**Appendix A: common quality parameters for the analytical methodologies**

**Linear range** Is the analyte concentration range where the response of the analytical methodology is linear and proportional to the amount of the target analyte.

**Limit of Detection** Is the amount of analyte giving a *significative different* signal from the blank sample. With significative differences, the statistical meaning is considered.

$$Y\_{\rm LoD} = Y\_{\rm blank} + \mathbf{3S} \tag{15}$$

where YLoD is the signal of the minimum amount of analyte, Yblank is the signal given by the blank sample and then, the LoD from the calibration curve can be calculated simply by

$$\mathbf{L}\mathbf{o}\mathbf{D} = \mathbf{3}\mathbf{S}/\mathbf{m} \tag{16}$$

being *m*, the sensitivity of the analytical methodology or the slope of the linear regression.

**Accuracy** Indicates how close to the *real value* is the results obtained with the analytical methodology. The evaluation of the method accuracy can be performed by using a certificate material, spiking a real sample, comparing the developed methodology with other analytical techniques, etc.

**Relative Standard Deviation** Gives a measurement of the precision of the methodology, especially useful when it has to be compared with other analytical methodologies available for the same analyte. It can be calculated by

$$\text{RSD} = \mathbf{100} \ast \mathbf{S} / \overline{\mathbf{x}} \tag{17}$$

where *x* is the mean value of *N* replicates.

*Fluoride*
