Dear Andrew (& others)
We have experienced similar problems with acidification. We too were using
manual injections as we were experiencing blockages in the tubing of the
acid pump - probably as we were only using it intermittently (every couple
of months or so).
Firstly, yes you can clean out the needle. We had some success with heating
it gently and using a small stiff piece of wire (try the smaller of the two
tools that you should have for removing the ferrules from the gas bench).
However, as this points to a problem in acidification, it is probably not
the final answer.
We tried all sorts of methods of manual injection - the trick is to be able
to retract the syringe plunger slightly once the acid drops are added to
avoid the drop of acid on the back of the septum. This is easier said then
done with phosphoric acid, but using a small gas tight syringe and having
the acid warm does improve your chances.
We came up with another solution that seems to work. We add the acid in
small teflon (because it is easy to machine) cups into the sample
extertainers after loading the carbonate but prior to flushing. We also
place a small glass "straw" ~1 cm long into the acid to break the surface
tension (we use sections of the glass capillaries that are used to draw up
blood). We flush the samples as normal in the gas bench at 72C and simply
tip the acid after flushing into the exertainer, making sure that it
interacts with the sample (I had taken to doing this anyway when injecting
the acid as there is little acid in a large exertainer).
Pros: you never get acid on the septum and you make fewer hole in the septum
Cons: it is a bit fiddly and occasionally some sample sticks to the bottom
of the cups
However, results are excellent and it is well worth a try...
Dr Ian Cartwright
Reader: Groundwater & Stable Isotope Geochemistry
School of Geosciences
Clayton 3800, Australia
tel: +61 (0)3 9905-4887 / 4879
fax: +61 (0)3 9905-4903
e-mail: [log in to unmask]
water, our greatest resource...
From: Stable Isotope Geochemistry [mailto:[log in to unmask]]On
Behalf Of Andrew Campbell
Sent: Wednesday, 3 December 2003 3:53 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Gas Bench Needle
I know there has been some discussion of clogged Finnigan Gas Bench needles
in the past but the Isogeochem Archive seems to be down so I can not check
it. I have used my gas bench for hundreds of CO2-H2O equilibrations with
no trouble. Now I am trying to run carbonates for the first time. So far
I have found it easier to manually flush the vials and then manually inject
the acid. After that I will use the autosampler to extract the CO2. Just
as I was starting I managed to break the gas bench needle by forgetting to
put the lid on the heating block. So I installed a new needle (which was a
trial in itself trying to get the old graphite ferrule out.) Now after
running only about 10 samples the needle is clogged with crystallized
phosphoric acid. I could easily see it in the needle with my hand
lens. We have been attempting to inject the acid toward the edge of the
septum (away from where the sampling needle enters) and then wipe off the
drop of excess acid that seems to accumulate there. There is also a drop
on the bottom side of the septum that we can not get at.
So I have two questions:
1)Has anyone had any luck unclogging the needle when it has acid in it?
2) What steps do I need to take to keep this from happening again.?
Dr. Andrew R. Campbell
Professor of Geology
Dept. of Earth and Environmental Science
New Mexico Tech
Socorro NM 87801
Research office: 505-835-5327
Chairman's office: 505-835-5635
e-mail: [log in to unmask]