US 20070018629 A1
Disclosed is an apparatus for generating a periodically varying electrical signal for creating a periodically varying electrical field between electrodes of an ion mobility spectrometer. The apparatus includes an output port. A first tuned circuit is provided for being electrically coupled to an external power source and for, in isolation, providing a first periodically varying electrical signal having a first frequency. The first tuned circuit is coupled to the output port for providing an output electrical signal having a component at the first frequency thereto. A second tuned circuit is provided for being electrically coupled to an external power source and for providing a second periodically varying electrical signal having a second frequency different from the first frequency. The second tuned circuit is coupled to the first tuned circuit for varying the output electrical signal about the first periodically varying electrical signal.
1. An electromagnetic transformer comprising:
a secondary winding comprising a plurality of turns of a first wire wound defining a core and having an approximately uniform spacing between adjacent turns;
a first primary winding comprising at least one turn of a second wire wound around the core and spaced apart from both the core and the secondary winding; and,
a second primary winding comprising at least one turn of a third wire wound around the core in parallel with the first primary winding and spaced apart from both the core and the secondary winding.
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a third primary winding comprising at least one turn of a third wire wound around the core and spaced apart from both the core and the secondary winding; and,
a fourth primary winding comprising at least one turn of a fourth wire wound around the core in parallel with the third primary winding and spaced apart from both the core and the secondary winding.
11. An apparatus according to
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The instant invention relates generally to high field asymmetric waveform ion mobility spectrometry (FAIMS), more particularly the instant invention relates to waveform generator electronics based on tuned LC circuits.
High sensitivity and amenability to miniaturization for field-portable applications have helped to make ion mobility spectrometry (IMS) an important technique for the detection of many compounds, including narcotics, explosives, and chemical warfare agents as described, for example, by G. Eiceman and Z. Karpas in their book entitled “Ion Mobility Spectrometry” (CRC, Boca Raton, 1994). In IMS, gas-phase ion mobilities are determined using a drift tube with a constant electric field. Ions are separated in the drift tube on the basis of differences in their drift velocities. At low electric field strength, for example 200 V/cm, the drift velocity of an ion is proportional to the applied electric field strength, and the mobility, K, which is determined from experimentation, is independent of the applied electric field. Additionally, in IMS the ions travel through a bath gas that is at sufficiently high pressure that the ions rapidly reach constant velocity when driven by the force of an electric field that is constant both in time and location. This is to be clearly distinguished from those techniques, most of which are related to mass spectrometry, in which the gas pressure is sufficiently low that, if under the influence of a constant electric field, the ions continue to accelerate.
E. A. Mason and E. W. McDaniel in their book entitled “Transport Properties of Ions in Gases” (Wiley, New York, 1988) teach that at high electric field strength, for instance fields stronger than approximately 5,000 V/cm, the ion drift velocity is no longer directly proportional to the applied electric field, and K is better represented by KH, a non-constant high field mobility term. The dependence of KH on the applied electric field has been the basis for the development of high field asymmetric waveform ion mobility spectrometry (FAIMS). Ions are separated in FAIMS on the basis of a difference in the mobility of an ion at high field strength, KH, relative to the mobility of the ion at low field strength, K. In other words, the ions are separated due to the compound dependent behavior of KH as a function of the applied electric field strength.
In general, a device for separating ions according to the FAIMS principle has an analyzer region that is defined by a space between first and second spaced-apart electrodes. The first electrode is maintained at a selected dc voltage, often at ground potential, while the second electrode has an asymmetric waveform V(t) applied to it. The asymmetric waveform V(t) is composed of a repeating pattern including a high voltage component, VH, lasting for a short period of time tH and a lower voltage component, VL, of opposite polarity, lasting a longer period of time tL. The waveform is synthesized such that the integrated voltage-time product, and thus the field-time product, applied to the second electrode during each complete cycle of the waveform is zero, for instance VHtH+VLtL=0; for example +2000 V for 10 μs followed by −1000 V for 20 μs. The peak voltage during the shorter, high voltage portion of the waveform is called the “dispersion voltage” or DV, which is identically referred to as the applied asymmetric waveform voltage.
Generally, the ions that are to be separated are entrained in a stream of gas flowing through the FAIMS analyzer region, for example between a pair of horizontally oriented, spaced-apart electrodes. Accordingly, the net motion of an ion within the analyzer region is the sum of a horizontal x-axis component due to the stream of gas and a transverse y-axis component due to the applied electric field. During the high voltage portion of the waveform an ion moves with a y-axis velocity component given by vH=KHEH, where EH is the applied field, and KH is the high field ion mobility under operating electric field, pressure and temperature conditions. The distance traveled by the ion during the high voltage portion of the waveform is given by dH=vHtH=KHEHtH, where tH is the time period of the applied high voltage. During the longer duration, opposite polarity, low voltage portion of the asymmetric waveform, the y-axis velocity component of the ion is vL=KEL, where K is the low field ion mobility under operating pressure and temperature conditions. The distance traveled is dL=vLtL=KELtL. Since the asymmetric waveform ensures that (VHtH)+(VLtL)=0, the field-time products EHtH and ELtL are equal in magnitude. Thus, if KH and K are identical, dH and dL are equal, and the ion is returned to its original position along the y-axis during the negative cycle of the waveform. If at EH the mobility KH>K, the ion experiences a net displacement from its original position relative to the y-axis. For example, if a positive ion travels farther during the positive portion of the waveform, for instance dH>dL, then the ion migrates away from the second electrode and eventually will be neutralized at the first electrode.
In order to reverse the transverse drift of the positive ion in the above example, a constant negative dc voltage is applied to the second electrode. The difference between the dc voltage that is applied to the first electrode and the dc voltage that is applied to the second electrode is called the “compensation voltage” (CV). The CV prevents the ion from migrating toward either the second or the first electrode. If ions derived from two compounds respond differently to the applied high strength electric fields, the ratio of KH to K may be different for each compound. Consequently, the magnitude of the CV that is necessary to prevent the drift of the ion toward either electrode is also different for each compound. Thus, when a mixture including several species of ions, each with a unique KH/K ratio, is being analyzed by FAIMS, only one species of ion is selectively transmitted to a detector for a given combination of CV and DV. In one type of FAIMS experiment, the applied CV is scanned with time, for instance the CV is slowly ramped or optionally the CV is stepped from one voltage to a next voltage, and a resulting intensity of transmitted ions is measured. In this way a CV spectrum showing the total ion current as a function of CV, is obtained.
In FAIMS, the optimum dispersion voltage waveform for obtaining the maximum possible ion detection sensitivity on a per cycle basis takes the shape of an asymmetric square wave with a zero time-averaged value. In practice this asymmetric square waveform is difficult to produce and apply to the FAIMS electrodes because of electrical power consumption considerations. For example, without a tuned circuit the power that is required to drive a capacitive load of capacitance C, at frequency f, with a peak voltage V and a 1:1 duty cycle square wave, is V2fC. Accordingly, if a square wave at 750 kHz, 4000 V peak voltage 1:1 duty cycle is applied to a 20 picofarad load, the theoretical power consumption will be 480 Watts produced by the sum of the squares of the voltage changes on the capacitive load of 40002+40002 multiplied by f*C. If, on the other hand, a waveform is applied via a tuned circuit with Q factor (Bandwidth 3 dB/Frequency) of 200, the power consumption is reduced to less than 2.5 Watts. Theoretically the power is P(cosΘ)) where Θ is the angle between the current and the voltage applied to the capacitive load, and P is 2V2fC. This power consumption approaches zero if the current and voltage are out of phase by 90 degrees, as they would be in a perfectly tuned LC circuit with ideal components. Similarly, if the waveform is asymmetrical with duty cycle of 2:1, as for example in a FAIMS application, then the theoretical power consumption is reduced to 333 Watts, produced by the sum of squares of the voltage changes on the capacitive load of 40002+20002+(20002−13332) times f*C.
Since a tuned circuit cannot provide a square wave, an approximation of a square wave is taken as the first terms of a Fourier series expansion. One approach is to use:
In practice, both the optimization of the LC tuning and maintenance of the exact amplitude of the first and second applied sinusoidal waves and the phase angle between the two waves is required to achieve long term, stable operation of a FAIMS system powered by such an asymmetric waveform generator. Accordingly, feedback control is required to ensure that the output signal is stable and that the correct waveform shape is maintained.
In U.S. Pat. No. 5,801,379, which was issued on Sep. 1, 1998, Kouznetsov teaches a high voltage waveform generator having separate phase correction and amplitude correction circuits. This system uses additional components in the separate phase correction and amplitude correction circuits, thereby increasing complexity and increasing the cost of manufacturing and testing the devices. Furthermore, this system cannot be implemented in the control software, making it difficult to vary certain operating parameters during use.
It is an object of the instant invention to provide an asymmetric waveform generator based on LC tuning electronics that overcomes the limitations of the prior art.
In accordance with an aspect of the instant invention there is provided an apparatus for generating a periodically varying electrical signal for creating a periodically varying electrical field between electrodes of an ion mobility spectrometer, comprising: an output port; a first tuned circuit for being electrically coupled to an external power source and for, in isolation, providing a first periodically varying electrical signal having a first frequency, the first tuned circuit coupled to the output port for providing an output electrical signal having a component at the first frequency thereto; and, a second tuned circuit for being electrically coupled to an external power source and for providing a second periodically varying electrical signal having a second frequency different from the first frequency, the second tuned circuit coupled to the first tuned circuit for varying the output electrical signal about the first periodically varying electrical signal.
In accordance with another aspect of the instant invention there is provided an electromagnetic transformer comprising: a secondary winding comprising a plurality of turns of a first wire wound defining a core and having an approximately uniform spacing between adjacent turns; a first primary winding comprising at least one turn of a second wire wound around the core and spaced apart from both the core and the secondary winding; and, a second primary winding comprising at least one turn of a third wire wound around the core in parallel with the first primary winding and spaced apart from both the core and the secondary winding.
Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which similar reference numbers designate similar items:
The following description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and the scope of the invention. Thus, the present invention is not intended to be limited to the embodiments disclosed, but is to be accorded the widest scope consistent with the principles and features disclosed herein.
As is noted above, the waveform that is applied in FAIMS is a combination of two sinusoidal waves of frequency omega (ω) and two times omega (2ω). The two sinusoidal waves are of amplitudes that differ by a factor of two and that are offset by a phase shift of π/2, resulting in a waveform that is defined by, for example, Equation 1, below:
In practice the application of two sinusoidal waves of frequency ω and 2ω is used to generate a waveform with the shape shown at
Referring now to
Inductors IN1 and IN2 are arranged in series with each other, and the input pulses of a first frequency are approximately identical and in phase. In other words, in this example, PP1 and PP2 are identical, and PM1 and PM2 are identical, however the positive-going (PPn) and negative-going (PMn) pulses are applied alternatively in a push-pull manner, not simultaneously to the inductors. The combined inductances of IN1 and IN2 are selected to oscillate in tuned resonance with a capacitance of C3 combined in parallel with the capacitance of the rest of the circuit attached to the secondary windings of IN3 and IN4, namely C4, and FAIMS load plus all other stray capacitances throughout the circuit. C1 and C2 do not contribute to the tuning as they are bypass capacitances for the DC voltages B1 and B2. C5 does not contribute to the tuning of IN1 and IN2 as it is balanced across IN3 and IN4. For example, if the combined inductance of IN1 and IN2 is 0.45 mH then the circuit will oscillate at 750 kHz if the capacitance of C3 in parallel with the rest of the circuit is 100 pF.
The secondary windings of IN3 and IN4 are in series, but the center tap between these inductors is attached to the secondary of IN1 and IN2. This means that the combined oscillation of the IN3 and IN4 is around the floating voltage provided from IN1 and IN2. It is therefore possible for IN3 and IN4 to oscillate at a second frequency that is independent of the first frequency of oscillation of IN1 and IN2. The secondary windings of inductors IN3 and IN4 are coupled with three capacitors in a symmetrical arrangement. One capacitor, C5, is parallel to the inductors IN3 and IN4, whereas the other two capacitors, C4 and the FAIMS load, are each in series with ground or with some other dc potential, for example B1 in
Tuning of IN3 and IN4, in concert with their capacitive load including C5, C4 and the FAIMS load, is made possible through adjustment of C5. Simultaneously, adjustment of C3 is required to ensure that the tuning of IN1 and IN2 with the remaining circuit is retained. Advantageously the computer control of this circuit is possible by using adjustable capacitors whose capacitance is changed by motors activated electronically.
Advantageously, the two frequencies applied to the two tuned circuits may be adjusted independently with the input signal provided to the other of the two tuned circuits disabled or fixed. In other words, if the inputs PP1, PP2, PM1 and PM2 are all reduced to zero, the application of PP3, PP4, PM3, and PM4 activates the LC oscillation at a frequency defined by the values of the inductances and capacitances attached to IN3 and IN4. The tuning of this part of the circuit is adjusted by changing the input frequency and voltages applied to PP3, PP4, PM3, and PM4, as well as by adjusting the variable capacitor C5. Similarly, with the inputs PP3, PP4, PM3, and PM4 set to zero, the oscillator defined by IN1, IN2 and their capacitive load, is activated by applying PP1, PP2, PM1 and PM2. Adjustment of this LC oscillation is achieved by changing the voltage and frequency applied to PP1, PP2, PM1 and PM2, and by adjusting variable capacitor C3. If both oscillators are independently optimized to maximum efficiency, quality value Q, the phase shift between the oscillations are adjusted by digital control of the phase difference between the PP1, PP2, PM1, PM2 relative to PP3, PP4, PM3, PM4 inputs.
Optionally, in a microprocessor controlled system it is not necessary to zero the other frequency to tune each resonant circuit. In this case, the data processing system extracts the amplitude of each frequency from the combined waveform.
Of course, a person skilled in the art will appreciate that optionally the sinusoidal drive waveforms are applied to a not illustrated conventional version of primary coil on the inductors IN1, IN2, IN3 and IN4. For maximum control over the drive waveforms, additional electronics, optionally including digital synthesis of the sinusoidal waveforms, may be also utilized.
Referring now to
Referring now to
Advantageously, a plurality of primary windings 60, 62 and 64, 66 as shown at
Beyond three or four parallel sets of primary windings wound around the core 32, the efficiency of coupling does not further increase significantly since the coupling is over 90% with three sets of parallel primary windings. Additional sets of parallel primary windings (beyond three or four) also have the detrimental effect of increasing the stray capacitance between the primary and secondary windings.
Advantageously, the cut toroid-shaped core results in a small instrument package. Optionally, the core is provided in the form of a bar, or another suitable shape.
The entire contents of U.S. patent application Ser. No. 10/529,309 filed on Mar. 25, 2005, are hereby incorporated by reference.
Numerous other embodiments may be envisaged without departing from the spirit and scope of the instant invention.