APPARATUS AND METHOD EMPLOYING ION SHUTTERS FOR ION SEPARATION
The disclosure relates to methods and apparatus for ion separation, for example using time of flight and ion mobility spectrometry. Ion shutters for use in IMS cells and to methods of operating them are also disclosed. The shutter includes a first shutter electrode and a second shutter electrode. A barrier voltage between these two electrodes is controlled to open and close the shutter to allow ions of interest to pass through the shutter in a drift direction. The control involves varying both: (a) the voltage of the first shutter electrode; and (b) the voltage of the second shutter electrode.
- 1-33. -33. (canceled)
- 34. An ion shutter for an ion mobility spectrometer (IMS) cell, comprising a first shutter electrode and a second shutter electrode, each of the first and second shutter electrodes comprising elongate conductive elements, wherein the first shutter electrode is spaced apart from the second shutter electrode in a drift direction of the IMS cell and the elongate conductive elements of the first shutter electrode are aligned with those of the second shutter electrode in the drift direction.
- 35. An ion mobility spectrometer (IMS) cell comprising:
an ion shutter comprising a first shutter electrode and a second shutter electrode, each of the first and second shutter electrodes comprising elongate conductive elements, wherein the first shutter electrode is spaced apart from the second shutter electrode in a drift direction of the IMS cell and the elongate conductive elements of the first shutter electrode are aligned with those of the second shutter electrode in the drift direction; a first shutter voltage provider configured to vary the voltage of the first shutter electrode; and a second shutter voltage provider configured to vary the voltage of the second shutter electrode, wherein the first shutter voltage provider and the second shutter voltage provider are configured to control a barrier voltage between the first shutter electrode and the second shutter electrode by varying the voltage of the first shutter electrode and the voltage of the second shutter electrode thereby to control the passage of ions of interest through the ion shutter in a drift direction.
- View Dependent Claims (36, 37, 38, 39, 40, 41, 42, 43, 44)
- 45. A method of controlling an ion shutter of an ion mobility spectrometer (IMS) cell, the method comprising controlling a barrier voltage between a first shutter electrode and a second shutter electrode to open and close the ion shutter to allow ions of interest to pass through the ion shutter in a drift direction by varying the voltage of the first shutter electrode;
varying the voltage of the second shutter electrode, wherein the first shutter electrode and the second shutter electrode are coplanar.
- View Dependent Claims (46, 47, 48, 49, 50, 51)
The present disclosure relates to apparatus and methods, more particularly to methods and apparatus for ion separation, for example using time of flight spectrometry, and still more particularly to ion shutters for use in IMS cells and to methods of operating them.
Ion mobility spectrometers (IMS) can identify material from a sample of interest by ionising the material (e.g., molecules, atoms, and so forth) and measuring the time it takes the resulting ions to travel a known distance under a known electric field. Each ion'"'"'s time of flight is associated with the ion'"'"'s mobility. An ion'"'"'s mobility relates to its mass and geometry. Therefore, by measuring the time of flight of an ion it is possible to infer its identity. These times of flight may be displayed graphically or numerically as a plasmagram.
Some IMS cells include detectors which collect ions to measure their time of flight so they can be identified, this may be done in the presence of a drift gas so that mobility effects can separate the ions. Some IMS cells may separate ions according to their time of flight so that ions having selected times of flight (implying a selected range of ion mobilities) can be provided to other detector instruments, such as mass spectrometers, for further analysis. One example of this technique is known as IMS-MS, in which an IMS cell is used as an ion filter to select ions from a sample. The selected ions are then provided to a mass spectrometer. In such ion identification or filtering methods, groups of ions can be released from a reaction region by opening an ion shutter and/or passed into an inlet of a mass spectrometer.
The reaction region of an IMS cell has a finite length, and in the time interval for which the shutter is held open, ions which may be distributed around the reaction region must travel (at least partially) across that reaction region to reach the shutter. The inventor in the present case has appreciated that this means that holding the shutter open only for a short interval reduces the number of slow moving ions that are able to travel through it in that interval. The inventor in the present case has recognised that this may reduce sensitivity of the detector to slow moving ions. After ions have passed through the shutter, their motion along the drift chamber is dependent upon the voltage profile in that drift chamber. He has further appreciated that the action of closing the shutter may modify the profile voltage near to the shutter. If so, the back of a group of ions in the drift chamber may experience a profile voltage different from that which the front of that group did when at that same position. The inventor concludes from this that this can retard or accelerate the ions at the back of the group relative to the rest of the group.
This leads him to believe that these factors may adversely affect the resolution and sensitivity of an IMS cell.
Aspects and embodiments of the invention are set out in the appended claims and aim to at least partially address problems such as those described above. These and other aspects and embodiments of the invention are also described herein.
Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:
In the drawings like reference numerals are used to indicate like elements.
In some embodiments the shutter electrodes 105 may be spaced apart in the direction of travel of the ions. In these embodiments, the voltage of the shutter that is nearest to the ions of interest (e.g. ions in the reaction region 102 prior to opening of the shutter, and in the drift region 104 after closing of the shutter) may be controlled to match the profile voltage in the IMS cell 100. This may enable ions in the reaction region to more closely approach the shutter 105 prior to its opening, and may reduce disturbances in the profile voltage in the drift chamber 104 due to closing of the shutter.
Other voltage control schemes may be applied to the shutter electrodes 105. In some schemes, opposing variations in the voltage of the first shutter electrode 106 and the second shutter electrode 107 are used to vary the barrier voltage. Such embodiments may at least partially avoid changes in the mean voltage of the shutter due to the action of opening and closing of the shutter 105. For example the electric field due to the shutter 105 at a point that is farther from the shutter than the spacing between conductors of the shutter may vary less than the change in barrier voltage would cause to be the case in the conventional case of using one fixed voltage electrode and one moving voltage electrode, for example the mean voltage of the shutter may remain constant, for example constant enough to avoid disturbances in the profile voltage. The shutter electrodes may be either coplanar or non-coplanar.
The shutter electrodes 106, 107 may each comprise elongate conductors, and the elongate conductors of the first shutter electrode 106 may be aligned in the drift direction with the elongate conductors of the second shutter electrode 107. The elongate conductors of each shutter electrode 106, 107 may be arranged as a grid, such as a mesh, for example a triangular, rectangular, hexagonal, or other regular or irregular mesh. As will be explained later, the shutter electrodes 106, 107 need not be separated in the drift direction. For example they may be coplanar, in which case the elongate conductors may be interdigitated, for example they may be interwoven.
The IMS cell of
As illustrated a voltage profile provider 202 is arranged to provide a spatially varying voltage profile along the IMS cell 100. The voltage profile in the drift region 104 may be applied using a series of drift electrodes 120a, 120b, 120c and 120d spaced apart along the drift region 104. Although not illustrated in
The shutter 105 has two closed states. In a first one of the closed states, the voltage of the first shutter electrode 106 is controlled to match the profile voltage at the location of the first shutter electrode. In a second closed state the voltage of the second shutter electrode 107 is controlled to match the profile voltage at the location of the second shutter electrode. This can control the passage of ions of interest through the shutter 105 in a manner selected to reduce disturbances of the profile voltage around the ion shutter 105 (a) in the reaction region in the first closed state and (b) in the drift chamber in the second closed state. As explained below, the shutter may also have a reset state in which the voltage of neither shutter electrode matches the profile voltage.
In operation, a substance of interest is introduced to the reaction region where it can be ionised. With ions in the reaction region, the shutter 105 is held in the first closed state. To open the shutter 105 to release ions from the reaction region 102, the second shutter voltage provider 204 then matches the voltage of the second shutter electrode 107 to the profile voltage. To close the shutter after ions of interest have passed into the drift chamber, the shutter 105 is switched into its second closed state. An example of such operation will be described in greater detail below with reference to
Although not yet mentioned, it will be appreciated in the context of the present disclosure that the IMS cell 100 may be configured to provide a flow of drift gas in a direction generally opposite an ion'"'"'s path of travel to the detector 118. For example, the drift gas can flow from adjacent the detector 118 toward the shutter 106. As illustrated, a drift gas inlet 122 and drift gas outlet 124 can be used to pass drift gas through the drift region. Example drift gases include, but are not limited to, nitrogen, helium, air, air that is re-circulated (e.g., air that is cleaned and/or dried) and so forth. Drift electrodes 120a, 120b, 120c and 120d may be arranged to guide ions toward detector 118, for example the drift electrodes 120a, 120b, 120c and 120d may comprise rings which may be arranged around the drift region 104 to move ions onto the detector 118. Although the example of
As shown, the first shutter electrode 106 is coupled to first shutter voltage provider 206 and the second shutter electrode 107 is coupled to second shutter voltage provider 204. In the example shown in
In the example illustrated in
In the state, illustrated in
The ion shutter remains in the open state shown in
In the first closed state illustrated in
As illustrated in
After a selected time interval in this second closed state, the ion shutter is reset. This time interval may be selected to provide a sufficient amount of time for the low mobility ions to travel to a part of the drift chamber in which the electric field matches the profile voltage, for example a part of the drift chamber in which the difference from the profile voltage due to the second shutter is much less than the profile voltage, for example so that the effect of this difference on time of flight of the ions is not measureable at the operating resolution of the IMS cell. This time may be selected based on the time of flight to the drift electrode 120a that is closest to the second shutter. Other, longer, times may also be used, for example long enough to allow the ions of interest to travel the length of the drift chamber. For example the time interval may be selected based on the cycle time of the IMS cell, and/or based on the longest expected time taken for ions of interest to reach the detector 118.
As illustrated in
The example illustrated in
The method illustrated in
A group of ions of interest is thus released to travel from the reaction region, through the open shutter. To close the shutter again behind this group, at time t2, the first shutter electrode is changed from the profile voltage while the second shutter remains at the profile voltage. The shutter may remain in this second closed state for an interval, t2 to t3, selected to be long enough to allow the ions of interest to travel to a part of the drift chamber in which the electric field matches the profile voltage, for example a part of the drift chamber in which the difference from the profile voltage due to the second shutter is much less than the profile voltage, for example so that the effect of this difference on time of flight of the ions is not measureable. This time may be selected based on the time of flight to the drift electrode 120a that is closest to the second shutter. Other, longer, times may also be used, for example long enough to allow the ions of interest to travel the length of the drift chamber.
The shutter may then be reset from the second closed state, at time t3, by increasing the barrier voltage. For example, the second shutter electrode voltage may be changed to increase the barrier voltage. The first shutter electrode voltage may be changed less than the second electrode voltage while doing this, for example it may be held constant as illustrated during the period t3 to t4 illustrated in
It will be appreciated that
The plots in
As shown in
By making opposing variations in this way, the average voltage of the shutter may change by less than the change in barrier voltage when opening and closing the shutter. For example, the changes on the individual shutter electrodes may at least partially cancel each other out.
These different voltage control schemes may be used with a variety of different configurations of shutter.
As illustrated in
It is not however necessary that all shutters of the present disclosure have this arrangement. For example, the methods and apparatus described above with reference to
As illustrated in
With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and/or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit. For example the voltage providers may be provided by a single drive circuit having multiple output channels, or separate drive circuitry may be provided for each. The voltage providers may comprise amplifiers arranged to provide switchable voltages, which may be fixed or arranged to vary relative to particular reference voltages. For example, the profile voltage of the IMS cell may be used as a reference voltage of the voltage providers which drive the shutter electrodes. The voltage providers described herein may comprise an AC power supply, which may comprise one or more step-up or step down transformers, the voltage providers may also comprise DC power supplies such as batteries or fuel cells or capacitive power stores. Combinations of AC and DC power may be used and the voltage provider may comprise an inverter for providing an AC voltage based on a DC power supply. In some embodiments the voltage providers may comprise rectifiers for providing DC voltage based on an AC power supply. Any combination of AC and DC power supply and voltage providing components may be used. In some embodiments the voltage provider may also operate as a current source.
Although in the above examples the ion shutter is illustrated between the reaction region and the drift chamber, a shutter may also be provided in place of, or to couple the IMS cell to a detector. This may permit operation of the shutter to select ions of particular mobilities (e.g. having particular time of flight along the cell). This can enable ions to be filtered before they are provided to a detector such as a mass spectrometer.
Where reference is made to electrodes it will be appreciated that any arrangement of conductors may be used, for example electrodes may comprise metals or other conductors and may be at least partially exposed and/or partially insulated.
The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.
In some examples, one or more memory elements can store data and/or program instructions used to implement the operations described herein. Embodiments of the disclosure provide tangible, non-transitory storage media comprising program instructions operable to program a processor to perform any one or more of the methods described and/or claimed herein and/or to provide data processing apparatus as described and/or claimed herein.
The activities and apparatus outlined herein may be implemented with fixed logic such as assemblies of logic gates or programmable logic such as software and/or computer program instructions executed by a processor. Other kinds of programmable logic include programmable processors, programmable digital logic (e.g., a field programmable gate array (FPGA), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM)), an application specific integrated circuit, ASIC, or any other kind of digital logic, software, code, electronic instructions, flash memory, optical disks, CD-ROMs, DVD ROMs, magnetic or optical cards, other types of machine-readable mediums suitable for storing electronic instructions, or any suitable combination thereof.