Description of Instrumentation Capabilities and Data Reduction Protocols Employed During In-Situ
LA-MC-ICP-MS Analyses


Antonio Simonetti

Associate Professor, Department of Civil and Environmental Engineering and Earth Sciences,
University of Notre Dame


In recent years, techniques for isotopic measurement using the multiple collector (MC-) inductively
coupled plasma mass spectrometry (ICP-MS) have been developed to a level where the typical
within-run (internal) precision and between-run (external) precision are comparable to TIMS
(thermal ionization mass spectrometry). Another significant advantage of the MC-ICP-MS is the
capability to couple a laser ablation system and perform in-situ spatially resolved isotopic
measurements. This innovation has led most notably to the combined U-Pb and Lu-Hf isotopic analysis
of zircon (ZrSiO[4]), as well as studies of a range of other isotopic systems, including both
radiogenic ones (e.g. Sr, Nd) and ‘non-traditional’ stable isotopes (e.g. Cu, Fe). There are many
advantages to be gained by LA-MC-ICP-MS analysis, the most important being the potential
information that can be obtained at the high spatial resolution. As with other microanalytical
techniques, laser ablation-MC-ICP-MS generates data that can be interpreted within spatial context
and integrated with microstructural and other geochemical datasets. Other advantages associated
with LA-MC-ICP-MS analyses include the removal of the sample digestion and chemical purification
procedures, high-sample throughput, and little or no memory. Despite the advances in recent years
there remains a perception that the accuracy and precision of the in-situ measurements suffer in
comparison to solution measurements because of matrix effects and isobaric interferences. Other
issues that also need to be addressed for LA-MC-ICP-MS analyses are the availability of suitable
reference materials, especially for matrix-matching in studies of mass-dependent isotopic
fractionation, and laser-induced isotopic fractionation. There are many factors that contribute to
the accuracy and precision of in-situ measurements, and these can be considered as the interplay
between parameters related to the sample, the laser operating conditions and processes in the mass
spectrometer.


The main purpose of my lecture is to examine the role of mass bias on the accuracy and precision of
isotopic analysis by LA-MC-ICP-MS, which includes a brief review of the theoretical basis for
isotopic normalisation using a ratio of the same element (internal normalisation) and investigates
the factors that affect the magnitude of the mass fractionation. I also discuss the techniques of
isotopic normalization using a different element (external normalisation) and ‘standard-sample’
bracketing. I also discuss the potential issues related to laser-induced isotopic fractionation and
plasma loading, which relate to the laser operating conditions and the transient nature of the
signal.