A recent article by Girouard and Zagury (doi:10.1016/j.scitotenv.2008.12.019) on the available of arsenic bound in soil highlights a recurring theme in soil environmental toxicology. Our current measurement techniques for contaminants in soil may be significantly underestimating the risk associated with a polluted soil to small children. Typically, we recommend that soils be sieved to pass a 2 mm sieve, or in other words, only particles less than 2 mm are analyzed. Sometimes we suggest that soils should be sieved to pass a 0.25 mm sieve. The reason we do this is that we know that concentrations of pollutants increase as we sieve to smaller and smaller sizes. However, what we don’t know is how ‘available’ pollutants in these smaller size classes would be to small children, or in other words, how easily can pollutants in small sized particles enter our bloodstream. The work by Girouard and Zagury indicates that the answer to this question is: more easily. Thus, the smaller the particles the more risk is posed to small children who might accidentally eat these soils. (as an aside, I’m sure you are wondering how much soil a children will eat. We estimate around 100 mg per day but in some cases, children can eat up to 13 g). As a society we need to revist how we assess risk from contaminated soils and ask the question: are we being protective enough?

A recent article by Dr. Gan’s group in Environmental Science and Technology (DOI: 10.1021/es802966z) investigates how available phenanthreneis to bacteria.  What the group did was measure phenanthrene degradation and also calculate the amount of phenanthrene freely dissolved in soil pore water.  To measure the amount of phenanthrene in pore water, they used solid phase microextraction fibers.  Importantly they checked many of the assumptions about SPMEs and their ability to measure freely dissolved phenanthrene.  Once they did this, they found that the freely dissolved concentration of phenanthrene could only predict the initialy 144 h of degradation but after that was not predictive at all.  This suggests that microorganisms can access phenanthrene directly from soil without the phenanthrene dissolving into pore water.  This is not a brand new observation by Dr. Gan’s group has been able nicely pin it down and also investigate the influence of other soil factors like organic carbon and surface area on bacterial use of phenanthrene.

Why do we care about this?  The concept of chemical activity versus bioaccessibility hinges on this very same issue.  Chemical activity can be linked to the freely dissolved chemical concentration in pore water.  In contrast, bioaccessibility is an estimate of how much of a contaminant an organism can access. One good rule of thumb put forward by Reichenberg and Mayer (2006) is that bioaccessibility is required  when organisms can specifically access a chemical whereas chemical activity is more important for passive uptake systems.  Typically for bacteria we think of them as passive systems for polyaromatic hydrocarbons.  The toxicity of PAHs to organisms is often thought to be due to non-polar narcosis in which the PAHs disrupt the lipid membrane (nitrifiers are an important exception to this).   Based on this recent publication by Dr. Gan, we are going to need to rethink this idea. If microorganisms can specifically uptake the toxicants, then we need some sort of estimate of bioaccessibilty for microorganisms.  This will allow us to modify site specific clean up scenarios to protect biogeochemical cycling at impacted sites.

Well it has been a frustrating week.  Methanol can be substituted but peaks broaden and more non-polar substances co-elute.  The Thermo guideline is a good first step but I think that the method development is going to take longer than I had initially hoped.   Hopefully the acetonitrile shortage will alleviate soon and we can go back to the original solvents.

So, if you are like our lab and use HPLC technology, then you know there is an acetonitrile shortage.  Acetonitrile is a by produce of the automative industry.  As you may have heard, this industry is in free fall and as a result acetonitrile has increased in price 6 to 8 fold and normally can’t be bought.  This is a problem becuase acetonitrile is the solvent of choice for most HPLC applications.  Thus, one can think of HPLC’s as running on acetonitrile.

Luckily, I work in an academic environment so I can change solvents.  Others who need to work with certified methods are stuck and can try to save acetonitrile usage by switching to smaller columns.  If you are going to change solvents, there are two major things to consider.  The first is that other solvents like methanol absorve in the UV spectrum which can pose a problem for UV/VIS detectors.  The second is that methanol has a slightly different polarity than acetonitrile and also results in higher backpressure.  Luckily, Thermo has posted an excellent user guide for replacing acetonitrile with other solvents. Today we are going to try to replace our PAH HPLC method that is acetonitrile based with a methanol based method. PAHs are detected using a fluorescence detector so the UV absorbance of methanol is not a big concern.  Hopefully it will go well and tommorrow I can post the results in case others need to change their PAH methods over.

It is funny that as scientists we normally don’t primarily think of ourselves as consumers and thus are somewhat disconnected from the overall economy.  The acetonitrile shortage is a good example of how interconnected our economy actually is.