Number 43 August, 2001

When Good Radiochemicals Go Bad

One reason we try to recover very old inventory forms for H-3, C-14, and other long-lived radionuclides is to satisfy documentation requirements that say we have to keep track of who uses what, how much, and when. Another reason is that good radionuclides go bad. We know that you are interested in using only the highest-quality reagents in your research. What you may not know is that the tritium or carbon-14 sitting on the shelf may be falling apart. All radiochemicals deteriorate over time, at rates that may surprise you.

The 2001 Amersham Pharmacia Biotech catalogue (p. 268) has a good discussion of storage and stability of radiochemicals. Highlights include:

• decomposition is accelerated by free radicals formed as a result of the energy released in radioactive decay,
• compounds of high molar specific activity decompose faster,
• even slight deviations from optimal storage conditions may accelerate decomposition,
• carbohydrates and amino acids are excellent substrates for microbial growth, leading to contamination,
• slow freezing can accelerate the rate of self-radiolysis in aqueous solutions of tritiated compounds, and
• exposure to bright light can increase decomposition.

A table of observed decomposition rates of chemicals labeled with selected radionuclides under optimum storage conditions may make you think twice about using that old vial of tritium sitting on the shelf.

In summary, very old stocks of radiolabeled chemicals (and carrier-free materials, too) are liabilities because they are poor research tools. See vendor catalogues and product technical information sheets for more information about storage and stability of radiolabeled chemicals, and let us pick up your old stocks and inventory forms. If you must maintain old materials be sure you have inventory forms that match them.

Meanwhile...

Heisenberg is out for a drive when he's stopped by a traffic cop. The cop says "Do you know how fast you were going?" Heisenberg says "No, but I know where I am."