Time-of-fight mass spectrometers generally use some form of a microchannel plate detector. This type of detector has a rise time in the range of 150 ps to 1 ns, and a pulse width (FWHM) from 400 ps to 1.5 ns. Generally, one should choose a preamplifier with a rise time similar to that of the detector. Depending somewhat on the input characteristics of the subsequent analyzer, a preamplifier voltage gain in the range of x10 to x100 will be needed.

Sometimes a TOF-MS will employ a fast plastic scintillator followed by a fast photomultiplier tube (PMT). The output rise time of this combination is circa 2 ns, and the pulse width is typically several nanoseconds (FWHM). A preamplifier with a 1 ns rise time and a voltage gain of x10 to x100 is usually ideal for this application.

Scanning quadrupoles and magnetic-sector/electrostatic-sector mass spectrometers often employ Channeltrons® and electron multipliers. These detectors have typical rise times in the range of 1 to 2 ns and FWHM pulse widths of the same magnitude. A preamplifier with a 1 ns rise time and a voltage gain of x10 to x100 is recommended for this simple counting application

Most of the input noise of the preamplifier is caused by the thermal noise generated in the 50 W input load resistor,  although the first amplification stage may contribute additional noise. With proper matching of the preamplifier to the detector, this input noise should not be a limiting factor for processing signals in mass spectrometry. It is best to choose a preamplifier rise time that is similar to the detector rise time. If the preamplifier rise time is longer than the detector rise time, the resulting rise time will be limited by the preamplifier. If the preamplifier rise time is much shorter than the detector rise time, the detector will limit the rise time, and the extra bandwidth of the preamplifier will increase its input noise unnecessarily. An estimate of the resulting rise time can be obtained from

where t_rD is the rise time of the detector, t_rP is the rise time of the preamplifier, and t_rR is the resulting rise time of the combination at the preamplifier output.

To precisely determine the required gain, the amplitude of the detector output pulses can be measured with the 50 W input of an oscilloscope. The oscilloscope must have a rise time that is less than the detector rise time. Compare this amplitude to the amplitude required by the analyzer that will be driven by the preamplifier. The ratio of these two voltage amplitudes is the required gain.

With the exception of the Model 9305-P, all the fast-rise-time preamplifiers employ AC coupling in their signal path. For such preamplifiers, there is always a slight undershoot on the opposite side of the baseline following each pulse. The magnitude of this undershoot and the time it takes to return to the baseline is determined by the low-frequency roll-off of the preamplifier. With time digitizers, or in simple counting applications, the pulse repetition rate is rarely high enough for this undershoot to perturb the processing of the next pulse. But, with a digital signal averager, this undershoot shows up as an undershoot in the spectral baseline following each peak. The Model 9326-P was designed to render this undershoot negligible in the digital signal averager application by incorporating an extremely-low , <10 kHz, low-frequency roll-off. Hence, the 9326-P is recommended for use with the FASTFLIGHT series of digital signal averagers.

Additional information on preamplifiers can be found by clicking here, while a selection-guide chart can be located by clicking here.