Ion transport through capillary atmospheric pressure interfaces of mass spectrometers

The transmission efficiency of ions/charged droplets traveling through typical atmospheric pressure interfaces (API) of mass spectrometers is investigated to advance the understanding of charge transport in micro capillaries. The charge transfer efficiency for various capillary dimensions and same flow conductance was focused upon. Quantitative measurements of current transmitted through the API and the current lost to the wall of the API were collected to assess the charge transmission efficiency of capillaries. Further, simulation strategies were developed and implemented to run a numerical simulation of the charge transport phenomenon for APCI ionization sources. Simulations were done using Fluent 6.3. The inlet of a typical lab-scale mass spectrometer was characterized for efficiency at different conditions of back pressure, capillary length and capillary flow conductance. It is found that within the range of vacuum pressures used in the present experiments, higher back pressures are conducive for higher efficiency in charge transport. Further, long capillaries of same diameter have lower magnitude of transmitted current. A computational model for simulating ion transport through capillaries was developed which shows good agreement with the experimental results. The computational model accounts for the effects of varying pressure and temperature scales on the diffusion loss of ions. The theoretical foundation for the model was based on hard ball model for gaseous collisions. Our model suggests that diffusive losses are predominant source of ion loss. Ion transport in capillaries of same flow conductance but varying diameters and lengths was studied to determine an effective interface between ambient and vacuum in mass spectrometers. Small inner diameter capillaries of shorter length have greater transmission efficiency. This provides a framework for selecting efficient interface. Charged droplets and dry ion transmission mechanism is completely different. Experimental studies reveal that the efficiency of charge droplet transport is higher as compared to that of dry ions. This is due to lower mobilities of large sized droplets and shielding of ions from discharge to walls by the solvent. A preliminary charge droplet transport model is developed and preliminary results for the same are reported