High-resolution ion mobility spectrometry
A supersonic gas jet having gas molecules with substantially equal velocities is formed by directing the gas through a Laval nozzle into an evacuated chamber. A field barrier having a substantially constant height across a cross-section of the supersonic gas jet is formed by respectively applying potentials U 2 , U 3 and U 4 to an arrangement of three apertured diaphragms R 2 , R 3 and R 4 , which are respectively separated by distances d 2 and d 3 , where (U 4 −U 3 )/(U 3 −U 2 )=d 3 /d 2 . The ions in the supersonic gas jet are directed against the field barrier, where ions with a mobility below a mobility threshold are pushed over the field barrier, and where ions with a mobility higher than the mobility threshold are held back by the field barrier.
1. A method for dividing ions in a gas according to their mobility, comprising:
forming a supersonic gas jet, having gas molecules with substantially equal velocities, by directing the gas through a Laval nozzle into an evacuated chamber;
forming a field barrier, having a substantially uniform height across a cross-section of the supersonic gas jet, by respectively applying potentials U 2 , U 3 and U 4 to an arrangement of three apertured diaphragms R 2 , R 3 and R 4 , which are respectively separated by distances d 2 and d 3 , where (U 4 −U 3 )/(U 3 −U 2 )=d 3 /d 2 ; and
directing the ions in the supersonic gas jet against the field barrier, where ions with a mobility less than a mobility threshold are pushed over the field barrier, and where ions with a mobility higher than the mobility threshold are held back by the field barrier.
2. The method of claim 1 , where the apertured diaphragms R 2 , R 3 and R 4 have substantially equal aperture diameters.
3. The method of claim 1 , further comprising measuring an ion current of the ions pushed over the field barrier.
4. The method of claim 3 , further comprising performing the measuring of the ion current with an ion detector without mass separation.
5. The method of claim 3 , further comprising performing the measuring of the ion current with a mass analyzer.
6. The method of claim 4 , further comprising:
providing a continuous current of the ions from an ion source;
pushing the ions with mobility less than the mobility threshold over the field barrier with the supersonic gas jet;
measuring passing ions in the form of a total ion current curve as a function of the height of the field barrier; and
generating a mobility spectrum by differentiating the total ion current curve.
7. The method of claim 5 , further comprising:
providing a continuous current of the ions from an ion source;
pushing the ions with mobility less than the mobility threshold over the field barrier with the supersonic gas jet;
measuring a series of mass spectra of passing ions as a function of the height of the field barrier;
extracting ion current curves for ions of individual mass ranges from the mass spectra; and
generating mass separated mobility spectra by differentiating the ion current curves.
8. The method of claim 7 , further comprising:
setting the height of the field barrier to measure fragment ion spectra of ions with lowest mobility; and
lowering the field barrier to measure mixtures of ions of the lowest mobility and ions of higher mobility.
9. An ion mobility spectrometer, comprising:
an ion source that generates ions in a gas in a high pressure region of the ion mobility spectrometer;
a Laval nozzle that generates a supersonic gas jet from the gas with the ions in a lower pressure region of the ion mobility spectrometer;
an arrangement of three or more apertured diaphragms R 2 , R 3 and R 4 through which the supersonic gas jet moves axially, where the apertured diaphragms are respectively separated by distances d 2 and d 3 ;
a power supply that respectively supplies the apertured diaphragms with potentials U 2 , U 3 and U 4 , where (U 4 −U 3 )/(U 3 −U 2 )=d 3 /d 2 ; and
an ion detector that measures a current of the ions passing the apertured diaphragms.
10. The ion mobility spectrometer of claim 9 , where the Laval nozzle is located between a region maintained at atmospheric pressure and a first vacuum chamber, or in a wall located between two vacuum chambers of a differential pumping system.
11. The ion mobility spectrometer of claim 9 , further comprising an additional RF quadrupole rod system located between the Laval nozzle and the arrangement of apertured diaphragms.
12. The ion mobility spectrometer of claim 9 , where the ion detector comprises a mass spectrometer.
13. The ion mobility spectrometer of claim 12 , where the mass spectrometer comprises a time-of-flight mass spectrometer with orthogonal ion injection.
14. An ion mobility spectrometer, comprising:
an ion source that generates ions in a gas in a high pressure region of the ion mobility spectrometer;
a Laval nozzle that generates a supersonic gas jet from the gas with the ions in a lower pressure region of the ion mobility spectrometer;
a coaxial arrangement of three or more apertured diaphragms R 2 , R 3 and R 4 through which the supersonic gas jet axially moves;
a power supply that respectively supplies the apertured diaphragms with potentials U 2 , U 3 and U 4 such that a field barrier is generated at the apertured diaphragm R 4 having a substantially uniform height across a cross-section of the supersonic gas jet, and such that ions in the supersonic gas jet are directed against the field barrier, where ions with a mobility less than a mobility threshold are pushed over the field barrier, and where ions with a mobility higher than the mobility threshold are held back by the field barrier; and
an ion detector that measures a current of the ions passing the apertured diaphragms.