Features

For large organic molecules, such as those produced in MALDI experiments, 20kV post acceleration is achieved by having the sample at 20kV and the detector at ground potential. In many experiments however it is not possible or convenient to have a sample at high potential to achieve such high post-acceleration potentials. Furthermore, for large non-organic clusters, there are calculations for MCP detectors predicting that a modest post-acceleration energy of 10kV is capable of producing very worthwhile increases in conversion efficiency, compared to having no post-acceleration. For instance, 80% detection efficiency is predicted for Cr clusters of ~5,000 m/z when post-accelerated to 10keV, compared to ~2% for clusters of the same m/z post-accelerated to 2keV. [I. S. Gilmore and M. P. Seah, Int. J. Mass Spectrom. 202 (2000) 217.]

By having a conversion dynode at -10kV, it is possible for ions of low flight energy to be accelerated in the last short section of their flight and gain a high momentum as they strike the conversion dynode.

In this detector embodiment, the rear of the multiplier chain is held at ground potential (signal is at ground potential), and the front of the multiplier stack therefore is raised to whatever gain is required. For instance, in a relatively new detector, the conversion dynode would be at -10kV, the front of the multiplier at -2.1kV and the rear of the multiplier at ground. As the detector ages with time, the front potential would be raised to be more negative, e.g. -2.5kV (and so on).

In the new detector, the conversion dynode is hand-polished, and floated to -10kV. The conversion dynode is also unusual because of its large opening of 25mm x 22mm. The entrance is gridded with high transmission mesh (91%).

The detector is mounted on a 150mm OD CF flange with feedthroughs for the 10kV post-acceleration voltage, the front of the multiplier and the signal (rear of multiplier). For applications where the TOF-MS does not have a lined flight tube, it is possible to house the detector inside an enclosed earthed shield. We recommend the use of a Kore close-coupled pre-amplifier that has been designed to fit onto the detector flange.

The signal produced by the detector is still of low amplitude, and so an extra stage of amplification is required to boost the signal to a level that can be detected by suitable electronics. This is why we supply detectors with matched pre-amplifiers. These pre-amplifiers are very fast and are ‘close-coupled’ which means that they need to be mounted directly onto the detector flange.

The pre-amplifier is designed to mount directly to the flange of the detector and provide signal amplification and 50 ohm cable drive for all time-of-flight applications. Seen is the pre-amplifier housing and the separate 10kV SHV feedthrough that brings in the post-acceleration voltage.

The connections are made in such a way as to allow fast disassembly from the flange for baking. Pre-amplifier power (±5V) needs to be supplied to a 3-pin DIN connection.

Features:

• Very high speed
• Drives standard 50 Ohm cables
• Fast de-mount from flange for baking

A measure of protection is provided at the input of the amplifier by a very fast diode, primarily to speed up the recovery from excessively large input signals that often occur in a time-of-flight experiment. However, inevitably with research equipment, unusual experiments may lead to unintended high voltage ‘flashovers’. Perfect protection from these is incompatible with very high-speed performance, so to minimise the pain associated with such events the semiconductor components on the pre-amplifier board are mounted in ultra low profile sockets and can therefore be changed relatively easily.

This pre-amplifier can have either digital or analogue outputs. Please specify which you would like at the time of ordering.