Combusting filters

Combustion of quartz and borosilicate filters used to extract sample material from air or water

Loading quartz or borosilicate/Pyrex filters into tubes for combustion can present unique challenges, the foremost being cracking of the tube during combustion.

A general recommendation is to only use quartz filters. If you are stuck with borosilicate filters, there are a few tweaks to normal handling that can improve your chances of achieving full sample combustion without the combustion tube breaking/melting.

Recommendations for handling quartz filters:

Depending on the size of the filter in use, you will most likely need to use a 9 mm diameter quartz tube for combusting the sample instead of the more commonly used 6 mm diameter.

If the filter diameter is 55 mm or less you may be able to roll the filter up and drop it directly into a 6mm quartz tube for combustion (using clean gloves, of course). Larger filters will require cutting the filter into thin strips, using a razor blade and forceps. This will enable you to get all, or a portion, of the filter into the combustion tube. Do not pack the filter too tightly or you may have issues trying to get all of the contents on the filter to combust.  Distributing the CuO throughout a portion of the length of the tube aides in more efficient combustion.

Recommendations for handling borosilicate filters:

If borosilicate filters are used you might find you need to double tube the filter, this is really only possible if the filter diameter is not much larger than 35-55 mm so that it will fit inside a 6mm tube. Again do not pack the filter tightly.

The 6 mm tube is then placed inside the 9 mm tube which will be sealed under vacuum.

If the filter is too large to double tube, or the filter is small and there is no other need to double tube (besides the fact that the filter will melt and react with the combustion tube at high temperatures) we recommend lowering the combustion temperature to 500 ͦC. This should combust the sample but keep the filter from melting and reacting with the quartz tube.

Combusting at such a low temperature, or having a really packed tube might lead to partial oxidation of the samples. You can usually tell if the sample isn’t fully combusted because you will still see sample left on the filter. If this happens just re-combust the sample as is. It should be fully combusted after two rounds of combustion.