Improved detectability of wavelength modulation diode laser absorption spectrometry applied to window-equipped graphite furnaces by 4th and 6th harmonic detection

Wavelength modulation-diode laser absorption spectrometry (WM-DLAS) is a technique for sensitive detection of atoms and molecules in gas phase, whose detectability often has been found to be limited by background signals originating from multiple reflections between surfaces in optical components, so-called etalon effects. This work investigates the dependence of the detectability of the WM-DLAS technique on the order of the harmonic detected when the technique is combined with a window-equipped graphite furnace (GF). It is shown that although the analytical WM-signal, SrAS,nf, for a particular detection order, n, is decreasing with increased detection order, i.e. SrAS,2f>SrAS,4f>SrAS,6f, the signal-to-background ratio, (SrAS/SrBG)nf, and thereby also the detectability, are related in the reverse order, i.e. as (SrAS/SrBG)6f≫(SrAS/SrBG)4f≫(SrAS/SrBG)2f. This implies that it is more advantageous to detect higher even order components, e.g. the 4th or the 6th, than the 2nd when the WM-DLAS technique is applied to a window-equipped GF.