Calibration
Equipment Required
4-2
Model 42
i
Trace Level Instruction Manual
Thermo Fisher Scientific
●
Zero gas generator
●
Gas phase titrator
A zero air source, such as a Thermo Scientific
Model 111 Zero Air Supply
or
Model 1160 Zero Air Supply
, free of contaminants such as NO, NO
2
, and
O
3
is required for dilution, calibration, and gas phase titration.
The zero air source should be at an elevated pressure to allow accurate and
reproducible flow control and to aid in subsequent operations such as
drying, oxidation, and scrubbing. An air compressor that gives an output of
10 psig is usually sufficient for most applications.
Several drying methods are available. Passing the compressed air through a
bed of silica gel, using a heatless air dryer, or removing water vapor with a
permeation dryer are three possible approaches.
NO is usually oxidized to NO
2
in order to ease its scrubbing. Oxidation
can be accomplished by either ozonation or chemical contact. During
ozonation, the air is passed through an ozone generator. The O
3
that is
produced reacts with the NO to form NO
2
. Care must be taken to allow
sufficient residence time for the ozonation reaction to go to completion.
Chemical oxidation is accomplished by passing the air stream through a
reacting bed. Such agents as CrO
3
on an alumina support or Purafil® are
very efficient at oxidizing NO to NO
2
. The chemical contact approach has
the advantage of needing no electrical power input for its application.
Fixed bed reactors are commonly used in the last step of zero air generation
to remove the remaining contaminants by either further reaction or
absorption. Table 4-1 lists materials that can be effective in removing
contaminants.
Table 4–1.
Scrubbing Materials
To Remove
Use
NO
2
Soda-Lime (6-12 mesh), Purafil
Hydrocarbons
Molecular Sieve (4A), Activated Charcoal
O
3
and SO
2
Activated
Charcoal
Zero Gas Generator
Compression
Drying
Oxidation
Scrubbing
