Many wine labs now have improved quality control systems in place and routinely analyse samples in duplicate and run the same control wine sample each day, plotting the results on control charts. These are great ways to monitor repeatability and reproducibility, two very important quality parameters that indicate how precise a method is. A third parameter of equal importance is trueness, or to put another way, ensuring the result obtained is close to the actual value. Your results might be precise, but if they are not close to the true value, then this is an indication of systematic bias in the method.
One of the best ways to determine trueness is to purchase certified reference materials (CRM’s). Unfortunately this is also the most expensive way to determine trueness, and suitable CRM’s that match your sample matrix and concentration range can be difficult to find.
An alternative way to monitor trueness is to perform routine spike analysis, which is adding a known amount of standard to one of your samples, analysing the spiked and not spiked sample, then calculating the percentage recovery. A practical example for doing this is outlined below:
Test: Malic acid determined by discrete analyser, calibration range 0 – 3g/L
Procedure: Analyse sample (let’s call this result y)
Add 1mL of the top calibration standard (3g/L) to 4mL of sample
(this is the spiked sample)
Analyse this spiked sample (let’s call this result z)
Calculation: % Recovery = z x 100
(0.8 x y) + (0.2 x 3)
Interpretation: For most tests, a recovery of between 95-105% is acceptable.
Depending on the uncertainty of the method, a recovery of between 90-110% may also be accepted.