Basis: The ambient air sample is mixed with an ionized reagent, and
the resulting mixture is introduced directly into a mass spectrometer,
where ions are focused with ion optics (magnetic lenses), analyzed by a
magnetic quadrupole and detected with a commercial ion multiplier. The
multiplier connects to a discriminator/preamplifier which converts ion
signals into TTL pulses. A computer is required to convert the signals
to a graphic spectrum presentation (See Figure 1) (Ref. 34).
Figure 1: Chemical Ionization Mass Spectrometer.
Range: Ambient air levels of .30 pptv (one part in 1012)
to 1000 pptv (1000 parts in 1012, or one part in 109) (Ref. 34).
Minimum Detection Level: in ambient air .15 pptv in one second
of integration (Ref. 34).
Operating Temperature: February to May in Boulder, Colorado,
including temperatures above 20°C (Ref. 34).
Known Interference:
High levels of formic acid (HCO2H) from on site sources
were found to be a possible source of interference if measurement of
formic acid as an ambient air pollutant were to be attempted.
Internal (within instrument) levels of nitric acid were high
enough (10 to 20 percent of ambient air levels) to interfere with
measurement. Interference was solved with a nylon wool scrubber.
Sensitivity to nitric acid is a function of temperature within the
instrument because the reaction is in equilibrium, and the equilibrium
constant is sensitive to temperature changes. A large flow of dry
N2 is used to bring the incoming ambient air to the internal
temperature
Notes of Interest:
This method was developed to measure gas phase nitric acid, but can
also be used to measure formic acid. Nitric acid is not only an abundant
pollutant, but also acts as a sink for NOx in the troposphere,
so that its role is complicated, and probably important in air quality
modeling (Ref. 35).
Huey et al. (Ref. 34) report that typical uncertainty for ambient
air monitoring is +/- 35 percent for any given measurement. A 25 percent
error occurs in the calibration for, which arises from fluctuations
in the permeation rate and the temperature of the flow tube.
The method described is an in situ method but it is intended for use
in the field where ambient air is drawn directly into the measurement
equipment, avoiding canister storage. However, as described, a computer is
directly connected to the mass spectrometer, and some type of protective
housing is necessary for the complete system.
Terms pertinent to the description of CIMS and the instrument
schematic in Figure 1:
Ion multiplier -- an ion multiplier senses (detects) highly focused
ions from a quadrupole mass filter. The output of the ion multiplier
is an electrical signal which can feed into electronic equipment for
conversion to recording or display (Ref. 36, Vol. 6, Page 204).
Ion lenses -- both electrostatic and magnetic lenses provide either
converging or refracting action on charged particles. The power of these
lenses is a function of their electrostatic or magnetic field and their
accelerating voltage (Ref. 36, Vol. 6, Page 204).
TTL pulses -- refers to bipolar transistor-to-transistor logic,
where the first transistor performs an "AND" operation on the inputs
and the second transistor performs an inversion. TTL transistor pairs
are operated in the saturation mode, and this overdriving helps to limit
noise in the output signal (Ref. 36, Vol. 10, Page 181).
Quadrupole mass filter -- a strong focusing magnetic lens where the
converging action on the particle stream is directly proportional to
the strength of the magnetic field (Ref. 36, Vol. 6, Page 204)
