Investigation of nitrogen addition, position effect and mismatch intensity effect in Li isotopic analysis by nanosecond laser ablation multi-collector inductively coupled plasma mass spectrometry

Abstract

The accurate calibration of Li isotope ratio analysis was implemented by a simple and easy-to-operate sample-standard bracketing (SSB) calibration method using nanosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (ns-LA-MC-ICP-MS). The method requires stable instrumental conditions to ensure similar or same mass bias for the sample and the bracketing standard. To stabilize the instrumental conditions and reduce the matrix effect, N2 was added at 4 ml min−1 into the central gas, at a cost of 35–42% reduction in intensity of 7Li. Additionally, to reduce the mass bias difference between the sample and the bracketing standard, their positions in the ablation cell and intensity mismatch effect were also evaluated. An offset of up to 5‰ in δ7Li was observed if the sample and standard were too far apart in the laser ablation cell, and our study demonstrated that the position effect was effectively reduced/eliminated when the sample and standard were placed close to each other, or a volume-optional and low-memory (VOLM) cell was applied for analysis. Furthermore, the intensity mismatch effect was evaluated by changing three laser ablation parameters (spot size, energy and repetition rate). To reduce the intensity mismatch effect, Li content of the selected standard should be close to that of sample, and the Li intensity of sample and standard should be matched by adjusting laser repetition rate. According to the investigation of the above influencing factors on the Li isotope ratio measurements, we can optimize an effective analytical scheme of Li isotope for more geological materials using ns-LA-MC-ICP-MS.