Mass spectrometers are used in a wide variety of applications. For example, mass spectrometers are used to analyze organic materials, such as pharmaceutical compounds, environmental compounds and biomolecules. Mass spectrometers have found particular applicability in DNA and protein sequencing. In these and other applications there is an ever-increasing demand for mass accuracy, as well as comparatively high resolution of analysis of sample ions by the mass spectrometer.
Time-of-flight mass spectrometry (TOFMS) involves accelerating ions through the same potential using an electric field. The TOFMS then measures the time of travel (i.e., flight in the electric field) of the ions to a detector. If the ions are of the same charge, their kinetic energy is the same and their velocities depend on their mass. Thus, the particles of differing masses may be resolved based solely on their velocity and hence, their time of flight. TOFMS devices have a resolving power on the order of 103 to 104. However, in many applications a higher resolving power is desirable.
Fourier transform ion cyclotron resonance (FTICR) MS, measures mass by detecting the image current produced by ions cyclotroning in the presence of a magnetic field. Instead of measuring the deflection of ions with a detector such as an electron multiplier, the ions are injected into a Penning trap (a static electric/magnetic ion trap) where they effectively form part of a circuit. Detectors at fixed positions in space measure the electrical signal of ions which pass near them over time producing cyclical signal. Since the frequency of an ion's cycling is determined by its mass-to-charge ratio, the frequency can be deconvoluted by performing a Fourier transform on the signal. FTICR MS has the advantage of high sensitivity (since each ion is ‘counted’ more than once) and much high resolution and thus precision FTICR mass spectrometer provides a comparatively high resolving power (on the order of 106).
While FTICR MS provides comparatively high resolving power, these types of mass spectrometers are comparatively complex, large and expensive. Notably, FTICR MS devices require a rather large magnetic field and magnetic. These magnets may be superconducting magnets requiring sufficient cooling to achieve and maintain superconducting conditions. As such, not only is the size and complexity of the FTICR MS great, the capital and operating costs are often high. Alone of in combination, these factors render the FTICR MS impractical in many applications.
Another type of mass spectrometer that has garnered attention recently is known as an orbitrap. In a known orbitrap, ions are electrostatically trapped in an orbit around a central, spindle-shaped electrode. The electrode confines the ions so that they both orbit around the central electrode and oscillate back and forth along the central electrode's long axis. This oscillation generates an image current in the detector plates which is recorded by the instrument. The frequencies of these image currents depend on the mass to charge ratios of the ions in the orbitrap. Mass spectra are obtained by Fourier transformation of the recorded image currents.
Like FTICR MS devices, orbitraps have a comparatively high mass accuracy, a comparatively high sensitivity and a good dynamic range. However, unlike the FTICR MS, the orbitrap does not require a large magnet and ancillary equipment.
While promising, known orbitraps normally comprise machined components to generate the electric fields for the trap. Once made, these components cannot be easily altered or tuned after fabrication. As such, if there are manufacturing inconsistencies or flaws, for example, little relief is available and the manufacturing yields are adversely impacted. Moreover, if the geometry of the machined pieces is flawed the electric field may be irreparably flawed. This results in poor performance.
In addition to the noted shortcomings of known orbitraps, ion injection can be rather difficult. In particular, in known orbitraps, the outer conductor has tapered ends and is maintained at a particular voltage, which may be rather large. Ions injected into the orbitrap from the tapered ends can be ejected by the large field created by the outer conductor. Thus, many known orbitraps require elaborate ion optics and ion injectors to introduce the ions into the trap effectively. Clearly, these injection facilitating devices can be costly and can add to the complexity of the orbitrap MS.
There is a need, therefore, for resonator structure and filter that overcomes at least the shortcoming of known optical encoders discussed above.