Hi,
we measure suspended particles from GF/F filters a lot and are therefore
using steel inserts (high temperature, low C steel). After up to 30 filters
(25mm diameter) we replace the liner with another one. The liners have small
holes, however not in the lower section where the glas filters accumulate.
Since these inserts are made by our institute workshop we have to drill the
melted glas out of the liner after every use. This makes their use a little
unconvenient. However, they last for app. 50-70 uses.
The insert seems not at all to affect the isotope values, because we find no
difference with and without the insert in the combustion column. Only
samples below 0.5 micromol have some random variation and the blank
correction suggested by Fry et al (1992) cannot be applied.
Cheers
Maren


Maren  Voss
Baltic Sea Research Institute
Seestr. 15
18119 Rostock
Germany
TEL. +49/381-5197209
FAX +49/381-5197440
e-mail:[log in to unmask]

----- Original Message -----
From: Paul Brooks <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, June 22, 2000 8:49 PM
Subject: Re: quartz liners etc


> >Dear Isogeochemers,
>
>
> We suck out the ash from the furnace on the EA with a stainless steel tube
> connected to two different flask that remove the large and then small
parts
> of the ash.  Then the air goes through a GAST 23 series pump with 10
micron
> filters onthe inlet and outlet, and is then vented into a fume hood.  We
do
> not attempt to suck out al the ash down to the level of  the quartz wool,
> but leave about 5-10mm of ash on the surface of the quartz wool.  We have
> the stainless steel tube marked so that we know how far into the
combustion
> tube we are.  This ash layer rapidly becomes hard and seems to protect the
> quartz wool, and chromium below.
>
> We usually suck out the ash at the start of the working day, and then run
> up to 200 organic samples through the instrument before the next morning
> when we would suck out the ash again.  For soils we would only run 60 soil
> samples before sucking out the ash.  We have found that leaving soils in
> for any length of time results in them forming into a large solid block of
> ash that cannot be removed. Therefore the rule here is that soil ash is
> removed immediately after the run or first thing the next morning.  I will
> bring photographs of the suction system and a stainless steel tube to the
> Canadian conference in August.
>
> Out of interest we use 100 mL min-1 Helium flow rate with an analysis time
> of 6 minutes per sample on the Europa Sci. 2020 and 8.3 minutes on the
> Carlo Erba 1500/Delta XL system.  In practice throughput of the unknown
> samples is similar on each system as we have to run more standards with
the
> Europa system since it doesn't have reference gas injection.
>
> We usually use 9x5 tins for all samples, and can still get up to 200 plant
> material samples as heavy as 15mg each burned without sucking out the
> ash.  Presumably with organic samples most of the ash buildup is tin
oxide.
>
> We routinely replace our combustion tube every 2000 samples or so.  At
this
> age there does not appear to be any reason to keep them longer as the cost
> per sample by that time is only on the order of US$0.10 per sample for the
> combustion tube, and it doesn't seem worth the risk of loosing samples by
> trying to hang on to the combustion tube any longer.  However, the
> combustion tube frequently seem as though it will go longer than this.
>
> As to safety of either technique I find it hard to believe that the liners
> are really all that much safer than sucking out the ash.  I have never
> cracked a combustion tube while sucking out the ash.  I would think that
> most of the hazard is from the chromium oxide.  We pack our combustion
> tubes in a hood, and use a funnel with a long piece of tubing on it to put
> in the chromium oxide, so that it doesn't touch the sides.  Even so, there
> are traces of Chromium oxide on the sides of the top of the combustion
> tube.  Either putting in a quartz liner or suction tube risks contacting
> some of this material and spreading it around.  I would have thought that
> taking a red hot liner out with would also be dangerous, especially since
> it might have chromium oxide on its surface.  Then the ash must still be
> disposed of somehow, which in a suction system is taken care of
> automatically with little handling.  We do not wash any of our parts as
> this means the water from the washing must be collected and disposed of as
> chemical waste.  No water that has been in contact with chemicals like
> chromium oxide, or even tine oxide, can be poured down our sink.  This
> means that washing the tubes out is not an option for us.
>
> We have tried to use quartz liners whenever analyzing glass filter paper,
> with mixed results.  This is necessary as it is virtually impossible to
> suck out molten glass from the furnace.  We are trying to switch all out
> work requiring filter papers to quartz fiber ones that seem to work well
> and suck readily out of the furnace.
>
>    With the inserts we would start to get trailing peaks or other problems
> that removing the insert seemed to solve.  I have been told by quartz
liner
> users to either have them long enough so that they sit out of the top of
> the combustion tube by a few mm and are pressed in by the sampler, or
short
> enough so that they don't.
>
> I would certainly like to have the option of using quartz liners.  Is
there
> any agreement on if they should be rise above the quartz tube and be
pushed
> down by the sampler, or lower than the sampler?  Should they have slots
cut
> in them?  How many samples do users get before they need to be
> changed?  What flow rate and cycle time for C and N analysis do users with
> quartz liners achieve?  For users who do wash their tubes, how do they get
> rid of the chemical waste?
>
> Thanks to all who have contributed to this discussion.
>
> Paul.
>