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.
Samples are normally analyzed in triplicate. While the precision and accuracy of any one measurement is quite good, many samples are somewhat heterogeneous and require multiple measurements to achieve a best overall assessment of the materialís composition.
© 2002 Woods Hole Oceanographic Institution.