[Polarlomvi]

Hi!

I have conducted a ecotoxicological experiment in soil. The soil was spiked with a concentration of a pesticide. I analysed the soil concentration at Day 0 and again at the end of the experiment at day 28. One of the reviewers of my paper suggested to use time-weighted averages (TWA) as the pesticide is degraded rather quickly. However, I don't see how this can be done. As far as I know, the TWA is calculated as followes:

TWA = (t1c1 + t2c2 +...+ tntn) / (T1 + t2 +...+ tn)

Where t is the duration of the time interval and c the concentration measured in the respective interval.

I haven't got any intervals, so I really don't know what to do!?

Does anyone have a suggestion? Is there a possibility to take kinetics into account (1. order)?

Thank you very much in advance!

[Stepan]

If you know order of the reaction, you can estimate TWA. TWA=integral(C(t)dt)/(t2-t1) (note integral is taken from t1 to t2)

[Polarlomvi]

If you know order of the reaction, you can estimate TWA. TWA=integral(C(t)dt)/(t2-t1) (note integral is taken from t1 to t2)

Thank you for your reply!
I don't know the order but would take 1. order as I don't know anything else about the degradation of my compounds. But how does the formula account for the order of the reaction?

[Stepan]

dC/dt=k*Cⁿ where you assume n=1

Usually it is done another way around. You measure concentration at time zero and at a few more points of time (including your final test). With this measurements on hands you: a) calculate order of the reaction and discuss possible mechanisme; and b) can numerically calculate TWA (if needed).   

[Polarlomvi]

Thank you, Stepan! Yes, I know that my measurments are not sufficient for real TWA.