Method and Apparatus for Operating a Vacuum Interface of a Mass Spectrometer

Abstract

A method is disclosed for operating a mass spectrometer vacuum interface, the vacuum interface comprising an evacuated expansion chamber downstream of a plasma ion source wherein the expansion chamber is pumped by an interface vacuum pump to provide an interface pressure in the chamber; the method comprising using a controller to automatically, or according to user input, control the throughput of the interface vacuum pump to control the interface pressure dependent on one or more operating modes of the spectrometer.

1 . A method of operating a mass spectrometer vacuum interface, the vacuum interface comprising an evacuated expansion chamber downstream of a plasma ion source at atmospheric or relatively high pressure, the expansion chamber having a first aperture that interfaces with the plasma ion source to form an expanding plasma downstream of the first aperture and a second aperture downstream of the first aperture from the plasma for skimming the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump to provide an interface pressure in the chamber;
 the method comprising using a controller to automatically control the throughput of the interface vacuum pump to control the interface pressure dependent on one or more operating modes of the spectrometer.   
     
     
         2 . A method according to  claim 1  wherein the throughput of the interface vacuum pump is controlled depending on one or more operating conditions of the plasma ion source and/or one or more elements of interest to be mass analysed by the spectrometer. 
     
     
         3 . A method according to  claim 2  wherein the plasma ion source is an inductively coupled plasma (ICP) ion source and the one or more operating conditions comprises a plasma temperature, plasma torch position and/or plasma gas flow. 
     
     
         4 . A method according to  claim 1  wherein the controller comprises a computer and associated controlling electronics interfaced to the interface vacuum pump. 
     
     
         5 . A method according to  claim 1  wherein the interface vacuum pump is a fore vacuum pump for a high vacuum pump that pumps a high vacuum region of the mass spectrometer and the interface pressure in the chamber is controlled to be in a range 0.1-10 mbar. 
     
     
         6 . A method according to  claim 1  further comprising acquiring data from the mass spectrometer at the controller and using the data to adjust the throughput of the vacuum pump in order to optimise the detection sensitivity of the spectrometer for at least one element. 
     
     
         7 . A method according to  claim 1  further comprising changing the plasma ion source operating conditions from a first operating condition to a second operating condition, or vice versa, and respectively automatically controlling the throughput of the interface vacuum pump from a first throughput when operating the plasma at the first operating condition to a second throughput when operating the plasma at the second operating condition, wherein the first and second throughputs are different to each other. 
     
     
         8 . A method according to  claim 7  wherein the first operating condition is a hot plasma condition and the second operating condition is a cold plasma condition. 
     
     
         9 . A method according to  claim 7  wherein the first throughput provides a first interface pressure in the interface vacuum stage to optimise detection sensitivity for at least one element of the sample being mass analysed using the first operating condition and the second throughput provides a second interface pressure to optimise detection sensitivity for at least one element being mass analysed using the second operating condition. 
     
     
         10 . A method according to  claim 9  wherein the at least one element whose detection sensitivity is optimised under the first operating condition is different to the at least one element whose detection sensitivity is optimised under the second operating condition. 
     
     
         11 . A method according to  claim 9  wherein the first and second interface pressures are controlled to be substantially the same. 
     
     
         12 . A method according to  claim 1  wherein more than two different operating conditions are provided, each having a respective vacuum pump throughput set by the controller. 
     
     
         13 . A method according to  claim 1  further comprising providing a pressure gauge in the expansion chamber, (i) measuring the interface pressure using the pressure gauge, (ii) providing signals representative of the measured pressure to the controller, and (iii) comparing the measured pressure to a set pressure using the controller and if the controller determines that there is a difference between the measured and set pressures, the controller adjusts the throughput of the vacuum pump to reduce the difference between the measured pressure and set pressure, wherein the steps (i)-(iii) are repeated in a feedback loop to maintain the interface pressure substantially at the set pressure. 
     
     
         14 . A method according to  claim 1  wherein the controller is connected to a Visual Display Unit (VDU), such that the initially automatically controlled throughput of the pump and/or the interface pressure are displayed to a user, and wherein by means of a graphical user interface (GUI) displayed on the VDU and an input device, the user overrides the automatic control and manually sets a particular throughput of the vacuum pump and/or a particular interface pressure. 
     
     
         15 . An apparatus for operating a mass spectrometer vacuum interface, comprising:
 a plasma ion source for generating a plasma at atmospheric or relatively high pressure;   an evacuated expansion chamber downstream of the plasma ion source, the expansion chamber having a first aperture that interfaces with the plasma ion source for forming an expanding plasma downstream of the first aperture and a second aperture downstream of the first aperture for skimming the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump to provide an interface pressure in the expansion chamber; and   a controller configured to automatically control the throughput of the vacuum pump dependent on one or more operating modes of the spectrometer.   
     
     
         16 . An apparatus according to  claim 15  wherein the controller is configured to automatically control the throughput of the vacuum pump dependent on a plasma condition and/or a measurement mode. 
     
     
         17 . An apparatus according to  claim 15  wherein the plasma ion source is an inductively coupled plasma (ICP) ion source. 
     
     
         18 . An apparatus according to  claim 15  wherein the controller comprises a computer and associated controlling electronics interfaced to the interface vacuum pump. 
     
     
         19 . An apparatus according to  claim 15  wherein the throughput of the vacuum pump can be continuously or quasi-continuously adjusted by the controller across a range of throughput speeds. 
     
     
         20 . An apparatus according to  claim 15  wherein the interface vacuum pump is a fore vacuum pump for a high vacuum pump that pumps a high vacuum region of the mass spectrometer. 
     
     
         21 . An apparatus according to  claim 15  wherein the controller is configured to automatically adjust the throughput of the interface vacuum pump from a first throughput when operating the plasma ion source at a first operating condition to a second throughput when operating the plasma ion source at a second operating condition, wherein the first and second throughputs are different to each other. 
     
     
         22 . An apparatus according to  claim 21  wherein the first operating condition and the second operating condition differ in the temperature of the plasma, preferably wherein the first operating condition is a hot plasma condition and the second operating condition is a cold plasma condition. 
     
     
         23 . An apparatus according to  claim 21  wherein the first throughput provides a first interface pressure in the interface vacuum stage to optimise detection sensitivity for at least one element of the sample being mass analysed using the first operating condition and the second throughput provides a second interface pressure to optimise detection sensitivity for at least one element being mass analysed using the second operating condition. 
     
     
         24 . An apparatus according to  claim 23  wherein the at least one element whose detection sensitivity is optimised under the first operating condition is different to the at least one element whose detection sensitivity is optimised under the second operating condition. 
     
     
         25 . An apparatus according to  claim 23  wherein the controller controls the first and second interface pressures to be substantially the same. 
     
     
         26 . An apparatus according to  claim 15  wherein a pressure gauge is located in the expansion chamber and a feedback loop is provided between the pressure gauge and the controller so that the controller can continuously adjust the throughput of the vacuum pump to maintain the interface pressure at a set pressure. 
     
     
         27 . An apparatus according to  claim 15  wherein the controller is connected to a Visual Display Unit (VDU), such that the throughput of the pump and/or the interface pressure are displayed to a user, and wherein by means of a graphical user interface (GUI) displayed on the VDU and an input device, the user can command the controller to set a particular throughput of the vacuum pump and/or a particular interface pressure. 
     
     
         28 . (canceled) 
     
     
         29 . (canceled) 
     
     
         30 . (canceled) 
     
     
         31 . (canceled)