Characterization of inductively coupled plasma time-of-flight mass spectrometry in combination with collision/reaction cell technology – insights from highly time-resolved measurements

Abstract

We present results from studies investigating the capabilities of inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS) in combination with collision/reaction cell technology (CCT). Experiments were carried out using various sample introduction techniques including high- and low-dispersion laser ablation and microdroplet generation. Specifically, we investigated H₂ as a reaction gas and He as a collision gas for their effects on reduction of background species and interferences, limits of detection (LODs), quantification capabilities, and structure of short transient signals. With H₂ as reaction gas, argon-based ions were suppressed. Ar⁺ and Ar₂⁺ signals could be attenuated to intensity levels in the hundreds and single-digit counts per second range, respectively. Selective suppression of Ar-based ions gives access to the most abundant isotopes of Ca and Se. Furthermore, the attenuation of the Ar⁺ signals allows the instrument to be operated without m/z-selective attenuation of the ion beam prior to TOF analysis, which enhances transmission of isotopes near m/z 40. It was also found that application of flow rates ≤4 mL min⁻¹ of H₂, He or mixtures of these gases results in collision-induced focusing, and enhances sensitivities by a factor of 1.5 to 2 and mass resolving power by up to 16%. Use of CCT enabled an improvement in the LOD for ⁴⁰Ca of more than three orders of magnitude. The LOD for ⁸⁰Se was improved by more than one order of magnitude. For many of the elements contained in NIST SRM 610, LODs were lower by a factor of two to four. However, for most elements, improvement in quantification accuracy was not observed. Experiments with microdroplet sample introduction demonstrated that the mass-dependent ion transit times between the ICP and the TOF extraction region are affected by the amount of He buffer gas in the collision cell. Changes in transit times, as well as signal broadening, were observed on a time scale of tens to hundreds of μs. The duration of individual aerosol plumes from low-dispersion laser ablation, however, remained practically unaffected from collisional effects and imaging at 100 Hz laser repetition rate with baseline-separated aerosol plumes is possible.