K. W. M. Siu

Electrospray mass spectrometry: a study on some aqueous solutions of metal salts.

Aqueous metal salt solutions were used as models to probe the origin of the species observed in the electrospray mass spectrum. A qualitative or semiquantitative correlation among different species was observed between electrospray responses and calculated equilibrium aqueous solution concentrations. Quantitative correlations were obtained, however, when ions that were identical in charge and similar in type were selected for comparison. In these experiments the ions experienced very similar electrospray-related processes and their effects on the responses were canceled in a comparison of these ions. Consequently, the relative abundances of these ions in the electrospray mass spectrum closely matched the calculated relative abundances in aqueous solution. Our results suggest that the basic principle that determines ionic distribution in the electro spray mass spectrum in aqueous solution chemistry.

Determination of arsenic species by high-performance liquid chromatography-inductively coupled plasma mass spectrometry

Arsenic(III), AsV, monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA) and arsenobetaine (AsB) were separated by high-performance liquid chromatography (HPLC) and determined on-line by inductively coupled plasma mass spectrometry (ICP-MS). Two forms of HPLC were used: ion pairing and ion exchange, with absolute detection limits for arsenic ranging between 50 and 300 pg. These detection limits were independent of the arsenic species when peak area was used for quantification. Anion pairing was found generally to be more sensitive to changes in the matrix of the sample injected. Anion exchange was more tolerant because of the higher buffering capacity of the mobile phase. Cation pairing was found suitable for the determination of DMAA and AsB in a biological sample containing high concentrations of salts.

Are the electrospray mass spectra of proteins related to their aqueous solution chemistry

Institute for Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada The shape of the profile described by the relative abundances of multiply charged ions of proteins in the electrospray mass spectrum can be described by means of one or more Gaussian functions. An aqueous solution equilibrium model of the distribution of multiply charged ions of equine cytochrome c and myoglobin has been shown to match qualitatively the shape of the distribution of these ions in an electrospray mass spectrum. Monotonic functions such as the quadrupole mass spectrometric transmission efficiency may alter the centroid of the profile, but the shape of the ion abundance pattern appears to be controlled by the aqueous solution chemistry of the proteins. NRCC No. 32938.

Applications of ICP-MS in marine analytical chemistry

SummaryThe versatility of ICP-MS in marine analytical chemistry is illustrated with applications to the multielement trace analysis of two recently released marine reference materials, the coastal seawater CASS-2 and the non-defatted lobster hepatopancreas tissue LUTS-1, and to the determination of tributyltin and dibutyltin in the harbour sediment reference material PACS-1 by HPLC-ICP-MS. Seawater analyses were performed after separation of the trace elements either by adsorption on immobilized 8-hydroxy-quinoline or by reductive coprecipitation with iron and palladium. Simultaneous determination of seven trace elements in LUTS-1, including mercury, by isotope dilution ICP-MS, was achieved after dissolution by microwave digestion with nitric acid and hydrogen peroxide. Butyltin species in PACS-1 were separated by cation exchange HPLC of an extract of the sediment; method detection limits for tributyltin and dibutyltin in sediment samples are estimated to be 5 ng Sn/g and 12 ng Sn/g, respectively.

Combined Ion Mobility/Time-of-Flight Mass Spectrometry Study of Electrospray-Generated Ions

A commercially available ion mobility spectrometer was interfaced to a custom-built linear time-of-flight (TOF) mass spectrometer for the purpose of examining electrospray-generated plumes. Ionic species that were separated in the ion mobility spectrometer could be selectively determined with the TOF mass spectrometer. Tetraalkylammonium salts, a peptide, and proteins were examined. Their ion mobility spectra typically comprised a few peaks; some of these mobility-resolved species produced characteristic electrospray ions, while others of lower relative mobility did not. The TOF mass spectra of cytochrome c, injected from the ion mobility spectrometer at an indicated temperature of 90 °C or lower, showed signs that were characteristic of protein-solvent clustering.

Extraction of butyltin species and their gas chromatographic determination as chlorides in a sediment certified reference material for trace metals, PACS-1☆

Abstract A gas chromatographic method has been developed for the determination of butyltin species in sediments. The butyltin species are separated as chlorides by using a DB-608 open tubular column after their extraction from the sediment using a combination of sonication in methanolic HCl and solvent extraction. Two extractants are possible: toluene—isobutyl acetate—tropolone and hexane—isobutyl acetate. The efficiencies for the first extractant are: tributyltin, 94.4 ± 4.7%; dibutyltin, 94.9 ± 2.2%; and monobutyltin, 86.3 ± 4.2%. The absolute detection limits are about 30 pg tin. Using a 1-g sample, the relative detection limits are about 30 ng tin per g sediment. These may be lowered to 3 ng tin per g by starting with a 4-g sample and adding a concentration step. The reference material PACS-1 was found to contain 1.08 ± 0.31 μg tin per g of tributyltin and 1.13 ± 0.30 μg tin per g of dibutyltin.

Electrospray interfacing for the coupling of ion-exchange and ion-paring chromatography to mass spectrometry☆

Abstract Electrospray was evaluated as an interface for coupling ion-exchange and ion-pairing chromatography to mass spectrometry. Effective separation and good sensitivity were demonstrated for a liquid flowrate as high as 50 μl/min and a matrix ion concentration as high as 10 mM. Minimum detectable amounts were estimated to be five to ten times poorer than those obtained in the absence of ion-exchange buffers or ion-pairing reagents. The minimum detectable amount for arsenobetaine after a dynamic ion-exchange separation was estimated to be about 20 pg.

Physico-chemical characterization of metal binding proteins using HPLC-ICP-MS, HPLC-MA-AAS and electrospray-MS

A size exclusion HPLC method was developed and interfaced with ICP-MS detection for determining the metal profiles of commercially available rabbit liver metallothioneins (MT) and metallothionein-like proteins (MLP) extracted from fresh water mussels and hemolyzed osprey blood. The redox state of the cysteine residues was indirectly evaluated via a cadmium saturation approach in the presence or absence of a reducing agent, followed by HPLC-microatomization (MA)-AAS and HPLC-ICP-MS analyses. An electrospray-MS protocol was also developed to accurately measure the molecular weight of rabbit MT isoform II. Nanogram quantities of Cd-MT/MLP were poorly chromatographed on silica based supports. A copolymeric styrene-divinylbenzene size exclusion support provided a symmetrical peak (rabbit MT standard) and linear HPLC-MA-AAS calibration curves [r=0.9988; from the LOD (27 ng, as protein) to about 300×LOD], indicating negligible losses of Cd during the chromatography of trace quantities. Co-injection of Cd2+ saturated samples with beta-mercaptoethanol (BMSH) was essential to repress Cd2+-support interactions which otherwise induced an undesirable metalaffinity retention mechanism. In the presence of added Cd2+, 22 mmol/L BMSH did not significantly compete for Cd2+ specifically bound to MT, while preventing non-specific binding to non-thiolic complexing sites. Crude mussel and osprey blood MLP extracts (in cold, deoxygenated Tris-HCl buffer) were obtained by ultracentrifugation (145,000 g) and thermocoagulation/centrifugation, respectively. Incubation with BMSH was prerequisite to obtain a maximum saturation of mussel and osprey blood MLP by Cd2+, even for samples conserved (−80° C) in the presence of BMSH (22 mmol/L). These observations indicated that a major proportion of the cysteine residues present in these MLP were oxidized. The assumption of a fully reduced MT/MLP pool binding metals in a definite stoichiometry has been the basis of several quantitative metal binding assays involving the saturation of the thiolic complexing sites with a metallic marker (Ag+, Cd2+, or Hg2+). Since thiolic agents may interfere, the metal saturation protocols do not include a reducing step to ensure that all cysteines in a MT/MLP extract are available for co-ordination. Given that variations in the redox state of crude MT/MLP extracts may compromise the accuracy of metal saturation assays, it is concluded that the preparation of reference samples certified for total metallothionein content would be desirable.

Marine biological reference materials for methylmercury: analytical methodologies used in certification

SummaryThe methylmercury concentrations in three existing marine biological certified reference materials — TORT-1, DORM-1 and DOLT-1 — are determined by gas chromatography with electron capture detection, cold vapour atomic absorption spectrometry and inductively coupled plasma mass spectrometry after selective isolation of methylmercury. Two such procedures were used. These and the three analytical techniques are evaluated and compared. The certified methylmercury concentrations are: TORT-1, 0.128 ± 0.014; DORM-1, 0.731±0.060; and DOLT-1, 0.080 ± 0.011 Μg Hg/g dry weight.

Electrospray mass spectrometry of some proteins and the aqueous solution acid/base equilibrium model in the negative ion detection mode

Abstract Basic solutions of myoglobin, β-lactoglobulin, pepsin and ubiquitin have been examined by means of electrospray mass spectrometry in the negative ion detection mode. The distribution of protein ions in the mass spectra was found to correlate well with the distribution of protein species in solution calculated from published titration data. These results lend further credibility to an earlier proposed aqueous solution acid/base equilibrium model, which relates the “bellshape” ion distribution observed in the electrospray mass spectrometry of proteins to the distribution of protein ions in solution.