Characterization of 2f-, 4f-, and 6f-background signals in wavelength modulation diode laser absorption spectrometry in graphite furnaces

A study of the origin, magnitude, and drifts of the 2f-, 4f-, and 6f-harmonic background signals from the wavelength modulation diode laser absorption spectrometry (WM-DLAS) technique applied to both window-equipped and open graphite furnaces (GFs) is presented. A previously developed theoretical description of the WM-DLAS technique was used to analyze the magnitudes as well as the drifts of the signals. The various nth harmonic WM background signals from a window-equipped GF system, SBG,nWE, were found to be related as SBG,2WE≫SBG,4WE≫SBG,6WE. The 2nd and 4th harmonic background signals were dominated by multiple reflections from within the windows, whereas the 6th harmonic was affected only in part. The 2nd and 4th harmonic therefore showed large drifts, originating from changes of the thickness of the GF windows induced by the intermittent heating of the graphite cuvette. The limits of detection (LODs) for Rb in a window-equipped GF system scaled roughly as LOD2fWE≈10LOD4fWE≈100LOD6fWE with LOD6fWE being approximately or above 140 fg. WM background signals were found also in windowless GF systems as well as in systems stripped of all optical components. The 2f-background signals from those systems were significantly larger than the 4f- and 6f-background signals and had contributions from non-linearities in the electronic equipment, an etalon in the laser diode, and ‘ripple’ from the ramp generator (providing scanning of the center frequency). The 4f- and 6f-background signals were dominated by the ‘ripple’ from the ramp generator under scanned conditions and by non-linearities in the electronics under fixed frequency conditions. None of the 4f- and 6f-signals was affected by the laser diode etalon. For open GF systems, the LODs for Rb were found to be similar for the three harmonics (approximately or above 15 fg).