Improved methods for signal processing in measurements of mercury by Tekransup?/sup 2537A and 2537B instruments

摘要

Atmospheric Hg measurements are commonly carried out using Tekransup?/sup Instruments Corporation's model 2537 Hg vapor analyzers, which employ gold amalgamation preconcentration sampling and detection by thermal desorption (TD) and atomic fluorescence spectrometry (AFS). A generally overlooked and poorly characterized source of analytical uncertainty in those measurements is the method by which the raw Hg atomic fluorescence (AF) signal is processed. Here I describe new software-based methods for processing the raw signal from the Tekransup?/sup 2537 instruments, and I evaluate the performances of those methods together with the standard Tekransup?/sup internal signal processing method. For test datasets from two Tekransup?/sup instruments (one 2537A and one 2537B), I estimate that signal processing uncertainties in Hg loadings determined with the Tekransup?/sup method are within ±[1?%?+? 1.2?pg] and ±[6?%?+?0.21?pg], respectively. I demonstrate that the Tekransup?/sup method can produce significant low biases (≥??5?%) not only at low Hg sample loadings (&??5?pg) but also at tropospheric background concentrations of gaseous elemental mercury (GEM) and total mercury (THg) (～??1 to 2?ng?msup?3/sup) under typical operating conditions (sample loadings of 5–10?pg). Signal processing uncertainties associated with the Tekransup?/sup method can therefore represent a significant unaccounted for addition to the overall ?～??10 to 15?% uncertainty previously estimated for Tekransup?/sup-based GEM and THg measurements. Signal processing bias can also add significantly to uncertainties in Tekransup?/sup-based gaseous oxidized mercury (GOM) and particle-bound mercury (PBM) measurements, which often derive from Hg sample loadings?&?5?pg. In comparison, estimated signal processing uncertainties associated with the new methods described herein are low, ranging from within ±0.053?pg, when the Hg thermal desorption peaks are defined manually, to within ±[2?%?+?0.080?pg] when peak definition is automated. Mercury limits of detection (LODs) decrease by 31 to 88?% when the new methods are used in place of the Tekransup?/sup method. I recommend that signal processing uncertainties be quantified in future applications of the Tekransup?/sup 2537 instruments.