Headspace techniques dynamic method

Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

The extent of oxidative deterioration will determine the acceptability of a food product. Because of this, methods for determining the degree of oxidation are very useful to the food industry. There are many possible methods that can be utilized (see Commentary) however, due to the stability of some of the end products, and their direct relationship with rancidity, headspace GC provides a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic, and solid-phase microextraction (SPME) methods. 

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

Headspace Sampling Technique. The method used a new gas chromatographic desorption - concentration - GC introduction device (D.C.I.) based on dynamic headspace analysis and available from Delsi Instruments (Paris, France). This apparatus made it possible to isolate volatiles from both solid and liquid samples (4). 

A method for the automated analysis of volatile flavor compounds in foods is described. Volatile compounds are removed from the sample and concentrated via the dynamic headspace technique, with subsequent separation and detection by capillary column gas chromatography. With this method, detection limits of low ppb levels are obtainable with good reproducibility. This method has experienced rapid growth in recent years, and is now in routine use in a number of laboratories. 

The use of pervaporation as an alternative to the headspace technique is worth separate discussion. This is, in fact, one of the most promising uses of this approach, as revealed by two existing methods for mercury speciation and VOC analysis in solid samples that exemplify the advantages of pervaporation over static and dynamic head-space modes. Both methods were developed by using the overall assembly depicted in Fig. 4.24A, by which the analytical process was developed in the following four steps ... 

Yes there is and it is Method 5021 from the recently updated SW-846 series of methods published by the Office of Solid Waste at EPA. The method uses the static HS technique to determine VOCs from soil or other solid matrix. This section will focus on some of the details of this method because it includes many of the quality control (QC) features that were absent in the method just discussed. This method also introduces some experimental considerations with respect to trace VOC analyses of soil samples (34). The method is applicable to a wide range of organic compounds that have sufficiently high volatility to be effectively removed from soil samples using static HS techniques. The method is used in combination with a determinative technique that is described in the 8000 series. The method cautions the user to the fact that solid samples whose organic matter content exceeds 1% or for compounds with high octanol/water partition coefficients may yield a lower result for the determination of VOCs by static HS in comparison to dynamic headspace (P T). It is... 

The search of adequate extraction techniques allowing the identification and quantification of wine volatile compounds has attracted the attention of many scientists. This has resulted in the availability of a wide range of analytical tools for the extraction of these compounds from wine. These methodologies are mainly based on the solubility of the compounds in organic solvents (liquid-liquid extraction LLE, simultaneous distillation liquid extraction SDE), on their volatility (static and dynamic headspace techniques), or based on their sorptive/adsorptive capacity on polymeric phases (solid phase extraction SPE, solid phase microextraction SPME, stir bar sorptive extraction SBSE). In addition, volatile compounds can be extracted by methods based on combinations of some of these properties (headspace solid phase microextraction HS-SPME, solid phase dynamic extraction SPDE). 

Analyte recoveries in P T experiments can vary widely due to matrix effects, purging efficiency, volatiUty, purge ceU design, choice of adsorbent, isolation temperature, and many other factors. Quantification with the various headspace techniques always requires method development in terms of extraction time and temperature in order to avoid degradation. With dynamic headspace (DHS) nearly 100% recovery of volatiles is possible provided headspace temperature is appropriate to remove most of the analyte in a reasonable time. Kolb et al. [32] have outlined the prospects of quantitation by means of headspace techniques. 

There are two methods of analysing the volatiles of a sample which have very different requirements concerning the instrumentation the static and dynamic (purge and trap, P T) headspace techniques. The areas of use overlap partially but the strengths of the two methods are demonstrated in the different types of applications. 

The SPME method should be regarded as an alternative to the conventional static or dynamic headspace technique, and at the same time serves as a substitute... 

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

The analyses of the flavour composition of yellow passion fruits were performed by four dilferent isolation techniques, namely vacuum headspace sampling (VHS), the dynamic headspace method, simultaneous distillation and extraction at atmospheric pressure, and simultaneous distillation and extraction under reduced pressure [62]. Significant differences were found not only in the chemical composition of the resultant extracts but also in their sensory properties. The most representative and typical extract was obtained by VHS. 

Of these methods, dynamic headspace sampling is probably the least well known, yet it has a number of advantages over other techniques in use. 

Headspace Extraction Headspace (HS) extraction is a well-known method of sample preparation and is frequently used in many laboratories, especially in industrial applications. It involves a partitioning equilibrium between the gas phase and a sample (liquid or solid). In this technique, an aliquot of gas phase is sampled into GC. There are two types of analysis, static and d3Uiamic. In the static version, when the equilibrium is reached, the gas phase is injected into GC. In dynamic analysis, the volatiles are exhaustively extracted by the stream of gas. However, matrix effects result in decreased sensitivity for certain substances, especially polar and hydrophilic samples. A comprehensive book describing HS techniques was presented by Kolb [31]. 

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