Orbitrap

In mass spectrometry, Orbitrap is an ion trap mass analyzer consisting of an outer barrel like electrode and a coaxial inner spindle like electrode that traps ions in an orbital motion around the spindle. The image current from the trapped ions is detected and converted to a mass spectrum by first using the Fourier transform of time domain of the harmonic to create a frequency signal which is converted to mass.

In the Orbitrap, ions are trapped because their electrostatic attraction to the inner electrode is balanced by their inertia. Thus, ions cycle around the inner electrode on elliptical trajectories. In addition, the ions also move back and forth along the axis of the central electrode so that their trajectories in space resemble helices. Due to the properties of the quadro logarithmic potential, and references therein. However, if ion packets are injected away from the minimum of the axial potential (which corresponds to the thickest part of either electrode), this automatically initiates their axial oscillations, eliminating the need for any additional excitation. Furthermore, the absence of additional excitation allows the detection process to start as soon as the detection electronics recover from the voltage ramp needed for ion injection.

Axial oscillations of ion rings are detected by their image current induced on the outer electrode which is split into two symmetrical pick up sensors connected to a differential amplifier. By processing data in a manner similar to that used in Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS), the trap can be used as a mass analyzer. Like in FTICR MS, all the ions are detected simultaneously over some given period of time and resolution can be improved by increasing the strength of the field or by increasing the detection period. The Orbitrap differs from FTICR MS by the absence of a magnetic field and hence has a significantly slower decrease of resolving power with increasing m/z.

Currently the Orbitrap analyzer exists in two variants: a standard trap and a compact high field trap. In practical traps, the outer electrode is sustained at virtual ground and a voltage of 3.5 or 5 kV is applied to the inner electrode only. As a result, the resolving power at m/z 400 and 768 ms detection time can range from 60,000 for a standard trap at 3.5 kV to 280,000 for a high field trap at 5 kV and with enhanced FT processing. Like in FTICR MS the Orbitrap resolving power is proportional to the number of harmonic oscillations of the ions; as a result, the resolving power is inversely proportional to the square root of m/z and proportional to acquisition time. For example, the values above would double for m/z 100 and halve for m/z 1600. For the shortest transient of 96 ms these values would be reduced by 8 times, whereas a resolving power in excess of 1,000,000 has been demonstrated in 3 second transients. The Orbitrap analyzer can be interfaced to a linear ion trap (LTQ Orbitrap family of instruments), quadrupole mass filter (Q Exactive family) or directly to an ion source (Exactive instrument, all marketed by Thermo Fisher Scientific). In addition, a higher energy collision cell can be appended to the C trap, with the further addition of electron transfer dissociation at its back. Most of these instruments have atmospheric pressure ion sources though an intermediate pressure MALDI source can also be used (MALDI LTQ Orbitrap). All of these instruments provide a high mass accuracy (<2–3 ppm with external calibrant and <1–2 ppm with internal), a high resolving power (up to 240,000 at m/z 400), a high dynamic range and high sensitivity. and are also used in life science mass spectrometry such as metabolism, metabolomics, environmental, food and safety analysis. Most of them are interfaced to liquid chromatography separations, secondary ion and ambient ionization methods. They have also been used to determine molecular structures of isotopically substituted molecular species.