Quantitative Solvent and Thermal Extraction

Principles and Characteristics As quantitative analysis is best carried out with a sampling technique that assures complete transfer of an analyte from the sample matrix to the analytical instrument direct in-polymer analysis is ideal (no sample preparation). Also analyses in the polymer melt (in-process analysis, tfr. Chp. 7) and the dissolution/precipitation technique (cfr. Chp. 3.7 of 

Soxhlet Largest weight of sample Unfavourable extraction efficiencies Concentration step required for low level additives 

US Least traumatic Low extraction efficiencies for some additives 

SFE Not particularly traumatic Very low, unacceptable, extraction efficiencies for some components (matrix dependency) 

PFE Total extraction Analyte stability problems (at high T) 

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The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

Ease of Detection and Stability. Numerous compounds are thermally produced in foods, but not all are suitable as chemical markers of sterility. Some of the compoimds that need to be ruled out include volatiles and unstable intermediates that rapidly undergo subsequent reactions. Preferably, the marker compound should be easily extracted wi an aqueous solvent and easily determined without many additional operations. The marker should also be stable during analysis. In situ analysis would be ideal however, accurate quantitation by simple in situ methods, such as surface fluorescence or near infrared measurements, is questionable. 

Volatile ginger oil obtained from steam distillation has been the subject of many research studies (5-12). However, the thermal degradative effects of steam distillation upon volatile and nonvolatile components of ginger were seldom discussed. Recently, Moyler (1) compared the advantages of liquid carbon dioxide extraction over conventional methods such as solvent extraction or steam distillation by showing reconstructed GC chromatograms which clearly displayed the differences. However,quantitative results showing the differences were not mentioned. 

The solvent selectivity should be as high as possible in order to avoid coextraction of useless or disturbing by-products. If the desired extract is rich in valuable ingredients the direct use is possible without further refining coimected not only to additional processing costs but also to thermal stress and product losses. At the same time and mostly in contradiction to the previous point, a high capacity is required for a fast extraction and for limiting the amount of solvent flow that is necessary for quantitative extract recovery. 

The analysis of 1,1,1-trichloroethane in occupational air samples can be accomplished by NIOSH method 1003 (NIOSH 1987). The sample is obtained in the field with a pumping system to pass a measurable quantity of air ( 3 L) through a tube loaded with a solid sorbent, such as charcoal. Extraction of the tube with the solvent CS2 liberates the 1,1,1 -trichloroethane collected, an internal standard is added, and quantitation is then achieved by GC. For packed column analysis, an OV-101 column using a flame ionization detector (FID) is given as the preferred choice (alternates, including capillary columns, are acceptable). For the estimation of low levels of 1,1,1 -trichloroethane in ambient air, thermal desorption following collection ofthe sample in an absorbent trap is the method of choice (Frank and Frank 1988). 

In sorbent tube sampling (Figure 3.5), volatile and semi-volatile compounds are pumped from the air and trapped on the surface of the sorbent. By sampling a measured quantity of air (typical volumes of 10-500 m3), quantitative sampling is possible. The sorbent tube is then sealed and transported back to the laboratory for analysis. Desorption of volatile and semi-volatile compounds takes place either by the use of organic solvents (solvent extraction) or heat (thermal desorption), followed by analysis using gas chromatography (see later). 

Thermal desorption, solvent elution, and solvent extraction are used in VOC preparation schemes for samples collected on solid sorbents. Thermal desorption methods require determination of the sensitivity of the target analytes to the desorption temperature. It is also critical to remove all traces of O2 from the gas used to purge the sorbent and transfer the analytes to the preconcentration trap. Quantitative recovery of monoterpenes from Tenax is accomplished at thermal desorption temperatures of 250°C. ° Multibed sorbent tubes consisting of graphitic carbon solids and molecular... 

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