Supramolecular structure of humic acids by TEM with improved sample preparation and staining

Understanding the role of the dissolved organic matter in the environment will benefit from the characterization of its components at the supramolecular scale. With its very high spatial resolution the transmission electron microscope (TEM) allows the determination of colloidal size, structure, and shape on aggregated substances as well as single particles. It also allows the determination of the chemical composition if coupled with an X-ray detector. However, TEM preparation artifacts may hamper the relevance of such observations. This study demonstrates the capacity of TEM for the size and shape analysis of humic substances. Three sample preparation techniques (ultramicrotomy sectioning after embedding into a hydrophilic resin, drop deposition, and absorption) were evaluated with the Fluka humic acid (HA). Additionally, several staining agents (cesium hydroxide, lead citrate, uranyl acetate, and ruthenium tetraoxide) were used to increase the humic acid contrast for TEM observations. An improved simple preparation method with selective staining of the HA is proposed which permits the imaging of the humic acid macromolecules. The results show that the Fluka humic acid is composed mainly of fibrils generated by the interconnection of basic molecules of about 20 nm diameter with some isolated larger macromolecules (30-200 nm).