A recovery control in analytical chemistry is a sample spiked with a known amount of analyte. It is used to assess the recovery of the method, which is done by comparing how much of the analyte is detected by the method to the known concentration at the start. This way the amount of analyte lost during the sample preparation can be determined. It is similar to a standard in that the analyte is added in a known concentration, but in contrast to a standard, the recovery control goes through the whole sample preparation protocol. It is also in the same matrix as actual samples, e.g. milk, which is not always the case with standards.
Several recovery controls would often be included, so you’re also able to estimate the variation of the method from start to finish. To estimate the recovery, you would first calculate the concentration based on the measured response, which in LC-MS/MS would be the peak area. The mean of the found concentrations divided by the known spiked amount will yield the recovery.
In some methods, you need to take dilution or concentration factors in the sample preparation into account as well.