This application relates in general to ion traps, and in particular, to ion traps operating at atmospheric pressures.
Instrumentation for trace compound detection and identification are perpetually pushed for increased sensitivity, lower detection limits, higher resolution and smaller physical size. The most sensitive instruments available include mass spectrometers (MS), which require high vacuum and are bulky with typical volumes on the order of 0.2 m3. MS instruments can be either linear quadrupole or quadrupole ion traps. A second type of instrument, but which operates at atmospheric pressure, is the ion mobility spectrometer (IMS). Although the IMS and its closely related cousin, the Differential Mobility Spectrometer (DMS), have demonstrated sub-ppb detection capabilities, resolution between peaks is often poor, and for unknown multi-analyte mixtures, often peaks, and thus species, cannot be resolved.
The operating principle for mass spectrometers is to separate species by a mass to charge ratio, m/z, typically by employing an (oscillating) electric quadrupole field, and sometimes in conjunction with applied magnetic fields. The equation of motion for the ion species is:
The operating principle for ion mobility spectrometers is separation according to the ion's drift velocity through the ambient background gas under an applied electric field. The drift velocity is proportional to the electric field according to
{right arrow over (v)}(t)=κ(E){right arrow over (E)}({right arrow over (x)},t)
where κ(E) is the ion mobility. Ion mobility κ(E) is often approximated as a constant, but it is actually a power series in even powers of E (so that the drift velocity is an odd power of {right arrow over (E)} hence antisymmetric; when the electric field changes sign the velocity does too, which motion in an isotropic, homogeneous medium should do). Comparing the above two equations, and noting that the velocity is just the first time derivative of the position, the acceleration for an ion in a mass spectrometer is proportional to the electric field, whereas for an ion mobility spectrometer, the velocity is proportional to the electric field. That means the two principles of operation give rise to very different ion motion dynamics.
And to satisfy Laplace's equation, the condition
λ+σγ=1
follows. For a quadrupole ion trap λ=σ=1, γ=−2. Writing the resulting potential in cylindrical coordinates gives:
For an arbitrarily chosen fixed value of Φ(r,z), the relationship between r and z define hyperbola, which are the shape of the ideal endcap and ring electrodes. A plot of the potential is shown in
φ0(t)=U+V cos(ωt)
This oscillating potential alternately produces a trapping then anti-trapping potential in both the radial and axial coordinates. If the frequency ω and potential V are judiciously chosen, the excursion of the ion motion will always be less than the position of the electrodes, making a stable trap. This can be visualized by imagining the potential in
Collisions between ions and helium atoms remove energy from the ions, i.e., energetically cooling and confining them closer to the center of the trap.
A scan of the m/z ratios of all the different species of ions simultaneously held in the trap is made by sweeping the endcap-ring potential amplitude V and the frequency ω. An alternative process is to excite resonant ions using a small (few hundred millivolts) rf field across the two endcaps. This drives resonant ions into larger amplitude orbits where they are then forced into a detector electrode or channeltron type amplifier for current measurement.
By superimposing a small DC electric field across the parallel plates with the use of a compensation voltage Vc, the net trajectory can be made parallel to the plates. Resonant ions then pass through the filter and are detected upon exit by detection electrodes. As the compensation voltage is swept, peaks appear corresponding to different ion species.
An embodiment of the present invention described herein combines elements of both the mass spectrometer and the ion mobility spectrometer. The ion trajectories in atmosphere indicated in
Because ion mobility is a function of the electric field strength, and the electric field increases linearly in both the radial and axial directions (but at different rates), ion trajectories are calculated numerically. A Monte Carlo simulation for this trap yields necessary performance characteristics. Once ions are trapped they may be selectively made to have larger excursion from the trap center until they strike a detector (step 710); or, laser spectroscopy may be performed for species identification (step 706). The detector may be a biased electrode, or a pair of biased electrodes, for sensing ions of + and − charges. Laser spectroscopy essentially recycles each ion's contribution to the signal so that extremely low concentrations are detectable. Alternatively, the ions may be released into a mass spectrometer (see step 707), or an ion mobility spectrometer (step 708). The spectrum may be collected and analyzed in step 709.
Another embodiment of an ion trap operating at atmospheric pressures uses the trap as an acoustic cell, which measures the concentration and types of ions that are created, which in turn permits a measure of trace chemicals in the atmosphere being probed.
A problem with this approach is that the frequency (color) of the light may be hard to achieve except when using broad band light sources. However, in this case, the selectivity of the PAS will be degraded because of the broad spectral band of light used. Some telecom lasers exist in wavelengths useful for some analytes, but other analytes require expensive lasers. It may be preferential to excite the sound waves without using a light source. An ion trap acoustic cell addresses this issue.
Referring to the schematic illustrated in
The electrodes 501-504 may be mounted on stiff, insulating walls 505. One or more holes (not shown) may be positioned on the cell walls 505 to allow gas to come in and out of the cell 500 (see step 803). A hole in one or more electrodes 501-504 may be used to position a microphone or sound transducer 506 to monitor for sound waves (see step 808). Ni-63 beta sources or other radioactive materials 507 may be placed inside the cell 500 to provide a source of electrons to create ions in the gas (see step 802). Other sources of electrons or ions may also be used, such as UV light, corona discharge, dielectric barrier discharge, or insulating barrier discharge. By changing the frequency of the oscillating RF electric potentials on the electrodes and by changing the voltage of the oscillating RF electric fields with control circuitry 508, specific ions can be trapped (see step 804). By modulating the RF on and off at acoustic frequencies (see step 807), sound waves are created in the cell 500 (see step 806). The intensity of the sound waves is proportional to the concentration of the analyte that is ionized. By sweeping through the parameters of the trapping field, the cell 500 can select which ions are trapped. The Ni-63 beta source 507 may be constantly creating ions. The ion trap 500 improves the sensitivity of acoustic cells since the trap 500 concentrates specific ions while more ions are continuously being made.
As noted previously, an advantage is that light sources are no longer needed. Selectivity of ions is performed by sweeping the electrical parameters of the trap 500 with the control circuitry 508. This opens the detection to a broad range of analytes (see step 809).
Referring to
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/116,970.
Number | Date | Country | |
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61116970 | Nov 2008 | US |