A student determines the amount of tetrachlorethylene in a contaminated well water using a purge and trap technique. The student takes 150.0 mL of the well water, spikes the well water with 100 microlitres of 50 ng carbon tetrachloride/mL and purges the sample with helium and traps the volatile compounds on a Tenax stationary phase. The phase is heated and the compounds transferred to a gas chromatograph equipped with an electron capture detector. The gas chromatogram gave peak areas of 120 for tetrachloroethylene and 78 for carbon tetrachloride. The student then took 150.0 mL of ultrapure water and spike it with 100 microlitres of 50 ng/mL tetrachloroethylene and 100 ng carbontetrachloride /mL, performed the purge and trap experiment on this sample and obtained areas of 72 for tetrachloroethylene and 148 for carbontetrachloride.
What is the concentration of tetrachloroethylene in the well water?