There is a bit of confusion as to how best to determine POC in (UK blanket bog) peat stream water (flow).
For example, which filter to use and how to determine C (C/N analyser or other) and what volumes are necessary?
It would be nice to see what other people are doing as in the literature quite often old publications are quoted using LOI methods which are relevant to mineral soil carbon determination etc.
Also nice would be to see what ranges are to be expected as there is not much published material (most is on DOC).
So far we are using about 50- 500 ml of stream water, filter through pre-ash filters (GF/F 0.7 µm glass microfibre filters) using a vacuum pump and then burn the lot in a C/N analyser.
What you are describing is pretty standard (GF/F, Elemental analyzer), but you might look into some of the labs that run stable isotope analysis (MS) downstream of the standard elemental analyzers, relatively inexpensive these days and could give you C, N, S content and isotope ratios too
You can use the fractionation in heavy liquids (e.g. ZnCl2, sodium polytungstate etc.) for separation of POC.
I agree, the most common method involves filtering your water samples with a fiberglass filter (typically GF/F). Then you grind all the sample (filter+sample) and put it in tin capsules for later isotopic analyses (keep the samples in a dessicator while waiting for analysis). If you suspect that the water sample may contain lots of suspended carbonates, then you should try to eliminate the carbonate prior to isotopic analyses (usually with acid vapor fumes inside a glass chamber). Hope this helps
I agrree with the above use of CNS analysers on a conc HCl fumed GF/F fliter. However, beware 1. inline combinations with stable isotopes as the CNS analyser associated with the Mass isotope ratiio spec is not as accurate, 2. using tin cups maybe problimatical as their are porous if you use 10 % HCl directly into the cups to rid relatively high carbonate materials, so as not to loose any associated DOC from the particulate matrix i.e. the cups can leak material much to the annoyance of the analyst as well as the bias in the results from the intial weighing, therefore use silver cups just as cheap, 3. best to use quartz filters especailly with isotope anlysis and it doesnt form a slag like glass and increases the combustion columns life and gives a better result by not trapping umcombusted materialwithin the glass slag matrix as well as recuing any isotope fractionation as this treatment stage.
All this can be discussed with you analyst or central science analytical lab section, should you have one,, such a rapore in my experience will ultimately give good and cheaper results.
Hope that Helps
Barry
You may consider the approach described in the paper listed below. Briefly, passing the water with GF/F and then 0.45 um filters. Both filrates are tested for TOC. The differences in TOC between two filtrates are considered POC. I used this approach to examine POC and DOC from peat soil extracts.
Chow et al. 2005. Fitler pore size selection to characterizing dissolved organic carbon and trihalomethane precursors from soils. Water Research 39: 1255-1264.
I have used POC dichromate methods in sulfuric acid with spectrometric determination and glucose as a standard for organic carbon, If you are interest I can send you detail protocol
I have distinguised colloidal from dissolved material by ultracentrifugation. I first removed larger particles by filtering over 1.2 µm. The filtrate comprises both dissolved and colloidal material. The colloidal fraction was removed through ultracentrifugation (assuming a density of 1.2 g/cm³ and a size of 10 nm). I have to add that this was done with soil suspensions, not with peat stream water. However, the principles still hold...
Attached is a publication you might find useful. It also includes references which might be helpful.
POC by filter method is best I think. For density separation with polytungstate, there are some mass contamination issues, even with high wash rates. Colleagues of mine have observed (using NIR) residual contamination peaks form the polytungstate on POM fractions, which if you go down to the small masses you might get in peat waters (compared to soil fractionations), you could have quite a bit error. The GF/F is a good approach, so i think you're doing a good job.
cheers
rob
Your choice of filters sounds good, but do not put them directly in the CN analyzer. You must first remove inorganic carbon. The best proven way to do this is through acid fumigation, as opposed to direct addition of acid. Roll up your filters into silver capsules (25mm filters work best, 47's are too large), and put them into a well plate. place a few drops of milli-q water in the silver capsules (tin will crumble in acid) so your sample is completely submerged. This will aid in complete acid diffusion throughout the sample. Then In a glass desiccator, place a petri dish of 12N HCL under the sample tray, let the acid fumes build up for an hour or so, and place the well plate in for 6-8 hours (8 is preferable). The reference I believe is Hedges and Stern 1996. I normally leave the cover over the well plate to prevent contamination, and just prop it up with a glass pipette to make sure the fumes get in. After 8 hours, dry the samples at 60C. It may take overnight or a bit longer. Then roll up the silver capsules into larger tin capsules. Roll those up as well and drop in the elemental analyzer
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