Current mass spectrometry strategies for selenium speciation in dietary sources of high-selenium
Analytical and Bioanalytical Chemistry, Apr 20, 2005
This document reviews the most relevant mass spectrometry approaches to selenium (Se) speciation ... more This document reviews the most relevant mass spectrometry approaches to selenium (Se) speciation in high-Se food supplements in terms of qualitative and quantitative Se speciation and Se-containing species identification, with special reference to high-Se yeast, garlic, onions and Brazil nuts. Important topics such as complexity of Se speciation in these materials and the importance of combining Se-specific detection and molecule-specific determination of the particular species of this element in parallel with chromatography, to understand their nutritional role and cancer preventive properties are critically discussed throughout. The versatility and potential of mass spectrometric detection in this field are clearly demonstrated. Although great advances have been achieved, further developments are required, especially if "speciated"certified reference materials (CRMs) are to be produced for validation of measurements of target Se-containing species in Se-food supplements.
The selection of an appropriate isotope dilution mass spectrometry technique for the practical ap... more The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes.
high background signal in the analyses due to the use of different antimony species in the produc... more high background signal in the analyses due to the use of different antimony species in the production of organic compounds like phthalic or 4-hydroxy benzoic acid. More data of the distribution and concentration range of antimony species in the environment -soil, water, biomasshave to be collected. There is evidence for methylated antimony compounds in nature. There has to be focused on. An important field must be the view of the solubility and the bioavailability of the compounds. Therefore it is necessary to develop a variety of different methods for the determination. Especially the sample preparation and extraction techniques have to be optimized.
Human serum is routinely measured for total calcium content in clinical studies. A definitive hig... more Human serum is routinely measured for total calcium content in clinical studies. A definitive high-accuracy and low-uncertainty method is required for reference measurements to underpin medical diagnoses. This study presents a novel octopole collision cell ICP-MS, high-accuracy, methodology and comparison of that technique with double-focusing sector field ICP-MS and an ICP-OES method. Double-matched isotope dilution mass spectrometry (IDMS) was employed for ICP-MS techniques and an exact matching bracketing technique using scandium as an internal standard was used for ICP-OES analysis. Medium resolution mode was utilised for double-focusing sector field ICP-MS analysis to resolve the dominant interferences on the 44 Ca/ 42 Ca isotope pair. Hydrogen reaction gas was employed to chemically resolve a number of polyatomic interferences predominantly through charge transfer reactions in the octopole collision cell. Comparison data presented for NIST CRM 909b human serum analysis from all three techniques demonstrates highest accuracy (99.6%) and lowest uncertainty (1.1%) for octopole collision cell ICP-MS. Data from ICP-OES using a non-IDMS technique produces comparably accurate data and low-uncertainties. The much higher total expanded uncertainties for double-focusing sector field ICP-MS compared with octopole collision cell data are explained by lower precision on the measurement of the 44 Ca/ 42 Ca isotope ratio. Data for octopole collision cell ICP-MS submitted for an international blind trial comparison (CCQM K-14) demonstrated excellent agreement with the mean of all participants with a low expanded uncertainty.
Spectrochimica Acta Part B: Atomic Spectroscopy, 2006
High accuracy procedures for the determination of arsenic are needed for assigning reference valu... more High accuracy procedures for the determination of arsenic are needed for assigning reference values to certified reference materials (CRMs). There are a number of problems associated with the determination of total arsenic by inductively coupled plasma-mass spectrometry. Arsenobetaine (AsB) (the major species in fish) gives an enhanced response (9%) when compared to inorganic arsenic(V) and is very difficult to decompose. Chloride causes interference at m/z 75 by the formation of ArCl + and chloride levels can be significant in marine fish. Also residual carbon in digests can lead to enhancement of arsenic signals by charge transfer effects. This can easily lead to erroneous quantification when compared to standards that do not contain carbon. This newly developed procedure overcomes these issues by complete mineralisation of the matrix leaving insignificant amounts of residual carbon and by removal of chlorine by evaporation. A detection limit of 30 ng/g was achieved. Recoveries for the following CRMs: DORM 2 100.1 ± 4.3%, SRM1548 96.1 ± 4.6%, BCR 422 103.6 ± 6.2% and SRM2976 105.9 ± 6.2% were obtained. The digestion procedure uses open vessel wet digestion with a mixture of sulfuric and nitric acid held at 300°C. The decomposition of AsB was confirmed by speciation analysis of the digest. Carbon (m/z 13) was monitored to demonstrate the efficiency of the digestion. Instrumentation for the reduction of ArCl + interference was not required and this view is supported by the recovery data. Measurements were performed by external calibration using tellurium as an internal standard.
Isotope ratio mass spectrometry (IRMS) and multicollector inductively coupled plasma mass spectro... more Isotope ratio mass spectrometry (IRMS) and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) are highly important techniques that can provide forensic evidence that otherwise would not be available. MC-ICP-MS has proved to be a very powerful tool for measuring high precision and accuracy isotope amount ratios. In this work, the potential of combining isotope amount ratio measurements performed by MC-ICP-MS and IRMS for the detection of counterfeit pharmaceutical tablets has been investigated. An extensive study for the antiviral drug Heptodin™ has been performed for several isotopic ratios combining MC-ICP-MS and an elemental analyser EA-IRMS for stable isotope amount ratio measurements. The study has been carried out for 139 batches of the antiviral drug and analyses have been performed for C, S, N and Mg isotope ratios. Authenticity ranges have been obtained for each isotopic system and combined to generate a unique multiisotopic pattern only present in the genuine tablets. Counterfeit tablets have then been identified as those tablets with an isotopic fingerprint outside the genuine isotopic range. The combination of those two techniques has therefore great potential for pharmaceutical counterfeit detection. A much greater power of discrimination is obtained when at least three isotopic systems are combined. The data from these studies could be presented as evidence in court and therefore methods need to be validated to support their credibility. It is also crucial to be able to produce uncertainty values associated to the isotope amount ratio measurements so that significant differences can be identified and the genuineness of a sample can be assessed.
Simultaneous identification of selenium-containing glutathione species in selenised yeast by on-line HPLC with ICP-MS and electrospray ionisation quadrupole time of flight (QTOF)-MS/MS
J. Anal. At. Spectrom., 2006
ABSTRACT A method based on the on-line combination of ion-pair reversed-phase HPLC with ICP-MS an... more ABSTRACT A method based on the on-line combination of ion-pair reversed-phase HPLC with ICP-MS and ESI-Q-TOFMS/MS was proposed for the identification of a number of Se-containing compounds in selenised yeast aqueous extracts without any pre-treatment or fractionation of the water extract. The on-line combination of reversed phase HPLC using a gradient of up to 40% methanol in 0.1% formic acid with ICP-MS was used for detection of multiple Se-containing species within a single chromatographic run of the water extract. The methanol load to the plasma was reduced two-fold by using post-column effluent dilution with ultra-pure water. This, along with a careful optimisation of the ICP parameters, allowed concentrations of up to 40% methanol in the mobile phase to be well tolerated by the ICP system with no significant changes in sensitivity or occurrence of plasma instability. Direct analysis of the water extracts by HPLC with on-line ESI-TOFMS allowed the accurate molecular mass determination of a range of Se compounds detected by HPLC-ICP-MS, including previously reported (m/z 198, 433 and 563) and unknown Se species. This was achieved thanks to the relatively high selectivity and compatibility of the chromatographic method with ESI-TOFMS. The accurate molecular mass data obtained by HPLC-ESI-Q-TOFMS/MS measurements of the [M + H]+80Se and [M + H]+78Se ions and their associated product ions allowed structural characterisation of the compounds. Two of these selenium compounds with m/z 402 and 661 for the [M + H]+80Se ions, identified as methyl Se-glutathione and a Se-glutathione species with a S–Se bridge between glutathione and Se-cysteinyl-gamma-glutamylglycine (GS-SeG) in selenised yeast, are reported here for the first time. Their empirical formulae were confirmed by the experimental data (measured accurate mass) with an error from calculated exact mass (of the derived empirical formulae) lower than 5 parts per million (ppm) for most of the ions detected. The presence of both of these Se compounds in two further Se-yeast supplements was confirmed by the same methodology.
Selenium speciation analysis of selenium-enriched supplements by HPLC with ultrasonic nebulisation ICP-MS and electrospray MS/MS detection
J. Anal. At. Spectrom., 2004
Size-exclusion, anion-exchange and reversed-phase ion-pair (RP-IP) HPLC were used in combination ... more Size-exclusion, anion-exchange and reversed-phase ion-pair (RP-IP) HPLC were used in combination with ICP-(Q)MS for selenium (Se)-specific detection and quantitation of Se-compounds in extracts of Se-yeast and Se-methylselenocysteine (SeMC) based supplements. On-line ...
High-accuracy IDMS analysis of trace elements in wheat flour for the provision of reference values to a proficiency testing scheme
Journal of Analytical Atomic Spectrometry, 2005
... Richard B. Myors*a, Annette L. Nolana, Shane Askewa, David L. Saxbya, Ruth Hearnb and Lindsey... more ... Richard B. Myors*a, Annette L. Nolana, Shane Askewa, David L. Saxbya, Ruth Hearnb and Lindsey G. Mackaya. a ... values and has been applied previously by national measurement institutes to provide reference values for similar reference materials, such as rice flour.3 In the ...
Identification of water-soluble gamma-glutamyl-Se-methylselenocysteine in yeast-based selenium supplements by reversed-phase HPLC with ICP-MS and electrospray tandem MS detection
Journal of Analytical Atomic Spectrometry, 2005
ABSTRACT Identification of gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC) in water-solu... more ABSTRACT Identification of gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC) in water-soluble yeast fractions was accomplished by on-line reversed-phase (RP) HPLC with ICP-MS and electrospray ionisation (ESI) tandem MS. The sample was leached with water using accelerated solvent extraction (ASE) and the aqueous extracts were directly analysed using on-line RP HPLC-ICP-MS. HPLC was carried out with 0.1% formic acid in methanol–water (2 + 98, v/v) solution. Se-specific detection of the chromatographic effluent by ICP-MS allowed identification of γ-glutamyl-SeMC on the basis of comparison of retention times with a matching standard. Aqueous extracts of three different Se–yeast supplements (1291, 1550 and 1983 μg g−1 Se in the dry sample) were analysed by HPLC-ICP-MS for their γ-glutamyl-SeMC content (as Se). A significant variation of the Se speciation in the water extracts appeared to occur with an increase in the total Se concentration from 1550 to 1983 μg g−1. Apparently, the contribution of water-soluble SeMC and selenomethionine (SeMet) increased whereas the contribution of Se incorporated into γ-glutamyl-SeMC decreased with the increasing total Se content. Verification of the presence of γ-glutamyl-SeMC in the water-soluble yeast extract, on the basis of molecular mass determination for the [M + H]+ 80Se ions (m/z 313) and detection of its product ions using on-line RP HPLC-ESI-MS/MS, is reported here for the first time. This was achieved without the need for a cleanup of the aqueous yeast extract. The presence of γ-glutamyl-SeMC might be relevant to the anticarcinogenic potential of selenised yeast since this species is believed to serve primarily as a carrier of SeMC, which appears to be easily converted in animals and possibly humans to methylselenol. This Se metabolite is thought to be an effective anticarcinogen.
The high-accuracy analysis of lead in wine by exact-matching isotope dilution mass spectrometry using ICP-MS
Journal of Analytical Atomic Spectrometry, 2005
ABSTRACT An isotope dilution technique was developed for the analysis of Pb in wine using an exac... more ABSTRACT An isotope dilution technique was developed for the analysis of Pb in wine using an exact-matching isotope dilution mass spectrometry (IDMS) approach. A metrological approach was used to examine potential biases and ensure that this potentially primary method of analysis provided appropriate accuracy, and an expanded uncertainty at the 95% confidence level of <2% was achieved. The accuracy of this method was demonstrated in an international CCQM intercomparison (CCQM-P12) involving other national metrology institutes. The result submitted for this intercomparison was in very good agreement with results from the majority of other laboratories, all of whom used IDMS.
Development of a high ratio isotope dilution mass spectrometry calibration technique for chemical metrology
Journal of Analytical Atomic Spectrometry, 2005
IDMS with calibration by “exact” or “approximate signal matching” methods offers a number of adva... more IDMS with calibration by “exact” or “approximate signal matching” methods offers a number of advantages for chemical metrology applications where measurements of low uncertainty are required for analytes at low concentrations in complex matrices. These methods have been applied extensively but are unsuitable for some important applications because in many cases, the concentration of the spike isotope in each measured blend of a sample or standard should be as close as possible to the concentration of the natural analyte isotope. In this paper we discuss recent work investigating the extent to which it is possible to move away from this ideal 1∶1 blend isotope ratio whilst retaining the key benefits of the approximate matching technique. This has been used for the first time for applications where the sample has a very high analyte concentration, which would require unacceptably high amounts of spike with the existing method. The methodology has been validated for sulfur using a well-characterised candidate matrix reference material – NIST SRM1624d. This is a diesel fuel sample that contains approximately 4000 μg g−1 sulfur. Results were obtained using a ratio for 32S/34S of 14, achieving accuracy and uncertainty comparable with the original method using a blend isotope ratio of 1.
Investigation of the selenium species distribution in a human B-cell lymphoma line by HPLC- and GC-ICP-MS in combination with HPLC-ESIMS/MS and GC-TOFMS after incubation with methylseleninic acid
J. Anal. At. Spectrom., 2007
ABSTRACT A methodology using the on-line combination of micro-HPLC with ICP-MS and electrospray i... more ABSTRACT A methodology using the on-line combination of micro-HPLC with ICP-MS and electrospray ionisation (ESI) MS/MS, GC-ICP-MS and GC-TOFMS is proposed for investigation of the Se species distribution in lysates and headspace of human lymphoma cell lines after incubation with methylseleninic acid (MSA), which is known as a precursor of methylselenol, a metabolite with anti-cancer properties. The HPLC-ICP-MS results showed significant differences in speciation results of control (untreated) and Se-exposed DHL-4 cells, revealing the presence of three Se compounds in the cell extracts after 10 min incubation with MSA. These compounds were preliminarily identified as Se-methylselenocysteine (SeMC), selenomethionine (SeMet) and gamma-glutamyl-methylselenocysteine (γ-glutamyl-SeMC) by HPLC-ICP-MS on the basis of retention matching with standards. Quantification of Se-compounds in cell lysates was performed by reversed-phase HPLC coupled directly to ICP-MS. Calibration was by the standard additions technique, using peak area measurements of the chromatographic signals by monitoring the 82Se signal. Verification of the presence of SeMC, the major Se compound in the cell extracts, was achieved by on-line HPLC-ESIMS/MS in selected reaction monitoring (SRM) mode. Total selenium concentrations in cell lysates were determined by flow injection analysis coupled with ICP-MS. The capabilities of solid phase micro-extraction (SPME)-GC in combination with ICP-MS and GC-TOFMS were investigated for studying the distribution of volatile Se species in the headspace of the lymphoma DHL-4 cell cultures. A cryogenic oven cooling (COC)-GC system, which led to a significant improvement in the chromatographic performance (retention time, peak shape and consequently, chromatographic selectivity) of compounds with a low boiling point such as dimethylselenide (CH3SeCH3) in comparison with conventional GC was employed for the first time. A number of volatile Se species could be detected by GC-ICP-MS, from which dimethylselenide and dimethyldiselenide (CH3SeSeCH3) could be identified by COC-GC-TOFMS in the headspace of DHL-4 cell lines exposed to methylseleninic acid.
The efficiency of enzymatic hydrolysis and leaching with water using accelerated solvent extracti... more The efficiency of enzymatic hydrolysis and leaching with water using accelerated solvent extraction (ASE) or boiling was investigated for quantitative Se speciation in selenized potatoes using reversed phase HPLC coupled to ICP-MS. Preliminary identification of selenomethionine (SeMet), Semethylselenocysteine (SeMeCys), and selenate in extracts of potato skin and flesh was achieved using complementary reversed phase and anion-exchange HPLC-ICP-MS and retention time matching with standards. The quantitative speciation data revealed a higher percentage of selenomethionine (73% of the total Se) found in the flesh in comparison with skin (containing 21% of the total Se as SeMet). ASE and boiling in water were found to be similar in terms of Se extraction efficiency and profiles. However, ASE was found to be more efficient than boiling with respect to sample cleanup and reduced sample handling. The presence of SeMet at parts per billion levels in selenized potatoes was confirmed by reversed phase HPLC with online ESI MS/MS.
Systematic comparison of delta34S measurements by multicollector inductively coupled plasma mass spectrometry and evaluation of full uncertainty budget using two different metrological approaches
Rapid communications in mass spectrometry : RCM, 2008
A systematic comparison of delta34S measurements by multicollector inductively coupled plasma mas... more A systematic comparison of delta34S measurements by multicollector inductively coupled plasma mass spectrometry (MCICPMS) using two different standardisation approaches and two different reference materials as working standards is reported for the first time. Full uncertainty budgets have been calculated using the Kragten approach. Internal standardisation by measuring Si ratios and external standardisation by bracketing samples with either NIST RM8553 or NIST RM8554 have been compared to correct for the effects of mass bias. The delta34S value and the associated uncertainty ( per thousand) were found to be slightly different when using different approaches. Corrected 34S:32S ratios for NIST RM8553 and NIST RM8554 differed from those previously reported by 0.1 to 0.5% when using external standardisation. 34S:32S ratios for NIST RM8553 and NIST RM8554 differed from those previously reported by 0.06 to 0.15% when using Si internal standardisation. This indicated that in order to minim...
Journal of agricultural and food chemistry, Jan 14, 2010
In this work, a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) was eval... more In this work, a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) was evaluated for the direct measurement of sulfur stable isotope ratios in beers as a first step toward a general study of the natural isotope variability of sulfur in foods and beverages. Sample preparation consisted of a simple dilution of the beers with 1% (v/v) HNO(3). It was observed that different sulfur isotope ratios were obtained for different dilutions of the same sample indicating that matrix effects affected differently the transmission of the sulfur ions at masses 32, 33, and 34 in the mass spectrometer. Correction for mass bias related matrix effects was evaluated using silicon internal standardization. For that purpose, silicon isotopes at masses 29 and 30 were included in the sulfur cup configuration and the natural silicon content in beers used for internal mass bias correction. It was observed that matrix effects on differential ion transmission could be corrected adequately u...
Current mass spectrometry strategies for selenium speciation in dietary sources of high-selenium
Analytical and Bioanalytical Chemistry, Apr 20, 2005
This document reviews the most relevant mass spectrometry approaches to selenium (Se) speciation ... more This document reviews the most relevant mass spectrometry approaches to selenium (Se) speciation in high-Se food supplements in terms of qualitative and quantitative Se speciation and Se-containing species identification, with special reference to high-Se yeast, garlic, onions and Brazil nuts. Important topics such as complexity of Se speciation in these materials and the importance of combining Se-specific detection and molecule-specific determination of the particular species of this element in parallel with chromatography, to understand their nutritional role and cancer preventive properties are critically discussed throughout. The versatility and potential of mass spectrometric detection in this field are clearly demonstrated. Although great advances have been achieved, further developments are required, especially if "speciated"certified reference materials (CRMs) are to be produced for validation of measurements of target Se-containing species in Se-food supplements.
The selection of an appropriate isotope dilution mass spectrometry technique for the practical ap... more The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes.
high background signal in the analyses due to the use of different antimony species in the produc... more high background signal in the analyses due to the use of different antimony species in the production of organic compounds like phthalic or 4-hydroxy benzoic acid. More data of the distribution and concentration range of antimony species in the environment -soil, water, biomasshave to be collected. There is evidence for methylated antimony compounds in nature. There has to be focused on. An important field must be the view of the solubility and the bioavailability of the compounds. Therefore it is necessary to develop a variety of different methods for the determination. Especially the sample preparation and extraction techniques have to be optimized.
Human serum is routinely measured for total calcium content in clinical studies. A definitive hig... more Human serum is routinely measured for total calcium content in clinical studies. A definitive high-accuracy and low-uncertainty method is required for reference measurements to underpin medical diagnoses. This study presents a novel octopole collision cell ICP-MS, high-accuracy, methodology and comparison of that technique with double-focusing sector field ICP-MS and an ICP-OES method. Double-matched isotope dilution mass spectrometry (IDMS) was employed for ICP-MS techniques and an exact matching bracketing technique using scandium as an internal standard was used for ICP-OES analysis. Medium resolution mode was utilised for double-focusing sector field ICP-MS analysis to resolve the dominant interferences on the 44 Ca/ 42 Ca isotope pair. Hydrogen reaction gas was employed to chemically resolve a number of polyatomic interferences predominantly through charge transfer reactions in the octopole collision cell. Comparison data presented for NIST CRM 909b human serum analysis from all three techniques demonstrates highest accuracy (99.6%) and lowest uncertainty (1.1%) for octopole collision cell ICP-MS. Data from ICP-OES using a non-IDMS technique produces comparably accurate data and low-uncertainties. The much higher total expanded uncertainties for double-focusing sector field ICP-MS compared with octopole collision cell data are explained by lower precision on the measurement of the 44 Ca/ 42 Ca isotope ratio. Data for octopole collision cell ICP-MS submitted for an international blind trial comparison (CCQM K-14) demonstrated excellent agreement with the mean of all participants with a low expanded uncertainty.
Spectrochimica Acta Part B: Atomic Spectroscopy, 2006
High accuracy procedures for the determination of arsenic are needed for assigning reference valu... more High accuracy procedures for the determination of arsenic are needed for assigning reference values to certified reference materials (CRMs). There are a number of problems associated with the determination of total arsenic by inductively coupled plasma-mass spectrometry. Arsenobetaine (AsB) (the major species in fish) gives an enhanced response (9%) when compared to inorganic arsenic(V) and is very difficult to decompose. Chloride causes interference at m/z 75 by the formation of ArCl + and chloride levels can be significant in marine fish. Also residual carbon in digests can lead to enhancement of arsenic signals by charge transfer effects. This can easily lead to erroneous quantification when compared to standards that do not contain carbon. This newly developed procedure overcomes these issues by complete mineralisation of the matrix leaving insignificant amounts of residual carbon and by removal of chlorine by evaporation. A detection limit of 30 ng/g was achieved. Recoveries for the following CRMs: DORM 2 100.1 ± 4.3%, SRM1548 96.1 ± 4.6%, BCR 422 103.6 ± 6.2% and SRM2976 105.9 ± 6.2% were obtained. The digestion procedure uses open vessel wet digestion with a mixture of sulfuric and nitric acid held at 300°C. The decomposition of AsB was confirmed by speciation analysis of the digest. Carbon (m/z 13) was monitored to demonstrate the efficiency of the digestion. Instrumentation for the reduction of ArCl + interference was not required and this view is supported by the recovery data. Measurements were performed by external calibration using tellurium as an internal standard.
Isotope ratio mass spectrometry (IRMS) and multicollector inductively coupled plasma mass spectro... more Isotope ratio mass spectrometry (IRMS) and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) are highly important techniques that can provide forensic evidence that otherwise would not be available. MC-ICP-MS has proved to be a very powerful tool for measuring high precision and accuracy isotope amount ratios. In this work, the potential of combining isotope amount ratio measurements performed by MC-ICP-MS and IRMS for the detection of counterfeit pharmaceutical tablets has been investigated. An extensive study for the antiviral drug Heptodin™ has been performed for several isotopic ratios combining MC-ICP-MS and an elemental analyser EA-IRMS for stable isotope amount ratio measurements. The study has been carried out for 139 batches of the antiviral drug and analyses have been performed for C, S, N and Mg isotope ratios. Authenticity ranges have been obtained for each isotopic system and combined to generate a unique multiisotopic pattern only present in the genuine tablets. Counterfeit tablets have then been identified as those tablets with an isotopic fingerprint outside the genuine isotopic range. The combination of those two techniques has therefore great potential for pharmaceutical counterfeit detection. A much greater power of discrimination is obtained when at least three isotopic systems are combined. The data from these studies could be presented as evidence in court and therefore methods need to be validated to support their credibility. It is also crucial to be able to produce uncertainty values associated to the isotope amount ratio measurements so that significant differences can be identified and the genuineness of a sample can be assessed.
Simultaneous identification of selenium-containing glutathione species in selenised yeast by on-line HPLC with ICP-MS and electrospray ionisation quadrupole time of flight (QTOF)-MS/MS
J. Anal. At. Spectrom., 2006
ABSTRACT A method based on the on-line combination of ion-pair reversed-phase HPLC with ICP-MS an... more ABSTRACT A method based on the on-line combination of ion-pair reversed-phase HPLC with ICP-MS and ESI-Q-TOFMS/MS was proposed for the identification of a number of Se-containing compounds in selenised yeast aqueous extracts without any pre-treatment or fractionation of the water extract. The on-line combination of reversed phase HPLC using a gradient of up to 40% methanol in 0.1% formic acid with ICP-MS was used for detection of multiple Se-containing species within a single chromatographic run of the water extract. The methanol load to the plasma was reduced two-fold by using post-column effluent dilution with ultra-pure water. This, along with a careful optimisation of the ICP parameters, allowed concentrations of up to 40% methanol in the mobile phase to be well tolerated by the ICP system with no significant changes in sensitivity or occurrence of plasma instability. Direct analysis of the water extracts by HPLC with on-line ESI-TOFMS allowed the accurate molecular mass determination of a range of Se compounds detected by HPLC-ICP-MS, including previously reported (m/z 198, 433 and 563) and unknown Se species. This was achieved thanks to the relatively high selectivity and compatibility of the chromatographic method with ESI-TOFMS. The accurate molecular mass data obtained by HPLC-ESI-Q-TOFMS/MS measurements of the [M + H]+80Se and [M + H]+78Se ions and their associated product ions allowed structural characterisation of the compounds. Two of these selenium compounds with m/z 402 and 661 for the [M + H]+80Se ions, identified as methyl Se-glutathione and a Se-glutathione species with a S–Se bridge between glutathione and Se-cysteinyl-gamma-glutamylglycine (GS-SeG) in selenised yeast, are reported here for the first time. Their empirical formulae were confirmed by the experimental data (measured accurate mass) with an error from calculated exact mass (of the derived empirical formulae) lower than 5 parts per million (ppm) for most of the ions detected. The presence of both of these Se compounds in two further Se-yeast supplements was confirmed by the same methodology.
Selenium speciation analysis of selenium-enriched supplements by HPLC with ultrasonic nebulisation ICP-MS and electrospray MS/MS detection
J. Anal. At. Spectrom., 2004
Size-exclusion, anion-exchange and reversed-phase ion-pair (RP-IP) HPLC were used in combination ... more Size-exclusion, anion-exchange and reversed-phase ion-pair (RP-IP) HPLC were used in combination with ICP-(Q)MS for selenium (Se)-specific detection and quantitation of Se-compounds in extracts of Se-yeast and Se-methylselenocysteine (SeMC) based supplements. On-line ...
High-accuracy IDMS analysis of trace elements in wheat flour for the provision of reference values to a proficiency testing scheme
Journal of Analytical Atomic Spectrometry, 2005
... Richard B. Myors*a, Annette L. Nolana, Shane Askewa, David L. Saxbya, Ruth Hearnb and Lindsey... more ... Richard B. Myors*a, Annette L. Nolana, Shane Askewa, David L. Saxbya, Ruth Hearnb and Lindsey G. Mackaya. a ... values and has been applied previously by national measurement institutes to provide reference values for similar reference materials, such as rice flour.3 In the ...
Identification of water-soluble gamma-glutamyl-Se-methylselenocysteine in yeast-based selenium supplements by reversed-phase HPLC with ICP-MS and electrospray tandem MS detection
Journal of Analytical Atomic Spectrometry, 2005
ABSTRACT Identification of gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC) in water-solu... more ABSTRACT Identification of gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC) in water-soluble yeast fractions was accomplished by on-line reversed-phase (RP) HPLC with ICP-MS and electrospray ionisation (ESI) tandem MS. The sample was leached with water using accelerated solvent extraction (ASE) and the aqueous extracts were directly analysed using on-line RP HPLC-ICP-MS. HPLC was carried out with 0.1% formic acid in methanol–water (2 + 98, v/v) solution. Se-specific detection of the chromatographic effluent by ICP-MS allowed identification of γ-glutamyl-SeMC on the basis of comparison of retention times with a matching standard. Aqueous extracts of three different Se–yeast supplements (1291, 1550 and 1983 μg g−1 Se in the dry sample) were analysed by HPLC-ICP-MS for their γ-glutamyl-SeMC content (as Se). A significant variation of the Se speciation in the water extracts appeared to occur with an increase in the total Se concentration from 1550 to 1983 μg g−1. Apparently, the contribution of water-soluble SeMC and selenomethionine (SeMet) increased whereas the contribution of Se incorporated into γ-glutamyl-SeMC decreased with the increasing total Se content. Verification of the presence of γ-glutamyl-SeMC in the water-soluble yeast extract, on the basis of molecular mass determination for the [M + H]+ 80Se ions (m/z 313) and detection of its product ions using on-line RP HPLC-ESI-MS/MS, is reported here for the first time. This was achieved without the need for a cleanup of the aqueous yeast extract. The presence of γ-glutamyl-SeMC might be relevant to the anticarcinogenic potential of selenised yeast since this species is believed to serve primarily as a carrier of SeMC, which appears to be easily converted in animals and possibly humans to methylselenol. This Se metabolite is thought to be an effective anticarcinogen.
The high-accuracy analysis of lead in wine by exact-matching isotope dilution mass spectrometry using ICP-MS
Journal of Analytical Atomic Spectrometry, 2005
ABSTRACT An isotope dilution technique was developed for the analysis of Pb in wine using an exac... more ABSTRACT An isotope dilution technique was developed for the analysis of Pb in wine using an exact-matching isotope dilution mass spectrometry (IDMS) approach. A metrological approach was used to examine potential biases and ensure that this potentially primary method of analysis provided appropriate accuracy, and an expanded uncertainty at the 95% confidence level of <2% was achieved. The accuracy of this method was demonstrated in an international CCQM intercomparison (CCQM-P12) involving other national metrology institutes. The result submitted for this intercomparison was in very good agreement with results from the majority of other laboratories, all of whom used IDMS.
Development of a high ratio isotope dilution mass spectrometry calibration technique for chemical metrology
Journal of Analytical Atomic Spectrometry, 2005
IDMS with calibration by “exact” or “approximate signal matching” methods offers a number of adva... more IDMS with calibration by “exact” or “approximate signal matching” methods offers a number of advantages for chemical metrology applications where measurements of low uncertainty are required for analytes at low concentrations in complex matrices. These methods have been applied extensively but are unsuitable for some important applications because in many cases, the concentration of the spike isotope in each measured blend of a sample or standard should be as close as possible to the concentration of the natural analyte isotope. In this paper we discuss recent work investigating the extent to which it is possible to move away from this ideal 1∶1 blend isotope ratio whilst retaining the key benefits of the approximate matching technique. This has been used for the first time for applications where the sample has a very high analyte concentration, which would require unacceptably high amounts of spike with the existing method. The methodology has been validated for sulfur using a well-characterised candidate matrix reference material – NIST SRM1624d. This is a diesel fuel sample that contains approximately 4000 μg g−1 sulfur. Results were obtained using a ratio for 32S/34S of 14, achieving accuracy and uncertainty comparable with the original method using a blend isotope ratio of 1.
Investigation of the selenium species distribution in a human B-cell lymphoma line by HPLC- and GC-ICP-MS in combination with HPLC-ESIMS/MS and GC-TOFMS after incubation with methylseleninic acid
J. Anal. At. Spectrom., 2007
ABSTRACT A methodology using the on-line combination of micro-HPLC with ICP-MS and electrospray i... more ABSTRACT A methodology using the on-line combination of micro-HPLC with ICP-MS and electrospray ionisation (ESI) MS/MS, GC-ICP-MS and GC-TOFMS is proposed for investigation of the Se species distribution in lysates and headspace of human lymphoma cell lines after incubation with methylseleninic acid (MSA), which is known as a precursor of methylselenol, a metabolite with anti-cancer properties. The HPLC-ICP-MS results showed significant differences in speciation results of control (untreated) and Se-exposed DHL-4 cells, revealing the presence of three Se compounds in the cell extracts after 10 min incubation with MSA. These compounds were preliminarily identified as Se-methylselenocysteine (SeMC), selenomethionine (SeMet) and gamma-glutamyl-methylselenocysteine (γ-glutamyl-SeMC) by HPLC-ICP-MS on the basis of retention matching with standards. Quantification of Se-compounds in cell lysates was performed by reversed-phase HPLC coupled directly to ICP-MS. Calibration was by the standard additions technique, using peak area measurements of the chromatographic signals by monitoring the 82Se signal. Verification of the presence of SeMC, the major Se compound in the cell extracts, was achieved by on-line HPLC-ESIMS/MS in selected reaction monitoring (SRM) mode. Total selenium concentrations in cell lysates were determined by flow injection analysis coupled with ICP-MS. The capabilities of solid phase micro-extraction (SPME)-GC in combination with ICP-MS and GC-TOFMS were investigated for studying the distribution of volatile Se species in the headspace of the lymphoma DHL-4 cell cultures. A cryogenic oven cooling (COC)-GC system, which led to a significant improvement in the chromatographic performance (retention time, peak shape and consequently, chromatographic selectivity) of compounds with a low boiling point such as dimethylselenide (CH3SeCH3) in comparison with conventional GC was employed for the first time. A number of volatile Se species could be detected by GC-ICP-MS, from which dimethylselenide and dimethyldiselenide (CH3SeSeCH3) could be identified by COC-GC-TOFMS in the headspace of DHL-4 cell lines exposed to methylseleninic acid.
The efficiency of enzymatic hydrolysis and leaching with water using accelerated solvent extracti... more The efficiency of enzymatic hydrolysis and leaching with water using accelerated solvent extraction (ASE) or boiling was investigated for quantitative Se speciation in selenized potatoes using reversed phase HPLC coupled to ICP-MS. Preliminary identification of selenomethionine (SeMet), Semethylselenocysteine (SeMeCys), and selenate in extracts of potato skin and flesh was achieved using complementary reversed phase and anion-exchange HPLC-ICP-MS and retention time matching with standards. The quantitative speciation data revealed a higher percentage of selenomethionine (73% of the total Se) found in the flesh in comparison with skin (containing 21% of the total Se as SeMet). ASE and boiling in water were found to be similar in terms of Se extraction efficiency and profiles. However, ASE was found to be more efficient than boiling with respect to sample cleanup and reduced sample handling. The presence of SeMet at parts per billion levels in selenized potatoes was confirmed by reversed phase HPLC with online ESI MS/MS.
Systematic comparison of delta34S measurements by multicollector inductively coupled plasma mass spectrometry and evaluation of full uncertainty budget using two different metrological approaches
Rapid communications in mass spectrometry : RCM, 2008
A systematic comparison of delta34S measurements by multicollector inductively coupled plasma mas... more A systematic comparison of delta34S measurements by multicollector inductively coupled plasma mass spectrometry (MCICPMS) using two different standardisation approaches and two different reference materials as working standards is reported for the first time. Full uncertainty budgets have been calculated using the Kragten approach. Internal standardisation by measuring Si ratios and external standardisation by bracketing samples with either NIST RM8553 or NIST RM8554 have been compared to correct for the effects of mass bias. The delta34S value and the associated uncertainty ( per thousand) were found to be slightly different when using different approaches. Corrected 34S:32S ratios for NIST RM8553 and NIST RM8554 differed from those previously reported by 0.1 to 0.5% when using external standardisation. 34S:32S ratios for NIST RM8553 and NIST RM8554 differed from those previously reported by 0.06 to 0.15% when using Si internal standardisation. This indicated that in order to minim...
Journal of agricultural and food chemistry, Jan 14, 2010
In this work, a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) was eval... more In this work, a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) was evaluated for the direct measurement of sulfur stable isotope ratios in beers as a first step toward a general study of the natural isotope variability of sulfur in foods and beverages. Sample preparation consisted of a simple dilution of the beers with 1% (v/v) HNO(3). It was observed that different sulfur isotope ratios were obtained for different dilutions of the same sample indicating that matrix effects affected differently the transmission of the sulfur ions at masses 32, 33, and 34 in the mass spectrometer. Correction for mass bias related matrix effects was evaluated using silicon internal standardization. For that purpose, silicon isotopes at masses 29 and 30 were included in the sulfur cup configuration and the natural silicon content in beers used for internal mass bias correction. It was observed that matrix effects on differential ion transmission could be corrected adequately u...
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Papers by Ruth Hearn