Easy Visualization Tool To Ensure Proper Tracer Addition during Isotope Dilution Mass Spectrometry Elemental Analyses: Application to Total and Metallomic Speciation (Postcolumn IDMS) of Iron, Zinc, and Copper in a Commercial Infant Formula

Isotope dilution mass spectrometry (IDMS) is used worldwide for precise quantitative total and speciation analysis. It relies on the accurate quantification of an analyte or a species in a single experiment. The minimization of error propagation during the addition of the tracer solution ensures measurement accuracy. Its verification in the total analysis can be easily accomplished by simple calculations. Nevertheless, it is hardly applicable to transient signal chromatographic analyses. Generally speaking, scientists sometimes struggle to justify the isotopic tracer concentration selected for their chromatographic studies by inductively coupled plasma mass spectrometry. Often, the tracer concentration selected is the one that ensures a close-to-1-ratio in species-specific isotope dilution analysis, which usually leads to good results. In species-unspecific isotope dilution analysis, using postcolumn IDMS methodology, the criteria is generally one "at middle-height of the peak with maximum intensity." However, this approach is debated for the quantification of several species presenting important intensity variations. In this situation, the following question emerges: What is the perfect tracer concentration to be used, that is able to quantify the majority of the species eluted from the chromatographic column in an accurate manner? The tool presented in this article (using a milk-based sample) intends to be a fairy godmother that acts as a wise advisor for the in-field scientist that needs to deal with total or postcolumn IDMS measurements. The approach aims to study the tool's versatility (applicable to any element) and readiness (easily applicable after reading this manuscript) while acting as a facilitator (simplifying laboratory experiments).