What method to use for chlorinated herbicides

If you have been involved for very long in environmental remediation under the Resource Conservation and Recovery Act (RCRA), you probably are accustomed to having your chlorinated herbicides analyzed by EPA Method 8151. There are some drawbacks to this method, but until fairly recently it was considered the only option to analyze for chlorinated herbicides.

So what are the problems with this method? The most glaring issue is the potential for false positives. Method 8151 uses gas chromatography with an electron capture detector. The electron capture detector (ECD) only responds to molecules that have an affinity for grabbing electrons. Halogens like chlorine, bromine and fluorine respond best on this type of detector, but other elements also show up. Molecules containing oxygen will show up on an ECD. If something shows up on a GC/ECD run, there is no way to know for certain that it is in fact your analyte of interest.

Below is an example of a chromatogram. Time is displayed along the horizontal axis and abundance along the vertical axis. Here we see a peak at about 6 minutes. This is how long it took this compound to travel through the GC column. We know it elutes at 6 minutes (which we can compare to our reference standard) and we know that it responds to an electron capture detector, but we don’t have additional evidence to prove it is in fact the chlorinated herbicide we are looking for. For this reason, the method specifies a dual column confirmation. You have to run it again on a different type of GC column, resulting in the analytes leaving the column (eluting) at different times and sometimes in a different order, and see if it shows up at the expected retention time on both columns (Click on the image for a magnified view, then use the back arrow when done).

This works fine for clean samples like drinking water. If you only see one or two peaks in your chromatogram, they probably won’t show up at the expected retention time on both columns unless they truly are your analyte of interest, but what if there are a lot of peaks in your sample? The chromatogram shown below is what I would call a picket fence chromatogram. Just like that wooden fence in your front yard, it has a lot of peaks. It is not uncommon to have many long-chain carboxylic acids or other oxy-hydrocarbons that show up on the ECD detector. If your sample produces this type of “picket fence” chromatogram, and if one of the peaks shows up in the expected retention time on the first column, it’s very likely that one of those other peaks will show up in the expected retention time of your herbicide on the second column. It will show up as a positive on your report, but it leaves a lot of uncertainty over whether or not you really have chlorinated herbicides in your sample. If your permit requires additional monitoring based on a positive herbicide result, the consequences of a false positive can be financially devastating.

A better technology is LC/MS—liquid chromatography with a mass spectrometer detector by EPA Method 8321. Here you will still have a chromatogram showing separation of the analytes based on how long it takes them to leave the column, but now there is the additional piece of information coming from the mass spectrometer. We can measure the mass, or more correctly the mass-to-charge ratio, of the molecule after it has been ionized. Even if something that is not a chlorinated herbicide leaves the column at the expected time of your analyte, it is very unlikely this imposter would have the same mass-to-charge ratio as your analyte of interest. False positives are therefore eliminated by choosing this newer technology.

Another advantage is that this technique requires a lot less sample volume. If you have a monitoring well that feeds out at a trickle, imagine the time saved by filling up a 40-mL vial instead of waiting to collect a full liter.

If you need testing for chlorinated herbicides, talk to one of our project managers for more information.