GE MOA 280i - 6. CALIBRATION; Zero Gas Calibration

GE Analytical Instruments ©2006 DLM 14291 Rev. A
6-1
6. CALIBRATION
To use the NOA for measurement of gas-phase NO, a calibration is required. The
stability of the calibration depends on many factors including ambient
temperature humidity, flow into the NOA, line voltage fluctuations, and
contamination of the reaction chamber. Most laboratories calibrate the analyzer at
least once a day or each time they make measurements. The calibration consists
of two parts; measurement of an offset for the ppb and ppm ranges using a gas
containing <1 ppb NO (“zero air”) and measurement of response factors for the
ppb and ppm ranges using a gas that contains a known concentration of NO. The
calibration should be done using the gas-sampling package with the Nafion drier.
Use of the drier will ensure that the relative humidity of the calibration gases is
the same as the humidity in the sample gas.
Note:
The NOA must be in the Start Mode to perform the Calibration.
Zero Gas Calibration
There is always a background signal from the PMT and the ozone in the NOA. This
background or offset is measured by running a gas that contains < 1 ppb NO, and is
subtracted from the PMT signal for standards and samples. Air containing < 1 ppb NO
can be produced using the Zero Air Filter from Sievers Instruments (Sievers Part No.
ACT 01400). The NOA draws room air through a bed containing KMnO
4
and activated
carbon. NO is oxidized to NO
2
by the KMnO
4
and NO and NO
2
absorbed by the carbon.
The KMnO
4
is converted to MnO
2
(brown color) and the filter should be replaced when
all of the purple KMnO
4
has turned brown. The lifetime of the filter will depend on the
frequency of use and the levels of NO in ambient air, but typically the filter is good for
5 years.
Most gas companies supply so-called “zero air” that can also be used for the zero gas
calibration. Cylinders of breathing air, nitrogen or oxygen may or may not contain NO
at ppb levels and should not be used for the Zero Gas Calibration unless they contain <
1 ppb of NO.