Louise Lapierre <74267.2135 at CompuServe.COM> wrote:
>I would like to know the relative advantages and disadvantages of
>large volume sampling of water and the differences of
>centrifugation vs filtration on the detectability of substances.
>Please forward me any thoughts or references.
>I am presently preparing sampling of rivers and would like to
>choose between different matrices.
It will depend very much on the chemicals / substances that you
want to detect & their ambient concentrations. That can be
difficult to get a handle on without some reconnaissance sampling,
if there's no prior sampling data available. High vol samples can
be anything from a few L to 5000 L or more depending on the ambient
levels of the target substance.
You may want to consider something like the Golden (APLE) extractor
developed at the Canada Centre for Inland waters.
The main disadvantage of high vol sampling & subsequent lab procedures
is that it's expensive. Generally you can't come close to the spatial
and temporal coverage that you can with routine methods.
OTOH for certain objectives, specifically mass flow / mass balance
calculations, there is no substitute for measuring the ambient
Assuming you're after organic contaminants, study of physico-chemical
properties (Kow, K_d, vapor pressure, solubility,etc) & usage/release/
occurrence data are helpful.
If you're after low level contaminants, eg, PCBs, TCDD/PCDF, etc,
you may little choice but to go high volume if you want to obtain
quantifiable lab measurements. A data base full of non-detections
doesn't give you much information other than an upper limit on potential
As for filtration versus centrifugation, both have been used. I've
not seen a good discussion of the relative merits of the two. With
centrifugation, there are 2 approaches (1) pump the sample into a
stainless steel tank, haul to the lab & centrifuge there; or (2)
in situ centrifugation. With the latter, you can assure that you
recover enough solids to obtain all the lab analyses that you want.
With the former, you're stuck with a fixed volume of water that may
not be enough to get all the measurements your after.
Centrifugation has one disadvantage that particles are smashed in the
process; so that, particle size analysis of the recovered solids is
Something to note is that centrifuge recovery efficiency is generally
a function of both solids concentration and the organic C fraction of the
solids. For low SS / high POC waters, you may have to process very
large volumes of water to obtain sufficient solids for analysis.
While you're at it don't forget to get the basic ancillary data,
i.e., suspended solids, DOC, TOC (or POC on the particles). I
just waded through extensive literature on global organochlorine
levels, and the majority of investigators have failed to collect
or report SS, DOC, POC which severely limits the use of the data for
comparative study and extrapolation.
Keep in mind that the lower the ambient concentrations of the target
substance, the greater the risks of sample contamination in the field
and the lab. Stringent QC/QA procedures may be required.
Byron Bodo 240 Markham St. tel: (416) 967 7286
|> |> Toronto, ON fax: (416) 967 9004
|> |> Canada M6J 2G6 email: bodo at io.org