Reverse Kendrick Mass Defect Analysis: Rotating Mass Defect Graphs to Determine Oligomer Compositions for Homopolymers

A new approach to determining the repeat unit compositions of homopolymers is reported in which a mass defect graph is rotated to zero slope to give a graph identical to a Kendrick mass defect graph. Because the Kendrick mass defect (KMD) is directly related to the elemental composition of the base unit, the process can be reversed. A mass defect graph (fractional m/z plotted against exact m/z) of a homopolymer can be rotated until the slope of the data points is zero. This is equivalent to finding a new constant factor by which the measured exact masses would have to be multiplied to create a Kendrick mass defect graph with zero slope. The elemental composition of the repeat unit can be determined by matching the new factor against the calculated factors for candidate compositions. This approach provides some benefits over simply looking for pairs of peaks corresponding to oligomer units. The primary benefit is to assist in visualization of the data. Rotating the data points corresponding to polymer masses to zero slope makes it easier to visualize the polymer data, and it facilitates the graphical isolation of polymer masses from background interferences. The repeat unit composition is determined not from a single pair of peaks but from multiple data points, and systematic errors in mass assignment can be visualized as deviations from linearity. Resolution-enhanced KMD graphs can be constructed for the calculated repeat unit composition by using fractional base units.