Academic literature on the topic 'Flash column chromatography'

"freezing out, adsorption and absorption. After concentrating, separation is achieved by classical methods such as gas chranatography (GC) or high pressure liquid chranatography (HPLC). Identification is based mainly on mass spectrometry, infra-red spectrometry and chrcmatographic data. 3. RESULTS The primary goal of these methods is to concentrate all volatile com­ pounds, mainly volatile organic compounds or VOCs, present. This mixture of VOCs, containing odorous ccmpcunds, next to a large majority of unodo-rous substance, then is analysed. This chemical analysis is based on the separation of these hundreds of compounds by gas chranatography, is hampe­ red by large amounts of water, which is always present in air, and which is also freezed out or adsorbed. The only way to escape more or less this difficulty is to use a rather apolar adsorbant, in casu Tenax GC or similar materials (e.g. Chranosorb 102) (5). A second limitation is the fact that no material will ever be capable of adsorbing all odorous com­ pounds completely, and permit to desorb then afterwards completely. For compounds with very low boiling point, e.g. hydrogen sulphide, strong ad-sorbants are necessary, while for odorants with high boiling point, e.g. skatol or the sesquiterpenes, thermal desorption is difficult with strong adsorbant s. So a compromise has to be accepted, or several complementa­ ry adsorbants have to be used. At this moment this compromise for concen­ trating all odorous substances is found in the adsorbant mentioned, kno­ wing that the most volatile compounds might escape partly. Many systems have been described and even carenercialised, but we use a home-built sy­ stem, which is schematically represented in figure 1 (6). On an outer side wall of the gas chromatograph (GC) an oven in which the Tenax-adsorp-tion-sampling tubes fit is constructed. Connections with pressurized he­ lium (transfer gas) is provided and their is a connection with a high tem­ perature resistant sixway valve, which replaces the normal GC-injector. During thermal desorption (position 1 in figure 1) the transfer gas, car­ rying desorbed volatiles, passes the sixway valve, a cold trap (stainless steel loop cold with liquid air) and enters the ambient air. The helium carrier gas is connected to the GC-column via the sixway valve. After the desorption stage which usually takes about 45 minutes, with a desorption oven temperature of 220°C for 30 minutes at least, the sixway valve is switched (position 2 in figure 1). At that moment transfer gas flows through the sixway valve directly into the ambient whereas the carrier gas passes the cold trap before entering the GC-column. The liquid air is removed from the cold trap and the latter is quickly heated by a high in­ tensity fload light. In this way condensed compounds are flash-evaporated and injected into the GC-system. Concentrating odorants by adsorption-desorption techniques produces a terribly complex mixture of VOCs, which is separated by gas chranato­ graphy. Fortunately this technique allows formidable separation power, but still then the result is not always sufficient far a clear-cut odour analysis. In figure 2 the GC-analysis is shown of an air sample in the neighbourhood of a rendering plant, showing a great number of VOCs; however almost all of them are hydrocarbons produced by cars and heating systems and sane other products, which do not contribute to the odour. Very small peaks of odorants are detected, which shows the difficult task of odour ana­ lysis with a general concentrating technique. Of course this analysis is far more relevant if emission gases are examined as is demonstrated in fi­ gure 3 (7). Part of these difficulties can be overcane if the odorants can." In Odour Prevention and Control of Organic Sludge and Livestock Farming. CRC Press, 1986. http://dx.doi.org/10.1201/9781482286311-75.