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 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 Analog television has been replaced by digital transmission, and controlled fusion remains firmly entrenched in the future.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.
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:
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).
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 10 11- or a 10 12-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 10 11-Ω amplifier in the FC system was used to produce a precision of 0.2%; for smaller samples, the 10 12-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 (10 6 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.
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).
(1) K.L. Busch, Spectros. 15(6), 28–32 (2000).
(2) P.T. Farnsworth, U.S. Patent No. 1,969,399, "Electron Multiplier," filed August 7, 1934.
(3) M.L. Alexandrov, L.N. Gall, N.V. Krasnov, L.R. Lokshin, and A.V. Chuprikov, Rapid Comm. Mass Spectrom. 4, 9–12 (1990).
(4) M.G. Inghram and R.J. Hayden, "A Handbook on Mass Spectroscopy," in Nucl. Sci. Ser. Report No. 14 (National Academy of Sciences, Washington, D.C., 1954), pp. 39-41.
(5) D.C. Imrie, J.M. Pentney, and J.S. Cottrell, Rapid Comm. Mass Spectrom. 9, 1293–1296, (1995).
(6) L.M. Welsh, K.H. Berkner, S.N. Kaplan, and R.V. Pyle, Phys. Rev. 158, 85–92 (1967).
(7) J.F. Seamans and W.K. Kimura, Rev. Sci. Instrum . 64, 460–469 (1993).
(8) J.D. Thomas, G.S. Hodges, D.G. Seely, N.A. Moroz, and T.J. Kvale, Nucl. Instrum. Meth. Phys. Res. A 536, 11–23 (2005).
(9) C.E. Sosolik, A.C. Lavery, E.B. Dahl, and B.H. Cooper, Rev. Sci. Instrum. 71, 3326–3330 (2000).
(10) A.J.H. Boerboom, Org. Mass Spectrom. 26, 929–935 (1991).
(11) M.E. Wieser and J.B. Schwieters, Int. J. Mass Spectrom. 242, 97–115 (2006).
(12) I.T. Platzner, K. Habfast, A.J. Walder, and A. Goetz, "Modern Isotope Ratio Mass Spectrometry," in Chemical Analysis, vol. 145 (John Wiley, New York, New York, 1997).
(13) The Encyclopedia of Mass Spectrometry, Volume 5: Elemental and Isotope Ratio Mass Spectrometry , D. Beauchemin and D.E. Matthews, Eds. (Elsevier, New York, New York, 2010).
(14) K.L. Ramakumar and R. Fiedler, Int. J. Mass Spectrom. 184, 109–118 (1999).
(15) J.V. Cordaro, S.R. Johnson, M.K. Holland, and V.D. Jones, "Extending the Useful Life of Older Mass Spectrometers," SRNS-STI-2010-00340, available at http://sti.srs.gov/fulltext/SRNS-STI-2010-00340.pdf|~sti.srs.gov/fulltext/SRNS-STI-2010-00340.pdf .
(16) H. Kipphardt, P. De Bievre, and P.D.P. Taylor, Anal. Bioanal. Chem. 378, 330–341 (2004).
(17) R. Santamaria-Fernandez and R. Hearn, Rapid Comm. Mass Spectrom. 22, 401–408 (2008).
(18) M. Krummen, D.G. Burgess, E. Wapelhorst, D. Hamilton, and J.B. Schwieters, Thermo Fisher Scientific, Application Note 30193.
(19) C.A. Bower, K.H. Gilchrist, M.R. Lueck, and B.R. Stoner, Sens. Actuators, A: Phys. 137, 296–301 (2007).
(20) R.B. Darling, A.A. Scheidemann, K.N. Bhat, and T.-C. Chen, Sens. Actuators, A: Phys. 95, 84–93 (2002).
(21) K. Knight, R.P. Sperline, G.M. Hieftje, E. Young, C.J. Barinaga, D.W. Koppenaal, and M.B. Denton, Int. J. Mass Spectrom. 215, 131–139 (2002).
(22) A. Scheidemann, R.B. Darling, F.J. Schumacher, and A. Isakharov, J. Vac. Sci. Tech. A 20, 597–604 (2002).
(23) D.W. Koppenaal, C.J. Barinaga, M.B. Denton, R.P. Sperline, G.M. Hieftje, G.D. Schilling, F.J. Andrade, and J.H. Barnes, IV, Anal. Chem. 77, 419A–427A (2005).
(24) J.H. Barnes, IV, and G.M. Hieftje, Int. J. Mass Spectrom. 238, 33–46 (2004).
(25) M.A. Grayson, J. Amer. Soc. Mass Spectrom. 22, 1301–1308 (2011).
(26) M. Dole, L.L. Mack, R L. Hines, L.D. Ferguson, and M.B. Alice, J. Chem. Phys. 49, 2240–2249 (1968).
(27) News Feature, "Scientific Exploration: What a Long, Strange Trip It's Been," Nature 454, 24–25 (2008).
(28) http://www.faraday-cup.com/index.html|~http://www.faraday-cup.com/index.html .