Time to Change the Way you Extract Water Samples?
Posted on July 1st, 2010
If you are an environmental analyst workin with GC-MS, sample preparation by solid phase extraction is an attractive alternative to liquid-liquid extraction when working with water samples. Unfortunately it doesn’t always work out in practice. There are a number of reasons SPE goes wrong (many of which are avoidable). One such problem is that if the sample contains any particulate material, this will either result in very slow flow-rates (if you are lucky) or will block the cartridge completely (if you are not). You could filter the sample of course, but you may then have to separately extract the solids (the Water Framework Directive requires this nowadays).
There has always been a way around this problem and that is to use SPE discs rather than cartridges. Discs have a much greater surface area and are much more permeable than cartridges. Samples flow through the extraction media much faster, the solids have less of a tendency to bung-up it up and when you elute the media with solvent, you extract the solids from the sample at the same time.
Why then didn’t the use of discs for the extraction of environmental water samples taken-off in Europe the way that it has in the USA?
There are two reasons. Firstly, in the past, although discs handle particulates better than cartridges, they weren’t perfect in that respect and they didn’t solve the particulate problem sufficiently well. Secondly, compared to cartridges, discs were expensive at £8-10 each, and although it is possible, in theory, to reuse discs, this never turned out to be a practical proposition.
However, things have changed and within the last few months several labs are looking at discs afresh and finding that they are working out this time around.
So what has changed?
Firstly, a series of incremental developments have transformed the efficiency with which discs can handle very dirty samples, this makes life much easier AND enables larger sample volumes to be extracted without a serious time penalty.
Secondly, and most importantly, one disc manufacturer (Horizon Technology) has now realised that discs will only sell in Europe if they are priced sensibly. We sell Atlantic discs in the UK and a new pricing structure that has recently come into play means that it is now possible for you to buy discs in quantity for less than a fiver per disc. OK, they still cost a bit more than SPE cartridges, but hey, SPE cartridges don’t work at all for gunky samples.
Suddenly, some analysts are back using a discs again and speaking very enthusiasticly about how well they work.
If you want to give this a go, I have a limited supply of free-trial packs of Atlantic discs.
Email us on email@example.com and one of our technical staff will arrange to give you a trial pack of the most suitable type for your application and some vital tips on how best to use them.