Re: purging FTIR

"Richard" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> "Marvin Margoshes" <[EMAIL PROTECTED]> wrote in message
news:<[EMAIL PROTECTED]>...
> > "rrs" <[EMAIL PROTECTED]> wrote in message
> > news:[EMAIL PROTECTED]
> > > What's the best way to purge an FTIR to minimize water vapor and CO2
> >  Peaks.
> > > I know running a background scan would nullify the effect of CO2 and
> > > moisture but nothing beats a "clean" scan w/ minimal CO2 and moisture
> > > interference.
> > >
> > >
> > Pass some dry gas through the spectrometer.  N2 would work, if clean and
> > dry.
>
> How do you purge the optical bench?  Most IR spectrometers I am
> familiar with prefer to be N2 purged because water vapor degrades
> their optics.  That same purge should exit through the sample chamber
> to enable a water vapor and CO2 free background.
> Richard

If you have access to a liquid N2 dewar, then that's the way to go.  Better
still if you have a bulk N2 installation to take advantage of, but that's a
rarity in most analytical labs.  An effective N2 purge is not feasible with
cylinders due to the volume requirements.  A common strategy in trace gas
analysis is to place all of the wetted parts of the analytical system
(typically a GC) inside a gas tight purge box with a constant flow of He,
thereby minimizing the effects of leaks.  I could see how that strategy
could work well with FTIR systems if your goal is to really get the CO2 &
H2O background down.  The size of the enclosure required would mean a high
volume source of N2 would be needed.  Alternatively, one could build a dual
bed molsieve trap in front of an air compressor, but the controls necessary
to make that work are probably not worth it if the necessary
equipment/expertise is not already available.  In the long run such a scheme
would be cheaper if not quite as effective as liquid N2...

Scott McFarlane