Extension of the dynamic range of the wavelength-modulated diode laser absorption spectrometry technique

A new methodology for increasing the dynamic range of the Wavelength-Modula ted Diode Laser Absorption Spectrometry (WM-DLAS) technique has been develo ped. It is based upon the general fact that the sample optical thickness (S OT) is a multi-valued function of the WM-DLAS signal. The methodology prese nted here is built upon detection of the 2f-WM signal used in its most sens itive mode of operation, i.e. optimised for detection of small sample amoun ts. It does not, therefore, require any prior knowledge of the analytical c ontent of the sample, nor does it sacrifice the high sensitivity of the tec hnique in order to make use of the extended dynamic range capability. The p ilot transition in this investigation is the 780-nm transition in Rb. Simul ations from a previously developed simulation program provide the basis fur a parameterisation of the 2f-WM-DLAS signal strength with respect to SOT a nd temperature, which then is used for evaluation of the actual SOT values from measurements in a graphite furnace. The rime-integrated SOT-curves pro vide an entity that corresponds to integrated absorbance and is, therefore, directly proportional to the amount of analyte in the furnace for low as w ell as high analyte masses. This new methodology has extended the dynamic r ange of the 2f-WM-DLAS technique to approximately 20 integrated absorbance units (s), implying that the technique is capable of detecting Rb atoms in graphite furnaces over six orders of magnitude (from a few femtograms to se veral nanograms) with no need for any changes of the laser settings. (C) 20 00 Elsevier Science B.V. All rights reserved.