Ion trap mobility spectrometer

Abstract

An ITMS includes an inlet for receiving a sample that will be tested for a substance of interest. The inlet communicates with an ionization chamber and a drift chamber communicates with the downstream end of the ionization chamber. A first grid electrode extends across the downstream end of the ionization chamber and a second grid electrode is slightly downstream from and parallel to the first grid electrode. A slight potential bias is applied to the first grid electrode to hold the ions in the potential well between the first and second grid electrodes. However a pulse is applied periodically to the first grid electrode to accelerate ions into the drift chamber. The accumulation of the ions in the potential well prior to generation of the pulse results in a thinner band of ions ejected into the drift chamber and hence achieves higher resolution.

1 . An ion trap mobility spectrometer comprising: 
 an inlet for receiving a sample to be tested for the presence of at least one substance of interest;    an ionization chamber having an upstream end communicating with the inlet and a downstream end;    a drift chamber having an upstream end communicating with the downstream end of the ionization chamber and a downstream end opposed to the upstream end thereof;    a collector electrode in proximity to the downstream end of the drift chamber for collecting ions passing through the drift chamber;    first and second grid electrodes aligned substantially parallel to one another substantially at an interface between the downstream end of the ionization chamber and the upstream end of the drift chamber; and    a controller for applying a slight potential bias to the first grid electrode relative to the second grid electrode during an ion accumulation phase for accumulating ions between the first and second grid electrodes, the controller further being operative for applying a pulse to the first grid electrode for accelerating ions accumulated between the first and second grid electrodes into the drift chamber and towards the collector electrode.    
   
   
       2 . The ion trap mobility spectrometer of  claim 1 , wherein the second grid electrode and the upstream end of the drift chamber are maintained at substantially a common potential when the pulse is applied to the first grid electrode.  
   
   
       3 . The ion trap mobility spectrometer of  claim 2 , wherein a voltage of approximately 1,000 volts is applied to the second grid electrode when the pulse is applied to the first grid electrode for accelerating ions between the first and second grid electrodes into the drift chamber.  
   
   
       4 . The ion trap mobility spectrometer of  claim 1 , wherein the ionization chamber includes a substantially cup-shaped chamber wall formed from a conductive material.  
   
   
       5 . The ion trap mobility spectrometer of  claim 1 , wherein the controller is operative for applying a negative bias to the first grid electrode for a positive ion mode of operation and for applying a positive bias to the first grid electrode for negative ion mode operation.  
   
   
       6 . A method for operating an ion trap mobility spectrometer to determine whether trace amounts of substances of interest are present in a sample, the ion trap mobility spectrometer having an inlet for receiving a sample to be tested for the trace amounts of the substance of interest, an ionization chamber communicating with the inlet, first and second substantially parallel grid electrode disposed sequentially substantially at a downstream end of the ionization chamber and a drift chamber downstream from the second grid electrode, said method comprising: 
 operating the ion trap mobility spectrometer during an ion accumulation phase by applying a potential bias to the first grid electrode relative to both peripheral walls of the ionization chamber and the second grid electrode for defining a potential well between the first and second grid electrodes that accumulates a narrow band of ions therein;    applying a pulse to the first grid electrode while maintaining the second grid electrode substantially at a potential defined by the drift chamber for accelerating the narrow band of ions from the potential well defined between the first and second grid electrodes.    
   
   
       7 . The method of  claim 6 , wherein the step of applying a potential bias to the first grid electrode comprises applying a negative bias to the first grid electrode for a positive ion mode of operation to detect narcotics in the sample.  
   
   
       8 . The method of  claim 6 , wherein the step of applying a potential bias to the first grid electrode comprises providing a positive bias to the first grid electrode for operating the ion trap mobility spectrometer in a negative mode to test samples for the presence of explosives.  
   
   
       9 . The method of  claim 6 , wherein the step of applying a pulse comprises a pulse for a duration of 0.1-0.2 mS.  
   
   
       10 . The method of  claim 9 , further comprising adjusting the second grid electrode to a voltage substantially corresponding to voltage existing in adjacent portions of the drift chamber while the pulse is applied to the first grid electrode.