Hi Toti,
I have tried something similar - using a single combustion column for
sulphur analysis, I put a granulated drying agent at the very bottom of the
tube so that no water was to ever make it into the plumbing. I'm not sure
now what dryer I used, maybe calcium chloride. I think that I got the idea
from some ancient textbook on elemental analysis - so can't give you a
reference on that one. 
There was certainly no problems with restriction of the flow with that
packing, no different from having the dryer in a separate tube in that
respect.
Robert
  

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Toti Larson
Sent: Tuesday, 6 February 2007 1:31
To: [log in to unmask]
Subject: [ISOGEOCHEM] Mg-perchlorate trap

Isotopists,

Is there a maximum temperature that a Mg-perchlorate trap will work to
effectively remove water in an He-continuous flow? In an EA, for example,
can you pack the top of the copper reactor with Mg-perchlorate (well above
the hot zone/furnace, and separated from the copper with quartz chips/quartz
wool), rather than having a separate Mg-perchlorate trap prior to the GC. At
first glance the only problem I can see is that the Mg-perchlorate will be
hotter at that position than it would be later down the line, and maybe the
trap is not effective at those temps. By touch, the outside temp of the
quartz tube is ~50°C in the region I am talking about (maybe its hotter in
the center of the tube?). Is there a flow concern I don't know about? I know
this probably sounds like a really bad idea, so don't ask why I want to do
such a thing. Briefly, I want to pass gas from my EA to a GCMSD setup, and
want to minimize the number of fittings (i.e. 
leaks) that the Mg-perchlorate trap would present.

Toti