Purge and trap devices

To satisfy the Resource Conservation and Recovery Act (1977) and its amendment for hazardous and solid waste (1984), the 80(K) Series Methods have been designed to analyze solid waste, soUs, and groundwater. In particular, methods 8240/8260 require the use of a purge-and-trap device in conjunction with packed or capillary GC/MS, respectively, for the analysis of purgeable organic compounds. Methods 8250/8270 concern analyses for the less-volatile bases, neutrals, and acids by GC/MS after extraction from the matrix by an organic solvent. 

Target compounds are specified for each Series Method. Volatile compounds that need to be analyzed can be extracted from the matrix by a purge-and-trap device. 

Bianchi AP, Varney MS, Phillips J. 1991. Analysis of industrial solvent mixtures in water using a miniature purge-and-trap device with thermal desorption and capillary gas chromatography-mass spectrometry. J Chromatogr 557(l-2) 429-439. 

Because of the differences in the construction of various purge and trap devices, actual recoveries may vary significantly from those shown in Figure 3 and Table I. Therefore it is required that individual investigators determine recoveries of compounds to be measured as a function of flow rate with their apparatus. Operation in the optimum flow rate range will assure maximum sensitivity and precision for the compounds measured. 

Target compounds are specified for each Series Method. Volatile compounds that need to be analyzed can be extracted from the matrix by a purge-and-trap device.-----------------------------------------... 

The operator will need room to store samples, along with syringes and other tools, before and after injection. It is best to leave at least a 2 x 2-ft working surface for this purpose. The instrument model will define the space requirement of the gas chromatograph, but in most cases, a 3-ft-wide space is adequate. Add two additional feet for computer controls and other ancillary devices (autosampler controls, purge-and-trap devices, sample concentrators, etc.). Thus, most gas chromatographs and associated devices will require about 6-8 linear feet of counter space. For most labs, this means no more than three to four gas chromatographs on a 20-24-ft (6-7-m) bench. 

Fig. 1.1 Aqueous Twister and purge- and-trap Tenax sampling devices for trapping volatiles...

Zero-headspace procedures involve the collection of a soil sample with immediate transfer to a container into which the sample fits exactly. The only space for gases is that within the soil pores. The volume of sample collected depends on the concentration of volatiles in the soil. It is imperative that the container employed can be interfaced directly with the gas chromatograph. Several commercial versions of zero-headspace sampling devices are available. The sample is transported to the laboratory at 4°C, where it is analyzed directly by purge-and-trap gas chromatography (EPA 5035) or other appropriate techniques, such as vacuum distillation (EPA 5032) or headspace (EPA 5021). 

Solvent extraction procedures involve collection of sample by an appropriate device and subsequent immediate placement into a borosilicate glass vessel, which contains a known quantity of ultrapure methanol. The bottle is then transported to the laboratory at 4°C, and the methanol fraction analyzed by purge-and-trap gas chromatography (or a similar procedure). 

Collection of analytes from a stream of gas on a sorbent bed and their release by thermal decomposition priori to the final determination stage, Membrane Extraction with Sorbent ) Interface - MESI), (Hollow Fiber Sampling Analysis -HFSA), (On-line Membrane Extraction Microtrap -OLME), (Membrane Purge and Trap - MPT), (Pulse Introduction Membrane Extraction - PIME), (Semi Permeable Membrane Devices - SPMD)... 

Purge and trap injectors are equipped with a sparging device by which volatile compounds in solution are carried into a low-temperature trap. When sparging is complete, trapped compounds are thermally desorbed into the carrier gas by rapid heating of the temperature-programmable trap. 

The basic set-up for headspace analysis comprises an HS element — the characteristics of which depend on the particular mode used for pretreatment and a gas chromatograph or, less often, an alternative detector for measurement. Static and dynamic headspace (purge and trap included) differ in the type of equipment required multiple headspace uses the same automated device as static headspace. 

This relatively simple technique can provide sensitivity similar to the above dynamic purge-and-trap analysis. Headspace is a sampling device used in tandem with a GC installation (Figure 21.6). It is reserved for the analysis of volatile compounds present in a matrix, which cannot be itself directly analysed by... 

If the analytes of interest are volatile or semivolatile, solvent extraction is not always necessary, and head-space techniques (HS) can be applied for the analysis, typically utilizing GC as the final analytical step. HS analysis can be defined as a vapor-phase extraction, involving ftrst the partitioning of analytes between a non-volatile liquid or solid phase and the vapor phase above the liquid or solid. The vapor phase is then transferred further and either analysed as vapor or (ad)sorbed to an (ad)sorbent. The head-space techniques have been widely utilized in the analysis of volatiles, such as fi agrances and aroma compounds, in various food and agricultural samples (81-84). The dynamic head-space (DHS), or purge-and-trap technique, is easily coupled on-line with GC. In an on-line system, desorption of trapped analytes for subsequent analysis is usually performed using on-line automated thermal desorption (ATD) devices. 

Figure 3.3a shows a schematic of a purge-and-trap system for the collection of volatiles contained in an aqueous sample. Clean air is bubbled through the water sample to purge the water of the volatiles and then carry the volatiles into the IMS. If the concentrations of the analytes are too low for direct detection, a trap (preconcentrator) device is inserted between the purged water sample and the IMS. Figure 3.3b is a schematic of an exponential dilution system used to calibrate IMS instruments for... 

Figure 2 shows a schematic diagram of a typical purge-and-trap system. It consists of two major components a purging device and a sorbent trap. The two parts are connected with each other and to the analytical instrument through transfer lines, with a six-port switching valve controlling the flow path. 

Curie-point pyrolysis/combustion chamber in which sub-/u.gs of plastic are burned in air atmosphere in the chamber the exhausted combustion gas is then cryofocused by a purge-and-trap (PT) device and analysed by GC-MS. In this way air-pyrograms are easily obtained. Lehrle et al. [598] have proposed PyGC-MS in oxidative pyrolysis conditions. Pyrolysis is then performed whilst air still remains within the pyrolysis chamber (before complete evacuation). This approach offers advantages over the so-called Enclosed Curie-Point (ECP) pyrolysis method for polymer and oil oxidation studies [511]. The latter procedure suffers from the limitation that secondary reactions may arise due to the fact that it is effectively a two-stage process, in which the analysis follows the oxidation stage. 

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