Detecting Ions in Mass Spectrometers with the Faraday Cup

Nov 01, 2011
Volume 26, Issue 11

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 Analog television has been replaced by digital transmission, and controlled fusion remains firmly entrenched in the future.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.

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."
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 an instrument.

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:

Figure 2: Schematic diagram of a simple FC detector.
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.

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