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 daltons to a few million daltons. 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 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 ) = L m 2 U + 2 L m 2 m U U m q {\displaystyle t(U)={\frac {L{\sqrt {m}}}{\sqrt {2U}}}\ +{\frac {2L_{m}{\sqrt {2mU}}}{U_{m}q}}\ }
where L is the path length of the ions in a field-free space, Lm is the length of ion mirror, Um 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
d t d U = 0 {\displaystyle {\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. Um = U). From here it follows that
L m = L 4 {\displaystyle 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 Em in the mirror of a single-stage reflector should be
E m = 4 U L {\displaystyle 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
d t t = k d 2 t d U 2 {\displaystyle {\frac {dt}{t}}=k{\frac {d^{2}t}{dU^{2}}}} . where k is a constant depending on the parameters of the single-stage reflector.
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