Patent Application
20180261442
Not Applicable
Not Applicable
Not Applicable
The invention relates to the field of electronics, and specifically to the electronic circuitry used to generate high voltage RF signals for driving quadrupole and ion trap mass spectrometers. The invention finds its greatest application in the field of portable mass spectrometers, where the efficiency and compact design of the RF generator provides distinct size, weight and cost advantages over conventional RF generator designs.
Fundamentally, the practice of mass spectrometry involves the manipulation and control of individual molecules of a substance to determine the mass-to-charge ratio, the chemical formula, the chemical structure, the isotopic ratio, the relative concentration and the amount of a specific compound present in a sample. Typically, a mass spectrometer performs this analysis by placing the sample molecules into an ionized state. Once the molecules are ionized, they may be controlled through the application of external electric and/or magnetic fields. The behavior of the ionized molecules can then be recorded and analyzed in a variety of ways to determine their mass/charge ratios and the mass/charge ratios of the fragments formed from the breakup of the original molecules of the sample. From this basic mass/charge data, information relating to the formula, structure, isotopic ratio and amount of material present can be calculated or deduced.
The mass spectrometer itself is typically composed of several different components, often operating under different pressures and different temperatures. A typical mass spectrometer includes an inlet or separation system within which the sample to be analyzed is separated into its fundamental chemical components prior to being mass analyzed. The separation device (typically a gas or liquid chromatograph), performs a chemical separation process and then directs the components of the sample into an ion source. The ion source is used to place the sample molecules into an ionized state whereupon the individually charged molecules may then be controlled through use of externally applied electric and/or magnetic fields.
After the ionized molecules leave the ion source they are directed into the analyzer, where the mass/charge ratios of the individual molecules will be measured. This analysis step may involve the use of a single or multiple set of quadrupole rods, a magnetic and/or electric analyzer, an electric trapping device, an ion mobility cell, a time-of-flight measuring device, or any combination or concatenation of these and other analyzers.
Many types of mass spectrometry analyzers require an RF generator capable of generating a high voltage RF signal. The quadrupole analyzer requires a bipolar RF signal to be applied to two of the four quadrupole rods comprising the analyzer. For the linear ion trap mass spectrometer, a bipolar RF signal must also be applied to two of the four quadrupole rods comprising the analyzer. For the three-dimensional ion trap mass spectrometer a unipolar RF signal must be applied to the ring electrode. For the rectilinear ion trap a bipolar RF signal must be applied to two of the four flat plate electrodes comprising the analyzer. For a cylindrical ion trap a unipolar RF signal must be applied to the center cylindrical conductor.
After passing through the analyzer of the mass spectrometer the ions are then detected. The detection process may involve the use of an electron multiplier, a Faraday cup detector, or a special charge-coupled device detector. The ion detection process may even involve the measurement of image currents generated externally to the analyzer due to the cyclic movement of the ions within the analyzer itself.
There are several types of mass spectrometers that utilize a high voltage RF field within the analyzer. One type of analyzer (the quadrupole mass filter) comprises a set of four or more conductive rods arranged in a parallel fashion, in which the sample molecules pass down the central axis of the rod assembly. This type of analyzer is typically used as a mass filter, in which ions are selectively removed from the sample stream, leaving a resulting mass spectrum to be recorded by an external detector.
Another type of analyzer that utilizes a high voltage RF field is the linear ion trap. This type of analyzer is similar to the quadrupole mass filter, except that the sample ions do not pass through the analyzer, and are instead “trapped” within the rod assembly through use of electric fields created at each end of the rod assembly. These trapped ions are then ejected sequentially in increasing mass/charge ratios from the rod assembly, generating a mass spectrum.
An additional type of analyzer requiring the use of a high voltage RF field is often referred to as the three-dimensional ion trap. This device is fundamentally a linear ion trap in which the cross-sectional geometry of the analyzer is rotated through a full 360 degrees, creating an enclosed trapping device comprising two “end cap” electrodes at opposite ends of the analyzer, with a “ring” electrode in the center. Sample molecules are either injected into the three-dimensional ion trap in an ionized state, or are injected as neutral molecules and then ionized internally after they have been injected into the ion trap.
The geometry of the quadrupole mass filters and ion traps can vary substantially. The quadrupole mass filter and linear ion trap can comprise conductive rods that are round, hyperbolic, or completely flat. Likewise, the three-dimensional ion trap can be constructed from round, hyperbolic, or cylindrical geometries.
The mass spectrometers based upon the quadrupole mass filter, or ion trap design, constitute a very large number of commercial mass spectrometers in current use. All of these instruments require the use of a high voltage RF power supply capable of supplying a fixed frequency signal that can be quickly varied in amplitude to match the desired scan rate of the instrument or to select a different operating condition.
The disclosed RF generator design represents an efficient method of generating a high voltage RF signal of several thousand volts peak-to-peak. The design requires very few components and maintains excellent stability over the entire amplitude range of its output.
The disclosed RF generator circuit is driven by a low voltage (0 to 10 volts peak-to-peak) RF signal representing the desired frequency and amplitude of the output signal. The output of the RF generator will have the same frequency as the input driving signal, but will have a voltage output of roughly a thousand times that of the input signal.
The disclosed RF generator circuit is biased as a class AB amplifier in which the amplifier components themselves conduct for slightly more than one half of the input sinusoidal waveform period. By comparison, the typical class A amplifier will always conduct current and dissipate power throughout the entire input waveform cycle, even when the input signal is zero. The typical class B amplifier is composed of two different stages, in which one stage will conduct only during the positive portion of the input signal and the other stage will conduct only during the negative portion of the input signal. This configuration improves upon the efficiency of the class A amplifier at the expense of linearity. However, the class AB amplifier design permits the amplifier circuit to maintain both high efficiency and linearity, which represents an optimum design for an RF generator used in a mass spectrometer, especially for a portable mass spectrometer where it is desired to minimize power consumption while maintaining adequate linearity of the RF generator signal.
The initial input drive signal, representing the frequency and amplitude of the RF output signal, is split into two separate channels by two separate rectifier circuits. The positive rectifier circuit generates the positive component of the RF signal while the negative rectifier circuit generates the negative component of the RF signal.
Each of the positive and negative rectified signals drive a bias voltage circuit which allows exact control over the offset of positive and negative signals. These offset adjustments are important in achieving optimum linearity of the generated RF output signal.
The output of each of the positive and negative bias voltage circuits drive a separate current amplifier composed of an input buffer operational amplifier and a power MOSFET, which achieves a current amplification of approximately one amp of current for each one volt of RF input signal. In addition, the two current amplifier circuits employ a simplified feedback control loop permitting adequate stability of the generated RF signal with a very limited number of components.
The output of the two current amplifier circuits are combined to drive the primary winding of the final air-core step-up transformer. The secondary winding of the step-up transformer generates the final RF output signal which will normally be used to drive the electrodes of a quadrupole or ion trap mass spectrometer. Additionally, the RF generator may be used to drive an array of miniaturized 3-dimensional quadrupole ion trap mass spectrometers or linear ion trap mass spectrometers.
The RF generator circuit primarily comprises four different sections, as illustrated in
The input driving signal is shown in
While the frequency of the Vin signal used to drive the RF generator has a fixed frequency, the amplitude of the Vin signal will normally vary from zero volts to a maximum input of ten or fifteen volts peak. The desired output of the RF generator will depend upon the physical size of the quadrupole or ion trap electrodes and the intended application of the mass spectrometer. Typical ion trap mass spectrometers could be driven by an RF signal having an amplitude of a few hundred volts, or as much as ten thousand volts peak-to-peak.
The RF driving signal, as shown in
The positive rectifier circuit is shown in detail in
The positive bias circuit shown in
In a similar manner the negative bias circuit shown in
The offset voltages shown in
The output of the positive channel bias circuit shown in
The output of the negative channel bias circuit shown in
The disclosed current amplifier design with the suggested values and components will produce an amplifier design generating approximately one amp of current at the output of the power MOSFET devices for a 1 volt input. These values relate to a particular application and can be expected to change with differing design criteria.
The output of the two current amplifiers, shown in
The secondary winding of the step-up transformer comprises the two inductors L2 and L4, with a center tap conductor point connected to ground. The capacitive load of the ion trap electrodes is represented by C5 and C6 in
The turns ratio and the actual number of turns of the T1 step-up transformer will be primarily dependent upon the desired output voltage of the RF generator. This could vary from a few hundred volts for smaller quadrupole or ion trap mass spectrometer analyzers, to as much as ten thousand volts for larger analyzers. This would allow for a turns ratio that would typically range from 1:100 (primary to secondary) to 1:1000 (primary to secondary). However, for specialized applications the optimum turns ratio could be more or less than these values.
The drawing in
After ions have been contained within the trapping volume at the center of the quadrupole rod set, as shown in
Table 1 lists five types of mass spectrometer configurations that can be driven with an RF generator of the type described herein. The first is the Quadrupole Mass Spectrometer, in which each of the two opposite pairs of rods of the quadrupole analyzer are connected together and the resulting two connections are driven by an RF and DC signal. The RF and DC potentials are increased in proper proportion to generate a mass spectrum by allowing only ions having a particular m/z value to pass through the analyzer at a given time, thus generating a mass spectrum.
The quadrupole linear ion trap has a similar construction to that of the quadrupole mass analyzer, but the quadrupole linear ion trap does not require the use of a DC potential. In addition, the quadrupole linear ion trap requires the use of two “endcaps”, or additional quadrupole segments to contain ions injected into the center of the quadrupole trapping volume. The quadrupole rods themselves may be circular, or preferably, hyperbolic in shape.
The rectilinear ion trap is constructed in a very similar manner to that of the quadrupole linear ion trap, but instead of four circular or hyperbolic rods, four flat conductive plates are used to construct the analyzer.
The 3-D (3-Dimensional) ion trap is constructed from two hyperbolic-shaped endcaps and a hyperbolic-shaped ring electrode. In this configuration, the RF generator only needs to supply one unipolar RF output signal to the ring electrode.
The 3-D cylindrical ion trap is constructed in a very similar manner to the 3-D ion trap, except that the two hyperbolic endcaps are replaced with simple flat circular plates with an aperture in the center to permit the entry and exit of ions, and the hyperbolic-shaped ring electrode is replaced with a simple circular ring electrode.
These descriptions represent most of the configurations used by quadrupole and ion trap mass spectrometers. Their exact detail is not pertinent to the description of the RF generator circuit described herein, but serves to demonstrate that the described RF generator circuit may be used to control a variety of mass spectrometer configurations, and may also be used to supply an RF signal to any type of instrument or device requiring a compact RF generator that can produce a variable high voltage signal at a low output current.
The RF generator circuit is designed to handle a wide range of operating frequencies. A typical operating frequency would be 1 MHz, but the circuit could also be used for frequencies up to 10 MHz. For frequencies above approximately 6 MHz, the feedback loop incorporates an additional current path. For the positive signal component, feedback flows back through C1 to the inverting input of U7, as shown by the dashed arrow 1004. For the negative signal component, feedback flows back through C3 to the inverting input of U8, as shown by the dashed arrow 1008.
The feedback control loop of the disclosed circuit is able to maintain stability and linearity of the RF generator amplitude by incorporating very fast power MOSFETS and sampling the RF output at the input to the primary winding of the final step-up transformer. This allows the RF generator design to operate with sufficient stability to control a portable mass spectrometer, without the need for a sampling circuit connected to the secondary winding of the RF generator final step-up transformer. Since a portable mass spectrometer typically presents a small load for an RF generator (often less than 20 pf.), the disclosed design is capable of generating an RF signal with excellent stability through use of the feedback loop described, which originates at the input to the primary winding of the final step-up transformer.
The component values described herein represent sample values that can be used to construct an operating RF generator circuit. Various optimizations and adjustments may be made to allow the RF generator performance to be more compatible with any particular application.
