In this video, Dr Fiona Petchey, from the Waikato Radiocarbon Dating Unit based at the University of Waikato, explains the use of standards in the carbon-14 dating process. As well as the sample to be dated, two other standard samples are also analysed – one a modern sample and the other older than 60,000 years with virtually no carbon-14 activity remaining. By doing this, a check can be made on the accuracy of their testing method.
Point of interest
Why can carbon-14 dating be used to track the age of modern wines but not wines 100-200 years old?
DR FIONA PETCHEY
We use several different sorts of standards. The two most common are a modern standard, which is called ANU Sucrose, and we know exactly the age of that standard. The other standard we use is a background standard that is beyond the limits of radio carbon, so beyond 60,000 years old, and those two standards are always measured at the same time as an unknown or commercial sample that we don’t know the age of. That enables us to basically figure out how much C-14 is in the sample, and it also enables us to monitor our own fluctuating levels of C-14 contamination, or what we call ‘memory effect’ in the lines in the lab, and develop a database of how those ages vary.
The precision of a date is the standard error associated with a date. When we report a radio carbon age, we report a date that says something like 400 years plus or minus 40 years. So the plus or minus 40 is our precision, and that is determined from our background standards and our replica analyses. And if we can’t reproduce our standards and get the same value every time, it obviously is very inaccurate.
So when people ask us, “How accurate are you?”, well, we do repeated measurements on multiple samples, and we can prove that we are reproducible at the level of precision that we give, therefore we are accurate at the level of precision that we put out on our dates.