Imagine you’re on a world trip. You find yourself in a vast, sprawling city. You drift about, your path seemingly changing at random, following flows of people and being guided by the shapes of the metropolis. Strange scents fill your nostrils, sometimes alluring, other times bitter and harsh. A clear, open square beckons. You join a smattering of fellow wanderers crossing the square, which seems free of distractions and the chaos of the city. Groups of people are milling closely around some attractions on the other side.
You get closer; you’re curious. What are these things? There are too many people clustered around the first few, so you slip through the crowd until you come across an opening. You see what’s there, realising too late what’s going on. The ploy has worked. You are trapped.
The situation seems terrifying: stuck in a throng of trapped people, in a foreign city. You’ll be relieved to know this doesn’t happen to real people: it’s the story of ions in an emerging type of chemical sampling device called DGT (Diffusive Gradients in Thin Films).
We understand our chemical world by looking at where different types of chemicals are, and how much of each is present. However, in most cases, it’s not easy to measure the chemicals as they are in the environment. We try to take ‘samples’ of the environment, bring them back to the lab and then run them through chemical tests or machines to find out what’s there.
The easiest, and most common, type of sampling is by taking a ‘grab’ of something, say, filling a bottle of water from a river or scooping a shovel of soil into a bag. The grab sample is taken back to the lab and analysed. Most monitoring will look for key indicators of contamination or pollution and compare these to regulated limits. If they’re low, everything is fine. Right?
Not so fast. The world is ever-changing: a bottle of water we fill from a river might be very different from the real situation in the river, by the time we’ve carried it back to the lab. The temperature would have changed, likely along with oxygen content and other factors. Not only that, the river itself might change quite quickly: the Thames, for example, is tidal, so it changes direction every few hours!
That’s where DGT comes in. It slowly, and specifically, accumulates certain types of chemicals from the water over time. So far, devices have been developed for a range of important chemicals, such as toxic heavy metals and sulfides. The most recent versions, described by Gold Coast researchers William Bennett, Jared Panther and co-workers in two 2010 papers, focus on arsenic, selenium and phosphorous, which are important for human and environmental health.
Unlike a grab sample, DGT is targeted to certain chemicals, gives an average concentration over time (rather than a potentially unrepresentative snapshot) and accumulates lots of the chemical in a special ‘binding phase’. This last feature is particularly useful, because some toxic chemicals are only present in very low concentrations normally, which makes them hard to detect. DGT gradually builds up enough of the target chemical to easily analyse.
The technique described by Bennett and Panther uses titanium dioxide nanoparticles, which are basically a fancy white powder. The powder strongly traps a range of arsenic, phosphorus and selenium compounds. The powder is mixed into a thin circle of gel, which is then covered by another, clear gel layer. The two gels are fitted into a case, and deployed in the environment. After a period of time – anywhere between six hours to a few days – it’s picked back up, and the little gel disc with the binding powder is removed for analysis.
There’s a trick to it, though. The researchers need to relate the concentration from the binding gel to the concentration in the river. Think back to when you were wandering about in the city earlier. If you put an ‘attraction’ on a really busy street, it will quickly get filled up, but if it’s down a side alley, business will be slower. Similarly, if we just drop our binding gel straight in to a river, it might sit and accumulate slowly in a stagnant spot, or collect chemicals really quickly in a fast-flowing section.
DGT elegantly avoids this problem by putting in the second, clear gel layer. That’s like the big, open square I talked about earlier. Imagine a crowd dispersing across an open space, with no paths to guide them. If you place an attraction on the far side, you’re more likely to get an even, steady stream of people wandering over to check it out. The same idea applies, as chemicals diffuse evenly across the clear gel – no matter what’s going on outside the sampler – before being trapped in the binding gel. Throw in a bit of maths and voila, we can work out the concentrations from the river!
The method has proved its worth in a range of applications so far, such as looking at wide-scale spreading of metals from mining operations. While it is more expensive than grab sampling, the quality of the information DGT collects is much higher. For me, a method that gives us a better tool which we can use to understand the world around us, and make good decisions about how we manage potentially dangerous chemicals, is a winner.
Oh, and don’t be too hesitant when crossing open squares in foreign cities. DGT devices for people sampling haven’t been invented… yet.
Bennett, W., Teasdale, P., Panther, J., Welsh, D., & Jolley, D. (2010). New Diffusive Gradients in a Thin Film Technique for Measuring Inorganic Arsenic and Selenium(IV) Using a Titanium Dioxide Based Adsorbent Analytical Chemistry, 82 (17), 7401-7407 DOI: 10.1021/ac101543p
Panther, J., Teasdale, P., Bennett, W., Welsh, D., & Zhao, H. (2010). Titanium Dioxide-Based DGT Technique for In Situ Measurement of Dissolved Reactive Phosphorus in Fresh and Marine Waters Environmental Science & Technology, 44 (24), 9419-9424 DOI: 10.1021/es1027713