How can spike recovery be over 100%?
A spike recovery value of >100% does not mean that you added too much or the recovery process is too good. All it means is that the analytical method which is being followed "in letter and spirit" gives a positive error. There may be a bias in the method or the precision of the method is poor. We cannot say just by looking at one number. At this stage it is assumed that there was no operator error; only the analytical method is to be blamed. A single number like 115% in analytical chemistry has no meaning. Whoever called it a good recovery process needs to revisit that statement.

Whenever we develop a new method, recovery is done is several matrices and even different types of samples. As a result one gets a range of recovery values in different matrices or for different samples. It is better to say that a method shows gives a recovery between 95% to 115%. This statement is far more meaningful than just stating that a recovery of 115% is good. Certainly, that method must be pretty good.