Novel HPLC Technique for Determining NO 2 -N and NO 3 -N Levels in Water as an Alternative to SKALAR Cadmium Reduction Method

Abstract

A novel technique was developed on High Performance Liquid Chromatography (HPLC) instrument that features a photodiode Array UV detector to determine the nitrite-nitrogen and nitrate-nitrogen levels at Qurryah Seawater Treatment Plant located along the Arabian Gulf seawater, as well as in waste water and groundwater in the eastern province of the Kingdom of Saudi Arabia at very low concentrations. The primary objective of this research is to identify a cost-effective alternative that is sufficiently accurate for measuring trace levels of nutrients in water, serving as a replacement for the traditional SKALAR cadmium reduction method used for quantifying nitrite-nitrogen and nitrate-nitrogen levels in water. The HPLC instrument utilized in this study was the 8998 Photodiode Array detector from Waters Alliance e2695. A Phenomenex Luna C-18 HPLC Column (5 µm, 250 × 4.6 mm) at an operating temperature of 40˚C was employed for the separation of analytes. The prepared aqueous solutions included solvent A (washing) at a 50:50 v/v% ratio of CH 3 OH:H 2 O, solvent B (seal wash) at a 10:90 v/v% ratio of CH 3 OH:H 2 O, and solvent C (needle wash) at a 75:25 v/v% ratio of Acetonitrile (ACN):H 2 O, while solvent D, which represents the mobile phase, consisted of a 55:45 v/v% mixture of Acetonitrile (ACN):H 2 O at a flow rate of 1.0 mL/min. These conditions ultimately facilitated optimal analyte elution and separation. Acidified deionized water with a pH of 2.5 was filtered through a 0.22 µm membrane, and the resulting deionized water filtrate was used to prepare the aqueous solution for solvent D (mobile phase) at a 55:45 v/v% ratio of ACN:H 2 O. The mobile phase was maintained at a flow rate of 1.0 mL/min, with an injection volume of 20 µL in the method. The results obtained from the proposed HPLC method were found to be comparable to those from the traditional SKALAR Cadmium reduction method (ASTM D3867), which was conducted using a continuous flow analyzer for determining trace levels of nitrate and nitrite-nitrogen in water. High-Performance Liquid Chromatography (HPLC) consistently shows a lower percentage of Relative Standard Deviations (%RSD) when analyzing QA NO 3 -N samples, achieving a %RSD of 1.32% compared to the 2.45% %RSD recorded with the SKALAR instrument. For QA NO 2 -N samples, HPLC has proven to have a %RSD of 1.74%, while the SKALAR method produced a significantly higher %RSD of 4.58%. This study concludes that the HPLC instrument is cost-effective, offering an estimated annual savings of USD 20,000, while also being dependable, facilitating rapid analysis, and ensuring a fast turnaround time. It allows for increased throughputs, requires minimal maintenance, conserve water resources, safer, and does not create cadmium-contaminated wastes. When looking at the SKALAR instrument, HPLC is considered a more cost-effective and environmentally friendly choice.