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.
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