Space Charge Reduction in TOF-MS

Abstract

A mass spectrometer that includes a mass filter and a TOF mass analyzer receives the ion beam from an ion source device that ionizes a compound of a sample. The mass filter selects a precursor ion mass range and the mass analyzer mass analyzes the mass range. A continuous flow of selected precursor ions is maintained between the mass filter and the mass analyzer. A first set of parameters is applied to the mass spectrometer to produce a resolution above a first resolution threshold. A space charge effect is detected by determining if the measured TIC exceeds a TIC threshold or the measured resolution is less than the first resolution threshold. If a space charge effect is detected, at least one precursor ion transmission window with a width smaller than the mass range is applied to the ion beam by the mass filter and mass analyzed to reduce the space charge.

Claims

1. A system for detecting and reducing space charge effects in time-of-flight (TOF) mass spectrometry (MS) analysis, comprising: an ion source device that ionizes a compound of a sample, producing an ion beam; a mass spectrometer that includes a mass filter and a TOF mass analyzer, receives the ion beam from the ion source device, is operated to select a precursor ion mass range of the ion beam using the mass filter and to mass analyze the selected mass range using the mass analyzer, producing a precursor ion mass spectrum for the mass range, is operated to maintain a continuous flow of selected precursor ions between the mass filter and the mass analyzer, and is operated with a first set of parameters to produce precursor ion peaks for the compound in the mass spectrum with a sensitivity above a first sensitivity threshold and a resolution above a first resolution threshold; and a processor that detects a space charge effect by determining if a total ion current (TIC) received from the mass spectrometer is greater than a TIC threshold or if a precursor ion peak of the mass spectrum received from the mass spectrometer has a resolution that is less than the first resolution threshold, and if a space charge effect is detected, reduces the space charge effect by instructing the mass filter to apply to the ion beam at least one precursor ion transmission window that has a width smaller than the mass range and that is positioned to include at least one precursor ion of the compound that is multiply charged and instructing the mass analyzer to mass analyze the precursor ions of the ion beam selected by the at least one precursor ion transmission window, producing a precursor ion mass spectrum for the at least one precursor ion transmission window.

2. The system of claim 1, wherein the least one precursor ion transmission window is positioned to only include the at least one precursor ion of the compound that is multiply charged.

3. The system of claim 1, wherein the least one precursor ion transmission window is positioned to further include one or more additional precursor ions of the sample that are multiply charged.

4. The system of claim 1, wherein if a space charge effect is detected, the processor further instructs the mass filter to apply to the ion beam one or more additional precursor ion transmission windows that along with the at least one precursor ion transmission window are positioned to span the entire mass range, wherein each window of the one or more additional precursor ion transmission windows also has a width smaller than the mass range, wherein the processor further instructs the mass analyzer to mass analyze the precursor ions of the ion beam selected by each window of the one or more additional precursor ion transmission windows, producing a precursor ion mass spectrum for each window, and wherein the at least one precursor ion transmission window and the one or more additional precursor ion transmission windows comprise a plurality of precursor ion transmission windows.

5. The system of claim 4, wherein the processor positions the plurality of precursor ion transmission windows in a stepwise fashion across the mass range.

6. The system of claim 5, wherein at least two windows of the plurality of precursor ion transmission windows overlap.

7. The system of claim 5, wherein at least two windows of the plurality of precursor ion transmission windows have different widths.

8. The system of claim 5, wherein all of the windows of the plurality of precursor ion transmission windows have the same width.

9. The system of claim 4, wherein all of the windows of the plurality of precursor ion transmission windows have the same width and wherein the processor positions the plurality of precursor ion transmission windows in a scanning fashion across the mass range so that each following window is offset from a preceding window across the mass range by the same offset amount, each following window overlaps a preceding window by the same overlap amount, and the offset amount is smaller than the overlap amount.

10. The system of claim 1, wherein if a space charge effect is detected, the processor further, before instructing the mass filter to apply the at least one precursor ion transmission window to the ion beam and instructing the mass analyzer to mass analyze the precursor ions of the ion beam selected by the at least one precursor ion transmission window, instructs the mass spectrometer to use a second set of parameters for a mass spectrometry (MS) survey scan of the mass range that produces precursor ion peaks with a sensitivity above a second sensitivity threshold and a resolution above a second resolution threshold, wherein the second sensitivity threshold is less than the first sensitivity threshold and the second resolution threshold is less than the first resolution threshold, and instructs the mass filter to again select the mass range of the ion beam and the mass analyzer again to mass analyze the selected mass range, producing a precursor ion survey mass spectrum for the mass range.

11. The system of claim 10, wherein the processor instructs the mass spectrometer to use a first set of parameters again after the MS survey scan.

12. The system of claim 10, wherein the processor further selects the at least one precursor ion of the compound that is multiply charged from the precursor ion survey mass spectrum.

13. The system of claim 12, wherein the processor determines that the at least one precursor ion is multiply charged by comparing the mass-to-charge ratio (m/z) value of the at least one precursor ion to the m/z value of one or more other precursor ions of the precursor ion survey mass spectrum and determining that the m/z value of at least one precursor ion of the one or more other precursor and the m/z value of the at least one precursor ion are multiples of the same mass.

14. A method for detecting and reducing space charge effects in time-of-flight (TOF) mass spectrometry (MS) analysis, comprising: ionizing a compound in an ion source producing an ion beam; receiving the ion beam from the ion source in a mass spectrometer that includes a mass filter and a TOF mass analyzer, selecting a precursor ion mass range of the ion beam in the mass filter, mass analyzing the selected mass range in the mass analyzer, producing a precursor ion mass spectrum for the mass range, maintaining a continuous flow of selected precursor ions between the mass filter and the mass analyzer in the mass spectrometer, and applying a first set of parameters to the mass spectrometer to produce precursor ion peaks for the compound in the mass spectrum with a sensitivity above a first sensitivity threshold and a resolution above a first resolution threshold; detecting a space charge effect by determining if a total ion current (TIC) received from the mass spectrometer is greater than a TIC threshold or if a precursor ion peak of the mass spectrum received from the mass spectrometer has a resolution that is less than the first resolution threshold; and if a space charge effect is detected, reducing the space charge effect by having the mass filter apply to the ion beam at least one precursor ion transmission window that has a width smaller than the mass range and that is positioned to include at least one precursor ion of the compound that is multiply charged and mass analyzing in the mass analyzer, the precursor ions of the ion beam selected by the at least one precursor ion transmission window, producing a precursor ion mass spectrum for the at least one precursor ion transmission window.

15. A computer program product, comprising a non-transitory tangible computer-readable storage medium whose contents include a program with instructions that are executable on a processor for detecting and reducing space charge effects in time-of-flight (TOF) mass spectrometry (MS) analysis, wherein the instructions are executable on the processor to: provide a system, wherein the system comprises one or more distinct software modules, and wherein the distinct software modules comprise a control module and an analysis module; instruct an ion source device to ionize a compound of a sample using the control module, producing an ion beam; instruct a mass spectrometer that includes a mass filter and a TOF mass analyzer to receive the ion beam from the ion source device, instruct the mass filter to select a precursor ion mass range of the ion beam, instruct the mass analyzer to mass analyze the selected mass range, produce a precursor ion mass spectrum for the mass range, maintaining a continuous flow of selected precursor ions between the mass filter and the mass analyzer, and apply a first set of parameters to the mass spectrometer to produce precursor ion peaks for the compound in the mass spectrum with a sensitivity above a first sensitivity threshold and a resolution above a first resolution threshold using the control module; detect a space charge effect by determining if a total ion current (TIC) received from the mass spectrometer is greater than a TIC threshold or if a precursor ion peak of the mass spectrum received from the mass spectrometer has a resolution that is less than the first resolution threshold using the analysis module; and if a space charge effect is detected, reduce the space charge effect by instructing the mass filter to apply to the ion beam at least one precursor ion transmission window that has a width smaller than the mass range and that is positioned to include at least one precursor ion of the compound that is multiply charged and instructing the mass analyzer to mass analyze the precursor ions of the ion beam selected by the at least one precursor ion transmission window using the control module, producing a precursor ion mass spectrum for the at least one precursor ion transmission window.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The skilled artisan will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way.

[0049] FIG. 1 is a block diagram that illustrates a computer system, upon which embodiments of the present teachings may be implemented.

[0050] FIG. 2 is an exemplary series of plots showing a precursor ion spectrum that includes large multiply charged ions with a high charge state and a zoomed-in section of the same spectrum showing the isotopes of one particular highly charged ion upon which embodiments of the present invention may be implemented.

[0051] FIG. 3 is an exemplary series of plots showing a precursor ion spectrum and a zoomed-in section of the same spectrum for the same sample protein and using the same mass spectrometer of FIG. 2 but with the ITC parameter of the mass spectrometer reduced to 3% upon which embodiments of the present invention may be implemented.

[0052] FIG. 4 is an exemplary diagram of the full spectrum of FIG. 2 and a precursor ion transmission window showing how the window is used to select a single precursor ion for MS mass analysis, in accordance with various embodiments.

[0053] FIG. 5 is an exemplary series of plots showing a precursor ion spectrum and a zoomed-in section of the same spectrum for the same sample protein and using the same mass spectrometer of FIG. 2 but after mass filtering one precursor ion using the single precursor ion transmission window of FIG. 4, in accordance with various embodiments.

[0054] FIG. 6 is an exemplary diagram of the full spectrum of FIG. 2 and a precursor ion transmission window showing how the window is used to select at least two precursor ions for MS mass analysis, in accordance with various embodiments.

[0055] FIG. 7 is an exemplary diagram of the full spectrum of FIG. 2 and a series of precursor ion transmission windows stepped across the mass range showing how the stepped precursor ion transmission windows are used to select all the precursor ions of the mass range for MS mass analysis, in accordance with various embodiments.

[0056] FIG. 8 is an exemplary diagram of the full spectrum of FIG. 2 and a series of precursor ion transmission windows scanned across the mass range showing how the scanned precursor ion transmission windows are used to select all the precursor ions of the mass range for MS mass analysis, in accordance with various embodiments.

[0057] FIG. 9 is a schematic diagram showing a system for detecting and reducing space charge effects in time-of-flight mass spectrometry (TOF-MS) analysis, in accordance with various embodiments.

[0058] FIG. 10 is a flowchart showing a method for detecting and reducing space charge effects in TOF-MS analysis, in accordance with various embodiments.

[0059] FIG. 11 is a schematic diagram of a system that includes one or more distinct software modules that performs a method for detecting and reducing space charge effects in TOF-MS analysis.

[0060] Before one or more embodiments of the present teachings are described in detail, one skilled in the art will appreciate that the present teachings are not limited in their application to the details of construction, the arrangements of components, and the arrangement of steps set forth in the following detailed description or illustrated in the drawings. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

DESCRIPTION OF VARIOUS EMBODIMENTS

Computer-Implemented System

[0061] FIG. 1 is a block diagram that illustrates a computer system 100, upon which embodiments of the present teachings may be implemented. Computer system 100 includes a bus 102 or other communication mechanism for communicating information, and a processor 104 coupled with bus 102 for processing information. Computer system 100 also includes a memory 106, which can be a random-access memory (RAM) or other dynamic storage device, coupled to bus 102 for storing instructions to be executed by processor 104. Memory 106 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 104. Computer system 100 further includes a read only memory (ROM) 108 or other static storage device coupled to bus 102 for storing static information and instructions for processor 104. A storage device 110, such as a magnetic disk or optical disk, is provided and coupled to bus 102 for storing information and instructions.

[0062] Computer system 100 may be coupled via bus 102 to a display 112, such as a cathode ray tube (CRT) or liquid crystal display (LCD), for displaying information to a computer user. An input device 114, including alphanumeric and other keys, is coupled to bus 102 for communicating information and command selections to processor 104. Another type of user input device is cursor control 116, such as a mouse, a trackball or cursor direction keys for communicating direction information and command selections to processor 104 and for controlling cursor movement on display 112.

[0063] A computer system 100 can perform the present teachings. Consistent with certain implementations of the present teachings, results are provided by computer system 100 in response to processor 104 executing one or more sequences of one or more instructions contained in memory 106. Such instructions may be read into memory 106 from another computer-readable medium, such as storage device 110. Execution of the sequences of instructions contained in memory 106 causes processor 104 to perform the process described herein. Alternatively, hard-wired circuitry may be used in place of or in combination with software instructions to implement the present teachings. Thus, implementations of the present teachings are not limited to any specific combination of hardware circuitry and software.

[0064] The term computer-readable medium or computer program product as used herein refers to any media that participates in providing instructions to processor 104 for execution. The terms computer-readable medium and computer program product are used interchangeably throughout this written description. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and precursor ion mass selection media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device 110. Volatile media includes dynamic memory, such as memory 106.

[0065] Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, digital video disc (DVD), a Blu-ray Disc, any other optical medium, a thumb drive, a memory card, a RAM, PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, or any other tangible medium from which a computer can read.

[0066] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor 104 for execution. For example, the instructions may initially be carried on the magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 100 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector coupled to bus 102 can receive the data carried in the infra-red signal and place the data on bus 102. Bus 102 carries the data to memory 106, from which processor 104 retrieves and executes the instructions. The instructions received by memory 106 may optionally be stored on storage device 110 either before or after execution by processor 104.

[0067] In accordance with various embodiments, instructions configured to be executed by a processor to perform a method are stored on a computer-readable medium. The computer-readable medium can be a device that stores digital information. For example, a computer-readable medium includes a compact disc read-only memory (CD-ROM) as is known in the art for storing software. The computer-readable medium is accessed by a processor suitable for executing instructions configured to be executed.

[0068] The following descriptions of various implementations of the present teachings have been presented for purposes of illustration and description. It is not exhaustive and does not limit the present teachings to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practicing of the present teachings. Additionally, the described implementation includes software but the present teachings may be implemented as a combination of hardware and software or in hardware alone. The present teachings may be implemented with both object-oriented and non-object-oriented programming systems.

Reducing Space Charge Effects Through Mass Filtering

[0069] As described above, recent TOF mass spectrometers can measure ion peaks with greater sensitivity and resolution than previous mass spectrometers. Due to the higher sensitivity and resolution of these new TOF mass spectrometers space charge effects have become more important.

[0070] One proposed solution to the space charge problem is to retune the TOF mass analyzer when a space charge effect is detected. Usually, this is achieved by throttling the total ion current (TIC) either by various beam focusing means or by changing the duty cycle of ion introduction. Unfortunately, both changes to the ion beam and changes to the duty cycle can lead to reduced sensitivity and dynamic range.

[0071] For example, as described above, a comparison of FIGS. 2 and 3 shows that attenuating the ion beam improved the resolution but at the expense of sensitivity. Consequently, there is a need for additional systems and methods for reducing space charge effects.

[0072] In addition, methods for reducing space charge effects are only needed in special instances of particular compounds analyzed by particular instruments. Thus, space charge reduction should be coupled to the detection of space charge effects. As a result, additional systems and methods are needed for detecting and reducing space charge effects without significantly reducing the sensitivity or resolution of a TOF mass spectrometer.

[0073] In various embodiments, a space charge effect is detected when the total ion current (TIC) is greater than a predetermined threshold or when a precursor ion peak of a compound of interest has a resolution that is less than a predetermined threshold. In response to the detected space charge effect, in various embodiments, the space charge effect is reduced using mass filtering. Mass filtering reduces the total number of ions that are mass analyzed thereby reducing the total space charge and increasing the resolution.

[0074] Unfortunately, mass filtering also reduces the mass range and therefore the sensitivity. However, in mass filtering, the sensitivity for the selected peaks is not compromised. This is advantageous in two ways. First, a mixture of two compounds can have a first compound that is very abundant and not of interest and second compound that is present at trace levels and is of interest. In this case, filtering the ions of no interest (the first compound) can yield better sensitivity than trying to meet space charge requirements for all ions.

[0075] Second, not all ion signals from the same compound are of equal value. Specifically, in TOF mass spectrometers lower charge states are generally easier to resolve for large multiply charged compounds. This comes from the fact that the spacing between isotopes in the time domain is larger for lower charge states and onset of peak coalescence happens later. Therefore, it is possible that some of the charge states present in the mass spectrum are of not of good use and still contribute to the space charge problem. Consequently, mass filtering allows the selection of only those charge states that can be well resolved.

[0076] More specifically, in various embodiments, a precursor ion transmission window is used to select just a portion of the precursor ions of a mass range and thereby reduce the overall space charge. One precursor ion transmission window can be used, producing one precursor ion mass spectrum, or two or more precursor ion transmission windows can be used over the entire mass range, producing two or more precursor ion mass spectra. Further, one or more precursor ion transmission windows can be used in a targeted manner or a non-targeted manner.

Targeted Approach

[0077] For example, in one targeted approach, a lower sensitivity and lower resolution MS survey scan can be used to locate precursor ions across a mass range. This is similar to the MS survey scan for an IDA method, as described above. Unlike an IDA method, however, a single precursor ion transmission window is used to select or filter one or more precursor ions for a high sensitivity and high-resolution MS scan rather than an MS/MS scan.

[0078] Returning to FIG. 3, spectrum 310 depicts a lower sensitivity MS scan like an MS survey scan. In the targeted approach, a precursor ion peak, which has an intensity above a predetermined threshold, such as peak 311 can be selected for mass filtering. Just peak 311 can be selected by a single precursor ion transmission window or two or more peaks such as peaks 311, 312, and 313, can be selected by a single precursor ion transmission window.

[0079] FIG. 4 is an exemplary diagram 400 of the full spectrum of FIG. 2 and a precursor ion transmission window showing how the window is used to select a single precursor ion for MS mass analysis, in accordance with various embodiments. Spectrum 210 of FIG. 4 shows the precursor ions of carbonic anhydrase II that would be measured with a high sensitivity across the entire mass range. Mass range 410 shows a single precursor ion transmission window 411 that can be applied to select the ion of carbonic anhydrase II represented by peak 211 of spectrum 210.

[0080] FIG. 5 is an exemplary series 500 of plots showing a precursor ion spectrum and a zoomed-in section of the same spectrum for the same sample protein and using the same mass spectrometer of FIG. 2 but after mass filtering one precursor ion using the single precursor ion transmission window of FIG. 4, in accordance with various embodiments. Precursor ion spectrum 510 now includes just one ion of carbonic anhydrase II. Peak 211 has an m/z value of 968.54. Thus, peak 211 represents a precursor ion of carbonic anhydrase II with a charge state of +30. All other precursor ion peaks have been filtered out by the single precursor ion transmission window of FIG. 4.

[0081] Zoomed spectrum 520 depicts the zoomed-in region around peak 211 between 968.2 and 968.8 m/z. In this region, the isotopes of the precursor at m/z value of 968.54 can be seen. For example, peak 521 represents an isotope of precursor ion of peak 211.

[0082] Due to the mass filtering using the single precursor ion transmission window of FIG. 4, the peaks in zoomed spectrum 520 are improved even over the peaks of zoomed spectrum 320 of FIG. 3. In fact, the resolution of the peaks in spectrum 520 has now improved to 100,000 (FWHM). In other words, using mass filtering produced a reduction of the space charge so that a large multiply charged ion that is highly charged was mass-analyzed with a high resolution.

[0083] High sensitivity for the entire mass range, of course, was lost by using such a small precursor ion transmission window or bandpass filter. However, as described above, this sensitivity can be regained by reconstructing other multiply charged ions of the compound of interest from the single ion measured.

[0084] Reconstruction, however, is based on knowing that a particular precursor ion is a multiply charged ion. As a result, some type of a priori knowledge or some type of analysis is needed to determine that the precursor ion is multiply charged. A priori knowledge can include input from a user that provides the mass and potential charge state of compounds of interest.

[0085] Alternatively, measurements of at least two separate ions can be used to determine if the ions are the same compound with multiple charges. As a result, instead of mass filtering using a precursor ion transmission window that selects only a single ion, a precursor ion transmission window can be used that selects at least two separate ions. Selecting just a few precursor ions of the mass range is unlikely to increase the space charge large enough to again reduce resolution.

[0086] FIG. 6 is an exemplary diagram 600 of the full spectrum of FIG. 2 and a precursor ion transmission window showing how the window is used to select at least two precursor ions for MS mass analysis, in accordance with various embodiments. Spectrum 210 of FIG. 6 shows the precursor ions of carbonic anhydrase II that would be measured with a high sensitivity across the entire mass range. Mass range 610 shows a single precursor ion transmission window 611 that can be applied to select two ions of carbonic anhydrase II represented by peak 211 and peak 212 of spectrum 210.

[0087] The m/z values of peak 211 and peak 212 are then used, in various embodiments, to verify that that the precursor ions represented by these peaks are multiply charged ions. For example, m/z values of peak 211 and peak 212 are multiplied by various charge values. In this case, multiplying the m/z value of peak 211 by +30 and multiplying the m/z value of peak 212 by +31 would show that both ions have the same mass of carbonic anhydrase II, which is on the order of 29 kDa. Thus, these peaks are verified as peaks of a multiply charged ion.

Non-Targeted Approach

[0088] In a non-targeted approach, in various embodiments, multiple precursor ion transmission windows that are each smaller than the mass range are stepped or scanned across the mass range. Using precursor ion transmission windows that are smaller than the mass range reduces space charge and therefore allows high resolution to be maintained. Using multiple precursor ion transmission windows across the mass range allows multiply charged ions to be determined from at least two ion measurements. As a result, sensitivity can be regained by reconstructing all of the multiply charged ions from the measured ions.

[0089] FIG. 7 is an exemplary diagram 700 of the full spectrum of FIG. 2 and a series of precursor ion transmission windows stepped across the mass range showing how the stepped precursor ion transmission windows are used to select all the precursor ions of the mass range for MS mass analysis, in accordance with various embodiments. Spectrum 210 of FIG. 7 shows the precursor ions of carbonic anhydrase II that can be measured with a high sensitivity across the entire mass range. Mass range 710 shows how four precursor ion transmission windows 711, 712, 713, and 714 are position across the mass range of spectrum 210 to select in a stepwise fashion all of the ions of carbonic anhydrase II of spectrum 210.

[0090] The precursor ion of peak 211, for example, is selected and mass analyzed using precursor ion transmission window 712, and precursor ion of peak 212 is selected and mass analyzed using precursor ion transmission window 713. Limiting the number of ions in each precursor ion transmission window reduces the space charge and allows the resolution to be maintained.

[0091] The precursor ion transmission windows, as shown in FIG. 7, can have different or variable widths. In various alternative embodiments, each of the precursor ion transmission windows can have the same width.

[0092] Similarly, the precursor ion transmission windows shown in FIG. 7 do not overlap. In various alternative embodiments, adjacent precursor ion transmission windows can be overlapped.

[0093] Although using multiple precursor ion transmission windows is described as a non-targeted approach, targeted or predetermined information can be used to determine the position of the precursor ion transmission windows. For example, each of the precursor ion transmission windows shown in FIG. 4 is positioned to include at least four multiply charged ions of the compound of interest. As in the targeted approach and as described above, information about a compound of interest can be obtained from a priori knowledge about the compound or additional and preceding measurements such as an MS survey scan.

[0094] FIG. 8 is an exemplary diagram 800 of the full spectrum of FIG. 2 and a series of precursor ion transmission windows scanned across the mass range showing how the scanned precursor ion transmission windows are used to select all the precursor ions of the mass range for MS mass analysis, in accordance with various embodiments. Spectrum 210 of FIG. 8 shows the precursor ions of carbonic anhydrase II that can be measured with a high sensitivity across the entire mass range. Mass range 810 shows how a precursor ion transmission window is scanned across the mass range of spectrum 210 24 times to select all of the ions of carbonic anhydrase II of spectrum 210.

[0095] This scanning of precursor ion transmission windows can be thought of as scanning a single precursor ion transmission window 24 times or positioning 24 different precursor ion transmission windows across the mass range. Each precursor ion transmission window has the same width. Each precursor ion transmission window selects the precursor ions within the window and the TOF mass analyzer mass analyzes the precursor ions of each window, producing a precursor ion mass spectrum for each precursor ion transmission window.

[0096] As described above, in scanning SWATH, the precursor ions of each precursor ion transmission window are fragmented. In various embodiments, however, the precursor ions of the scanned precursor ion transmission windows described herein are not fragmented. Instead, these precursor ions are simply mass analyzed using the TOF mass analyzer.

[0097] Precursor ion transmission windows are scanned so that each following window is offset from a preceding window across the mass range by the same offset amount 810. Each following window overlaps a preceding window by the same overlap amount 820. The offset amount 810 is smaller than the overlap amount 820.

[0098] The non-targeted approach provides at least two additional benefits. First, it allows the reconstruction of multiply charged ions that may be found in low abundance. In other words, the survey scan of the targeted approach may not detect ions that have a lower abundance compared to other ions in the spectrum. Secondly, the nontargeted approach allows the sample data to be interrogated after acquisition for additional compounds of interest without having to reanalyze the sample.

[0099] The non-targeted approach uses multiple precursor ion selection windows like the DIA methods described above. However, the nontargeted approach described herein differs significantly from previous DIA methods in that an MS scan is performed for each window rather than an MS/MS scan. U.S. Pat. No. 11,069,517 (hereinafter the '517 Patent) describes using scanning precursor ion selection windows in an MS scan of a targeted IDA method. However, the '517 Patent is directed to filtering out contaminants, such as adducts or product ions of precursor ions that are produced by some form of unintentional spontaneous fragmentation within the mass spectrometer, before fragmentation. The '517 Patent is not directed to the space charge problem in MS analysis. In addition, no charge space problem was previously encountered with TOF mass spectrometers that provided lower sensitivity and resolution. In other words, both the targeted and nontargeted approaches described herein were not thought to be necessary or useful until the space charge problem was encountered with recent high sensitivity and high-resolution TOF mass spectrometers.

[0100] Previously, mass filtering has been used in trapping mass spectrometers to reduce space charge during the MS/MS analysis of fragment or product ions. Excess space charge is a common problem in ion traps. For example, enhanced resolution (ER) MS/MS scans have been available for mass spectrometers that include ion traps for some time. In addition, U.S. Pat. No. 9,318,310 describes a method for operating a mass spectrometer to mass filter fragment or product ions in an ion trap to reduce space charge in the trap.

[0101] As described above, however, space charge is also becoming a problem involving recent TOF mass spectrometer systems without a pre-trap. These non-trapping TOF mass spectrometer systems or TOF systems that are not multi-reflecting are typically continuous ion flow systems where there is a continuous flow of ions between a mass filter and the TOF mass analyzer. Using mass filtering to reduce space charge in a continuous flow system where there is a continuous flow of ions between a mass filter and the TOF mass analyzer has not previously been contemplated and would not have been for at least the reasons described above.

System for Detecting and Reducing Space Charge Effects

[0102] FIG. 9 is a schematic diagram 900 showing a system for detecting and reducing space charge effects in time-of-flight mass spectrometry (TOF-MS) analysis, in accordance with various embodiments. The system of FIG. 9 includes ion source device 910, time-of-flight (TOF) mass spectrometer 920, and processor 930.

[0103] Ion source device 910 continuously receives and ionizes a compound 901 of a sample, producing an ion beam. Ion source device 910 can include any type of source compatible with the purposes described herein, including for example sources that provide ions through electrospray ionization (ESI), matrix-assisted laser desorption ionization (MALDI), ion bombardment, application of electrostatic fields (e.g., field ionization and field desorption), chemical ionization, etc. Ion source device 910 is shown as being a device separate from mass spectrometer 920. However, in various embodiments, ion source device 910 can be part of mass spectrometer 920.

[0104] Mass spectrometer 920 includes at least mass filter 921 and TOF mass analyzer 923. Mass filter 921 is shown as a quadruple ion guide in FIG. 9. Mass filter 921, however, can be any type of device used to filter or select precursor ions in a mass spectrometer. Mass spectrometer 920 can also include other devices between mass filter 921 and TOF mass analyzer 923, such as collision cell 922. However, as described herein any devices between mass filter 921 and TOF mass analyzer 923 are used to maintain a continuous flow of precursor ions between mass filter 921 and TOF mass analyzer and are not used to fragment precursor ions.

[0105] Mass spectrometer 920 receives the ion beam from ion source device 910. Mass spectrometer 920 is operated to select a precursor ion mass range of the ion beam using mass filter 921 and to mass analyze the selected mass range using mass analyzer 923, producing a precursor ion mass spectrum for the mass range. Mass spectrometer 920 is operated to maintain a continuous flow of selected precursor ions between mass filter 921 and mass analyzer 923. Mass spectrometer 920 is operated with a first set of parameters to produce precursor ion peaks for the compound with a sensitivity above a first sensitivity threshold and a resolution above a first resolution threshold. In other words, mass spectrometer 920 is operated with a first set of parameters in a first mode to analyze the compound with a high sensitivity and high resolution.

[0106] Processor 930 can be, but is not limited to, a computer, a microprocessor, the computer system of FIG. 1, or any device capable of sending and receiving control signals and data from mass spectrometer 920 and processing data. Processor 930 is in communication with ion source device 910 and at least mass filter 921 and TOF mass analyzer 923 of mass spectrometer 920.

[0107] Processor 930 detects a space charge effect by determining if a TIC received from mass spectrometer 920 is greater than a TIC threshold or if a precursor ion peak of the mass spectrum received from mass spectrometer 920 has a resolution that is less than the first resolution threshold. If a space charge effect is detected, processor 930 reduces the space charge effect by instructing mass filter 921 to apply to the ion beam at least one precursor ion transmission window. The at least one precursor ion transmission window has a width smaller than the mass range. The at least one precursor ion transmission window is positioned to include at least one precursor ion of the compound that is multiply charged. Processor 930 further instructs TOF mass analyzer 923 to mass analyze the precursor ions of the ion beam selected by the at least one precursor ion transmission window, producing a precursor ion mass spectrum for the at least one precursor ion transmission window.

[0108] In various embodiments, the least one precursor ion transmission window is positioned to only include the at least one precursor ion of the compound that is multiply charged as shown in FIG. 4. In various alternative embodiments, the least one precursor ion transmission window is positioned to further include one or more additional precursor ions of the sample that are multiply charged as shown in FIG. 6.

[0109] In various embodiments, if a space charge effect is detected, processor 930 further instructs mass filter 921 to apply to the ion beam one or more additional precursor ion transmission windows that along with the at least one precursor ion transmission window are positioned to span the entire mass range. Each window of the one or more additional precursor ion transmission windows also has a width smaller than the mass range. Processor 930 further instructs TOF mass analyzer 923 to mass analyze the precursor ions of the ion beam selected by each window of the one or more additional precursor ion transmission windows, producing a precursor ion mass spectrum for each window. The at least one precursor ion transmission window and the one or more additional precursor ion transmission windows make up a plurality of precursor ion transmission windows.

[0110] In various embodiments, processor 930 positions the plurality of precursor ion transmission windows in a stepwise fashion across the mass range as shown in FIG. 7. In various embodiments, at least two windows of the plurality of precursor ion transmission windows overlap.

[0111] In various embodiments, at least two windows of the plurality of precursor ion transmission windows have different widths. In various alternative embodiments, all of the windows of the plurality of precursor ion transmission windows have the same width.

[0112] In various embodiments, all of the windows of the plurality of precursor ion transmission windows have the same width. Processor 930 positions the plurality of precursor ion transmission windows in a scanning fashion across the mass range so that each following window is offset from a preceding window across the mass range by the same offset amount. Each following window overlaps a preceding window by the same overlap amount. The offset amount is smaller than the overlap amount.

[0113] In various embodiments, if a space charge effect is detected, processor 930 further, before instructing mass filter 921 to apply the at least one precursor ion transmission window to the ion beam and instructing TOF mass analyzer 923 to mass analyze the precursor ions of the ion beam selected by the at least one precursor ion transmission window, instructs mass spectrometer 920 to use a second set of parameters for an MS survey scan of the mass range.

[0114] The second set of parameters produces precursor ion peaks with a sensitivity above a second sensitivity threshold and a resolution above a second resolution threshold. The second sensitivity threshold is less than the first sensitivity threshold and the second resolution threshold is less than the first resolution threshold. After the MS survey scan, processor 930 instructs mass filter 921 to again select the mass range of the ion beam and TOF mass analyzer 923 again to mass analyze the selected mass range, producing a precursor ion survey mass spectrum for the mass range.

[0115] In various embodiments, processor 930 instructs mass spectrometer 920 to use a first set of parameters again after the MS survey scan.

[0116] In various embodiments, processor 930 further selects the at least one precursor ion of the compound that is multiply charged from the precursor ion survey mass spectrum.

[0117] In various embodiments, processor 930 determines that the at least one precursor ion is multiply charged by comparing the m/z value of the at least one precursor ion to the m/z value of one or more other precursor ions of the precursor ion survey mass spectrum and determining that the m/z value of at least one precursor ion of the one or more other precursor and the m/z value of the at least one precursor ion are multiples of the same mass.

Method for Detecting and Reducing Space Charge Effects

[0118] FIG. 10 is a flowchart showing a method 1000 for detecting and reducing space charge effects in TOF-MS analysis, in accordance with various embodiments.

[0119] In step 1010 of method 1000, an ion source device is instructed to ionize a compound of a sample using a processor, producing an ion beam.

[0120] In step 1020, a mass spectrometer that includes a mass filter and a TOF mass analyzer is instructed to receive the ion beam from the ion source device using the processor. The mass filter is instructed to select precursor ion mass range of the ion beam using the processor. The mass analyzer is instructed to mass analyze the selected mass range using the processor, producing a precursor ion mass spectrum for the mass range. The mass spectrometer is instructed to maintain a continuous flow of selected precursor ions between the mass filter and the mass analyzer using the processor. Finally, a first set of parameters is applied to the mass spectrometer to produce precursor ion peaks for the compound in the mass spectrum with a sensitivity above a first sensitivity threshold and a resolution above a first resolution threshold using the processor.

[0121] In step 1030, a space charge effect is detected by determining if a TIC received from the mass spectrometer is greater than a TIC threshold or if a precursor ion peak of the mass spectrum received from the mass spectrometer has a resolution that is less than the first resolution threshold using the processor.

[0122] In step 1040, if a space charge effect is detected, the space charge effect is reduced by instructing the mass filter to apply to the ion beam at least one precursor ion transmission window that has a width smaller than the mass range and that is positioned to include at least one precursor ion of the compound that is multiply charged using the processor. In addition, the mass analyzer is instructed to mass analyze the precursor ions of the ion beam selected by the at least one precursor ion transmission window using the processor, producing a precursor ion mass spectrum for the at least one precursor ion transmission window.

Computer Program Product for Detecting and Reducing Space Charge Effects

[0123] In various embodiments, a computer program product includes a non-transitory tangible computer-readable storage medium whose contents include a program with instructions being executed on a processor so as to perform a method for detecting and reducing space charge effects in TOF-MS analysis. This method is performed by a system that includes one or more distinct software modules.

[0124] FIG. 11 is a schematic diagram of a system 1100 that includes one or more distinct software modules that performs a method for detecting and reducing space charge effects in TOF-MS analysis. System 1100 includes control module 1110 and analysis module 1120.

[0125] Control module 1110 instructs an ion source device to ionize a compound of a sample, producing an ion beam.

[0126] Control module 1110 instructs a mass spectrometer that includes a mass filter and a TOF mass analyzer to receive the ion beam from the ion source device. Control module 1110 instructs the mass filter to select precursor ion mass range of the ion beam. Control module 1110 instructs the mass analyzer to mass analyze the selected mass range, producing a precursor ion mass spectrum for the mass range. Control module 1110 instructs the mass spectrometer to maintain a continuous flow of selected precursor ions between the mass filter and the mass analyzer. Finally, control module 1110 applies a first set of parameters to the mass spectrometer to produce precursor ion peaks for the compound in the mass spectrum with a sensitivity above a first sensitivity threshold and a resolution above a first resolution threshold.

[0127] Analysis module 1120 detects a space charge effect by determining if a TIC received from the mass spectrometer is greater than a TIC threshold or if a precursor ion peak of the mass spectrum received from the mass spectrometer has a resolution that is less than the first resolution threshold.

[0128] If a space charge effect is detected, control module 1110 reduces the space charge effect by instructing the mass filter to apply to the ion beam at least one precursor ion transmission window that has a width smaller than the mass range and that is positioned to include at least one precursor ion of the compound that is multiply charged. In addition, control module 1110 instructs the mass analyzer to mass analyze the precursor ions of the ion beam selected by the at least one precursor ion transmission window, producing a precursor ion mass spectrum for the at least one precursor ion transmission window.

[0129] While the present teachings are described in conjunction with various embodiments, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art.

[0130] Further, in describing various embodiments, the specification may have presented a method and/or process as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the various embodiments.