ABSTRACT Rationale Nitrite anion is among the highly reactive anions and converts the secondary amines to their corresponding carcinogenic N‐nitroso metabolites in the human body. Nitrite ion is highly abundant in natural waters and enters the body mainly through the consumption of water and beverages. The concentration of nitrite ion varies according to the nature of the soil, habitat and pollution in various geographical locations. Hence, it is important to assess the quality of water using novel analytical techniques. Methods In the present work, the concentration of nitrite ion from the collected water samples was monitored by N‐nitrosation reaction with 1,2,3,4‐tetrahydroquinoline (THQ) followed by quantification by liquid chromatography–high‐resolution mass spectrometry using atmospheric chemical ionization on octadecyl stationary phase using water:methanol (both contain 0.2% formic acid) gradient elution. The N‐nitroso THQ levels were specifically monitored using high resolution selected ion monitoring. Results The developed method was validated over the range of 1–5000 ng/mL, with recoveries of 81%–119%. The limit of detection (LOD) and limit of quantification (LOQ) of the method were 1 and 2 ng/mL, respectively, yielding good linearity ( R 2 > 0.9996) and precision with a relative standard deviation less than 7% ranging from 3.2% to 6.2%. The water samples collected across 60 different areas showed nitrite ion concentrations ranging from 28.5 to 3177.0 ng/mL. Conclusion The derivatization procedure adopted in the present method is rapid and converts THQ to a stable NTHQ derivative in a single step. The results showed that water samples collected from residential areas, oceans, and mountains had the highest nitrite ion concentration. However, the clean, continuously flowing waters showed the lowest contamination, as the river beds act as effective filters for nitrite anions; hence, the water collected from these regions might be relatively clean compared to other regions.