was used to prepare the 50, 100, 200, 500, and 1000 ng samples by. pipetting 1, 2, 4, 10, and 20 μL, respectively into HS crimp-top vials. The 2000 ng/μL stock solution was used to prepare the 2000, 4000, 8000, and 10 000 ng samples by pipetting 1, 2, 4, and 5 μL, respec-. tively into HS crimp-top vials.