Patent Description:
An ELIT-MS is a type of mass spectrometer that achieves a high mass resolution. An ELIT-MS includes an ELIT for performing mass analysis of ions. In an ELIT, electric current induced by oscillating ions in the trap is detected. The measured frequency of oscillation of the ions is used to calculate the mass-to-charge ratio (m/z) of the ions. For example, a Fourier transform is applied to the measured induced current.

<NPL>, (the "Dziekonski Paper") describes an exemplary ELIT.

<FIG> is a three-dimensional cutaway side view of an exemplary conventional ELIT <NUM>. ELIT <NUM> is similar to the ELIT of the Dziekonski Paper. ELIT <NUM> includes first set of reflectron plates <NUM>, pickup electrode <NUM>, and second set of reflectron plates <NUM>. First set of reflectron plates <NUM> and second set of reflectron plates <NUM> include plate electrodes with holes in the center. Note that the end electrodes of first set of reflectron plates <NUM> and second set of reflectron plates <NUM> do not include holes in the center. However, this is only for simulation purposes. In an actual device, these end electrodes can include holes in the center for the introduction and/or removal of ions from ELIT <NUM>.

In ELIT <NUM>, ions are introduced axially and oscillate axially between first set of reflectron plates <NUM> and second set of reflectron plates <NUM>. Pickup electrode <NUM> is used to measure the induced current produced by the oscillating ions. A Fourier transform is applied to the induced current signal measured from pickup electrode <NUM> to obtain the oscillation frequency. From the oscillation frequency or frequencies, the m/z of one or more ions can be calculated.

<FIG> is an exemplary plot <NUM> showing how ion energy and oscillation frequency are related in an ELIT. An ion is trapped in an ELIT by the voltages applied to the reflectron plates and the electric field they produce. The relative trapped kinetic energy of the ion is set by the voltage difference between the injection device and the field free region of the ELIT.

<FIG> is an exemplary plot <NUM> of the electric field produced in a conventional ELIT by the voltages applied to the reflectron plates. Reflectron plates <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are biased with voltages of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> V, respectively. Similarly, reflectron plates <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are biased with voltages of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> V, respectively. Note that depending on the charge of the ions the reflectron plates can be biased negatively or positively.

The voltages applied to the reflectron plates at either end of the ELIT produce an electric field <NUM>. Electric field <NUM> can be expressed as E = <NUM>/eL, where K is the kinetic energy of ions (ZeV), L is length <NUM>, and e is a single proton charge of <NUM> ×<NUM>-<NUM>. It should be specified that this assumes a perfectly linear electric field in the reflectrons as is shown in <FIG>. Electric field <NUM> is measured in V/m. Length <NUM> is typically on the order of <NUM>, for example. Electric field <NUM> causes ions to oscillate axially along path <NUM> between the reflectron plates at either end of the ELIT. Essentially, the voltages applied to the reflectron plates at either end of the ELIT produce a potential well for ions.

<FIG> is an exemplary diagram <NUM> of the potential well produced by voltages applied to the reflectron plates at either end of an ELIT. Path <NUM> depicts the voltages experienced by ions in potential well <NUM>.

Ideally, the trajectory of ions in an ELIT can be expressed as a semi-sinusoidal waveform where the frequency, f, is equal to <MAT>, when the electric field in the reflectrons is linear and follows E = 4KleL.

<FIG> is an exemplary annotated plot of the semi-sinusoidal trajectory of an ion in an ELIT, in accordance with various embodiments. Semi-sinusoidal trajectory <NUM> shows the position of an ion with respect to time.

The sinusoidal trajectory of an ion in an ELIT is detected by measuring the induced current on a pickup electrode, such as pickup electrode <NUM> of <FIG>. Unfortunately, however, the induced current in a conventional ELIT is not a sinusoid. The frequency, f, of the induced current in a conventional ELIT is, for example, <MAT>, when using a single detector, positioned at the center and when the electric field in the reflectrons is linear and follows E = 4KleL.

<FIG> is an exemplary plot <NUM> showing the induced current for an ion in a conventional ELIT. Plot <NUM> shows that induced current <NUM> for an ion is not a perfect sinusoid. Because induced current <NUM> is not a perfect sinusoid, when a Fourier transform is applied to induced current <NUM>, not just one frequency is obtained. In other words, the Fourier transform of induced current <NUM> produces a fundamental frequency and higher order harmonics.

<FIG> is an exemplary plot <NUM> showing the fundamental frequency and higher order harmonics obtained by applying a Fourier transform to the induced current for an ion in a conventional ELIT. In plot <NUM>, fundamental frequency <NUM> is calculated for the ion of <FIG>. However, higher order frequencies or harmonics <NUM> are also found. In addition, some of the higher order frequencies are found with higher amplitudes than fundamental frequency <NUM>.

As described above, the frequencies calculated from the induced current in an ELIT are used to determine the m/z values of ions. For example, the m/z value of an ion is calculated from the oscillation frequency, f, of an ion in an ELIT according to m/z = eV/<NUM>f<NUM>L<NUM>, under the assumptions of the previous equations. As a result, higher order frequencies can be misidentified as fundamental frequencies and, in turn, incorrect m/z values. Also, higher order frequencies of one ion can interfere with fundamental frequencies of other ions confounding the identification of the correct m/z values of those ions.

Consequently, there is a need for improved ELIT systems and methods that can reduce the higher order harmonics obtained from an ELIT.

An electrostatic linear ion trap (ELIT) for measuring induced current of one or more ions and reducing higher order frequency harmonics of the induced current by combining the induced current with measurements from reflecting reflectron plates is disclosed. A method for measuring induced current of one or more ions and reducing higher order frequency harmonics of the induced current by combining the induced current with measurements from reflecting reflectron plates in an ELIT is also disclosed.

The ELIT includes a first set of reflectron plates, a cylindrical pickup electrode, a second set of reflectron plates, a voltage power supply, and measurement circuitry. The plates of the first set of reflectron plates each includes holes in the center and are coaxially aligned along a central axis. The first set of plates includes a first inlet plate followed by a first plurality of reflection plates followed by a first plurality of trapping plates.

The cylindrical pickup electrode is positioned so that a first end of the pickup electrode is adjacent to the first inlet plate of the first set of plates. The pickup electrode is coaxially aligned with the first set of plates along the central axis.

The plates of the second set of reflectron plates also each includes holes in the center and are coaxially aligned along the central axis. The second set of plates includes a second inlet plate followed by a second plurality of reflection plates followed by a second plurality of trapping plates. The second set of plates is positioned so that the second inlet plate is adjacent to a second end of the cylindrical pickup electrode.

The voltage power supply applies separate voltages to one or more plates of the first set of plates and to one or more plates of the second set of plates. These voltages are applied in order to trap and then oscillate one or more ions between the first set of plates and the second set of plates. The one or more ions have been received along the central axis through the holes of the first set of plates, for example.

The measurement circuitry is used to measure a first induced current from the cylindrical pickup electrode, a second induced current from one or more plates of the first set of reflectron plates, and a third induced current from one or more plates of the second set of reflectron plates. The measurement circuitry combines the first measured induced current with the second measured induced current and the third measured induced current to determine an induced current of the one or more ions. The use of the second measured induced current and the third measured induced current in addition to the first measured induced current reduces higher order frequency harmonics of the induced current.

In various embodiments, the one or more plates of first set of reflectron plates include the first inlet plate and one or more plates of the first plurality of reflection plates, and the one or more plates of second set of reflectron plates include the second inlet plate and one or more plates of the second plurality of reflection plates. The first measured induced current is combined with the second measured induced current and the third measured induced current by summing the second measured induced current, the third measured induced current, and twice the first measured induced current.

In various embodiments, the one or more plates of first set of reflectron plates include one or more plates of the first plurality of trapping plates and the one or more plates of second set of reflectron plates include one or more plates of the second plurality of trapping plates. The first measured induced current is combined with the second measured induced current and the third measured induced current by subtracting the second measured induced current and the third measured induced current from the first measured induced current.

These and other features of the applicant's teachings are set forth herein.

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.

As described above, in an ELIT, ions are introduced axially and oscillate axially between a first set of reflectron plates and a second set of reflectron plates. A pickup electrode is used to measure the induced current produced by the oscillating ions. A Fourier transform is then applied to the induced current signal measured from the pickup electrode to obtain the oscillation frequency. From the oscillation frequency or frequencies, the mass-to-charge ratio (m/z) of one or more ions can be calculated.

Unfortunately, however, the induced current measured for each ion is typically not a perfect sinusoid. As a result, higher order harmonics or frequencies are found for each ion. These higher-order harmonics can result in the misidentification of the m/z value for an ion. Also, higher order harmonics or frequencies of one ion can interfere with fundamental frequencies of other ions confounding the identification of the correct m/z values of those ions.

In various embodiments, higher order harmonics are reduced by measuring the induced current on the reflectron plates as well as on the pickup electrode and summing these induced currents. It is theorized that the short pickup electrode at the center of a conventional ELIT, such as the one shown in <FIG>, does not adequately measure the induced current for the entire trajectory of an ion resulting in a non-sinusoidal measured induced current. More specifically, the short pickup electrode at the center of the ELIT does not adequately measure induced current when an ion is close to or inside the reflectron plates.

<FIG> is an exemplary annotated plot <NUM> of the semi-sinusoidal trajectory of an ion in an ELIT, in accordance with various embodiments. Semi-sinusoidal trajectory <NUM> shows the position of an ion with respect to time. Straight lines <NUM> and <NUM> delimit portions of semi-sinusoidal trajectory <NUM> where the ion is between the reflectron plates. The location between the reflectron plates in an ELIT can also be referred to as the field free region. So, straight lines <NUM> and <NUM> also delimit regions of semi-sinusoidal trajectory <NUM> where the ion is in the field free region.

Arrow <NUM> points to a parabola of semi-sinusoidal trajectory <NUM>. The parabolas of semi-sinusoidal trajectory <NUM> represent the trajectory of the ion when the ion is within the reflectron plates of the ELIT.

<FIG> is an exemplary plot <NUM> of the amplitude of the induced charge versus position measured at an ideal pickup electrode of a theoretical ELIT that provides the semi-sinusoidal ion trajectory, in accordance with various embodiments. Plot <NUM> shows, for an ideal pickup electrode, intensity of induced charge <NUM> to obtain perfect sinusoidal induced current. In the field free region, the intensity of induced charge <NUM> has the form, <MAT> where x is the position (parameter) from the center of the field free region, and X<NUM>: position of the inlet plates of the reflectors (<NUM> and <NUM>) from the field free region. In the reflectors, the intensity of induced charge <NUM> has the form, <MAT> here Xmax is the position that the ions can be reached (or maximum distance) from the center of the field free region. In the case of an example, X<NUM> is <NUM>, a half of the length of the field free region, L = <NUM>. Note that when the ion is between -<NUM> and +<NUM>, the amplitude is positive. This is when the ion is between the reflectron plates or in the field free region. When the position of the ion is less than -<NUM> or greater than +<NUM>, the ion is within one of the two sets of reflectron plates and the amplitude is negative. The ideal pick up profile <NUM> gives perfect sinusoidal induced charge when an ion is traveling the ideal ELIT electrode that produced semi-sinusoidal trajectory in <FIG>. The induced current is also perfect sinusoidal because the induced current is equivalent to the differentiated induced charge by time.

<FIG> is an exemplary plot <NUM> of the amplitude of the induced charge versus time measured at an ideal pickup electrode of a theoretical ELIT, in accordance with various embodiments. Plot <NUM> shows that an ideal pickup electrode can produce a measured induced current <NUM> that is almost a perfect or ideal sinusoid. Plot <NUM> can be compared to plot <NUM> of <FIG>, which shows a non-ideal sinusoid produced by a conventional pickup electrode.

Performing a Fourier transform on the almost perfect or ideal sinusoidal, such as measured induced current <NUM>, greatly reduces higher order harmonics. <FIG> is an exemplary plot <NUM> showing the fundamental frequency and higher order harmonics obtained by applying a Fourier transform to the measured induced current for an ion in a theoretical ELIT that includes an ideal pickup electrode, in accordance with various embodiments. In plot <NUM>, fundamental frequency <NUM> is calculated for the ion of <FIG>. Higher order frequencies or harmonics <NUM> are also found. However, higher order harmonics <NUM> have a much smaller amplitude than fundamental frequency <NUM>. In other words, by using an ideal pickup electrode, the higher order harmonics can be significantly reduced. Plot <NUM> can be compared to plot <NUM> of <FIG> to see how an ideal pickup electrode can reduce higher order harmonics.

<FIG> is an exemplary plot <NUM> of the amplitude of the induced charge versus position measured at the short pickup electrode of the conventional ELIT of <FIG> superimposed on the plot of the amplitude of the induced charge versus position of <FIG>, which is for a theoretical ELIT with an ideal pickup electrode, in accordance with various embodiments. Induced charge <NUM> is measured at the short pickup electrode of the conventional ELIT of <FIG>. Induced charge <NUM> is for a theoretical ELIT with an ideal pickup electrode.

Comparing induced charge <NUM> and induced charge <NUM> shows how the conventional ELIT of <FIG> might be improved to reduce higher order harmonics. In particular, the amplitude of induced charge <NUM> is <NUM> when the position of the ion is less than -<NUM> or greater than +<NUM>. This is when the ion is within one of the sets of reflectron plates. As a result, no or very little induced charge is being measured in the conventional ELIT of <FIG> when an ion is within one of the sets of reflectron plates. However, as induced charge <NUM> shows, an ELIT with an ideal pickup electrode would measure induced charge in this region. Consequently, the conventional ELIT of <FIG> can be improved by measuring the induced charge within the sets of reflectron plates.

Further, the comparison of induced charge <NUM> and induced charge <NUM> shows that, when an ion is between - <NUM> and +<NUM> or in the field free region of the conventional ELIT of <FIG>, induced charge <NUM> is still less than ideal induced charge <NUM>. Consequently, the conventional ELIT of <FIG> can also be improved by optimizing induced charge measurement in the field free region.

<FIG> is a three-dimensional cutaway side view <NUM> of an ELIT for measuring induced current of one or more ions and reducing higher order frequency harmonics of the induced current by combining the induced current with measurements from reflecting reflectron plates, in accordance with various embodiments. The ELIT of <FIG> includes first set of reflectron plates <NUM>, cylindrical pickup electrode <NUM>, second set of reflectron plates <NUM>, voltage power supply <NUM>, and measurement circuitry <NUM>.

The plates of first set of reflectron plates <NUM> each includes holes in the center and are coaxially aligned along central axis <NUM>. First set of plates <NUM> includes first inlet plate <NUM> followed by a first plurality of reflection plates and, in turn, followed by a first plurality of trapping plates. The first plurality of reflection plates include plates <NUM>, <NUM>, <NUM>, and <NUM>. The first plurality of trapping plates include plates <NUM>, <NUM>, <NUM>, and <NUM>. Plate <NUM> is not part of the ELIT and is only used for simulation purposes.

Cylindrical pickup electrode <NUM> is positioned so that a first end of pickup electrode <NUM> is adjacent to first inlet plate <NUM> of first set of plates <NUM> and pickup electrode <NUM> is coaxially aligned with first set of plates <NUM> along central axis <NUM>.

The plates of second set of reflectron plates <NUM> also each includes holes in the center and are coaxially aligned along central axis <NUM>. Second set of plates <NUM> includes second inlet plate <NUM> followed by a second plurality of reflection plates and, in turn, followed by a second plurality of trapping plates. The second plurality of reflection plates include plates <NUM>, <NUM>, <NUM>, and <NUM>. The second plurality of trapping plates include plates <NUM>, <NUM>, <NUM>, and <NUM>. Plate <NUM> is not part of the ELIT and is only used for simulation purposes. Second set of plates <NUM> is positioned so that second inlet plate <NUM> is adjacent to a second end of cylindrical pickup electrode <NUM>.

Voltage power supply <NUM> applies pulsed voltages to one or more plates of first set of plates <NUM> and one or more plates of second set of plates <NUM> are held at their static trapping potentials. In this manner, the accepted m/z range of the device is extended. In this case, voltage power supply <NUM> applies separate voltages to nine plates of first set of trapping plates <NUM> and to nine plates of second set of plates <NUM>. Inlet plates <NUM> and <NUM> can have a zero voltage, for example. These voltages are applied in order to trap and then oscillate one or more ions between first set of plates <NUM> and second set of plates <NUM>. The one or more ions have been received along central axis <NUM> through the holes of first set of plates <NUM>, for example.

Voltage power supply <NUM> can be one power supply with multiple outputs that can supply multiple different voltages as shown in <FIG>. In various other embodiments, voltage power supply <NUM> can be two or more separate power supplies.

<FIG> is an exemplary plot <NUM> of the electric field produced in the ELIT of <FIG> by the voltages applied to the reflectron plates, in accordance with various embodiments. Reflectron plates <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are biased with increasingly higher positive voltages for positively charged ions or increasingly lower negative voltages for negatively charged ions. Similarly, reflectron plates <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are biased with the same increasingly higher positive voltages for positively charged ions or increasingly lower negative voltages for negatively charged ions.

The voltages applied to the reflectron plates at either end of the ELIT produce an electric field <NUM>. Electric field <NUM> causes the one or more ions that are introduced axially into the ELIT to oscillate along path <NUM> between the reflectron plates at either end of the ELIT. Essentially, the voltages applied to the reflectron plates at either end of the ELIT produce a potential well for the one or more ions.

<FIG> is an exemplary diagram <NUM> of the potential well produced by the electric field of <FIG>, without the focusing lenses, showing how an ion is received into the potential well of the ELIT, in accordance with various embodiments. Path <NUM> depicts the path followed by an ion <NUM> that is introduced axially into potential well <NUM>. The electric field walls of potential well <NUM> are lowered, for example, to allow ion <NUM> to be introduced.

<FIG> is an exemplary diagram <NUM> of the potential well produced by the electric field of <FIG>, without the focusing lenses, showing how an ion is trapped in the potential well of the ELIT, in accordance with various embodiments. Path <NUM> depicts the oscillating path followed by ion <NUM> when ion <NUM> is trapped in potential well <NUM>. The electric field walls of potential well <NUM> are raised, for example, to trap ion <NUM> in potential well <NUM>.

<FIG> is an exemplary diagram <NUM> of a portion of an ELIT-MS showing how an ion is introduced into an ELIT from a quadrupole, in accordance with various embodiments. For example, ion <NUM> is ejected from quadrupole <NUM> along path <NUM> and injected into ELIT <NUM>. Ion <NUM> is injected into ELIT <NUM> along central axis <NUM> through the holes of first set of reflectron plates <NUM>.

Returning to <FIG>, measurement circuitry <NUM> is used to measure first induced current <NUM> from cylindrical pickup electrode <NUM>, second induced current <NUM> from one or more plates of first set of reflectron plates <NUM> and third induced current <NUM> from one or more plates of the second set of reflectron plates <NUM>. Measurement circuitry <NUM> combines first measured induced current <NUM> with second measured induced current <NUM> and third measured induced current <NUM> to determine an induced current of the one or more ions. The use of second measured induced current <NUM> and third measured induced current <NUM> in addition to first measured induced current <NUM> reduces higher order frequency harmonics of the induced current.

Measurement circuitry <NUM> can be one circuit for detecting, filtering, and combining the measured induced currents or can be two or more separate circuits, for example.

Various additional embodiments also further reduce higher order frequency harmonics of the induced current.

In various embodiments, one or more plates of first set of reflectron plates <NUM> include first inlet plate <NUM> and one or more plates (<NUM>, <NUM>, and <NUM>) of the first plurality of reflection plates, and one or more plates of second set of reflectron plates <NUM> include second inlet plate <NUM> and one or more plates (<NUM>, <NUM>, and <NUM>) of the second plurality of reflection plates.

In various embodiments, first measured induced current <NUM> is combined with second measured induced current <NUM> and third measured induced current <NUM> by summing second measured induced current <NUM>, third measured induced current <NUM>, and twice first measured induced current <NUM>. In other words, first measured induced current <NUM> is multiplied by <NUM> and summed with second measured induced current <NUM> and third measured induced current <NUM> to calculate the induced current. The factor of <NUM> further reduces higher order frequency harmonics of the induced current.

In various embodiments, second measured induced current <NUM> and third measured induced current <NUM> are adjusted to have the same phase before second measured induced current <NUM> and third measured induced current <NUM> are summed with twice first measured induced current <NUM>. For example, the phase of second measured induced current <NUM> or third measured induced current <NUM> is shifted <NUM>° before second measured induced current <NUM> and third measured induced current <NUM> are summed with twice first measured induced current <NUM>.

In various embodiments, cylindrical pickup electrode <NUM> includes circular plate <NUM> in the middle of cylindrical pickup electrode <NUM> and circular plate <NUM> has a hole in the center. Circular plate <NUM> further reduces higher order frequency harmonics of the induced current.

In various embodiments, the diameter of cylindrical pickup electrode <NUM> is half the length of the distance between first set of plates <NUM> and the second set of plates <NUM>. In other words, the diameter of cylindrical pickup electrode <NUM> is half the length of the field free region. These dimensions further reduce higher order frequency harmonics of the induced current.

In various embodiments, the hole diameter of the one or more plates of the first plurality of reflection plates is larger than the hole diameter of the other plates of first set of plates <NUM>, and the hole diameter of the one or more plates of the second plurality of reflection plates is larger than the hole diameter of the other plates of second set of plates <NUM>. For example, as shown in <FIG>, the hole diameter of plates <NUM>, <NUM>, and <NUM>, from which induced current is measured, is larger than the hole diameter of plates <NUM>, <NUM>, and <NUM>. Similarly, the hole diameter of plates <NUM>, <NUM>, and <NUM>, from which induced current is measured, is larger than the hole diameter of plates <NUM>, <NUM>, and <NUM>. These dimensions further reduce higher order frequency harmonics of the induced current.

In various embodiments, first inlet plate <NUM> further includes first focusing lens <NUM> around the hole of first inlet plate <NUM> to focus the one or more ions radially. Similarly, second inlet plate <NUM> further includes second focusing lens <NUM> around the hole of second inlet plate <NUM> to focus the one or more ions radially.

<FIG> an exemplary plot <NUM> of the electric field produced in an ELIT without focusing lenses and shows how ions can disperse radially along the ion path without radial focusing. For example, without radial focusing, ions along ion path <NUM> begin to disperse radially within the reflectron plates in region <NUM>. This dispersion can result in the loss of ions and, therefore, a reduced signal.

Returning to <FIG>, in various embodiments, the ELIT further includes processing circuitry (not shown). This processing circuitry receives the induced current from measurement circuitry <NUM>, performs a Fourier transform on the induced current to determine one or more oscillation frequencies of the one or more ions, and calculates mass-to-charge ratios of the one or more ions from the one or more oscillation frequencies. The processing circuitry can include a general purpose processor, such as a computer, a microprocessor, microcontroller, or a digital signal processor. In various embodiments, the processing circuitry can also include a specific circuit developed for performing these functions.

<FIG> is an exemplary plot <NUM> showing the sinusoidal trajectory of an ion in the ELIT of <FIG>, in accordance with various embodiments. Sinusoidal trajectory <NUM> shows the position of an ion with respect to time. Comparing plot <NUM> to plot <NUM> of <FIG> shows that sinusoidal trajectory <NUM> in the ELIT of <FIG> is essentially equivalent to sinusoidal trajectory <NUM> of a conventional ELIT.

<FIG> is an exemplary plot <NUM> showing the first measured induced current for an ion in the ELIT of <FIG>, in accordance with various embodiments. Plot <NUM> shows that first measured induced current <NUM> for an ion is not a perfect of ideal sinusoid. Measured induced current <NUM> is similar to measured induced current <NUM> of <FIG> of a conventional ELIT but is not identical due to the changes made to the ELIT of <FIG>.

<FIG> is an exemplary plot <NUM> showing the sum of the second measured induced current and the third measured induced current for an ion in the ELIT of <FIG>, in accordance with various embodiments. In plot <NUM>, induced current <NUM> is the sum of second measured induced current <NUM> and third measured induced current <NUM> of <FIG> after an appropriate phase correction.

<FIG> is an exemplary plot <NUM> showing the sum of twice the first measured induced current of <FIG> and the sum of the second measured induced current and the third measured induced current of <FIG>, in accordance with various embodiments. In other words, induced current <NUM> is the sum of second measured induced current <NUM> of <FIG>, third measured induced current <NUM> of <FIG>, and twice first measured induced current <NUM> of <FIG>. More simply, induced current <NUM> of <FIG> is the overall induced current produced by measurement circuitry <NUM> of the ELIT of <FIG>.

<FIG> shows induced current <NUM> measured by the conventional ELIT of <FIG>. A comparison of induced current <NUM> of <FIG> with induced current <NUM> of <FIG> shows that the ELIT of <FIG> can produce an induced current measurement that is more sinusoidal in shape than the conventional ELIT of <FIG>.

<FIG> shows induced current <NUM> of a theoretical ELIT that includes an ideal pickup electrode. A comparison of induced current <NUM> of <FIG> with induced current <NUM> of <FIG> shows that the ELIT of <FIG> can produce an induced current measurement that is closer to an ideal sinusoidal shape than the conventional ELIT of <FIG>.

<FIG> is an exemplary plot <NUM> showing the fundamental frequency and higher order harmonics obtained by applying a Fourier transform to the measured induced current of <FIG>, in accordance with various embodiments. Plot <NUM> includes fundamental frequency <NUM> and higher order harmonics <NUM>.

Plot <NUM> of <FIG> shows the fundamental frequency and higher order harmonics obtained by applying a Fourier transform to the measured induced current for the conventional ELIT of <FIG>. A comparison of plot <NUM> of <FIG> with plot <NUM> of <FIG> shows that the ELIT of <FIG> is able to reduce the amplitudes of higher order harmonics.

Plot <NUM> of <FIG> shows the fundamental frequency and higher order harmonics obtained by applying a Fourier transform to the induced current of a theoretical ELIT with an ideal pickup electrode. A comparison of plot <NUM> of <FIG> with plot <NUM> of <FIG> shows that the ELIT of <FIG> is almost able to reduce the amplitudes of higher order harmonics as well as the theoretical ELIT with an ideal pickup electrode.

<FIG> is an exemplary plot <NUM> of the amplitude of the combined induced charge versus position produced by the measurement circuitry of the ELIT of <FIG> superimposed on the plot of the amplitude of the induced charge versus position of <FIG>, which is for a theoretical ELIT with an ideal pickup electrode, in accordance with various embodiments. Combined induced charge <NUM> is produced by the measurement circuitry of the ELIT of <FIG>. Induced charge <NUM> is for a theoretical ELlT with an ideal pickup electrode.

Combined induced charge <NUM> and induced charge <NUM> are very similar in shape. This shows that the ELIT of <FIG> is able to closely mimic a theoretical ELIT with an ideal pickup electrode.

Plot <NUM> of <FIG> shows the amplitude of the induced charge versus position measured at the short pickup electrode of the conventional ELIT of <FIG> superimposed on the plot of the amplitude of the induced charge versus position of <FIG>, which is for a theoretical ELIT with an ideal pickup electrode. A comparison of plot <NUM> of <FIG> with plot <NUM> of <FIG> shows that the ELIT of <FIG> is able to produce an induced charge much closer to an ideal induced charge than the ELIT of <FIG>.

<FIG> is an exemplary cross-sectional side view <NUM> of the ELIT of <FIG> showing some exemplary dimensions and biasing, in accordance with various embodiments. The dimensions shown in <FIG> are provided in millimeters.

<FIG> is an exemplary plot <NUM> of simulated measurements of resolution versus ion energy from the ELIT of <FIG> for a number of different ion beam energies and radii, in accordance with various embodiments. Region <NUM> shows that the ELIT of <FIG> is able to produce a resolution of greater that <NUM>,<NUM> when the ion beam energy is <NUM> eV and the ion beam radius is <NUM>. In other words, <FIG> shows that the ELIT of <FIG> can be used as a practical device.

<FIG> is a flowchart showing a method <NUM> for measuring the induced current of one or more ions in an electrostatic linear ion trap and reducing higher order frequency harmonics of the induced current by combining the induced current with measurements from reflecting reflectron plates, in accordance with various embodiments.

In step <NUM> of method <NUM>, one or more ions are received along a central axis through holes in the center of a first set of reflectron plates. The plates of the first set of plates are coaxially aligned along the central axis. The first set of plates includes a first inlet plate followed by a first plurality of reflection plates followed by a first plurality of trapping plates.

A cylindrical pickup electrode is positioned so that a first end of the pickup electrode is adjacent to the first inlet plate of the first set of plates. The pickup electrode is coaxially aligned with the first set of plates along the central axis.

A second set of reflectron plates with holes in the center are coaxially aligned with the pickup electrode along the central axis. The second set of plates includes a second inlet plate followed by a second plurality of reflection plates followed by a second plurality of trapping plates. The second set of plates is positioned so that the second inlet plate is adjacent to a second end of the cylindrical pickup electrode.

In step <NUM>, separate voltages are applied to one or more plates of the first set of plates and to one or more plates of the second set of plates using a voltage power supply. These voltages are applied in order to trap and oscillate the one or more ions that have been received between the first set of plates and the second set of plates.

In step <NUM>, a first induced current is measured from the cylindrical pickup electrode, a second induced current is measured from one or more plates of the first set of reflectron plates, and a third induced current is measured from one or more plates of the second set of reflection plates using measurement circuitry. Further, the first measured induced current is combined with the second measured induced current and the third measured induced current to determine an induced current of the one or more ions and reduce higher order frequency harmonics of the induced current using the measurement circuitry.

In various embodiments, the one or more plates of the first set of reflectron plates include the first inlet plate and one or more plates of the first plurality of reflection plates and the one or more plates of the second set of reflectron plates include the second inlet plate and one or more plates of the second plurality of reflection plates.

In various embodiments, combining the first measured induced current with the second measured induced current and the third measured induced current includes summing the second measured induced current, the third measured induced current, and twice the first measured induced current.

Common-mode or environmental signals are induced along the signal path of a conventional Fourier transform ELIT from sources such as radiofrequency power supplies, mains voltage, turbomolecular pumps, etc. These noise sources generate peaks in the mass spectrum after Fourier transformation which do not result from the detection of an ion. Existing experimental detection schemes for a conventional electrostatic linear ion trap rely upon non-differential detection using a central pickup electrode.

In various embodiments, a technique is disclosed for differentially detecting the image current of an ion within an ELIT using an operational amplifier, thereby minimizing common-mode signals and false peaks in the mass spectrum. By utilizing detection electrodes near the ion turning point, or trapping electrodes in the reflectron, a nearly sinusoidal signal is preserved, thereby minimizing peaks corresponding to harmonic frequencies and simplifying data processing.

<FIG> is a two-dimensional cross-sectional view <NUM> of an ELIT for measuring induced current of one or more ions and reducing higher order frequency harmonics of the induced current by combining the induced current with measurements from trapping reflectron plates, in accordance with various embodiments. The ELIT geometry utilized is similar to the geometry of <FIG>. The induced current is monitored in two places along the axis of the ELIT. Central electrode <NUM> is capacitively coupled to input <NUM> (A) of differential transimpedance operational amplifier <NUM>. Trapping reflectron plates <NUM>, <NUM>, and <NUM> on both side of the ELIT are capacitively coupled to input <NUM> (B) of differential amplifier <NUM>. By utilizing the trapping reflectron plates to measure the induced current, a signal is still detected while the ion is turning around.

The measured induced image current out of differential amplifier <NUM> is the difference between the two inputs, i.e., A-B, which is Fourier transformed and calibrated to generate a mass spectrum. The magnitude of the induced current (><NUM> f A/charge at m/z <NUM>) is virtually identical to the induced current measured from <FIG>, as described above, thereby preserving the signal integrity. In <FIG>, the induced current measured is the sum of twice the current measured from the central electrodes (2A1) and the current measured from the inlet plate and three of reflecting reflectron plates on both sides of the ELIT (A2), or the sum 2AI+A2.

The detected noise of the measurement technique of <FIG> is reduced by a factor of sqrt(<NUM>/<NUM>) relative to the (2AI+A2) detection scheme of <FIG>, increasing the signal-to-noise of the measurement by the same factor. This minimizes the number of charges that need to be injected and thereby reduces adverse effects that could arise from space charge (e.g., peak splitting, frequency drifts, coalescence).

In Fourier transform mass spectrometry, differential detection minimizes common-mode signals from environmental sources (e.g., mains voltage, RF pickup, or pumps). Additionally, by using the trapping reflectron electrodes near the ion turning points as detectors, nearly sinusoidal signals are observed, minimizing harmonic content and false peaks. This also allows for standard FFT processing which can easily display the derived mass spectrum in real-time and allows the user to know exactly how the mass spectrum is generated (software transparency). In summary, differential detection lowers the noise floor of the induced image charge measurement, reduces the number of charges that need to be injected, reduces space charge effects, reduces common-mode noise, provides a real-time mass spectrum, and generates a mass spectrum of higher integrity.

<FIG> is a three-dimensional cutaway side view <NUM> of an ELIT for measuring induced current of one or more ions and reducing higher order frequency harmonics of the induced current by combining the induced current with measurements from trapping reflectron plates, in accordance with various embodiments. Like the ELIT of <FIG>, the ELIT of <FIG> includes first set of reflectron plates <NUM>, cylindrical pickup electrode <NUM>, second set of reflectron plates <NUM>, voltage power supply <NUM>, and measurement circuitry <NUM>.

Measurement circuitry <NUM> is used to measure first induced current <NUM> from cylindrical pickup electrode <NUM>, second induced current <NUM> from one or more plates of the first set of reflectron plates, and third induced current <NUM> from one or more plates of the second set of reflectron plates. Measurement circuitry <NUM> combines first measured induced current <NUM> with second measured induced current <NUM> and third measured induced current <NUM> to determine an induced current of the one or more ions. The use of second measured induced current <NUM> and third measured induced current <NUM> in addition to first measured induced current <NUM> reduces higher order frequency harmonics of the induced current.

In various embodiments, one or more plates of first set of reflectron plates <NUM> include one or more plates (<NUM>, <NUM>, and <NUM>) of the first plurality of trapping plates and one or more plates of second set of reflectron plates <NUM> include one or more plates (<NUM>, <NUM>, and <NUM>) of the second plurality of trapping plates.

In various embodiments, measurement circuitry <NUM> combines first measured induced current <NUM> with second measured induced current <NUM> and third measured induced current <NUM> by subtracting second measured induced current <NUM> and third measured induced current <NUM> from first measured induced current <NUM>.

In various embodiments, measurement circuitry <NUM> includes differential transimpedance amplifier <NUM>. Cylindrical pickup electrode1230 is capacitively coupled to a first input of differential transimpedance amplifier <NUM> and the one or more plates (<NUM>, <NUM>, and <NUM>) of the first plurality of trapping plates and the one or more plates (<NUM>, <NUM>, and <NUM>) of the second plurality of trapping plates are each capacitively coupled to a second input of differential transimpedance amplifier <NUM> to perform the subtraction.

<FIG> is an exemplary plot <NUM> showing the combined induced current <NUM> measured by the ELIT of <FIG> by subtracting second measured induced current <NUM> and third measured induced current <NUM> from first measured induced current <NUM> of <FIG>, in accordance with various embodiments.

A comparison of combined induced current <NUM> of <FIG> with induced current <NUM> of <FIG> shows that the ELIT of <FIG> can also produce an induced current measurement that is more sinusoidal in shape.

<FIG> is an exemplary plot <NUM> showing the fundamental frequency and higher order harmonics obtained by applying a Fourier transform to the measured induced current of <FIG> with their amplitudes plotted on a logarithmic scale, in accordance with various embodiments. Plot <NUM> includes fundamental frequency <NUM> and higher order harmonics <NUM>. Both <FIG> and <FIG> show that the ELIT of <FIG> is able to reduce higher order harmonics relative to the fundamental frequency.

Returning to <FIG>, in step <NUM>, a first induced current is measured from the cylindrical pickup electrode, a second induced current is measured from one or more plates of the first set of reflectron plates, and a third induced current is measured from one or more plates of the second set of reflectron plates using measurement circuitry. Further, the first measured induced current is combined with the second measured induced current and the third measured induced current to determine an induced current of the one or more ions and reduce higher order frequency harmonics of the induced current using the measurement circuitry.

In various embodiments, the one or more plates of the first set of reflectron plates include one or more plates of the first plurality of trapping plates and the one or more plates of the second set of reflectron plates include one or more plates of the second plurality of trapping plates.

In various embodiments, combining the first measured induced current with the second measured induced current and the third measured induced current includes subtracting the second measured induced current and the third measured induced current from the first measured induced current.

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.

Claim 1:
An electrostatic linear ion trap (<NUM>) for measuring induced current of one or more ions and reducing higher order frequency harmonics of the induced current by combining the induced current with measurements from reflecting reflectron plates, said electrostatic linear ion trap comprising:
a first set of reflectron plates (<NUM>) with holes in the center that are coaxially aligned along a central axis (<NUM>), wherein the first set of plates (<NUM>) includes a first inlet plate (<NUM>) followed by a first plurality of reflection plates (<NUM>, <NUM>, <NUM>, <NUM>) followed by a first plurality of trapping plates (<NUM>, <NUM>, <NUM>, <NUM>);
a cylindrical pickup electrode (<NUM>) positioned so that a first end of the pickup electrode is adjacent to the first inlet plate (<NUM>) of the first set of plates and the pickup electrode is coaxially aligned with the first set of plates along the central axis;
a second set of reflectron plates (<NUM>) with holes in the center that are coaxially aligned with the pickup electrode along the central axis (<NUM>), wherein the second set of plates includes a second inlet plate (<NUM>) followed by a second plurality of reflection plates (<NUM>, <NUM>, <NUM>, <NUM>) followed by a second plurality of trapping plates (<NUM>, <NUM>, <NUM>, <NUM>) and wherein the second set of plates is positioned so that the second inlet plate (<NUM>) is adjacent to a second end of the cylindrical pickup electrode;
a voltage power supply (<NUM>) configured to apply separate voltages to one or more plates of the first set of reflectron plates and to one or more plates of the second set of reflectron plates in order to trap and oscillate one or more ions that have been received along the central axis through the holes of the first set of plates between the first set of plates and the second set of plates; and
measurement circuitry (<NUM>) configured to measure a first induced current from the cylindrical pickup electrode, a second induced current from one or more plates of the first set of reflectron plates, and a third induced current from one or more plates of the second set of reflectron plates and to combine the first measured induced current with the second measured induced current and the third measured induced current to determine an induced current of the one or more ions and reduce higher order frequency harmonics of the induced current.