot of the nitrogen spectrum, along with nearby iron and chromium spectra.
The first-order spectrum is affected by iron and chromium, because the iron and chromium spectra overlap with the nitrogen spectrum in the middle due to inadequate resolution. In contrast, using the second-order spectrum provides twice the resolution and eliminates the effects of iron and chromium on nitrogen. In addition, continuous spectra of silicon are also known to exist near the nitrogen spectrum. Their effect can approximately be cut in half as well, by using the second-order line.
Since the second-order spectral line for nitrogen at 149.2nm appears where it overlaps with the first-order
spectral line at 298.4nm, measuring the second-order spectral line also measures the interfering first-order spectral line. Therefore, high sensitivity can be achieved by using a special detector (photomultiplier) that provides lower measurement sensitivity than previous models for first-order spectral lines, but high measurement sensitivity for second-order spectral lines, as shown below.
Discharge conditions were also considered and sensitivity was increased by using a 5 times higher energy spark discharge than for conventional discharge conditions.