Purification of a Total Lipid Extract with Column Chromatography

Column chromatography is a flexible technique for purifying the complex mixture of compounds found in sediment. Mixtures separate as they move through the column and are collected in fractions, each containing a different chemical class of compounds. Therefore, column chromatography is often used as an additional purification step after initial isolation of the desired compound. Organic extracts such as total lipid extracts may be complex mixtures of many compounds. Some purification techniques, such as saponification, introduce compounds that can damage analytical instruments and so must be removed before analysis. This video is part of a series on lipid extraction, purification, and analysis from sediments. Once a total lipid extract is collected from a sedimentary sample, column chromatography is used to purify both alkenones and GDGTs, depending on the desired analysis.

In column chromatography, a mixture of chemical compounds is loaded onto a solid stationary phase such as silica gel. A mobile phase such as an organic solvent is then used to elute, or remove, the compounds from the column. The order in which the compounds are eluted depends on the strength of the interactions of the compounds with the silica gel and with the eluent.

The eluate is collected in fractions, each containing different compounds from the mixture. Depending on the properties of the compounds, a single solvent may provide sufficient separation and elute all of the compounds of interest. Otherwise, multiple solvents are used to elute each compound of interest in turn.

Polar compounds, which have an uneven distribution of charge, adsorb strongly to the polar silica gel, whereas apolar compounds adsorb weakly. Polar solvents have greater affinity for silica gel and therefore are more powerful eluents than apolar solvents. Thus, apolar solvents elute only apolar compounds, whereas polar solvents elute both apolar and polar compounds.

When the desired compounds are moderately polar, apolar compounds should be washed off the column with an apolar solvent before a polar solvent is used. To avoid eluting unwanted highly polar compounds such as acids, a polar eluent should not have more eluting power than needed for the most polar desired compound.

Now that you understand the principles of column chromatography, let's go through a procedure for purification of lipid biomarkers from a total lipid extract by silica gel column chromatography.

To remove organic contaminants, combust borosilicate glass pipettes, borosilicate glass vials, and glass wool for 6 h at 550 ?C. Once the glassware is ready for use, set up a rack to hold pipettes and vials. Obtain pipette bulbs, clean tweezers, a laboratory spatula, silica gel, hexane, dichloromethane, and methanol. With clean tweezers, place a small tuft of glass wool into the mouth of a pipette. Gently push the glass wool to the bottom of the pipette with the stem of another pipette to form a loose plug. Carefully load silica gel into the pipette until half full. Secure the pipette upright in the rack. Secure a 4-mL borosilicate glass vial below the tip of the pipette for waste collection. In another borosilicate glass vial, suspend up to 10 mg of dry sample from the saponification process in hexane. If the sample sticks to the walls of the vial, sonicate the vial for 5 min. The chromatography procedure can now begin.

To begin the chromatography, wash the silica gel column with 3 volumes of hexane to remove air bubbles and impurities. Then, replace the waste vial with an empty vial for the apolar fraction. With a glass pipette, load the sample onto the column and allow the suspension to soak into the silica gel. Work quickly so the column does not dry out during the procedure. Rinse the sample vial twice with small portions of hexane and transfer each rinse to the column. Continue adding hexane to the column until the collection vial is nearly full. Allow all of the hexane to finish entering the silica gel. Then, exchange the filled vial with an empty vial for the mid-polar fraction. Next, rinse the sample vial with DCM and add it to the column 3 times. Continue adding DCM to the column until the collection vial is nearly full. Allow the DCM to finish soaking into the silica gel and then exchange the filled vial with an empty vial for the polar fraction. Repeat this process with methanol.

Once the vial is nearly full, allow the methanol to finish dripping into the vial and then cap all of the vials. The mid-polar fraction contains the desired alkenones, while the GDGTs are in the polar fraction. For particularly dirty or complex alkenone samples, the mid-polar fraction must be further purified with urea adduction, before analysis.

Column chromatography is widely used in chemistry as an analytical and purification technique.

Carbon nanotubes, or CNTs, are increasingly used in many industries, but there is growing concern of their effects on human health. Varying the properties of CNTs changes how they behave in water and soil. To investigate how well porous media like sand and dirt retain CNTs, a column was prepared with porous soil as the stationary phase. First, fractions were collected during loading of the CNT solution onto the column to analyze the transport of CNTs through soil. Then, the CNTs still adsorbed to the soil were eluted and the fractions analyzed for the amount of CNTs that had remained in soil. The results lend insight into the relationship between CNT surface functionalization and their transport mechanisms in the environment.

Column chromatography can be operated on both large and small scales, and is therefore used when designing syntheses for industrial applications. Spider silks have excellent tensile strength and ductility, but cannot be harvested on an industrial scale. Following silk protein synthesis, the recombinant silk proteins are purified by affinity chromatography, in which the stationary phase is designed to trap only the desired molecule. Thorough washing and elution grants the pure protein fractions needed to spin spider silk in large?scales.

Many stationary phases are available for column chromatography. One stationary phase may not be suitable for all potential products of a synthesis with a broad substituent scope, such as this iodoaziridine synthesis. Crude product is mixed with various stationary phases and decomposition assessed by proton NMR. As proton NMR is highly sensitive, many stationary phases can be screened for product decomposition using a small amount of crude product. Column chromatography is then performed with the optimal stationary phase, allowing purification of novel and highly reactive compounds.

You've just watched JoVE's introduction to column chromatography for the purification of a total lipid extract. The following video will demonstrate how to further purify complex mixtures containing alkenones.

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