Modulated glow discharge source with supplementary microwave excitation

Spectral interferences occurring in a microwave-boosted glow discharge source due to emission lines of the carrier gas and to band systems of molecular species were investigated. A method is described which greatly reduces such interferences. It is shown that analytical lines are strongly dependent on the current in the dc discharge whereas Ar I lines are scarcely influenced and OH bands are influenced to only a small extent. By modulating the dc current with the microwave discharge running continuously and using a lock-in amplifier, selective amplification of analyte atomic lines is achieved. In this way, spectral interferences by Ar I lines on Fe I lines and interferences by lines in OH bands on the Bi I 306.771 nm and Al I 308.216 and 309.271 nm analytical lines, which occur even when using a high resolution monochromator, are greatly reduced.