Dear Blair,

> Thank you for your helpful comments on the exetainers. The Isochem
> product you are using sounds comparable to the Europa exetainer.  Do you
> have a web site or email address for them?  I wonder if their exetainers
> have specifications similar to the Europa product described by Steve
> Brooks in his reply to my inquiry on ISOGEOCHEM?

Thanks for your kind reply. Unfortunately, apart from phone and fax
numbers I haven't any further info (URL or e-mail) for Isochem (who
are part of CK Gas Products). I shall endeavour to find out.

However, I have some info / comments re specifications given by Steve
Brooks in his posting to the list.

(1) If one wants/needs to use evacuated sample tubes one is well
advised to evacuate them prior to usage and not to rely on the
pre-evacuated bit.

(2) Since I do not like my septa pre-pierced, the first thing to go
before using my containers are the caps-cum-septa. So far, I
experienced no problems with the caps-cum-septa as supplied by
Isochem. To the naked eye they are dry and clean; and as far as
bulging (or coring) goes, almost any septum will do that when
excessively tightened.

(3) If I have to evacuate my tubes I use either side vented needles
(of course with these evacuation takes a heck of a time) or push the
needle through with the needle guard in place so not to punch a piece
of material out of the septum.

(4) When worried about residual 13C signature but not about vacuum I
flush my containers with dry, oil free nitrogen (99.999%). Before
doing that, a good wash and rinse (which also removes the
labels) followed by careful baking in a muffle furnace (400 C) gets
rid of any organic carbon.

(5) I fail to see as to why one would need a desactivated/coated
glass surface for breath tests that are only interested in measuring
CO2. The moisture (water) with its dissolved CO2 should stay
equilibrated with the free CO2 in breath since you don't
change temperature or partial pressure (Henry's Law) when collecting
and storing your samples. So, whether the water clings to the glass
surface or runs down, collecting at the bottom of the tube, shouldn't
make a difference.

However, if activity of the glass surface is a concern, desactivation
can be easily achieved by treating the inside of the tube with 5-10%
DMDCS in toluene (or heptane).

DMDCS stands for dimethyldichlorosilane and is fairly cheap (GBP8.90
for 100 ml from Fluka). You can buy ready to use solutions (often
sold under fancy names) for more extortionate prices but preparing a
10% solution is hardly rocket science.

However, the protective layer is sensitive to hydrolysis and wears
off over time (which is especially true for samples with a high
moisture content). Which is why one should replace desactivated
GC injector liners on a regular basis. At room temperature on the
other hand, this process takes quite  a while and should not lead to
contamination of one's sample with organic carbon from the
"protective" layer. Still, if contamination with carbon from sources
other than one's sample is a concern, I would use clean glass (see
above) and nothing else.

Another way of going about it, desactivation that is, is to coat the
glass with a thin (nanometer) layer of GC stationary phase material
such as OV1701 or OV225. This has the advantage of desactivating the
surface while making it water repellent without being susceptible to
hydrolysis. Again, this is something one can do easily in one's lab.

At the end of the day, it's up to you to decide which product
and protocol you are most happy with, giving you reliable and
reproducible results.


Best wishes,

                         Wolfram


***********************

Dr. W. Meier-Augenstein, CChem MRSC
Senior Research Fellow

University of Dundee, Dept. of Anatomy & Physiology,
OMS, Small's Wynd, DUNDEE  DD1 4HN, United Kingdom

Tel.: +44-(0)1382-34/5124, /4574, /4968
Fax:  +44-(0)1382-34/5514

e-mail: [log in to unmask]

URL: http://www.dundee.ac.uk/anatphys/wma/wolfram.htm


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