Progress Report on the Magnetic-Sector ICP-MS
(Plasma 54)

Janne Blichert-Toft Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France

Béatrice Luais Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France

Chloé Maréchal Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France

Francis Albarède Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France

The Plasma 54 is essentially the combination of a conventional multiple-collector mass spectrometer similar to those used for TIMS with an ICP torch-box front end such as those used for ICP-MS with quadrupole detection. At the junction of the two parts, a quad lens stack reshapes the ion beam from a circular plasma to rectangular in order to force it into the defining slit, while an electrostatic sector ensures the energy focussing required by the energy dispersion in the plasma source. This mass spectrometer combines the advantage of maximum ionization over the whole mass range with the precision of a state-of-the-art low-mass resolution magnetic filter. Most of the work carried out so far on the Plasma 54 in Lyon concerns the radiogenic isotopes of Hf and Nd as well as the stable isotopes of Cu and Zn. Three different nebulisation devices were investigated. The standard (Meinhardt) nebulizer provides a very stable beam and a precision better than 100 ppm on isotopic ratios but with a rather low sensitivity (50 ppm for Hf and Nd, 5 ppm for Cu and Zn) inadequate for most sample analyses. The Mistral nebulizer combines HT vaporization and vapor chilling. It improves the sensitivity by a factor of 10 but at the expense of a rather noisy beam and a large memory effect. The microconcentric nebulizer (MCN) of CETAC reduces the uptake rate and yields the same efficiency as the Mistral (500 ppm for Hf and Nd, 50 ppm for Cu and Zn) but with a stable beam and excellent precision. The exponential fractionation law is extremely efficient in reducing the mass bias from more than 1% per amu to less than 50 ppm. Interelement mass fractionation corrections allow the isotopic analysis of Cu isotopes using a Zn standard and vice versa. It was found that energy dispersion in the plasma is more critical than expected, especially in the HT dry plasma created by the Mistral and, to a lesser degree, the MCN. A careful alignment of the mass spectrometer must take into account both the ion mass and energy. Isobaric interferences have proven to be smaller than in quadrupole ICP-MS even within the range of Ar compounds. The background at masses that are essential to transition element isotopic analyses, such as mass 56, is small and dominated by metal ions and not molecular compounds. It has further been observed that precise and sensitive measurements critically depend on good sample purification. For example, trace amounts of Ti reduce Hf transmission by a factor of 2 during a 10 mn run. We have completed extensive separation protocols for Lu, Hf, Sm, Nd, and transition elements which allow the precise isotopic analysis of less than 100 ng of sample. Elemental ratios likewise can be measured with accuracy. This is, for example, the case of the U/Pb ratio which can be statically measured to 0.2% using the off-axis collector devised to cover a larger mass range (20%) than the span of the axial Faraday array (10% of the mass). We expect to be able to report on Lu-Hf chronology of chondrites, Nd isotope stratigraphy of the Piton de la Fournaise volcano over the last 500 ky, and the first stable isotope measurements of Cu and Zn in igneous, hydrothermal, and oceanic samples by the time of the conference.