Getting accurate and reproducible trace element analysis results

Challenge 2: Interferences

Since the early days of ICP-MS in the mid-1980s, interferences with this technology have been well studied, characterised and documented. The most immediately noticeable interferences are those that form as a result of reactions between the plasma gas (Ar) and the sample constituents in the high temperature environment of the plasma. As a noble gas, Ar is not expected to be reactive, but in an up to 10,000 °C plasma with acid vapours, water and other sample components being present, it forms a range of products, primarily including ArO⁺, ArN⁺, ArC⁺, ArCl⁺ and ArAr⁺. These so-called polyatomic interferences interfere with Fe, Mn, Cr, As and Se respectively. Additional, less abundant interferences from ArOH⁺, ArNH⁺, etc. are also observed and, when high amounts of sample matrix components such as Na are present, other species such as ArNa⁺ (which interferes with Cu) are also seen. Aside from Ar-based interferences, samples containing high amounts of S, P, Cl and C (e.g. from sulphuric, phosphoric and hydrochloric acids and organic sample components) produce a plethora of interferences including SO⁺, SO₂⁺, PO⁺, PO₂⁺, ClO⁺ and CO⁺ (which interfere with Ti, Zn, Cu, V and Si), as well as less abundant hydride adducts of all of these species (i.e. SOH⁺, SO₂H⁺, etc.).

With all these interferences (and more), you might be wondering how you can get any sensible results at all out of an ICP-MS. The answer is that today’s ICP-MS instruments (such as the Thermo Scientific iCAP RQ single quadrupole ICP-MS) (see Fig. 2)come complete with an advanced system known as collision/reaction cell technology that significantly reduces the majority of common polyatomic interferences while retaining sufficient sensitivity for the interfered elements to allow them to be accurately quantified. This technology consists of a cell placed between the interface of the instrument (where the plasma meets the rest of the mass spectrometer) and its quadrupole mass analyser.

Gallery

This QCell in the iCAP RQ ICP-MS is filled with a continuous, low flow of (usually) He gas. As the ion beam from the interface enters the cell, polyatomic interference ions and analyte ions collide with the He gas and lose kinetic energy. Even though the polyatomic ions have the same mass as the element with which they interfere, they have a larger cross-sectional area, which means that they undergo more collisions than the analyte ions do. The result is that they lose more kinetic energy than the analytes, allowing them to be rejected from the ion beam using a simple voltage barrier between the collision cell and the quadrupole mass spectrometer. The analyte ions can then be measured with much less interference — in fact, in many cases effectively interference free. This collision cell ion energy filtering approach to interference removal is known as kinetic energy discrimination (KED). The whole process is illustrated in Figure 1.

In a few cases, use of H₂ (a reactive gas), in the cell of single quadrupole ICP-MS instruments can be helpful, because certain interferences can be removed by chemical reactions with this gas, while the analytes with which they interfere don’t react (or at least hardly react), leading to much improved accuracy and lower detection limits. However, use of He and H₂ with single quadrupole instruments, although highly effective for a wide range of applications, has its limitations. There are some interferences that are either so intense or so difficult to resolve, that even this powerful technology is defeated.

The types of interferences that go beyond the capabilities of single quadrupole ICP-MS include the so-called isobaric interferences, consisting of isotopes of an element that have the same mass as isotopes of another element (such as ⁸⁷Rb and ⁸⁷Sr) and intense interferences generated by samples containing very high levels of matrix elements, such as metal alloy digests. Examples of this latter kind of interference include CoO⁺, which interferes with the monoisotopic As, and ZrO⁺/ZrOH⁺ which interfere with Cd. Other interferences which cause problems with single quadrupole instruments include doubly-charged rare earth elements, such as Nd²⁺ and Gd²⁺ which interfere with As and Se respectively. This is when triple quadrupole ICP-MS technology, such as that provided in the iCAP TQ ICP-MS, comes to the fore.

Triple quadrupole systems allow mass filtering of the ion beam emanating from the plasma before the beam enters the collision/reaction cell, which greatly enhances the selectivity of the instrument for interference removal and opens the door to using a wider range of reactive cell gases (such as O₂ and NH₃) than can be used with single quadrupole instruments. These additional gases allow more complex chemical reactions to be used in the cell to achieve far better removal of challenging interferences, such as CoO⁺, Nd²⁺ and Gd²⁺. Triple quadrupole instruments achieve their superior interference removal capability in one of two ways, depending on the interference to be resolved: the first way is ‘on-mass’ analysis and the second is ‘mass-shift’ analysis.

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