Transcending Resolution Limits in HPLC and Diffusion NMR Nouran A. Hamed, Alexandria K. Shread, Gareth A. Morris, Mathias Nilsson Analytical Chemistry, 2024 Mixture analysis is crucial in many areas of chemistry, and a wide variety of separation methods are in use. A common method for physical separation is high-performance liquid chromatography (HPLC), but resolution is a problem: chemically similar species coelute. An alternative approach is diffusion-ordered NMR spectroscopy (DOSY), in which the signals of mixture components are separated according to the diffusion coefficient. Again, separation is limited if species diffuse similarly or have overlap in their NMR spectra. Using the two techniques in combination can resolve both NMR spectra and the elution profiles of individual components, even where both techniques fail when used in isolation. Recording diffusion NMR data as a function of HPLC retention time gives a three-dimensional (3D) data set that can be analyzed using multiway statistical methods. PARAFAC analysis of diffusion NMR data measured from HPLC eluate for commercial "monoacetin" (a mixture of glycerol and its mono-, di-, and triacetates) yielded fully resolved and quantitative NMR spectra and elution profiles for all four components, whereas neither HPLC nor diffusion NMR applied independently was able to resolve the components.
Spectroscopic methods for determination of critical micelle concentrations of surfactants; a comprehensive review Mokhtar M. Mabrouk, Nouran A. Hamed, Fotouh R. Mansour Applied Spectroscopy Reviews, 2023 The applications of surfactants in various fields are gaining more attention, which makes full characterization of surfactants of growing interest. It is fundamental to measure the critical micelle concentration (CMC) as a parameter for characterizing surfactants. Spectroscopic methods for determination of CMC are more common, easier to perform, and in certain applications more accurate. In this review, different spectroscopic techniques and methods used for determination of CMC are discussed. These methods include direct UV/VIS Spectroscopy, which studies liquid surface curvature in thin wells using vertical detecting light beam with the wavelength set at 900 nm. The indirect UV/VIS Spectroscopic methods include using surface plasmon resonance or surface-enhanced Raman scattering of metal nanoparticles. Direct spectrofluorometric methods measure CMC based on the intrinsic fluorescence of the tested surfactants, and it was reserved for surfactants with intrinsic fluorescence such as Triton-X100. Indirect spectrofluorometric methods include measuring the change in fluorescence intensity, spectral shape, lifetime, polarization, or the solvatochromic shift of surfactant-dye solution. Other spectroscopic methods have been reported such as X-ray diffraction, nuclear magnetic resonance spectroscopy and small-angle neutron scattering. This review article discusses the spectroscopic methods developed for CMC determination with emphasis on the principle, applications, advantages, and limitations of each method.
