Highly sensitive determination of dissolved ultra-trace phosphorus by ICP-MS with methane mixed plasma

Abstract

Phosphorus concentrations in water are usually determined using phosphomolybdenum blue (PMB) spectrophotometry. However, the measurement of extremely low phosphorus concentrations by this method is limited for various reasons. This work systematically developed a highly sensitive analytical method for the measurement of dissolved ultra-trace phosphorus by inductively coupled plasma mass spectrometry with methane mixed plasma (CH₄-ICP-MS). We determined the optimal radio frequency (RF) power and verified the sensitizing effect of carbon-containing plasma, showing that the addition of 6.0 mL min⁻¹ methane can significantly improve the sensitivity of phosphorus detection. There was no mass spectral interference in the determination of phosphorus in medium resolution mode. The species of phosphorus, dissolved organic carbon (DOC) concentrations ≤15.0 mg L⁻¹, and salinity ≤3.4 had no effect on the determination of phosphorus. DOC concentrations >15.0 mg L⁻¹ had some inhibitory effect on the determination, but microwave digestion eliminated the effect of organic matter with good reproducibility. Salinity levels >3.4 have an inhibitory effect on the determination and the samples need to be diluted or desalinated for accurate determination. The limit of detection (LOD) for the determination of phosphorus by CH₄-ICP-MS is 7.3 ng L⁻¹. The accuracy and precision of this method are 2.5% and 1.3%, respectively. The determination precision values for phosphorus in drinking water and in natural samples such as Arctic glacier meltwater, lake water, river water, seawater and rain water were all within 3.5%, and the spike recovery rates were 101.2–103.8%. The method developed here thus has a low LOD and high accuracy. Using the proposed method, ultra-trace phosphorus in low-phosphorus samples such as polar water can be directly measured, which simplifies the sample pretreatment process, and the sample is not easily contaminated by reagents.