Dedicated Solution for Hexavalent Chromium Analysis

Hexavalent chromium is highly toxic and carcinogenic. It can severely contaminate water bodies and pose significant risks to human health, making it a key pollutant under strict regulatory control in water quality monitoring. HJ1470-2026 "Water Quality — Determination of Hexavalent Chromium — Post-Column Derivatization Ion Chromatography Method" is the latest ecological and environmental testing standard, which imposes stringent requirements on detection accuracy and analytical procedures.

Leveraging its inherent technical strengths, Wayeal has developed a dedicated, customized testing instrument specifically tailored to this standard. The instrument features optimized derivatization, separation, and detection modules in accordance with the standard's technical parameters. It enables accurate analysis of various water sample types, delivering stable and reliable data that are fully compliant with the standard specifications. This customized dedicated instrument provides a professional solution for environmental monitoring, effluent testing, and other related applications, thereby reinforcing the safeguards for water quality safety.

Keywords: hexavalent chromium, post-column derivatization, UV detection, environment.

1. Experimental Principle

Using ammonium sulfate and ammonia water as the eluent, the hexavalent chromium separated by the ion chromatography column reacts with the chromogenic reagent diphenylcarbazide to form a purple-red compound. This compound exhibits characteristic absorption at 540nm in the visible region. Detection is performed at this characteristic wavelength, with qualitative analysis based on retention time and quantitative analysis based on peak area.

2. Instruments and Reagents

2.1 Configuration List

Table 1 Instrument Configuration List

3. Experiment Method

3.1 Solution Preparation

3.1.1 Eluent: Weigh 66g of ammonium sulfate and dissolve it in an appropriate volume of water. Add 14mL of ammonia water, mix well, then dilute with water to 2,000mL and mix thoroughly. The pH of this solution ranges from 8 to 9.

3.1.2 Slowly add 28mL of sulfuric acid to 300mL of water, then dilute with water to 500mL. Allow to cool to room temperature and set aside. Weigh 0.50g of diphenylcarbazide and dissolve it in 100mL of methanol. Transfer the diphenylcarbazide-methanol solution into the sulfuric acid aqueous solution, and dilute with water to 1,000mL. Transfer the resulting solution to the derivatization reagent bottle. It can be stored at room temperature under light-protected, sealed conditions for 3 days, or under refrigeration at 4 °C or below with protection from light and sealed for up to 30 days.

3.2 Experimental Conditions

3.2.1 Chromatography Conditions

Chromatographic column: HS-5A-Cr (4 mm × 250 mm);

Eluent flow rate: 1.0 mL/min; column temperature: 30 °C;

Injection volume: 200 μL;

Detector: UV detector;

Derivatization reagent flow rate: 0.33 mL/min;

Derivatization reaction coil: 375 μL;

Derivatization temperature: 35 °C.

3.3 Experiment Result

3.3.1 Standard Curve

Table 3 Gradient Concentration Table for Cr⁶⁺ Standard Curve

Fig 2 Chromatogram of Cr⁶⁺ Standard Curve

Fig 3 Linearity at an Injection Volume of 200μL

Analysis: Under these chromatographic conditions, the calibration curve for Cr⁶⁺ exhibited good linearity.

3.3.2 Repeatability

Experimental Design: In this experiment, the stability of the method was validated by determining the repeatability of six consecutive injections of Cr⁶⁺ standards at concentrations of 2μg/L, 50μg/L, and 500μg/L.

Test Results and Chromatogram Analysis:

Fig 4 Overlaid Chromatograms of Six Replicate Determinations of 0.02mg/L Cr⁶⁺ at an Injection Volume of 200μL

Table 4 Repeatability for 0.02mg/L Cr⁶⁺ at an Injection Volume of 200μL

Fig 5 Overlaid Chromatograms of Six Replicate Determinations of 0.10mg/L Cr⁶⁺ at an Injection Volume of 200μL

Table 5 Repeatability for 0.10 mg/L Cr⁶⁺ at an Injection Volume of 200μL

Fig 6 Overlaid Chromatograms of Six Replicate Determinations of 1.0mg/L Cr⁶⁺ at an Injection Volume of 200μL

Table 6 Repeatability for 1.0mg/L Cr⁶⁺ at an Injection Volume of 200μL

Analysis: From the repeatability chromatograms and the resulting data, it can be observed that under these chromatographic conditions, the RSD of retention time ranges from 0.043% to 0.197%, and the RSD of peak area ranges from 0.357% to 0.440%. These results indicate good repeatability and stable instrument performance.

3.3.3 Sample Analysis

Sample Test Results and Chromatogram Analysis:

Fig 7 Overlaid Chromatograms of Samples at an Injection Volume of 200μL

Table 7 Sample Analysis Results with an Injection Volume of 200μL

3.3.4 Theoretical Detection Limit

Fig 8 Peak height of 0.2μg/L Cr⁶⁺ at an Injection Volume of 200μL

Fig 9 Baseline Noise

LOD = (3 × 0.0074 mAU × 0.2μg/L) / 0.0065 mAU = 0.068μg/L

Analysis: Under these chromatographic conditions, the theoretical limit of detection (LOD) for Cr⁶⁺ is 0.068μg/L.

3.3.5 Spiking Recovery

Fig 10 Chromatogram of 2μg/L Spiked Sample

Fig 11 Chromatogram of 50μg/L Spiked Sample

Fig 12 Chromatogram of 500μg/L Spiked Sample

Spiking recovery 1 = ((2.008 − 0.0) / 2) = 100.40%

Spiking recovery 2 = ((50.025 − 0.0) / 50) = 100.05%

Spiking recovery 3 = ((502.503 − 0.0) / 500) = 100.50%

Analysis: From the test results, it can be seen that the spiking recoveries range from 100.05% to 100.40%, indicating good recovery performance.

4. Coclusion

This method was validated using the Wayeal IC-Cr-6000 system in accordance with the environmental standard "Water Quality — Determination of Hexavalent Chromium — Post-Column Derivatization Ion Chromatography Method" (HJ1470-2026). Based on the results of linearity, repeatability, detection limit, and spiking recovery, the method fully meets the requirements specified in the standard.