Time-of-Flight (TOF) mass analyzers

TOF mass analyzers are not among the most popular GC-MS instruments but they offer tremendous benefits for analysis.  TOF mass spectrum acquisition rates are orders of magnitude faster than other types of mass spectrometers, and since no ions are lost as a result of mass filtering (as in quadrupoles) and most reach the detector, the sensitivity of TOF instruments approaches that of triple-quads operated in MRM mode. In other words, using the GC-TOF technique untargeted unknowns can be detected and identified by recording the full mass spectrum at sensitivity close to that of a triple-quad operated in the most sensitive mode but for targeted analysis.

The TOF is the only analyzer capable to acquire data fast enough (up to 50 spectra/sec) for the GC×GC  separations.

In our laboratory, TOF mass analyzers are available in Leco's Pegasus 4D BT and Agilent QTOF 7200 instruments.

Principles of TOF operation

Ions, generated in the ion source, are transferred through the interface of ion optics into the orthogonal accelerator and are mass analyzed. This means that the ions are sent in a direction perpendicular to their flight from the ion source (and/or through the quadrupole and collision cell in QTOF). The orthogonal direction minimizes the effects of ion generation and transmission on mass measurement because any unwanted ions are pushed or ejected from the direction of travel by pulsed voltages applied onto deflection plates.

The nearly parallel beam of ions passes into the time-of-flight ion pulser. The ion pulser is a stack of plates, each one (except the back plate) with a center hole. The ions pass into this stack from the side just between the back plate and the first plate with its center hole. To start the flight of the ions to the detector, a high voltage (HV) pulse is applied to the back plate. The applied pulse accelerates the ions through the stack of pulser plates, acting as a rapid-fire ion gun. The charged plates of the pulser send the charged ions through the flight tube.

In the TOF, the m/z is determined by measuring its time-of-flight (i.e. its travel time from the accelerator to the detector).

Ions are accelerated through a flight tube by an electric field of known strength. This acceleration results in an ion having the same kinetic energy as any other ion that has the same charge. The velocity of the ion depends on the mass-to-charge ratio. The time that it subsequently takes for the particle to reach a detector at a known distance can be measured. This time will depend on the mass-to-charge ratio of the particle (heavier particles reach lower speeds). From this time and the known experimental parameters, one can find the mass-to-charge ratio of the ion using the following relationships:

Assuming that d/2KE remains constant, the mass of the ion  can be determined by measuring the time it takes to fly through the flight tube.

Thus, the m/z values calculated from the time-of-flight equation give the masses of the ions directly. The time-of-flight is measured as the time between the moment when the push pulse is triggered and the moment when the signal maximum from the corresponding ion packet is detected. The time-of-flight of the heaviest ion possible for detection (m/z=1500) is approximately 50 μsec. This is the time required to acquire the complete mass spectrum of the sample ionized in the ion source and introduced into the orthogonal accelerator. In addition, the complete mass spectrum is obtained by sampling all ions in the orthogonal accelerator simultaneously. These two important features, simultaneous sampling of all ions for each mass spectrum and a very short acquisition time for the mass spectrum create the advantage that Time-of-Flight Mass Spectrometry has over other mass spectrometry techniques.

Reflectron

Ions of the same mass may leave the pulser at different positions, or they may have a range of kinetic energies depending on several factors. This spread of energy results in differences in the time-of-flight of ions with the same m/z ratio, which in turn, affects the mass resolution of the mass spectrometer.  To reduce these differences in the times-of-flight and thus improve mass resolution, an electrostatic device called a reflectron or a dual-stage ion mirror is placed in the drift region of the instrument. Ions with higher kinetic energy will penetrate further into the ion mirror, causing them to travel a further distance. This equalizes the arrival time of the ions to the detector.

Ion Detection

Ion detector in Agilent TOF

At the surface of the ion detector is a microchannel plate (MCP), which is a very thin plate containing a set of microscopic tubes that pass from the front surface to the rear of the plate. When an ion hits the front surface of the MCP, an electron escapes and begins the process of electrical signal amplification. As freed electrons collide with the walls of the microscopic tubes, an ever-increasing cascade of electrons travels to the rear of the plate. Roughly 10 times more electrons exit the MCP than incoming ions contact the surface. These electrons are then focused onto a scintillator, which, when struck by electrons, produces a flash of light. The light from the scintillator is focused through two small lenses onto a photomultiplier tube (PMT), which produces the electrical signal read by the data system. The reason for producing an optical signal from the MCP electrons is because the output of the MCP is at roughly - 6000 volts. The light produced by the scintillator passes to the PMT, which has a signal output at ground potential.

Ion detector in Leco Pegasus BT

In the Pegasus BT, a micro-channel plate (MCP) chevron stack is used for ion detection. Ions striking the MCP channel surface knock out several electrons due to the Ion-electron Emission Effect. These created electrons, called secondary electrons, are accelerated by the electrical field applied to the MCP, collide with the MCP channel surface, and create even more electrons through an electron-electron emission effect. Then new electrons are accelerated again and strike the MCP channel surface to create another generation of electrons. This process is repeated many times, creating an avalanche of electrons that result from a single ion strike. Thus, the signal from the single ion is amplified up to 107 times, which makes it much easier to detect ions in order to electronically process the resulting signal.

The amount of amplification, also known as the detector gain, depends upon the detector voltage applied across the detector and the condition of the surface of the MCP channels, that is, the ability of the channel surfaces to generate enough secondary electrons in ion-electron and electron-electron events. This ability to generate electrons may change due to surface degradation and modification from the bombardment by charged particles, adsorption of residual gas molecules, or moisture adsorption and contamination.

The degree and rate of such degradation, also known as detector aging, depends upon detector usage conditions. In order to prevent exposing the ion detector to the highly abundant ions created from the carrier gas and residual gas, all undesirable, low m/z ions are deflected by applying an electrical pulse to a deflection plate, located after the ion source. The electrical pulse has an appropriate delay and duration (set automatically by the software) after each sampling of the ion source in order to deflect all undesired, low m/z ions.