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This study presents a rapid and highly sensitive method for the determination of trace rubidium (Rb) and cesium (Cs) in high-salinity brines using inductively coupled plasma mass spectrometry (ICP-MS) equipped with an all-matrix sampling (AMS) device. The AMS system achieves online gas dilution by vertically introducing argon gas into the brine sample flow, effectively reducing the severe matrix suppression effect caused by 35 g·L −1 salinity to an intermediate level. Experimental results demonstrated that the signal suppression induced by coexisting cations (K + , Na + , Ca 2+ , Mg 2+ ) in actual brine samples was minimal (< 1.5%), thereby eliminating the need for conventional matrix matching or standard addition approaches. Accurate quantification was achieved through a straightforward calibration process based on standard curves (Rb: 5–400 μg·L −1 ; Cs: 5–400 μg·L −1 ; R 2  > 0.999), enhanced by dynamic internal standard correction using yttrium (Y) and rhodium (Rh), along with optimized instrument parameters (RF power and nebulizer gas flow rate). The method demonstrates excellent limits of detection (LOD: 0.039 μg·L −1 for Rb; 0.005 μg·L −1 for Cs), high precision (RSD < 5%), and acceptable recovery rates (85%–108%). The accuracy was further validated through comparison with AAS standard addition for high-concentration samples (> 200 μg·L −1 ), yielding consistent recoveries (98.6% –114%) and inter-method deviations ≤ 12.2%. Additionally, the simplification of the sample pretreatment procedure—from a traditional multi-step dilution to a single-step dilution without acid-washed containers—enhances analytical efficiency by over 70%. This approach provides a robust, sensitive, and operationally efficient solution for the analysis of extreme high-salinity environmental samples.