Abstract:

A gas-liquid separator (GLS) for the removal of carbon dioxide in the eluent of an ion chromatography (IC) system was developed. Firstly, the microporous hollow fiber polypropylene tube (PP-T) was inserted into the polytetrafluoroethylene tube (PTFE-T) with suitable inner diameter to form a double casing structure and wound into a spiral shape. Each terminal of the PP-T is 3 cm longer than that of the PTFE-T. Then the terminals of PP-T and PTFE-T were fixed respectively on two horizontal joints equipped with tee joints, and the regenerated liquid pipeline was fixed on a vertical arm interface equipped with tee joints. When the carbonate eluent flows into PP-T from the upstream suppressor, the inhibited carbonate solution will decompose and produce CO₂, which then escapes through PP-T and enters the flowing absorption solution located in the annular space between PP-T and PTFE-T, so as to achieve continuous and effective CO₂ removal. The production conditions and operating conditions of GLS were optimized, including the length of PP-T, the inner diameter of PTFE-T, the operating temperature, the type, concentration and flow rate of the absorption solution. The experimental results show that when 40 mmol/L potassium hydroxide solution is used as absorption solution, the removal efficiency of CO₂ can reach more than 98% at the flow rate of 1 mL/min. Higher operating temperature is helpful to improve the removal efficiency of CO₂ and reduce the baseline noise. The optimized operating temperature in this study was 40 ℃. When applying this GLS to the IC system for carbonate eluent, using a mixed eluent of 1.8 mmol/L potassium carbonate-3.2 mmol/L potassium bicarbonate (1∶1, v/v), the background conductance signal decreased from 41.6 mV without GLS to 5.5 mV after using GLS. Using a mixed standard solution of common anions (F^-、Cl^-、 ${{\mathrm{NO}}_2}^{-}$、Br^-、 ${{\mathrm{NO}}_3}^{-}$、 ${{\mathrm{SO}}_4}^{2-}$) at 150 μmol/L as the reference sample, the signal-to-noise ratio (SNR) after applying the GLS has been significantly improved by a factor of 6.01 to 11.8 times. In addition, the GLS can also be used to remove CO₂ from large volume samples in hydroxide eluent system, reducing the interference of ${{\mathrm{CO}}_3}^{2-}$ on the separation of other anions.

Key words: ion chromatography (IC), gas-liquid separator (GLS), carbon dioxide removal, anion analysis, suppressor