How Dirty Are You? Part 5…Micropipette Tips

Mouth Pipetting
Source: http://collections.nlm.nih.gov/catalog/nlm:nlmuid-101446257-img

Pipettes are common tools for drawing and dispensing precise volumes in chemical, biological and medical labs. Thanks to the invention of micropipettes and other techniques, we don’t do mouth pipetting anymore.

Positive displacement pipettes are particularly useful because the air gap in standard micropipettes are replaced by capillary pistons, which greatly improve the accuracy for handling aggressive liquids, such as organic solvents, and viscous solutions.

It sounds like the perfect way to transfer problematic solvents, however, the question of cleanliness has not be addressed. This reminds me of the blog series from my colleague Julie Kowalski, “How Dirty Are You?” So maybe it is time to add another subject to it.

To mimic the worst case scenario, I soaked the positive displacement pipette tips (50-250 µL) into 10 mL of various organic solvents in glass centrifuge tubes and shook them for 30 min. The organic solvents are toluene, acetone, dichloromethane, hexane, and acetonitrile. The 0.5 ppm triphenylphosphate (TPP) was prepared as the internal standard. The solvents were then completely dried under N₂ at 50 °C after extraction. Finally, 100 μL of the corresponding solvent was added for reconstitution using glass syringes.

For comparison, another set of samples were prepared by pipetting 250 μL of solvents 10 times. Blanks went through the entire preparation without soaking the micropipette tips.

Each sample was tested on a Rxi-5ms (30 m x 0.25 mm x 0.25 µm) using a Shimadzu GC-MS-QP2010 Plus instrument.

The 30 min soaking results are shown below. I was not surprised to see a contamination peak at higher intensity than the internal standard (TPP) for all solvents. What really surprised me was the absence of phthalate.

So, what is the contamination? It was identified as “Erucylamide” in the NIST MS search library with a relatively good match (>90%). Erucylamide is often employed as an antiblocking/smoothing agent for plastic products, such as PE, PP, and PVC. Therefore, it is a very common contaminant in plastic packages and products.

So, what is the worst solvent? Toluene produced the most intense signals. Acetonitrile and acetone generated relatively similar backgrounds which were cleaner relative to toluene. It pretty much followed the polarity trend except for hexane (see the picture below).

From the chromatograms below soaking for 30 min produced much higher signals than 10-time pipetting. For disposable pipette tips, many people only do one aspiration for each sample, sometimes two if doing sample priming. Therefore, you will get a cleaner background less times you use the micropipette.

The good news is the major ions of erucylamide (m/z 59/72/55) are relatively small, which may not interfere with your target in either full range scan or specific transitions. For anyone who may still be concerned about this contamination, I hope this blog will make you feel better.