Differential trapped ion mobility filter

A mass spectrometric system comprising an ion source, a mass analyzer with an ion detector, and at least a first trapped ion mobility filter located downstream of said ion source and/or upstream of said mass analyzer, wherein said first trapped ion mobility filter 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 filter region therebetween,
• a first generator that causes a first axial force to be exerted on 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, wherein, at substantially any time, the direction of said first axial force does not change along said axis in said filter region, and wherein said alternating separating voltage 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 said 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, and
• an electrical controller which communicates with said first generator and said second generator to configure said first axial force and said second axial force such that ions with a first specific range of differential mobility are trapped within said filter region of said ion channel and ions with a second specific range of differential mobility, which is different from said first specific range of differential mobility, are transmitted through said ion channel.

A method for analyzing ions using a first trapped ion mobility filter, 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 filter 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, wherein, at substantially any time, the direction of said first axial force does not change along said axis in said filter region,
• 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,
• configuring said first axial force and said second axial force such that ions with a first specific range of differential mobility are trapped within said filter region of said ion channel and ions with a second specific range of differential mobility, which is different from said first specific range of differential mobility, are transmitted through said ion channel.