作図法による高級ジメチルパラフィンの保持指標の推定
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概要
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A graphical method for estimating retention indices in gas chromatography of higher dimethyl paraffins than C<SUB>10</SUB> was investigated.<BR>In this method, the retention indices of these compounds are indirectly obtained in the form of "retention index component of methyl radical", (Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB>), of methyl paraffins, which are calculated by subtracting linear carbon number, (<I>x</I>), multiplied by a factor of 100 from retention indices of mono- or dimethyl paraffins.<BR>Three rules are derived from the relationship between Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> and <I>x</I>, those are as follows;<BR>(1) Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> values of monomethyl paraffin homologues in regard to substituted position of a methyl group tend to decrease exponentially with increase of the <I>x</I>, and to approach to respective converged values. These values of 2-, 3-, 4-, 5-, and 6-methyl paraffin homologues, for example, are 63.5, 68.8, 57.9, 50.9, and 46.4, respectively, for a squalane column at 80°C. The same tendency was observed in the case of dimethyl paraffins.<BR>(2) when a dimethyl paraffin has one or more methylene groups between the two methyl groups, its Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> value is approximately equal to a sum of the converged Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> of the two corresponding monomethyl paraffins, and the difference between them approaches to zero with increase of <I>x</I>.<BR>(3) the Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> value of <I>k</I>, <I>k</I>-dimethyl paraffin (<I>k</I>≥3) is approximately equal to the double of Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> of <I>k</I>-monomethyl paraffin having the same carbon number, and the difference between them approaches to zero with increase of <I>x</I>.<BR>For this work, the retention indices of lower monoor di-methyl paraffins than C<SUB>10</SUB> were cited from literatures, and calculated their Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> for a squalane column at 80°C. These values are plotted against <I>x</I>, and extrapolated on the basis of the rules mentioned above. The Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB>-<I>x</I> curves can be drawn accurately by drawing the straight lines which denote the rate of variation of Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB>-<I>x</I> curves.<BR>In the case of monomethyl paraffin homologues such as 6- or 7-methyl, of which retention data is not at hand, their Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB>-<I>x</I> curves can be estimated by using Δ<I>I</I><SUP>Me</SUP><SUB>R</SUB> values of another lower monomethyl paraffin homologues such as 5- or 4-methyl. The similar estimation technique can be applied to dimethyl paraffins.<BR>By comparing the retention indices of C<SUB>13</SUB> dimethyl isomers calculated by means of this method with experimental values in a literature, the standard deviation of the differences between them is within 2 i. u.
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