Head-space techniques

In an important next step, it has been found that flowers and other plant parts can be analyzed by using head space techniques without removing them from the living plant (3). It was immediately observed that there are remarkable differences in the volatile compositions observed from Hve and picked flowers. This is exemplified for jasmine flowers in Table 3. Reconstitutions produced from this information have provided perfumers with novel and fresh notes for use in their creations. This technique continues to be appHed to many kinds and varieties of flowers, leaves (herbs, spices), and fmits. The reasons for the remarkable differences observed are not known. 

Knudsen J. T., Tollsten L. and Bergstrom L. G. (1993) Floral scents - a check-list of volatile compounds isolated by head-space techniques. Phytochemistry 33, 253-280. 

P. Bellenger, F. Pradier, M. Sinegre, and D. Pradeau, Determination of residual ethylene oxide in non-reusable plastic medical devices by the head-space technique, Sci. Technol. Pharm., 72 37-39 (1983). 

At this time there are no reliable measurements of the volatile fraction as defined by any method. Russian workers (10) have estimated by difference that the fraction removed by evaporation of acidified seawater at 60°C is about 15% of the total DOC. Work has been done on identifying and measuring specific compounds removed from seawater by sparging 11) and by the head space technique (J2), but hard numbers and reliable distributions for the whole volatile fraction await the development of a simple, accurate technique. 

Reproducibility can be a significant problem with manual head-space techniques therefore, rigorous efforts must be made to standardize all times, temperatures, and procedures. 

An American Chemical Society Symposium was devoted to head-space techniques, and the proceedings have been published (7). 

The barrier performance of rigid containers was measured on both a second modified MOCON instrument and by a head space technique in which nitrogen purged and sealed containers were stored in an oxygen environment for periods of time. Containers were generally measured at 23°C with 100% relative humidity (RH) inside and either 50 or 75 RH outside. Container oxygen transmission rates (OTR) are expressed as cm30 /container-day-atm. 

This overview is focused on the on-line coupling of pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), supercritical fluid extraction (SEE) and sonication-assisted extraction (SAE) with liquid and gas chromatography for the analysis of solid agricultural and food samples. In addition, head-space techniques and direct thermal extraction are discussed. 

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. 

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). 

Volatile components of packaging materials in foods can be detected and quantified using head-space techniques. As an example, vinyl chloride as a component of poly(vinyl chloride) (PVC) has been determined in olive oil after storage in PVC containers. 

On Figure 4 a chromatogram of mix of volatile compoimds, precursors and degradation products of some warefare chemicals, introduced by Head-space technique is shown. It demonstrates both the possibility for successful use of this technique for analysis of these compounds m samples with liquid and solid matrice and their good separation usung PoraPlot Q GC column. 

In addition, the possibility for gas samples analysis using Head-space technique should be mentioned as well. In this case the sampling and sample tran ortation is easy and it replaces... 

The Gas Chromatographic Determination of Trichloroacetic Acid in Urine After Tri- and Perchloroethylene Exposure Using Head-Space Technique Z. Med. Laboratoriumsdiagn. 18(2) 123-127 (1977) CA 87 79152d... 

Kepner, R.E. Maarse, H. and Strating, J. Gas chromatographic head space techniques for the quantitative determination of volatile components in multicomponent aqueous solutions. Analytical Chemistry 1964, 36 (1), 77-82. 

Karppinen K, Hokkanen J, Tolonen A, Mattila S, Hohtola A (2007) Biosynthesis of hyperforin and adhyperforin from amino acid precursors in shoot cultures of Flypericum perforatum. Phytochemistry 68 1038-1045 Kaufman TS, Riiveda EA (2005) The quest for quinine those who won the battles and those who won the war. Angew Chem Int Ed 44 854-885 Klein RM (1987) The green world an introduction to plants and people. HarperCollins, New York Knudsen JT, Tollsten L, Bergstrom G (1993) Floral scents a checklist of volatile compounds isolated by head-space techniques. Phytochemistry 33 253-28... 

This head space technique is capable of detennining a range of different types of volatiles in polystyrene in amounts down to 0.002%. 

Some proposed analytical methods are based on the use of distillation or volatilization steps. Head-space technique has been extensively used for GC determination (as for example in the above-mentioned article by Wala Jerzykiewicz and Szymanowski, 1998). 

Wala-Jerzykiewicz, A., W. Jerzykiewicz, A. Sobczynska, J. Szymanowski, Toxic contaminants in polyoxyethylene alkylamines, Tenside, Swfactants, Deterg., 1999,56,173-177. Gaia, G., E. Moretti, A. Penati, GC determination of 1,4-dioxane in ethoxylated products using the head-space technique, Riv. Ital. Sostanze Grasse, 1989,66,575-580. 

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