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Suggestions
for Elemental Analysis The
importance of accurately and precisely weighing your samples can not
be overstressed. In spite of the use of an apparently crude
detection technique (TCD), determinations can be made that are good
to 0.1%. For user weighed samples, we have available a microbalance
and cleaned boats. Regardless of what balance you use, take your time.
Wait for the balance to fully stabilize before you tare the boat, and
again, wait until the balance has come to rest before you record the
sample weight. Weights should be determined to the nearest microgram.
Obviously, you will not know the exact carbon content of your samples,
or you would not be requesting this analysis. However, most users have
a rough idea of the amount of carbon present, based on the sample type.
In general, the system runs best with the smallest sample loading consistent
with having about 20 to 30ug of organic carbon present in the sample,
but large quantities of low carbon sample can result in technical difficulties.
As a practical guide, if your material is expected to be about 30% organic
carbon then a 100mg
sample is about correct, 3% organic carbon materials should use around
800mg samples, but a 0.2% organic carbon material
would use a 3mg sample. Please
alert Facility personnel to any inorganic carbonate which may be present
in your samples. Many samples contain inorganic carbonates which, if
not properly removed, can result in grossly overstated organic carbon
values. If inorganic carbonates are present, samples must be weighed
into pre-cleaned silver boats. After weighing, samples will
be exposed to an HCl rich atmosphere which should cause the release
of most of the carbonate. After gaseous acidification, each sample will
be further acidified with 2N HCl until all signs of CO2
production cease. Samples are then dried in a warm oven prior to analysis.
Samples that have been acidified with sulfurous acid will not be accepted
for analysis. Sulfurous acid is incredibly difficult to fully remove
and its presence in the elemental analyzer carousel results in permanent
corrosion of surfaces which need to form air-tight seals. Be
aware that large amounts of high carbonate samples can be quite difficult
to acidify. There is a tendency for the surface to crust over during
gaseous acidification which then stops acidification of lower layers
of the sample. For very high carbonate samples, it may be better to
work towards the lower end of the desired organic carbon range. If
you are providing your own boats, please contact us for further instructions
on proper cleaning.
© 2002 Woods Hole Oceanographic Institution.
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