Francis William Aston's first mass spectrographs used photographic film to detect ions passed through the instrument, and
photoplates continue to be used for ion detection in spark-source and glow discharge ionization instruments today. However,
electron multipliers and photomultiplier detectors are installed in many modern beam instruments that are used for organic
and bio-organic analysis, providing gains in excess of 10
6. Specialized detection systems are used in instruments when, for example, precision abundance measurements or position-sensitive
detection is required. In this column, we review the Faraday cup detector for mass spectrometry.
An earlier column discussed the operation of electron multipliers used as detectors in mass spectrometry (MS) (1). You may
remember that the electron multiplier was invented by P.T. Farnsworth (2), who also invented analog television, and in his
later years, a device that claimed to provide controlled fusion. Analog television has been replaced by digital transmission,
and controlled fusion remains firmly entrenched in the future. However, the electron multiplier became an extraordinarily
useful device and is widely used in mass spectrometers. Despite this, the electron multiplier detection process is subject
to a mass-discrimination effect (3). Additionally, because the detector produces a signal for both fast-moving ions and neutral
particles, it also produces detector "noise" unrelated to the mass-selected ions.
In this column, as well as the next installment, we describe two detector systems used in MS — the Faraday cup and position-sensitive
array detectors. The Faraday cup (FC or FEC, for Faraday electrometer cup) is very simple in concept. Coupled with modern
electronics, the Faraday cup is singularly useful in producing high precision measurements in isotope ratio mass spectrometers.
Array detectors (which can include arrays of miniaturized Faraday cups) are used not only as detectors for dispersive-beam
instruments, but also as developmental tools for characterizing the position and cross section of a beam of ions as it traverses
Figure 1: Photograph of a cylindrical FC detector. The ions enter the cup through the aperture on the right. An electron suppression
plate surrounds the ion entrance aperture and keeps secondary electrons emitted from the impacted surface within the confines
of the device. Adapted from the public domain figure provided in the wikipedia entry on "Faraday cup."
A Simple Cup
The design of a Faraday cup is remarkably simple; it is indeed a cup. The metal cup (Figure 1 is a photograph and Figure 2
is a schematic) is placed within a vacuum system to intercept a beam of charged particles (electrons or ions). The charge
on each particle (approximately 1.6 × 10-19 C) is passed to the metal on neutralization of the impacting ion. The cup is an element in a circuit; the current flow through
the circuit can be very accurately measured and is directly proportional to the number of ions that have been intercepted
by the Faraday cup. A current of 1 nA in the circuit corresponds to the arrival of several billion singly charged ions per
second at the Faraday cup. Let's do the calculation, remembering that 1 A corresponds to a current of 1 C/s:
Because the detection is based solely on the charge, FC-based detectors exhibit no mass discrimination, which is an advantage
in high precision measurements. Additionally, ions of higher charge states produce a correspondingly larger signal. Errors
in the current measurement are reduced with the addition of an electron suppressor plate to the cup, as shown in Figure 1.
The suppressor plate reduces losses because of backscattering of the incident ions and also reduces the probability of escape
for secondary electrons that may be released on ion impact. Commercial FC detectors may have a weak magnetic field to prevent
secondary electrons from leaving the Faraday cup (4), and they may operate with a slight positive bias on the impacted surface
to reduce secondary electron emission. As expected, the limit of detection for a Faraday cup depends on the sensitivity of
the electrometer in the circuit that it is connected to. The current passes through a circuit resistor, and the generated
difference in voltage is measured (V = IR). Even relatively simple circuits and low cost amplifiers can provide a 10 mV signal for a picoampere of input current. Revisiting
equation 1 above reveals that measuring microvolts corresponds to a few thousand ions. Therefore, the FC detector can be used
for high sensitivity analyses. The ability to avoid a scanning mass analysis is also advantageous, as we will describe shortly.
The noise associated with the electronics necessary for "amplification" of the weak signal (usually involving a high-ohm resistor)
is compensated for by a measurement time that can last several hundred seconds.
Figure 2: Schematic diagram of a simple FC detector.