Study of long-term determination accuracy for rees in geological samples by inductively coupled plasma quadrupole mass spectrometry

For ICP-QMS analysis, daily optimization of operating parameters is mandatary. With the spray chamber temperature set at 2 °C, the effects of Ar gas flow rates (including nebulizer gas flow, plasma gas flow and auxiliary gas flow), nebulizer inserting depth, sampling depth and sampling flow are fully studied.

To optimize nebulizer gas flow, its influences on the ratios of oxide formation, hydroxide formation and doubly charged species were tested. Taking Ce as the representative element, the results are shown in Figure 1. It can be seen that the ratios of CeO⁺/Ce⁺ and CeOH⁺/Ce⁺ slightly increase with an increase in the nebulizer gas flow from 0.6 to 0.8 L/min, with corresponding values well under 3.0%, and then ascend sharply. However, the ratio of Ce²⁺/Ce⁺ first goes up to 0.75 L/min, showing a maximum value of 3.3%, and then decreases to a value lower than 2.5% under 0.85 L/min of nebulizer gas flow. Additionally, the corresponding signal intensity of Ce shows an obvious increment in the range from 0.6 to 0.85 L/min and then an apparent decrement from 0.85 to 1.2 L/min. Thus, 0.85 L/min of nebulizer gas flow is recommended for the subsequent testing. Here, the effect study for plasma gas flow rate and auxiliary gas flow rate showed that the optimum values were 14.5 L/min and 0.8 L/min, respectively. To further enhance the sensitivity and precision of REEs quantification, the possible effect of nebulizer inserting depth in the spray chamber was also investigated. Results revealed that the signal intensities of REEs were greatly influenced by the nebulizer inserting depth, showing that REEs signals sharply increased with a decrease in the nebulizer inserting depth from 36 to 30 mm, and then declined slightly. Thus, a nebulizer inserting depth of 30 mm was chosen as the optimal depth. After taking signal stability and efficiency of reagents into consideration, the other ICP-QMS operating parameters, 90 of sampling depth and 1.0 mL/min of peristatic pump, were selected in the subsequent experiments.

In order to test the long-term instrumental operation stability of ICP-QMS, the effect of the chosen daily gas flow rates (0.85 L/min of nebulizer gas flow, 14.5 L/min of plasma gas flow and 0.8 L/min of auxiliary gas flow) on the ratios of Ce²⁺/Ce⁺, CeO⁺/Ce⁺ and CeOH⁺/Ce⁺ and Ce signal intensity for 10 ng/mL of REEs standard solution were also monitored over seven months. Results showed that Ce signal intensity was relatively stable around the level of 4.2 × 10⁴ CPS, and all the ratio values of Ce²⁺/Ce⁺, CeO⁺/Ce⁺ and CeOH⁺/Ce⁺ were less than 3.5% across the whole assay period, with inter RSDs well below 6.5% (n = 50), confirming the reasonability of the daily operating parameters of this ICP-QMS instrument for REEs quantification.