A quick reminder…I was warned against using Parafilm® because it would cause contamination. At the time, I was working with acetonitrile and I didn’t think there was much risk of contamination. We decided to perform a quick test to see what would happen if we exposed Parafilm® to different solvents. See the original blog for more details.
There were two questions posed in the last blog…
1. Which solvent produced the most intense signal?
2. Which solvent produced the smallest signal?
The solvents tested included (in alphabetical order) acetone, acetonitrile, ethyl acetate, hexane, isopropyl alcohol, methanol, methylene chloride and toluene.
The Answers…Drum roll please…
Hexane produced the most intense signal.
Acetonitrile produced the least intense signal.
Although soaking Parafilm® in solvent is not typical, it does show that background contamination can occur if Parafilm® contacts solvents. Characteristically, hydrocarbon type contamination is observed as in the chromatograms below. My initial thought was to look at solvent polarity and assume that Parafilm® would be most soluble in a non-polar solvent. Polarity was also a key consideration of the people who sent me their answers. (THANK YOU for the emails!) One person actually used polarity and dielectric permittivity (ɛ). I have to confess I had to google dielectric permittivity because I forgot what it was exactly.
Signal intensity, generally but not strictly, follows the polarity (and dielectic constants which had a similar trend as polarity) of the solvents tested with the most non-polar solvents producing the most intense signals. Also, you will see from the chromatograms below that hexane, methylene chloride and toluene show high contamination at very similar levels. After these three solvents, there is a significant decline in contamination. Methanol and acetonitrile produce the lowest signals – about two orders of magnitude lower than hexane, methylene chloride and toluene.
Samples of solvents post- Parafilm® soak were tested with a 50 ppm hydrocarbon standard as a signal reference, see the blue dashed line in each chromatogram. Click on the image to the right for a closer view.
Here are the chromatograms from highest to lowest contamination signal…
It is difficult to see the level of contamination for these more polar solvents…so let’s take a closer look.
For solvents that produced relatively low signal, the y-axis scale is changed and 5 and 0.5 ppm hydrocarbon standards were used for reference levels.
If you didn’t guess the correct answers, don’t worry, you aren’t alone. Only 8% of people had the correct answers.
My bet was on methylene chloride producing the highest signal and methanol producing the lowest…so i was close.
Remember that you are unlikely to see contamination from Parafilm® especially if there is no or little actual contact with your sample. But be aware that if contact with your sample does occur non-polar solvents can produce significant contamination.
At least we know what the contamination chromatogram looks like and this will likely be the most useful thing I have learned from this experiment… especially as we continue to use increasingly sensitive detectors that can easily detect trace level contamination.