Resolving Isomeric Structures in Natural Complex Carbohydrates: A Comparative Study of SLIM and Cyclic Ion Mobility...

Recent advances in ion mobility mass spectrometry (IMS) have led to the development of high-resolution platforms such as cyclic IMS and SLIM IMS (structures for lossless ion manipulations), both offering enhanced capabilities for resolving complex biomolecular conformations.

In this study, these two IMS technologies were evaluated for the characterization of complex natural polysaccharides, using homogalacturonan oligosaccharides (HGOS) derived from plant cell wall pectin as a model system. HGOS were generated through enzymatic depolymerization of the parent polymer and analyzed on both platforms. This approach enabled, for the first time, a direct comparison of mass spectra and ion mobility maps for identical species with varying degrees of polymerization (DP) and methylation (DM).

As expected, single-pass analyses on the 10-m SLIM IMS provided higher resolving power, allowing a greater number of IMS features to be separated for a given species (e.g., DP5DM4) compared to single-pass cyclic IMS experiments. However, IMS features unresolved in cyclic IMS single-pass mode could be accessed through multi-pass separations. A key distinction between the platforms lies in ion formation and charge state distributions. The SLIM IMS, retrofitted to an Agilent 6560, utilizes a commercial ion source and ion optics that result in the formation of larger and higher charged species, with more cation adducts. While beneficial for resolution, this distribution can dilute signal intensity across multiple charge states, impacting sensitivity. In contrast, the cyclic IMS employs a Z-spray source that favors lower charge states, facilitating more straightforward diagnostic interpretation and characterization. Despite the superior intrinsic IMS resolution of the 10-m SLIM, the combination of ionization behavior, instrument tuning requirements, and data processing complexity makes the cyclic IMS more practical for routine analysis.

Both IMS platforms demonstrated strong capabilities for resolving HGOS structures, highlighting their complementarity and effectiveness in studying complex polysaccharide mixtures.