Mass spectrometer with tandem ion mobility analyzers

1. A method for analyzing ions in a mass spectrometer comprising an ion source, first and second trapped ion mobility spectrometry (TIMS) analyzers, an ion gate located between the first and second TIMS and a mass analyzer, the method comprising the steps of:

transferring ions from the ion source to the first TIMS analyzer;

driving the ions transferred to the first TIMS analyzer by a first gas flow against a first counter-acting electric DC field barrier such that the ions are trapped and spatially separated according to their mobilities at different positions along a ramp of the first electric DC field barrier at which a friction force of the first gas flow equals the counter-acting force of the first electric DC field barrier;

temporally separating ions according to mobility in the first TIMS analyzer by adjusting a height of the first electric DC field barrier or the velocity of the first gas flow;

selecting at least one ion mobility species of interest by adjusting the transmission of the ion gate during the temporal ion separation in the first ion mobility analyzer;

transferring the selected ion mobility species of interest to the second TIMS analyzer;

driving the ions transferred to the second TIMS analyzer by a second gas flow against a second counter-acting electric DC field barrier such that the ions are trapped and spatially separated according to their mobilities at different positions along a ramp of the second electric DC field barrier at which a friction force of the second gas flow equals the counter-acting force of the second electric DC field barrier; and

temporally separating ions according to mobility in the second TIMS analyzer by adjusting a height of the second electric DC field barrier or the velocity of the gas flow.

2. The method according to claim 1 , wherein the temporal separation in the first TIMS analyzer, the selecting of ion mobility species of interest and the transfer of the selected ion mobility species of interest to the second TIMS analyzer are repeated and the second TIMS analyzer is operated to accumulate the repeatedly transferred ion mobility species of interest prior to temporally separating them according to mobility in the second TIMS analyzer.

3. The method according to claim 1 , wherein the transmission of the ion gate is adjusted while the ions are separated in the first TIMS analyzer such that the transmission for a highly abundant ion mobility species of interest is lower than the transmission of less abundant ion mobility species of interest.

4. The method according to claim 3 , wherein an opening interval of the ion gate for the highly abundant ion mobility species of interest is shorter than an opening interval for the less abundant ion mobility species of interest.

5. The method according to claim 1 , wherein the transmission of the ion gate is adjusted while the ions are separated in the first TIMS analyzer such that a total space charge of the ion mobility species transferred to the second TIMS analyzer and/or a local space charge of the ion mobility species transferred to the second ion mobility TIMS analyzer is below a predetermined threshold.

6. The method according to claim 1 , wherein the ion gate is opened and closed once while the ions are separated in the first TIMS analyzer such that the transferred ion mobility species of interest are substantially from a single mobility range.

7. The method according to claim 6 , wherein the electric field ramp of the second TIMS analyzer is adjusted such that the transferred ion mobility species of interest of the single mobility range spread substantially over a full width of the electric field ramp.

8. The method according to claim 1 , wherein the ion gate is opened and closed several times while the ions are separated in the first TIMS analyzer such that the transferred ion mobility species of interest are substantially from several disjoint mobility ranges.

9. The method according to claim 1 , wherein the selected ion mobility species which are separated in the second TIMS analyzer are further analyzed by acquiring mass spectra or acquiring fragment mass spectra.

10. A mass spectrometer comprising an ion source, first and second trapped ion mobility spectrometry analyzers (TIMS), an ion gate located between the two ion mobility analyzers and a mass analyzer,

wherein the first and second TIMS analyzers each comprise a gas flow and a counter-acting electric DC field barrier configured to spatially separate ions according to their mobilities at different positions along a ramp of the electric field barrier at which a friction force of the first gas flow equals the counter-acting force of the electric DC field barrier and to temporally separate ions according to mobility by adjusting a height of the electric DC field barrier or a velocity of the gas flow, and

wherein the ion gate is configured to vary the transmission of ion mobility species of interest during operation of the first mobility analyzer.

11. The mass spectrometer according to claim 10 , wherein the ion gate is an ion-optical einzel lens or a Bradbury-Nielsen grid.

12. The mass spectrometer according to claim 11 , further comprising a DC generator and/or an RF generator connected to the Bradbury-Nielsen grid.

13. The mass spectrometer according to claim 10 , further comprising a fragmentation cell between the second TIMS analyzer and the mass analyzer.