Are You Contaminating Your Samples During Evaporation?

“Water in my extracts again?!?!”

How many of you have been in that position?  You’ve worked hard to extract your samples, you’ve dried your extracts to remove the last droplets of water from your organic solvent – only to add that water back in during your evaporation step!  There are fewer frustrating situations than losing a set of extracts in this manner.

If you’re like me, you work hard, follow all the precautionary step-by-step procedures to carefully produce extracts in a timely fashion.  It’s frustrating to think that a whole day’s work can be ruined with just a few milliliters of water.  When you see the water, you make an attempt to remove it and save your extracts, but there’s no guarantee that it’ll work.  Is there any way to avoid this?

The first time I found water in my extracts was when I was concentrating samples that had been extracted per EPA Method 625.1.  I loaded my acetone- and methylene chloride-containing extracts into evaporation tubes, loaded the tubes into my nitrogen bath concentrator, closed the lid and off I went.  The first extract concentrated to the final point and was translucent, but when methylene chloride was added to dilute the extract to 1 mL, it became cloudy.

The extract turned cloudy due to the presence of water, but I couldn’t figure out where the water had come from.

“I must have dripped some water from one of the other tubes into this one.  How clumsy of me!”

I moved on to the next extract.  It too had evaporated down to the final volume I was looking for and it too was translucent.  When I added methylene chloride to bring it to a 1 mL volume, it turned cloudy like the first extract.

I started to get suspicious.

I moved on to the third extract.  Can you guess what happened?

That’s right.  The same cloudy solution developed.

I knew something wasn’t right.  It couldn’t have been a coincidence that all three samples ended up with water-containing extracts.  It was time for an experiment to investigate.  I filled three evaporation tubes with the following:

  1. Methylene chloride – no other solvents, just DCM
  2. A mixture of acetone/methylene chloride
  3. An extract that mimics that from samples processed per EPA Method 625.1

When the evaporation was complete, I diluted each to 1 mL with methylene chloride.  The results were as follows:

  1. Methylene chloride extract – clear
  2. Acetone/methylene chloride extract – cloudy
  3. Method 625.1 extract – cloudy

This is where I had my breakthrough.  The only extract to remain clear was the one that didn’t have any acetone in it.

Acetone was the problem!

Why does the presence of acetone produce extracts with water in them?  Good question!

It turns out that when you use a water bath evaporator, the heated bath causes water molecules to evaporate from the surface of the bath.  At the same time, acetone is evaporating from the extracts in your evaporation tube.  Both solvents are polar and are miscible with each other so the evaporating acetone attracts the evaporating water molecules and you end up with water in your evaporation tubes – and solvents that are no longer dry.

Can this be prevented?

Of course it can!  And it’s a really simple fix.  A piece of aluminum foil will do the trick!

Lesson learned – if you’re evaporating extracts that contain acetone, cover your evaporation tubes with aluminum foil.  Don’t forget to poke a hole in the aluminum foil to let nitrogen from the nozzles get into the tube and evaporate the solvent.  Read this tech tip for a show and tell on using aluminum foil to cover your evaporation tubes.

From now on, your extracts will be safe and you won’t have to worry about losing an entire day’s worth of extractions!

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