Differential trapped ion mobility separator

A mass spectrometric system comprising an ion source, a mass analyzer with an ion detector, and at least a first trapped ion mobility separator located downstream of said ion source and/or upstream of said mass analyzer, wherein said first trapped ion mobility separator comprises:

• an ion channel in which ions move along an axis between a first end of said ion channel, at which ions are introduced into said ion channel, and a second end of said ion channel, said ion channel containing a gas through which the ions pass, wherein the ion channel is supplied with radially confining voltages for preventing the ions from escaping the ion channel laterally,
• at least a first electrode and a second electrode arranged spaced apart from each other along said axis of said ion channel for defining an ion separation region therebetween,
• a first generator that causes a first axial force to be exerted on the ions along said axis by applying a separating voltage to said first electrode and said second electrode to generate an alternating axial electric field, the first axial force having an effect on the movement of the ions that is dependent on differential mobility by virtue of its interplay with said gas, wherein said separating voltage is an alternating voltage and is applied so that for a first time interval an electric field with a first field strength is generated, and for a second time interval, following said first time interval, an opposing electric field with a second field strength is generated, which is lower in magnitude than the first field strength, said first time interval lasting a shorter time span than said second time interval,
• a second generator that causes a second axial force to be exerted on the ions along said axis, which is counteracting said first axial force at least temporarily, wherein said first generator and said second generator are configured such that at least one of the axial forces is changing in strength along the axis for trapping ions along said axis at mobility dependent positions where a force equilibrium of said first axial force and said second axial force exists for the ions, and
• an electrical controller which communicates with said first generator and said second generator and varies at least one of said first axial force and said second axial force over time, such that the trapped ions are driven progressively to one of said first end and said second end of said ion channel as a function of their differential mobility.

A method for analyzing ions using a first trapped ion mobility separator, comprising the steps of:

• providing an ion channel in which ions move along an axis between a first end of said ion channel, at which ions are introduced into said ion channel, and a second end of said ion channel, said ion channel containing a gas through which the ions pass, wherein the ion channel is supplied with radially confining voltages for preventing the ions from escaping the ion channel laterally,
• providing at least a first electrode and a second electrode arranged spaced apart from each other along said axis of said ion channel for defining an ion separation region therebetween,
• generating a first axial force that is imparted to the ions along said axis by applying an alternating separating voltage to said first electrode and said second electrode to generate an alternating axial electric field, the first axial force having an effect on the movement of the ions that is dependent on differential mobility by virtue of its interplay with said gas,
• applying said alternating separating voltage so that for a first time interval an electric field with a first field strength is generated, and for a second time interval, following said first time interval, an opposing electric field with a second field strength is generated, which is lower in magnitude than the first field strength, said first time interval lasting a shorter time span than said second time interval,
• generating a second axial force that is imparted to the ions along said axis and that counteracts said first axial force at least temporarily,
• changing at least one of the axial forces in strength along the axis for trapping ions along said axis at mobility dependent positions where a force equilibrium of said first axial force and said second axial force exists for the ions, and
• varying at least one of said first axial force and said second axial force over time, such that the trapped ions are driven progressively to one of said first end and said second end of said ion channel as a function of their differential mobility.