Evaluation of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the analysis of organoarsenic compounds of environmental interest

Laser ablation based bioimaging with simultaneous elemental and molecular mass spectrometry: towards spatially resolved speciation analysis

RATIONALE

Biological functions of metals are not only specified by the element itself, but also by its chemical form and by its organ, cell and subcellular location. The developed laser ablation based setup enables spatially resolved analysis with simultaneous elemental and molecular mass spectrometry (MS) and promises therefore localization, identification and quantification of metal or heteroelement-containing species in biological samples such as tissue sections.

METHODS

A UV laser ablation (LA) system is hyphenated in parallel both with an elemental and a molecular mass spectrometer via flow splitted transfer lines to simultaneously obtain data from both of the mass spectrometers. Elemental MS was performed using inductively coupled plasma (ICP)-MS, whereas atmospheric pressure chemical ionization (APCI)-MS with an orbitrap mass analyzer was utilized for molecular MS.

RESULTS

Simultaneous elemental and molecular MS imaging with high lateral resolution down to 25 µm was presented for the staining agents eosin Y and haematoxylin as well as for the chemotherapy drug cisplatin in thin tissue sections. For molecular MS, target compounds were identified by their exact masses and by characteristic fragment ions.

CONCLUSIONS

The first simultaneous elemental and molecular MS imaging approach based on laser ablation sampling was introduced for spatially resolved speciation analysis. The combination of the advantages of LA-ICP-MS such as low detection limits and high spatial resolution with information on the chemical identity promises not only localization of metals, but also identification of metal species in biological samples. Therefore, this novel technique opens up new possibilities to address complex challenges in life science research.

High-performance liquid chromatography-inductively coupled plasma mass spectrometry based method for the determination of organic arsenic feed additives and speciation of anionic arsenics in animal feed

A novel method has been developed to detect two organic arsenic animal feed additives including roxarsone and p-arsanilic acid, as well as other arsenic species such as arsenite, dimethylarsinic acid, monomethylarsonic acid, arsenate, and 4-hydroxyphenylarsonic acid, by using high-performance liquid chromatography coupled to an inductively coupled plasma mass spectrometer (HPLC-ICP-MS). The influence of the type and concentrations of ion-pairing reagents on the separation efficiency of the different arsenic compounds was examined. The effects of the mobile phase pH on the retention of arsenic species on the chromatography column were studied. When a gradient elution procedure was used, the best separation of the seven arsenic species could be achieved in <20 min with a mobile phase consisting of 8% methanol and 92% aqueous tetrabutylammonium hydroxide (4 mM, pH 6.25) followed by 92% trifluoroacetic acid aqueous solution (0.1%, pH 2.0). Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used as an assistant tool to screen arsenobetain (AsB) in the feed samples by monitoring the reaction at m/z 179-->120. The extractions of arsenic compounds from formula feed samples were studied, and results showed that the extraction with methanol/water (1:1) mixture yielded the most efficient percent compound recovery and the fastest extraction time for all arsenic species. Under optimum conditions, the limits of detection were <1.7 microg of As kg(-1), and the recoveries of all seven arsenic species were >78.5% with the relative standard deviation of <10%. The ion-pair reversed phase HPLC-ICP-MS method was then successfully applied to the speciation of arsenic in feedstuff and formula feed samples.

Environmental proteomics and metallomics

Monitoring environmental pollution using biomarkers requires detailed knowledge about the markers, and many only allow a partial assessment of pollution. New proteomic methods (environmental proteomics) can identify proteins that, after validation, might be useful as alternative biomarkers, although this approach also has its limitations, derived mainly from their application to non-model organisms. Initial studies using environmental proteomics were carried out in animals exposed to model pollutants, and led to the concept of protein expression signatures. Experiments have been carried out in model organisms (yeast, Arabidopsis, rat cells, or mice) exposed to model contaminants. Over the last few years, proteomics has been applied to organisms from ecosystems with different pollution levels, forming the basis of an environmental branch in proteomics. Another focus is connected with the presence of metals bound to biomolecules, which adds an additional dimension to metal-biomolecule and metalloprotein characterization - the field of metallomics. The metallomic approach considers the metallome: a whole individual metal or metalloid species within a cell or tissue. A metallomic analytical approach (MAA) is proposed as a new tool to study and identify metalloproteins.

Microbial methylation of metalloids: arsenic, antimony, and bismuth

A significant 19th century public health problem was that the inhabitants of many houses containing wallpaper decorated with green arsenical pigments experienced illness and death. The problem was caused by certain fungi that grew in the presence of inorganic arsenic to form a toxic, garlic-odored gas. The garlic odor was actually put to use in a very delicate microbiological test for arsenic. In 1933, the gas was shown to be trimethylarsine. It was not until 1971 that arsenic methylation by bacteria was demonstrated. Further research in biomethylation has been facilitated by the development of delicate techniques for the determination of arsenic species. As described in this review, many microorganisms (bacteria, fungi, and yeasts) and animals are now known to biomethylate arsenic, forming both volatile (e.g., methylarsines) and nonvolatile (e.g., methylarsonic acid and dimethylarsinic acid) compounds. The enzymatic mechanisms for this biomethylation are discussed. The microbial conversion of sodium arsenate to trimethylarsine proceeds by alternate reduction and methylation steps, with S-adenosylmethionine as the usual methyl donor. Thiols have important roles in the reductions. In anaerobic bacteria, methylcobalamin may be the donor. The other metalloid elements of the periodic table group 15, antimony and bismuth, also undergo biomethylation to some extent. Trimethylstibine formation by microorganisms is now well established, but this process apparently does not occur in animals. Formation of trimethylbismuth by microorganisms has been reported in a few cases. Microbial methylation plays important roles in the biogeochemical cycling of these metalloid elements and possibly in their detoxification. The wheel has come full circle, and public health considerations are again important.