Problem - Dissolving drugs - (Sep/27/2016 )

Greetings,

I am so sorry for bothering everyone with another question in less than 2 months time.

Yesterday, I dissolved a drug (came in 50 ug) to 100 000 nM concentration.

After calculating, I was supposed to add around 1.2 ml of DMSO to the tube. So I aliquoted 1.2 ml using a 5 ml syringe and poked into the tube from the cap. I think my hands were shaking and not realizing that my needle was poked in the wrong area (side of the cap instead), - until like a few seconds after I found it was hard to dislodge the DMSO liquid (there was a pressure acting against the syringe). As far as I concern, the liquid did not come out from the syringe since it was not an open area where the needle was wrongly poked, if u get what i m trying to say.

After getting everything right, I dissolved the drug and mixed them. When i wanted to transfer the stock into different tubes, I realised the stock solution was less than 1.2 ml (like 1.0 ml instead).

I am really confused right now whether should I further add another 200 ul to compensate the lost liquid. Or was the liquid spilled somewhere when I was mixing the DMSO with drugs. I couldnt remember much. 

What is your advice, to check on this? Pls and thank you.

I will be more careful after this. 

Thank you in advance!

The only true way to test would be to do a chemical assay for the drug or something like HPLC where you can determine the concentration.

If, as you say, you were poking the needle into the cap without penetrating, then no liquid should have come out of the syringe until you removed the needle from the cap, however, if you didn't return the syringe to the 1.2 ml position, then you might have had positive pressure on the needle and lost some of the DMSO that way. Another consideration is that you didn't account for the volume of the needle when withdrawing the DMSO, and just sucked it up with the syringe alone... some DMSO will be retained in the needle, which might account for your loss. When aliquoting you usually end up with less than what you thought as some of the solution will be retained on the sides of the bottle and in the cap, you could try spinning down the bottle (gently... it's likely to be glass) and see what you get.

A fourth consideration is that the solution volume might be less than the sum of the solutes, but this is not very likely as you usually only see this when mixing two liquids (e.g. methanol and water), and is unlikely to account for >20% of the total volume.

Thank you for the reply! @bob1

The problem is that I could not spin down the glass bottle, otherwise I would have spinned the bottle down (when the drug was still in powder form) and opened the cap to dissolve the drug with using a micropipettor instead - which im more familiar with. I used a syringe to ensure the powder did not "fly off" as soon as I open the cap without spinning down - since it only came in 50 ug. 

So, in order for me to determine the exact concentration of the drug right now would be running HPLC with the known concentrations of the same drug? Like I need purchase another bottle and dissolve it correctly?  That would be a wastage right, as one bottle is actually sufficient for me to complete the whole project. 

But thanks anyway! 

I was meaning spin down the bottle now- there'll be some fluid retained in the bottle from surface tension.  BTW, you did the correct procedure by injecting the DMSO.

Thank you very much for the advice!