Atmospheric pressure ion focusing device employing nonlinear DC voltage sequences

Apparatus comprise an electrode arrangement comprising a plurality of electrodes defining a volume, an ion entrance, and an ion exit, and a voltage source coupled to the plurality of electrodes and configured to apply a nonlinear DC voltage sequence to the electrodes between the ion entrance and the ion exit that directs ions through the volume with the volume at a pressure of at least 1 Torr. Ions can be focused using nonlinear DC voltage sequences, including at atmospheric pressure. Related methods are also disclosed.

FIELD

The present disclosure relates to apparatus and methods for ion movement and manipulation.

BACKGROUND

Ion mobility and mass spectrometers are widely used in laboratories to radially confine and analyze ions. Spectrometers can be used for the analysis of complex mixtures, biological samples, and explosives, as well as for pharmaceutical and illicit drugs. However, nearly all spectrometers must operate within under very low pressure conditions using elaborate vacuum systems in order to both minimize ionic interactions with background gas molecules and to precisely control ion motion. These vacuum systems are often bulky, expensive, and power-intensive, all of which are significant barriers to successfully commercializing miniature and portable ion mobility and mass spectrometer systems. Thus, there exists a need for ion focusing without the attendant drawbacks of existing systems.

SUMMARY

According to an aspect of the disclosed technology, apparatus include an electrode arrangement comprising a plurality of electrodes defining a volume, an ion entrance, and an ion exit; and a voltage source coupled to the plurality of electrodes and configured to apply a nonlinear DC voltage sequence to the electrodes between the ion entrance and the ion exit that directs ions through the volume with the volume at a pressure of at least 1 Torr. In further examples, the volume is at a pressure of at least 50 Torr. Representative examples can have the pressure of the volume at or near an atmospheric pressure.

In some examples, the nonlinear DC voltage sequence is configured to focus the ions at the ion exit. In further examples, the nonlinear DC voltage sequence is defined by a voltage difference between adjacent electrodes that increases nonlinearly from the ion entrance to the ion exit. In some examples, at least a portion of the nonlinear DC voltage sequence includes a quadratic DC voltage sequence. In other examples, at least a portion of the nonlinear DC voltage sequence includes an exponential DC voltage sequence. In further examples, the nonlinear DC voltage sequence includes a quadratic DC voltage sequence along a first length and an exponential DC voltage sequence along a second length adjacent to the first length.

In some examples, the apparatus can include an ion receiver coupled to the ion exit. In further examples, the ion receiver is an ion analyzer or is coupled to an ion analyzer. In other examples, the ion receiver is a collection plate, an ion mobility spectrometer, or a mass spectrometer. In some examples, the apparatus can include an ion source coupled to the ion entrance. In further examples, the ion source is an electrospray ionization source, a plasma ionization source, an atmospheric pressure chemical ionization source, an atmospheric pressure photoionization source, electron impact ionization source, or a combination thereof.

In some examples, apparatus can include a housing supporting the electrode arrangement, wherein the housing includes a gas port configured to receive a drift gas source and a heat source configured to heat the electrode arrangement and the volume. In further examples, the plurality of electrodes are circular electrodes and the electrodes are evenly spaced along, and concentrically arranged about, a common axis. In other examples, the common axis is bent or curved. In other examples, the electrodes have a non-circular cross section.

According to a further aspect of the disclosed technology, methods include applying a nonlinear DC voltage sequence to a plurality of electrodes of an electrode arrangement, wherein the plurality of electrodes define a volume, ion entrance, and ion exit, and wherein the application of the nonlinear DC voltage sequence is configured to direct ions through the volume between the ion entrance and ion exit with the volume at a pressure of at least 1 Torr. In further examples, the volume is at a pressure of at least 50 Torr. In selected examples, the volume is at a pressure of at or near atmospheric pressure.

In some examples, methods include introducing the ions into the volume through the ion entrance. In further examples, methods include focusing the ions at the ion exit using the applied nonlinear DC voltage sequence. In other examples, the pressure is at least atmospheric pressure. In further examples, the methods include directing ions focused at the ion exit to a focused ion beam target.

In some examples, the nonlinear DC voltage sequence is defined by a voltage difference between adjacent electrodes that increases nonlinearly from the ion entrance to the ion exit. In further examples, the nonlinear DC voltage sequence comprises a first portion defining a quadratic DC voltage sequence and a second portion defining an exponential voltage sequence. In some examples, methods include receiving ions at the ion exit by an ion receiver. In further examples, methods include directing the received ions to an ion analyzer, wherein the ion analyzer is a mass spectrometer or an ion mobility spectrometer.

In some examples, the nonlinear DC voltage sequence comprises a quadratic sequence, an exponential sequence, a cubic sequence, and/or a complex wave function sequence. In other examples, methods include varying the nonlinear DC voltage sequence over time with ions in the volume. In further examples, methods include injecting a drift gas into the volume with the ions in the volume. In some examples, methods include applying heat to the electrode arrangement and/or the volume. In further examples, methods include introducing a drift gas to the volume. In other examples, methods include receiving ions at an ion exit of the electrode arrangement.

DETAILED DESCRIPTION

Some examples are described in relation to one or more longitudinal and lateral directions generalized to correspond to ion movement or confinement. Directions typically apply to ion movement, trapping, and confinement and are provided by electric fields produced by one or more electrodes that are arranged to define one or more volumes or channels of various shapes, sizes, and configurations. A direction can correspond to a single path, multiple paths, bi-directional movement, inward movement, outward movement, or a range of movements. Actual ion movement paths vary and can depend on the various characteristics of the electrode and the positional, polarity, kinetic, or other characteristics of the ions received in a confinement volume. Directions referred to herein are generalized and actual specific particle movements typically correspond to electric fields produced and the electrical mobilities of the ions propagating in relation to the electric fields.

Transitioning the analysis of ions from a vacuum to atmospheric pressure conditions is a considerable challenge in analytical chemistry. Primarily, the analysis of ions is typically performed within a vacuum because few ways to effectively focus ions at elevated pressures exist. For instance, in order to radially confine ions, most conventional ion mobility and mass spectrometers use high voltage radiofrequencies (RF) (e.g., electrodes possessing 180-degree phase-shifted RF waveforms) at vacuum pressures of 10 Torr or less. While these processes that utilize RF voltages at lower pressures can radially confine ions and prevent their diffusion toward discharging surfaces, these processes are highly ineffective at higher pressures (e.g., greater than or equal to 1 Torr) and/or under low electric field strength, drift gas density ratios (e.g., low E/N conditions). Any attempt of using these systems and/or methods at atmospheric pressure results in improper function and/or significant ion losses.

Alternatively, the present disclosure utilizes spatially, nonlinearly distributed direct current (DC) voltages to provide radially inward confinement (i.e., spatial ion focusing) of ions at atmospheric pressure (e.g., approximately 760 Torr). The use of such spatial ion focusing can, among other things, increase signal intensity, provide an effective ion source interface for spectrometer devices (e.g., ion mobility spectrometry), and/or produce a suitable ion source for atmospheric pressure ion manipulation in surface functionalization and soft-landing processes (e.g., the deposition of ions on a particular surface).

As described herein, the present disclosure is directed to systems and methods of manipulating ions, including the use of a nonlinear sequences to form a voltage gradient to direct ions linearly along a longitudinal path of electrode arrangement. In some examples, ion focusing (i.e., confinement) is provided by electric fields formed by a nonlinear DC voltage gradient applied to the system. In representative examples, ion focusing is provided by a nonlinear DC gradient that can include one or more nonlinear mathematical functions to achieve radial ion focusing under atmospheric pressure with minimal to no loss of ions. In some examples, the ions are manipulated under atmospheric conditions for delivery to an ion analytical device, for example, a mass spectrometer, an ion mobility spectrometer, inlet capillaries, an optical device, and/or any other analytical devices, including those already operating under atmospheric conditions. In some examples, an ion source including an electrospray ionization source, a plasma ionization source, an atmospheric pressure chemical ionization source, an atmospheric pressure photoionization source, and/or corona discharge ionization source can deliver ions into the system.

The systems and methods of the present disclosure successfully achieve ion focusing at atmospheric pressure and/or other elevated pressures (e.g., pressures greater than 1 Torr, 50 Torr, 100 Torr, 500 Torr, atmospheric, above atmospheric, etc.) by applying a nonlinear sequence of DC voltages to an electrode arrangement, for example, to a conventional stacked ring focusing device. The radial confinement at atmospheric pressure is achieved by the nonlinear DC voltage gradient created by the nonlinear sequences which produce an electric filed gradient that changes as a function of distance. As a result, rather than being time dependent, such as a RF system which establishes a pseudopotential well in the time domain, a nonlinear DC voltage gradient establishes a pseudopotential well in the space domain, allowing for significant spatial ion focusing at atmospheric pressure without any need for operating in a vacuum. This ability to focus ions at atmospheric pressure, for example, can improve the transmission of ions to atmospheric pressure interfaces between ion sources and mass spectrometers. Additionally, spatial ion focusing can produce tightly collimated ion beams for interfacing with ion manipulation devices operating under atmospheric pressure conditions.

To successfully focus ions at atmospheric pressure, the present disclosure utilizes exponential, quadratic, cubic, and/or other nonlinear mathematical sequences to create a voltage gradient along an electrode arrangement. By using a nonlinear sequence, such as an exponential and/or quadratic sequence, voltage differences between adjacent electrodes form an electric field gradient which is constantly changing along the length of an electrode arrangement to direct ions introduced into the system linearly through the electrodes. For example, as described herein, an exponential sequence applied to an electrode arrangement forms relatively low voltage differences between adjacent electrodes at the beginning of the system; whereas, the voltage differences further along and toward the end of the system become increasingly greater causing the ions to intensely focus.

FIG. 1shows an example ion focusing system100that can be used to achieve ion focusing at pressures above very low-pressure conditions, including at atmospheric pressure. The ion focusing system100can include an electrode arrangement102(e.g., such as a drift tube) that is situated to collect ions104from an ion source106and to direct the ions104to an ion receiver108. In representative examples, the electrode arrangement102includes a plurality of electrodes, e.g., electrodes103a-103d, each being adjacently spaced from each other by a distance110along a longitudinal axis112. A voltage source113is coupled to the electrode arrangement102and configured to apply different voltages to different individual electrodes or groups of electrodes of the electrode arrangement102(e.g., through wiring or traces of a printed wiring board). In representative examples, the spatial arrangement of the electrodes and the voltage applied to the electrodes is selected to define a non-linear voltage gradient along the longitudinal axis112that moves the ions104through a volume116defined by the electrode arrangement102, from an ion entrance114and to an ion exit118.

The ion focusing system100typically includes a housing120which can house and/or support the electrode arrangement102and other components, such as the ion source106and ion receiver108. In some examples, fittings (not shown) can be used to removably couple the ion source106to the ion entrance114and/or removably couple the ion receiver108to the ion exit118. In some examples, the housing106can also house and/or support other system components, such as a gas port121that can be coupled to a gas source122. The gas source122can supply one or more gases124(e.g., a drift gas) that can be introduced to the volume116through the gas port121. In some examples, the housing can also house a heating block126configured to apply heat to the plurality of electrodes of the electrode arrangement102, volume116, or other components of the ion focusing system100, as for example, to improve desolvation of the ions104that move through the ion focusing system100.

In examples, the ion focusing system100can include an ion control system environment128in communication with the electrode arrangement102, ion source106, ion receiver108, voltage source113, gas source122, housing120, and/or the heating126, and that is operable to control collection and/or manipulation of the ions104. The ion control system environment128can include one or more computing devices and/or controllers that include at least a processor130and a memory132. Computing devices can include desktop or laptop computers, mobile devices, tablets, industrial control systems, programmable logic controllers (PLCs), systems-on-a-chip, etc. The processor130can include one or more CPUs, GPUs, ASICs, FPGAs, MCUs, PLDs, CPLDs, etc., that can perform various data processing or I/O functions associated with the ion focusing system100. The memory132can be volatile or non-volatile (e.g., RAM, ROM, flash, hard drive, optical disk, etc.) and fixed or removable and is coupled to the processor130. The memory132can provide storage for various processor-executable logic instructions and program modules which when executed by the processor130, cause the ion focusing system100to generate, move, focus, and/or manipulate the ions104. Storage can also be provided with one or more other computer-readable media. One or more system buses134can provide a communication path between various environment components, such as between processor and I/O communication modules. The ion control system environment128can also be situated in a distributed form so that applications and tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules and logic can be located in both local and remote memory storage devices. For example, the ion receiver108can be an ion analyzer or coupled to an ion analyzer140which can send control signals to the ion control system environment128or can correspond to the ion control system environment128. In some examples, various components can be combined, such as by having the voltage source113include the processor130and memory132.

In representative examples, the memory132includes DC voltage control logic136that controls the electric potential applied to the electrode arrangement102such that the ions are directed through the volume116of the electrode arrangement102. The DC voltage control logic136can apply a specified single sequence or multiple sequences such that a nonlinear DC voltage gradient can include one or more sequences along different portions of the electrode arrangement102, or such that a DC voltage gradient varies over time, e.g., between different types of nonlinear DC voltage gradients, linear to nonlinear, nonlinear to linear, etc. In some examples, gradient profile, selection, and/or variation can be applied based on feedback from one or more system components, such as thermal feedback, ion analyzer or ion source gating, pulsing, packet formation or sensing, or other signaling, etc. In additional examples, gradient profile, selection, and/or variation can be applied based on other characteristics, such as predetermined user selection, ion species characteristics, ion exit aperture, etc.

In further examples, the memory132can include ion analyzer logic138to detect and/or receive parameters from the ion analyzer140coupled to the ion receiver108. In some examples, the ion analyzer140can be configured to apply (or to communicate with the computing device so that the DC voltage control logic136applies) a particular nonlinear DC voltage sequence and/or series of sequences to the electrode arrangement102in accordance with a specified analytical process and/or processes (e.g., a nonlinear DC voltage gradient for time-of-flight measurement of a particular ion sample). In additional examples, the memory132can include ion source logic142that controls generation and/or emission of ions from the ion source106. The ion source logic142can also be synchronized to the DC voltage control logic136to queue or position the ions104in the ion focusing system100so that groups of the ions104can be controllably released or directed into the volume116through the ion entrance114. In some examples, the memory132includes gas control logic146which can be in communication with one or more gas control valves of the gas source122and/or gas port121in order to introduce the gas124into the ion focusing system100. In further examples, the memory132can include heat control logic148in communication with the heating block126to control a temperature of the volume116, the electrode arrangement102, the gases124, or other components of the ion focusing system100. In further examples, DC voltages, ion characteristics, and/or other system parameters or performance outputs can be displayed on a display144and can be controlled by one or more input/output devices and/or operator (e.g., with a keyboard, mouse, or other interactive device, including the display144). In other examples, the DC voltage control logic136, ion source logic142, analyzer logic138, and/or gas control logic146can all be accessed and manipulated through user control by way of the display144.

In different examples, the electrode size and spacing of the electrode arrangement102can be varied based on electric field requirements for different ions (e.g., with different m/z ratios), coupled efficiencies, and input parameters for an ion analyzer140(e.g., timing, density, particle energy). Electrode spacing typically provides a minimum non-conductive area between electrodes to reduce the probability of electrical shorts or interference. In typical examples, the spacing110can correspond to a minimum spacing and electrode size that is on the order of millimeters, including as low as approximately 0.5 mm, though other dimensions are possible. Also, circular and/or other geometric shapes can simplify construction and can define system features or boundaries (e.g., apertures).

As shown inFIG. 1, the electrodes of the electrode arrangement102have a circular shape of constant diameter and the longitudinal axis112is straight, defining a cylindrical shape for the volume116. In additional examples, individual electrodes and/or electrode arrangements of ion focusing systems can have different geometries and/or different configurations. For example,FIG. 2Ashows an example electrode200A having a square shape so as to define a planar square-shaped cross-section of an associated electrode arrangement volume202A. Non-square shaped cross-sections can be defined as well, e.g., with electrode200A extending with a rectangular shape.FIG. 2Bshows an example electrode set200B defining a cross-section of an electrode arrangement volume201B. The electrode set200B includes four electrodes202B-208B, with electrodes202B,204B situated parallel and opposed to each other and with electrodes206B,208B also situated parallel and opposed to each other but perpendicular to the electrodes202B,204B. In some examples, one set of parallel electrodes, e.g., electrodes202B,204B, can be configured to receive voltages defining a non-linear voltage gradient into or out of the plane ofFIG. 2B, and the other set of parallel electrodes206B,208B can be configured as guard electrodes, e.g., with a static DC voltage. Guard electrodes can be situated to prevent ions from leaving the electrode arrangement volume201B. Guard electrodes can also extend into or out of the plane ofFIG. 2B, e.g., a substantial portion or an entire length of the electrode arrangement volume201B. In some examples, each of the electrodes202B-208B can have a common voltage. In additional examples, one or more of the electrodes202B-208B can have other electrical potentials applied that are different from guard voltages or non-linear gradient voltages.FIG. 2Cshows a square-shaped set of electrodes200C defining a cross-section of an electrode arrangement volume201C. The set of electrodes200C includes four ‘L’ shaped electrodes202C-208C, though more or fewer electrodes are possible.FIG. 2Dshows a circular set of electrodes200D defining a cross-section of an electrode arrangement volume201D. The set of electrodes200D includes curved electrodes202D-216D, which can be arranged symmetrically as shown or non-symmetrically (e.g., with an odd number of electrodes). In some examples, selected ones, sets, or all of the electrodes202D-216D are configured to provide a non-linear voltage gradient in the electrode arrangement volume201D. Selected sets can include one or more adjacent and/or opposing pairs of electrodes, by way of example.FIG. 2Eshows an example electrode200E having an elliptical shape defining an elliptical cross-section of an electrode arrangement volume202E.

FIG. 2Fshows an example of an electrode arrangement200F having a plurality of electrodes202F arranged along a longitudinal axis204F so as to define a curved electrode arrangement volume206F. Thus, in various examples, electrode arrangements can define bent, folded, or other non-straight configurations along which electrodes are arranged and configured to direct ions. For example, in order to avoid problems associated with neutral transmission (e.g., ghost peaks, peak broadening, inability to focus neutral species), the curved path configuration shown inFIG. 2Fcan allow ions of interest to be transmitted along the longitudinal axis204F as the ions are directed 180-degrees (or another angle) from an initial direction of travel.

FIGS. 3-12show how a nonlinear DC voltage gradient can be used to avoid using RF electrode confinement and overcome extensive vacuum requirements. For example,FIG. 3shows the profile of an ion focusing device300having an electrode arrangement302with one hundred electrodes in a drift tube configuration having a total length352(e.g., 100 mm). In some examples, the ion focusing device300can be similar to components of the ion focusing system100. As shown, each of the individual electrodes of the electrode arrangement302(e.g., with electrodes303a-303dforming a small subset) can be equally spaced by a distance310or pitch (e.g., of 0.5 mm) along a longitudinal axis305. The electrodes can have a circular shape defining an inner diameter354(e.g., of 50 mm) and corresponding tubular volume306for an interior of the ion focusing device300. The electrodes can also have an outer diameter356(e.g., of 55 mm), and a width (e.g., of 0.5 mm). A focusing electrode358(e.g. with a length360extending in the direction of the longitudinal axis305(e.g., of about 6 mm) can be situated at an ion entrance314of the electrode arrangement302and volume306. The focusing electrode358can also surround an ion source362(e.g., an ESI emitter) and provide an initial, relatively large, voltage. The higher voltage can be configured to provide a voltage gradient to assist with ions entering the volume306and to reduce radial dispersion of ions emitted into the volume306. In some examples, the voltage applied to the focusing electrode358is significantly higher than a voltage applied to a first electrode at the ion entrance314of the electrode arrangement302. The focusing electrode358can be a single electrode or multiple electrodes, with multiple electrodes held at a common potential or at different potentials (e.g., a gradient). During operation the electrodes arrangement302directs received ions from the ion entrance314through the volume306to an ion exit316.

During vacuum based ion spectrometer processes (e.g., less than about 50 Torr, 20 Torr, 10 Torr, 5 Torr, 1 Torr, etc.), a linear voltage sequence can be applied to the system of electrodes (e.g., 1000, 900, 800, 700, 600, etc.). However, in various examples herein, ion focusing is obtained at elevated pressures, such as greater than 1 Torr, 50 Torr, 100 Torr, 500 Torr, 760 Torr (atmospheric), or higher, by applying nonlinear DC voltage sequences to the electrodes of the electrode arrangement302. For example, rather than providing a linear voltage sequence with respect to distance, such as 1000, 900, 800, 700, 600, etc., and spatial positions 0, 1, 2, 3, 4 (arb. units), nonlinear voltage sequences are provided, such as 1000, 900, 790, 670, 540, etc., at the same spatial positions 0-4.

FIG. 4andFIG. 5are examples of nonlinear voltage sequences that can be applied to the electrode arrangements102,302and other electrode arrangements.FIG. 4shows various power sequences402a-402e(such as defined by an exponential) along with a linear sequence404plotted in a graph400, andFIG. 5shows various quadratic sequences502a-502ealong with a linear sequence504plotted in a graph500. The power series examples402a-402efollow a power expression of the form y=axb+c and can be described by equation 1 below:

Where VElecis the voltage applied to electrode n, Vinis the voltage applied to the first electrode of an electrode arrangement, order is the sequence order, and Vstepis a number added to the order every time the electrode number increases. Values of Vstepcan be determined by rearranging equation 1 to obtain an expression such as the one shown in equation 2, where Vstepis a function of the sequence order and the initial voltage Vin:

Similarly, the quadratic sequences502a-502ecan follow a quadratic equation of the form y=ax2+bx+c and can be described by equation 3:

As with the power sequence, an expression for Vstepfrom the quadratic sequence can be obtained by rearranging equation 3 to obtain equation 4:

In applying equations 2 and 4 to the one-hundred electrodes of the electrode arrangement302of the focusing device300, values for Vstepcan be determined for example, by setting n=101 to account for the voltage applied to both the electrode arrangement302and to an ion receiver318(e.g., a collection plate, capillary, or other ion receiver) such that VElec=V101. For example, Table 1 lists the values of Vstepfor both the power sequences402a-402eand the quadratic sequences502a-502ein an 100-electrode system (e.g., n=101) with different initial voltages (V) Vin(2 kV, 3 kV, and 4 kV), appropriate values for the order, and a voltage of 15 V applied to the ion receiver318.

The values for Vsteplisted in Table 1, can be substituted into the power and quadratic sequences of equations 1 and 3, respectively, to generate the graphs400,500that are illustrative of VElechas a function of the number of electrodes n. The graphs400,500further show that the nonlinear DC voltage sequences create a voltage difference ΔV, ΔV′ between adjacent electrodes that is constantly changing along the length of the electrode arrangement. Both power and quadratic sequences establish relatively low voltage differences ΔV, ΔV′ between electrodes at the beginning of the electrode arrangement but can have larger voltage differences further along and toward the end of the electrode arrangement. For example, in contrast to the linear sequence404shown inFIG. 4, where the voltage difference between individual electrodes is constant, the power sequence402aof an order 1.01 generates an 18 V voltage difference ΔV between the first two electrodes of the electrode arrangement and a 46 V difference ΔV between electrodes99and100. Further, this effect the power sequence has on voltage difference can be enhanced by increasing the sequence order, leading to the creation of significantly low voltage differences between electrodes at the beginning of the electrode arrangement and significantly high voltage differences between electrodes at the end of the electrode arrangement. For example, in contrast to the power sequence402a, the power sequence402eof an order 1.05, establishes a 1 V difference ΔV between the first and second electrodes and a 137 V difference ΔV between the final two electrodes of the electrode arrangement102. As observed in the graph400, the power sequences402b-402dfor varied order values between the sequences402aand402eillustrate how the voltage differences vary as the number of electrodes of the electrode arrangement102increases (e.g., as its length increases).

Similarly,FIG. 5shows that like trends can be present in quadratic sequences502a-502e. However, as shown in the graph500, the voltage differences ΔV′ between electrodes are not as large in comparison to those found in the power sequences402a-402e. As a result, the quadratic sequences502a-502ediffer less from the voltage differences observed in the linear sequence504than that of the power sequences402a-402e, as the number of electrodes in the electrode arrangement increases. Consequently, the quadratic sequences502a-502etend to lead to larger voltage differences ΔV′ at the beginning of an electrode arrangement and lower voltage differences near the end of the electrode arrangement, as compared to the power sequences402a-402e.

FIGS. 6 and 9show experimental ion trajectory results of application of both linear and nonlinear DC voltage gradients to an electrode arrangement, such as the electrode arrangement302. The experiments used 10,000 ions (e.g., 1000 ions per run over 10 runs) with a mass to charge ratio (m/z) of 100 per experiment, resulting in a total space charge of 2×10−11C per run (e.g., 2×10−14C per ion).

First, a linear voltage gradient (e.g., order of 1.00) was applied to the electrode arrangement302ofFIG. 3to obtain reference ion trajectory graphs600a,600g,600mand900a,900g,900mfor the three starting voltages of 2 kV, 3 kV, and 4 kV. As shown in bothFIGS. 6 and 9, as the ions are introduced into the electrode arrangement302, radial expansion of the ions is observed throughout the length of the electrode arrangement302with narrower profiles, showing similar effects, obtained with increasing voltage.

The resulting trajectory profiles for power sequence nonlinear voltage application for the three initial voltages and their respective orders 1.00, 1.01, 1.02, 1.03, 1.04, and 1.05 are shown in ion trajectory graphs600b-600f,600h-600l,600n-600r. As can be seen in ion trajectory graph600b, when low power orders are used, the ions maintain a relatively straight trajectory through the electrode arrangement102. At higher orders, such as order 1.05 shown in ion trajectory graph600f, the ions initially radially expand after entering the electrode arrangement302but subsequently converge, or focus, at more distant positions from an ion entrance. This initial radial expansion of the ions results from the smaller voltage differences between adjacent electrodes at the beginning of the electrode arrangement302. However, instead of continuing to radially expand, as is produced with a linear sequence (e.g.600a,600g,600m), the ions traveling along the electrode arrangement302exhibit significant radial focusing as the voltage differences steadily increase. As observed in ion trajectory graphs600f,600l, and600r, the largest amount of focusing occurs near the ion exit316of the electrode arrangement302, where the largest voltage differences between electrodes are present. Consequently, the initial radial expansion at high orders of the power sequence also causes significant peak tailing, which can be seen in the power sequence drift time graphs700a-700rinFIG. 7. Using the ion focusing device300as an example, due to the low voltage differences between electrodes nearer to the ion entrance314of the electrode arrangement302, the ions pushed to the periphery of the electrode arrangement302during the initial radial expansion travel longer paths from ion entrance314to ion exit316than those initially closer to the longitudinal axis305during expansion, causing longer drift times to reach the ion receiver318. In contrast, minimal tailing is observed in the quadratic sequence drift time graphs1000a-1000rshown inFIG. 10. As described herein, because the quadratic sequence results in larger voltage differences between adjacent electrodes at the beginning of the focusing device, ions undergo minimal initial radial expansion and thus, exhibit little peak tailing, if any at all.

FIG. 8shows a series of ion intensity plots800a-800rshowing variation in focusing obtained with power nonlinear DC voltage gradient sequences and linear voltage gradient sequences. For example, the nonlinear DC voltage of the power sequence of order 1.05 (e.g.,800f,800l,800r) produced focusing that was approximately 2.5 times greater than the focusing achieved by the linear sequences (e.g., of order 1.00 shown in800a,800g,800m). InFIG. 8, the spot diameter in mm of the ions leaving the electrode arrangement302is indicated by the values outside of the parenthesis, while the values inside parenthesis indicate the ion transmission as a percentage. As shown inFIG. 8, the spot diameters of the linear voltage gradients800a,800g,800mare approximately between 2.25 to 2.5 larger than the spot diameters of the nonlinear DC voltage gradients associated with ion intensity plots800f,800l,800r. Other spot diameters are obtained by varying power sequence orders. Use of larger power orders can result in ion loss due to considerable ion expansion near the ion entrance, such as the traces shown in the ion trajectory plots600c-600f. In some examples, related ion losses are reduced by increasing an initial voltage of the electrode arrangement, such as that shown for600h-600land600n-600r. In further examples, losses can be used as a filter of ions with selected initial trajectories, such as trajectories that are angled with respect to an axis of the focusing device.

FIGS. 9 and 11show the ion trajectories900a-900rand ion intensity plots1100a-1100rproduced by applying the quadratic series voltages, using sequence order values of 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5, and the same initial voltages used for the power sequence examples inFIGS. 6 and 8. The quadratic series produced minimal to no ion losses as the ions are directed along through the volume306by the electrode arrangement302. For example, ion trajectories900h-900land900n-900rshow that ions experience little to no radial expansion after being introduced into the electrode arrangement302. The ions subsequently travel the length of the volume306defined by the electrode arrangement302in a relatively straight line, rather than radially expanding as observed with the linear voltage sequence. This lack of initial ion defocusing (e.g., radial expansion) within the volume306of the electrode arrangement302results from the large voltage differences formed between the electrodes at the beginning of the electrode arrangement302, which almost entirely avoids ion losses. This is in contrast to the small voltage differences at the beginning of the electrode arrangement302observed when applying one of the example power sequence voltage gradients, which as mentioned above, can produce considerable ion expansion after the ions are introduced. It should be noted, that a combination of Vinequal to 2 kV, VElec=Viol equal to 15 V, and an order of 0.5 produces a positive voltage gradient at the beginning of the device, meaning ion transmission through device is not possible, as denoted by the N/P at ion trajectory900f.

In comparison to the power sequence examples, quadratic sequence examples can be configured to provide a modest amount of ion focusing. As shown inFIG. 11, focusing effects from application of quadratic sequence voltages can be approximately 1.4 times larger than the linear voltage gradient, as opposed to the 2.5 times achieved by applying power sequence examples. For example, when a sequence order of 0.5 and an initial voltage (Vin) of 3 kV are used, the spot diameter produced by the nonlinear quadratic sequence is approximately 23.5 as shown in ion intensity plot1100lversus the 33.2 shown in ion intensity plot1100gobtained with a linear voltage sequence. In examples, in contrast to the power sequence, as the order of the quadratic sequence is increased, drift time distributions can be observed without significant changes in peak widths, as shown inFIG. 10. Overall drift times for quadratic sequence examples can be considerably lower than those observed in power sequence examples (e.g.,FIG. 7), and can be attributable to the lack of initial ion radial expansion, associated with larger voltage differences between electrodes nearer the entrance of the volume316and related reduction in ion travel distances.

The effects of applying a nonlinear DC voltage demonstrate that the nonlinear DC voltage sequences cause the electric field gradient within an ion propagation volume to change as a function of distance along an electrode arrangement (i.e., spatially). For example, the linear, power (1.05), and quadratic (0.5) gradients shown in the contour plots1200,1202,1204ofFIG. 12show that the radial motion of ions are not affected by the electric field gradient when a linear sequence is applied but do become affected when the nonlinear power or quadratic sequence are applied. Vertical lines1206of the contour plot1200show that the electric field exhibits little to no radial change along the length of the electrode arrangement (e.g., electrode arrangement102or302) such that any radial expansion experienced by the ions introduced into the system are likely due to diffusion and overall space charge.

In contrast, the contour plots1202and1204for the power and quadratic nonlinear sequences, respectively, show that the electric field gradient of either sequence experiences significant radial change as a function of distance. The change in the electric field gradient of a nonlinear DC voltage sequence can be seen in the contour plot1202for the power sequence. For example, the contour lines1208of contour plot1202curve strongly outward initially near the ion entrance114of the electrode arrangement102and then shift vertically around an inflection point1210(e.g., change in the curvature) located at the 25thelectrode. The shift of the contour lines1208at the inflection point1210is consistent with the ion trajectories and ion intensity plots shown inFIGS. 6 and 8. For example, the shape of the contour plot1202support the observed initial defocusing effect nearer to the ion entrance of the electrode arrangement302, as the electric field gradient initially pushes a number of the ions outward when the power sequence is applied. Similarly, after the contour lines1208of electric field gradient shifts at the inflection point1210, the contour lines1208progressively begin to curve inward as the number of electrodes increases, supporting the observed strong focusing effects associated with the power sequence voltages (e.g., evidenced by the ion intensity plots ofFIG. 8).

With reference to the contour plot1204associated with the quadratic sequence voltage, a similar field shape can be observed in contour lines1212though with less variation than the contour lines1208of the power sequence. Rather, the contour lines1212show a curvature outward and inward along the length of the electrode arrangement302that is less pronounced. This change in the electric field gradient produced by the quadratic sequence voltage is consistent with the ion trajectories and ion intensity plots ofFIGS. 9 and 11, supporting the smaller defocusing effect nearer the entrance of the ion volume316and less focusing overall as compared to the power sequence.

Rather than forming a pseudopotential in time, as the systems using RF waveforms do, the constant change observed in the electric field gradient as a function of distance (though not necessarily the same change per unit distance), according to some described examples, establishes a pseudopotential well in space allowing for the spatial ion focusing at atmospheric pressure. As described herein, the pseudopotential well deepens as a function of distance, thereby providing increased ion focusing at larger distances traveled by the ions into the device.FIG. 12further illustrates this increased ion focusing at larger distances traveled.

FIG. 12shows the potential energy surfaces formed by the linear contour plot1200, power contour plot1202(e.g., 1.05), and quadratic contour plot1204(e.g., 0.5) sequences at two different regions of the electrode arrangement302, with the first region1214being located between electrodes20and25and with the second region1216being located between electrodes80and85. For the linear voltage sequence, a potential energy “hill”1218(i) is observed in the first region1214and flattens along the length electrode arrangement302where only a small potential hill1218(ii) is observed in the second region1216. The small “hill”1218(ii) observed at the second region1216further indicates that charge repulsion and diffusion are predominate factors affecting radial motion of the ions in a linear voltage sequence.

Similarly, the potential energy surfaces of the power and quadratic sequences both produce a hill within the first region1214, as can be seen in hills1218(iii),1218(v), similar to the linear sequence hill1218(i); however, rather than a second hill at the second region1216, potential wells1218(iv),1218(vi) are observed. For example, when the power sequence is applied, the hill1218(iii) formed by the potential energy surfaces at the first region1214begins to flatten toward the inflection point1210, but then forms into a potential well that deepens toward the second region1216and end of the electrode arrangement302. This deepening potential well observed can produce the ion focusing effect observed inFIGS. 6 and 8toward the end of the electrode arrangement302. Similarly, the potential energy surfaces observed for the quadratic sequence show a hill1218(v) in the first region1214and a potential well1218(vi) in the second region1216. However, the potential well1218(vi) observed with the quadratic sequence is more shallow along the electrode arrangement302than that of the one observed with the power sequence, which corresponds to the reduced ion focusing effect demonstrated earlier with regard to the ion trajectories and ion intensity graphs ofFIGS. 9 and 11.

The following figures show representative methods for implementing the nonlinear DC voltages of the present disclosure in order to utilize the foregoing ion focusing effects. It should be noted that although the methods described herein are listed in a sequence, the various steps listed do not necessarily have to follow the sequence but may be undertaken in any manner suitable for the intended use.

FIG. 13shows a representative method of applying a single nonlinear voltage sequence to focus ions in an environment at 1 Torr or greater, such as at atmospheric pressure. At1302, an operator can use a display or other interface, or initiate through a pre-programmed ion focus routine, to select a nonlinear voltage sequence. In some examples, the nonlinear voltage sequence can be a power or quadratic nonlinear voltage sequence, or another nonlinear voltage sequence, including but not limited to, a cubic function, a complex wave function, and/or any polynomial of order “n.”FIGS. 16 and 17, for instance, schematically depict examples of a cubic voltage sequence1600and a polynomial sequence1700of order n=4, respectively. In some examples, a coupled device, such as an ion receiving apparatus, mass spectrometer, ion mobility spectrometer, etc., can specify and/or control selection of the nonlinear voltage sequence based on, e.g., a specified analytical process specified by the operator and/or automated output. In some examples, the selected nonlinear voltage sequence is associated with measurement of ion focusing intensity or time-of-flight measurements. At1304, the selected nonlinear DC voltage gradient is applied to an electrode arrangement defining an ion focusing volume. The applied voltages form voltage differences between adjacent electrodes of the electrode arrangement to produce a nonlinear voltage gradient within the ion focusing volume.

At1306, ions are introduced into the ion focusing volume through an ion entrance and at1308, the electrode arrangement with applied nonlinear voltage sequence focuses ions radially as the ions propagate along a direction of a longitudinal axis of the ion focusing volume. In representative examples, the ions are introduced at predetermined times in relation to the applied nonlinear voltage gradient. The ions are focused within the volume with the volume at pressures that are larger than substantial vacuum conditions, such as at 1 Torr or greater, 50 Torr or greater, 100 Torr or greater, 200 Torr or greater, 500 Torr or greater, 760 Torr or greater, etc.

As described herein, as the ions are introduced into the electrode arrangement at1306, the ions are directed and focused at1308through the volume defined by the electrode arrangement and applied nonlinear voltage sequence by the electric field gradient formed by the voltage differences. At1310, the ions exit the ion focusing volume and are received by an ion receiver, such as an ion analyzer, mass spectrometer, ion mobility spectrometer, collection plate, current detector, ion conduit, etc. In examples, the ions can be captured by an ion receiver. In further examples, the ion receiver is and/or can be coupled to an ion analyzer, such as a mass spectrometer and/or an ion mobility spectrometer, including spectrometers with an atmospheric pressure interface. In other examples, the ion analyzer can be another analytical device and/or can be an optical device to focus the captured ions into an ion beam for atmospheric pressure ion manipulation, such as in surface functionalization and/or soft-landing processes, including for deposition of the ions. In further examples, the ion receiver can be any one and/or combination of various ion manipulation devices, such as an ion confinement apparatus, ion focusing device, ion mobility device, and/or ion mass spectrometer, all of which can operate under atmospheric pressure.

FIG. 14depicts an example method1400of generating a nonlinear DC voltage gradient with one or more nonlinear sequences, such as a combination of two or more sequences. By using two or more nonlinear voltage sequences concurrently, or varying one or more over time, various benefits can be obtained. For example, ions exhibit an initial radial expansion and intense focusing when a power sequence is applied, but exhibit little to no initial radial expansion and moderate focusing when a quadratic sequence is applied. Thus, in some examples, a first nonlinear DC voltage gradient is based on a quadratic sequence and can be applied to a first portion of an electrode arrangement, and a second gradient based on a power sequence can be applied to a second portion of the electrode arrangement.

For example, at1402,1404an operator and/or controller can select a quadratic sequence to be applied to a first portion of an electrode arrangement and a power sequence to be applied to a second portion of the electrode arrangement, e.g., as illustrated inFIG. 15with first portion1500, second portion1502, of electrode arrangement1504. At1408, a voltage gradient is generated in an ion volume with the selected nonlinear sequences applied to each respective portion of the electrode arrangement, such as generating and applying the selected quadratic sequence to first portion1500and the power sequence to second portion1502. In some examples, at1410, heat and/or gas is introduced into the volume by the heating block or gas source (including, e.g., multiple gases), respectively. Heat can be used to incorporate constant or variable temperature conditions and gas can be used to alter separation efficiency for molecules. In some examples, introduced gas and/or heat can be selected and/or controlled to increase desolvation of the ions introduced into the volume.

At1412, as ions are introduced at the ion entrance of the electrode arrangement, the combination of nonlinear DC voltage sequences generated and applied at1408focus the ions radially along a direction of a longitudinal axis of the ion focusing volume defined by the electrode arrangement. In this way, the combination of nonlinear DC voltages reduce a radial expansion of the ions under a quadratic voltage sequence at the first portion1500(e.g., up to the inflection point1220) and increase focusing of the ions under the power voltage sequence through the section portion1502(e.g., after inflection point1210). Thus, in this example, ion losses can be reduced through the varying of nonlinear voltage sequences along the length of an electrode arrangement. Such a distribution of the sequences could be helpful, for example, when a higher flux of ions is directed through the ion volume defined by the electrode arrangement. At1418, as the ions propagate along the longitudinal axis of the electrode arrangement and are received at the ion exit at1416, the ions can be captured, delivered to an ion analyzer, such as an atmospheric ion spectrometer, and/or be focused as to become an ion source for other processes.

In other examples, the quadratic sequence and power sequence can be switched to create radial expansion of ions at the beginning of the electrode arrangement and moderate focusing through the rest of the arrangement, such as to study the effects on ions propagating along a particular length of an electrode arrangements. Some method examples can include, for example at1406, one or more additional sequences (e.g., n number of nonlinear sequences) in addition to the first and second nonlinear voltage sequences to achieve a desired ion focusing and/or defocusing effect. In further examples, the voltage sequences can vary over time, such as based on propagation of an ion packet through the volume. As one example,FIG. 18shows a DC voltage sequence varying between different nonlinear DC voltage sequences over a time t0-t6.

Although ion focusing was demonstrated herein with the power sequence and quadratic sequence, the advantages of the system and methods of the present disclosure are not limited to such sequences but rather may be used with other nonlinear polynomial functions of order “n,” in accordance with the system and methods described herein.

Having described and illustrated the principles of the disclosed technology with reference to the illustrated embodiments, it will be recognized that the illustrated embodiments can be modified in arrangement and detail without departing from such principles. For instance, elements of the illustrated embodiments shown in software may be implemented in hardware and vice-versa. Also, the technologies from any example can be combined with the technologies described in any one or more of the other examples. It will be appreciated that procedures and functions such as those described with reference to the illustrated examples can be implemented in a single hardware or software module, or separate modules can be provided. The particular arrangements above are provided for convenient illustration, and other arrangements can be used.

In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only representative examples and should not be taken as limiting the scope of disclosure. Alternatives specifically addressed in these sections are merely exemplary and do not constitute all possible alternatives to the embodiments described herein. For instance, various components of systems described herein may be combined in function and use. We therefore claim all that comes within the scope of the appended claims.