Analytical characteristics and applications of ICP-MS in speciation analysis

ICP-MS is widely used for elemental detection in combination with various methods for separation, including GC, HPLC, CE and ion chromatography. Detection limits are extremely low, in the nanogram per liter range.

Numerous applications of speciation analysis have been reported, mainly in the field of biological systems, food analysis and environmental chemistry. References to publications and information on species certified reference materials are found at the website of the European Virtual Institute for Speciation Analysis (EVISA): http://www.speciation.net/

Upon heating to 5000 K in the plasma most elements are present as single-charged ions (+1) which can be transferred to a mass spectrometer. The inorganic mass spectrum shows the intensities of nuclides and a few poly atomic species in the m/z range up to about 300. The intensities correspond to the concentrations of the elements in the sample solution.

Molecular information is lost in atomic spectroscopy detection methods because the sample must be heated to a temperature that is high enough to generate atoms and/or ions of the elements of interest. However, the combination of both organic and inorganic mass spectrometry coupled to separation techniques can offer full analytical characterization of all species.

The degree of ionization, α, varies from 100% for alkali and alkali earth elements to about 30% for elements with a high ionization energy (e.g. As and Se).

where [M] is the atom density and [M⁺] is the ion density. The Saha equation shows that the ionization equilibrium depends on the ionization energy V_ion and ionization temperature T_ion

where n_e is the electron density, m_e is the electron mass, T_e is the electron temperature, h is the Planck constant, Z_a and Z_i the partition function for the atom and ion, respectively and k is the Boltzmann constant. The ICP is an excellent source for the formation of single-charged ions

ICP-MS instrument: The ions formed in the plasma are extracted via a conical water-cooled sampler into the first vacuum stage where a pressure of ~100 Pa is maintained and then pass through the skimmer placed a few cm behind the sampler. Ion lenses focus the ion beam in the high vacuum (~10^-3 Pa) provided by a turbomolecular pump. The detector measures the ion intensities.

ICP-MS Instrument

 The ICP-MS instrument consists of the following components:

• Torch

• Power supply and load coil

• Sample introduction system (nebulizer and spray chamber)

• Mass spectrometer (quadrupole, sector field, time of flight or multicollector)

• Detection system (electron multiplier, Faraday cup)

More information on the inductively coupled plasma is found in the section on ICP-OES.

The mass spectrometers used in ICP-MS are basically the same as used in GC/MS and LC/MS.

We prefer to measure the intensities of selected isotopes with high(est) abundances unless interferences occur at a specific m/z.

Mass spectrum for several elements

• Isobaric interferences are caused by overlap between signals from nuclides having the same mass number but different atomic numbers (isobaric nuclides or isobars). The relative isotopic abundance of the naturally occurring elements is well known and therefore potential isobaric interferences are predictable. They can often be corrected by mathematical correction equations, measuring the interfering nuclide at another mass which is free of interference.

• Spectral interferences. Polyatomic or molecular spectral interferences can be generated by either argon-, solvent or sample-based ionic species, e.g. the interference of ⁴⁰Ar¹⁶O⁺ on ⁵⁶Fe⁺, ⁴⁰Ar³⁵Cl⁺ on ⁷⁵As⁺ or ⁴⁰Ar⁴⁰Ar⁺ on ⁸⁰Se⁺. Polyatomic spectral interferences can be minimized or eliminated by the application of correction equations, cool plasma technology or matrix separation.

• Oxides, hydroxides, doubly charged species. Oxides and hydroxides may occur at 16 or 17 mass units higher than the analyte mass and create spectral overlap problems. Doubly charged ions may cause interferences at half of their mass. The formation of oxides and double-charged ions is related to the ionization conditions in the plasma and can be minimized by appropriate optimization of the experimental conditions, e.g. nebulizer gas flow and RF power. Such interferences are typically observed for rare-earth and refractory elements (e.g. CaO⁺, BaO⁺, CeO⁺, CaOH⁺, POH⁺, BrH⁺, Ba²⁺).

• Matrix interferences. The signal may be influenced by the sample matrix, e.g. due to changes in droplet formation or transport efficiency or changes in the plasma ionization conditions. Such problems can usually be solved by internal standardization.

Resolving power

For many applications it is sufficient to measure m/z values without decimal positions, which can easily be achieved with a quadrupole mass spectrometer. The resolution would then increase from 7 for ⁷Li⁺ up to 208 for ²⁰⁸Pb⁺. This would not be enough to separate the peaks of ⁵⁶Fe (m/z = 55.934939 and ⁴⁰Ar¹⁶O (m/z = 55.957298) or ⁷⁵As (m/z = 74.921594 and ⁴⁰Ar³⁵Cl (m/z = 74.931236). Better resolution can be achieved with time of flight and sector field mass spectrometers. For further information on mass spectrometers see Chromedia sections on GC/MS and LC/MS.

Separation of As from ArClSeparation of ⁷⁵As⁺ from ⁴⁰Ar³⁵Cl⁺ using the high resolving power (R = 10 000) of a sector field ICP-MS instrument (Thermo Finnigan).