Axial inductively coupled plasma time-of-flight mass spectrometry using direct liquid sample introduction

Abstract

This report is concerned with the first investigation of a direct injection high efficiency nebulizer (DIHEN) as a microliter-solution introduction system for axial inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS). The effect of rf generator frequency on analytical performance indices is investigated at both 27 and 40 MHz at operating conditions suitable for the DIHEN: a solution uptake rate of 100 µL min⁻¹, nebulizer gas flow rate of 0.2 L min⁻¹, and rf power from 800 to 1700 W. Four observations indicate an increased secondary discharge at 27 MHz compared to 40 MHz for the DIHEN compared to conventional solution introduction: (1) higher background count rates, (2) faster and wider ion deflection pulse times, (3) increased doubly-to-singly charged ion ratios (Ba²⁺/Ba⁺ = 0.03 vs. 0.02), and (4) reduced oxide-to-metal ion ratios (CeO⁺/Ce⁺ = 8.5 vs. 51) at 27 MHz in comparison with 40 MHz. Analytical figures of merit achieved, including detection limits, sensitivity, and precision using DIHEN-ICP-TOFMS are superior in both normal and cool plasma conditions to those obtained with a conventional nebulizer-spray chamber arrangement operated at a solution uptake rate of 1 mL min⁻¹. For example, typical precision (percent relative standard deviation, %RSD) for the DIHEN at 27 MHz is less than 0.6% for an integration time of 10 s. The signal intensity is nearly a factor of 2 higher with the DIHEN compared to the conventional nebulizer-spray chamber arrangement. Isotope ratio precision acquired by the DIHEN-ICP-TOFMS system closely approximates values predicted by Poisson counting statistics for both steady-state and transient signals. Finally, fast transient analysis (ca. 12.75 ms) is demonstrated for Sn and Cr isotopes in human lung fibroblast cells.