Abstract:

Urea is a simple organic compound that is widely used in both the industry and daily life. Compared with conventional methods, the preparation of urea by electrochemical synthesis is more environmentally friendly and sustainable. However, after the reaction, low amounts of urea and high concentrations of inorganic ions, including \mathrm{N} {{\mathrm{O}}_3}^{-}, \mathrm{N} {{\mathrm{O}}_2}^{-}, \mathrm{N} {{\mathrm{H}}_4}^{+}, \mathrm{H} {{\mathrm{CO}}_3}^{-}, and \mathrm{C} {{\mathrm{O}}_3}^{2-}, with a comparatively high concentration of \mathrm{N} {{\mathrm{O}}_3}^{-}, remain in the solution. These ions tend to interfere with the urea measurements. Thus, detecting traces of urea in highly concentrated ionic matrices is challenging. Several urea detection methods, such as infrared spectrometry, the urease method, and high performance liquid chromatography (HPLC), are available. Among these methods, HPLC shows greater sensitivity and accuracy. However, urea is difficult to separate from \mathrm{N} {{\mathrm{O}}_3}^{-} on reversed-phase C18 columns. Porous graphitic carbon (PGC) columns have lower column loss and better baseline stability at low UV absorption wavelengths than ordinary reversed-phase C18 columns. In this study, a qualitative and quantitative analytical method based on a PGC column was established to detect urea from inorganic ions. The separation of urea from other ions was successfully achieved on a Hypercarb^TM PGC column (100 mm×4.6 mm, 5 μm). Experimental investigations were performed under the optimal chromatographic conditions, and gradient elution was performed with a H₂O-25 mmol/L methanesulfonic acid aqueous solution as the mobile phase (initial mobile phase volume ratio, 98∶2). The column temperature was 30 ℃, the flow rate was 1.0 mL/min, and the injection volume was 25.0 μL. A diode array detector with a detection wavelength of 190 nm was selected because of the low UV absorption wavelengths of urea and impurity ions. The electrolyte product was passed through a 0.22 μm aqueous-phase filter membrane, and the resulting filtrate was analyzed under the optimized conditions. The results showed that urea was well separated from the other ions in the filtrate within 15 min. The urea measurements were unaffected by other ions present in the electrolyte. Moreover, none of the retention times of potentially interfering ions overlapped with those of urea. The background noise remained low and the baseline was smooth, even at a low detection wavelength of 190 nm. Methodological verification experiments showed that urea had a good linear relationship in the mass concentration range of 0.1-100 mg/L (r²≥0.9988). The limits of detection and quantification were 0.028 and 0.093 mg/L, respectively. When the electrolyte product was spiked with urea at three levels, the average recoveries were in the range of 112.0%-118.4%. Finally, an actual electrolyte sample was analyzed, and the results showed that urea could be detected quantitatively in this sample. The established method requires fewer pretreatment procedures because the samples only need to be filtered and analyzed. It also has a short analysis time and yields accurate, reliable, and specific results. Moreover, at an ultralow detection wavelength of 190 nm, the method demonstrated high sensitivity to urea and ensured a low background noise and baseline stability. Owing to the specific retention effect of the PGC column, the separation of urea at a high \mathrm{N} {{\mathrm{O}}_3}^{-} concentration was achieved without interference. Thus, the developed method can be applied for the detection of trace urea and other related ions in urea-containing electrolyte products.

Key words: porous graphitic carbon column, high performance liquid chromatography (HPLC), urea, anions