JOCHEN VOGL and WOLFGANG PRITZKOW
BAM Federal Institute for Materials Research and Testing
Unter den Eichen 87, 12205 Berlin, Germany
IDMS Principle
The principle of IDMS is surprisingly simple. A sample with known isotopic composition of the analyte element, but unknown element content is mixed with a known amount of spike. This spike contains the analyte element in a different isotopic composition: Ideally enriched in the rarest natural isotope. After complete mixing of sample and spike the so-called sample-spike blend or isotope diluted
sample gained a new isotope ratio being between the isotope ratio of the sample and that of the spike. This blend isotope ratio directly reflects the analyte concentration in the sample. The principle is demonstrated for the example of antimony
As IDMS only requires isotope ratio determination and mass measurements, the advantages compared to other methods become apparent: After equilibration losses of analyte do not affect the accuracy of the analytical result, because the measurand, the blend isotope ratio, is equal in all sub-samples of the blend. Thus extended sample preparation schemes like trace-matrix separations can be (ICPMS) or have to be (TIMS) carried out. Here a remark seems necessary as many readers today are familiar with ICPMS but only few with TIMS: Performing IDMS with TIMS requires an analyte isolation step, as evaporation, ionization and thus fractionation in TIMS dramatically change with
matrix. Because analyte isolation is mandatory for TIMS and thus an intrinsic component, the measurement procedure "TIMS" is largely unaffected by matrix. In ICPMS isotope ratio measurements are in fact largely unaffected by matrix due to the ionization conditions, as both isotopes will be affected in the same way. This is valid as long as quadrupole ICPMS (Q-ICPMS) is concerned. Aiming at highest
precision and accuracy by applying high resolution (HR) or multi-collector (MC) ICPMS trace-matrix separation is necessary, as matrix components can slightly influence mass discrimination [31-32]. IDMS offers the possibility to determine major (%) to ultra-trace concentrations (< ng·kg-1) of elements, species and compounds in various matrices. The fact that IDMS has the potential to produce analytical results of highest accuracy and precision, metrologically denoted to as smallest uncertainties, makes it most suitable as reference method for RM characterisation. When using ICPMS also multielement IDMS is available, although this is rarely used.
multi-element spikes, risk of cross contamination by other spikes and different chemical behaviour of the analytes. In trace matrix separation each step increases the blank and reduces the analyte recovery, so that commonly 2, 3 or maximum 6 analytes can be separated in one procedure.
Although disadvantages occur in IDMS also, fortunately they are more than balanced by the advantages. Nevertheless they have to be considered.IDMS is a destructive method and two isotopes, ideally completely free from any isobaric or polyatomic interference, must be available. Even mono-isotopic elements can be determined, if they feature a longliving radionuclide such as129I. The most importanti issue, however, is the mixture between sample and spike isotopes. A complete isotopic equilibrium between sample and spike isotopes is preferred.
However, a complete mixing between sample and spike analyte is already sufficient, as long as sample and spike analyte are present in the same species. Most commonly this is ensured by using a digestion step in elemental analysis. Even if sample and spike analyte are being present in different species, IDMS may work as long as the species conversion can be accomplished after the digestion step (more details in
the example chapter). If no sample treatment, especially separation, is being carried out after spiking, as is the case for some ICPMS analysis, sample and spike even can be present in different species without being converted. However, in this case losses of analyte do bias the result and it has to be guaranteed that the different species are being ionized with the same efficiency. In terms of speciation analysis
the above described principles do apply in the same way. The spike availability is no issue for RM characterisation, because most labs produce and characterise their own spikes or they purchase it from institutes like NIST [33] or IRMM [34].
source :Journal of metrology society if india Vol 25 no 3, 2010 pp 135-164
BAM Federal Institute for Materials Research and Testing
Unter den Eichen 87, 12205 Berlin, Germany
IDMS Principle
The principle of IDMS is surprisingly simple. A sample with known isotopic composition of the analyte element, but unknown element content is mixed with a known amount of spike. This spike contains the analyte element in a different isotopic composition: Ideally enriched in the rarest natural isotope. After complete mixing of sample and spike the so-called sample-spike blend or isotope diluted
sample gained a new isotope ratio being between the isotope ratio of the sample and that of the spike. This blend isotope ratio directly reflects the analyte concentration in the sample. The principle is demonstrated for the example of antimony
As IDMS only requires isotope ratio determination and mass measurements, the advantages compared to other methods become apparent: After equilibration losses of analyte do not affect the accuracy of the analytical result, because the measurand, the blend isotope ratio, is equal in all sub-samples of the blend. Thus extended sample preparation schemes like trace-matrix separations can be (ICPMS) or have to be (TIMS) carried out. Here a remark seems necessary as many readers today are familiar with ICPMS but only few with TIMS: Performing IDMS with TIMS requires an analyte isolation step, as evaporation, ionization and thus fractionation in TIMS dramatically change with
matrix. Because analyte isolation is mandatory for TIMS and thus an intrinsic component, the measurement procedure "TIMS" is largely unaffected by matrix. In ICPMS isotope ratio measurements are in fact largely unaffected by matrix due to the ionization conditions, as both isotopes will be affected in the same way. This is valid as long as quadrupole ICPMS (Q-ICPMS) is concerned. Aiming at highest
precision and accuracy by applying high resolution (HR) or multi-collector (MC) ICPMS trace-matrix separation is necessary, as matrix components can slightly influence mass discrimination [31-32]. IDMS offers the possibility to determine major (%) to ultra-trace concentrations (< ng·kg-1) of elements, species and compounds in various matrices. The fact that IDMS has the potential to produce analytical results of highest accuracy and precision, metrologically denoted to as smallest uncertainties, makes it most suitable as reference method for RM characterisation. When using ICPMS also multielement IDMS is available, although this is rarely used.
multi-element spikes, risk of cross contamination by other spikes and different chemical behaviour of the analytes. In trace matrix separation each step increases the blank and reduces the analyte recovery, so that commonly 2, 3 or maximum 6 analytes can be separated in one procedure.
Although disadvantages occur in IDMS also, fortunately they are more than balanced by the advantages. Nevertheless they have to be considered.IDMS is a destructive method and two isotopes, ideally completely free from any isobaric or polyatomic interference, must be available. Even mono-isotopic elements can be determined, if they feature a longliving radionuclide such as129I. The most importanti issue, however, is the mixture between sample and spike isotopes. A complete isotopic equilibrium between sample and spike isotopes is preferred.
However, a complete mixing between sample and spike analyte is already sufficient, as long as sample and spike analyte are present in the same species. Most commonly this is ensured by using a digestion step in elemental analysis. Even if sample and spike analyte are being present in different species, IDMS may work as long as the species conversion can be accomplished after the digestion step (more details in
the example chapter). If no sample treatment, especially separation, is being carried out after spiking, as is the case for some ICPMS analysis, sample and spike even can be present in different species without being converted. However, in this case losses of analyte do bias the result and it has to be guaranteed that the different species are being ionized with the same efficiency. In terms of speciation analysis
the above described principles do apply in the same way. The spike availability is no issue for RM characterisation, because most labs produce and characterise their own spikes or they purchase it from institutes like NIST [33] or IRMM [34].
source :Journal of metrology society if india Vol 25 no 3, 2010 pp 135-164
