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Dilution help

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Mbradley62:
Hello! I have a TNF-a stimulation that has a stock concentration of 10ug/mL. I want to have a final concentration in my 100uL well of 10ng. I have no idea how to solve for the amount of PBS I need to add to the stock so that after I add to the well it is at the 10ng concentration. Any one can help explain?

Hunter2:
Convert  10 μg/ ml to ng/μl to have the same unit like 10 ng/100 μl

How much is it.

Mbradley62:
1 want to add between 1-8ul to the well

Hunter2:
10 μg/ml = 10 ng/ μl

To get 10 ng/100 μl  you have to add 1 μl  of the stock to 99 μl of the solvent.

Babcock_Hall:
When I teach dilutions, I suggest to students that they draw a picture of what is going on.  With respect to your example, I suggest starting with the idea that the mass of material in the pipet tip (1) is equal to the mass of material in the well (2).  Putting this idea into a formula:  Mass1 = Mass2.   The mass of TNFα is equal to the concentration of a solution times the volume of the same solution.  Try writing an equation that expresses this idea.  Making a picture minimizes plugging incorrect numerical values into a formula.

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