Detecting Ions in Mass Spectrometers with the Faraday Cup

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 Analog television has been replaced by digital transmission, and controlled fusion remains firmly entrenched in the future.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

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.

Detection Efficiency

An early concern in the analysis of higher mass biomolecules was detection efficiency. Electron multipliers, for example, exhibit a threshold velocity for emission of secondary ions. Ions must impact the first emissive surface with sufficient velocity for the electron cascade to be initiated. Higher mass ions move more slowly than lower mass ions; remember that the acceleration potential of the source represents the potential energy drop that is transformed into kinetic energy of the ions. Various designs incorporating higher accelerating potentials near the detector were explored. The performance of the Faraday cup in these high-mass applications, using a matrix-assisted laser desorption–ionization (MALDI) source coupled to a time-of-flight (TOF) mass spectrometer, was also explored (5). The detector response was found to be about 50 ns, sufficient for high-mass applications, and ions with masses as high as 300,000 Da were observed.

FC Applications in MS

FC applications in MS are a small subset of the broader suite of applications in charged-particle detection. Larger Faraday cups for scaled up beam instruments, such as particle accelerators (6), have larger interception surfaces and may need separate cooling systems because the beam current is so high. Developments in new designs (7,8) are continually reported in the research literature, reflecting specialized applications, miniaturization, improved ion modeling simulations, and improved electronics and measurement processes (9).

A single FC detector can be used in a scanning mass spectrometer, or in a TOF instrument (5). Of course, multiple detectors can be used simultaneously or configured into an array. The advantages of such systems have long been clear (10), even as technology to create them lags our desire to deploy them. Consider one singular advantage of a multiple-detector system. In a scanning beam mass spectrometer, the mass analyzer is scanned to mass-select the ions that are collected at the single-channel detector, which could be a single Faraday cup. The detected signal is linked in software to the scan function to create a time or intensity file that is eventually plotted as the mass spectrum. In a dispersive-beam instrument, no scanning of the mass analyzer is required, and the mass spectrum (or selected ions within the mass spectrum) is assembled from the combined signals of multiple collectors or an array detector. Aston's mass spectrographs were dispersive instruments, with the ion signals assuming the shape of a parabola on the photographic film. Dispersive instruments with multiple FC collectors are commonly used in modern isotope ratio measurements. The mass analyzer is not scanned, but instead disperses the ion beams from an ionization source to a suite of detectors, each located at the proper position to collect the ions of only one mass (a simple schematic is shown in Figure 3). The geometry of such a device is easily calculated, given the strengths of the mass analyzing field, the physical size of the instrument, the intercept angle of the FC detectors, and the resolving power needed.

Figure 3: Schematic diagram of a dispersive-beam instrument that sends ions of different masses to separate FC detectors. In isotope ratio instruments, the mass separation between adjacent Faraday cups is usually 1 or 2 Da, and adjacent Faraday cups correspond to the various isotopes of a single atomic elemental ion.

Wieser and Schwieters (11) have reviewed the development of multiple collector instruments for isotope ratio measurements, and the applications in geological and cosmological sample analysis are simply extraordinary. Such applications require the utmost care in sample collection, sample preparation, and measurement. Proper use of certified standards and calibration procedures is mandatory and attests to the accuracy and precision of results (12–14). As an example of the robustness of an FC detection system, a 30-year old mass spectrometer was upgraded (15) to better than original performance specifications using the original seven FC collectors (one fixed and six movable collectors). Because of improved electronics, the sample loading requirement was reduced. The instrument is used for uranium and plutonium isotope ratio analysis, and the two instruments in the facility analyze 8000 samples annually. The authors note that the ion stack at the front end of the instrument requires regular care and replacement, much more so than the FC detection system.

Isotope Ratio Measurements

Isotope ratio measurements, which are founded in authoritative metrology and the statistical underpinnings for accurate and precise measurement, have been described (16) and clearly linked through calibration to standards. This field of analysis (no pun intended) represents an exact science that should serve as a quality example for all of analytical MS. The importance of standard materials for the isotope-ratio community has been emphasized repeatedly. For example, Santamaria-Hernandez and Hearn (17) described inductively coupled plasma–mass spectrometry (ICP-MS) measurements using FC detectors, comparing sulfur isotope ratios in accepted and suggested standards. It is a testament to modern instrumentation that the primary contribution to the total uncertainty in the reported measurements for sulfur in samples, such as methionine, is the stated uncertainty in the standards themselves.

A commercial manufacturer's application note for an instrument used for argon isotope ratio measurement describes strategies for high-precision measurements of argon in air samples using different available detection systems (18). The instrument described consisted of five separate Faraday cups and a compact discrete dynode electron multiplier used in ion counting mode. The electronics for the Faraday cups used either a 10¹¹- or a 10¹²-Ω amplifier. The multiplier was used in an ion-counting mode, using peak jumping from one ion mass to another. For larger samples (that is, 4.2 × 10^-13mol), the 10¹¹-Ω amplifier in the FC system was used to produce a precision of 0.2%; for smaller samples, the 10¹²-Ω amplifier produced nearly equivalent results. The ion counting system can be used for still smaller samples, but the data must be treated to correct for dead time of the counting data and the inaccuracies associated with the jump. The FC data showed a linear and robust response over the dynamic range necessary for common argon isotope-ratio measurements. Many isotope-ratio instruments are equipped with both FC and electron multiplier detectors. The gain achieved with the latter (10⁶ or higher) facilitates measurements with very low signal levels. But the simplicity and stability of FC detectors provides excellent precision in the simultaneous measurements of multiple argon isotopes.

Size and Other Parameters

How small can a Faraday cup be? The relevant metric is the size and shape (cross-section) of the ion beam that the cup is intended to collect, and the spacing between ion beams of different masses at the desired mass resolving power. Alternatively, an FC array can be used as a position-sensitive detector. Array and position-sensitive detectors will be described in greater detail in the next installment of this column. Microfabricated Faraday cups must also confront the issue of secondary electron emission from impact, and usually do so through geometry rather than the fitting of a discrete electron suppressor or use of an auxiliary weak magnetic field. Bowers and colleagues (19) describe a dense one-dimensional array of miniature Faraday cups in which 64 cups with widths of 15–45 µm are separated by a spacing of 5 µm. The cups can be up to eight times as deep as they are wide, minimizing cross talk in the separate detection channels. Similar microfabricated FC arrays have been described by others (20,21).

Photoplate detectors used for MS require the use of scanners to retrieve the integrated and recorded signal. Similarly, array detectors, including FC arrays, require an electronic readout process. Scheidemann and colleagues (22) describe a system in which only a low overhead time is used for the read-and-reset cycle, and a 99.7% ion collection efficiency is achieved for a 64-cup array. A five-decade dynamic range is also demonstrated. The array is used as a position-sensitive detector in a dispersive compact mass spectrometer with a 200-Da mass span.

Conclusion

The Faraday cup is but one of a host of different detectors used for MS (23), and FC arrays are only one type of array detector (24). The Faraday cup has been associated with MS since the first instruments were assembled and will continue to be used as a specialized detector. The robustness of an FC detector system is a consequence of its inherent simplicity. As discussed in this column, the electronics can be updated and the physical act of intercepting a charged particle beam needs no rejuvenation. As noted (and shown with a reproduced figure) in Grayson's recent article about J.B. Fenn (25), the early electrospray ionization device described by Dole (26) used an FC detector. It was the use of a retarding variable potential plate placed directly in front of the entrance to the Faraday cup that allowed the high-mass and highly charged ions to be detected, a simple and unambiguous measurement with profound implications. Simple, basic measurements are almost always at the forefront of exploration. The plasma spectrometer on the Voyager instruments (now in an interstellar place) is equipped with a FC detector. Although the gold-plated record encoded with information sent out with Voyager is well-known to the public, a recent article attests to the fact that the FC detector carries the inscribed names of those who worked on the plasma spectrometer (27). Extraterrestrials may not be able to decipher the names, but with a little study should be able to discern the purpose of the FC detector. Those of us closer to home may be interested in reading about The Faraday Cup Award (28).

Kenneth L. Busch has not inscribed his name on any hardware currently traversing interstellar space. Nevertheless, he has often been described as "spacey," given his oft-repeated prediction that preparative MS in near-Earth orbit (especially for the production of isotope-enriched or isotope-depleted materials) will be a major growth industry by 2100. When this prediction turns out to be true, he asks only for a special mention at the 150th annual ASMS meeting, and a toast with the Faraday Cup. This column is the sole responsibility of the author, who can be reached at WyvernAssoc@yahoo.com.

Kenneth L. Busch

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