Interfaces headspace

Figure 8.26(A) is an example of a valve type interface [329]. Helium carrier gas is provided to the headspace saiq)ler and is split into two flow paths. One path is flow-controlled and provides a constant flow of carrier gas which passes from the headspace unit through the heated transfer line to the gas chromatograph. The second flow path is pressure-regulated and, in the standby mode, the seunple loop and seuapling needle are flushed continuously by the helium flow. At a time determined by the operator, the sampling needle pierces the septum and helium pressurizes the headspace vial to any desired pressure. The headspace gas is then allowed to vent through the sample loop. Once filled, the sample loop is placed in series with the normal carrier gas flow and its contents are driv Bbhrough the heated...

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). 

Luo, Y.Z. and J. Pawliszyn. 2000. Membrane extraction with a sorbent interface for headspace monitoring of aqueous samples using a cap sampling device. Anal. Chem. 72 1058-1063. 

Membrane SLM MESI MMLLE MIMS PME HF-LPME MASE LGLME Headspace-solid phase dynamic extraction (HS-SPDE) Purge-and-membrane extraction Headspace/membrane extraction w. sorbent interface (HS-MESI) Headspace/membrane inlet mass spectrometry (HS-MIMS)... 

SPME (Figure 2.48) can be conducted as a direct extraction in which the coated fiber is immersed in the aqueous sample in a headspace configuration for sampling air or the volatiles from the headspace above an aqueous sample in a vial (headspace SPME analyses are discussed elsewhere) or by a membrane protection approach, which protects the fiber coating, for analyses of analytes in very polluted samples [136]. The SPME process consists of two steps (Figure 2.49) (a) the sorbent, either an externally coated fiber or an internally coated tube, is exposed to the sample for a specified period of time (b) the sorbent is transferred to a device that interfaces with an ana-... 

Residual solvents are amenable to GC/MS analysis because they possess sufficient volatility to enter the gas phase without thermolytic decomposition, and they do not typically interact with the sample introduction system (i.e., GC separation column, detector interface, or detector) in such a manner as to cause irreversible surface adsorption or surface catalyzed decomposition. The application of GC/MS for identification of residual solvents is best illustrated with an example. Fig. 1 shows a total ion chromatogram (TIC) from the headspace GC/MS analysis of a drug substance dissolved in a relatively high boiling solvent... 

In addition to the headspace module, headspace-based methods use three different types of ancillary units, namely devices for conducting preliminary steps such as dilution, suspension or concentration an instrument for detection (and, usually, also individual separation) of the target species and an interface between the headspace module and the detector or chromatograph. 

Automated Procedures At least four commercial automated instruments exist which will concentrate headspace volatiles on adsorbents and thermally desorb them into a gas chromatograph. Times, temperatures, and gas flow rates are accurately controlled. Manual handling steps which can introduce variability are eliminated. Since these instruments are automated, sample throughput is enhanced they can be interfaced to high resolution gas chromatographs. 

The newly developed solid-phase microextraction (SPME) technique, first reported by Pawliszyn in 1989, is increasingly used for the gas chromatographic determination of a wide variety of volatile and semivolatile organic compounds in water or aqueous extracts of different substrates. Basically, it involves the extraction of specific organic analytes directly from aqueous samples or from the headspace of these samples in closed vials. The extraction is achieved onto a fused-silica fiber coated with a polymeric liquid phase. After equilibration, the fiber containing the absorbed or adsorbed analyte is removed and thermally desorbed in the hot injector port of a gas chromatograph or in an appropriate interface of a liquid chromatograph. ... 

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