ION MOBILITY SEPARATION SYSTEM WITH ROTATING FIELD CONFINEMENT
An ion mobility separator includes an ion path with a central axis along which ions travel, the ion path containing a gas. A first force is applied to the ions in a first axial direction, and a second force that varies spatially along the ion path is applied to the ions in second axial direction opposite the first axial direction. A rotating confinement field has a radially-inhomogeneous electric potential with relative maxima and minima that rotate about the central axis as a function of time, the confinement field exerting a radial confinement force on the ions in a radial direction toward the central axis. The ion mobility separator may be operated at elevated pressures including ambient pressure and higher. The first and/or second axial forces may be a constant or gradient gas flow, a constant or gradient electric field or an axial component of the rotating confinement field.
1 . A trapped ion mobility separator comprising:
an ion path along which ions travel from an entrance to an exit along a first axial direction relative to a central axis of the ion path, the ion path containing a gas through which the ions pass;
a first force-generating apparatus that exerts a first force on the ions in the first axial direction;
a second force-generating apparatus that exerts a second force on the ions in a second axial direction opposite to the first axial direction, wherein at least one of the first and second forces varies spatially along the first axial direction such that ions are trapped and separated by ion mobility along said first axial direction during an accumulation phase, and wherein at least one of the first and second forces is varied during an elution phase to increase a magnitude of the first force relative to the second force over time such that the ions are progressively driven to the exit of the ion path as a function of ion mobility; and
a rotating confinement field-generating apparatus that generates a radially-inhomogeneous electric potential that exerts a confinement force on the ions in a radial direction toward said central axis, relative maxima and minima of said electric potential rotating about said central axis as a function of time.
2 . The trapped ion mobility separator according to claim 1 , wherein the pressure of the gas in the ion path is higher than 5,000 Pa.
3 . The trapped ion mobility separator according to claim 1 , wherein the force that varies spatially along the first axial direction comprises a gradient along a first portion of the ion path that flattens to a plateau of substantially constant force in the vicinity of the exit of the ion mobility separator.
4 . The trapped ion mobility separator according to claim 1 , wherein the first force and second force are of different respective types, each being generated by one of a gas flow, an electric DC field and an axial component of the rotating confinement field.
5 . The trapped ion mobility separator according to claim 1 , wherein the trapped ion mobility separator is arranged such that such that
K
o
p
o
p
T
T
o
m
q
≪
τ
R
o
F
where p is pressure of the gas, p o is normal pressure, T is temperature of the gas, T o =normal temperature, K o is normalized ion mobility, m is mass, q is charge and τ RoF is a time constant of the rotating confinement field which specifies how fast the rotating confinement field changes at a given position.
6 . The trapped ion mobility separator according to claim 5 , wherein the trapped ion mobility separator is arranged such that such that
K
o
p
o
p
T
T
o
U
R
o
F
f
R
o
F
≤
c
R
o
F
where c RoF is a confinement constant, K o is normalized ion mobility, p is pressure of the gas, p o is normal pressure, T is temperature of the gas, T o is normal temperature, U RoF is the potential difference between the maxima and minima of the electric potential rotating about the central axis and f RoF is the angular frequency of the rotating confinement field.
7 . The trapped ion mobility separator according to claim 1 , wherein the rotating confinement field-generating apparatus comprises a plurality of radially-segmented electrodes each having a minimum of four segments.
8 . The trapped ion mobility separator according to claim 7 , wherein each electrode has eight radial segments.
9 . The trapped ion mobility separator according to claim 7 , wherein one of two different electrical potentials is applied to each of the segments of each radially-segmented electrode, and wherein a distribution of the electrical potentials applied to the segments of each electrode is symmetric with respect to the central axis at any given point in time.
10 . The trapped ion mobility separator according to claim 7 , wherein one of two different electrical potentials is applied to each of the segments of each radially-segmented electrode, and wherein a distribution of the electrical potentials applied to the segments of each electrode is asymmetric with respect to the central axis at any given point in time.
11 . The trapped ion mobility separator according to claim 1 , further comprising an ion trap that is located upstream of the ion mobility separator and that comprises a rotating confinement field-generating apparatus that generates a radially-inhomogeneous electric field that exerts a confinement force on the ions in a radial direction toward a central axis of the ion trap, relative maxima and minima of said electric potential rotating about the central axis of the ion trap as a function of time.
12 . The trapped ion mobility separator according to claim 1 , further comprising an ion funnel that is located at the entrance or exit of the trapped ion mobility separator and that comprises a rotating confinement field-generating apparatus that generates a radially-inhomogeneous electric field that exerts a confinement force on the ions in a radial direction toward a central axis of the ion funnel, relative maxima and minima of said electric potential rotating about the central axis of the ion funnel as a function of time.
13 . A method for analyzing ions using a trapped ion mobility separator comprising:
providing an ion path along which ions travel from an entrance to an exit of the separator along a first axial direction relative to a central axis of the ion path, the ion path containing a gas through which the ions pass;
generating a first force that acts on the ions in the first axial direction;
generating a second force that acts on the ions in a second axial direction opposite to the first axial direction, wherein at least one of the first and second forces varies spatially along the first axial direction such that ions are trapped and separated by ion mobility along said first axial direction;
varying at least one of the first and second forces to increase a magnitude of the first force relative to the second force over time such that the ions are progressively driven to the exit of the ion path and separated as a function of ion mobility; and
confining the ions using a rotating confinement field-generating apparatus that generates a radially-inhomogeneous electric potential that exerts a confinement force on the ions in a radial direction toward said central axis, relative maxima and minima of said electric potential rotating about said central axis as a function of time.
14 . The method according to claim 13 , wherein the separated ions are detected by an ion detector.
15 . The method according to claim 13 , wherein the separated ions are further analyzed as a function of mass in a mass analyzer located downstream of the trapped ion mobility separator.
16 . The method according to claim 13 , wherein the separated ions are fragmented into fragment ions and the fragment ions are further analyzed as function in a mass analyzer located downstream of the trapped ion mobility separator.
17 . The method according to claim 16 , wherein the separated ions are filtered according to mass prior to the fragmentation and/or selected prior to the fragmentation.
18 . The method according to claim 13 , wherein ions of specific ion mobility are selected, the selected ions are activated or fragmented in a downstream activation/fragmentation cell and the activated/fragmented ions are further analyzed according to ion mobility.
19 . The method according to claim 13 , wherein the trapped ion mobility separator is operated such that
K
o
p
o
p
T
T
o
m
q
≪
τ
R
o
F
where p is pressure of the gas, p o is normal pressure, T is temperature of the gas, T o =normal temperature, K o is normalized ion mobility, m is mass, q is charge and τ RoF is a time constant of the rotating confinement field which specifies how fast the rotating confinement field changes at a given position.
20 . The trapped ion mobility separator according to claim 19 , wherein the trapped ion mobility separator is operated such that
K
o
p
o
p
T
T
o
U
R
o
F
f
R
o
F
≤
c
R
o
F
where c RoF is a confinement constant, K o is normalized ion mobility, p is pressure of the gas, p o is normal pressure, T is temperature of the gas, T o is normal temperature, U RoF is the potential difference between the maxima and minima of the electric potential rotating about the central axis and f RoF is the angular frequency of the rotating confinement field.
21 . The method according to claim 13 , wherein ions from an ion source are accumulated in an ion trap located upstream of the trapped ion mobility separator while ions which have been provided earlier from the ion source are analyzed in the trapped ion mobility separator.