All Principal Investigators actively using radioactive material must insure that routine wipe tests are conducted in their laboratories. Normally these wipe tests are conducted on a monthly basis at a minimum. However, if researchers use large amounts of radioactive material in a laboratory, it may be necessary to conduct more frequent wipe tests. See the Albert Einstein College of Medicine Radiation Safety Manual for frequency of wipe based on amount of radioactive material.
Demonstration (Video): Conducting a Wipe Test
- Background Sample (Blank): A clean cotton swab or filter paper used to determine the number of counts corresponding to a contamination free surface.
- Control Sample: A scintillation vial containing a known amount of radioactive material, typically less than 0.1 microCurie, and a clean cotton swab or filter paper.
- Disintegration per minute (dpm): the number transformation from the radioisotope to its progeny isotope in a one minute period. For example, P-32 decays to S-32 at a certain rate that can be described in dpm.
- Inactive Status: Principal Investigators who are not using radioactive material for an extended period may choose to place their license on inactive status. When the license is on inactive status, the Principal Investigator does not need to perform wipe tests in his/her laboratory.
A map of the lab should be created showing benches, refrigerators, freezers, sinks, fume hoods, and biosafety cabinets. Equipment used in radioactive research such as centrifuges, hybridization ovens and water baths should be drawn on the map as well. The map should indicate areas to be checked for contamination using the wipe test method. Use numbers to correspond to the various locations. The number of wipes to be conducted depends on the size of the laboratory and the number of laboratory benches used for radioactive research. For example, if two benches are used for radioactive research than those two benches should be monitored for contamination each month. Here are some typical locations that should be monitored for contamination using the wipe test method:
- Laboratory benches used for radioactive material research
- Floors in front of benches
- Equipment used with radioactive material
- Door handle to freezer and/or refrigerator used for storing radioactive material
- Sink used for disposal of radioactive material
- Cover to radioactive waste containers
- Fume Hoods used for radioactive material research
- Laboratory door handle
In addition, you must monitor equipment used in radioactive research, including:
- Hybridization ovens
- Water baths
- Scintillation counters and/or gamma counters
- Plexiglas shields
A cotton swab or filter paper may be used to wipe the surface of the target area. Wipe approximately 100 cm2 of the surface. Perform the wipe test on all designated areas on the map using a new swab/filter paper for each location. When analyzing the wipes, a background sample and control must be included. The background and control are used to determine if a particular wipe is contaminated and the amount of contamination.
Place the swabs/filter papers into scintillation vials and add the scintillation fluid. Cap the vials and record the number for that wipe on the cap. Place the vials into a scintillation rack and place the rack into the scintillation counter. Set the scintillation counter to detect the radioisotopes used in the laboratory and run the samples. The results of the analysis is typically printed out on paper. It displays the sample number, the time, the counts and the count rate in cpm.
As a rule-of-thumb if any of your sample results are 3 to 4 times background, it is considered contaminated. For example if your background sample is 10 cpm and you have a wipe from your lab bench that is 50 cpm, than the bench is contaminated. If you find that one of the locations is contaminated you must determine the amount of contamination on the wipe using the background and control results. To do this you may use the Calibration Work Sheet, inserting the numbers from the results of analysis in the liquid scintillation counter. Locate the surface that is contaminated and take the necessary steps to decontaminate the area in accordance with Section 20.0 of the Radiation Safety Manual titled "Emergency Response".
The results of wipe tests must be kept in the Radiation Safety Manual for a minimum of five years. The records must contain the laboratory map, the results from the scintillation counter and Calibration Work Sheets for contaminations.