Depth profiling of organic coatings with GD-OES—investigation of molecular emission and interference

A low alloy steel sheet with a coating system incorporating an epoxy-based top coating Bonazinc™ is analysed with a DC glow discharge optical emission spectrometer system. The CCD type spectrometer records complete spectra in the range 160–460 nm. At a constant discharge power of 14 W, the discharge parameters were varied in three steps: 1000 V 14 mA, 700 V 20 mA, and 500 V 28 mA. Substantial molecular emission was observed in several parts of the spectrum, identified as originating from the diatomic molecules CH, OH, NH and probably CO. The observed molecular bands overlap several atomic emission lines, causing line interference at these spectral positions. The most intense molecular emission was observed at the condition 1000 V 14 mA, corresponding to the lowest pressure and highest plasma impedance. The results also show that the highest signal to background, in the Bonazinc™ coating, for atomic emission lines is obtained at high pressure–low impedance discharge conditions.