Model 405 nm NO
2
/NO/NO
x
Monitor Manual Rev. D
25
7.
CALIBRATION
7.1. Introduction
Every analytical instrument is subject to some drift and variation in response, making it
necessary to periodically check the calibration. Dynamic calibration is a multipoint
check where gas samples of known concentration are sampled by the instrument in
order to determine a calibration relationship. For more information on calibration of
NO
2
and NO monitors, please refer to the related information in Code of Federal
Regulations (Title 40, Part 50, Appendix F:
https://www.gpo.gov/fdsys/pkg/CFR-2011-
title40-vol2/pdf/CFR-2011-title40-vol2-chapI.pdf
Calibration is the process of adjusting the gain and offset of the Model 405 nm against
some recognized standard. The reliability of the data collected from any analytical
instrument depends on the accuracy of the calibration, which is largely dependent
upon its analytical traceability to a reference material or reference instrument
calibration.
The calibration of NO
2
and NO monitors using compressed gas mixtures is unreliable
because the gases are lost by reaction and adsorption in the cylinder and on gas
handling equipment. When concentration standards are required, NO
2
and NO
concentrations are generated by diluting a high concentration NO mixture and
converting a fraction of the NO to NO
2
via the gas-phase titration reaction with ozone.
The depletion of NO measured by the Model 405 nm provides the calibration for NO,
and the appearance of NO
2
provides the calibration of NO
2
. The concentration of
ozone used in the gas-phase titration (GPT) reaction can be measured using a
photometer with a NIST-traceable calibration to validate the difference in
concentrations for both NO
2
and NO.
Commercial NO
2
and NO calibrators have dynamic dilution manifolds with an internal
ozone generator and photometer. The calibrator generates NO
2
concentrations by the
technique of gas phase titration (GPT), in which standard ozone concentrations are
reacted with excess NO. Nitric oxide standards are generated by difference, where
standard ozone concentrations are reacted with excess NO and the change in NO
concentration is measured.
7.1.1. Equipment Required
This procedure requires the following equipment:
1. NO
2
and NO calibrator
2. Zero air source
3. Compressed NO standard (refer to the manufacturer's User Manual for the
calibrator)
4. Sampling lines (inert materials such as PTFE or FEP only)
