Nonpolar columns

On nonpolar columns, the compounds of a homologous series separate as a function of their boiling points, and linear relationships have been established between the logarithms of the retention volumes and the number of carbon atoms in the 2-, 4-, and 5-positions (see Fig. III-l). 

The accurate determination of relative retention volumes and Kovats indices is of great utility to the analyst, for besides being tools of identification, they can also be related to thermodynamic properties of solutions (measurements of vapor pressure and heats of vaporization on nonpolar columns) and activity coefficients on polar columns by simple relationships (179). 

The purity of a dicyclopentadiene stream may be expressed in terms of DCPD itself or in terms of available CPD monomer. Both analyses are deterrnined by gas chromatography (gc). The first analysis is capillary gc on a nonpolar column. The data from the analysis can be used to calculate the available CPD, assuming that all the DCPD and CPD codimers crack completely. In the second analysis the sample is charged to the gc equipment under temperature conditions (injection port 400°C) that cause essentially complete reaction of the dimers to monomers. 

For routine separations, there are about a dozen useful phases for capillary columns. The best general-purpose columns are the dimethylpolysiloxane (DB-1 or equivalent) and the 5% phenyl, 95% dimethylpolysiloxane (DB-5 or equivalent). These relatively nonpolar columns are recommended because they provide adequate resolution and are less prone to bleed than the more polar phases. If a DB-1, DB-5, or equivalent capillary column does not give the necessary resolution, try a more polar phase such as DB-23, CP-Sil88, or Carbowax 20M, providing the maximum operating temperature of the column is high enough for the sample of interest. See Appendix 3 for fused silica capillary columns from various suppliers. 

Early work relied on the use of packed columns, but all modern GC analyses are accomplished using capillary columns with their higher theoretical plate counts and resolution and improved sensitivity. Although a variety of analytical columns have been employed for the GC of triazine compounds, the columns most often used are fused-silica capillary columns coated with 5% phenyl-95% methylpolysiloxane. These nonpolar columns in conjunction with the appropriate temperature and pressure programming and pressure pulse spiking techniques provide excellent separation and sensitivity for the triazine compounds. Typically, columns of 30 m x 0.25-mm i.d. and 0.25-qm film thickness are used of which numerous versions are commercially available (e.g., DB-5, HP-5, SP-5, CP-Sil 8 CB, etc.). Of course, the column selected must be considered in conjunction with the overall design and goals of the particular study. 

Certain false positives are common (EPA 8020). For example, trimethylben-zenes and gasoline constituents are freqnently identified as chlorobenzenes (EPA 602, EPA 8020) becanse these componnds elnte with nearly the same retention times from nonpolar columns. Cyclohexane is often mistaken for benzene (EPA 8015/8020) becanse both compounds are detected by a 10.2-eV photoionization detector and have nearly the same elntion time from a nonpolar colnmn (EPA 8015). The two compounds have very different retention times on a more polar column (EPA 8020), but a more polar column skews the carbon ranges (EPA 8015). False positives for oxygenates in gasoline are common, especially in highly contaminated samples. 

The basic choices are the stationary phase, column diameter and length, and the thickness of stationary phase. A nonpolar stationary phase in Table 24-1 is most useful. An intermediate polarity stationary phase will handle most separations that the nonpolar column cannot. For highly polar compounds, a strongly polar column might be necessary. Optical isomers and closely related geometric isomers require special stationary phases for separation. 

The selectivity of a stationary phase for a particular compound can be measured in terms of the degree to which the retention index differs from the retention index obtained with a nonpolar column. As shown in Figure 3.10, the retention index for benzene was 649 on the squalane column. If the experiment is repeated in exactly the same manner but using dinonylphthalate as a stationary phase, the retention index is 733. This increase, Al, of 84 units of retention index indicates that the dinonylphthalate column will retard the benzene slightly more than will the squalane column. Under the same conditions, a highly polar phase such as SP-2340 would give a retention index of 1169 which would be a Al of 520 units. An excellent review of the retention index system has been presented by Ettre (22). 

For chemical substances to be analyzed by gas chromatography, they must be relatively nonpolar and volatile. Fatty acids are not readily vaporized at temperatures attainable in a gas chromatograph (up to 200°C). FAMEs have much lower boiling points than fatty acids. Polar fatty acids also have very long retention times unless extremely nonpolar column packings are used. 

Using a micropipet, add one drop of each n-paraffin from heptane to hexadecane into a 100-ml brown glass bottle containing 50 ml of an appropriate solvent (e.g., pentane for nonpolar columns). Seal with a Teflon-lined screw cap. Store at 4°C in the refrigerator for years. 

C7-C20 -alkane hydrocarbon standard for a nonpolar column, or C9-C30 for a polar column (hydrocarbon standard see recipe)... 

Working solution dilute 1 ml of stock solution to 10 ml For nonpolar columns use a standard composed of C7-C18 n-alkanes For polar columns use a standard composed of C9-C30 -alkanes Over a period of time, preferential evaporation of the lower molecular weight alkanes will occur. Unless known weights of each alkane are required, this is not a serious problem. Using a working solution minimizes this problem and also avoids contamination. When the quality of the standard becomes unacceptable, a new sample can be diluted from the stock solution. 

On a 30-m nonpolar column, the chromatogram for a hydrocarbon standard C7 to C2owith a temperature program of 2°C/min starting at 35°C will take 90 min to run with an alkane eluting every 6 to 8 min. With arate of 6°C/min, the run will take 40 min, with an alkane eluting every 2 to 3 min. 

Retention changes work exactly the same with reverse-phase column as with normal-phase columns. Increasing the polarity difference between column and mobile phase increases the /c s of the components. However, since the column is nonpolar, we now must add more of the polar solvent to make compounds stick tighter. On our reversed-phase column, our dye mixture would also elute in opposite order, the more polar red dye would have less affinity for the nonpolar column and would elute before the nonpolar blue dye. By controlling the column nature, you control the elution order. Figure 4.6 illustrates the effect of solvent polarity changes on a separation. 

Assuming we have selected the proper mode of chromatography, will the mixture dissolve in the mobile phase Ion-exchange columns must be run in polar-charged solvents. Size separation columns are not, in theory, affected by solvent polarity, and size columns for use in both polar and nonpolar solvents are available. In partition chromatography, we have nonpolar columns that can be run in polar or aqueous solvents, and polar columns that are only run in anhydrous, nonpolar solvents. Intermediate columns such as cyanopropyl or diol can be run in either polar or nonpolar solvents, although often with differing specificity. An amino column (actually a propylamino) acts in methylene chloride/hexane like a less polar silica column but in acetonitrile/water... 

When I make a diagram of column polarities versus solvent polarities, I tend to think of the columns as being a continuous series of increasing polarity from Cis to silica C18, phenyl, C8, cyano, C3, diol, amino, and silica (Fig. 5.5). Under that, I have their solvents in opposite order of polarity from hexane under Ci8 to water under silica hexane, benzene, methylene chloride, chloroform, THF, acetonitrile, i-PrOH, MeOH, and water. The cyano column and THF are about equivalent polarity. In setting up a separation system, we cross over nonpolar columns require polar mobile phase and vice versa to achieve a polarity difference. 

To make a separation, I look at the polarity of the compound I want (X) and its impurity (Y). Like attracts like. Let s assume that compound X is more nonpolar then its impurity Y. On a Ci8 column, the nonpolar compound sticks tightest to the nonpolar column and elutes last the more polar impurity comes off first. Running the same separation on a silica column in a nonpolar solvent, we should expect a reversal. The polar impurity Y sticks to the polar column, while the nonpolar compound X washes out first in the nonpolar solvent. By thinking about the polarities involved in the separation, we can control the separation. 

C8 (Octyl)—Nonpolar column or packing with 8 carbon hydrophobic hydrocarbon chain bound to silica. 

Cig (Octyldecyl)—Nonpolar column or packing with an 18 carbon hydrophilic hydrocarbon chain bound to silica. Used for reversed phase separations. (See ODS.)... 

Reverse-Phase Chromatography—Separation mode on bonded phase columns in which the solvent/column polarities are the opposite of normal-phase separations. Polar compounds elute before nonpolar compounds, Nonpolar columns require polar solvents. 

Aldehydes and ketones are polar compounds that can be separated on a polar or an intermediate polar column. Polyethylene glycol (PEG)-type phase, such as Carbowax 20 M, Supelcowax 10, VOCOL, DBWax, or equivalent are suitable for the purpose. Compounds may also be separated according to their boiling points on a nonpolar column. A 60 m long, 0.53 mm ID and 1 pm film or other appropriate dimension methyl silicone capillary columns, such as SPB-1, DB-1, or DB-5. 

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