Search: dimethyl sulfide

DMS and the precursors of DMS (DMSP) are obtained from a finely ground malt sample by means of cold extraction. This aqueous extract is then boiled under alkaline conditions  which converts the DMS precursors (DMSP) to free DMS. The DMS is determined using gas chromatography by means of the headspace technique.

Free DMS is determined in wort and beer with a gas chromatograph using the headspace method. Calibration is performed at various concentrations within the relevant range. The relative peaks corresponding to the concentrations are evaluated.

A (modified) Congress wort is produced from malt samples prior to analysis. NDMA present in the Congress wort is extracted using dichloromethane followed by concentration of the eluate. The determination is performed with a gas chromatograph using a packed Carbowax 20M column with a specific TEA detector (thermal energy analyzer); nitrosodipropylamine (NDPA) serves as an internal standard.

This detector analyzes nitrosamine according to the following procedure:

After exiting the GC column, the separated substances are heated to 500 °C in a pyrolyzer. At this temperature, the N-NO bond of the nitrosamine is broken, thus forming an NO radical (NO۰): 

The gas mixture then flows through a special filter (CTR^TM gas stream filter), which only allows the carrier gas and the NO radicals to pass. After exiting the filter, the NO radicals flow into a reaction chamber along with ozone, which is created by a special generator. The following chemical reactions take place in the chamber: 

These NO radicals react with ozone, forming nitrogen dioxide in an excited state (NO₂•). The NO₂• molecules decompose spontaneously to form nitrogen dioxide in its common form (NO₂), emitting radiation (h•ν) with a wavelength of approx. 600 nm. 

Dimethyl sulfide precursors (DMSP) in the wort are converted to dimethyl sulfide (DMS) by heating under aqueous, alkaline conditions. The DMS is determined using gas chromatography by means of the headspace technique.

The NDMA is extracted from the wort (plant wort) and beer on Extrelut®, Tox Elut® or comparable material using dichloromethane and the eluate is then concentrated. The determination is carried out by gas chromatography with the specific TEA detector ("Thermal Energy Analyzer"). Nitrosodipropylamine (NDPA) or nitrosodiisopropylamine (NDiPA) is used as the internal standard (ISTD). This detector detects nitrosamines according to the following scheme: After exiting the GC column, the separated substances first enter a pyrolysis oven, where they are heated to around 500 °C. The detector is used as an internal standard (ISTD). At this temperature, the (N-NO) bond of the nitrosamines breaks down, forming an NO radical (NO^-):
 

The gas mixture then passes through a special filter (CTR Gas Stream Filter), which only allows the carrier gas and NO radicals to pass through. The NO radicals and ozone produced by a special generator then flow into a reaction chamber, where the following reaction takes place:

These NO radicals react with ozone to form nitrogen dioxide in an excited state (NO₂^-) and oxygen. The NO2- decomposes spontaneously into ordinary nitrogen dioxide (NO₂) by emitting radiant energy (h-ν) at a wavelength of around 600 nm.

In an acidic sulfurous solution containing phosphorus, nitrate ions react with 2,6-dimethylphenol to produce 4-nitro-2,6-dimethylphenol.

The method is suitable for determining approx. 0.5–25 mg/l NO₃^-. If the ratio of chloride to nitrate ions is larger than 10, this may interfere with the reaction (if this is the case, according to DEV D 9-3, follow DIN 38405); likewise, nitrite ions exceeding approx. 0.2 mg/l may also interfere with the reaction. For their removal, refer to Remarks below.