DMSO stock solutions

[shemesh199]

Most of the insoluble pharmaceutical compounds are dissolved in DMSO as stock solutions. Periodically these stock solutions(EX: 100mM of Lipophilic Drug) which are prepared in 100% DMSO must be quantitatively assayed. One question which comes to mind is what is the best way to shoot these samples. I have found in the literature to perform a large dilution with mobile phase and shoot (EX: 1X100 Dilution) Are there any ideas or people with LC-MS-MS experience in the testing of libraries from DMSO stocks? What is the best way to proceed.

Thanks

[gxch43]

We usually dilute the DMSO stock solution using ACN:Water(25%:75%), The initial concentration is about 5mg/mL, the final concentration is about 500ng/mL, every compound is fine.

[shemesh199]

Yep, that makes sense. Let me ask an additional inquiry. Say you are taking a stock 50mM of Drug X. You perform a dilution and run the assay. You come out with a concentration (EX ng/mL), after you multiply by the dilution factor this should directly correlate with the prepared stock. Seems that I am having trouble, I did calculate the weighed out concentration to be around 10.86mg dissolved in 500uL of DMSO to equal ~50mM of my compound. However, after performing the dilute and shoot my calculated concentration only arrived to ~28mM. The compound concentration was around 21.72mg/mL dissolved in DMSO, so I performed a 1X100 Dilution in DMSO, then I further performed a 1X200 dilution with mobile phase. Something is not adding up here. Basically I was expecting around a 1,000ng/mL value, I did arrive only at ~600ng/mL or so. Could that be the way the drug was weighed out, or dissolved? Something really fishy going on. Any suggestions, advice, tips or tricks?