*3.2. Device Measurements Validation*

Nitrate concentrations in wastewater obtained from the standardized experimental method and automatically through the device are shown in Figure 11a. In spite of the observed variations between nitrate concentrations detected by the sensor and those analyzed by applying the experimental method, from a statistical level, the test performed indicates that there are no significant differences between the two methods (*p* = 0.432). In this sense, note that the average concentration in the eight days of sampling was approximately 4.43 mg/L and 4.58 mg/L for the sensor and the experimental method, respectively.

**Figure 11.** Concentration in wastewater obtained from standardized experimental method (blue sky) and automatically from the enhanced portable sensor (blue navy).

A similar behaviour is observed for the case of nitrite. In Figure 11b, the concentrations of this parameter obtained from the sensor and those analyzed by applying the experimental method in the eight days of sampling are shown. From a statistical level, there are also no significant differences between the two methods (*p* = 0.395). The average concentration is approximately 2.90 mg/L and 3.09 mg/L for the sensor and the experimental method, respectively.

The study of the analytical validation parameters of both methods (LD (mg/L), LQ (mg/L), Range (mg/L), Linearity (determination coefficient (R2)), Repeatability (relative standard deviation (RSD%) of peak area and retention time), and Accuracy (%)) shows that the results achieved in the sensor validation [14] are very similar to those obtained from the standardized experimental method (Table 3). In this context, Figure 11 shows that the differences observed between the sensor data and the experimental data (0.2–2.8 mg/L and 0.2–2.2 mg/L for nitrate and nitrite, respectively) are within what is expected, taking into account the working ranges of each of the parameters for each of the methods of analysis. Therefore, the concentrations analyzed by the device are valid and demonstrate the potential of the system for portable analysis under real conditions, as the validation has been conducted in a wastewater treatment plant effluent, with the influence that the matrix effect can have on this type of waters.


**Table 3.** Analytical validation parameters for nitrate and nitrite obtained of device and standardized experimental method.
