# Nitrite: From Application to Detection and Development
## 2 Traditional Detection Methods
In recent years, as detection requirements have rapidly increased, more and more studies have shown that ion chromatography is seldom used as a stand-alone detection method, but rather in combination with other instruments or methods for the detection of target substances. For example, UPLC-MS (Ultra Performance Liquid Chromatography-Mass Spectrometry) can be used for the detection of nitrite in a variety of pharmaceutical products [20]. The gas-chromatography-negative chemical ionization-mass spectrometry derivatization method can also make up for the shortcomings of ion chromatography detection methods. The proposed ion chromatography-mass spectrometry method solves the ion suppression problem of MS (mass spectrometry) detection. The loss of sensitivity due to the accumulation of trace salts in the ion source is avoided [21]. The combined method of HPLC (high-performance liquid chromatography) and IC (ion chromatography) for the determination of nitrite can achieve rapidity and accuracy and avoid the problem of labor intensity [22]. The four-stage liquid flow path of the HPLC-IC system is shown in Figure 2.  
The combination of ion chromatography and automated techniques can improve reproducibility and selectivity. Portable automated ion chromatography systems with dual capabilities are now available for the analysis of freshwater nitrite, achieving an analytical range of 0.1 mg/L-40 mg/L NO\({}_{2}\)\({}^{-}\) for marine analyses, as well as nitrite analysis [17]. A smart wastewater quality monitoring system is shown in Figure 3. The range of analysis is suitable for freshwater analysis. In today's era of big data, the empowerment of smart devices in detection has led to a significant improvement in the detection of traditional methods as well. Related research has developed a low-cost water quality testing device consisting of a nitrate and nitrite analyzer based on a novel ion chromatography method. In addition, the analytical device was integrated using an IoT software platform based on the middleware called thethings.io, which was found to be reliable after careful comparisons [23]. Ion chromatography, as a representative of the traditional method, has the following advantages: (1) high sensitivity and accuracy [16]. (2) The detection is standardized, and there are relevant national standards at home and abroad to regulate it [12]. (3) Pre-treatment of the sample is required [18]. In short, when using ion chromatography for the determination of nitrite, investigators have improved the method mainly in terms of the following aspects. (1) Select suitable eluent. (2) Adjust the flow rate of eluent. (3) Change the ionic strength to improve the stability of nitrite. (4) Control the column temperature. (5) Choose a suitable detector. (6) Improve the pretreatment step. All these steps should be adjusted according to the specific experimental conditions and experimental objectives and verified by experiments. For the determination of nitrite by ion chromatography, we need to try many times in practice so that we can find the best combination of parameters. Figure 2: Schematic diagrams of the liquid flow paths in the four stages of the HPLC-IC system. The system is connected by two six-way valves and one enrichment column. (**a**) Sample loading. (**b**) Separation of nitroaromatic compounds and anions. (**c**) Anion enrichment in the AG20 column. (**d**) Determination of nitroaromatic compounds and anions.