Fast Measurements for Sulfur Dioxide by Atmospheric Pressure Ionization Mass Spectrometry

 Alan Bandy and Donald Thornton, Drexel University

     A quadrupole mass spectrometer with a nickel-63 (Ni-63) source is being used for the determination of sulfur dioxide (SO₂) at the low part-per-trillion by volume (pptv) with sampling rates of 1 to 25 Hz. As an internal standard, high isotopic purity S-34 SO₂ is added to the ambient air as it is drawn into the Teflon sampling line by a Metal Bellows pump. The Teflon sampling manifold can be pressure controlled by either a manual valve or an electrical valve under pressure sensor feedback control. Mass flow devices were used to control and measure the gas flows.

    A portion of the sampled air is drawn through a Nafion dryer and then into the Ni-63 source. A pinhole aperture admits the ionized air into a low pressure (3-5 torr) declustering region prior to high vacuum region of the mass spectrometer. In the presence of carbon dioxide and ozone, the ion cluster SO₅^- is formed.

    The mass spectrometer is operated in a mode so that the ion signal for the ambient ion is measured followed by an equal time period for measuring the ion signal for the isotopically labeled internal standard. A pulse counting ion multiplier is used for measuring the ion signals. Time periods for the ion signal measurements can be varied from 20 ms to 1 second. The mode of operation during Trace P was 20 ms for the ambient and 20 ms for the internal standard. The count data was logged by the TAMMS data system to insure that the fast SO₂ measurements were recorded on the same time base as the data for air motion sensors to allow for flux determinations. The data were also monitored on the Drexel data system to provide real time estimates of the SO₂ concentrations.