Inductively coupled plasma mass spectrometry (ICPMS)

is the method of choice for (trace) elemental analysis. The technique was commercialized in 1983 and shows an unparalleled growth, as it combines simultaneous multi-element analysis at the ultra-trace level with unrivalled sample-throughput. ICPMS today has a wide diversity of applications ranging from environmental, biological, chemical, geochemical, metallurgical and pharmaceutical research, to research in semiconductors, nuclear science and monitoring, and forensic and medical science.

Samples to be analysed must be either in liquid or solid form. In liquid form, samples are typically diluted with 2 % nitric acid, pumped into a nebulizer and aspirated as fine aerosol with argon gas. Fine droplets of the aerosol (1–2 %) are separated in a spray chamber from larger droplets (>98 %) and transferred into the plasma torch via a sample injector. The plasma is energized by inductively heating argon gas with an electromagnetic coil, and made electrically conductive by a sufficient concentration of ions and electrons. It typically reaches temperatures of 6000 °C to 9000 °C, which is sufficient to ionize over 90 % of all elements.

Atomic and small polyatomic ions produced in the ICP pass through the sampling interface consisting of two metallic cones, the sampling and the skimmer cones (orifice radius approximately 1 mm). Once pulled in, the ions converge through the ion lens, are sorted for mass by a mass discriminator (e.g., a quadrupole analyser or a laminated sector-field magnet) before they reach the ion detectors. A rotary gear pump ventilates the sampling interface to 10² to 10^-2 Pa and turbo molecular pumps pump the mass analyser and detector unit to 10^-3 to 10^-7 Pa.

Solution-mode ICPMS regularly runs fully automatized using an auto-sampler unit with a sample through-put of around 3 to 8 samples per hour. Reference solutions and reagent blank are analysed in the same analytical sequence to allow quantification of element concentrations and background signals, respectively. The limit of detection for most elements lies in the range of ppt to ppq (10^-12 to 10^-15 g ml^-1) and is for many elements limited by the purity of the lab environment and the reagents.

Many samples may typically be dissolved by acid digestion and analysed from solution, as described above (e.g., soils, minerals, biological, and synthetic materials). However, in other cases acid digestion of solids may not be desirable, and instead in-situ sampling by laser ablation is employed. Here the material is removed from a solid surface by irradiating it with a focused pulsed UV laser beam in a confined sample cell. The ablated material is transported as an aerosol in a stream of helium, subsequently mixed with argon, which is then directly injected into the ICP, where it is ionized. The laser beam irradiates the sample surface typically with an energy density of 2 to 6 J cm^-2, at 5–10 Hz, and spot diameters of 10 to 150 µm, which results in ablation rates of <1 µm s^-1. Ideally the sample surface is polished and flat (e.g., embedded in epoxy resin and polished). Laser ablation (LA) ICPMS combines high spatial resolution with very high sample-throughput of approximately 40 spot analyses per hour. Line profiles, raster areas or spots of ablation are programmed and run automatically over 3 to 12 hours, in which the laser unit triggers the ICPMS analysis. For quantification and quality control, sample analyses are bracketed by a variety of reference materials.

ICPMS types

differ by their mass analyser and mass resolution. Currently the most common one uses a quadrupole analyser (Q-ICPMS) to discriminate the ions by mass-to-charge ratio. Sector-field ICP mass spectrometers are double-focusing instruments using a combination of a static electric and a magnetic sector as mass analyser. Sector-field mass spectrometers are capable of high-resolution (HR) ICPMS with a resolving power of up to 10,000 (m/Dm), approximately 30 times higher than that of Q-ICPMS, allowing for a better mass separation and the resolution of isobaric interferences. Most ICPMS instruments operate with a single collector (SC) and thus can analyse only one mass at a time. However, due to their fast scan speed, it is possible to detect more than 50 masses (isotopes/elements) in less than 0.5 s, thus almost simultaneously. Beside elemental concentrations SC ICPMS are also used for the determination of isotope ratios (e.g. ²⁰⁶Pb/²⁰⁴Pb, ⁸⁷Sr/⁸⁶Sr). FIERCE currently operates two sector field HR ICPMS machines, a Thermo Element 2 and an Element XR, used for solution and laser ablation analysis.

Multi-collector ICPMS

is the instrument of choice for the precise and accurate analysis of isotope ratios for most existing elements (except for H, C, N, O, Ar) in solids and solutions. The detector array in a MC-ICPMS instrument typically carries between 9 and 11 faraday detectors. MC-ICPMS is more sensitive than single-collector ICPMS and allows for high mass resolution of up to 10,000 with flat-top mass peaks that are required for precise and accurate isotope-ratio measurements. Faraday collectors coupled with 10¹⁰ to 10¹³ Ω amplifiers can be operated over a dynamic range of 8 orders of magnitude (e.g., 0.3 mV to 500 V signals). In addition, up to 7 parallel secondary ion counters enable the detection of isotopes with low abundances of 1 ppt (10^-12 g ml^-1) down to sub-ppq levels (<10^-16 g ml^-1).

At our lab, we operated Neptune No. 1 for 19 years after which is was dismantled and replaced by a new Thermo Neptune Plus, installed at FIERCE in August 2018.