Thermo scientific LTQ Orbitrap Velos
The Thermo scientific LTQ Orbitrap Velos is a hybrid mass spectrometer, which means that it combines two different types of mass analyzers. These analyzers act together to form a powerful instrument that is able to provide significant amount of information about molecules by using tandem mass spectrometry in conjunction with the high resolution accurate mass measurements of the molecular weight of the molecules. The tandem mass spectra can be obtain simultaneously with the high resolution mass spectra of undissociated ions on a time frame compatible with liquid chromatography. The method of choosing the precursors for tandem mass spectrometry can be done by either by pre-programming the spectrometer or by using the data itself to determine the experiments to do (This method is called data dependent analysis).
The first analyzer is a dual pressure linear quadrupole ion trap (Velos portion), this is a low resolution mass spectrometer. The ion trap is based on trapping ions using radiofrequency (RF) electric fields to form a "trap". By changing the RF and direct current (DC) voltages, the ions could be manipulated. These manipulations include isolation of a ion with a specific mass to charge ration (m/z), activation by putting kinetic energy into the ion with the specific m/z to cause fragmentation by collision with a background gas (Helium is usually in the trap) , which is collision induced dissociation. The mass analysis is done in this analyzer by selectively ejecting ions out of the trap into a pair of electron multipliers. This process is fast and allows for multiple tandem mass spectra to be obtained while a full scan is being performed in the Orbitrap portion of the instrument. This instrument allows for multiple stages of tandem mass spectrometry by selecting a fragment ion formed in a tandem mass spectra experiment, and doing the same CID experiment on the fragment. When the separation for the different stages of tandem mass spectrometer is done in the same analyzer and the different stages of tandem mass spectra are separated by time, it is called "tandem in time”. "Tandem in time" experiments are limited by the number of ions formed. While doing tandem mass spectrometer in different analyzers for each stage of tandem mass spectrometry are called "tandem in space". Because a different analyzer is needed for each stage of tandem mass spectra in "tandem in space" experiment, the number of stages is limited by the design of the instrument. The Velos uses two ion traps, one that can efficiently fragment and isolation efficiently, which is maintain at a higher pressure, another that can be used to scan more efficiently that is maintain at a lower pressure.
The second type of analyzer is the Orbitrap. which is a purely electrostatic trap, consisting of a cylindrical outer electrode and a barrel inner electrode that hold DC potentials to trap the ions in circular orbits around the inner electrodes. The ions are collected in a device called a c-trap, which is an electrostatic trap in the shape of a C. The ions from the C-trap are then injected simultaneously into the orbitrap off axis. The ions than undergo axial motion at frequency related to the square route of the m/z of the ions. The frequency of the motion is measured to provide the m/z of the ions. The instrument is capable of resolutions up to 60,000 at m/z 200. The mass range of the instrument is up to 4000.
The LTQ Orbitrap Velos is an instrument that transmits the ions from the Velos to the Orbitrap. The Velos is actually used to control the number of ions going into the Orbitrap portion of the instrument, which improves the mass accuracy. This improvement is because the mass calibration is slightly dependent on the number of ions in the trap, because of a process called space charge. While decreasing the number of ions in the cell increases mass accuracy, the dynamic range (ratio of biggest peak to smallest peak) of the instrument is decreased also. The Velos is also used to perform CID, to fragment the ions. The ion trap could take several CID spectra (at low resolution) while the Orbitrap portion is taking a high resolution mass spectrum of the full spectrum of possible precursors. In addition to CID in the ion trap, fragmentation can be done by sending the ions isolated in the ion trap through the C-trap into a set of quadrupoles that are at potential which increases the kinetic energy of ions, but a potential at the end of the quadrupoles causes the fragment ions to return to the C-Trap. The ions are than analyzed with the Orbitrap.
These capabilities are especially useful in doing LC-MS runs, by using the computer to look at the Orbitrap data to decide which CID spectra obtain. This method is called data dependent acquisition. The Orbitrap portion could also be used to get exact masses on the CID fragments formed in the Velos.