# Homework Help: How to calculate

1. Jul 23, 2008

### raida

I have thymocin that allready diluted and written on it 25ug/ml and under that written
5ng/200ul. for my experiment I need 100ng. Can you please tell me how to calculate?
Thanks

2. Jul 23, 2008

### mgb_phys

25ug/ml and 5ng/200ul aren't the same concentration.
Since 200ul is 1/5 of a ml the second number is equal to 25ng/ml so one of the labels is wrong - perhaps the first one should be 25ug/l ?

Assuming the 5ng/200ul is correct, then if you need 100ngyou need 4x 200ul, ie 800ul.

3. Jul 23, 2008

### raida

I think the second number should be 5ug/200ml.
Thanks

4. Jul 25, 2008

### chemisttree

Let's assume that the correct concentration is 25ug/mL and you need 100 ng for your experiment. We know that 1 nanogram = 0.001 ug, so 100 ng would be how many ug? Call that result 'X ug'

Once you have that, you need to determine what pipette size you have available. Most labs have at least a 100 uL (0.1 mL) and you probably have more selection than that. If you want to express the concentration of your analyte in terms of ug/mL, you should have a solution of X ug/0.100 mL or 10X/mL if you want to use a 100 uL pipette.

Do the math to this point and post your results if you need any further help.

5. Jul 26, 2008

### raida

Thanks for your reply. Please check if I'm correct. I have 25ug/ml and I need 100ng/ml in 600ul solution.
25ug/ml x v1= 0.1ug/ml x 600ul
v1= 2.4ul
I take 2.4ul from the stock solution and add 598ul of PBS. Is that the correct way?
Thanks

6. Jul 27, 2008

### Staff: Mentor

I just skimmed and numbers look OK, but you will be not able to take 598 uL of anything and mix it with 2.4 uL of something else. You have to prepare at least several mililiters of the solution. And 598 uL - while generally correct - doesn't make sense in lab practice, if anything, that's just 600 uL. You can't measure such volumes with such accuracy.

7. Jul 28, 2008

### raida

Thank you very much.

8. Jul 31, 2008

### chemisttree

If I were doing this, I would dilute 2 mL of the stock solution to 10 mL using a 2 mL volumetric pipette, a 10 mL volumetric flask and PBS as the diluent. This would give you 10 mL of 5 ug/mL. Label this new sample "Dilution A - 5 ug/mL". You would then dilute 2 mL of Dilution A to 10 mL using another clean 2 mL volumetric pipette, a 10 mL volumetric flask and PBS as diluent. This is now 1 ug/mL... label it "Dilution B - 1 ug/mL".

Pipette out 100 uL of Dilution B and you have 0.1 ug (100 ng) of analyte.

9. Jul 31, 2008

### raida

Thank you very much for your answer. Can you please explain to me this technique, the way you did the dilution,for my next experiments.
Thanks again

10. Aug 1, 2008

### chemisttree

In your example the amount of analyte is 100 ng. You can deliver this from a diluted solution that you have complete control over. I suggested that you might want to use a common micropipette found in the lab... 100 uL. To use this pipette you need a solution that will contain 100 ng per 100 uL or a 1.0 ug/mL solution.

Your problem becomes,"How do I go from 25 ug/mL to 1 ug/mL?" You will notice that to go from 25 to 1 you can either divide by 25 or you can divide by 5 and then divide by 5 again. So you could either dilute 1 mL of stock solution to 25 mL using a 1.0 mL volumetric pipette and a 25 mL volumetric flask or you could do the dilution I suggested.