Chemical compounds decompose over time during storage. The rate this takes place is specific to the compound. However, compounds labeled with radioisotopes typically decompose faster than their unlabeled counterparts due to radiolytic decomposition. The shelf-life of a radiochemical compound, the time during which a labeled compound may be used with confidence, is important to the user.
As a general guide, if stored under optimum conditions, H-3 and C-14 labeled compounds will typically decompose at an initial rate of 0.1 – 0.5% per month for the first 6 months. This rate may increase significantly with greater age or if stored in conditions other than those specified in the safety data sheet. Recommended storage conditions are normally included with each order.
| Isotope | Typical Observed Decomposition Rates |
|---|---|
| H-3 | 1-3% per month |
| C-14 | 1-3% per year |
| P-32 | 1-2% per week |
| S-35 | 2-3% per month |
| I-125 | 5% per month |
While every radiochemical is shipped with specific storage instructions, here are a few principle guides that will help minimize decomposition.
- Always consult the Safety Data Sheet that accompanies each radiochemical for the specific storage and handling recommendations for that product.
- Store at the temperature indicated on the Safety Data Sheet.
- Store at the lowest specific activity required for use, a compound at high specific activity will decompose faster due to radiolytic decomposition than at low specific activity.
- Store as solutions – this effectively disperses the labeled molecules, decreasing the effect of radiolytic decomposition.
- If repeated use is anticipated, avoid reopening of vials and warming/cooling cycles. Sub aliquot in a number of vials keeping those to be used later in the refrigerator or freezer until required.
- Store material in the dark and keep protected from the adverse effects of nearby chemicals.
- Do not freeze the solution unless advised to do soon the safety data sheet. If freezing the solution, it should be done rapidly, as a small volume.
- Add free radical scavengers or other stabilizers – when compatible with the use. Adding a free radical scavenger (e.g., 2 – 3% ethanol added to an aqueous solution) can lead to an increased shelf-life.
- Recheck the purity of your radiolabelled products at appropriate intervals. Always measure the purity of a radiolabelled product that has been stored for 6 months or more. Consult the product data sheet for the appropriate analytical method.
- Additional information on managing radiochemical decomposition is available at https://hs.shef.ac.uk/attachments/185.
Sources: Radiation Review – UW-Madison Safety Department 2/2000; Radiochemical Product Information, Komabiotech 2005