Hi Toti
Mg-perchlorate decomposes at 250oC.  When hydrated it melts at 185oC.  Saturated it can go liquid at below 50oC and completely invade and crystallise in the pipework all the way through the GC column and into the TCD.  It happened more often, and in many labs, then one might expect.

Gilles 

Gilles St-Jean
Chercheur / Research Scientist
Université d'Ottawa / University of Ottawa
Sciences de la Terre / Earth Sciences
140 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5
Tel: 1-613-562-5800 xt 6830 (Bureau / Office)
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-----Message d'origine-----
De : Stable Isotope Geochemistry [mailto:[log in to unmask]] De la part de Toti Larson
Envoyé : 5 février 2007 19:31
À : [log in to unmask]
Objet : [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