Hello Jamie-Lee Fisher

The volume of TNF-α solution should be such that the final concentration in the well must be 10ng/ml. For instance, make a stock solution of TNF-α of a higher concentration and add the desired volume of stock to the well already containing fixed amount of growth media so that finally the concentration of TNF-α in the well is 10ng/ml.

Alternatively, you may make a working solution namely, growth media containing 10ng/ml TNF-α and add to the well directly which does not contain any previously added solution.

Best.