Reply to “Comment on Comparison of direct and indirect measures of transport efficiency in single particle inductively coupled plasma mass spectrometry” by H. Goenaga-Infante

This paper provides a rebuttal to the comments on our original research article entitled “Comparison of Direct and Indirect Measures of Transport Efficiency in Single Particle ICP-MS” recently published by Goenaga-Infante in Spectrochimica Acta Part B: Atomic Spectroscopy.

The intent of our study is not to condemn the dynamic mass flow (DMF) method but rather to explore the use parameters under which it can and cannot be applied. As explained in the original paper, the goal of this study is to evaluate the ranges of use conditions under which measures of transport efficiency (TE) yield accurate and reproducible results for spICP-MS measurements of particle number concentration (PNC) and particle size (diameter, PS) of various gold nanoparticle (AuNP) suspensions. The evaluation was performed through a systematic comparison of three methods for the measurement of TE: the particle frequency (TEF), particle size (TES), and DMF methods using three types of spray chambers operated at cooled and ambient temperature conditions and employing different ICP-MS platforms. While we appreciate thorough critical and constructive comments on our paper, we strongly disagree that the conclusions and highlights are not supported by the content and findings about the features and benefits of the DMF method.

The discussion of three additional independent studies in this field by other spICP-MS expert groups that have been published after the submission of our manuscript is included in this paper. While the DMF approach has been used in two interlaboratory comparisons for TE determination in spICP-MS measurements of PNC, only one laboratory, namely the laboratory led by Goenaga-Infante, provided results using the DMF method in both projects. A method cannot be considered validated if successful results have only been demonstrated within one laboratory.

The paper by Goenaga-Infante claims that the main use of DMF, the assignment of a SI traceable PNC value to new commercial nanomaterials (NMs), has not been highlighted explicitly in our original paper. The advantages and disadvantages of the DMF, TES, and TEF methods to measure TE were presented throughout the manuscript and are clearly summarized in the conclusion. The assignment of PNC that is metrologically traceable to the SI implies that all known or suspected uncertainty components including bias are taken into account. Our research shows that under some use conditions, the biases inherent to the DMF method are not yet fully understood and cannot be accounted for. While Cuello-Nuñez et al. Journal of Analytical Atomic Spectrometry, 2020, DOI: https://doi.org/10.1039/c9ja00415g state that cooled conditions (2 °C) were used in their work, they do not define the level of bias that can be expected if different spray chamber temperatures are used, and they do not provide recommendations on the optimal use conditions that define the field of applicability of the DMF method. Furthermore, besides the use of a Scott-type spray chamber cooled to 2 °C, neither the composition material of the spray chamber nor the volume of the Scott-type spray chamber were specified. It is important to note that in our original research, fifteen of the nineteen experiments were conducted using a cooled spray chamber. Also, the four different ICP-MS platforms were used in the standard configuration and with additional spray chamber options encountered in common ICP-MS usage. The value of our work is that it extends the study of Cuello-Nuñez et al. to a wide variety of use conditions routinely encountered in spICP-MS analysis.