GE Analytical Instruments ©2006 DLM 14291 Rev. A
6-9
To calibrate the ppb range, connect a gas containing <1 ppm NO to the sampling
tee, let it flow for 5 minutes, then select PPB for the Calibration Gas Units Menu.
To calibrate the ppm range, connect a gas containing >1 ppm NO to the sampling
tee, let it flow for 5 minutes, then select the PPM option in the Calibration Range
Select Menu (Figure 7-15). When the ranges are calibrated independently, only one
response factor will be calculated and displayed.
Accuracy of ppb level Measurements using ppm level Calibration
Chemiluminescence has been conclusively shown to provide a linear response over
6 orders of magnitude (0.5 ppb to 500 ppm for the NOA). This means that the error
associated with measurement of ppb levels of NO when calibrating with a ppm
level gas will be low (for the NOA typically <5% at 10 ppb). While gas standards are
commercially available in the 100-500 ppb range, they are expensive and not as
stable as gas standards containing >1 ppm NO. Gas standards containing <100 ppb
NO can be prepared by dilution, but special equipment is required to accurately
dilute gases. Most laboratories will get the most accurate (and less expensive)
measurements by using a 10-100 ppm level calibration gas and using the calibrate
PPM&PPB option.
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