Matrix-matched two-point calibration based on the standard dilution analysis method

Calibration by Proxy

Calibration by Proxy (CbPx) is a matrix-matched calibration method that utilizes multiple internal standard species to build a calibration curve. The technique requires only two solutions: solution 1 containing a sample solution and a suite of internal standards at known concentrations, and solution 2 identical to solution 1, plus an aliquot of a standard containing all analytes and the internal standards at the same concentration. The calibration curve is prepared by plotting the signal measured for each internal standard in solution 1 divided by the signal arising due to the aliquot of internal standard added to solution 2 on the y-axis. In this ratio, the sensitivity for each element cancels, because the sample matrix is equal between the solutions. Therefore, the y-axis value measured for a specific internal standard is identical to the value that would be measured for any other element present at the same concentrations in the two solutions. Hence, each internal standard serves as a proxy for analyte values. The concentrations of internal standards in solution 1 are plotted on the x-axis, and these correspond to any analytes present in solution 1 at the same concentration. CbPx is applied to the analysis of five certified reference materials by inductively coupled plasma optical emission spectrometry (ICP-OES). Percent recoveries for analytes range from 89 to 106%, with relative standard deviations on the order of 1%. A recommended working range for the method is developed through both theoretical simulation and experimental results and then exhibited through the analysis of off-the-shelf vitamin tablets.

Single-cell ICP-MS to address the role of trace elements at a cellular level

The heterogeneity properties shown by cells or unicellular organisms have led to the development of analytical methods at the single-cell level. In this sense, considering the importance of trace elements in these biological systems, the inductively coupled plasma mass spectrometer (ICP-MS) configured for analyzing single cell has presented a high potential to assess the evaluation of elements in cells. Moreover, advances in instrumentation, such as coupling laser ablation to the tandem configuration (ICP-MS/MS), or alternative mass analyzers (ICP-SFMS and ICP-TOFMS), brought significant benefits, including sensitivity improvement, high-resolution imaging, and the cell fingerprint. From this perspective, the single-cell ICP-MS has been widely reported in studies involving many fields, from oncology to environmental research. Hence, it has contributed to finding important results, such as elucidating nanoparticle toxicity at the cellular level and vaccine development. Therefore, in this review, the theory of single-cell ICP-MS analysis is explored, and the applications in this field are pointed out. In addition, the instrumentation advances for single-cell ICP-MS are addressed.