MS Detectors
Just as laser eye surgery has restored fading human vision, new technologies are needed to improve ion “chemical vision” detection.
David W. Koppenaal Charles J. Barinaga Pacific Northwest National Laboratory M. Bonner Denton Roger P. Sperline University of Arizona Gary M. Hieftje Gregory D. Schilling Francisco J. Andrade Indiana University James H. Barnes, IV Los Alamos National Laboratory
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n MS, detectors are the “eyes” of the instrument. New detectors and technologies are needed to correct, improve, and extend ion detection and, hence, our “chemical vision”. This report reviews current MS detector technology and provides a glimpse of what we hope future detectors will be capable of. The technology of MS can be divided into three processes: ion generation, ion separation, and ion detection. Ion generation and separation have received significant and focused attention. The first prevalent MS ionization source, the electron impact (EI) source, has given way to a variety of other specialized ionization sources, for example, spark source (SS), thermal ionization, chemical ionization, fast-atom bombardment, secondary ion, glow discharge, inductively coupled plasma (ICP), ESI, and MALDI. Similarly, a variety of ion (mass/charge) separators have been developed, including the seminal sector-field or magnetic MS, electrodynamic techniques (2D quadrupoles, 3-D quadrupole ion traps), combined magnetic and electrodynamic tech-
© 2005 AMERICAN CHEMICAL SOCIETY
niques (FT ion cyclotron resonance [ICR]), and drift-tube or TOF devices. Ion detection technology, by contrast, has received less attention. Traditional analog (Faraday) and electron multiplier (EM) detectors have been used for decades. Advances in this technology have occurred, but they have been incremental rather than revolutionary. Very few new detection approaches have been devised. In addition, detector technology is often given brief or no mention in MS texts and reviews. Thus, it has remained an area of rather understated importance, despite the fundamental need to better “see” ions. Only a few groups have recognized the demand for new MS detectors, and only one lone special journal issue on MS detectors has appeared (1–4). Some initially promising developments, including work on electro-optical arrays, never quite took hold in the MS community (5, 6). With continued advances in ionization and separation techniques and development of evermore-sensitive methods, detector technology will need to keep pace, and revolutionary new approaches will be required.
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Considerations and needs Several varieties of MS detectors currently exist; they vary primarily according to spectrometer design and/or analytical application requirements. The characteristics of an ideal MS detector are given in the box on page 424 A. Certain of these characteristics are common to all detectors, such as high sensitivity and linear, quantitative response. However, some detectors are designed for very specific functions or applications. Many MS detectors must be able to perform at very high count rates (>106 counts/s) with a minimal recovery time. TOF instruments require detectors with very rapid readout and response. Isotope-ratio mass spectrometers require highly stable, multiple, intercalibrated detectors that count or measure several ion masses simultaneously to provide extremely high precision ratio measurements (1 count/s), and relatively olution, and generally poor performance in elemental MS (15). large linear dynamic range (104–106). SEMs can also be operat- Although EOIDs have been made commercially available, hopes ed in a dual analog and pulse-counting mode, thereby achieving that this detector would achieve electronic photoplate perforan operative dynamic range of >108. Commercial improvements mance have not yet fully materialized (16, 17). Image-current detection. Detecting ions that are trapped yet have produced devices with narrow pulse widths and pulse width distributions, good storage and operational lifetimes, and many still in motion is possible by observing induced currents on adjasizes and configurations for use in a wide variety of applications cent electrodes or pickup antennas (Figure 1i). In the case of FTICR MS, the trapped or resonating ions are detected by the and instrument designs. image currents they induce on a pair of detection plates (two of the six sides) on a cubic ICR cell. In a given magnetic field, ions with Other common detectors Daly detector. The Daly detector, an early example of an electro- different m/z values orbit at different radii and frequencies, thus optical ion detector, is a combined ion and photon detection de- generating an rf signal whose frequency is related to the m/z of vice (Figure 1c). Ions are accelerated toward a high-voltage con- the ion. Because the electronic determination of frequency can be version dynode, and secondary electrons are accelerated in the very accurate, m/z can be determined with high resolution and opposite direction toward a scintillation or phosphor screen by high mass accuracy, especially with re-measurement techniques. the same potential field. A conventional PMT detects the intense These techniques can be used because detection of image currents photon flashes. The relatively fast response time (narrow pulse is nondestructive (uniquely so among MS detectors) and detected width for a single ion) produces good time resolution, and thus ions remain in the cell; the ions can thus be re-excited and re-deN O V E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
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The ideal MS detector Analytical attributes Unity ion-detection efficiency
tected multiple times until the desired measurement parameters Simultaneous detection are met (18). These deWide mass-range response tectors can actually Mass-independent response measure 10–100 ions Wide dynamic range with long acquisition Fast response and re-measurement Short recovery time techniques, especially High saturation level when few other ions are present. Image-curOperational attributes rent detection has also Long life been used with other Low maintenance 3-D, Paul-type ion trap Easy to replace mass spectrometers, Low replacement cost and more recently with Kingdon trap-type systems (19–23). We now turn our attention to the new types and configurations of MS detectors currently under investigation by several research groups. A few of these efforts hold significant promise. High stability
Cryodetectors Cryogenic detectors operate by detecting lattice excitations in superconducting (SC) thin films. When a particle or ion impinges onto a surface, energy is deposited; heat is generated; and neutrals, ions, electrons, and photons are sputtered. With a conventional detector at a normal temperature, the temperature rise associated with this process is indiscernible. However, with a small detector at a very low temperature (
