Mobility and mass measurement using time-varying electric fields
A method of ion mobility and/or mass spectrometry is disclosed in which the ion mobility and/or mass to charge ratio of an ion is determined using an algorithm or relationship that relates the transit time or average ion velocity of the ion through an ion separation device in which one or more time-varying electric field is used to separate ions passing therethrough to one or more parameters for the device, the mass to charge ratio of the ion and the ion mobility of the ion.
1. A method of ion mobility and/or mass spectrometry comprising:
providing an ion separation device in which one or more time-varying electric field is used to separate ions passing therethrough such that the ions exit the ion separation device at different times;
providing an algorithm or relationship that relates the transit time or average ion velocity of any given ion through the ion separation device to one or more parameters for the device, the mass to charge ratio of that ion and the ion mobility of that ion;
measuring the transit time or average ion velocity of an ion through the ion separation device; and
(i) determining the ion mobility of said ion from said algorithm or relationship by using the measured transit time or average ion velocity of the ion, a known mass to charge ratio of the ion, and known parameters for the device; or
(ii) determining the mass to charge ratio of said ion from said algorithm or relationship by using the measured transit time or average ion velocity of the ion, a known ion mobility of the ion, and known parameters for the device; or
(iii) wherein said measuring the transit time or average ion velocity of the ion through the ion separation device is performed whilst using a first set of operating conditions of the device associated with first known parameter values to obtain a first transit time or average ion velocity, and wherein the method further comprises measuring a second transit time or average ion velocity of the ion through the ion separation device whilst using second, different operating conditions of the device associated with second known parameter values, and determining both the ion mobility and mass to charge ratio of said ion from said algorithm or relationship by using the measured first and second transit times or average ion velocities and the first and second parameter values.
2. The method of claim 1 , comprising measuring the mass to charge ratio of the ion to determine said known mass to charge ratio in step (i); or measuring the ion mobility of the ion to determine said known ion mobility in step (ii).
3. The method of claim 1 , wherein step (iii) comprises: determining a first equation relating the first transit time or average ion velocity, the first known parameter values, the ion mobility of the ion, and the mass to charge ratio of the ion; determining a second equation relating the second transit time or average ion velocity, the second known parameter values, the ion mobility of the ion, and the mass to charge ratio of the ion; and solving said first and second equations simultaneously to determine the ion mobility and mass to charge ratio of said ion.
4. A method of calibrating an ion mobility and/or mass spectrometer comprising:
providing an ion separation device in which one or more time-varying, electric field is used to separate ions passing therethrough such that the ions exit the ion separation device at different times;
providing an algorithm or relationship that relates the transit time or average ion velocity of any given ion through the ion separation device to one or more parameters for the device, the mass to charge ratio of that ion or a function of mass and charge of that ion, and the ion mobility of that ion;
measuring the transit time or average ion velocity of an ion through the ion separation device; and
determining the parameter values for the device from said algorithm or relationship by using the measured transit time or average ion velocity of the ion, a known ion mobility of the ion, and a known mass to charge ratio of the ion or a known value of the function of mass and charge of the ion.
5. The method of claim 1 , wherein the ion separation device is a travelling wave separation device, wherein voltages are successively applied to electrodes of the device so as to form a wave of potential barriers that move in a first direction along the device so as to urge ions in the first direction through a gas.
6. The method of claim 5 , wherein said algorithm or relationship is, or comprises a term of the form:
T
=
L
μ
(
1
-
ω
0
-
α
2
ω
0
2
(
1
-
ω
0
)
)
where T is the transit time of any given ion through the ion separation device, L is the length of the ion separation region in the device, μ is the velocity in the first direction of the potential barriers,
ω
0
=
1
-
γ
2
,
α
=
2
π
μ
λ
K
m
q
,
γ
=
2
π
V
0
μ
λ
K
,
V 0 is the amplitude of each potential barrier, λ is the wavelength between adjacent potential barriers of said wave of potential barriers, K is the ion mobility of the ion, and m/q is the mass to charge ratio of the ion.
7. The method of claim 4 , wherein the ion separation device is a travelling wave separation device, wherein voltages are successively applied to electrodes of the device so as to form a wave of potential barriers that move in a first direction along the device so as to urge ions in the first direction through a gas;
said algorithm or relationship is, or comprises a term of the form:
T
=
L
μ
(
1
-
ω
0
-
α
2
ω
0
2
(
1
-
ω
0
)
)
where T is the transit time of any given ion through the ion separation device, L is the length of the ion separation region in the device, μ is the velocity in the first direction of the potential barriers,
ω
0
=
1
-
γ
2
,
α
=
2
π
μ
λ
K
m
q
,
γ
=
2
π
V
0
μ
λ
K
,
V 0 is the amplitude of each potential barrier, λ is the wavelength between adjacent potential barriers of said wave of potential barriers, K is the ion mobility of the ion, and m/g is the mass to charge ratio of the ion; and
the step of determining the parameters for the device comprises determining one or more operational parameters for the device such as: μ; L; λ; V 0 ; μ/L; μ/λ; or V 0 /(μλ).
8. The method of claim 5 , wherein said algorithm or relationship is, or comprises a term of the form:
v
_
ion
=
c
2
K
2
+
c
42
K
4
(
m
q
)
2
+
c
4
K
4
+
c
6
K
6
or
wherein said algorithm or relationship is:
v
_
ion
=
c
2
K
2
/
3
+
c
4
K
4
/
3
+
c
6
K
6
/
3
+
c
8
K
8
/
3
+
c
82
K
8
/
3
(
m
q
)
2
where v ion is the average ion velocity through the separation device defined by the length L of the ion separation region in the separation device divided by the transit time T of any given ion through the ion separation region; c 0 are constants; K is the ion mobility of the ion, and m/q is the mass to charge ratio of the ion.
9. The method of claim 4 , wherein the ion separation device is a travelling wave separation device, wherein voltages are successively applied to electrodes of the device so as to form a wave of potential barriers that move in a first direction along the device so as to urge ions in the first direction through a gas;
said algorithm or relationship is, or comprises a term of the form:
v
_
ion
=
c
2
K
2
+
c
42
K
4
(
m
q
)
2
+
c
4
K
4
+
c
6
K
6
;
or
wherein said algorithm or relationship is:
v
_
ion
=
c
2
K
2
/
3
+
c
4
K
4
/
3
+
c
6
K
6
/
3
+
c
8
K
8
/
3
+
c
82
K
8
/
3
(
m
q
)
2
where v ion is the average ion velocity through the separation device defined by the length L of the ion separation region in the separation device divided by the transit time T of any given ion through the ion separation region; c 0 are constants; K is the ion mobility of the ion, and m/q is the mass to charge ratio of the ion; and
the step of determining the parameters for the device comprises determining the constants c n .
10. The method of claim 1 , wherein the ion separation device is a trapped ion mobility separation device, wherein a gas flow urges ions in one direction, and a first electric field urges the ions in the opposite direction so as to trap the ions in a first region, and the first electric field is varied with time so that different ions exit the first region at different times, optionally wherein the ions exit the first region at different times into a second region in which a second electric field is maintained, wherein the second electric field is varied with time.
11. The method of claim 10 , wherein said algorithm or relationship is, or comprises a term of the form:
T
=
1
β
(
E
0
-
v
g
K
)
+
α
′
-
2
L
p
β
K
-
α
′2
where T is the transit time of any given ion through the ion separation device, β is the rate of change of the second electric field in the second region, E 0 is the initial value of the second electric field in the second region, v g is the velocity of the gas flow, K is the ion mobility of the ion, α′ is the product of the ion mobility and mass to charge ratio of the ion, and L p is the length of the second region.
12. The method of claim 4 , wherein the ion separation device is a trapped ion mobility separation device, wherein a gas flow urges ions in one direction, and a first electric field urges the ions in the opposite direction so as to trap the ions in a first region, and the first electric field is varied with time so that different ions exit the first region at different times, optionally wherein the ions exit the first region at different times into a second region in which a second electric field is maintained, wherein the second electric field is varied with time;
said algorithm or relationship is, or comprises a term of the form:
T
=
1
β
(
E
0
-
v
g
K
)
+
α
′
-
2
L
p
β
K
-
α
′2
where T is the transit time of any given ion through the ion separation device, β is the rate of change of the second electric field in the second region, E 0 is the initial value of the second electric field in the second region, v g is the velocity of the gas flow, K is the ion mobility of the ion, α′ is the product of the ion mobility and mass to charge ratio of the ion, and L p is the length of the second region; and
the step of determining the parameters for the device comprises determining one or more operational parameters for the device such as: E 0 ; β; v g ; L p ; E 0 /β; v g /β; or L p /β.
13. The method of claim 1 , wherein the algorithm or relationship further relates the transit time or average ion velocity of any given ion through the ion separation device to a function of mass and charge of the ion, or to the mass and charge of the ion.
14. The method of claim 13 , wherein the algorithm or relationship relating the transit time or average ion velocity of any given ion through the ion separation device to the mass and charge of the ion includes a term of the form e cm a /q b , wherein m is the mass of the ion, q is the charge of the ion and a, b and c are free parameters.
15. The method of claim 1 , further comprising determining the mass of the ion additionally to the determination of the mass to charge ratio, and using the determined mass in combination with the mass to charge ratio to determine the charge of the ion.
16. The method of claim 15 , wherein determining the mass of the ion comprises measuring the ion mobilities for the ion through three or more gases of different polarisability and determining the mass of the ion using a relationship or algorithm relating the measured ion mobilities for the three or more gases to the mass of the ion.
17. The method of claim 1 , wherein step (iii) comprises performing measurements with two or more different gases in the device, wherein the measurements performed with at least one of the gases are performed using different operating conditions of the device.
18. A method of ion mobility and/or mass spectrometry comprising:
providing an ion separation device in which one or more time-varying electric field is used to separate ions passing therethrough such that the ions exit the ion separation device at different times;
providing an algorithm or relationship that relates the transit time or average ion velocity of any given ion through the ion separation device to one or more parameters for the device, the ion mobility of that ion and a function of mass and charge of that ion;
measuring the transit time or average ion velocity of an ion through the ion separation device; and
(i) determining the ion mobility of said ion from said algorithm or relationship by using the measured transit time or average ion velocity of the ion, a known value of the function of mass and charge of the ion, and known parameters for the device; or
(ii) determining a value of the function of mass and charge of said ion from said algorithm or relationship by using the measured transit time or average ion velocity of the ion, a known ion mobility of the ion, and known parameters for the device; or
(iii) wherein said measuring the transit time or average ion velocity of the ion through the ion separation device is performed whilst using a first set of operating conditions of the device associated with first known parameter values to obtain a first transit time or average ion velocity, and wherein the method further comprises measuring a second transit time or average ion velocity of the ion through the ion separation device whilst using second, different operating conditions of the device associated with second known parameter values, and determining both the ion mobility and a value for the function of mass and charge of said ion from said algorithm or relationship by using the measured first and second transit times or average ion velocities and the first and second parameter values.
19. An ion mobility and/or mass spectrometer comprising:
an ion separation device comprising electrodes, at least one voltage supply connected to the electrodes for generating one or more electric field in the device, and an ion detector;
a memory for storing an algorithm or relationship that relates the transit time of any given ion through the ion separation device to operational parameters for the device, the mass to charge ratio of that ion or a function of mass and charge of that ion, and the ion mobility of that ion;
a processor having access to said memory; and
a controller configured to perform a method as claimed in claim 1 .
20. An ion mobility and/or mass spectrometer comprising:
an ion separation device comprising electrodes, at least one voltage supply connected to the electrodes for generating one or more electric field in the device, and an ion detector;
a memory for storing an algorithm or relationship that relates the transit time of any given ion through the ion separation device to operational parameters for the device, the mass to charge ratio of that ion or a function of mass and charge of that ion, and the ion mobility of that ion;
a processor having access to said memory; and
a controller configured to perform a method as claimed in claim 4 .
21. The method of claim 1 , wherein the step of providing an algorithm or relationship comprises an analytic or perturbative approach.
22. The method of claim 4 , wherein the step of providing an algorithm or relationship comprises an analytic or perturbative approach.
23. The method of claim 18 , wherein the step of providing an algorithm or relationship comprises an analytic or perturbative approach.