Headspace techniques static method

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). 

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 ... 

The headspace technique, a static gas extraction method, is particularly suitable for the enrichment of volatile compounds. It enables the analysis of solid and liquid samples by direct sampling from the gas phase, and can be directly combined with gas chromatography. This principle is based on the distribution of analyte between the matrix and gas phase. It has been used successfully to determine volatile sulfur compounds from various matrices, such as wastewater, body fluids, plants, and animal fatty tissue. 

The difficulty that the EPA has had with the static headspace technique might be seen in the comment from Method 3810 (SW-846 3rd edition). This method was eliminated in the recently published final update (III) and replaced by Method 5021. The authors who wrote Method 3810 state (33) ... 

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... 

Method 5021 describes the automated static-HS technique. Static HS has been introduced in this book from a theoretical viewpoint. A soil sample is placed in a tared septum-sealed vial at the time of sampling. A matrix modifier containing internal and/or surrogate standards is added. The sample vial is placed into an automated equilibrium headspace sampler. The vial s temperature is elevated to a fixed value that does not change over time and the contents of the vial is mixed by mechanical agitation. A measured volume of headspace is automatically introduced into a GC or a GC-MS. The method is automated and downtime is minimal. However, the cost of the automated system is appreciable. Contamination of the instru-... 

Several others techniques dealing with the injection problems have been developed. Among them the solid-phase microextraction method (SPME) and the full evaporation technique must be mentioned. According to Camarasu, the SPME technique seems to be very promising for RS determination in pharmaceuticals, with much better sensitivity than the static headspace technique. 

Current official GC methods are described in USP XXIII under chapter 467 Organic volatile impurities . Four methods (I, IV, V, VI) are mentioned. Methods I, V and VI are based on direct injection. They are suitable for water-soluble drugs and V for water insoluble drugs. Method IV describes the static headspace technique and is used for water soluble drugs. Method VI is very general and refers to the individual monograph which describes the chromatographic conditions (injection, column, conditions) which should be used. The main characteristics of these four methods are summarized in Table 16.2.2. 

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). 

There are many methods which enable determination of activity coefficients in infinite dilution. They are mostly based on differential ebulliometry or on gas chromatographic measurement of retention time, subsequently retention volume. The headspace chromatographic analysis is another popular technique which enables measurement of equilibrium compositions at given temperature. Some similarity with static methods may be found, however, degassing is not required since the pressure is not measured. The data may be obtained rather quickly, nevertheless their accuracy is not very high. Methods for measurement of activity coefficients in infinitely diluted solutions are not described here in detail because such data are not included in this volume. 

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. 

How then do the techniques differ For this, the terms recovery and sensitivity must be defined. For both methods, the recovery depends on the vapour pressure, the solubility and the temperature. The effects of temperature can be dealt with because it is easy to increase the vapour pressure of a compound by raising the temperature during the vaporization step. With the P T technique, the term percentage recovery is used. This is the amount of a compound which reaches the gas chromatograph for analysis relative to the amount which was originally present in the sample. If a sample contains 100 pg benzene and 90 pg reach the GC column, the percentage recovery is 90%. In the static headspace technique, a simple expression like this cannot be used because it is possible to use a large... 

The static headspace technique is very simple and quick. The procedure is well documented in the literature, and for many applications the sensitivity is more than adequate, so that its use is usually favoured over that of the P8dT technique. There are areas of application where good results are obtained with the static headspace technique which cannot be improved upon by the P8dT method. These include the forensic determination of alcohol in blood, of free fatty acids in cell cultures, of ethanol in fermentation units or drinks and residual water in polymers. This also applies to studies on the determination of ionization constants of acids and bases and the investigation of gas phase equilibria. 

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... 

Figure 7 illustrates the enormous advantage of the headspace SPME method as compared with the static headspace technique, using the example of analytical investigation of a dry apricot flavor. While the time expended is the same, the... 

Stuart et al. [127] studied the analysis of volatile organic compounds using an automated static headspace method. Recoveries decreased in the following order water, pure sand, sandy soil, clay and topsoil. A full evaporation technique that uses little or no aqueous phase and higher equilibration temperature gave the most reproducible analyte recoveries. 

Static headspace extraction is also known as equilibrium headspace extraction or simply as headspace. It is one of the most common techniques for the quantitative and qualitative analysis of volatile organic compounds from a variety of matrices. This technique has been available for over 30 years [9], so the instrumentation is both mature and reliable. With the current availability of computer-controlled instrumentation, automated analysis with accurate control of all instrument parameters has become routine. The method of extraction is straightforward A sample, either solid or liquid, is placed in a headspace autosampler (HSAS) vial, typically 10 or 20 mL, and the volatile analytes diffuse into the headspace of the vial as shown in Figure 4.1. Once the concentration of the analyte in the headspace of the vial reaches equilibrium with the concentration in the sample matrix, a portion of the headspace is swept into a gas chromatograph for analysis. This can be done by either manual injection as shown in Figure 4.1 or by use of an autosampler. 

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