The overall goal of this procedure is to synthesize, purify and isolate stable dimethyl monothiol arsenic acid and dimethyl dithioarsenic acid for use as standards in quantitative speciation analysis of thioarsenicals. This experimental guideline can help address key issues in quantitative thioarsenical analysis such as unreliability caused by errors in critical steps of the synthesis of thioarsenical standards. The main advantage of this modified technique is that it produces stable chemical standards that can be used to quantify thioarsenical content in samples.
To begin the DMMTA synthesis in a fume hood prepare solutions of dimethylmonothioarsinic acid, sodium sulfide and sulfuric acid in deionized water purged for 30 minutes with nitrogen gas. Add the DMA solution to the sodium sulfide solution. Rinse the DMA solution container with 10 milliliters of N2 purged water and add the rinse to the mixture.
Note the time at which the solutions were combined. Connect a three hole rubber stopper fitted with glass tubes to a nitrogen gas line. Stopper the flask and immediately begin flowing nitrogen gas through the flask ensuring that the solution does not splash under the flow then use acid resistant tubing to connect a 50 milliliter syringe containing the sulfuric acid solution to another of the glass tubes.
Slowly add the sulfuric acid solution, four to five milliliters at a time drop wise to obtain a cloudy white solution. Once the sulfuric acid has been added and one hour has elapsed since the DMA and sodium sulfide solutions were initially combined transfer the reaction mixture to separatory funnel containing 200 milliliters of diethyl ether. Shake the funnel for five to 10 minutes while regularly venting the funnel.
Collect the aqueous layer in a beaker and then collect the organic layer containing the DMMTA in a separate bottle. Extract the DMMTA from the aqueous layer with 200 milliliter portions of diethyl ether three more times then wash the combined ether layers with 100 milliliters of N2 purge deionized water. Collect the washed ethyl layer in a glass crystallizing dish and transfer the solution to a nitrogen filled glovebox.
Allow the solvent to evaporate under a flowing nitrogen atmosphere to obtain DMMTA as a white crystalline precipitate. Measure and record the mass of the crystallized DMMTA then dissolve the solid in 50 milliliters of N2 purged deionized water. Filter the solution through a 0.2 micron syringe filter and store the solution at four degrees celsius in the dark.
To begin the DMDTA synthesis prepare solutions of DMA, sodium sulfide and sulfuric acid in deionized water. Combine the the DMA and sodium sulfide solutions in a 250 milliliter flask. Rinse the container of DMA solution with 10 milliliters of deionized water and add the rinse to the flask.
Slowly add the sulfuric acid solution to the mixture four to five milliliters at a time drop wise. Monitor the appearance of the mixture as it becomes white to yellow and cloudy. Allow the mixture to sit uncovered overnight.
Then, pass the reaction mixture through C18 silicon based solid phase extraction cartridge to trap the DMDTA on the resin. Elute the DMDTA into a glass crystallizing dish with about one liter of a 10 millimolar pH 6.3 ammonium acetate solution. Transfer the eluate to a nitrogen filled glovebox.
Allow the solvent to evaporate under a flowing nitrogen atmosphere to obtain crystallized DMDTA as a white precipitate. Measure and record the mass of the crystallized DMDTA. Dissolve the solid in 50 milliliters of deionized water and filter the solution through a 0.2 micron syringe filter.
Store the solution at four degrees celsius in the absence of light. Electrospray ionization mass spectrometry was used to verify the successful synthesis of DMMTA and DMDTA from DMA and to confirm that dimethylarsinic acid was not the major product. HPLCICPMS also showed that DMA starting material had been consumed and that DMMTA and DMDTA had been produced.
After being stored at four degrees Celsius in the absence of light for 13 weeks, the DMMTA and DMDTA solutions showed only a 2.2 and a 5.8 percent change in the species distributions respectively. Before attempting this procedure remember to read the reference material and safety data. It is important to understand the mechanism of DMMTA and DMDTA synthesis when preforming this experiment.
