6.14: Inductively Coupled Plasma-Mass Spectrometry (ICP-MS): Interferences
Inductively coupled plasma–mass spectrometry (ICP–MS) is a highly selective and sensitive technique for accurate elemental analysis. Though the analysis of ICP–MS mass spectra is comparatively straightforward, it is affected by spectroscopic and non-spectroscopic interferences. Spectroscopic interferences arise when the plasma contains ionic species with an m/z value the same as the analyte ion. Spectroscopic interference can be categorized as isobaric, polyatomic ions, and refractory oxide ion interferences.
Isobaric interference occurs when the inductively-coupled plasma contains isobaric species of analytes. These species are different nuclides with the same mass number and so have the same m/z values as the analyte, assuming the same charge. For example, 40Ar+ and 58Fe+ generally overlap the peaks for 40Ca+ and 58Ni+, respectively. Quadrupole-based mass instruments, having a resolution of less than 1 u, are substantially affected by isobaric interference. However, it can be minimized using higher resolution instruments.
Another spectroscopic interference is observed when various plasma components interact with matrix or atmospheric components to form polyatomic species, which may undergo further fragmentation to form molecular ions having the same m/z value as the analyte ion. 40ArH+, 16O2+, and 40ArO+ are some potential interferents. A blank correction or using a different analyte isotope removes such interferences.
Another critical interference occurs when several analyte and matrix components form oxides and hydroxides whose peaks overlap with the analyte ions. The formation of oxides and hydroxides depends on various experimental factors, like the injector flow rate, the sample orifice size, the sampler skimmer spacing, etc. These interferences are more challenging to eliminate than isobaric and polyatomic ion interferences.
Non-spectroscopic interferences mainly include the matrix effect. It is associated with a high concentration of matrix species—generally, 500 to 1000 mg/mL—causing analyte signal reduction. Specific experimental conditions enhance the analyte signal. Matrix interferences can be eliminated by incorporating an internal standard with mass and ionization potential close to the analyte.
