Re: What does AFDW (ash-free dry weight) show?

Hi Marc:

You're quite right- Ash-Free Dry Weight is a measurement of the weight of organic material. In order to measure AFDW, one takes a sample (sediments, plankton, whatever) and removes all the water. The water is usually removed by baking the sample at a low temperature (around 60 degrees Celcius, which is a low enough temperature that combustion does not occur), followed by placing the sample in a sealed container with a dessicant (a chemical that "catches" water molecules). The resulting dry weight is the weight of both the organic and inorganic contents of the sample. Next, the dried sample is combusted (or, in other words, set on fire!). This is usually done by placing the sample in an oven (at greater than 450 degrees Celcius), in the presence of oxygen. In the oven, all of the organic content (carbon) is burned off- all of the carbon in the sample reacts with oxygen to produce carbon dioxide. Carbon dioxide is a gas, so it leaves the sample. The ash that is left over is thus the inorganic contents of the sample. The AFDW is then the dry weight (inorganic + organic contents) minus the weight of the ash (inorganic contents only). AFDW is therefore the weight of the organic content of the sample.

Hope that helps!
Rob Campbell, MAD Scientist