Ion Chromatography (IC)
Determination of Nitrite, Bromide and Nitrate in Seawater Using Ion Chromatography
May 20 2020
Author: Vadim Kraft, Philipp Jochems, Robert Ludwig, Brigitte Bollig, Gesa J. Schad on behalf of Shimadzu Europa GmbH
The determination of low levels of inorganic ions in seawater samples with high salt content is challenging. Ion chromatography (IC) with suppressed conductivity and UV/Vis detection has the advantages of high sensitivity and selectivity for analysis of standard anions including fluoride, chloride, nitrite, nitrate, bromide, phosphate and sulphate. High linearity, low limits of detection as well as excellent precision of retention time and peak area were demonstrated using IC. The method was applied for analysis of nitrite, nitrate and bromide in seawater samples.
Determination of inorganic ion concentration is important for water quality control. The quantity of nitric compounds especially has significant importance in shrimp and fish farming. Nitrite (NO2-) is an intermediate product in bacterial nitrification and denitrification processes in the nitrogen cycle . It appears as a result of feeding and can be highly toxic for aquatic animals, depending on the pH value. Nitrate (NO3-) is a main nitrogen source for phytoplankton growth in seawater , and the normally constant bromide/chlorinity ratio is helpful to determine groundwater contamination as well as seawater intrusion.
For determination of nitrite and nitrate, many different techniques are reported . Sensitive determination of traces of anions in samples with high salt content like seawater can be challenging. The ability of nitrite and nitrate as well as bromide to absorb UV light in the range around 210 nm in contrast to highly concentrated chloride is beneficial for analysis with required selectivity and high sensitivity by ion chromatography and UV detection [2,4].
The IC system with electrolytically regenerated suppression and UV detection provides sensitive measurements of anions in complex samples like seawater. In this work, standard anions including nitrite, nitrate and bromide were analysed by UV/Vis and conductivity detection after suppression of the eluent.
The Shimadzu modular LC system consisted of a solvent delivery pump, autosampler, column oven and an electrolytically regenerated suppressor as well as conductivity and UV/Vis detectors (see Figure 1). Analytical conditions used are shown in Table 1.
The ultrapure water (ASTM Type 1) for preparation of the eluent and dilution of standards was prepared by arium® pro water purification system from Sartorius. Anions standards of fluoride, chloride, nitrite, bromide, nitrate, phosphate and sulphate in water (1000 ppm, for IC) were purchased from Sigma Aldrich. Sodium carbonate (≥ 99.8%) and sodium bicarbonate (≥ 99.7%) for preparation of the eluent were purchased from Sigma Aldrich and Honeywell Fluka, respectively. Syringe filter (polyester, 45 µm pore size) for filtration of samples were purchased from Macherey-Nagel.
The seawater samples were diluted 1/20 (v/v) with ultrapure water and filtered through 0.45 µm PET syringe filters.
Figure 2 illustrates conductivity and UV/Vis chromatograms for the separation of seven standard anions on a Shim-pack IC-SA4 column. Nitrite, bromide and nitrate show absorption in contrast to fluoride, chloride, phosphate and sulphate ions that were determined using conductivity detection. As listed in Table 2, all anions were determined within 17.5 min with high precision for retention time (RSD ≤ 0.07%) and peak area (RSD ≤ 0.51%) for both detectors.
Late eluting peaks might appear broad. The dispersion effect for late eluting anions is especially strong which is characteristic for isocratic elution. In addition, low kinetics of ion-exchange interactions limit high separation efficiency in ion chromatography.
Linearity and limit of detection
For both detectors, linearity in the calibration range of 0.1-5 ppm obtained with external standards (0.1, 0.2, 0.5, 1, 5 ppm) of nitrite, bromide and nitrate was excellent with a coefficient of determination of R2 ≥ 0.9999 (see Figure 3 and Table 3).
Limits of detection (LOD) were determined as signal to noise (S/N) ratio of 3.3. As shown in figure 3 and table 3, the LOD for nitrite and nitrate detected with UV/Vis detection (0.9 ppb and 1.4 ppb, respectively) were lower compared to suppressed conductivity (2.9 ppb and 4.8 ppb respectively). In contrast to nitrite and nitrate, bromide had lower LOD with suppressed conductivity (5.3 ppb) compared to UV/Vis detection (12.8 ppb).
Analysis of seawater
Figure 4 shows separation of anions in diluted seawater using suppressed conductivity and UV/Vis detection. In the samples, bromide and nitrate were determined. While both anions could clearly be detected by UV/Vis, nitrite concentration was below LOD by conductivity detection.
In three individually collected seawater samples, the concentration of bromide was determined (by UV/Vis detection) using the external calibration method in the range of 65.1 to 65.9 ppm. The nitrate concentration (UV/Vis detection) was significantly lower in the range of 0.5 to 1.3 ppm (semi-quantitative determination). Nitrite could not be identified in the 20x diluted sample. Peak tracking was also supported by spiking the dilution with 0.1 ppm nitrite, 2.5 ppm bromide and 0.1 ppm nitrate (Figure 5).
Ion chromatography with electrolytically regenerated suppression and conductivity as well as UV/Vis detection provides sensitive measurements of anions in complex samples like seawater. Due to the different selectivity and high sensitivity, both detectors can be applied complementarily for qualitative and quantitative analysis of anions.
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2. K. Ito, R. Nomura, T. Fujii, M. Tanaka, T. Tsumura, H. Shibata, T. Hirokawa, Analytical and bioanalytical chemistry, 404(8) (2012), 2513.
3. M. J. Moorcroft, J. Davis, R. G. Compton, Talanta, 54(5) (2001), 785.
4. R. Wang, N. Wang, M. Ye, Y. Zhu, Journal of Chromatography A, 1265 (2012), 186.
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