Headspace

This method is quite reliable for repetitive analyses involving stable matrices. However, if the matrix composition fluctuates, then this disturbs the equilibrium factors and reduces the precision of the result, by consequence of an irregular calibration. In this case cartridge-base extraction would be preferred. 

The head space will be affected by the groundwater levels. This can affect the response zone of the well to ground gas, although the effects are not clearly xmderstood at present. Changes in groundwater levels can, however, cause suction or pressure within the head space if the groxmdwater level is above the top of the slotted section of standpipe. 

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Vreugdenhil A J and Butler I S 1998 Investigation of MMT adsorption on soils by diffuse reflectance infrared spectroscopy DRIFTS and headspace analysis gas-phase infrared spectroscopy HAGIS Appl. Organomet. Chem. 

Sample Preservation Without preservation, many solid samples are subject to changes in chemical composition due to the loss of volatile material, biodegradation, and chemical reactivity (particularly redox reactions). Samples stored at reduced temperatures are less prone to biodegradation and the loss of volatile material, but fracturing and phase separations may present problems. The loss of volatile material is minimized by ensuring that the sample completely fills its container without leaving a headspace where gases can collect. Samples collected from materials that have not been exposed to O2 are particularly susceptible to oxidation reactions. For example, the contact of air with anaerobic sediments must be prevented. 

Why is it necessary to collect samples such that there is no headspace (layer of air overlying the liquid) in the sample vial ... 

Due to the volatility of trihalomethanes, the presence of a headspace allows for the possible loss of analyte. 

Although aimed at the introductory class, this simple experiment provides a nice demonstration of the use of GG for a qualitative analysis. Students obtain chromatograms for several possible accelerants using headspace sampling and then analyze the headspace over a sealed sample of charred wood to determine the accelerant used in burning the wood. Separations are carried out using a wide-bore capillary column with a stationary phase of methyl 50% phenyl silicone and a flame ionization detector. 

Welch, W. G. Greco, T. G. An Experiment in Manual Multiple Headspace Extraction for Gas Ghromatography, ... 

The principle of headspace sampling is introduced in this experiment using a mixture of methanol, chloroform, 1,2-dichloroethane, 1,1,1-trichloroethane, benzene, toluene, and p-xylene. Directions are given for evaluating the distribution coefficient for the partitioning of a volatile species between the liquid and vapor phase and for its quantitative analysis in the liquid phase. Both packed (OV-101) and capillary (5% phenyl silicone) columns were used. The GG is equipped with a flame ionization detector. 

Dynamic headspace GC/MS. The distillation of volatile and semivolatile compounds into a continuously flowing stream of carrier gas and into a device for trapping sample components. Contents of the trap are then introduced onto a gas chromatographic column. This is followed by mass spectrometric analysis of compounds eluting from the gas chromatograph. 

Maturation is conducted in closed, full containers to prevent oxidation and aerobic growth of microorganisms. Etee air contact with low alcohol wine soon leads to vinegar. Except for those sherry types already mentioned, wines ate exposed to air minimally and temporarily. During transfers incident to bulk storage and processing, some air exposure is almost inevitable, mote in total the longer the wine is held. In the cases of white and pink table wines, it is ordinarily as neat zero as possible, and stainless steel or other impermeable containers, inert gas headspace, etc ate employed. Red wines withstand and even benefit from small but repeated exposures to air. 

Considerable developmental effort is being devoted to aerosol formulations using the compressed gases given in Table 4. These propellants are used in some food and industrial aerosols. Carbon dioxide and nitrous oxide, which tend to be more soluble, are often preferred. When some of the compressed gas dissolves in the product concentrate, there is partial replenishment of the headspace as the gas is expelled. Hence, the greater the gas solubiUty, the more gas is available to maintain the initial conditions. 

Deodorization can be carried out ki batch, continuous, or semicontkiuous systems. Figure 4 shows a typical design for a semicontkiuous deodorizer. The heated ok is passed through a series of trays under vacuum. Steam is passed through the ok through a steam sparge ki the bottom of the tray. Volatiles are carried through the headspace and condensed. In addition to fatty acids and compounds responsible for odor, some tocopherols and sterols are also distilled kito the condensate. The amount of tocopherols distilled depends on deodorization temperature and vacuum. 

Blood and urine are most often analyzed for alcohol by headspace gas chromatography (qv) using an internal standard, eg, 1-propanol. Assays are straightforward and lend themselves to automation (see Automated instrumentation). Urine samples are collected as a voided specimen, ie, subjects must void their bladders, wait about 20 minutes, and then provide the urine sample. Voided urine samples provide the most accurate deterrnination of blood alcohol concentrations. Voided urine alcohol concentrations are divided by a factor of 1.3 to determine the equivalent blood alcohol concentration. The 1.3 value is used because urine has approximately one-third more water in it than blood and, at equiUbrium, there is about one-third more alcohol in the urine as in the blood. 

When a comparative analysis of the headspace volatiles of living and picked osmanthus flowers was performed by the dynamic headspace trapping method using Tenax GC, even more dramatic differences were observed, shown in Table 20 (60). 

It is interesting to compare data for headspace volatiles of the living vs picked and both of these with respect to the direct analysis of the extracts shown ia Table 19. 

Component CAS Registry Number Stmcture number In distilled oil, % In headspace, %... 

Table 30. Comparison of Components of the Headspace of Living Ripe Cardamom Seed and Commercial Oil ...

Table 37. Comparison of the Headspace Components of Whole and Crushed Clove Bud with a Commercial Oil ...

Table 38. Comparison of Components of Authentic Coriander Seed Oil and Headspace Volatiles over Freshly Crushed Ripe Coriander Seeds ...

Juniper Oil. The best juniper oil [8012-91 -7] is obtained from the steam distillation of the ripe cmshed, dried berries of Juniperus communis L., a shmb which grows wild in many regions of Europe, Asia, Africa, and North America. However, most commercial juniperberry oil comes from the fermented fmits as a by-product of flavors for alcohoHc beverages such as gin, brandy, Hquors, cordials, and sloe-gin. This represents the actual commercial juniperberry oil, since very Httle tme juniperberry oil is produced. A comparison of the headspace volatiles of ripe juniperberries (85) with an authentic, freshly prepared juniperberry oil (86) is shown in Table 42. 

Table 42. Comparison of Headspace Volatiles of Ripe Juniperberries with an Authentic Juniperberry Oil...

Pimento Berry Oil. The pimento or allspice tree, Pimenta dioca L. (syn. P. officinalis, Liadl.), a native of the West Indies and Central America, yields two essential oils of commercial importance pimento berry oil and pimenta leaf oil. The leaf oil finds some use ia perfumery for its resemblance to clove leaf and cinnamon leaf oils as a result of its high content of eugenol. Pimento berry oil is an item of commerce with extensive appHcation by the flavor industry ia food products such as meat sauces, sausages, and pickles, and moderate use ia perfumery, where it is used primarily as a modifier ia the modem spicy types of men s fragrances. The oil is steam-distilled from dried, cmshed, fully grown but unripe fmits. It is a pale yellow Hquid with a warm-spicy, sweet odor with a fresh, clean topnote, a tenacious, sweet-balsamic-spicy body, and a tea-like undertone. A comparative analysis of the headspace volatiles of ripe pimento berries and a commercial oil has been performed and differences are shown ia Table 52 (95). 

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