Division 3.1.2.1 Recovery

If there is no CRM available, the recovery has to be determined by experiments using fortified blank matrix using, for example, the following scheme:

• select 18 aliquots of a blank material and fortify six aliquots at each of 1, 1,5 and 2 times the minimum required performance limit or 0,5, 1 and 1,5 times the permitted limit,

• analyse the samples and calculate the concentration present in each sample,

• using the equation below, calculate the recovery for each sample,

• calculate the mean recovery and CV from the six results at each level,

• % Recovery = 100 × measured content/fortification level.

This conventional method for the determination of recovery is a variant of the standard addition method described in 3.5, when:

• the sample is considered as a blank sample instead of a sample to be analysed,

• it is considered that yield ^f1 and recovery ^f2 are similar for the two test portions,

• the test samples have the same masses and the test portion extracts the same volumes,

• the amount of the calibration standard that is added to the second (spiked) test portion is noted x_ADD. (x_ADD = ρ_A.V_A),

• x1 is the measured value for the blank and x2 the measured value for the second (spiked) test portion,

• then, % Recovery = 100 (x₂ - x₁)/x_ADD.

When any of the above conditions is (or is supposed) not to be achieved, then the complete procedure for determination of the recovery by mean of the standard addition method as described in 3.5 has to be performed.