Calculation for PCR magnesium

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DrCl
DrCl's picture
Calculation for PCR magnesium

 Hi

My PCR kit has a buffer with 15mM MgCl2. I have an additional solution of MgCl2 at 25mM. Using this solution, I would like to increase the MgCl2 concentration of the PCR buffer to 50mM. What calculation do I need to perform to obtain this?

Thanks a lot!!

Sami Tuomivaara
Sami Tuomivaara's picture
Hi there,

Hi there,

You cannot make 50 mM buffer from two stocks of which concentrations are less than 50 mM. Only possibility is to use higher concentration buffer, for example mix equal volumes of 15 mM and 85 mM stocks, and you get 50 mM working solution. The higher the concentration of the other stock is, less you need to use it to bring the concentration of your final solution to 50 mM. The general equation to calculate the relationship between volume of your 15 mM PCR stock (V_pcr), volume of your higher concentration stock (V_stock) and the concentration of the high concentration stock (c_stock), final volume (V_final) and final concentration (c_final) is:

c_pcr * V_pcr + c_stock * V_stock = c_final * V_final

or

15 mM * V_pcr + c_stock * V_stock = 50 mM * V_final
15 mM * V_pcr + c_stock * V_stock = 50 mM * (V_pcr + V_stock)

From here you can solve for volume of the high concentration stock you need to bring the final concentration to 50 mM:

V_stock = 35*V_pcr / (c_stock - 50 mM)

In order to keep the total volume reasonably low, you should prepare the stock quite concentrated, but not so concentrated that your smallest pipette can't reach low volume needed.

Cheers,