Studying Structural Details in Complex Samples. I. Combining two Chromatographic Separation Methods with Ultrahigh Resolution Mass Spectrometry

The analysis of complex mixtures poses a challenge due to the high number of compounds present in a mixture, which often exceed the capabilities of analytical methods and instruments. Even more challenging is understanding the structural details of compounds within a complex sample. Most analytical methods provide just bulk information on complex samples, and individual structural details cannot be observed. High-resolution mass spectrometry, the best method to analyze complex samples, suffers from inherent problems for structural studies in complex systems because collision-induced fragmentation (CID) measurements cannot provide data from individual compounds alone. The combination of different steps of chromatographic separation, here the combination of size exclusion chromatography with argentation chromatography, provides sufficient reduction in complexity to implement a method that allows gaining structural details of individual compounds within a complex mixture. The combination of offline size exclusion chromatography followed by online argentation chromatography effectively creates fractions based on the respective properties of the compounds in the mixture (size and number of π-bonds and heteroatoms) and reduces matrix effects to a great extent. Mass spectrometry with ultrahigh resolution provides basic chemical information for each detected compound and also provides the opportunity to gain structural information from MS/MS experiments. The results indicate effectively separated sample fractions yielded by the chromatographic steps with tremendously decreased total numbers of compounds. Especially, argentation chromatography proved to be a valuable separation tool when it comes to heteroatom-containing constituents. In the end, the fragmentation experiments indicated high-quality data due to the clean ion isolation enabled by prior separation. The structural elucidations provided deep insights into the carbon space of crude oil.