In reply to the questions about the quartz reactors;

The main difference between an opaque or clear quartz tube is the opaque
tubes have extremely fine air bubbles in them. This is why the ID varies
from tube to tube, the wall thickness changes during production. Clear
tubes are always exactly the same wall thickness.

They have the same chemical resistance to substances combusted in the
tubes, so theoretically they should both give the same life in the
instrument. Practically they do show some differnces in certain
applications. The problem is not so much in the combustion as in the
sample that is being combusted. There are certain elements which will
attack the quartz at the temperatures you are using to combust samples.
The worst culprit is fluorine, which will degrade the quartz very
quickly in high concentrations. The applications where this is a problem
are usually in pharmaceuticals and organic chemicals, where there may be
fluorine present at 50-75%. To overcome this problem pure alumina is
used as a base for the oxidative catalysts (chromium oxide on alumina
and tungstic anhydride on alumina), or is added to the samples.

The high temperature of combustion (tin + oxygen = 140,000 calories)
will also degrade the quartz. That is what produces the "shattered
glass" effect you can see in the clear tubes. This is normal and will
have no effect on the quartz until it cools below about 300 degrees
centigrade. The sections of the tube which are degraded will then crack.
There is no advantage to a slow cool down, once the tube cools it will
crack if there is sufficient damage.

In those applications where the tubes are prone to cracking, the opaque
tubes sometimes last a bit longer. We are not sure why that occurs as
they should behave the same way in the instrument. Perhaps the air
bubbles in the opaque tubes provide a thermal shock barrier, but this is
only an assumption on my part.

Besides the nickel sheet mentioned by Peter Swart earlier, quartz
inserts are used to catch the sample residue and to protect the tubes.
Combustion occurs in the insert and then is removed while the tube
remains in the furnace, eliminating the problems mentioned above. The
disadvantages of the quartz inserts are the number of samples which can
be run. In the older Carlo Erba NA 1500's and the ANCA's, a 20 mm quartz
tube is used, so an insert of 15 mm OD can be fabricated which still
allows you to run a reasonable amount of samples before replacing it.
The newer CE Instrument analyzers use an 18 mm OD tube, with the clear
tubes having an ID of 15mm and the opaque around 14 mm. An insert of
13mm OD will allow you to run approximately 80 samples of soil or
sediment (20 mg each) or 150+ samples of plant or organic chemicals.
Steel inserts are also available, but they will produce the same effects
as the SS reactors, so are not a viable option for EA/IRMS.

I hope this was helpful.


Bruno Lavettre
Costech Analytical Technologies, Inc.
phone:  805-297-2395
fax:    805-297-5492
e-mail: [log in to unmask]
www.costechanalytical.com