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MCP based bipolar detection of high masses using 20kV post acceleration

Photo of 20kV detector assembly

A time-of-flight mass analyser has no inherent mass limitation, but the detection efficiency of a micro channel-plate detector assembly decreases with mass, because, at a given ion energy, heavier ions have a lower conversion efficiency to electrons. The relatively modest ion acceleration used in the standard detector assembly limits the mass range to around 5,000 amu for inorganic cluster-ions and less for large organic ions .

It is possible to increase the sensitivity for high-mass ions by using post-acceleration to increase the energy of the initial impact between ion and detector. One way to do this is to float the detector to a high voltage with respect to ground. There is then the question of managing the high electric fields created so as to avoid arcs/discharges and finding a way to couple the ion signal to a ground based detection system.

The first photograph shows the in-vacuo assembly which mounts a 25mm active area dual microchannelplate assembly from a high voltage insulator so it can be floated at up to 20kV.


Photo of air side cover

The second photo shows the grounded external safety cover. Inside are bias chains, capacitative coupling/decoupling and a fast preamplifier for driving a 50 Ohm signal cable.

To keep costs low, a single high-voltage supply is provided. A resistor chain inside the safety cover divides this voltage to provide post-acceleration and excitation for the multiplying detector. As the detector ages with time the excitation will need to be increased, which will tend to increase the post-acceleration also. Eventually, a simple resistor change is required to keep the highest voltage at no more than 20kV.

To switch the polarity of ions detected , the high voltage power supply is reversed and two leads on the detector are simply swapped.

Technical Specifications Summary

Type of Detector

High voltage Power supplies

Vacuum part

Detector Mounting



Last updated:15:12 24/07/2019

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