We commonly use a correction factor for the amount of weight lost to
handling. I'm actually surprised that your blanks didn't lose any
weight. How much water did you run through the blank filters?
If I could make a couple of suggestions:
-I ash filters at 450°C for 2-4 hrs when I do this. The lower
temperature leaves the filters a little less "crispy" and I think they
tend to hold up better and break less when you fold them (Especially
on Pall A/Es. I don't use the GF/Cs, but I do use GF/Fs and they seem
to handle combustion a little bit better than the Palls). 450°C
should take care of any OC on the filters. Much over 500°C and you
will end up with a hockey puck.
-Vacuum at a lower suction. I think Wetzel and Likens recommend about
half of what you were filtering at. I don't have my copy in front of
me, but 15psi sounds high.
-Try another 24hrs on the drying time to make sure there is no
residual moisture on the filter.
-Make sure you are running enough water through so that the filter is
almost clogged. The more weight you can put on the filter, the better.
Hope that helps.
Jesse
On Sep 7, 2009, at 6:00 PM, [email protected] wrote:
Hello all,
I recently had to filter water samples through glass fiber filters (I
used Whatman brand, GF/C filters). I combusted the filters at 500 C
for
an hour, let them cool down, ran the water samples through them with a
vacuum pump at 15 psi, dried them at 60 C for 24 hours, and then
combusted them at 500 C for 3 hr once more in order to obtain ash
free dry
mass. To my dismay, the filters with the ash weighed less than the
original weight of the filter after the initial combustion. Either my
scale has done something funny or the filters have lost mass during
the
course of this process. I tried running some blank filters (doing
everything the same way, except using pure water), and the filters
did not
lose any mass. Room temperature has stayed roughly the same during
the
course of this process.
Has anyone had any experience with this that might be able to
explain to
me what just happened?
Thank you,
Aaron Stoler
--
University of Pittsburgh
Department of Biology
101 Clapp Hall
Pittsburgh PA 15260
410.365.7700 (cell)
412.624.4458 (office)
[email protected]
www.pitt.edu/~abs45/index.html
Jesse Becker
Doctoral Student
Department of Biology
Freeman Aquatic Station
Texas State University - San Marcos
[email protected]
