Search: gas chromatography.

The method facilitates the determination of trace amounts of acrylamide monomers in aqueous matrices and is based upon the bromination of the acrylamide double bond. The reaction product (2,3-dibromopropionamide) is extracted from the mixture with ethyl acetate after precipitation with sodium sulfate. The extract is purified in a Florisil column and analyzed using gas chromatography (GC/ECD). The detection limit in aqueous matrices is approx. 0.032 µg/l.

Due to physico-chemical properties of these substances, a number of effective enrichment processes are available for analysis using gas chromatography and can be summarized as follows:

• pentane extraction

• adsorption onto solid materials using thermal desorption (purge and trap)

• headspace techniques

Pentane extraction
The sample is cooled with ice and extracted using chilled pentane. Subsequently, the pentane phase is separated with a micro separator.

Purge and Trap
The purge gas, as a rule, the carrier gas of the gas chromatograph, passes through the exhaust vessel filled with the water sample. Through stripping, the volatile substances are driven out and then accumulate on the sorbent, e.g., Tenax. After the stripping process is complete, the substances are thermally desorbed by rapidly heating the adsorber column. They are then conveyed to the gas chromatograph through a heated transfer tube.

Headspace techniques
The static headspace method is an ideal technique for the analysis of the volatile substances found in water, due to the simple sample preparation and the substantial sensitivity of the analysis. A further advantage of this procedure is that particulate matter as well as other substances present in the sample with a low volatility and high molecular weight do not interfere with the analysis, since they are not carried by the steam into the headspace and are therefore do not reach the separation system. Moreover, the high degree of automation combined with the aforementioned short time required for sample preparation allows for a rapid, precise and user-friendly analysis for water samples.

Gas chromatography
For the gas chromatographic analysis, an electron capture detector (ECD) is employed due to its high selectivity and high sensitivity. If the ECD is combined with a flame ionization detector (FID), methylene chloride, benzene and its homologues can also be analyzed. With the aid of cryo-focusing, this method can be adapted to detect more volatile substances, such as vinyl chloride or chlorofluorocarbons.

Refer to W.010.03.150 - Trihalogenmethane (THM) in Trinkwasser

Volatile aroma compounds are driven out of the sample through steam distillation. The ethanolic distillate is saturated with NaCl. Potassium hydrogen sulfite is added to separate carbonyl groups that might interfere with the analysis. The extraction of the aroma compounds is performed by shaking out with dichloromethane and the phases separated by centrifuging.

Volatile aroma compounds are driven out of the sample through steam distillation. The ethanol distillate is adjusted to be alkaline and saturated with NaCl. The extraction of the aroma compounds is performed by shaking out with dichloromethane and the phases separated by centrifuging. The organic phase is further concentrated in a stream of nitrogen gas. An ammonia solution is added to remove the acids, because the acids would co-elute, thus preventing quantification of the target substances.

The sample freed of the dissolved carbon dioxide through shaking and filtration of the liquid.