Dynamic head-space

Piet, G. J., Luijten, W.C.M.M. van Noort, P.C.M. (1985a) Dynamic head-space determination of volatile organic halogen compounds in water. In Fishbein, L. O Neill, I. K., eds. Environmental Carcinogens. Selected Methods of Analysis, Vol. 7, Some Volatile Halogenated Hydrocarbons (lARC Scientific Publications No. 68), Lyon, lARC, pp. 331-343... 

It should be noted that the problem of very slow diffusion in and out of a bulky solid matrix makes any type of gas extraction impractical and the results dubious. In other words, these restrictions apply not only to the MHS approach but also to dynamic head-space processes in general. 

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 ... 

Dynamic Head-Space Extraction and Thermal Desorption GC... 

If the analytes of interest are volatile or semivolatile, solvent extraction is not always necessary, and head-space techniques (HS) can be applied for the analysis, typically utilizing GC as the final analytical step. HS analysis can be defined as a vapor-phase extraction, involving ftrst the partitioning of analytes between a non-volatile liquid or solid phase and the vapor phase above the liquid or solid. The vapor phase is then transferred further and either analysed as vapor or (ad)sorbed to an (ad)sorbent. The head-space techniques have been widely utilized in the analysis of volatiles, such as fi agrances and aroma compounds, in various food and agricultural samples (81-84). The dynamic head-space (DHS), or purge-and-trap technique, is easily coupled on-line with GC. In an on-line system, desorption of trapped analytes for subsequent analysis is usually performed using on-line automated thermal desorption (ATD) devices. 

Figure 1.3 Dynamic head space sampling systems for volatile collection. A Closed-loop stripping system according to Donath and Boland, B Semi-open collection system. The direction of the air flow is indicated by arrows.

Saito, Y., Tanemura, L, Sato, T., and Ueda, H. (1999). Interaction of fragrance materials with 2-hydroxypro-pyl-beta-cyclodextrin by static and dynamic head-space methods. International Journal of Cosmetic Science, 21(3), 189-198. 

F. Angerosa, L. Di Giacinto, R. Wo, S. Cumitini, Sensory evaluation of virgin olive oils by artificial neural network processing of dynamic head-space gas chromatographic data. J. Sd. Food Agric. 72,323-328 (1996)... 

In order to validate the system, external sampling under pressure during the reaction was performed. The reactant contents were analyzed by dynamic head space gas chromatography and the polymer molecular weight distribution by size exclusion chromatography with triple detection. 

The dynamic head-space technique is recommended, because thermodynamic equilibrium can be reached with all if the volatile compounds, decreasing the risk of not detecting important components in the samples. Moreover, it avoids having non-volatile compounds in the chromatograph (Nunez. 1986). To obtain better results, it is necessary to control the operating conditions, like agitation time, vial temperature, heat time, sample volume take in the head space, sample injected in the GC, etc. (Bylaite. 2006). 

Studies of other alcoholic beverages like wine have used dynamic head-space to quantify and identify volatile compounds (Leino. 1993, Mestres. 1998). More than 45 volatile compounds have been identified, some of which had very low concentrations. Head space analysis is a good method for separating, quantifying, and identifying compounds and for determining the authenticity of the wines being tested (Etievant. 1986, Savchuk. 2001). 

Figure 2 Concepts of static headspace (A), dynamic head-space (B), and purge and trap (C) sampling systems.

Sublimation is often the mechanism by which preconcentration of an analyte is effected, although this fact is frequently not appreciated. Dynamic head-space concentration from solid samples such as plant materials, foodstuffs, or polymeric materials occurs by sublimation of the volatile components. Indeed, given appropriate apparatus that can be operated at different temperatures for dynamic headspace concentration, the heat of sublimation can be determined for various compounds. 

Possible reasons for the final rapid failure of the vessel may be structural instability of the vessel, rapid overpressurization due to a dynamic head space impact of the two-phase swell initiated upon a depressurization (initiated by the formation of a thermal crack or tear which arrests), or the rapid quenching of its crack tip, due to the two-phase discharge, that results in large local thermal stresses that cause the uncontrolled vessel failure. 

Figure 32. Principle of static (A) and dynamic head space...

Schnable, J.G. Dussert, B. Suffet, I.H. and Herzt, C.D. Comparison of quarter-hourly on-line dynamic head-space analysis to purge-and-trap analysis of varying volatile organic compounds in drinking water sources. Journal of Chromatography 1990, 513, 47-54. 

More material is obtained by dynamic head-space analysis or by solid phase microextraction (SPME). In the dynamic procedure the headspace volatiles are flushed out, adsorbed and thus concentrated in a polymer, as outlined in 5.2.1.2. However, it is difficult to obtain a representative sample by this flushing procedure, a sample that would match the original headspace composition. A model system assay (Fig. 5.7) might clarify the problems. Samples (e) and (f) were obtained by adsorption on different polymers. They are different from each other and differ from sample (b), which was obtained... 

Dynamic head-space by transfer-line technology... 

There are several approaches to managing water vapor in dynamic head-space analyses, including selection of a trapping medium that is hydrophobic, trapping the water independently of the analytes (11), venting the water independently, and combinations of these. 

J. S. Elmore, M. A. Erbahadir, and D. S. Mottram, Comparison of dynamic head-space concentration on Tenax with solid phase microextraction for the analysis of aroma volatiles, J. Agiic. Food Chem. 45 2638 (1997). 

One alternative to dynamic headspace is the closed loop headspace collection device, developed by Brunke et al. (9), shown in Fig. 4. This device places a flower within a collection vessel, and the volatiles are collected on an appropriate trapping material. The difference between this method and conventional dynamic headspace is that the closed loop technique constantly circulates the air within the device using an in-line pump. Results obtained with the closed loop system are similar to those obtained with conventional dynamic head-space. 

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