## 3 Results and Discussion  
### Validation of Multi-Elemental ICP-OES Analysis of Donkey Milk  
The elemental analysis of donkey milk samples was proven according to the EURACHEM guidelines [37]. Table 1 summarizes the figures of merit related to this methodology.  
For every investigated element, linearity and sensitivity were obtained using analytical calibration curves as the Determination Coefficient (R\({}^{2}\)), and Limit of Detection (LOD) and Quantification (LOQ), respectively. The observed R\({}^{2}\) values are greater than 0.9995, except for Fe (R\({}^{2}\) = 0.9983), indicating a good linearity of the relationship between the analytical response and the concentration for all the elements in the working range investigated. LOD and LOQ values were both determined according to the Background Equivalent Concentration 0 BEC0 concept. The relations LOD = 3-RSDblank-BEC/100 and LOQ = 10-RSDblank-BEC/100 were used, where RSDblank is the Relative Standard Deviation (n = 10) of the blank and BEC is equivalent to the concentration producing twice the intensity of the background. The observed LOQ values (from 0.06 to 13.78 mg/L) are largely below the mean native levels of each target element in donkey milk samples (see below), except Fe, which in some samples presents a content comparable to the corresponding LOQ value. The sensitivity is generally sufficient to ensure a reliable determination of the detected elements in the donkey milk samples. Trueness was evaluated by a spike recovery analysis. In order to assess the mean recoveries (R (%)) of the target elements, six authentic samples and six fortified ones were examined (Table 1). Relative Standard Deviation (RSD (%)), which was calculated using data from six procedural replicates, was used to measure the precision of the ICP-OES method. Comparison of the two parameters mentioned above with the permitted values listed in the specialized literature [38; 39] proves that the seven investigated elements are assessable with reasonable accuracy and precision. In fact, as depicted in Table 1, the macro-elements Ca, K, P, Mg, and Na show R (%) values from 99 to 105% which fall within the recovery benchmark ranges of 90\(-\)105% or 97-107% for 100 ppm and 1000 ppm concentration levels, respectively. On the other hand, the RSD% threshold values according to the AOAC and Horwitz criteria are, respectively, 5.3 and 8% for 100 ppm concentration levels and 5.7 and 3.7% for 1000 ppm. It follows that precision is good for K, Mg, and Na, and acceptable for Ca and P. Both recoveries of Fe (95%) and Zn (91%), whose expected concentrations in the real samples are close to 100 ppb and 1 ppm, respectively, fall within the acceptable range (80-110%), as well as RSD% of Zn (4%) which is largely below the AOAC threshold and Horwitz threshold values (11 and 16%, respectively). It must be noted that a greater RSD% value (19%) is observed in the analysis  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Element** & **\(\lambda\) (nm)** & **R\({}^{2}\)** & **LOD (mg/L)** & **LOQ (mg/L)** & **R (\%)** & **RSD (\%)** \\ \hline Ca & 315.887 & 0.9999 & 1.93 & 6.43 & 99 & 6 \\ K & 766.491 & 0.9998 & 4.13 & 13.78 & 101 & 2 \\ Mg & 280.271 & 0.9997 & 1.27 & 4.24 & 100 & 2 \\ P & 213.618 & 0.9999 & 2.59 & 8.63 & 100 & 5 \\ Na & 588.995 & 0.9999 & 0.82 & 2.73 & 105 & 1 \\ Fe & 259.940 & 0.9983 & 0.04 & 0.12 & 95 & 19 \\ Zn & 206.200 & 0.9997 & 0.02 & 0.06 & 91 & 4 \\ \hline \end{tabular}
\end{table}
Table 1: Detection wavelength (\(\lambda\)) of the analyzed elements, determination coefficient (R\({}^{2}\)) of calibration curves and limit of detection (LOD) and quantification (LOQ), mean recovery (R) observed in the analysis of the enriched samples (n = 6), and relative standard deviation (RSD).  
of Fe, although this value is below the [PERSON] reference value (22.6%) and only slightly higher than the AOAC threshold (15%).