Objective To establish a method for determination of ammonium ions in fine particulate matter (PM_2.5) by a fully automated flow injection analyzer.

Methods Quartz filter membrane used for atmospheric PM_2.5 sampling was subjected to ultrasonic extraction with ultrapure water and centrifugation; the supernatant was taken for dilution and then injected into a flow injection analyzer for measurement of mass concentration of ammonium ions.

Results Under the best optimized condition (30-minute ultrasonic water bath at 20 ℃, buffer solution pH 5.2, and reaction temperature of flow injection analyzer 50 ℃), the standard curve of ammonium ions showed a good linearity in the range of 0 to 1.0 mg/L, with a linear correlation coefficient of 0.999 9. When the sampling volume was 135 m³ (standard condition), the detection limit of the method was 0.026 μg/m³; the spike recovery rates for low-, medium-, and high-concentration samples were from 90.0% to 110%; the relative standard deviations were from 1.0% to 4.1%.

Conclusion This method has high precision and accuracy, which makes it suitable for analysis and testing of water-soluble ammonium ions in atmospheric PM_2.5 samples.