Electrostatic Linear Ion Trap Optimization Strategy for High Resolution Charge Detection Mass Spectrometry.

Single ion mass measurements allow mass distributions to be recorded for heterogeneous samples that cannot be analyzed by conventional mass spectrometry. In charge detection mass spectrometry (CD-MS), ions are detected using a conducting cylinder coupled to a charge sensitive amplifier. For optimum performance, the detection cylinder is embedded in an electrostatic linear ion trap (ELIT) where trapped ions oscillate between end-caps that act as opposing ion mirrors. The oscillating ions generate a periodic signal that is analyzed by fast Fourier transforms. The frequency yields the m/z, and the magnitude provides the charge. With a charge precision of 0.2 elementary charges, ions can be assigned to their correct charge states with a low error rate, and the m/z resolving power determines the mass resolving power. Previously, the best mass resolving power achieved with CD-MS was 300. We have recently increased the mass resolving power to 700, through the better optimization of the end-cap potentials. To make a more dramatic improvement in the m/z resolving power, it is necessary to find an ELIT geometry and end-cap potentials that can simultaneously make the ion oscillation frequency independent of both the ion energy and ion trajectory (angular divergence and radial offset) of the entering ion. We describe an optimization strategy that allows these conditions to be met while also adjusting the signal duty cycle to 50% to maximize the signal-to-noise ratio for the charge measurement. The optimized ELIT provides an m/z resolving power of over 300 000 in simulations. Coupled with the high precision charge determination available with CD-MS, this will yield a mass resolving power of 300 000. Such a high mass resolving power will be transformative for the analysis of heterogeneous samples.

Ion-Ion Interactions in Charge Detection Mass Spectrometry.

Charge detection mass spectrometry (CDMS) is a single-particle technique where the masses of individual ions are determined by simultaneously measuring their mass-to-charge ratio (m/z) and charge. Ions are usually trapped inside an electrostatic linear ion trap (ELIT) where they oscillate back and forth through a detection cylinder, generating a periodic signal that is analyzed by fast Fourier transforms. The oscillation frequency is related to the ion's m/z, and the magnitude is related to the ion's charge. In early work, multiple ion trapping events were discarded because there was a question about whether ion-ion interactions affected the results. Here, we report trajectory calculations performed to assess the influence of ion-ion interactions when multiple highly charged ions are simultaneously trapped in an ELIT. Ion-ion interactions cause trajectory and energy fluctuations that lead to variations in the oscillation frequencies that in turn degrade the precision and accuracy of the m/z measurements. The peak shapes acquire substantial high and low m/z tails, and the average m/z shifts to a higher value as the number of trapped ions increases. The effects of the ion-ion interactions are proportional to the product of the charges and the square root of the number of trapped ions and depend on the ions' m/z distribution. For the ELIT design examined here, ion-ion interactions limit the m/z resolving power to several hundred for a typical homogeneous ion population.