Swimming against the current - sacrificing unit mass resolution in ICP-MS to improve figures of merit

Abstract

While quadrupoles provide unit mass resolution across the entire mass range, alternative analysers, (sector field and time-of-flight) were developed to increase mass resolution and to improve selectivity for interfered analytes. Increasing mass resolution is inherent with a loss in ion transmission and therefore, sensitivity is sacrificed in a trade-off for selectivity. In inductively coupled plasma-mass spectrometry (ICP-MS), the analysis of some elements may benefit from this increased selectivity, but many elements are not interfered, and their trace analysis would rather benefit from increased sensitivity. Performing low resolution ICP-MS can be achieved in the bandpass mode, in which scanning properties of the quadrupole are altered to boost ion transmission. The aim is to gain sensitivity at the expense of mass resolution and therefore selectivity. Here, unit mass resolution is deliberately given up to trigger two main effects: first, an increased bandwidth improves ion transmission for each isotope. Second, isotopic signals start to convolute and can be analysed at one point. This enables the simultaneous transmission and acquisition of isotopes from polyisotopic elements increasing sensitivity substantially. Both effects are cumulative and can enhance limits of analysis as long as potential interferences are considered and avoided. This perspective details technicalities, the potential, and pitfalls of the bandpass mode for ICP-MS. Applications and future potential are discussed for different hyphenated techniques and modes, namely LC-ICP-MS, LA-ICP-MS and SP ICP-MS. Distinct effects and strategies in the bandpass mode are detailed and provide options to develop dedicated methods tailored for certain elements, samples, and scientific questions. However, the loss of selectivity is inherent with a longer list of potential interferences, which is discussed for specific examples.