[confusedcollegestudent]

The spike recovery equation is usually
% recovery = [(c_{spiked sample} - c_{unspiked sample})/c_added] x 100%
My professor wrote this equation in our lab manual about spike recoveries and provided the following equation:
% recovery = [(c_{spiked sample} - c_{unspiked sample})/Δc_theoretical] x 100%
And said that Δc_theoretical is the theoretical change in concentration calculated based on the known concentration and volume of the spiking solution and the known volume of the sample.
Does anyone know what this means? I thought that c_added was just the concentration added. Do I need to perform some sort of dilution calculation here? I'm very confused, especially because this form of the equation has never appeared in the textbook or any lectures.