Isotopic dilution with inductively coupled plasma mass spectrometry
One of the priorities of LNE's chemical metrology laboratory is research and development of primary reference methods and studies of their metrological qualities. Isotopic dilution with inductively coupled plasma mass spectrometry (ICP-MS), with a metrological approach, is used to analyse trace elements in small concentrations (µg/l or less) in solution, with a relative uncertainty of 1 to 5%.
A laboratory and clean rooms (ISO 7 and ISO 8 class) for preparation of samples are wholly dedicated to this activity.
A liquid sample is ionized in an argon plasma at a temperature approaching 10,000 K, which is high enough to break the molecules and ionize the atoms.
Ions produced by the ICP source are pumped through an interface with sampler and skimmer cones and a vacuum system. An ionic optical system, comprising a series of lenses with different potentials, focuses the ion beam on the spectrometer. The PQ ExCell mass spectrometer is a quadrupole instrument (figure 1):
Figure 1b. Operating principle
Figure 1a. PQ ExCell mass spectrometer
The Axiom mass spectrometer is a double focusing (magnetic and electrostatic) instrument with a single collector (figure 2):
Figure 2b. Operating principle
Figure 2a. Axiom mass spectrometer
Isotopic dilution with ICP-MS has been validated at international level in comparisons between national metrology institutes performed by the Consultative Committee for Amount of Substance - Metrology in Chemistry (CCQM) of the International Weights and Measures Office BIPM. Mutual recognition arrangements between laboratories and degrees of equivalence on measurement capacities have been established on the basis of key comparisons. The CMCs (Calibration and Measurement Capabilities) declared by national metrology institutes may be consulted on the BIPM website.
So far LNE has concentrated on ICP-MS for analysis of heavy metals in fields linked to the environment and agri-food sectors, particularly for CRM1 certification in the European IRMM-BCR2 framework. The wide range of samples and matrices analysed confirms the primary nature of this reference method.
Isotopic dilution with ICP-MS is now being used in national test programmes organized to supply reference values. This method is of great interest to programme organizers, as it will ensure traceability of assigned values and allow analysis laboratories to detect any bias in the methods they use. After an initial experiment completed in the context of the METREAU applied water metrology project, other experiments have been carried out in inter-laboratory comparisons organized by BIPEA3 (analysis of Se and Hg in feed water, circuit 34 2004), EMD4 (analysis of Cd, Ni and Pb in atmospheric particles, LCSQA project 2005) and AFSSA5 (analysis of Hg, Cd and Pb in fish and meat products, EIL 2005). This is undoubtedly one of the activities that LNE's chemical metrology laboratory will be developing in the future. These partnerships with test programme organizers place France well ahead of its European neighbours in the field of test result traceability, which is set to become a key application of metrology for analysis laboratories. They also open the way for new traceability procedures in chemical analysis by incorporating three essential components:
LNE is accreditated by French accreditation committee COFRAC for analysis of heavy metals in environment and food matrices (accreditation no. 2-54, technical annex 9).
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