## 4 Discussion
A study by [PERSON] [10] also suggested that a controlled and vigorous calibration of the FT-IR increases the ability to quantify and identify an unknown sample. Studies have also revealed that PCA methods require more calibration standards to increase its prediction ability [10; 18; 20]. This could have been the reason the PCA as compared to the PLS model results in Table 3 were mostly higher (for all the heavy metals except Ag). Moreover, in cases where regions in a spectrum do not contain absorptions of the analyzed components, better results are obtained by using the PLS method as it is more suitable in quantifying components in water [21; 22; 23]. [PERSON] et al., [19]; [PERSON] and [PERSON], [33] and [PERSON] et al., [34] also used an ATR-FTIR spectroscopy and their studies had good regression models and high correct identification rates ofsamples with the PCA method. This might have been the reason that the PCA method (Table 2), although it had some limitations (i.e., for Cd, Cu, Pb and Zn), was more capable of detecting Ag with values similar to the AAS results (Table 4) as compared to the PLS method. Studies have also shown that Ag\({}^{+}\) ions are sorbed in the highest amount in soils and hence detect low concentrations in water sources [26]. Silver ions have the shortest ionic radii of all studied cations, which can influence their highest sorption rate. Instead, Pb\({}^{2+}\) and Ag\({}^{+}\) ions are sorbed better on soils [34]. These effects were observed in our study because the results showed that heavy metal concentrations in groundwater sources and surface water sources were similar to boreholes and rivers, respectively. [PERSON] et al. [26] and [PERSON] et al. [2], in studies for heavy metal adsorption in solution, were able to detect Cd, Cu and Pb with the FT-IR spectroscopy. Their findings on Cd, Cu and Pb also agreed with the results obtained in this study. Analytical measurement techniques generally are closer in determination of most chemical elements [19; 33; 34; 35]. However, as in the present study, some mean deviations might also occur based on several factors. [PERSON] et al. [35], in a study comparing the AAS and the FT-IR of metal ions (Ag, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Pd and Zn) in tap, Nile, waste, sea water samples, human urine and milk, suggested that interfering effects of different foreign ions can cause measurement variation from different analytical methods. In the present study, this might also have been the cause in some of the deviations which were seen between the PLS and PCA values (Table 3) and AAS values in Table 4. Their findings showed that variations in the detection of the metal ions between the different analytical methods were influenced by the function of pH, mass of ion exchanger at equilibrium, cations and anions in solution. Similar to this study, high levels of Zn ions above 84 \(\mu\) g L\({}^{-1}\) were also observed as compared to all their studied metal ions with the FT-IR compared to the AAS with no detection. Cd, Ag and Pd ions were not detected in their tap water with the AAS as compared to other sources [35]. In this study, the methods used detected these metals using the FT-IR and AAS techniques. [PERSON] et al. [36], in a study measuring Cadmium (II) in tap, bottle and pier water samples, also had comparable results between the new MID-FTIR-PLS PIM based-sensor and F-AAS analytical methods. Other studies argue that analytical procedures for determining heavy metal ions can be strongly interfered with matrix constituents such as alkali and alkaline earth elements [35; 36; 37; 38; 39]. In this present study, groundwater sources similar to boreholes can also contain soluble salts in solution. The underlying geological substrata can contribute to the concentration of other interfering elements.