Ion-chromatography high-resolution MS (IC-HRMS) analysis of selenate and molybdate in the Zin River (נחל צין)

While hiking near the Zin River on the East side of Maale Akrabbim, which is in the Negev desert, in March and  February of 2023 and 2024, I collected water samples for curiosity, intending to explore them in our laboratory.  Due to the absence of agricultural fields around (anyway, its desert), and since the water was highly enriched with minerals, the IC-HRMS was chosen as a primary analytical technique. Indeed, common cations (determined by ICP analysis) and anions were detected, but to our surprise, unusually high concentrations of selenate and molybdate were found in the Zin River water.

Herein, it is demonstrated how IC-HRMS can be used for the detection and identification of individual oxoanions (speciation analysis), while only elements may be detected using the conventional ICP-MS or OES techniques.

Instrumental: Q-Exactive Plus hybrid FT mass spectrometer coupled with Dionex Integrion HPIC ion-chromatograph (IC) (Thermo Scientific). The IC is equipped with an automated eluent (KOH) generator and conductivity detector (CD). Chromatographic separations are carried out using hydroxide-selective AS19 Dionex ion-chromatography column.

Results and Conclusions

Concentrations of selenium and molybdenum in the collected water samples, determined using ICP-OES in the laboratory of Vasiliy Rosen were 5.61-5.85 mg/L (Se) and 3.8-3.92 mg/L (Mo) respectively. Both selenium and molybdenum are essential elements for humans. However, due to their rare occurrence in the ground and drinking water, selenium and molybdenum concentrations are not regularly monitored and additionally, there are only provisional guidelines for recommended values. According to WHO food-based dietary guidelines, the recommended intakes are 35-55 µg/day of  Se and 0.075-0.25 mg/day of Mo for adults. This gives recommended guidelines for upper values of Se, - 40 µg/L (5,600-5,800 µg/L was detected) and Mo, -  70 µg/L concentrations in drinking water. Excessive levels of selenium are toxic and may appear as the following symptoms, - loss of hair, nail abnormalities, mottled teeth, brittle hair, and others. Selenium compounds are metabolized in three ways: to specific selenoproteins, to nonspecific proteins, and excretory products. The nutritionally essential functions of selenium appear in some 25 selenoproteins, each of which contains selenium in the form of selenocysteine. In animals, molybdenum interacts in a complex manner with copper by a mechanism that is yet unknown.

Based on our findings and WHO recommendations, the Nahal Zin water shall be regarded as not drinkable for humans and other animals.