Testing for toxic organic contaminants in environmental samples often requires the following three sample preparation steps: 1) Extraction, 2) Concentration, and 3) Analysis. The goal during these three steps is to isolate the contaminants from the sample matrix and concentrate them so they can be detected and quantified. In this blog, I will discuss the importance of understanding how different types of concentration equipment can impact your analyte recoveries and the overall performance of your extraction procedure.
In this study, we looked at Hexachloroethane, which is a contaminant that is commonly screened for in the United States of America, following EPA methods 625.1 and 8270E. From my experience as an application scientist, this compound seems to be a recurring problem for many laboratories that are using EPA methods 625.1 and 8270E.
In fact, this problem came up so often, I decided to track the issue and investigate potential solutions. One of the most common sources of variability I saw at each of these labs was the type of concentration system being used. After comparing average percent recoveries that were obtained from concentrating extracted samples on two different concentrating instruments, I was able to determine that the type of concentrator used made a significant impact on Hexachloroethane recoveries. Instrument one was similar to a Kuderna-Danish (KD-Type) concentrator that uses a heating element in direct contact with a closed concentrator tube for reflux, applying heat directly to the sample extract while under vacuum. The second instrument was the TurboVap® II, which uses open concentration tubes in a heated water bath to provide consistent heat distribution across the entire evaporator tube. Looking at the data table below, you can quickly see an increase in average percent recovery for hexachloroethane by around 15%. That difference could be massive when you are dealing with specified project limits for determining the required percent recovery of your control spikes.
The third column of the table takes this study one step further by using the programmable method feature of the TurboVap® II. We developed this method using a gradient flow to gradually ramp up the nitrogen flow over time to reduce the time it takes to bring the sample to its final volume. As you can see, the results remain consistent on the TurboVap® while reducing the evaporation time by at least 30 minutes. We also show that the key parameter impacting the recoveries were related to the means by which the sample extract was heated. In this case concentration using a water bath as the heat source performed significantly better. For further information on how to set up the ramp method or its benefits, we have a few technical notes that go into further detail. You can download them from the link below.
In conclusion, we have had quite a few run-ins with hexachloroethane, and the concentrator has been noted to be the prime suspect when it comes to lower recoveries than the norm. If your lab is able to develop their own in-house limits, then it would be helpful to update those to reflect the change in application. However, if specific limits are required for this troublesome compound, you might find a water bath concentrator to be the solution.