CAUTION: Don’t mix concentrated nitric acid with organic solvents!

Published in Protocols & Methods

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Radiolabelling with copper-64 (or any other metal cation radioisotope) is done by attaching a metal chelating group to the probe of interest. In 2006, we worked with Thaddeus J Wadas & Carolyn J Anderson to publish a protocol for radiolabelling peptides with copper-64 which included a procedure for making sure that the reaction tubes and pippette tips used for the labelling were free from any other metals that might compete with copper-64 for coordination by the chelator (Box 1 in the protocol). This procedure involved washing the equipment with nitric acid, followed by rinses with ethanol followed by diethyl ether.

This is actually potentially hazardous, and the authors would like to make the following precautionary statement:

CAUTION:  When removing trace metal contaminants from pipette tips or reaction vials with 1:1 concentrated nitric acid:ddH2O, make sure no organic solvents like ethanol or diethyl ether are inadvertently mixed with the nitric acid waste.  An explosion occurred when this happened at a lab using this procedure.  Fortunately, nobody was injured.

In fact, it is possible (and advisable) to not use organic solvents for this process at all. The text for Box 1 should instead read:


Working on the tracer level requires that the reagents and vessels used be as free as possible of trace metals. To achieve this, pipette tips, reaction tubes and caps can be acid-washed by following the steps below.
Alternatively, trace metal free reaction tubes and pipette tips can be

Removing trace metal contaminants from reaction tubes, caps and tips
1. Soak the tubes and caps or tips in a 1:1 mixture of concentrated nitric
acid and ddH2O (greater than 18 MO resistivity) for several hours with
periodic mixing, and then drain.
2. Rinse the tubes with ddH2O, and drain.

Removing trace metal contaminants from reaction buffers and other
1. Prepare the solution(s) or buffer(s) to be used.
2. Add Chelex resin (10 g l-1) to these solutions.
3. Stir for several hours or overnight at room temperature.
4. Filter through a Corning 1-liter filter system (pore size 0.2 mm).


Related explosions:

It will probably not surprise you to know that this is not the first time that such things have happened with nitric acid and oxidisable organics. Here are some links to related anecdotes:

Safety Chat: Nitric Acid Waste
(Lawrence Berkeley National Laboratory, 2009)

Explosion at U. Maryland: Another Nitric Acid Oopsie
(University of Maryland, 2011)

The flaming apron that sparked the invention of gun cotton and the motion picture industry
(a kitchen in Switzerland, 1845)


Note added on 17 October 2013:

A corrigendum for this protocol has now been published (11 October 2013). Box 1 has been corrected in the pdf and the online version of the protocol.

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