Headspace volume

Another recently developed technique is headspace sorptive extraction (HSSE) with PDMS stir bars [552]. HSSE-GC was compared with SHS and HS-SPME. SBSE and HSSE extract organic analytes from aqueous or vapour samples. In SBSE, the stir bar is inserted into the aqueous sample and extraction takes place during stirring whereas in HSSE the glass rod is suspended within the headspace volume and sampling takes place during headspace equilibration. New trends are the development of selective sorbents. 

Combining solids that have previously been equilibrated at different relative humidities results in a system that is thermodynamically unstable, since there will be a tendency for moisture to distribute in the system so that a single relative humidity is attained in the headspace. As shown in Fig. 7, moisture will desorb into the headspace from the component initially equilibrated at a higher relative humidity and sorb to the component initially equilibrated at a lower relative humidity. This process will continue until both solids have equilibrated at the final relative humidity. The final relative humidity can be predicted a priori by the sorption-desorption moisture transfer (SDMT) model [95] if one has moisture uptake isotherms for each of the solid components, their initial moisture contents and dry weights, headspace volume, and temperature. Final moisture contents for each solid can then easily be estimated from the isotherms for the respective solids. 

Fig. 7 Schematic representation of moisture transfer between solid components A and B with (a) headspaces isolated from one another and (b) headspaces allowed to equilibrate. Ra and Rb = initial relative humidities above A and B VA and VB = headspace volumes above A and B Rf and VT = final relative humidity and headspace volume above A and B. (From Ref. 95.)...

Kwiatkowski, M. J., Skouroumounis, G. K., Lattey, K. A., and Waters, E. J. (2007). The impact of closures, including screw cap with three different headspace volumes, on the composition, colour and sensory properties of a Cabernet Sauvignon wine during two years storage. Aust. ]. Grape Wine Res. 13, 81-94. 

Lowest headspace volume required to perceive the odorant at the sniffing port The highest headspace volume was equated to a flavour dilution factor of 1. The flavour dilution factors of the other odorants were calculated on this basis. (Source [31])... 

The ratio of the sample volume to headspace volume should be large enough (close to 0.5) to minimize changes in water activity due to loss of water by vaporization. In samples of higher water content the ratio of sample to headspace volume could be lower. 

Another important factor is the volume of unfilled headspace designed into the bottle. Typically, this is not less than 5% of the total volume for an IPP carbonated product and ideally it is more like 7%. This is because as the product expands during pasteurising, the headspace becomes squeezed, and the smaller the headspace volume, the higher the internal pressure becomes. This could blow the closure off the bottle or, more typically, cause leakage of carbon dioxide or product or both. The loss of product gives rise to uneven fill levels, which could cause consumer concern over apparent low fills or, more seriously, draw attention from trading standards officers because of short volume fills. 

This procedure allows the differentiation of odor active compounds from odorless substances within a complex mixture of volatiles. For decades this procedure has been successfully applied for aroma analyses of foods (Grosch, 1993). The mixture of volatile compounds either collected in a purified organic solvent extract or in a defined headspace volume is separated into its different components by means of GC and the effluent gas flow at the end of the GC capillary column is split between a FID and an experienced test person s nose. By sniffing the column effluent, the human nose is able to perceive the odor active compounds contained in a complex mixture and the test person can mark the corresponding spot in the FID chromatogram recorded in parallel and attribute a certain odor quality. A sample GC—O chromatogram of a solvent extracted material is shown in Figure 8.7. 

Sample volume and headspace volume Vial shape Time between extraction and analysis Adsorption on sampling vessel or components Carrier gas flow rate... 

The sensors of the electronic nose are assembled in an array. The array is normally a small electronic unit that integrates the different sensors into a practical circuit card or another appropriate system that is easy to insert into the electronic nose instrument. If the array is to be used in a flow injection setup the unit also comprises a flow cell compartment with minimal volume. The system depicted in Fig. 2 shows how MOS and MOSFET arrays are integrated in a flow injection system [11]. Larger arrays can be integrated into silicon chips, as described for CP sensors where, for example an ASIC chip with 32 sensors has been fabricated with BiCMOS technology and having an area of 7 x 7 mm [18]. If the array is be inserted in the headspace volume of a bioreactor, the technical solution is a remote array probe that can be placed in a gas sample container [19]. 

Figure 1 Coexistence of the past and present techniques in F F industry classical enfleurage process (photo on the left) and a supercritical carbon dioxide extraction facility as modern factory equipment (on the right). The photo on the left shows a stock of jasmine flowers in the basket (center) that are spread upon a wooden frame (chassis) that secures a glass plate coated with fat. The chassis is then piled to allow diffusion of fragrant components (note that the fat is applied on both sides of the glass plate to gain access to the headspace volume made by the chassis underneath). Enfleurage process photo reproduced from E. Guenther, The Essential Oils with permission from Krieger Publishing Company Melbourne, FL, USA, 1948 (reprinted 2006) Vol. 1, p 192.

The correlations were for data obtained with model fluids in copper cans with headspace volume in the range 3-9% and the rate of heat transfer was independent of the radius of rotation in the range 0-14.9 cm. 

The gas volume Fj, in this step consists of two parts, namely one, F,, that is vented, and the other, F, which corresponds to the headspace volume above the sample in the vial ... 

In this step, the analyte concentration in the headspace will be C, the same as established for the expanded volume cf. eq. (4.14)]. Such a concentration can be expressed in terms of, and (the vial s headspace volume) ... 

In the example of baguette crust (Table 6.25), analysis started with a headspace volume of 20 mL and 13 odorants were revealed by GCO. The headspace sample was then reduced in a series of steps to determine the most potent, highly volatile odorants. GCOH of volumes of 10 and 2.5 mL indicated only 10 and 7 odorants, respectively (Table 6.25). After reduction to 0.2 mL, only 2,3-butanedione was found. According to this experiment, 2,3-butanedione was the most potent highly volatile odorant of baguette cmst. A comparison of Tables 6.24 and 6.25 shows that some odorants were detected by both GCOH and AEDA (e.g. l-octen-3-one, dimethyltri-sulphide, 2-acetyl-l-pyrroline). 

Lowest headspace volume required to perceive the odorant at the sniffing port. 

Table ED. Predominant Odorants Detected by Decreasing Dynamic Headspace Volumes of Rice Bran...

SPME is a multiphase equilibrium technique and, therefore, the analytes are not completely extracted from the matrix. Nevertheless, the method is useful for quantitative work and excellent precision and Unearity have been demonstrated. An extraction is complete when the concentration of analytes has reached distribution equilibrium between the sample and coating. This means that once the equihbrium is achieved, the amount extracted is independent of further increase in extraction time. If extraction is terminated before the equihbrium is reached, good precision and reproducibihty is still obtained if incubation temperature, sample agitation, sample pH and ionic strength, sample and headspace volume, extraction and desorption time are kept constant. The theory of the thermodynamic, kinetic and mass transfer processes underlying direct immersion and HS-SPME has been extensively discussed by Pawhszyn [2]. The sensitivity and time required to reach adsorption equilibriiun depends on the partition coefficients between the fiber and the analytes, and the thickness of the phase. Limits of detection and quantitation are often below 1 ppb. 

Static mode the sample (solid or liquid matrix) is placed in a glass vial capped with a septum such that the sample occupies only a part of the vial s volume. After thermodynamic equilibrium between the phases present (1/2 to 1 h), a sample of the vapour is taken. Under these conditions, the quantity of each volatile compound present in the headspace (volume above the liquid) will be proportional to its concentration in the matrix. After calibration (using methods of internal or external standards), it is possible to match the real concentrations in the sample with those of the vapours injected in the gas chromatograph (Figure 21.6). 

The important constraints within this model are the headspace volume above the liquid in the container and the volume of liquid in the container itself. Typically, larger ratios between headspace volumes and fluid volumes lead to higher internal pressures. 

The amount of analyte extracted in HS-SPME, N from the headspace volume above a volume V2... 

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