Headspace gas composition

Table 3. Effect of headspace gas composition on artemisinin and growth of cultures in gas fed systems. Data represent averages and standard deviations of 5-7 replicate flasks, each supplied with the given oxygen and carbon dioxide composition and various ethylene concentrations. Because regression analysis did not indicate ethylene to have significance in determination of these parameters at the 90% confidence level, all ethylene concentrations were lumped in the given oxygen and carbon dioxide concentrations...

Headspace gas composition changes dynamically due to several factors. Gas exchange with the enviromnent occurs over the packaging film if the partial pressure of a gas differs on the two sides of the film. The amount of gas that permeates the film can be calculated from equation 9.1 (Robertson, 1993) ... 

Hence, the standard additions method is unique in that it actually employs the very material under analysis as a reference matrix material, thus providing for efficient elimination of very complex matrix effects even when the final material is the result of a multi-step preparative procedure and the composition of the matrix of the original material is completely unknown. These advantageous features of the standard additions technique have been discussed and verified in context with quantitative headspace gas analysis [68]. 

Figure 20.4. Effect of gas composition (proportions of nitrogen and carbon dioxide) in the headspace atmosphere during mixing on the aeration of the dough. (From Campbell, G.M. (2003). Bread aeration. In Bread Making Improving Quality, Cauvain, S.P., ed., Woodhead Publishing, Cambridge, Figure 17.2, p. 360.)...

The headspace sampling technique can yield useful results if sufficient numbers of samples can be collected to use statistical populations to suggest anomalous areas. One should always exercise caution, however, with respect to characterisation of gas composition, since evaporation during the collection stage always occurs, resulting in the relative depletion of the lighter gases. 

If complete exclusion of CO2 and O2 is required, or if exact control of the gas composition in the reactor headspace is needed, the reactor must be completely sealed from the atmosphere and a gas trap must be used to vent the purge gas and to prevent contact between the reactor headspace and the outside atmosphere. This is normally required only when precise control over redox status is needed. 

Fig. 2. Gas isotope compositions, gas hydrate saturation and TOC in the Nankai Trough, (a) Depth-trends of of CH4 and CO2. Open symbols, PTCS samples solid symbols, headspace gas samples.

Table 1. Gas molecular composition and of methane in pressure-temperature-core sampler (PTCS) from the post-survey hole-2 and headspace gas samples from the main hole of the MITI Nankai Trough Well...

Headspace methods are used extensively for the determination of residual monomers and other residues in polymer compositions after dissolution or dispersion in a suitable solvent and equilibration in a sealed vial at constant temperature prior to chromatography of the headspace gas. For samples in the form of fine powders or thin films, the technique can be applied directly to the solid and liquid samples. (Tables 5.2 and 5.3). 

PVC compositions are typically analyzed as an indication of correct formulation or to deconstruct competitive compounds. Occasionally, analysis is required by specification, such as the level (usually zero detectable) of vinyl chloride monomer (VCM). The latter is carried out by headspace gas chromatography (GC) per ASTM D 3749 for resins, and ASTM D 4443 for compounds. Such analyses are routinely done by suppliers. The formulator in the field will usually send samples to a testing laboratory. This is also the case with trace analysis, often for levels of lead, cadmium, arsenic, or mercury. 

In this equation x, is the liquid perfume concentration, Mt the molecular weight, R the ideal gas constant, and T the absolute temperature. Equation 2 relates the liquid perfume composition, x, with the human sensory reaction of the evaporated perfume. A key factor of Equation 2 is the activity coefficient, y, because it represents the affinity of a molecule to its neighboring medium. High value of y means an increased inclination for a given substance to be released from the mixture and low value of y means a low concentration in the headspace. This means that the OV values of a particular component can change if it is diluted in different solvents or mixed with different fragrance components. 

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