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I have benthic samples that were collected with an Ekman dredge from some small ponds. The samples contain quite a bit of coarse particulate organic matter (CPOM, basically dead leaves). I would like to quantify the amount of CPOM in the samples but they were preserved with 70% ethanol in the field.

Is there any evidence that ethanol preservation alters benthic CPOM mass?

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  • $\begingroup$ How do you plan to separate the POM from the rest of the sample? $\endgroup$ – Abe Nov 18 '13 at 6:26
  • $\begingroup$ We run the samples through a 1 mm sieve to separate CPOM from the remaining sample. The coarse fraction is then dried at 50 dC and ashed at 550 dC to determine LOI and % OM. $\endgroup$ – KennyPeanuts Nov 18 '13 at 17:19
  • $\begingroup$ This sounds like something that would be straightforward to test empirically. As a bonus, any effect of EtOH on your samples (bias) could (possibly) be quantified and accounted for, especially if the effect is small. The questions you are asking will inform how best to do this. But I do suspect EtOH might dissolve some hydrophobic compounds in the coarse fraction. Maybe try evaporating it first and adding water. Not ideal, but likely more informative than throwing samples out! $\endgroup$ – Abe Nov 18 '13 at 18:49
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    $\begingroup$ This paper implies the EtOH extractable fraction in willow leaf litter is small ~2% download.springer.com/static/pdf/61/… but this will vary by species (eg pine tar). Again, this effect can be measured on samples similar to yours. $\endgroup$ – Abe Nov 18 '13 at 18:57
  • $\begingroup$ @Abe Thanks for the paper, I look forward to reading it. We are actually testing this now. The initial tests with fairly old OM showed no effect but we are planning some tests with fresher OM to see if that makes a difference. $\endgroup$ – KennyPeanuts Nov 19 '13 at 2:04
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It's very likely that you should have at least minor changes in mass.

  • The reason is because ethanol can extract a number of substances from leaves (e.g. tannins, pigments, etc.).

However, you could still easily double check on your own.

You could still figure out the mass of the CPOM even without knowing the answer to this answer directly.

  1. Drain and collect the EtOH.

  2. Determine mass of separated CPOM.

  3. Determine the excess mass present due to any leaching/dissolving from the CPOM by using density = mass / volume (or ρ=M/V) with your collected volume of ethanol.

    • EtOH has a density of 0.789 g/mL at 20°C. 70% EtOH has a density of 0.885 g/ml at 20°C.

    • Determine the volume of the collected EtOH.

    • Determine the mass you'd expect of "pure" 70% EtOH given your volume (m = ρV )
    • Determine the mass of the collected EtOH
    • Subtract the expected mass from your calculated measured mass. The difference (if any) would represent mass leached/dissolved from the CPOM.
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  • $\begingroup$ But consider that EtOH evaporates quite rapidly which could give you a false reading. Why not try with some leaves of a not-that-important-sample? Weight them before, put them in alcohol for some weeks and weight after (let the alcohol attached to the leaf litter evaporate before weighting). $\endgroup$ – Stockfisch Mar 7 '17 at 15:10
  • $\begingroup$ Evaporation shouldn't matter because you would measure densities. Any CPOM mass would remain in the fluid. In fact, with evaporation, the concentration of remnant cpom would increase per ml, and so would likely make subtle density discrepencies even more apparent $\endgroup$ – theforestecologist Mar 7 '17 at 16:14
  • $\begingroup$ Ok, thanks for clarifying, I wasn't aware of that. $\endgroup$ – Stockfisch Mar 7 '17 at 19:25

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