Inductively Coupled Plasma-Mass Spectrometry (ICP-MS): Interferences

Spectroscopic interference occurs when ions in the plasma have the same m/z values as analyte ions, making them indistinguishable during mass spectrometry analysis. Three main types exist: isobaric species (different nuclides with identical mass numbers), polyatomic ions (formed from plasma and matrix species combinations), and refractory oxide and hydroxide ions. These interferences directly overlap analyte peaks, compromising measurement accuracy.

Isobaric interference arises when different nuclides possess the same mass number, producing identical m/z values as the analyte. For example, 40Ar+ overlaps with 40Ca+, and 58Fe+ overlaps with 58Ni+. Quadrupole-based mass analyzers with resolution less than 1 u are substantially affected. Higher resolution instruments can minimize these interferences by separating ions with similar m/z values.

Polyatomic ion interference results when plasma components combine with matrix or atmospheric species to form polyatomic species that fragment into molecular ions with the same m/z as the analyte. Common interferents include 40ArH+, 16O2+, and 40ArO+. Blank correction or selecting a different analyte isotope effectively removes these interferences from analysis.

Refractory oxide and hydroxide interferences form when analyte and matrix components combine to create oxides and hydroxides whose peaks overlap with analyte ions. Formation depends on experimental factors including injector flow rate, sample orifice size, and sampler-skimmer spacing. These interferences are more challenging to eliminate than isobaric or polyatomic ion interferences due to their complex formation mechanisms.

The matrix effect is a non-spectroscopic interference associated with high concentrations of matrix species, typically 500 to 1000 mg/mL, which generally causes analyte signal reduction. This interference suppresses the analyte signal, compromising measurement accuracy. Specific experimental conditions can enhance the analyte signal and help compensate for matrix effects during analysis.

Internal standards compensate for matrix effects by using an element with mass and ionization potential close to the analyte. As the matrix affects both the analyte and internal standard similarly, the ratio between them remains relatively constant, allowing accurate quantification despite matrix interference. This approach effectively eliminates matrix-induced signal reduction in ICP-MS measurements.

ICP-MS offers high selectivity and sensitivity for elemental analysis, but complex samples containing multiple matrix components present significant analytical challenges. While spectroscopic interferences can be addressed through isotope selection or higher resolution instruments, non-spectroscopic matrix effects require compensation strategies. Understanding both interference types is essential for accurate analysis of complex matrices.