Hi all,
Using an Ion Chromatography system
Now that is very interesting, we have an HPLC and a GC, but neither of those are of much use to me, an ion-exchange chromatography system (IC/IEC) would be more useful (it produces peaks for all the charged particles (ions) in the water as the stationary phase of the column retains the differing anions in the analyte for differing times), although you would need both anion and cation exchange columns to cover all the ions we are interested in.
Like Tom says the problem with
other methods of NO3- measurement is that other anions interfere with the NO3- measurement, which takes us right back to what the OP found originally. If he had access to an IEC system, the computer software to control it, a suitable column and active phase carrier he could have got a more accurate estimation.
Both BOD and COD are different in that they aren't quantitative tests, but provide an estimate of the potential demand for oxygen, which is however a pretty good measure of the organic
pollution of a water sample. The advantage that BOD offers over COD is that it is much more sensitive, you can think of COD as giving you a ball-park figure, and BOD as adding all the decimal places.
For rivers and streams, if you have a measure of BOD and carbonate hardness, you don't really need any other analytical measurements to give you an idea of water quality and likely biota. When you sample the invertebrate biota, if there is a big discrepancy between the species assemblage you would expect, based upon the water quality, and the species you find (or more importantly don't find), you can be pretty sure there has been a pollution incident. If any-one is interested there are more details here: <
RBMS - Title and Index to web pages>
To give some figures a really clean river will give a 5-day BOD below 1 mg/L. Most "clean" rivers in the UK rivers will have a BOD value of between 2 & 10 mg/L. Treated sewage about 20 mg/L and raw sewage anywhere from 200mg/L (winter) to 1000mg/l (summer).
Even though we can't measure BOD, we can still make use of it as a concept. When I add something to the tank I think of it in terms of BOD. If you feed your fish a protein or sugar rich food you've raised the BOD, because you've provided a substrate for bacteria, and you've increased the ammonia production of the fishes metabolism. Both of these activities lead to increased oxygen demand.
If you add fertilisers the situation is more complicated, you potentially increase the BOD, because nutrients become non-limiting in terms of plant growth and plants are leaky organisms, but much of that increased metabolism is incorporated into new plant tissue. Photosynthetic organisms also differ from all other forms of life because they are net oxygen producers. EI and the Duckweed index both make use of healthy plant growth to maintain water quality, but EI is a much more intensive system, where inputs and plant growth rates are much higher, and regular large water changes, maintenance and plant pruning remove organic compounds from the system.
cheers Darrel
) but no good for contaminated water testing.
