*3.4.4 Accuracy assessed as percent recovery*

The accuracy of the method is determined in terms of percent recovery by adding in triplicate, known amounts (4, 8 and 12 μL) of a 1000 μg/L standard of cadmium to a thermoformed or flexible film sample prior to the digestion process. Following this, the respective readings (4 absorbance readings for cadmium) are taken using the atomic absorption equipment of the doped sample and of the sample with the standard added.

To calculate the percentage recovery, the cadmium concentrations in the samples are determined previously, as indicated in Eq. (2).

$$\text{Recovery} \left( \% \right) = \frac{\text{Concentration with stock added} - \text{Concentration with no stock added}}{\text{Concentration with stock added}} \text{\*100} \tag{2}$$

The percent recovery of Cd ranged between 96.23% and 97.31% for the MPER 183 thermoformed material, which makes it possible to conclude that the extraction method used is suitable, and that the determinations are therefore reliable [54, 70].

A statistical t-student test should be applied to determine whether there are significant differences between the value obtained from recovery and the 100% level. The experimental t-values are compared with the tabulated values (t(0.05, 8) = 1.860) for 8 degrees of freedom in our case. The results show that for all the recoveries of the thermoformed samples, there are significant differences between the average values and 100% since t calculated > t tabulated (11.489> 1.860) [54, 70].
