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ISOGEOCHEM  December 2003

ISOGEOCHEM December 2003

Subject:

Re: Raw dD memory correction

From:

Paul Brooks <[log in to unmask]>

Reply-To:

Stable Isotope Geochemistry <[log in to unmask]>

Date:

Mon, 8 Dec 2003 11:38:48 -0800

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (249 lines)

Willi,

We have been able to reduce the memory effect of the syringes to reliably
less than 1%, and most of the time in the 0.2 to 0.5% carryover range using
an ethyl ether rinse with waste to vacuum under a septa.  Furthermore,
there is no evidence that this memory effect extends beyond the next
sample.  I presented this at the conference in Winnipeg last August. It is
just one of several small method improvements that I haven't had time to
write up.

The solution to the memory problem is to dry out the syringe between each
sample.  We now do this by rinsing with Ethyl Ether between samples and
injecting to a waste under vacuum.

I should acknowledge John Morrison for originally suggesting the memory
effect was in the syringe, Gilles St-Jean and others at the 2002 Canadian
conference who suggested rinsing the syringe and emptying to waste under a
vacuum, and my colleague Richard Neese for suggesting the use of an azeotrope.

We use Hamilton #80300 10uL syringe with the Combi PAL autosampler
injecting to the H/Device.

We have a 2 mL vial set in position 96 in the autosampler tray filled with
fresh daily (from the main bottle) ethyl ether capped with a normal
septa.  It is important that the ethyl ether main bottle only be used for a
month, and be kept dry.  Dry ethyl ether will form a azeotrope with water
up to 1.2%.  This means that the ethyl ether will boil off at about 34.2
degrees C and carry with up to 1.2% water.  Therefore when the syringe is
rinsed in ethyl ether the ether combines with the water, and when the
syringe is injected into the vacuum the ether boils off taking the water
with it and leaving a dry syringe.

The waste is a tube attached to our building vacuum (about -0.8 bar) that
ends in a septa strapped to the end of the normal waste containers.  All
waste injections go to this septa.

The sequence of events is:

1) Rinse syringe twice with 1.0 uL sample and inject to waste vacuum.
2) Rinse syringe 10 times with 1.5 uL volume (this is where the syringe is
in the sample vial and the plunger goes back and forth 10 times in rapid
succession filling the syringe with 1.0 uL water.  This eliminates the air
bubbles that are often in the syringe, and produces a reliable 50-60%
volume of the sample bellows to achieve a 4 volt signal on the mass 2
collector).
3) Inject a 1 uL sample into the H/Device.
4) Fill the syringe twice with 8 uL ethyl ether and inject to the waste
vacuum.  It is interesting to note that even before the syringe plunger
descends, as soon as the syringe needle enters the septa with the waste
vacuum under it, the ethyl ether disappears as the ethyl ether boils off
under the vacuum.
5) Push syringe plunger back and forth 5 times in the vacuum to get rid of
any residual ether.

We always measure the syringe carryover on every analysis run and correct
for carryover before any other calculations are done as you describe
below.  However, the carryover is always less than 1% and usually 0.2 to
0.5%, and does not affect more than the next sample.

We have found it important to keep replacing the main bottle of ethyl ether
with a new one every month, otherwise the results seem to get very poor for
reasons unknown.

The only disadvantage of the method seems to be that the syringes only work
for about 150-300 injections.  Then on the rinse cycle when the plunger is
raised up to 8 uL and pushed out the plunger bends and the syringe is
useless.  One can take the syringe out of the autosampler between samples
and feel the syringe plunger getting harder and harder to move back and
forth as the number of samples injected increases.  A brown residue appears
in on the glass walls of the syringe.  This might be oxides from the metal
syringe plunger, or organic residue from the water, as we run a large
number of water samples extracted from plants.  Our solution to the problem
so far is to replace the syringe every 212 injections, two analysis runs or
two days analysis or 168 unknown samples.  The syringes cost US$20 each,
which is a bearable US$0.12 per sample.

We did try a SGE #002987 10uL gas tight syringe with Teflon plunger, but it
cost US$40 and lasted a reliable 400 injections, the same cost as the
Hamilton per sample.  Strangely the amount injected was not reliable, as
the sample bellow would close to 35-65% with the SGE syringe.  Looking at
it working, there was an air bubble between the plunger and the sample that
does not disappear completely when pushing the plunger up and down in the
sample fill the syringe, not surprising since it is a gas tight
syringe.  Therefore we have kept using the Hamilton syringes.

If anyone else tries this please let me know how it works for a potential
methods note, especially if they discover how to make the syringes last longer.

Cheers!

Paul.



At 08:24 AM 12/8/03 +0100, you wrote:
>Hi,
>I suspect variable memory to be the cause of the observed raw dD
>fluctuations. We have been struggling with memory in water analysis and
>it seems to vary quite a bit. There is changing memory in the syringe we
>use to inject into the TC/EA sytem. We see the same with the H-device.
>There is also memory associated with the chromium, the quartz and,
>probably most importantly, with the surface that the water can interact
>with before reduction.
>In our sequence runs we therefore always include a large isotope jump
>(>100 per mill for hydrogen) and follow the fate of the measurements for
>about 6 analyses. The memory correction (usually around 2-4 % of a
>mixture of the preceding measurements) is then modelled (e.g 2% of the
>previous analysis + 1% of the one before that) to correct the jump in an
>optimal way. The correction is then applied to all samples.
>The memory correction is applied prior to a drift correction that can be
>attributed to several drift sources including depletion of reference gas
>in the bellows (H-Device).
>The last step is the scaling to a given difference as mentioned by Paul
>Brooks.
>
>Regards Willi
>
>Paul Brooks wrote:
>
>>Dachun,
>>
>>We have see a similar effect here using a FM H/Device, and I know
>>another researcher using a Micromass continuous flow system who has a
>>similar effect.
>>
>>I assume the results you show below are then normalized to the correct
>>value of the standards.  However, that means that the difference between
>>them should be the same.  Your data shows the difference between the
>>standards has changed from -281 to -289, an 8 delta difference when I
>>presume the precision of your system is better than 1 delta unit.
>>
>>-360  -79     difference      -281
>>-354  -65     difference      -289
>>
>>
>>I assume that most analysts are using a two point calibration for D
>>analysis as recommended in:
>>
>>Brand, W. A, T. B. Coplen.  2001.  An interlaboratory study to test
>>instrument performance of hydrogen dual-inlet isotope-ratio mass
>>spectrometers.  Fresenius J. Anal. Chem. 370:358-362.
>>
>>Doing a two point calibration should take care of any change in each
>>standard if it is consistent over the course of the analysis run.
>>However, what seems to happen is that during the course of an analysis
>>using chromium,  if one calibrates with one standard, the other standard
>>of a different isotope ratio does not always drift or change at the same
>>rate.  I calibrate with a +3.5 standard and then, if analyzing samples
>>from 0 to -80 which is our usual range, us a -95 standard for the two
>>point calibration.  I was originally using two standards so close
>>together because memory effects were a problem from the syringe in the
>>auto-sampler, a problem we have now overcome.
>>
>>However, just calibrating with the 3.5 the -95 standard would change
>>during the course of a 23 hour 100 injection analysis, as shown below.
>>
>>
>>    run number   1       2       3       4        5       6       7      8
>>beginning of
>>run      -96.3   -95     -96     -96.2   -96.4   -94.9   -96.2   -95
>>
>>end of run    -93.6   -95     -94     -93.7   -93.5   -95.1   -93.8   -95
>>
>>difference    -2.7    0       -2      -2.5    -2.9    0.2     -2.4    0
>>
>>
>>One can see here that -95 standard has a tendency to be more negative at
>>the beginning of a run than at the end, but not consistently. I
>>eventually created a a spreadsheet that fits a curve to both the 3.5 and
>>-95 standards, and then for every injection does a two point fit between
>>the 3.5 and  -95 curve.  Quality controls since I started this procedure
>>have been excellent, better than plus or minus 0.4 delta units long term
>>external precision.
>>
>>I was able to modify my spreadsheet to work with 30 hour analysis runs
>>that a colleague was doing with a Micromass continuous flow system and
>>was seeing the same effect.  Therefore the effect would not seem to be
>>from the instrument, but some effect of the chromium.
>>
>>I would be interested in any other researchers who have seen a similar
>>effect.
>>
>>Paul Brooks.
>>
>>
>>
>>At 10:43 AM 12/5/03 -0800, you wrote:
>>
>>>Hi,
>>>
>>>I use IsoPrime to run dD of water by Cr reduction method. The raw
>>>values of the working standards sometimes fluctuate day by day. The
>>>values of  -360 and -79 for the first day can be -354 and -65 the next
>>>day. There is no big change of the machine condition except a slight
>>>shift of peak center. However, the calculated dD values of the
>>>repeated samples are perfectly match from day to day. It seems not a
>>>real big problem. I will feel better if more people telling me they
>>>have the similar experiences.
>>>
>>>Cheers.
>>>
>>>Dachun Zhang
>>>Zymax
>>
>>
>>
>>
>>Paul D. Brooks,
>>Center for Stable Isotope Biogeochemistry,
>>Dept. Integrative Biology MC3140,
>>3060 Valley Life Sciences Building,
>>UC Berkeley, Ca. 94720-3140.
>>
>>[log in to unmask]
>>
>>phone (510)643-1748,
>>FAX (510)643-1749
>>
>>http://ib.berkeley.edu/groups/biogeochemistry/
>
>--
>............................................................
>Willi A. Brand, Stable Isotope Laboratory      [log in to unmask]
>Max-Planck-Institute for Biogeochemistry
>
>!!! New !!!
>Beutenberg Campus
>Hans-Knoell-Str. 10, 07745 Jena, Germany      Tel +49-3641-576400
>P.O.Box 100164, 07701 Jena, Germany              Fax: +49-3641-57-70
>
>http://www.bgc-jena.mpg.de/pages/public/Iso+Gas-Lab/BGC_Gaslab.html
>http://www.bgc-jena.mpg.de/pages/public/Iso+Gas-Lab/BGC_Isolab.html
>............................................................



Paul D. Brooks,
Center for Stable Isotope Biogeochemistry,
Dept. Integrative Biology MC3140,
3060 Valley Life Sciences Building,
UC Berkeley, Ca. 94720-3140.

[log in to unmask]

phone (510)643-1748,
FAX (510)643-1749

http://ib.berkeley.edu/groups/biogeochemistry/

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