Principal Component Analysis Loadings plot shows good reproducibility of all the prepared electrodes, as repetitions for each electrode type are grouped in specific places of the plot (Fig3). It also shows that all of the electrode types provide unique information about the samples. Reproducibility of the syringe sensors was also confirmed through the correlation matrix, which indicated perfect correlation for the two K + DB sensors (1.00) and two NH4 + (1.00), three K + Val sensors (0.99-1.00), two Cat sensors (0.99), and two An sensors (0.99).