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10.Jul.2007 08:00:55 AM
|Subject:||NO2 calibration gas|
I have a cylinder containing 2000ppm NO2 + balance argon (supplied by Scott in March 2007). I use this to calibrate an FTIR, by diluting the mixture in a flow of high purity argon (99.998%, <3 ppm H2O). Recently, I have noticed peaks associated with HNO3 in the IR spectra (corresponding to a conc. of around 40 ppm).
My first thought is that there is some ingress of moisture; however most of the system is at 2 barg, with the exception of the FTIR. I have ruled out reaction of NO2 in the FTIR by first adsorbing HNO3 from the gas stream - at this point NO2, but no HNO3, appears in the IR spectra. Through a further process of elimination I believe I have narrowed the source of the HNO3 down to the cylinder or the regulator.
Looking back over 1 years worth of data, typically no HNO3 is observed during the calibration or it is so insignificant as to be barely detectable. It is only now that the problem is significant.
The regulator that I am using is a BOC model 1700S, which was suppllied specifically for this application. I have tried using a second, identical regulator, with no improvement. I have also tried using an old cylinder containing 1% NO2 (supplied by BOC) and also observe HNO3 in the IR sprectra.
I am at a loss as to what the problem may be so I would be very grateful for any thoughts that you may have!
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