Ion mobility spectrometer with virtual aperture grid
An ion mobility spectrometer does not require a physical aperture grid to prevent premature ion detector response. The last electrodes adjacent to the ion collector (typically the last four or five) have an electrode pitch that is less than the width of the ion swarm and each of the adjacent electrodes is connected to a source of free charge, thereby providing a virtual aperture grid at the end of the drift region that shields the ion collector from the mirror current of the approaching ion swarm. The virtual aperture grid is less complex in assembly and function and is less sensitive to vibrations than the physical aperture grid.
1. An ion mobility spectrometer, comprising:
an ionizing region for ionizing a sample vapor to form ions,
a drift tube comprising a drift region in which the ions drift under the influence of an electric field, established by a plurality of stacked electrodes, against a counter-flowing drift gas and are separated into at least one single ion swarm therein,
a gate for pulsing the ions into the drift region,
an ion collector for detecting the at least one single ion swarm at the end of the drift region to provide an ion mobility spectrum, and
wherein at least two of the stacked electrodes adjacent to the ion collector have an electrode pitch that less than the width of an ion swarm and each of the adjacent electrodes is connected to a source of free charge, thereby providing a virtual aperture grid at the end of the drift region that shields the ion collector from the mirror current of the ion swarm.
2. The ion mobility spectrometer of claim 1 , wherein the source of free charge comprises a low-pass electrical circuit that has a cut-off frequency below a minimum frequency component of the ion swarm.
3. The ion mobility spectrometer of claim 2 , wherein the source of free charge comprises a capacitor.
4. The ion mobility spectrometer of claim 3 , wherein the capacitor has a capacitance of greater than
C
>
2
L
2
RR
′
κ
m
i
n
V
m
ax
,
where C is the capacitance, L is the length of the drift tube, R is the resistance between the stacked electrodes, R′ is the desired resolution of the tube, κ min is the minimum mobility of the ions, and V max the power supply voltage on the stacked electrodes.
5. The ion mobility spectrometer of claim 3 , wherein the capacitor has a capacitance of greater than
C
>
2
L
RR
′
κ
m
i
n
E
,
where C is the capacitance, L is the length of the drift tube, R is the resistance between the stacked electrodes, R′ is the desired resolution of the tube, κ min is the minimum mobility of the ions, and E is the average field strength at the virtual aperture grid electrodes.
6. The ion mobility spectrometer of claim 1 , wherein the number of adjacent electrodes is five or less.
7. The ion mobility spectrometer of claim 1 , wherein the adjacent electrode pitch is less than
P
=
L
m
R
′
,
where P is the pitch, L is the length of the drift tube, R′ is the resolution of the tube, and m is the number of electrodes comprising the virtual aperture grid.
8. The ion mobility spectrometer of claim 7 , wherein m is five or less.
9. The ion mobility spectrometer of claim 1 , wherein the adjacent electrode pitch is less than 5 mm.
10. The ion mobility spectrometer of claim 1 , wherein the adjacent electrode pitch is less than 0.75 mm.