Porous nature, solvent-extracted

The nature of flavor compounds creates challenges for analysis. Aroma compounds must be volatile. They are usually present at very low concentrations in foods. Despite the fact that hundreds of volatile compounds are often present in a food, only a few may be odor-active. Gas chromatography has been an invaluable tool for separation and subsequent identification of volatile compounds. Concentration of flavor chemicals is often necessary since the compounds are usually present at low levels. Some methods of sample preparation are described in this handbook, including solid-phase microextraction (see Chapters 16, 20-22, 30, and 31), sorptive stir bar extraction (Chapter 32), absorption on a porous polymer (Chapters 21, 22, and 27), super-critical CO2 extraction (Chapter 22), simultaneous steam distillation (Chapter 31), accelerated solvent extraction (Chapter 35), simultaneous distillation extraction (Chapters 21 and 31), and direct gas injection with cryofocusing (Chapter 20). Sampling conditions are considered in Chapters 20, 23, and 24, and comparisons of some chemical detector sensitivities are made in Chapters 18, 23, and 27-29. 

This separation is based on the size of the porous, hydrophobic gels. The pore size must be greater than the pore size of the molecules to be separated. Gel-permeation cleanup (GPC) is used for cleaning sample extracts from synthetic macromolecules, polymers, proteins, lipids, steroids, viruses, natural resins, and other high molecular weight compounds. Methylene chloride is used as the solvent for separation. A 5 mL aliquot of the extract is loaded onto the GPC column. Elution is carried out using a suitable solvent, and the eluate is concentrated for analysis. 

The first example comes from the field of environmental chemical analysis and considers SLM enrichment and determination of triazine herbicides from natural water samples [141]. It shows how simple manipulation of donor and acceptor phase pH, the simplest manner of transforming the analyte into a transportable form, can lead to an SLM system with high extraction efficiency. A porous PTFE membrane impregnated with water immiscible dihexyl ether was used as an organic solvent. The obtained detection hmit of triazines ranged from 0.03 to 0.16 pg/1 in natural waters with 20 min extraction time using simple UV detection. 

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