Tim Catterick

Selenium speciation analysis of selenium-enriched supplements by HPLC with ultrasonic nebulisation ICP-MS and electrospray MS/MS detection

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 electrospray ionisation (ESI)-MS/MS combined with RP-IP HPLC allowed characterisation of such materials in terms of species identification. Accelerated solvent extraction (ASE) was evaluated for extraction of the Se-compounds from the complex matrices in water. Alternatively, digestion with proteolytic enzymes was used for yeast protein hydrolysis. The use of ultrasonic nebulisation in combination with HPLC-ICP-(Q)MS for Se speciation led to detection limits up to six-fold lower (as low as ng l−1 levels) than those obtained with pneumatic nebulisation. Such enhancement of the ICP-MS capabilities for Se detection, combined with an improved separation using the newly developed RP-IP-HPLC method, allowed quantification of minor Se species such as SeMC in yeast extracts. Identification of the main compounds in yeast and SeleniumMC™ tablet hydrolysates as SeMet and SeMC, respectively, was accomplished by on-line RP-IP HPLC with ESI-MS/MS. Identification of SeMC in yeast digests, on the basis of retention time, molecular mass determination for the [M + H]+ 80Se ions (m/z 184) and detection of its product ions, is reported here for the first time. The mass spectral confirmation for SeMC in yeast is of interest, as this species is believed to be metabolised in animals and humans to methylselenol (CH3SeH), an anti-carcinogenic Se-metabolite.

Speciation of organo-tin compounds using liquid chromatography–atmospheric pressure ionisation mass spectrometry and liquid chromatography–inductively coupled plasma mass spectrometry as complementary techniques

Abstract A liquid chromatographic method for the determination of dibutyltin (DBT), tributyltin (TBT), diphenyltin (DPhT) and triphenyltin (TPhT) in sediments has been developed, which is compatible with both atmospheric pressure ionisation (API) mass spectrometry and inductively coupled plasma (ICP) mass spectrometry. As a result of this development both techniques may be used for the complementary speciation of organo-tin compounds. The chromatographic system comprises of a Kromasil-100 5 μm C18 (150×2.1 mm) column and a mobile phase of 0.05% triethylamine in acetonitrile–acetic acid–water (65:10:25), at a flow-rate of 0.2 ml min−1. The optimisation of the LC–API-MS conditions is discussed, together with the analysis of a real sediment sample for DBT and TBT using selected ion monitoring (SIM).

Development of a Multi-element Hydride Generation–Inductively Coupled Plasma Mass Spectrometry Procedure for the Simultaneous Determination of Arsenic, Antimony and Selenium in Waters

A multi-element hydride generation–inductively coupled plasma mass spectrometry (HG–ICP-MS) method for the simultaneous determination of arsenic, antimony and selenium in water matrices has been developed. The method involves an off-line pre-reduction procedure for the reduction of Se VI to Se IV by HCl, combined with an on-line reduction of As V and Sb V to the trivalent state with thiourea and generation of the hydrides. Analytical characteristics include detection limits of 0.08 ng g -1 As, 0.06 ng g -1 Sb and 0.10 ng g -1 Se, linearity of four orders of magnitude and short and long term reproducibility of between 8 and 12%. Results from four reference water samples for As, Sb and Se showed data which were all within 10% of the target values. Interferences were minimal for As and Sb, whereas Cu 2+ , and to a lesser extent Ni 2+ and Cd 2+ , caused signal suppression effects on Se. Advantages over an alternative, single element, HG–AFS technique include speed of analysis (by a factor of two) and elimination of the conflicting chemistry requirements, traditionally found with sequential single element hydride generation methods.

Structured approach to achieving high accuracy measurements with isotope dilution inductively coupled plasma mass spectrometry

This study investigated a method for simplifying the implementation of ID-ICP-MS, to achieve high accuracy measurements. The method developed is an adaptation of an earlier methodology that utilised an iterative ‘matching’ procedure. While retaining the many advantages of this approach, we have removed the iterative component and simplified the ‘matching’ step between the spiked mass bias calibrant and the spiked sample. Based on a conventional analysis, an approximate ‘match’ (usually within 5%), was made between the spiked reference standard and the spiked sample. This provides the basis for calculating the final result using the normal IDMS equation, while retaining many of the benefits of the full ‘matching’ procedure. These benefits negate many sources of error, such as mass bias, detector dead time and characterisation of the spike material. Many common errors and necessary corrections are negated or eliminated when using the new procedure. Examples are given of the method as applied to the analysis of certified reference materials, such as waters and plastics as well as blind trial data. The accuracy achieved using this procedure, on blind trial solutions, is typically to within 1% (relative to concentration) at the 95% level of confidence.

Problems Encountered During the Development of a Method for the Speciation of Mercury and Methylmercury by High-performance Liquid Chromatography Coupled to Inductively Coupled Plasma Mass Spectrometry

A study was carried out to develop a method for the determination of methylmercury and inorganic mercury by coupling HPLC to ICP-MS. During the course of this work it was noted that the use of stainless-steel components in the HPLC system led to adsorption of the mercury compounds, which was more pronounced with mercury(II) chloride than methylmercury. In particular, the use of a stainless steel sample loop led to larger peaks for both compounds, but particularly inorganic mercury, compared with a PEEK loop. The type of loop used had no effect on the separation of these compounds on an ODS column using a mobile phase composed of 10 mM tetrabutylammonium bromide and 60% methanol. However, both of the two ODS stainless-steel columns tested adsorbed the compounds to some degree, leading to a slow bleed of mercury. The use of components made with materials other than stainless steel, e.g. , PEEK sample loops and glass-lined columns, and inclusion of a sulfur ligand in the eluent (2-mercaptoethanol) reduced these problems significantly.

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

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 SPECIATION OF MERCURY AND ORGANOMERCURY COMPOUNDS BY HIGH PERFORMANCE LIQUID CHROMATOGRAPHY/ATMOSPHERIC PRESSURE IONIZATION MASS SPECTROMETRY

A method for the structural confirmation of mercury (II) chloride and organomercury (II) chlorides (monomethylmercury chloride, monoethylmercury chloride and monophenylmercury chloride) employing liquid chromatography coupled to atmospheric pressure ionization mass spectrometry was developed. The chromatographic system comprised a Kromasil 100-5C18, 5μm (250 × 4.6 mm) column and a mobile phase of 2-mercaptoethanol (0.01% v/v) in methanol: water (1:1 v/v), at a flow rate of 1 mL min−1. The optimization of the API-MS conditions is discussed, together with the fragmentation patterns observed for standards. No peaks due to the molecular ion were present and the most intense peaks were due to [M + mercaptoethanol]+ or [M + 2 mercaptoethanol − H]+. The qualitative analysis of fish tissue spiked with methylmercury (II) chloride, using selected ion monitoring (SIM), is discussed.

Orthodox uncertainty budgeting for high accuracy measurements by isotope dilution inductively coupled plasma-mass spectrometry

A full uncertainty budget for the analysis of copper in dilute nitric acid solution by isotope dilution ICP-MS has been formulated and described. The budget was dominated by the contribution of the standard uncertainty of the measured isotope amount ratios of the sample blend and the blend used for mass bias correction. The contribution from other components such as weighing, isotopic composition, standard solution concentration, and dilution factors were not very significant. The accuracy of the method was demonstrated by the analysis of NIST SRM 3114 copper solution.

Simultaneous determination of arsenic, antimony and selenium in aqueous matrices by electrothermal vaporization inductively coupled plasma mass spectrometry

A method for the simultaneous determination of As, Se and Sb in aqueous matrices by ETV–ICP-MS has been developed. Complex interactions between the analytes and various chemical modifiers are described. A mixed modifier of Pd(NO 3 ) 2 –Ni(NO 3 ) 2 was finally selected for quantitative multi-element analysis. Criteria of merit of the proposed method include a 0–100 ng g -1 linear range for As and Sb and 0.5–100 ng g -1 for Se. Internal standardisation using Te was used to help produce repeatability of 3.9% for As, 6.9% for Se and 2.1% for Sb at the 1 ng g -1 level. Detection limits of 0.03, 0.08 and 0.01 ng g -1 for As, Se and Sb, respectively, are routinely achievable. The accuracy of the proposed method was demonstrated with the analysis of several reference waters. A severe negative interference on Se by high HCl concentrations could be overcome with the use of a Pd(NO 3 ) 2 –Mg(NO 3 ) 2 –ascorbic acid chemical modifier.

Isotope Dilution ICP-MS for Speciation Studies

III. SPECIATION IN PRACTICE . . . . . . . . . . . . . . . . . . . . . . . 104 A. Sampling and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . 105 B. Sample Preparation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 C. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 D. Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107