Measurement of external ion injection and trapping efficiency in the ion trap mass spectrometer and comparison with a predictive model

Abstract

The efficiency of injecting and trapping ions from an external ion source in a quadrupole ion trap mass spectrometer was measured and compared against a simulation employing a simple hard-sphere collision model. A commercial ion trap mass spectrometer was modified with the addition of a variable energy ion gun, split Faraday cup/electron multiplier detector, and the capability to measure the ion current incident on the ion trap inlet and exit end caps. The entire instrument was modeled using the ion optics simulation program SIMION and user programs were applied in SIMION which modeled the collisions between the injected ions and the helium buffer gas. The model was based on simple hard-sphere collision physics with randomized collision angles and collision probability converging to the mean free path (mfp); the helium buffer gas was assumed to be at rest relative to the injected ions. The measured trapping efficiency of Cs + with helium buffer gas at optimum conditions was 3–5%. The trapping efficiency predicted by the model and the dependence on buffer gas pressure (mfp) agreed with the measured efficiencies within the uncertainties of the model predictions and experimental variance. The fidelity of the simulation was shown to be sensitive to the spatial resolution of the model when predicting the dependence of trapping efficiency on q z. It was concluded that a hard-sphere collision model was adequate for predicting the effects of helium buffer gas pressure on ion injection and trapping within the range of injection energies and buffer gas pressures tested.