Affiliation: College of Arts and Sciences, Department of Chemistry
Mass spectrometry has become a powerful analytical technique because it provides high sensitivity, short analysis times, and provides quantitative measurements of chemical and biological systems. Mass spectrometry also provides a high degree of selectivity, separating ions based on their mass-to-charge ratio. Isobaric or isomeric ions which have the same mass-to-charge ratio are more difficult to distinguish with mass spectrometry. Methods have been developed for distinguishing isobaric/isomeric compounds, the most common of which is collision induced dissociation (CID). Isomeric ions can also be distinguished by unique reaction with other ions (ion-ion reactions) or molecules (ion-molecule reactions). One example of an ion-molecule reaction is the adduction of water to lithium cationized molecules, [M+Li]+, in a quadrupole ion trap, producing [M+Li+H2O]+ observed 18 mass-to-charge units higher than [M+Li]+. This water adduction reaction was used to distinguish several different monosaccharide isomers including an exhaustive list of D-pentoses and several biologically relevant hexoses, hexosamines, and N-acetyl hexosamaines. These isomers could be distinguished by at least one of two metrics of the water adduction reaction. The first metric is the water adduction reaction rate. The second metric is the fraction of [M+Li]+ that will not adduct water, even when allowed very long reaction times. This fraction is very reproducible and unique for different isomers. The chemistry behind the unreactive fraction is studied with a combination of density functional theory calculations and experimental results. Together the reaction rate and the unreactive fraction of ions were then used to determine the relative concentration of two different hexoses in a binary mixture and determine the anomeric ratio of glucose in different solvents. Water adduction was used to distinguish several different glucose-glucose disaccharides, determining both the linkage position and anomericity of the glycosydic linkage.