Reflectron

An ion mirror (right) attached to a flight tube (left) of the reflectron. Voltages applied to a stack of metal plates create the electric field reflecting the ions back to the flight tube. In this particular design, gaps between the mirror electrodes are too large. This can lead to a distortion of the field inside the mirror caused by a proximity of metal surface of the enveloping vacuum tube.

A reflectron (mass reflectron) is a type of time-of-flight mass spectrometer (TOF MS) that comprises a pulsed ion source, field-free region, ion mirror, and ion detector and uses a static or time dependent electric field in the ion mirror to reverse the direction of travel of the ions entering it. Using the reflectron, one can substantially diminish a spread of flight times of the ions with the same mass-to-charge ratio (m/z) caused by spread in kinetic energy of these ions measured at the exit from the ion source.

Development

The idea of improving mass resolution in TOF MS by implementing the reflection of ions from a region with retarding electric field (the ion mirror) has been first proposed by Russian scientist S. G. Alikhanov. In 1973, the dual-stage reflectron utilizing an ion mirror with two regions of homogeneous field was built in a laboratory of Boris Aleksandrovich Mamyrin. Mass resolution of the reflectron measured over broad mass range is much larger than that in a simpler (so-called linear) time-of-flight mass spectrometer comprising a pulsed ion source, flight tube, and ion detector. The masses of ions analyzed in the reflectron can span from a few Dalton to a few million Dalton. Sensitivity in the reflectron used for the analysis of ions produced in vacuum by photo or electron ionization, e.g., matrix-assisted laser desorption/ionization source, can be lower than in linear TOF MS due to post-source decay - a dissociation of vibrationally-excited molecular ions (often referred as metastable ions).

Single-stage reflectron

File:Reflectron ss.gif

Schematic drawing of a single-stage reflectron.

A single-stage reflectron is equipped with an ion mirror that has a single electric field region. The distribution of electric potential along the central axis of the ion mirror can be linear or non-linear. Also, the electric field in the mirror can be constant or time-dependent. In single-stage reflectrons with homogeneous field, a zero field in a field-free region of a flight tube and the homogeneous field inside the ion mirror are separated by highly transparent (~95%) metal grid. The grid position is then referred as the entrance (exit) to the ion mirror and is used to calculate the retarding electric field. The single-stage reflector utilizing homogeneous field can be used to attain high mass resolution in cases where the variation of energies of ions leaving the ion source is small (typically less than a few per cent). Time of flight t of the ions with mass m, charge q, kinetic energy U is

$t(U) = \frac{L}{\sqrt{2U}} \sqrt{\frac{m}{q}}\ + \frac{2 L_{m}\sqrt{2U}}{U_{m}} \sqrt{\frac{m}{q}}\$

where L is the path length of the ions in a field-free space, L_m is the length of ion mirror, U_m is the voltage applied across the mirror. To find a first-order compensation condition for flight time t with respect to spread dU in ion energy U, the following condition should be fulfilled

$\frac{dt}{dU} = 0$

Assume that the kinetic energy of the ions in the field-free region equals the ion potential energy near the stop point of the ions inside the mirror (we assume that this stop point is very close to the back electrode of the mirror, i.e. U_m = U). From here it follows that

$L_{m} = \frac{L}{4}$

In practice, the mirror length should be 10-20% longer to accommodate all ions whose kinetic energy is spread over some interval.

So, the electric field E_m in the mirror of a single-stage reflector should be

$E_{m} = \frac{4U}{L}$

In case of a wider variation of dU, the relative width of the time-of-flight peaks dt/t in such a reflectron is determined by the uncompensated part of the flight time t(U) proportional to the second derivative

$\frac{dt}{t} = k \frac{d^2t}{dU^2}$.

where k is a constant depending on the parameters of the single-stage reflector.

Dual-stage reflectron

File:Reflectron ds.gif

Schematic drawing of a dual-stage reflectron.

The mirror in a dual-stage reflectron has two regions (stages) with different fields. This makes it possible to zero both the first and second derivatives of t(U) with respect to energy U. That is why dual-stage reflectrons can compensate flight times over larger variations in ion kinetic energy compared to single-stage ones. This type of reflectrons is typically employed in orthogonal acceleration (oa) TOF MS. "Classic" (Mamyrin's) design includes two highly transparent conductive grids separating regions with homogeneous fields. Mass resolution in dual-stage reflectron is mainly determined by ion scattering on the grids, the spread of kinetic energy of ions leaving the pulsed ion source, and accuracy of mechanical alignment. To diminish effect of scattering, the length of the first deceleration region should be relatively large. Ion scattering makes using triple- and further stage reflectrons impractical.

The effect of ion scattering on mass resolution in single- and dual-stage reflectrons can be diminished by utilizing polarized grid geometry.

Gridless reflectron

The general theories behind the Gridless Reflectron are "The Common Theory of Space-and-Time-of-Flight Focusing of Ion Beams in Electrostatic and Magnetic Fields" and "The Central Particle Method", developed in 70-80s of last century by a group of scientists, headed by academician Kelman V.M. at MS Laboratory, Institute of Nuclear Physics (Alatau, Kazakhstan).

The design of the first TOF MS with gridless reflection was proposed by Sapargaliyev A.A. in 1987 in his doctoral thesis.

In 1988, the first Gridless Reflectron, TOF MS-F01B, with mass-analyzer containing axially symmetric 4-electrode ion mirror, was designed and tested at Kazakh State University. The analyzer of TOF MS-F01B was designed for a molecular beam epitaxy system.

In particular, reflection from ion mirror (with nonlinear electrostatic field) was used for simultaneous spatial and temporal focusing of ion beams. The Reflectron had an ability of filtering ions by energy and had resolving power 1000 at size: length 380 mm, diameter 60mm. In 1990 TOF MS-F01B was upgraded.

Post-source decay

A post-source decay (PSD) is a process specific to the ion source utilizing matrix-assisted laser desorption/ionization and operating in vacuum. In the post-source decay, parent ions (typically of several keV kinetic energy) fragment in a process of laser-induced fragmentation or high-energy collision-induced dissociation (HE CID). Time interval suitable for observation of the post-source decay in the reflectron starts after the precursors (parent ions) leave the ion source and ends prior to the moment when the precursors enter the ion mirror. The kinetic energy of fragment ions of mass m in the post-source decay significantly differs from that of parent ions of mass M and is proportional to m/M. So, the distribution of kinetic energies for the PSD ions is extremely large. Not surprisingly, it cannot be compensated in "classic" single or double-stage reflectrons. To achieve acceptable mass resolution for PSD ions which masses typically distributed over broad mass range, these ions are accelerated to energies substantially (at least, a factor of 4 ) exceeding the initial energy of precursor ions. Use of gridless curved-field mirror or that with time-dependent field also improves the mass resolution for fragment ions generated in the post-source decay.