Fractional interference order

The excess e < 1, also called fractional interference order, can be obtained from a comparison of (4.65) and (4.66) as... 

With normal-phase HPLC, oil samples were analyzed as is by simple dilution in n-hexane. A Du Pont Zorbax amino-bonded phase column, 25 cm x 0.46 cm ID, was used, with n-hexane and dichloromethane as solvents. For reversed-phase HPLC, Vydac 201TP5 columns were used (25 cm x 0.46 cm ID for analytical scale and 25 cm x 1 cm ID for preparative scale). Samples for reversed-phase HPLC were fractionated in order to remove the saturated hydrocarbons which can interfere with the separation mechanism. The samples dissolved in n-hexane were passed Baker silica solid-phase extraction cartridges. The PAH fraction was then collected by eluting with a 1 1 mixture of dichloromethane and methanol. Acetonitrile and dichloromethane were used in the HPLC gradient. 

Giddings pointed out (32) that separated compounds must remain resolved throughout the whole process. This situation is illustrated in Figure 1.5, where two secondary columns are coupled to a primary column, and each secondary column is fed a fraction of duration Ar from the eluent from the first column. The peak capacity of the coupled system then depends on the plate number of each individual separation and on At. The primary column eliminates sample components that would otherwise interfere with the resolution of the components of interest in the secondary columns. An efficient primary separation may be wasted, however, if At is greater than the average peak width produced by the primary column, because of the recombination of resolved peaks after transfer into a secondary column. As At increases, the system approaches that of a tandem arrangement, and the resolution gained in one column may be nullified by the elution order in a subsequent column. 

A quantitative yield of a pure gas is usually necessary for the mass spectrometric measurement in order to prevent not only isotope fractionation during sample preparation, but also interference in the mass spectrometer. Contamination with gases having the same molecular masses and similar physical properties may be a serious problem. This is especially critical with CO2 and N2O, (Craig and Keeling 1963), and N2 and CO. When CO2 is used, interference by hydrocarbons and a CS + ion may also pose a problem. 

As already mentioned, uncontrolled superposition of size exclusion and interactive retention must be avoided since the order of separation in both types of chromatography is different. This is of particular importance in copolymer fractionation where macromolecules are to be separated which differ in size and chemical nature. Because of the strong influence of the latter on adsorption phenomena the results would be illusive if part of the sample was separated by exclusion and another by adsorption. Large-pore packings provide a chance to avoid interference as, in this case, all the... 

The lipid fraction of foods containing the fat-soluble vitamins is composed mainly of triglycerides, with much smaller amounts of sterols, carotenoids, phospholipids, and minor li-poidal constituents. All of these substances exhibit solubility properties similar to those of the fat-soluble vitamins, and therefore they constitute a potential source of interference. A proportion of the indigenous fat-soluble vitamin content of a food is bound up with a lipoprotein complex, and hence the fat-protein bonds must be broken in order to release the vitamin. The protective gelatin coating used in certain proprietary vitamin premixes will need to be dissolved before commencing the analysis of supplemented foods. 

We had shown that it was possible to extract both A9-THC and ll-hydroxy-A9-THC from plasma with toluene. The next question was the problem of lipophiles. Again we found that in order to remove interferences to A9-THC determinations it was necessary to fractionate the toluene extract with Claisen s alkali in order to obtain the phenol fraction of plasma. We found that the extractions of A9-THC from toluene with Claisen s alkali is not quantitative, but it is reproducible. The loss of A9-THC in this reaction however, is compensated for by removal of interferences which were the limiting factors in the single extraction step. 

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