Isotope dilution analysis species-specific

P. Rodriguez-Gonzalez, M. Monperrus, J. I. Garcia Alonso, D. Amouroux and O. F. X. Donard, Comparison of different numerical approaches for multiple spiking species-specific isotope dilution analysis exemplified by the determination of butyltin species in sediments, J. Anal. At. Spectrom., 22, 2007, 1373-1382. 

Isotope ratio measurements were employed for the quantification of analytical data using the isotope dilution strategy. For example, isotope dilution analysis was developed by Sanz-Medel s group for the determination of selenomethionine in Se enriched yeast material by HPLC-ICP-MS using a Se-enriched selenomethionine spike obtained by growing yeast on a Se rich culture medium. For Cr(III)/Cr(VI) determination in yeast, Caruso et al employed the double spike species specific isotope dilution technique measured by HPLC-ICP-MS. The isotope pattern deconvolution approach applied in this work delivers a more intuitive and elegant solution to an otherwise complex data analysis without the need for iterative calculations as widely practised in double spike isotope dilution. The results are in exact agreement with the conventional isotope dilution calculations. ... 

With radioisotopes now available for many elements, the tracer technique became generally applicable. New variants were developed, such as neutron activation analysis, which was introduced in 1936 for the determination of dysprosium in rare-earth samples (Hevesy and Levi 1936) and subsequently became a widely used technique for sensitive trace analyses, particularly when much larger neutron fluxes became available with the advent of nuclear reactors. Another frequently applied method for trace determination is isotope dilution the species to be determined in the sample is diluted by addition of a known amount of the same species labeled with known specific activity. From the specific activity then resulting and measured, the original quantity of the species is derived, even if only a fi action of the species is finally recovered. The impact on biosciences was revolutionary, when suitable isotopes of key elements in the biosphere were soon discovered (Ti/2 = 10 min) was one of I. Curie and... 

The principle of isotope dilution analysis (IDA) [31], [47], [90]. [97], [98] involves measurement of the change in isotopic ratio when portions of a radiolabeled and nonlabeled form of the same chemical species are mixed. To perform a radioisotope IDA. an aliquot of a radioactive spike substance of known specific activity o,=A,/ni/ is... 

Clough, R., Belt, S.T., Fvans, E.H., and Catterick, T. (2003) Uncertainty contributions to species specific isotope dilution analysis. Part 2. Determination of mefhylmercury by HPLC coupled with quadrupole and multicollector ICP-MS. J. Anal. At. Spectrom., 18 (9), 1039-1046. 

The application of isotope dilution analysis to ICP-MS-based hyphenated techniques, for example HPLC-ICP-MS, can be divided into two diflerent modes the spedes-unspecific mode and species-specific mode. The two modes are illustrated in Figure 4.6. Because IDA-ICP-MS relies on the measurement of isotope ratios by mass spectrometry, the traditional interference to the accuracy and precision of IDA should all be considered and some biases need further correction in this case, such as spectral interferences, dead time of detector, mass discrimination, and statistic and stability of ion counting. 

Encinar et al. developed a method for the accurate determination of sele-noamino acids in human serum by species-specific isotope dilution analysis. A human serum was enzymatically digested, and then the selenoamino acid and carboxymethylated selenocysteine were separated and quantified by HPLC-ICP-MS. Quantification of selenomethionine was carried out by isotope dilution using a synthetic Se-labeled counterpart. The selenomethionine in samples was also measured by using selenomethionine as an internal standard. The instrumental detection limit was down to 75 fg Se and the precision was better than 5% RSD. ... 

Harrington et al. labeled a copper-containing protein rusticyanin (Rc) with Cu for use as a spike material in species-specific isotope dilution analysis, and... 

Later, Busto et al. used synthesized Fe-labeled transferrin to determine individual transferrin isoforms in human serum samples for the species-specific mode. The stability of the prepared proteins was tested for 1 week and no iron exchange had occurred. They concluded that the results are in good agreement with other calibration methods, e.g. the species-unspecific method however, the species-spedfic mode can offer better precision. Hoppler et al. also synthesized and characterized Fe-labeled Phaseolus vulgaris ferritin for isotope dilution analysis. " ... 

The development of species-specific isotope dilution analysis, i.e. using a spike with isotopically labelled species, has led to more precision and accuracy in the quantitative results, because with this technique, species loss, transformation or degradation during storage and sample preparation steps can be recognised and corrected for. Therefore, isotope dilution techniques for speciation are also discussed. 

The continuously aspirated aqueous standard of choice may also be used for simultaneous mass bias correction in species-specific isotope dilution analysis, e.g. by aspirating thallium... 

The application of isotopically labelled elemental compounds for species-specific isotope dilution analysis has greatly advanced during recent years. Species-specific isotope dilution has... 

The prerequisite for performing species-specific isotope dilution analysis is the availability of a pure species-specific standard, highly enriched in one element isotope, with known isotopic abundance and concentration. To date, only one isotopic MeHg standard is available a reference material provided by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium) under the name ERM-AE 670, certified for isotopic abundance and concentration. This reference material contains MeHg highly enriched in and comes as a 34.5 mg/L... 

Wahlen, R. and Wolff-Briche, C. (2003) Comparison of GC-ICP-MS and HPLC-ICP-MS for species-specific isotope dilution analysis of tributyltin in sediment after accelerated solvent extraction. AnaZ. Bioanal. Chem., 377,140. ... 

Mass spectrometry can be specific in certain cases, and would even allow on-line QA in the isotope dilution mode. MS of molecular ions is seldom used in speciation analysis. API-MS allows compound-specific information to be obtained. APCI-MS offers the unique possibility of having an element- and compound-specific detector. A drawback is the limited sensitivity of APCI-MS in the element-specific detection mode. This can be overcome by use of on-line sample enrichment, e.g. SPE-HPLC-MS. The capabilities of ESI-MS for metal speciation have been critically assessed [546], Use of ESI-MS in metal speciation is growing. Houk [547] has emphasised that neither ICP-MS (elemental information) nor ESI-MS (molecular information) alone are adequate for identification of unknown elemental species at trace levels in complex mixtures. Consequently, a plea was made for simultaneous use of these two types of ion source on the same liquid chromatographic effluent. 

In the last few years, we have seen the application of isotope dilution methodologies to some new analytical fields. One of these is elemental speciation , where the aim is to determine individual chemical species in which an element is distributed in a given sample. IDMS has also proved its usefulness in element speciation, in which either species-specific or species-unspecific spikes can be used. For example, species-specific IDMS is nowadays used in several laboratories as an effective tool to validate analytical procedures for speciation and to investigate and document eventual interconversion between species. In addition, the study of induced variations in the isotopic composition of a target element can also provide insight into various (bio)chemical and physical processes isotopic analysis is, therefore, also of increasing importance in biological studies. 

Uncontrolled species transformations during analysis form another source of error. For methylmercury determinations in sediments it was demonstrated that errors of up to 80% resulted from the formation of the compound from inorganic mercury during separation and analysis [28, 29], For the study of possible species transformations during analysis multiple isotope dilution could be used as a diagnostic tool for identifying the error and bias inherent in specific methods of storage, sample preparation and measurement [30, 31]. 

Isotopically labeled mono-, di-, and tributyltin have been obtained, allowing the use of species-specific isotope dilution techniques both in GC and LC separations. A comparative study revealed that the separation mode had no significant effect on mass fractions determined in the sediment sample however GC-ICP-MS analysis yielded about 2 orders of magnitude better absolute detection limit (0.03 pg Sn as tributyltin) with respect to reversed phase LC-ICP-MS. ... 

In addition, ICP-MS offers a unique characteristic of being able to provide isotopic information of an element. By utilizing isotope dilution (ID), GC/ICP-MS has been applied for accurate and precise determination of organotin and organomercury species. Not only is it possible to significantly improve the precision of determination, but it is also competent for studying transformation of species during analysis process by species-specific ID/GC/ICP-MS. ... 

Speciation Analysis Simultaneous Determination of Multiple Species of the Same Element At the beginning of the development of species-specific isotope dilution, only a single spike compound was used [77]. Later, species-spedfic IGP-IDMS has also been used for the determination of more than one spedes of the same element, using different spike spedes labeled with the same isotope. Examples include the determination of mono- and dimethylmercury (MMM and DMM) and of the three butyltin compounds TBT, DBT, and MBT [80-83]. Various spike isotopes, usually of medium natural abundance, such as Hg or ° Hg and Sn, Sn, or Sn, have been applied. The reason for selecting this type of spike... 

Chapters on sample introduction and hyphenated sample treatment and ICP systems have also been further updated since the last edition. No doubt that chromatographic, electrophoresis, flow injection and field flow fraction separations have extended ICP-MS (and AES) measurements as the mainstay of elanental specia-tion measurements in biological and environmental fields. Without the combination of these separation techniques and ICP measurements, elemental speciation applications would be severely hampered... if not impossible (Chapter 18). The ability to measure P and S with high sensitivity has opened up new opportunities in proteomics, for example. Species-specific and unspecific isotopic dilution (ID-MS) has been critical in quantifying speciation analysis and revealing recovery errors (Chapter 13). Species-specific techniques have been applied to identify species transformations, resulting in the development of multi-species methods whereas, hyphenated species-unspecific ICP-ID-MS determinations of heteroatoms such as sulfur have become a common quantification technique in proteomics. 

The main challenge when applying this method for protein analysis is the lack of commercial isotopically labeled proteins. Thus, most applications are focused on small molecules, such as organic mercury, organic tin and so on. However, there is increasing interest in the use of the ICP-MS linked system and species-specific isotope dilution for quantification of peptides or proteins due to the outstanding performance of ICP-MS. 

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