The possibilities of eliminating spectral interferences caused by molecular bands and lines of the plasma gas in a microwave-boosted glow discharge source (MB-GDS) have been investigated. Suitable modulation of one of the two excitation sources and phase sensitive amplification were used to selectively amplify the signals emitted by the sample atoms. The molecular bands or lines of the plasma gas were hardly influenced by the modulation and did not appear in the amplified spectrum. In particular, the OH line interference on the most important analytical lines of Mg, Bi and Sn and the interference of Ar II on Al were eliminated to a high degree. The standard deviation of the background after suppression of interfering lines was increased. The limits of detection (3s) for Al, Bi, Mg and Sn in samples of aluminum and copper were found to be in the range 0.1-9 mug g(-1).