Working practices when using radioactive isotopes

By law, undergraduate work with radioactive isotopes must be very closely supervised. In practical classes, the protocols will be clearly outlined, but in project work you may have the opportunity to plan and carry out your own experiments, albeit under supervision. Some of the factors that you should take into account, based on the assumption that your department and laboratory are registered for radioisotope use, are discussed below:

• Must you use radioactivity? If not, it may be a legal requirement that you use the alternative method.
• Have you registered for radioactive work? Normal practice is for all users to register with a local Radiation Protection Supervisor. Details of the project may have to be approved by the appropriate administrator(s). You may have to have a short medical examination before you can start work.
• What labelled compound will you use? Radioactive isotopes must be ordered well in advance through your department's Radiation Protection Supervisor. Aspects that need to be considered include:
  1. The radionuclide. With many organic compounds this will be confined to ³H and ¹⁴C (but see Table 35.2). The risk of a significant 'isotope effect' may influence this decision (see above).
  2. The labelling position. This may be a crucial part of a metabolic study. Specifically labelled compounds are normally more expensive than those that are uniformly ('generally') labelled.
  3. The specific activity. The upper limit for this is defined by the isotope's half-life, but below this the higher the specific activity, the more expensive the compound.
• Are suitable facilities available? You will need a suitable work area, preferably out of the way of general lab traffic and within a fume cupboard for those cases where volatile radioactive substances are used or may be produced.

Each new experiment should be planned carefully and experimental protocols laid down in advance so you work as safely as possible and do not waste expensive radioactively labelled compounds. In conjunction with your supervisor, decide whether your method of application will introduce enough radioactivity into the system, how you will account for any loss of radioactivity during recovery of the isotope and whether there will be enough activity to count at the end. You should be able to predict approximately the amount of radioactivity in your samples, based on the specific activity of the isotope used, the expected rate of uptake/exchange and the amount of sample to be counted. Use the isotope's specific activity to estimate whether the nonradioactive ('cold') compound introduced with the radio labelled ('hot') compound may lead to excessive concentrations being administered. Advice for handling data is given in Box 35.1.

Safety and procedural aspects
Make sure the bench surface is one that can be easily decontaminated by washing (e.g. Formica^®) and always use a disposable surfacing material such as Benchkote^®. It is good practice to carry out as many operations as possible within a Benchkote^®-lined plastic tray so that any spillages are contained. You will need a lab coat to be used exclusively for work with radioactivity, safety spectacles and a supply of thin latex or vinyl disposable gloves.

Fig. 35.3 Tape showing the international symbol for radioactivity.

Suitable vessels for liquid waste disposal will be required and special plastic bags for solids - make sure you know beforehand the disposal procedures for liquid and solid wastes. Wash your hands after handling a vessel containing a radioactive solution and again before removing your gloves. Gloves should be placed in the appropriate disposal bag as soon as your experimental procedures are complete.

It is important to comply with the following guidelines:

• Read and obey the local rules for safe usage of radiochemicals.
• Maximize the distance between you and the source as much as possible.
• Minimize the duration of exposure.
• Wear protective clothing (properly fastened lab coat, safety glasses, gloves) at all times.
• Use appropriate shielding at all times (Table 35.1).
• Monitor your working area frequently for contamination.
• Mark all glassware, trays, bench work areas, etc., with tape incorporating the international symbol for radioactivity (Fig. 35.3).
• Keep adequate records of what you have done with a radioisotope - the stock remaining and that disposed of in waste form must agree.
• Store radiolabelled compounds appropriately and return them to storage areas immediately after use.
• Dispose of waste promptly and with due regard for local rules.
• Make the necessary reports about waste disposal etc. to your departmental Radiation Protection Supervisor.
• Clear up after you have finished each experiment.
• Wash thoroughly after using radioactivity.
• Monitor the work area and your body when finished.