Sample preparation Isolation, Headspace sampling

Before any sample can be subjected to chromatography, some type of sample preparation is required, which can be as simple as filtration or an involved solid-phase extraction protocol. Sample preparation is that activity or those activities necessary to prepare a sample for analysis. The ultimate goal of sample preparation is to provide the component of interest in solution, free from interferences, and at a concentration appropriate for detection. Sample preparation can be divided into a number of classes of activities solvent extraction, sorbent extraction and compound isolation, headspace, and membrane separations, with each of these areas further divided into techniques that apply to the category of activities. 

Isolation of the products from complex matrixes (e.g. polymer and water, air, or soil) is often a demanding task. In the process of stability testing (10 days at 40 °C, 1 h at reflux temperature) of selected plastic additives (DEHA, DEHP and Irganox 1076) in EU aqueous simulants, the additive samples after exposure were simply extracted from the aqueous simulants with hexane [63]. A sonication step was necessary to ensure maximum extraction of control samples. Albertsson et al. developed several sample preparation techniques using headspace-GC-MS [64], LLE [65] and SPE [66-68]. A practical guide to LLE is available [3]. 

See also Air Analysis Sampling. Chromatography Overview Principles. Clinical Analysis Sample Handling. Drug Metabolism Metabolite Isolation and Identification. Extraction Solid-Phase Extraction. Food and Nutritional Analysis Sample Preparation. Forensic Sciences Volatile Substances. Headspace Analysis Purge and Trap. Perfumes. Sample Handling Sample Preservation Automated Sample Preparation. Sampling Theory. 

Samples containing nonvolatile materials present problems for gas chromatography. The non-volatiles cannot be injected into the GC as they will rapidly plug up the injection port and may destroy the GC column. Common sample preparation steps for isolating the volatiles from the nonvolatile sample matrix include liquid-liquid extraction, solid phase extraction (SPE), solid phase microextraction (SPME), supercritical fluid extraction (SFE), and headspace. SPE and SPME for GC are discussed in a recent review article by Penton [15]. 

Headspace techniques are often the method of choice since there is virtually no sample preparation involved. One simply places the food sample in a closed vessel, allows the headspace to equilibrate and then samples the headspace with a gas-tight syringe or an automated sampling system. Problems with sample carry over in the syringe and reproducibility favor automated systems for headspace sampling. The primary limitation of headspace sampling is a lack of sensitivity. One may not isolate sufficient quantities of indicator compoxmds to permit accurate and precise quantification. However, the simplicity, reproducibility and speed of this method make it exceptionally desirable for quality control purposes. 

Figure 5 Stacked chromatograms showing the GC analysis of a series of n-hydrocarbons isolated by headspace SPME at various temperatures. The sample was prepared by transferring l-pl of individual hydrocarbon standards into a 4-mL vial and sealing with a Teflon-coated septum. After 10 minutes equilibration at the indicated temperature, the headspace vapor was extracted using a 100-lim polydimethylsiloxane fiber and was analyzed by GC with flame ionization detection. The injector temperature for this sample was set at 300°C, and the fiber was desorbed for 1 minute.

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