Volatiles, determination

A measured volume of sample purged with an inert gas styrene purged out from water absorbed on a sorbent trap trap heated and back flushed with He analyte transported into the GC column separated from other volatiles determined on a PID, FID or a mass spectrometer. 

Although autoxidatlon of lipids in foods is generally considered as unwanted, certain products of lipid autoxidatlon at low concentrations are necessary to the characteristic odor and aroma properties of meats from different species (8.9.28 >. Therefore, the concentration and relative abundance of these chemicals in meat volatiles determine whether they play a desirable or an undesirable role in flavor characteristics of cooked meats. Thus, the origin of flavor and off-flavors developments, which are somewhat species-specific, are perhaps the same. So, in freshly cooked meats the specific flavor of meat which is species-specific develops and progression of autoxidatlon results in the formation of undesirable warmed-over flavor in cooked meats upon storage. 

Because of its volatility, determination of vinylidene chloride is best by gas chromatography using a variety of detectors, including flame ionization, electron capture, electrolyte conductivity, and mass spectrometry. The major limitation is interference by other constituents of the media being analyzed. Methods are available for quantifying environmental samples (air, water, soil sediment) and biological samples (breath, food, body tissues). 

C remaining after volatiles determination (i.e. coking potential)... 

There is more to doing a volatilization determination quantitatively than you might believe. The desired gas often must be separated from other gases and it must be dried. The Knorr alkalimeter shown in Figure 1-2 for the determination of COj in limestone is a good example of the precautions that must be taken to ensure an accurate analysis. CO2 from the air must be removed as well as other possible impurities HCl, SO, HjS, and water vapor. 

Amino acids. Sugars and Organic Acids were analysed as previously described (4). Total Volatiles determination. Volatiles composition and concentration in the samples of melon flesh were determined using a headspace sampler (HP 19393) coupled to a gas chromatograph (HP 5890). Approximately 5g of flesh was obtained by using a cork borer to remove a number of cores from around the equator of the fhiit. This was immediately sealed in a headspace vial, placed in the headspace sampler at a bath temperature of 100 C, equilibrated for 15min before analysis on an HP 5890 GC equipped with an FID detector and fitted with a 30m x 0.32mm i.d. OVl fused silica colunm. 

As for patulin, it is necessary to derivatize it with trimethylsilyl ether or heptafluorobutyrate to make it volatile. Determination of patulin through GC-MS detection and quantification of the heptafluorobutyrate derivative after treatment with hepta-fluorobutyrylimidazole has also been developed. Trimethylsiliyl can also serve as a derivatiza-tion agent for the detection of patulin through GC-ECD or GC-MS. However, it should be noted that LC methods are generally preferred over GC methods because of their easiness of application. 

The volatility determination was part of a program to determine devolatilization kinetics fen petroleum coke. Samples of the petroleum coke were reacted in the drop tube furnace at ten )eratuies fiom 1000"C to 1700 C, in an argon atmosphere, to determine the rate of devolatilization. The procedure for calculating the kinetics was as follows ... 

The increased attention paid to ecological and environmental issues in recent years has lead to a growing significance of residual volatile determination in polymer dispersions. Depending upon the production process, polymer dispersions may contain small quantities of residual monomers, monomer impurities, substances formed by the decomposition of the initiator or from chemical reactions between the various components in the reaction mixture. The European Union has defined such substances as volatiles, if they have a boiling point below 250 °C. 

Brunauer and co-workers [129, 130] found values of of 1310, 1180, and 386 ergs/cm for CaO, Ca(OH)2 and tobermorite (a calcium silicate hydrate). Jura and Garland [131] reported a value of 1040 ergs/cm for magnesium oxide. Patterson and coworkers [132] used fractionated sodium chloride particles prepared by a volatilization method to find that the surface contribution to the low-temperature heat capacity varied approximately in proportion to the area determined by gas adsorption. Questions of equilibrium arise in these and adsorption studies on finely divided surfaces as discussed in Section X-3. 

The extent of dissociation at a given temperature can be determined by measuring the density of the vapour. Since anhydrous sulphuric acid is less volatile than hydrogen chloride, ammonium sulphate does not readily sublime on heating some ammonia is evolved to leave the hydrogensulphate ... 

Distillation. If the impurities in a liquid are non-volatile, the liquid may be purified by direct distillation, the impurities remaining in the distilling-flask. This process is therefore essentially the same as a simple distillation for boiling-point determination, and has been already described on pp. 7-9. 

Beckmann rearrangement of benzophenone oxime to benz-anilide. Dissolve 2 g. of benzophenone oxime in 20 ml. of anhydrous ether in a small conical flask and add 3 g. of powdered phosphorus pentachloride (or 3 ml. of pure tbionyl chloride). Distil off the solvent and other volatile products on a water bath CAUTION ether), add 25 ml. of water, boil for several minutes and break up any lumps which may be formed. Decant the supernatant liquid, and recrystallise, in the same vessel, from boiling alcohol. The product is benzanilide, m.p. 163° confirm this by a mixed m.p. determination with an authentic specimen. 

Separations based upon differences in the physical properties of the components. When procedures (1) or (2) are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Sections 11,15 and 11,17) the degree of separation may be determined by the range of boiling points and/or the refractive indices and densities of the different fractions that are collected. A mixture of non-volatile sohds may frequently be separated by making use of the differences in solubilities in inert solvents the separation is usually controlled by m.p. determinations. Sometimes one of the components of the mixture is volatile and can be separated by sublimation (see Section 11,45). 

If the water insoluble mixture is a liquid, evaporate a small sample (say, 4 ml.) in an evaporating dish on a water bath in order to determine the amount of volatile components, if any. If the solvent distils at the temperature of the boihng water bath, it is advisable to distil ofiF this solvent on a water bath and to replace it by ether. 

The distillate may contain volatile neutral compounds as well as volatile acids and phenols. Add a slight excess of 10-20 per cent, sodium hydroxide solution to this distillate and distil until the liquid passes over clear or has the density of pure water. The presence of a volatile, water-soluble neutral compound is detected by a periodic determination of the density (see Section XI,2) if the density is definitely less than unity, the presence of a neutral compound may be assumed. Keep this solution Si) for Step 4. 

Step 4. The steam-volatile neutral compounds. The solution (containing water-soluble neutral compounds obtained in Step 1 is usually very dilute. It is advisable to concentrate it by distillation until about one-third to one-half of the original volume is collected as distillate the process may be repeated if necessary and the progress of the concentration may be followed by determination of the densities of the distillates. It is frequently possible to salt out the neutral components from the concentrated distillate by saturating it with solid potassium carbonate. If a layer of neutral compound makes its appearance, remove it. Treat this upper layer (which usually contains much water) with solid anhydrous potassium carbonate if another aqueous layer forms, separate the upper organic layer and add more anhydrous potassium carbonate to it. Identify the neutral compound. 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

With samples that are difficult to dissolve, the first approach is usually to try digesting the sample with an acid or base. Table 7.2 lists the most commonly used acids and bases and summarizes their use. Digestion is commonly carried out in an open container, such as a beaker, using a hot plate as a source of heat. The chief advantage of this approach is its low cost as it requires no special equipment. Volatile reaction products, however, are lost, leading to a determinate error if analyte is included among the volatile substances. 

When thermal or chemical energy is used to remove a volatile species, we call the method volatilization gravimetry. In determining the moisture content of food, thermal energy vaporizes the H2O. The amount of carbon in an organic compound may be determined by using the chemical energy of combustion to convert C to CO2. 

Determine the identities of the volatilization products and the solid residue at each step of the thermal decomposition. 

Representative Method Although each volatilization gravimetric procedure has its own unique characteristics, the following indirect method for the determination of Si in ores and alloys by formation of volatile SiF4 provides an instructive example of a typical procedure. 

Any acid-soluble materials present in the sample will react with HF or H2SO4. If the products of these reactions are volatile or decompose at the ignition temperature of 1200 °C, then the change in weight will not be due solely to the volatilization of SiF4. The result is a positive determinate error. 

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