I have a 2000ppm benzoic acid solution.I want to prepare a 20ppm benzoic acid solution.Is it better to

Question

1) do serial dilutions i.e. from 2000 to 200 to 20ppm
or
2) prepare 20ppm standard fresh when required( take 10ul of 2000ppm stock and top up to 1ml in a LC vial with 990ul 70% ethanol)

Answer 1

You want to dilute it 100 times, which means one part concentrated, 99 part dilutant.

I would do fresh, I always found the chance of contimination doubled with each dilution. Maybe I never had clean bottles.

I will see if I can get other views on this.

Answer 2

I have had the opposite experience from Bob Pursley. My problem has been that I never trusted the 10 ul pipets. Too large a relative error in 10 ul. Therefore, I always went the serial dilute route.