Determination of water-soluble ions in PM2.5 using capillary electrophoresis with resonant contactless conductometric detectors in a differential model

Abstract

Ten water-soluble ions (F⁻, Cl⁻, NO₂⁻, NO₃⁻, SO₄²⁻, K⁺, Na⁺, NH₄⁺, Ca²⁺, and Mg²⁺) in PM2.5 samples were determined by capillary electrophoresis (CE) with a resonant capacitively coupled contactless conductive detector (RC⁴D). The main component of the background electrolyte (BGE) solution was 20 mM 2-morpholinoethanesulfonic acid (MES)/L-histidine (His) (pH = 6.1). The modifiers of 1.5 mM 18-crown-6 and 20 μM CTAB were added to the BGE for the separation of cations and anions, respectively. The limit of detection for the ten ions is in the range of 3–20 μg L⁻¹. In the combination of dual-opposite end injection and end-to-end differential detection, simultaneous determination of the ten ions was performed in a BGE of 20 mM MES/His + 1.5 mM 18-crown-6 + 20 μM CTAB. A flow injection interface was employed for delay of sample injection without interrupting the electrophoresis. The negative peaks appeared in the early and later stages of electrophoresis increase the possibility of peak superposition in electropherograms. With two independent separation capillaries employed in end-to-end differential detection, simultaneous determination of anions and cations was performed with optimized BGE and better peak separation.