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-Modulated Diode Laser Absorption Spectrometry (WM-DLAS) technique has been developed. It is based upon the general fact that the sample optical thickness (SOT) is a multi-valued function of the WM-DLAS signal. The methodology presented here is built upon detection of the 2f-WM signal used in its most sensitive mode of operation, i.e. optimised for detection of small sample amounts. It does not, therefore, require any prior knowledge of the analytical content of the sample, nor does it sacrifice the high sensitivity of the technique in order to make use of the extended dynamic range capability. The pilot transition in this investigation is the 780-nm transition in Rb. Simulations from a previously developed simulation program provide the basis for a parameterisation of the 2f-WM-DLAS signal strength with respect to SOT and temperature, which then is used for evaluation of the actual SOT values from measurements in a graphite furnace. The time-integrated SOT-curves provide an entity that corresponds to integrated absorbance and is, therefore, directly proportional to the amount of analyte in the furnace for low as well as high analyte masses. This new methodology has extended the dynamic range 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 several nanograms) with no need for any changes of the laser settings.