ICP-MS

Inductively coupled plasma mass spectrometry (ICP-MS) is an instrument that provides atomic information, produces ions and separates them in the gas phase based on their mass-to-charge ratio. It is an analysis technique multi-elemental that allows to determine and quantify most of the elements of the periodic table at trace and ultra-trace level.

Calibration is established by a mathematical relationship between signal intensities and analyte concentrations. ICP-MS results are presented as elemental concentrations obtained after calibration. The usual practice is to report these quantitative values with the corresponding uncertainty, along with validation experiments, through certified reference materialsthat demonstrate that the method is fit for purpose. Depending on the calibration blank and the type of element to be quantified by ICP-MS, different limits of detection and limits of quantification can be obtained.

Although quantification is facilitated by the wide linear range of the quadrupole ICP-MS (9-12 orders of magnitude), matrix effects and instrumental drift can lead to measurement errors. In this sense, the external calibration must integrate a internal standard. As a prerequisite, the internal standard must be similar to the analyte, e.g. close in mass and ionization potential. It must also be free of polyatomic interferences and be absent in the samples.

Routine protocols use rare earth elements such as rhodium or rhenium as internal standards. The same amount of one or more internal standards is added to all solutions, including blanks, calibration standards and samples. As a result, the respective signal normalized to the internal standard is used to evaluate the data and thus compensate for instrumental signal drifts and systematic errors between samples and (usually aqueous) calibration standards. This quantification strategy is limited by moderate matrix effects and does not provide precious results when there is a pronounced difference between matrix composition and aqueous calibration standards. Often the composition of the matrix is also unknown which makes the choice of the internal pattern difficult. In such circumstances, a calibration curve is constructed based on the addition of the calibration standards to the sample at different concentration levels. This quantification strategy is called standard additions.