The calculation is as follows:
15µl of 20 mM what is equal 20 mmol/l = 0,02 µmol/µl = 0,3 µmol. This amount is dissolved in 15 µl + 285µl =300 µl. This gives a new concentration of 0,3 µmol/300µl = 0,001 µmol/µl = 1 µmol/ml = 1 mmol/l = 1 mM = 1000 µM It is ok!
The Acetonitrile is 20% . The dilution from 285 µl to 300 µl doesnt change much . It is 19%. DMSO is correct. 15 µl in 300 µl is 5%.
The second gives with 5 µl only 0.1 µmol. Dissolved in 5 µl +120 µl = 125 µl. The result is 0,0008 µmol/µl = 800 µmol/l = 800 µM It is ok.
Acetonitrile 120/125 *20% = 19,2% . DMSO gives 4,16%
The third would give, if you take 100 µl of the 1000 µM and dilute with 100µl solvents a concentration of the half what means 500 µM not 600 µM.
The Acetonitrile would be 19% *20 % divided by 2 gives 19,5% and the DMSO would stay at 5%. Both solution contain 5%.
ChemBuddy | 
Chem Reddit | 
Topic: Trying to keep the % DMSO constant in a calibration curve (Read 1452 times)