Separation techniques volatile organic compound

Historically, measurements have classified ambient hydrocarbons in two classes methane (CH4) and all other nonmethane volatile organic compounds (NMVOCs). Analyzing hydrocarbons in the atmosphere involves a three-step process collection, separation, and quantification. Collection involves obtaining an aliquot of air, e.g., with an evacuated canister. The principal separation process is gas chromatography (GC), and the principal quantification technique is wdth a calibrated flame ionization detector (FID). Mass spectroscopy (MS) is used along with GC to identify individual hydrocarbon compounds. 

It is the determination of volatile organic compounds produced from natural products that requires separation techniques that allow isolation of stereoisomers. The most commonly determined groups are the terpene and sesquiterpene species present in essential oils, which are used as key indicators of biological factors such as the growth season, geographic location, climate, etc. These species are also released directly into the atmosphere by very many plants and trees, and make a substantial contribution to global biogeochemical cycles. 

Distillation is a suitable technique for the isolation of volatile organic compounds from liquid samples or the soluble portion of solid samples [24,27-30]. The physical basis of separation depends on the distribution of constituents between the liquid mixture and the vapor in equilibrium with that mixture. The more volatile constituents are concentrated in the vapor phase, which is collected after condensation. The effectiveness of the separation is dependent on the physical properties of the... 

Chemical analysis of odorants in ambient air is hampered by the presence of a plethora of volatile organic compounds, which do not contribute to the odour. Nevertheless application of either powerful separation and identification techniques, such as the GC-MS combination, or specific GC-detection or absorption procedures allow qualitative and quantitative determination of odourants. Improvements are under way to achieve the sensitivity necessary for relevant immission concentrations, which go down to 0.1 ppb for some odorants. 

MDGC, and comprehensive two-dimensional GC, or GCxGC), faster separation techniques (fast GG), fast methods for quality assessment or process control in the flavour area ( electronic noses and fingerprinting MS) and on-line time-resolved methods for analysis of volatile organic compounds (VOGs) such as proton-transfer reaction MS (PTR-MS) and resonance-enhanced multi-photon ionisation coupled with time-of-flight MS (REMPI-TOFMS). The scope of this contribution does not allow for lengthy discussions on all available techniques therefore, only a selection of developments will be described. 

Purge-and-trap techniques in which volatile analytes are evolved from blood or urine in a gas stream and collected on a trap for subsequent chromatographic analysis have been developed. Such a technique employing gas chromatographic separation and Fourier transform infrared detection has been described for a number of volatile organic compounds in blood.6... 

Gas chromatography (GC) is a chromatographic technique that is used to separate volatile organic compounds. A gas chromatograph consists of a mobile (gas) phase, an injection port, a separation column containing the stationary phase, and a detector. The... 

Separation of ethylene, benzene, propanol, olefin, aromatic amines from organic liquid mixtures, of volatile organic compounds (VOC) and phenol from wastewater, were investigated (Table 5.11), using a rotating film module, spiral-type FLM, hoUow-fiber and layered LM techniques. High separation factors (>1000) in pUot- and industrial-scale experiments were found. 

The TVOC (Total Volatile Organic Compounds) guideline valne for the complex mixture is defined as the sum of the individual compounds separated and quantified by a gas-chromatographic technique (Seifert, 1990). 

This static trapping technique could be an excellent reconnaissance technique for many volatile organic compounds. In this study, the ion counts for the PCE were proportional to the surface fluxes from the plume contamination, falling off sharply at the plume edges. An attractive feature of the technique is the short analysis time — a few minutes per sample. However, as pointed out by the authors, the technique is limited in some instances by the lack of separation of the adsorbed components prior to the measurement by mass spectrometry. Apparently, the overlapping ions from the mixture of components can make some components unidentifiable in the... 

Gas Chromatography, The widely used technique of GLC for separation of volatile organic or volatile derivatives of nonvolatile compounds is capable of considerable sensitivity, but lacks specificity. Mass spectrometry (see Ligon, 1979) has added specificity to this chromatographic technique as well as sensitivities in the attomole (10 ) range. (See Figure 2) (Hunt and Crow, 1978). ... 

LC is a suitable technique for trace analysis of organic micropollutants. However, the separation power of common LC columns is insufficient for very complex mixtures. Online couphng of LC with MS will be an attractive tool for analysis of low-volatility organic compounds in samples. Howeveg cleanup of the sample prior to LC-MS analysis will be very necessary. The sensitivity is less than that obtained by capillary GC-MS combinations and at present there is no universal LC-MS system for all types of compounds. 

Steam distillation (Secs. 2.16 and 4.5) is a good technique for separating volatile organic compounds from nonvolatile organic and inorganic substances. However, the reaction mixture is acidic, and it must be made basic prior to performing the steam distillation so aniline is present as the free base, and the 4-aminophenol is converted to its water-soluble sodium phenoxide salt. The aniline and nitrobenzene are then removed from the reaction mixture by steam distillation. The nonvolatile salt of 4-aminophenol and the nonvolatile benzidine remain in the aqueous phase. 

A new technique, which is applicable for sampling in air and liquids or in the headspace above a liquid or a solid sample, is solid-phase micro-extraction (SPME). The mechanism of SPME, which has been developed by Pawliszyn et al. 1225], [226], is based on the partition equilibrium of the analytes between the sample or the head-space above the sample, respectively, and a fused silica fiber coated with a suitable stationary phase. The amount of analyte extracted by the fiber is proportional to the initial analyte concentration in the sample and depends on the type of fiber. After sampling, the fiber can be thermally desorbed directly into the injector of a gas chromatograph. SPME combines sampling, analyte enrichment, matrix separation, and sample introduction within one step [226]. Since its development, this innovative technique has found widespread use in environmental analysis. It has, for example, been applied in the determination of volatile organic compounds [227], 228]. phenols [229],... 

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