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.