Pentane interference

In Eq. (25) the first term represents the contribution to the entropy from rotational isomerism without regard to lattice interferences, and 0.86 R the decrease in entropy due to packing on the lattice . The values of A SD calculated by difference were found to lie between 1.7 and 2.0 cal deg-1 mole-1 for polyethylene, polyoxymethylene and polytetrafluoro-ethylene. According to this theory when E equals zero,/equals Vs instead of V3 as one would expect if pentane interference did not occur. Assuming that A SD is compensated for by 0.86 R, values of A SR computed from the first term on the right hand side of Eq. (25) agreed farily well with the observed values of (A S,)v. 

Fig. 3.4 Conformations of the n-pentane molecule. Pentane interference occurs in the gauche -gauche conformation.

Figure 13.4 shows the standard chromatogram obtained by using this method. In this sample, isoprene and major interference compounds, i.e. 2-methyl pentane and hexane, were spiked at 1 p.g 1 , plus DMS at lOpig 1 . ... 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

Reactions at Eqs. (12) and (13) are carried out in toluene solution. The yellow crystalline products are purified by recrystallization from pentane and stored in a dry atmosphere at —20 °C. Reaction at Eq. (14) takes place slowly at 20 °C in benzene. The sulfane product H2S is separated from the silylester side product by distillation together with the benzene solvent (in the case of n=2-4) or by phase separation (for n=4, 5). For certain preparative purposes the ester and the solvent may not interfere. If deuterated triflu-oroacetic acid is used in the reaction at Eq. (14) the fully deuterated sulfane is obtained [29]. 

Removal of FMOC-OH can be achieved by liquid-liquid extraction (normally into pentane).A more elegant method of preventing interference of FMOC-OH is to react excess FMOC with a very hydrophobic amine (e.g. I -aminoadamantane) to form a derivative which elutes after the peaks of interest. 

Aznarez et al. [2] have described a Spectrophotometric method using curcumin as chromopore for the determination of boron in soil. Boron is extracted from the soil into methyl isobutyl ketone with 2-methylpentane-2,4-diol. In this method 0.2-lg of finely ground soil is digested with 5ml concentrated nitric-perchloric acid (3 + 1) in a polytetrafluoroethylene lined pressure pump for 2h at 150°C. The filtrate is neutralized with 6M sodium hydroxide and diluted to 100ml with hydrochloric acid 1+l.This solution is triple extracted with 10ml of methyl isobutyl ketone to remove iron interference. This solution is then extracted with 10ml 2-methyl pentane-2,4 diol and this extract dried over anhydrous sodium sulphate. 

Methanol can be determined colorimetrically but must usually be distilled from the reaction mixture before being analyzed. Wood and Siddiqui (27) described a simple and precise spectrophotometric method for measuring methanol in an assay for PE. The method involved permanganate oxidation of methanol to formaldehyde and reaction of the formaldehyde with pentane-2,4-dione. The color reaction was developed directly in the PE reaction mixture without interference from the pectin. However, Termote et al. (28)... 

The stability of the hydrate structure depends heavily on the size of the entrapped guest molecules. Ultimately the molecules can become so large (pentane for example) that a stable hydrate cannot form. As noted earlier, many materials can interfere and prevent hydrate formation. 

These fractions are combined and carefully evaporated on a steam bath to eliminate the n-pentane, CHCI3, and most of the benzene prior to the IR measurement. A minor amount of benzene does not interfere with the measurements. The 10 ml of solvent is first evaporated to 2 ml and then replenished with 2 ml CCI4. A second evaporation to 0.5 ml cleans up the solution for a satisfactory IR measurement. 

Isolation of the hydrocarbons from other lipids The total lipid extract may be subjected to removal of elemental sulphur by passage through an activated copper column (Blumer, 1957) and then to chromatographic separation on adsorbent columns or thin layer plates. For column chromatography, silic el is used with a short alumina bed on the top of the silic el. Both adsorbents should be partially deactivated by the addition of water (2—5%) to prevent the formation of artifacts (Blumer, 1970). Elution with a non-polar solvent such as hexane or pentane and subsequently with mixtures of non-polar and polar solvents, e.g. benzene and methanol, permits the isolation of several fractions containing saturated, unsaturated, aromatic hydrocarbons and more polar compounds (methyl esters, alcohols, acids, phenols and heterocyclic compounds). The interference from esters encountered in the isolation of aromatic hydrocarbons can be avoided prior to separation by saponification of the esters of fatty acids, which are easily removed. 

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