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The Neptune MC-ICP-MS
Schematic setup of the Neptune instrument. From Weyer & Schwieters (2003)

ThermoFinnigan Neptune MC-ICP-MS

Introduction

 

The study of isotope compositions of different substances is a broad field of scientific research that helps to elucidate processes of e.g. dissolution and precipitation in environmental studies, may provide an age information of rocks and minerals, or the formation of Earth, Moon, and the solar system, just to name a few.

 


For isotope research, multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) represents a versatile and reliable tool for precise analyses of isotope ratios of various elements. The plasma source allows ionization of many elements, either being introduced via an aerosol or a solution.

The Institute of Mineralogy uses a ThermoFinnigan Neptune MC-ICP-MS instrument equipped with 9 Faraday detectors, one secondary electron multiplier (SEM, fixed), and two compact discrete dynode (CDD) SEMs for isotopes with very low signal intensities (< 106 cps; attached to the L4 and H4 Faraday detector, respectively). One amplifier with a 1012 Ω feedback resistor can be selected for a low signal intensity (typically < 5*10-13 A ≡ 50 mV), and nine amplifiers with 1011 Ω resistors are installed enabling measurement of signals of up to 5*10-10 A (≡ 50 V).

 

The Neptune offers analyses of solutions via a stable introduction system (SIS) quartz glass spray chamber combined with nebulizers made of PFA or glass, and solid samples like glasses, metals, or crystals via connection to the adjacent femtosecond laser ablation system. A Cetac Aridus II desolvating system and an ESI Apex system are available as well for an improved signal intensity and stability during analyses of solutions. An ESI SC2 DX autosampler system can be connected to the instrument additionally for large numbers of sample solutions to be run overnight. Cones made of Ni (types Jet, H, and X), Pt (type X), and Al (types H, X) are available.

 

Ongoing projects at the institute use the instrument for stable respective radiogenic isotope analyses of the elements Li, B, Mg, Si, Fe, Cu, Zn, Sr, Mo, Nd, Hf, Pb, and U.

Principle

Sources for sample material are aerosols produced by the femtosecond laser ablation system (using He as carrier gas), or dissolved material in an acidified solution (e.g., 3% HNO3) from which interfering elements were removed in the clean lab employing ion exchange resins.

 

The sample is ionized in an Ar plasma, the Transfer Optics accelerate the ions and shape and focus the ion beam. The Electrostatic Analyzer (ESA) produces an electric field that focuses ions with a different energy, thus improving abundance sensitivity. The magnet separates the ions depending on their respective mass and focuses them on different trajectories. The Neptune is designed with a normal Nier-Johnson geometry, i.e. the ESA is located in front of the magnet.

 

The high resolution mode is selectable by switching between the low and high-resolution entrance slit. This enables separation of polyatomic mass interferences from the signal of interest, e.g. separation of 56Fe from 40Ar16O. The Neptune at the institute offers a high mass resolution of ca. 11.000 (defined at 5 and 95 % peak height), which e.g. creates a wide 40Ar16O-free plateau for 56Fe measurements.

 

Different types of detectors (Faraday, SEM, CDD) and amplifier resistors (1011 Ω, 1012 Ω) allow analyses of a wide range of isotope abundances in a variety of matrices.