Atmospheric pressure ion source high pass ion filter

Abstract

For generation and delivery of ions from an ionization chamber through an ion entrance orifice to a mass analyzer operating at high vacuum, high pass ion filtration is effected within the ionization chamber by application of electrical potentials to an electrode associated with the ion entrance orifice and to an electrode between the ionization region and the ion entrance orifice to create a retarding electric field upstream from the ion entrance orifice. The retarding electric field hinders the movement to the ion entrance orifice of ions having drift velocities below a lower limit, and as the retarding voltage gradient is made steeper, the lower limit increases.

What is claimed is:  
     
         1 . Apparatus for delivering ions to a mass analyzer operating at vacuum, comprising: 
 an ionization chamber comprising a chamber wall enclosing an ionization region and having a sample inlet and an ion entrance orifice that communicates downstream with a vacuum chamber    a first electrode associated with the ion entrance orifice,    a second electrode having an orifice situated within the ionization chamber upstream from the ion entrance orifice, wherein the ion entrance orifice and the second electrode orifice are aligned on an ion beam axis and wherein the first and second electrodes are connected to a source of electrical potential,    whereby in operation an electrical potential difference applied across the second electrode and the first electrode retards movement to the ion entrance orifice of ions having drift velocities below a selected lower limit.    
     
     
         2 . Apparatus of  claim 1  wherein the ionization source is an atmospheric pressure ionization source.  
     
     
         3 . Apparatus of  claim 1 , further comprising a third electrode having an orifice situated within the ionization chamber upstream from the second electrode orifice.  
     
     
         4 . Apparatus of  claim 3  wherein a space between the third and the second electrodes is connected to a source of gas, providing a flow of gas in an upstream direction through the third electrode aperture toward the ionization region.  
     
     
         5 . Apparatus of  claim 1  wherein the ion entrance orifice comprises an aperture in a plate.  
     
     
         6 . Apparatus of  claim 1  wherein the ion entrance orifice comprises an axial bore in a conduit.  
     
     
         7 . Apparatus of  claim 6  wherein said conduit is a capillary.  
     
     
         8 . Apparatus of  claim 1 , further comprising a source of drying gas in the ionization chamber for directing a drying gas toward the ionization region.  
     
     
         9 . A method for delivering ions to a mass analyzer operating at vacuum employing apparatus including an ionization chamber formed of chamber walls enclosing an ionization region and having a sample inlet and an ion entrance orifice that communicates downstream with a high vacuum chamber, and including a first electrode associated with the ion entrance orifice and further including within the ionization chamber a second electrode having an orifice situated upstream from the ion entrance orifice, configured so that the ion entrance orifice and the second electrode orifice are aligned on an ion beam axis, the method comprising applying electrical potentials to the electrodes such that: 
 a potential difference across the second electrode and the ion entrance electrode reduces the flux to the ion entrance orifice of ions having lower drift velocities.    
     
     
         10 . A method for effecting ion filtration within an ionization chamber enclosing an ionization region and having an ion entrance orifice that communicates with a vacuum chamber, the method comprising applying electrical potentials to an electrode associated with the ion entrance orifice and to an electrode situated between the ionization region and the ion entrance orifice to create a retarding electric field upstream from the ion entrance orifice, whereby the retarding electric field hinders movement to the ion entrance orifice of ions having drift velocities below a lower limit.