Differential Mobility Spectrometry Method

Abstract

Methods and systems are provided herein for varying the CoV about a nominal CoV-apex while monitoring the ion of interest corresponding to the nominal CoV-apex as it is transmitted by a DMS. In various aspects, the CoV can be swept or stepped across a series of values during the injection of ions into the DMS such that a composite spectra of the ion of interest transmitted by the DMS (or its product ions following one or more stages of fragmentation) can be generated so as to include the transmission of the particular ion at a CoV with optimum sensitivity (i.e., if distinct from the CoV-apex), thereby improving the robustness, accuracy, and/or selectivity during experimental conditions relative to known DMS techniques, which typically used a fixed CoV value for each ion of interest.

Claims

1. A method of operating a system comprising a differential mobility spectrometer, the method comprising: (a) injecting a stream of ions into a differential mobility spectrometer during a first duration, the stream of ions containing or suspected of containing an ion species of interest; (b) determining a compensation voltage apex for the ion species of interest in the differential mobility spectrometer; (c) varying the compensation voltage applied to the differential mobility spectrometer over a range about the determined compensation voltage apex during said first duration; (d) detecting the ion species of interest transmitted by the differential mobility spectrometer during said first duration.

2. The method of claim 1, wherein the compensation voltage apex represents a nominal optimum for transmission of the ion species of interest based on operating conditions of the differential mobility spectrometer, optionally wherein the nominal optimum is provided by a look-up table.

3. The method of claim 1, wherein the compensation voltage apex for the ion species of interest is determined based on a theoretical optimum transmission compensation voltage.

4. The method of claim 1, wherein said range about the compensation voltage apex comprises a range between a first compensation voltage value less than said compensation voltage apex and a second compensation voltage value greater than said compensation voltage apex.

5. An apparatus for analyzing ions comprising: an ion source configured to generate ions from a sample containing or suspected of containing an ion species of interest; a differential mobility spectrometer configured to receive the plurality of ions from the ion source and transmit one or more ions species based on the differential mobility of the ion species therein; a detector for detecting the ion species transmitted by the differential mobility spectrometer; and a controller operatively coupled to the ion source, the differential mobility spectrometer, and the detector, and configured to: cause the ion source to inject a stream of ions into the differential mobility spectrometer during a first duration; determine a compensation voltage apex for the ion species of interest in the differential mobility spectrometer; vary the compensation voltage applied to the differential mobility spectrometer over a range about the determined compensation voltage apex during said first duration; and detect the ion species of interest transmitted by the differential mobility spectrometer during said first duration.

6. The apparatus of claim 5, wherein the compensation voltage apex represents a nominal optimum for transmission of the ion species of interest based on operating conditions of the differential mobility spectrometer.

7. The apparatus of claim 5, further comprising data storage associated with the controller, wherein the data storage is configured to store compensation voltage apex for one or more ion species under operating conditions of the differential mobility spectrometer.

8. The apparatus of claim 5, wherein the compensation voltage apex for the ion species of interest is selected based on a theoretical optimum transmission compensation voltage.

9. The apparatus of claim 5, wherein said range about the compensation voltage apex comprises a range between a first compensation voltage value less than said compensation voltage apex and a second compensation voltage value greater than said compensation voltage apex.

10. The apparatus of claim 9, wherein the controller is configured to discretely vary, in stepwise fashion, the compensation voltage applied to the differential mobility spectrometer between the first compensation value and the second compensation value.

11. The apparatus of claim 9, wherein the controller is configured to continuously vary the compensation voltage applied to the differential mobility spectrometer between the first compensation value and the second compensation value.

12. The apparatus of claim 5, wherein said range about the compensation voltage apex is predetermined.

13. The apparatus of claim 12, wherein said range represents ±1.0V of the compensation voltage apex, optionally wherein the compensation voltage is varied in stepwise increments of 0.1V.

14. The apparatus of claim 5, further comprising an input device configured to receive an input from a user regarding the compensation voltage apex, optionally wherein the input device is configured to receive an input from the user of said range about said compensation voltage apex.

15. The apparatus of claim 5, wherein said range is determined through a user-selected deviation from the compensation voltage apex.

16. The apparatus of claim 5, further comprising a fragmentor disposed between the differential mobility spectrometer and the detector, wherein the fragmentor is configured to fragment the ions transmitted by the differential mobility spectrometer, and wherein the detector is configured to detect the ion species of interest by detecting one or more fragments of the ion species of interest.

17. The apparatus of claim 15, wherein the controller is configured to calculate the m/z for each fragment detected by the detector and optionally wherein the controller is configured to generate a mass spectrum corresponding to the ion fragment m/z data during said first duration.

18. The apparatus of claim 16, wherein the fragmentor comprises a collision cell wherein beam-type fragmentation is implemented.

19. The apparatus of claim 16, wherein the fragmentor comprises a collision cell wherein resonance excitation for fragmentation is implemented.

20. The apparatus of claim 5, further comprising a drift gas supply for providing a drift gas for flowing through the differential mobility spectrometer and a modifier liquid supply for supplying a modifier liquid to the drift gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the applicant's teachings in any way.

[0019] FIG. 1, in a schematic diagram, illustrates an exemplary differential mobility spectrometer/mass spectrometer system in accordance with an aspect of various embodiments of the applicant's teachings.

[0020] FIG. 2A depicts an exemplary timing diagram for generating an asymmetric electric field in DMS of FIG. 1;

[0021] FIG. 2B depicts an exemplary timing diagram for operating the filter electrodes using a fixed CoV in the DMS of FIG. 1;

[0022] FIG. 2C, in schematic diagram, depicts exemplary paths for groups of ions within the DMS filter subjected to the combined electric fields of FIGS. 2A and 2B;

[0023] FIG. 3 is a flowchart showing an exemplary method for analyzing an ion of interest in the system of FIG. 1 in accordance with various aspects of the present teachings.

[0024] FIG. 4A depicts exemplary data regarding the relative intensity of the ion of interest at various CoV values utilized in determining the CoV-apex in accordance with various aspects of the present teachings.

[0025] FIG. 4B depicts one exemplary continuous technique for varying the CoV in method of FIG. 3 in accordance with various aspects of the present teachings.

[0026] FIG. 4C depicts one exemplary stepwise technique for varying the CoV in method of FIG. 3 in accordance with various aspects of the present teachings.

DETAILED DESCRIPTION

[0027] It will be appreciated that for clarity, the following discussion will explicate various aspects of embodiments of the applicant's teachings, while omitting certain specific details wherever convenient or appropriate to do so. For example, discussion of like or analogous features in alternative embodiments may be somewhat abbreviated. Well-known ideas or concepts may also for brevity not be discussed in any great detail. The skilled person will recognize that some embodiments of the applicant's teachings may not require certain of the specifically described details in every implementation, which are set forth herein only to provide a thorough understanding of the embodiments. Similarly it will be apparent that the described embodiments may be susceptible to alteration or variation according to common general knowledge without departing from the scope of the disclosure. The following detailed description of embodiments is not to be regarded as limiting the scope of the applicant's teachings in any manner.

[0028] In various aspects, methods and systems are provided herein for varying the CoV about the CoV-apex while monitoring the ion of interest corresponding to the nominal CoV-apex as it is transmitted by the DMS. In various aspects, the CoV can be swept or stepped across a series of values during the injection of ions into the DMS such that a composite spectra of the ion of interest transmitted by the DMS (or its product ions following one or more stages of fragmentation) can be generated so as to include the transmission of the particular ion at a CoV with optimum sensitivity (i.e., if distinct from the CoV-apex), thereby improving the robustness, accuracy, and/or selectivity during experimental conditions relative to known DMS techniques.

[0029] With reference now to FIG. 1, an exemplary differential mobility spectrometer/mass spectrometer system 100 in accordance with various aspects of applicant's teachings is illustrated schematically. As shown in FIG. 1, the differential mobility spectrometer/mass spectrometer system 100 generally comprises an ion source 103, a differential mobility spectrometer (DMS) 110, a first vacuum lens element 150 of a mass spectrometer (hereinafter generally designated mass spectrometer 150) in fluid communication with the DMS 110, and a controller 160 operatively coupled to the ion source 103, the DMS 110, and the mass spectrometer 150 for controlling operation of the system 100 as discussed otherwise herein.

[0030] In the exemplary embodiment depicted in FIG. 1, the differential mobility spectrometer 110 comprises a pair of opposed electrode plates 112 surrounded by an electrical insulator 114 that supports the electrode plates 112 and insulates them from other conductive elements. The electrode plates 112 surround a drift gas 116 that drifts from an inlet 118 of the differential mobility spectrometer 110 to an outlet 120 of the differential mobility spectrometer 110. The outlet 120 of the differential mobility spectrometer 110 releases the drift gas 116 into an inlet 154 of a vacuum chamber 152 containing the mass spectrometer 150. As will be appreciated by a person skilled in the art, the differential mobility spectrometer/mass spectrometer system 100 represents only one possible configuration for use in accordance with various aspects of the systems, devices, and methods described herein. By way of non-limiting example, the differential mobility spectrometer can be a differential mobility spectrometer, or FAIMS devices of various geometries such as parallel plate, curved electrode, or cylindrical FAIMS device, among others.

[0031] The differential mobility spectrometer 110 can be contained within a curtain chamber 130 that is defined by a curtain plate or boundary member 132 and is supplied with a curtain gas from a curtain gas supply 134. The pressure of the curtain gases in the curtain chamber 130 can be maintained at or near atmospheric pressure (i.e., 760 Torr). It will be appreciated by a person skilled in the art that the curtain gas supply 134 can provide any pure or mixed composition curtain gas to the curtain gas chamber via curtain gas conduits at flow rates determined by a flow controller and valves, for example. By way of non-limiting example, the curtain gas can be air, O.sub.2, He, N.sub.2, CO.sub.2, or any combination thereof. As shown in FIG. 1, the system 100 can also include a modifier supply 138 for supplying a modifier to the curtain gas. As noted above, the modifier agents can be added to the curtain gas to cluster with ions differentially during the high and low field portions of the SV. As will be appreciated by a person skilled in the art, the modifier supply can be a reservoir of a solid, liquid, or gas through which the curtain gas is delivered to the curtain chamber 130. By way of example, the curtain gas can be bubbled through a liquid modifier supply. Alternatively, a modifier liquid or gas can be metered into the curtain gas, for example, through an LC pump, syringe pump, or other dispensing device for dispensing the modifier into the curtain gas at a known rate. For example, the modifier can be introduced using a pump so as to provide a selected concentration of the modifier in the curtain gas. The modifier supply 138 can provide any modifier including, by way of non-limiting example, acetone, water, methanol, isopropanol, methylene chloride, methylene bromide, or any combination thereof. Optionally, the curtain gas conduit and/or curtain chamber 130 can include a heater for heating the mixture of the curtain gas and the modifier to further control the proportion of modifier in the curtain gas.

[0032] As will be appreciated by a person skilled in the art, the differential mobility spectrometer/mass spectrometer system 100 can additionally include one or more additional mass analyzer elements 150a downstream from vacuum chamber 152. By way of example, ions can be transported through vacuum chamber 152 and through one or more additional differentially pumped vacuum stages containing one or more mass analyzer elements 150a. For instance, in one embodiment, a triple quadrupole mass spectrometer may comprise three differentially pumped vacuum stages, including a first stage maintained at a pressure of approximately 2.3 Torr, a second stage maintained at a pressure of approximately 6 mTorr, and a third stage maintained at a pressure of approximately 10.sup.−5 Torr. The third vacuum stage can contain a detector, as well as two quadrupole mass analyzers with a collision cell (Q2) located between them. In some aspects, for example, the collision cell (Q2) can be operated as a fragmentor for fragmenting the ions transmitted by the differential mobility spectrometer 110, with the detector being configured to detect the ion species of interest by detecting one or more fragments of the ion species of interest. It will be apparent to those skilled in the art that there may be a number of other ion optical elements in the system. Other type of mass analyzer such as single quadrupole, ion trap (3D or 2D), hybrid analyzer (quadrupole-time of flight, quadrupole-linear ion trap, quadrupole-orbitrap), orbitrap or time-of-flight mass spectrometer, could also be used.

[0033] Ions 102 can be provided from an ion source 103 and emitted into the curtain chamber 130 via curtain chamber inlet 134. As will be appreciated by a person skilled in the art, the ion source can be virtually any ion source known in the art, including for example, a continuous ion source, a pulsed ion source, an atmospheric pressure chemical ionization (APCI) source, an electrospray ionization (ESI) source, an inductively coupled plasma (ICP) ion source, a matrix-assisted laser desorption/ionization (MALDI) ion source, a glow discharge ion source, an electron impact ion source, a chemical ionization source, or a photoionization ion source, among others. The pressure of the curtain gases in the curtain chamber 130 (e.g., ˜760 Torr) can provide both a curtain gas outflow out of curtain gas chamber inlet 134, as well as a curtain gas inflow into the differential mobility spectrometer 110, which inflow becomes the drift gas 116 that carries the ions 102 through the differential mobility spectrometer 110 and into the mass spectrometer 150 contained within the vacuum chamber 152, which can be maintained at a much lower pressure than the curtain chamber 130. For example, the vacuum chamber 152 can be maintained at a pressure of 2.3 Torr by a vacuum pump 156. As the curtain gas within the curtain chamber 130 can include a modifier, the drift gas 116 can also comprise a modifier.

[0034] As discussed otherwise herein, the electrode plates 112 can be coupled to a power source (not shown) such that an RF voltage (i.e., a separation voltage (SV)) can be applied to the electrode plates 112 to generate an electric force in a direction perpendicular to that of the drift gas flow 116. As a result, ions contained within the drift gas tend to migrate radially away from the axis of the drift tube by a characteristic amount during each cycle of the RF waveform due to differences in mobility during the high field and low field portions. A DC potential (i.e., a compensation voltage (CoV)), is applied to the electrode plates 112 to provide a counterbalancing electrostatic force to that of the SV. The CoV can be tuned so as to preferentially restore a stable trajectory to particular ions such that they will be sampled by the mass spectrometer 150 via its inlet 154. Depending on the application, the CoV can be set to a fixed value such that only ion species exhibiting a particular differential mobility are transmitted through the outlet 120 of the differential mobility spectrometer 110 (the remaining species of ions drift toward the electrodes 112 and are neutralized thereby). As will be appreciated by a person skilled in the art, the differential mobility spectrometer 110 can also operate in “transparent” mode, for example, by setting SV to zero such that substantially all ions are transmitted therethrough without experiencing a net radial force.

[0035] As noted above, the exemplary system 100 can additionally comprise a controller 160 for controlling operation thereof. By way of example, the controller 160 can include a processor for processing information. Controller 160 also includes data storage 162 for storing mass spectra, CoV-apex data (e.g., in a database or library), and instructions to be executed by processor, etc. Data storage 162 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor.

[0036] The controller 160 can also be operatively associated with an output device such as a display (e.g., a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information to a computer user) and/or an input device including alphanumeric and other keys and/or cursor control, for communicating information and command selections to the processor. Consistent with certain implementations of the present teachings, the controller 160 can execute one or more sequences of one or more instructions contained in data storage 162, for example, or read into memory from another computer-readable medium, such as a storage device (e.g., a disk). Implementations of the present teachings are not limited to any specific combination of hardware circuitry and software.

[0037] With reference now to FIGS. 2A-C, the exemplary DMS 110 separates the ions in a drift gas based on their mobility when subject to an asymmetric electric field generated between the parallel electrodes 112 through which the ions pass, typically, in a continuous manner. In DMS, the electric field waveform typically has a high field duration at one polarity and then a low field duration at an opposite polarity. The duration of the high field and low field portions can be applied such that the net force being applied to ions across the DMS filter electrodes is zero. Such an electric field can be generated, for example, through the application of RF voltages, often referred to as separation voltages (SV), across the drift tube in a direction perpendicular to that of a drift gas flow. Ions of a given species tend to migrate radially away from the axis of the transport chamber by a characteristic amount during each cycle of the RF waveform due to differences in mobility during the high field and low field portions. With specific reference to FIG. 2A, a plot 210 of an exemplary, time-varying, RF, and/or asymmetric high and low voltage waveform 214 that can be applied to generate an asymmetric electric field is depicted. Although the waveform of FIG. 2A is depicted as a square-wave function with a total period of 6 μsec, it will be apparent to those of skill in the relevant arts that other waveform shapes and periods are possible, including waveforms constructed by summation of two sine waves, by way of non-limiting example.

[0038] In DMS, a DC potential is also applied to the electrodes 112a,b, with the difference in DC potential between the electrodes commonly referred to as a compensation voltage (CoV) that generates a counteracting electrostatic force to that of the SV. Typically, as noted above, the CoV can be set to a fixed value to pass only an ion species with a particular differential mobility while the remaining species of ions drift toward the electrodes 112a,b and are neutralized thereat, as shown in FIG. 2B. The plot 220, for example, depicts exemplary DC offset voltages 212a,b that are applied to filter electrodes 112a,b, respectively, such that the CoV is fixed at the CoV-apex (e.g., +5V (i.e., 50V−45V) in this example).

[0039] With reference now to FIG. 2C, the combined effect of the electric fields generated by the waveforms in FIGS. 2A and 2B is shown in schematic representation for two species of ions. For the first species, the ion's mobility in the asymmetric electric field indicates a net movement 103 towards the bottom electrode 112b of the DMS 110 upon injection. (It should be appreciated in view of the depicted motion of the first species that, in a DMS, the ion's mobility is not constant under the influence of the low electric field compared to the high electric field.) However, for the second ion species, the +5V CoV applied to the filter electrodes 112a,b maintains the second ion species along a safe trajectory 104 for the ion through the DMS 110 (i.e., without striking one of the filter electrodes 112a,b) to allow, for example, detection by the detector 150 of FIG. 1. It is noted that trajectory 104 is shown schematically as being averaged over a full cycle of the waveform, and thus does not show the ion oscillations for each high and low portion of the waveform.

[0040] With reference now to FIGS. 3 and 4A-C, exemplary methods of operating the differential mobility spectrometer/mass spectrometer system 100 of FIG. 1. As shown at step 310 of the flow diagram of FIG. 3, the method can comprise determining the CoV-apex to be applied to the DMS 110. As will be appreciated by a person skilled in the art, the CoV-apex (e.g., +5V as shown in FIG. 2B) can be determined based on theoretical or empirical information based on the standard operating conditions of the DMS 110, with reference for example, to the SV to be applied to the DMS 110, the mobility of the ion of interest, the drift gas identity and flow rate, and the modifier. By way of example, the CoV-apex can be input by a user or determined automatically (e.g., using the controller 160). By way of example, after setting up the remainder of the operating parameters (e.g., SV), the controller 160 can access a library (e.g., data storage 162) of CoV-apex under the selected operating parameters. For example, as shown in the trace 401 of FIG. 4A, the library can contain data regarding the relative intensity of the ion of interest at various CoV values such that the CoV 402 corresponding to the maximum intensity of the transmitted ions representing the CoV-apex can be determined. Alternatively, it will be appreciated that the user can select a CoV-apex based on empirical data or experience, for example, of the user's particular system 100. In some aspects, the applied SV (e.g., an optimum SV) can be determined by the user or automatically by the controller 160, for example, by referencing a table or library of standard DMS operating parameters (e.g., based on theoretical or empirical data).

[0041] As shown at step 320, a range of the CoV about the CoV-apex can then be determined, again via user input or the controller 160. By way of example, a voltage difference between the lowest CoV value and the highest CoV value to be applied to the electrodes 112a,b can be specified by the user (e.g., within ±1V of the CoV-apex), by specifying the lower and upper bounds of the range (e.g., the CoV-apex can be one of these bounds), or by specifying a deviation from the CoV-apex (e.g., by % of CoV, by number of standard deviations). Alternatively, in some aspects, the range can be pre-determined by the system 100, for example, such that only the CoV-apex need be determined in step 310, with the range being determined automatically by the controller in step 320. As shown in FIG. 4A, for example, upon determining the CoV-apex 402 in step 310, the range of the CoV can be selected to correspond to the full-width at half-maximum (FWHM) of theoretical or historical data of the relative intensity of the transmitted ion, as shown by the dotted box 403 of FIG. 4A.

[0042] As shown in step 330, it can then be determined the manner in which the CoV applied to the electrodes will be varied across the range determined in step 320. For example, as shown in FIG. 4, the range of CoV determined in step 320 can be applied such that the CoV varies continuously (e.g., continuously increases as shown in FIG. 4B) or varies in a stepwise manner (as shown in FIG. 4C). By way of example, if the electrodes 112a,b are configured to have a stepwise CoV applied thereto, the number of increments in the range, the duration of increments, and/or the size of the increments can be pre-determined or selected (e.g., via user input or by the controller 160).

[0043] Upon setting these parameters and applying an initial SV and CoV to the electrodes 112a,b as determined in step 330, the ion source can be activated such that a stream of ions containing or suspected of containing the ion species of interest (i.e., the ion corresponding to the determined CoV-apex) can be continuously injected into the DMS 110 in step 340. As the ions generated by the ion source traverse the filter electrodes, the SV and CoV in combination will tend to neutralize extraneous ions while transmitting the ion of interest from the downstream end of the DMS 110 to the mass spectrometer 150, for example.

[0044] In step 350, as the sample ions continue to be injected (e.g., for a duration corresponding to a substantially constant ion concentration in an eluent), the CoV can be varied in accordance with the function determined in step 330. By way of example, if the CoV is to be varied continuously, the initial CoV (e.g., +4V) applied to the electrodes 112a,b can be linearly increased to the CoV value at the upper end of the range (e.g., +6V) over the selected duration, as shown in FIG. 4B. Alternatively, as shown in FIG. 4C, if the CoV is to be varied in a stepwise manner, the initial CoV (e.g., +4V) applied to the electrodes 112a,b can be increased incrementally to the CoV value at the upper end of the range (e.g., +6V) over the selected duration. Though four stepwise increments of +0.5V are depicted in FIG. 4C, it will be appreciated in light of the present teachings that any number and size of CoV increments can be applied to the electrodes during the duration to vary the CoV about the CoV-apex. By way of example, the system 100 default to adjust the CoV by 0.1V in each increment.

[0045] In step 360, the ions that pass through the DMS 110 can be transmitted to the detector (e.g., to detect the ion species of interest directly) or can be subject to one or more additional analysis steps. By way of example, the ions transmitted by the DMS 110 can be subject to fragmentation with the m/z transitions between the precursor ions and product ions (i.e., fragment ions) being monitored to confirm the identity of and/or quantify the ion of interest in the sample. In an MRM-triggered EPI mode, the ions transmitted by the DMS 110 can be transmitted to a fragmentor (e.g., Q2), fragmented therein, and if the detected MRM transitions meet threshold criteria, trigger the generation of additional MS/MS steps such that the fragments can be collected in another mass analyzer (e.g., Q3) and analyzed. In each aspect discussed above, the detector 150a can be configured to detect the ions transmitted thereto, such that the controller 160 can calculate the m/z of the ions and/or generate a composite mass spectra of the ion of interest (or a product ion mass spectra) acquired during the duration of the varying CoV in accordance with the present teachings.

[0046] It will be appreciated in light of the present teachings that the CoV-apex can be adapted to obtain maximum selectivity based on the results of the methods described herein. By way of example, in an adaptive CoV technique in accordance with the present teachings, it may be found that the optimum conditions may reside at a CoV that is on the boundary (or outside the boundary) of the experimental range of CoVs applied while collecting data from matrix samples such that the CoV-apex can be adapted (e.g., re-calibrated) following data collection (post-acquisition). By way of example, with reference again to FIG. 4A, based on the data obtained from utilizing the theoretical or nominal CoV-apex 401 and window 403, it may be determined that under experimental conditions, the actual CoV-apex 404, has shifted to a higher value (to the right). Accordingly, in some aspects, the processor 160 can adapt such that under similar experimental condition in a subsequent run, for example, the CoV apex can be revised to increase sensitivity.

[0047] It should be appreciated that numerous changes can be made to the disclosed embodiments without departing from the scope of the present teachings. While the foregoing figures and examples refer to specific elements, this is intended to be by way of example and illustration only and not by way of limitation. It should be appreciated by the person skilled in the art that various changes can be made in form and details to the disclosed embodiments without departing from the scope of the teachings encompassed by the appended claims.