基于多孔石墨碳柱的高效液相色谱法测定电化学合成尿素产物

Determination of electrosynthesized urea products by high performance liquid chromatography based on porous graphitic carbon column

电化学合成尿素法是一种相对绿色环保的尿素制备方法,其反应产物中,除目标产物尿素外,还含有大量NO_(3)^(-)、NO _(2)^(-)、CO _(3)^(2-)等杂质离子,这些离子易对尿素测定产生干扰。本文基于多孔石墨碳柱特殊的极性保留效应,并结合其柱流失远小于普通反相C18色谱柱、在极低紫外吸收波长下基线稳定的优势,利用多孔石墨碳柱建立了电合成尿素产物中尿素的高效液相色谱分析方法。经过色谱条件优化后,选择Hypercarb TM多孔石墨碳柱(100 mm×4.6 mm,5μm),采用水-25 mmol/L甲基磺酸溶液为流动相进行梯度洗脱分离,柱温为30℃,流速为1.0 mL/min,进样量为25.0μL,检测波长190 nm,可实现不受电解液中其他杂质(如NO_(3)^(-)、NO _(2)^(-)、CO _(3)^(2-)等离子)干扰的尿素测定。结果表明,单次样品分析可在15 min内完成,尿素在0.1~100 mg/L范围内线性关系良好,相关系数(r ^(2))不小于0.9988;方法的检出限(S/N=3)和定量限(S/N=10)分别为0.028 mg/L和0.093 mg/L,低、中、高3个水平下的加标回收率为112.0%~118.4%。采用所建立的方法对实际电化学合成尿素反应液样品进样分析,结果显示尿素色谱峰的峰形较为良好。该方法前处理简单,方便快捷,尿素与其他干扰离子实现了一定程度的分离,结果准确可靠,特异性好,可用于实际电合成尿素电解液产物中微量尿素及其他相关离子的检测。

Urea is a simple organic compound that is widely used in both the industry and daily life.It can be utilized as a raw material for products such as crop fertilizers,animal feed,and explosives,and for the treatment of skin diseases.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 NO_(3)^(-),NO _(2)^(-),NH _(4)^(+),HCO_( 3)^(-),and CO _(3)^(2-),with a comparatively high concentration of NO_(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 NO_(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,5μm).Experimental investigations were performed under the optimal chromatographic conditions,and gradient elution was performed with a H_(2)O-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 25.0μL.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 0.22μm 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(r^( 2)≥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 NO_(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.