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Isotope dilution analysis using ICP-MS detection for trace element speciation

(12.07.2020)

Accuracy of quantification in speciation analysis is critical dependent on the calibration strategy. Speciation analysis in general is more complex and time-consuming than more simple total element analysis. The greater complexity is broadening the possibilities for degrading accuracy due to additional error sources, enhanced sensitivity drift and species conversion. A calibration strategy that can help to reduce or even eliminate such degrading effects is the isotope dilution analysis.

Calibration strategies for ICP-MS:

The determination of chemical species in biological or environmental samples can be difficult and time-consuming. The accuracy and precision generally achieved for this type of work is not as good as that for total elemental analysis, because of the higher complexity of the methodology and the higher number of parameters to be controlled. In general, for quantification by ICP-MS detection, external calibration strategies together with internal standardization and standard addition procedures have been applied. Best accuracy can be obtained by using Isotope dilution mass spectrometry (IDMS).

The principle of IDMS:

IDMS is based on the fact that many elements have two or more stable isotopes, whose proportion in nature is constant. A known amount of a stable isotope is added to a sample, and the ratio between the isotopes can then be measured by MS. The approach is shown in the following figure:

The requirements for IDMS are the following:

target analyte element must have more than one isotope
all isotopes must be measured free of isobaric interferences (often requires high resolution MS
sample and spike must reach isotopic equilibrium (problematic for solid samples !)

Isotope dilution analysis can correct for interferences, analyte loss, and instrumental drift problems for effects taking place after the addition of the isotope spike. Two different modes can be distinguished for speciation analysis:

Species-specific isotope dilution
Species-unspecific isotope dilution

Both approaches have their pros & cons.

Species-specific isotope dilution:

the spike solution is added to the sample at the beginning of the analytical procedure
the spike solution must contain the species to be analysed in an isotopically labelled form

Advantages of species-specific isotope dilution:

uncertainty of concentration measurement depends only on the precision of isotope ratio measurements
no influence of instrumental intensity drift
no influence of the matrix effects
no influence of changing mobile phase (gradient elution)
no influence of sensitivity differences between species (ionization efficiency)
No influence of analyte loss during the analytical run (e.g. incomplete column yield !)
species transformation during analysis can be corrected by using different spike species

Problems:

requires an isotopically labelled standard for every target species
cannot be used for unknown species
cannot be used when isotopically labelled compounds are not available or cannot be synthesized

Species-unspecific isotope dilution:

the isotope tracer or spike is added after the complete separation of the naturally occurring species in the sample (post-column spiking)
the spike may exist in a chemical form different from that of the species to be determined

Advantages of species-unspecific isotope dilution:

uncertainty of concentration measurement depends only on the precision of isotope ratio measurements
no influence of instrumental intensity drift
no influence of the matrix effects
no influence of changing mobile phase (gradient elution)
chromatogram is absolutely scaled to an analyte mass-flow rate
only a single isotope standard is required for all analyte element species present
can be used for unknown species
can be used even when isotopically labelled compounds are not available or cannot be synthesized

Problems:

analyte loss before spike addition cannot be corrected (column yield !)
sensitivity differences between species (ionization efficiency) cannot be corrected (not a problem in ICP-MS)

Reviews on ICP-IDMS (for speciation analysis) (newest first)

M.F. Giné, A.P. Packer, Online isotope dilution and inductively coupled plasma mass spectrometry: from elemental to species quantification, J. Braz. Chem. Soc. 21 (2010) 575–589. DOI: 10.1590/s0103-50532010000400002

P. Rodríguez-González, J. Ignacio García Alonso, Recent advances in isotope dilution analysis for elemental speciation, J. Anal. At. Spectrom., 25/3 (2010) 239-259. DOI: 10.1039/b924261a

P. Rodríguez-González, J.M. Marchante-Gayón, J.I. García Alonso, A. Sanz-Medel, Isotope dilution analysis for elemental speciation: a tutorial review, Spectrochim. Acta B, 60 (2005) 151–207. doi: 10.1016/j.sab.2005.01.005

D. Schaumlöffel, R. Łobinski, Isotope dilution technique for quantitative analysis of endogenous trace element species in biological systems, Int. J. Mass Spectrom., 242/2 (2005) 217–223. DOI: 10.1016/j.ijms.2004.11.020

K.G. Heumann, L. Rottmann, J. Vogl, Isotope-dilution ICP–MS for trace element determination and speciation: from a reference method to a routine method, Anal. Bioanal. Chem., 378/2 (2004) 318–329. DOI: 10.1007/s00216-003-2325-z

M. Monperrus, E. Krupp, D. Amouroux, O.F.X. Donard, R.C. Rodríguez Martín-Doimeadios, Potential and limits of speciated isotope-dilution analysis for metrology and assessing environmental reactivity, Trends Anal. Chem., 23/3 (2004) 261-272. DOI: 10.1016/S0165-9936(04)00313-9

L. Rottmann, K.G Heumann, Development of an on-line isotope dilution tech- nique with HPLC/ICP-MS for the accurate determination of elemental species, Fresenius’ J. Anal. Chem., 350/4-5 (1994) 221–227. DOI: 10.1007/BF00322473

K.G. Heumann, L. Rottmann, J. Vogl, Elemental speciation with liquid chromatography–inductively coupled plasma isotope dilution mass spectrometry, J. Anal. Atom. Spectrom., 9/12 (1994) 1351–1355. DOI: 10.1039/JA9940901351