The presence of small amounts of C-14 in practically all carbon-bearing earth materials rises an additional concern for the radiocarbon dating method. In dating experiments, the standard practice is to count C-14 atoms in the sample and also in a "procedural blank" or calibration standard. Many laboratories use Precambrian graphite, purified natural gas, or optical grade calcite for the counting standard. This "blank" is thought to hold no C-14 whatsoever, so any counts recorded with it in place are assumed to come from the surrounding "background." These counts are subtracted from the count for the actual sample. However, we now find that the calibration blank itself may not be carbon-14-free. The counts obtained using this blank are not all coming from the background environment after all. Therefore, these counts should not be subtracted out during the sample dating procedure. The result of this incorrect counting is that many samples in past years have been carbon-14 dated older than they actually are.

/>Taken from, Thousands... Not Billions: Challenging an Icon of Evolution, Questioning the Age of the Earth, by Dr. Don DeYoung