Foreword
Quadrupole-based inductively coupled plasma–mass spectrometry (ICP-QMS) was introduced commercially in 1983. The technique immediately aroused considerable interest in the analytical community as it provided multielement capabilities and better limits of detection compared to atomic absorption spectrometry (AAS) and ICP optical emission spectroscopy (ICP-OES). In the three decades since it was introduced, ICP-MS has been widely adopted as the technique of choice for trace element analysis in samples ranging from ultrapure reagents used in the semiconductor industry, through food commodities, pharmaceutical products, body fluids and tissues, to waters, soils and rocks measured in the environmental monitoring and mining industries. The ever increasing demand to quantitatively determine (ultratrace) elements in various sectors and in a myriad of matrices ensures that ICP-MS is still gaining popularity.
As is the case with every other technique, ICP-MS has its drawbacks, and spectral interferences, especially those that jeopardize the accurate trace element determination at mass-to-charge ratios
≤80 amu, were rapidly recognized as the technique's proverbial Achilles’ heel. Early ICP-MS users learnt how to overcome this obstacle in some instances, but some important elements simply remained inaccessible.
With the introduction of sector-field ICP-MS with higher mass resolution, the majority of spectral interferences encountered can be resolved. Sector-field instrumentation is, however, substantially more expensive, and higher mass resolution comes at the cost of reduced ion transmission and thus, lower signal intensity. At that time, ICP-QMS had no “general approach” to offer as an alternative to overcome spectral overlaps. Cool plasma conditions formed a first step in the right direction, then with the introduction of multipole collision/reaction cells, the gap between sector-field and quadrupole-based ICP-MS instrumentation began to close.
These collision/reaction cells already provide a versatile means for tackling spectral overlaps, as interferences can be removed either through differences in reaction chemistry (with a reactive cell gas), or by kinetic energy discrimination using a bias voltage (when an inert cell gas is used).
With the introduction of the Triple Quadrupole ICP-MS (ICP-QQQ) instrument, equipped with an octopole collision cell located between two quadrupole mass filters, this approach has been further perfected. Specifically, the capabilities of chemical resolution based on selective reactions between ions extracted from the ICP and the molecules of a reactive gas can now be fully exploited. The tandem mass spectrometer configuration provides two separate mass selection steps, which provides full control over the ion/molecule chemistry that occurs in the cell. The first quadrupole allows only the ions of a given mass-to-charge ratio into the gas-pressurized octopole cell, rejecting all the ions at all other mass-to-charge ratios. The second quadrupole then selects only the ion of interest emerging from the cell, and rejects the ions at all other mass-to-charge ratios. This approach not only provides a clear insight into the cell processes – via precursor ion and/or product ion scanning – but also provides an elegant approach for solving even the most challenging cases of spectral overlap. More reactive gases (e.g., NH3 or O2) can now be used in a controlled way, with the certainty the reaction pathways and product ions formed in the cell will not be affected by changes in the sample matrix or by other coexisting analyte ions. This consistency of reaction processes also permits more complex, heavier reaction product ions and clusters to be exploited for ultra-trace element determination.
Even for such multi-element reaction product ions, the double mass selection preserves the original isotopic pattern, thereby facilitating spectral interpretation and isotope ratio determination, e.g., in the context of elemental assay via isotope dilution.
The full extent of the capabilities of Triple Quadrupole ICP‑MS still needs to be discovered, but it seems clear that with the introduction of the 8800 ICP-QQQ, the analytical community has entered a new era of ICP-MS. This handbook provides an overview of the analytical approaches developed using ICP-QQQ thus far.
Frank Vanhaecke
Department of Analytical Chemistry
Ghent University, Belgium