Method for linearization of ion currents in a quadrupole mass analyzer

Abstract

A method for linearizing the sensitivity of a quadrupole mass spectrometric system to allow the sensor to more accurately report partial pressures of a gas in high pressure areas in which the reported data is effected by a number of loss mechanisms. According to the invention, correction factors can be applied empirically or software in a quadrupole mass analyzer system can be equipped with correcting software to expand the useful range of the mass spectrometer.

What is claimed is:  
     
       1. A method for correcting a measurement of a partial pressure in a mass spectrometer system having means for measuring both  a measured partial pressure and means for measuring a measured total pressure in the millitorr range of a high vacuum, comprising the steps of: 
       measuring a total pressure ion current to obtain a measured total pressure ion current;  
       determining, based on said measured total pressure ion current, said measured total pressure;  
       measuring a partial pressure ion current to obtain a measured partial pressure;  
       determining a correction factor based on said measured total pressure ion current; and  
       correcting, using said correction factor, said measured partial pressure to obtain a corrected partial pressure.  
     
     
       2. A method according to  claim 1 , wherein a correction factor due to ion beam scattering is determined based on the relationship: 
       
         
           I corrected =I measured e KLP    
         
       
       in which I corrected  is linearized ion current, I measured  is measured ion current, K is a loss constant, L is the measured distance traveled by the ions through the mass spectrometer, and P is total sample gas pressure. 
     
     
       3. A method according to  claim 2 , wherein the correction factor e PLK  is determined by measuring the slope of a line obtained by the graphical output of In(S) versus system total pressure; said slope being equal to KL and wherein:        S   =       I   m   +       (     P                   P   m       )                       
       in which S=sensitivity which is equal to the ratio of ion current at a specified mass from a specific gas to the partial pressure of that gas, (PP m )=partial pressure of a specified gas and I m   + =measured ion current. 
     
     
       4. A method according to  claim 1 , wherein a correction factor based on non-gas scattering is determined by: 
       
         
           I corrected =I measured *F −1 (P)  
         
       
       wherein I corrected  is the corrected ion current, I measured  is ion current as measured by the mass spectrometer, and F −1 (P) is a correction factor based on sample gas pressure. 
     
     
       5. A method according to  claim 1 , wherein a correction factor based on columbic repulsion is determined by: 
       
         
           I corrected =I measured *F −1 (I m   + )  
         
       
       in which I corrected  is linearized ion current, I measured  is the measured ion current measured by the mass spectrometer for a transmitted ion mass, and F −1  is a correction factor which is dependent on ion current (I m   + ). 
     
     
       6. A method according to  claim 1 , wherein a total correction factor is determined based on the relationship: 
       
         
           CF TOTAL =e PKL *F 1 (P)*F 2 (I m   + )  
         
       
       in which 
       P=total pressure of gas  
       K=loss constant due to scattering  
       L=total ion distance traveled  
       F 1 (P)=pressure dependent correction factor due to non-gas scattering  
       and 
       F 2 (I m   + )=ion current correction factor due to columbic repulsion based on a measured positive ion current (I m   + ) for a transmitted ion mass, m.  
     
     
       7. A method according to  claim 1 , wherein said partial pressure measuring means are disposed within said mass spectrometer. 
     
     
       8. A method according to  claim 7 , wherein said total pressure measuring means are disposed within said mass spectrometer. 
     
     
       9. A mass spectrometer system comprising: 
       a mass spectrometer;  
       means for measuring a partial pressure and a total pressure in the millitorr range of a high vacuum in said mass spectrometer; and  
       means for correcting a measurement of a partial pressure using a correction factor based upon a measured total pressure ion current. 
     
     
       10. A mass spectrometer system according to  claim 9 , in which said total pressure measuring means are contained within said mass spectrometer.