calculate original conc - (Nov/10/2014 )

I took 2.5uL of a sample I assumed is 80ng/ul and diluted into 17.5uL water for total of 20uL. So the dilution would be 10ng/ul. However upon doing a Picogreen of the dilution, my sample quantifies at 21ng/ul. Is the original concentration 168ng/ul and not 80ng/ul (I calculated 21/0.125=168, where 2.5/20=0.125)?
 

Why don't you measure the sample with original concentration? E.g. with a nanodrop, then you need only 2 uL and get an impression which value comes closer to reality.

And which of the values I'd trsut more depends how your 80ng/ul "assumption" comes from (measurement or really assumption?), how pure the sample is (only DNA or also RNA, nucleotides, other contaminants?) and finally possible pipetting errors (not sure if you need to do dilutions for the picogreen measurement).

Hello. Please let me know if my calculation is correct. If I could measure my original stock accurately I wouldn't have posted a question. Thanks

Your calculation is correct, but there is no way for us to know which of your measurements is better. We have no idea where the 80 ng/ul number came from, or its reliability. If someone gave it to you and told you that number, then I would use your measurements, which are at least ones you have done.

That's ok. Thank you for checking the calculation