Study for developing an electronic tongue to discriminate three different classes of waters, by using common sensors and the principal component analysis

Figures & data

Figure 1. Representation of four used probes.

Table 1. Main analytical features of four utilised probes.

Sensor type	Measured parameter	Range	Precision	Sensitivity	Response time	Life time
Digital thermometer	Temperature (°C)	(−50 to +150)°C	±0.3°C	0.1°C (between −20 and +90°C)	10 s	1 year
Combined pH electrode	pH (pH units)	(0–12) pH units	±0.05 pH units	0.01 pH units (at 25°C)	20 s	6 months
Gas diffusion amperometric O2 probe	O2 (ppm O2)	(0–20) ppm O2	(±0.02) ppm O2	0.005 ppm O2 (between +15 and +40°C)	5 min	6 months
Enzymatic DMFC probe	Alcoholconcentration (M)	(5 × 10^−4 to 6 ×  10^−1) M	(±0.005) M	2 ×  10^−4 M (at room temperature)	20 min	About 2 weeks (if enzymatic); or >3 months (if not enzymatic)

Table 2. All experimental values of four measured different parameters in all the analysed natural water samples (each value is the mean of three determinations).

Rain, river, groundwater sample number	pH (pH units) SD % ≤ 2.3	Dissolved O2 (ppm) SD % ≤ 2.9	T (°C) SD % ≤ 1.7	Total alcohol concentration found value, using fuel cell (mg/L) SD % ≤ 2.8
1 (rain)	5.4	8.89	20.8	7.6
2 (rain)	5.6	8.38	21.3	0.0
3 (rain)	5.7	8.78	21.1	9.5
4 (rain)	5.6	8.65	21.2	7.2
5 (rain)	5.3	8.78	20.7	3.9
6 (rain)	5.4	8.81	20.9	0.0
7 (rain)	5.5	8.45	21.2	6.1
8 (rain)	5.5	8.53	21.1	5.3
9 (river)	7.6	8.50	19.2	0.0
10 (river)	7.5	8.40	19.3	3.7
11 (river)	8.3	8.70	19.3	5.2
12 (river)	7.8	6.50	19.2	0.0
13 (river)	7.7	6.01	19.4	3.6
14 (river)	7.6	7.60	19.5	3.0
15 (river)	7.8	7.90	19.3	0.0
16 (river)	7.9	8.10	19.2	0.0
17 (groundwater)	7.2	7.60	22.9	3.8
18 (groundwater)	7.6	5.40	23.1	3.1
19 (groundwater)	7.8	5.90	23.4	3.2
20 (groundwater)	8.0	6.70	23.3	3.2
21 (groundwater)	7.3	7.40	23.0	3.6

Table 3. Eigenvalues, RSS (residual sum of squares) and PRESS (predictive residual sum of squares), as a function of principal components number.

PCA: Principal components analysis (standardised)
		Crossvalidation
Number of PCs	Eigen values	RSS	PRESS
Total SS	100.0	11266.89	11266.89
#1	39.3	138.73	171.88
#2	28.0	68.04	82.84
#3	12.1	5.82	n/a

Figure 2. (a) Scores representation along PC1 and PC2 principal components (triangles = groundwater samples; diamonds = rain samples; circles = river samples); and (b) loadings representation.

Figure 3. (a) Scores representation along PC1 and PC3 principal components (triangles = groundwater samples; diamonds = rain samples; circles = river samples); and (b) loading representation.

Figure 4. (a) Scores representation along PC2 and PC3 principal components (triangles = groundwater samples; diamonds = rain samples; circles = river samples); and (b) loadings representation.