U の尿素の内転によって分岐、環状化合物の除去^k'₃₇ Paleothermometry

Urea forms a porous crystalline structure with long, narrow spaces in its lattice. Straight-chain or rarely-branching compounds like n-alkanes and alkenones fit into these spaces, while highly-branched and cyclic compounds are too large to enter. After washing with an apolar solvent, the excluded molecules separate from the included linear ones, enabling selective purification based on molecular size and shape.

Urea adduction purifies complex lipid extracts by isolating compounds of interest from interfering substances. In paleothermometry, it separates alkenones and other biomarkers from unwanted branched and cyclic compounds, enabling accurate measurement of paleoclimate proxies. This size-exclusion purification ensures that only target compounds are analyzed, improving data reliability for reconstructing past sea surface temperatures.

Straight-chain compounds trapped within the urea crystal lattice cannot be extracted directly with apolar solvents because they are physically enclosed. Dissolving the crystals in water releases the included molecules back into solution. An apolar solvent is then used to extract the desired compounds from the aqueous phase, separating them from the dissolved urea.

Highly branched isoprenoids produced by sea ice diatoms serve as proxies for seasonal sea ice presence at high latitudes. Cyclic compounds may reflect the occurrence of past fires in the sediment record. These excluded compounds, separated from linear biomarkers through urea adduction, offer complementary paleoenvironmental information alongside straight-chain proxies for ecosystem structure and temperature.

Urea adduction is optional when compounds of interest can be analyzed directly from unaltered extracted organic samples without interference. However, it becomes necessary in extreme cases, such as sediments from highly polluted areas like industrial estuaries, where unknown compounds coelute during analysis. Removing these interfering substances ensures accurate measurement of target biomarkers.

In the petroleum industry, urea adduction separates branched-chain alkanes from straight-chain alkanes based on their molecular shapes. Branched-chain alkanes are used as light lubricants, while straight-chain alkanes serve in alkylation processes. Successive urea adductions can achieve high purity separation, with studies demonstrating 99-percent purity separation of alkanes from kerosene.

After white precipitate forms, gently dry the urea crystals under nitrogen with gentle heat until all solvent evaporates. Rinse the crystals three times with hexane, transferring each rinse to a fresh vial as the non-adduct fraction. Dissolve the crystals in water, then extract with hexane three times, combining all hexane extracts as the adduct fraction ready for analysis.