Carbowax™ retention values

While the cx dependence on the number of C atoms is monotonic for Carbowax, with MEAB the a values alternate with an increase in retention exhibited by n-alkylbenze-nes having an odd numbers of C atom in the molecule and by dialkylbenzenes that have an odd number of C atoms in their longer alkyl chain (which is in agreement with a greater linearity (length) of these structures). Therefore, liquid crystals retain more selectively 1-propyl-2-pentylbenzene than 1,2-dibutylbenzene, compared with common phases, which has a positive effect on their separation,... 

A method to select the appropriate stationary phase for analysis of a sample mixture is to consider the polar characteristics of the analytes and select a stationary phase of similar polarity. An analyte with similar polar character to the stationary phase will be well retained, the principle of like attracts like applies, and useful retention is then likely to occur leading to adequate selectivity and separation of the analytes primarily on the basis of volatility. Conversely, if the solute is immiscible with the stationary phase then little or no retention difference will be obtained. Further useful indication of retention characteristics may be obtained by analysing a sample on a non-polar and polar column, for example, a dual column GC fitted with Apiezon/OVlOl and Carbowax 20M columns temperature programmed 50-220°C at 10°Cmin with a final hold of 10 min. The chromatogram will indicate the polarity of stationary phase required for the components and the analysis can be repeated with columns of differing polarity, e.g. OV17, OV1701. Tables of RIs for various classes of compounds have been published, mainly for squalane, the reference non-polar stationary phase and Apiezon L, with RI values for Carbowax 20M as a reference polar stationary phase [10]. 

Here c and are the concentrations of the substance under test in the stationary and mobile phase respectively, F" and F are the volumes of the stationary and mobile phase in the column, and is the retention time of an inert substance. From measurements of and f the capacity ratio k, and from this the partition coefficient K, can be calculated. In Figure 37 experimentally determined log k values are plotted against the density p and the pressure p of supercritical carbon dioxide at 40 C as a mobile phase for SFC experiments with alkanes having between 10 and 30 carbon atoms a chemically bonded stationary phase (Carbowax 400 on Porasil C) was used and further details are given in the caption of Figure 37. > The curves show that with increasing density the retention time f and the capacity ratio k decrease and consequently the... 

Table 2 Retention time (log k) and molecular interaction energy values for a polyethyleneglycol (Carbowax ) phase (CPWAX, DBWAX). Reproduced by permission of Institute for Chromatography, ref. 13.

Ge(CH3)3C4Hg is selectively dealkylated by SnCl4 In CH3NO2 to give Sn(CH3)Cl3 and Ge(CH3)2(C4Hg)Cl [18]. For its GLC retention times on squalene and Carbowax 4000 columns and a comparison between experimental and calculated values, see [6]. 

As expected the steric effects are in the order Pr >Pr >Et> Me. When the distance of mixed tri-B-alkylborazoles from the paraffin line are compared with those calculated by simply adding the individuals alkyl group values, very good agreement is obtained (Table 34). Thus, if the log t values of tri-B-alklyborazoles are known in squalane, their retention times in Carbowax 4000 can be quite accurately predicted. 

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