What is radiocarbon contamination?
Researchers utilize radiocarbon enriched reagents and substrates for various field and laboratory tracer experiments. Lab spaces, equipment and samples become contaminated with elevated levels of radiocarbon through contact with these tracer compounds. Even if it has been decades since radiocarbon tracer experiments were conducted in your lab or building, the tracer will still be potent since the half life of radiocarbon is 5,730 years.
Even if radiocarbon tracer has never been used in your lab, your lab and your samples may become contaminated through shared use of space or shared use of equipment. Most cases of contamination happen because the samples passed through a contaminated “common use” area (i.e. a freezer, refrigerator, centrifuge, balance, ball mill, etc.). If you are planning on utilizing natural abundance radiocarbon measurements in your research, exercise extreme caution when processing and storing your samples.
Why should I "swipe" to test for contamination?
If you submit contaminated samples to a graphite lab, you contaminate the graphite lab and risk spreading contamination to other submitters' samples that are being processed at the same time as yours. This can cause thousands of dollars in damages, as many pieces of equipment may not be cleanable and will have to be replaced. Through contamination of other's samples, you could inadvertently ruin a student's thesis work, or an essential component of a project. In rare cases where the level of contamination is extreme, you could contaminate and/or damage parts of the accelerator mass spectrometer.
It is possible for your samples to be only slightly contaminated. In these cases your samples will appear "younger" than they should be, but will still be within the range of natural abundance. This leads to improper conclusions regarding your data, and produces bad science.
How do I "swipe" to test for contamination?
Individual graphite labs each have their own protocols for producing swipes, but the general approach tends to be the same. Clean glass fiber filters are wetted with alcohol and swiped or rubbed on the surface of the item in question (e.g. centrifuge, countertop, freezer, door handle, etc.). The fiber filters are then graphitized after addition of dead or near-dead carrier carbon. Carrier carbon is necessary since there is not enough carbon by mass on the swipe to allow for an AMS measurement.
A "swipe kit" will be sent to you from a graphite lab. The swipe kit will include clean gloves, filters and vials for each swipe. Submitters provide their own isopropyl or ethanol.
Many labs utilize the swipe protocol developed by the WM Keck Carbon Cycle Accelerator Mass Spectrometry at UC Irvine attached below.
What if my lab is contaminated?
If your lab is severely contaminated, you will need to find a different clean space to conduct sample processing for projects requiring natural abundance. If your lab shows moderate levels of contamination, you may be able to clean your lab. Consult Zermeño et al., 2004 for guidance on cleaning up tracer contamination.