Comment on “Comparison of direct and indirect measures of transport efficiency in single particle inductively coupled plasma mass spectrometry” by K. E. Murphy, A. R. Montoro Bustos, L. L. Yu, M. E. Johnson, M. R. Winchester

This paper provides critical and constructive comments on the paper “Comparison of direct and indirect measures of transport efficiency in single particle inductively coupled plasma mass spectrometry” by K. E. Murphy, A. R. Montoro Bustos, L. L. Yu, M. E. Johnson, M. R. Winchester, which has been recently published in Spectrochimica Acta Part B. It details the author's reservations about the discussions and message of this paper. It calls the reader's attention to the paper conclusions and highlights which are not supported by the paper content/findings but they only mislead the reader with regards to the features and benefits of the dynamic mass flow (DMF) method reported for the first time by Cuello et al., Journal of Analytical Atomic Spectrometry, 2020, DOI: https://doi.org/10.1039/c9ja00415g and used by other groups and in interlaboratory comparisons at the metrological level for transport efficiency determination in spICP-MS measurements of particle number concentration.

The paper by Murphy et al. misses the key fact that DMF is not intended for use with any set up or condition but, if used under specified optimal operating set up and conditions (e.g. use of an ICP-MS system in equilibrium, a cooled spray chamber as reported by Cuello et al.) the method is invaluable for applications where use of the TEF and TES methods to determine transport efficiency is constrained by their reliance on reference materials which are limited or unavailable. The main use of DMF is the assignment of a SI traceable number concentration value to new, commercial nanomaterials and use those as quality control materials in spICP-MS experiments. This has not been highlighted explicitly in the paper under discussion. Instead, the findings rely heavily on data acquired by the DMF method using nineteen different conditions of which only one complies with the published recommendations and, unsurprisingly, for which the authors found the DMF to work. Indeed, some of the chosen operating conditions are unjustifiable given any knowledge of common ICP-MS usage. Yet this is used to justify condemning the DMF method and ignores the fact that close control of operating conditions is essential for many methods, particularly when striving for small uncertainty.