Evaluation of a commercial pulsed fluorescence detector for the measurement of low-level SO2 concentrations during the gas-phase sulfur intercomparison experiment

A modified pulsed fluorescence (PF) detector (Thermo Environmental Instruments, Model 43s) was used to measure low levels of SO2 in a rigorous, blind intercomparison experiment (Gas-Phase Intercomparison Experiment (GASIE)). The PF detector was able to detect as little as 30 pptv SO2 in a 25-min sampling interval. The coefficients of variation for measurements of approximately 30, 60, 200, 330, and 600 pptv were approximately 40, 9, 6.5, 3, and 3%, respectively. Overall uncertainty of the measurements at 30 pptv approaches +/- 100%. As inferred from GASIE results, the response of the PF detector may be reduced (quenched) by approximately 7% and 15% at water vapor mixing ratios of 1 and 1.5 mole percent (relative humidities of 35-50% at 20-25 degrees C and 1 atm), respectively. These results are uncertain, however, due to lack of extensive data. Post-GASIE tests point to moderate interferences from NO (rejection ratio of 35), CS2 (rejection ratio of 20), and a number of highly fluorescent aromatic hydrocarbons such as benzene, toluene, o-xylene, m-xylene, p-xylene, m-ethyltoluene, ethylbenzene, and 1,2,4-trimethylbenzene. Rejection ratios for these compounds increase from approximately 17-123 to circa 1200-3800 as the sample flow rate is decreased from 2000 to 300 standard cubic centimeters per minute (seem), and the hydrocarbons are more efficiently removed by the instrument's proprietary hydrocarbon ''kicker'' membrane. At a flow rate of 300 seem and a pressure drop of 645 torr across the kicker, the interference from ppmv levels of many aromatic hydrocarbon was eliminated entirely. None of the tested interferants were removed by the carbonate-impregnated paper filter used to zero the instrument during GASIE; thus they induced no net response in the PF detector. These results illustrate the importance of using a selective zeroing method to scrub SO2 without removing potential interferants from the sample flow, thus preserving the overall composition of the sampling matrix.