Image charge detection mass spectrometry: pushing the envelope with sensitivity and accuracy

A novel image charge detection mass spectrometer (CDMS) with improved sensitivity and mass accuracy is described. The improved detector design and method of data analysis allow us to measure a reliable mass for a single macroion that is an order of magnitude smaller than previously achieved with CDMS. The apparatus employs an image charge detector array consisting of 22 detectors. The detectors are divided into two groups that can be floated at different potentials. The signals from the detector array are analyzed using a correlation approach to yield the velocities in the two groups of detectors and the charge. These quantities, together with the voltage difference between the two groups of detectors, provide a value for the mass. The mass, m/z, and charge distributions recorded for 300 kDa poly(ethylene oxide) (PEG) are presented. The mass distribution shows a peak at around 300 kDa with a width close to that expected from the polymer size distribution. In addition, there are broad peaks in the mass distribution at around 100 and 500 MDa. The 300 kDa ions have m/z ratios of ～2 kDa/e, and the 100 and 500 MDa ions have m/z ratios of ～40 kDa/e. The 100 and 500 MDa ions probably result from PEG aggregates that are either present in solution or the residue of large electrospray droplets.