14 Spectroscopy 21(7) July 2006 www.spectroscopyonline.com
Mass Spectrometry Forum Electron Ionization Sources: The Basics The ion source is the heart of the mass spectrometer. In the ion source, ions are created from gas-phase neutral sample molecules, or preformed ions are extracted from solu- tions, and then sent into the mass analyzer of the instrument. The electron ionization (EI) source was the first source widely used for organic mass spectrometry (MS), and design, development, and optimization of this source are all the result of the work of many of the early pioneers of MS.
Kenneth L. Busch
he sensitivity of a mass spectrometer (the source and optimization are overlooked in the fact that the EI combined with the remainder of the instrument, but source is so common. Lack of current research literature on Tsometimes also the source itself) is defined strictly in the design of the EI source reflects the long history of suc- units of C/ g, where C represents the charge in coulombs cessful development of this source for organic analysis, the carried by ions that can be created from 1 g of sample in- tendency for design studies to be completed now in con- troduced to the source. The sensitivity of any MS measure- junction with instrument manufacture (and therefore to be ment is predicated first upon instrument sensitivity (includ- proprietary), and the attention given to new ionization ing factors such as source performance, mass analyzer methods. However, more specialized applications of the EI transmission, and ion detection efficiency), but also ex- source often require revisiting these fundamental design pa- pands to include sample preparation, signal-to-noise dis- rameters; examples of these applications include portable crimination, and matrix effects in real samples. The central instruments and instruments for analysis in extreme envi- position of source performance in this scheme should be ronments. Importantly, an appreciation of the factors con- clear. For EI sources, source design and operational details sidered in the design of the EI source, the balances reached directly affect source performance. It is useful then to in accord with the transfer of sample molecules into the broadly consider the 50-year history of the EI source in source, and the transfer of ions out of it informs our de- terms of sensitivity, and its optimization, but also to include signs and evaluations of other ionization sources. In this all the other factors that go into the design and construction column, we begin a brief overview of some of these factors of an ion source. and balances; then in the next column, we revisit some of We begin with the essential elements of the EI source. Es- the “older” scientific literature and hopefully enlighten the sential components of an EI source assembled into the basic understanding of source design for mass spectrometers. layout can be found in schematics in introductory texts or The basic function of an EI source is to create ions from manufacturer’s literature on the web. Often it seems that neutral gas-phase sample molecules M, according to the short shrift is given to the EI source, and details of design equation: www.spectroscopyonline.com July 2006 21(7) Spectroscopy 15
2 ( W/kT) the minimum amount of energy re- js = A T e o → . M e filament M + e mol- quired to remove an electron from the ecule e filament surface of a metal. The relationship where jS is the current density of the between work function and tempera- emission (mA/mm2), A is Richardson’s where the electrons causing molec- ture is described in “older” research constant (approximately 1202 ular ionization are emitted usually literature (3,4) and is summarized in mA/mm2 K2), T is temperature (K), W from a heated metal filament. In most the Richardson–Dushman equation is the work function of the cathode EI sources, the filament is made of (5,6), which relates the emitted cur- material (J), and k is the Boltzmann tungsten or rhenium, fabricated as a rent density to the work function (W) constant (1.38066 10 23 J/K). wire, a coil, or sometimes a ribbon. and temperature (T) of the emitting The filament operates in a vacuum Figure 1 is a photograph of such a fila- material: such that the heated metal does not ment, and the coiled structure can be immediately oxidize, become brittle, clearly seen. The coil (or multiple rib- bons) provides a greater surface area from which thermionic emission of electrons can occur. Specialized mate- rials that have been used, often in the higher operating pressures of residual gas analyzers, include thoriated irid- ium (thoria coated onto a supporting iridium wire) and yttriated iridium. Other specialized applications demand other filament materials; these are compiled in Manura’s authoritative texts on the subject (1,2). Tungsten is cheapest, but it produces carbon diox- ide by reactions of hydrocarbons at the heated metal surface. This is not usually an issue in organic MS, which usually does not scan to that low of a mass or attach high interpretive value to the ion at m/z 44, but the interfer- ence is something to be aware of in other applications. Rhenium filaments produce carbon dioxide to a lesser ex- tent due to the lower work function of rhenium and the resultant lower oper- ating temperature. However, rhenium filaments emit stable rhenium oxide negative ions that can be a potential mass interference in some analyses. The filament is heated by a robust power supply that passes current suffi- cient to heat the filament to its operat- ing temperature. Because the power supply provides 3–4 amperes at 70 V relative to ground (more on that later), great care must be taken to avoid personal contact with the out- put. The current passage raises the temperature of the filament to a point at which the work function of the heated metal is exceeded, and elec- trons are then released from the sur- face of the metal in a process of thermionic emission. Remember that the work function is defined simply as Circle 15 16 Spectroscopy 21(7) July 2006 www.spectroscopyonline.com
stant. This period is reached when a balance is achieved between heat gen- eration due to the passage of the fila- ment current and loss of heat to the source block. The current passed through the filament heats not only the wire itself, but through radiation and some convection, it also heats sur- rounding metal. The source block is the metal “container” into which gas- phase sample molecules and electrons are introduced and contained, and from which ions are extracted into the mass analyzer. The internal volume of a typical EI source block is on the order of 1 mL; for miniaturized in- struments it can be less. It is instruc- tive to calculate the mean free path of an ideal gas at the pressure and tem- perature at which the EI source oper- ates and compare that value with the dimensions of the source block itself. The source block is heated by radiative emission from the filament and by dedicated source heaters as well to minimize the time required to reach an equilibrium operating temperature and to maintain source cleanliness. The normal operating temperature is 180–220 °C, depending on what sorts of samples are being brought into the source. Why is the source block heated? Re- call that hot metal often catalyzes or- ganic sample decomposition, so it Figure 1: Photograph of a coiled filament used in an electron ionization source. Look closely to seems that this would be a bad idea see the ion burn behind the filament. Cropped from an original photograph taken by Frederick from the start. In this case, however, Strobel, Manager of the Mass Spectrometry Facility at Emory University, and used with the source is heated to prevent con- permission. densation of gas-phase sample mole- cules onto the “cold” metal walls, lead- ing to source contamination through and fracture, much as an incandescent columns, the pressure of the reagent sample cross talk, as well as a generally lightbulb filament will burn out in a gas in the source is about 1 torr. Resul- “dirty source” through condensation few seconds if the surrounding glass tant changes in basic design and oper- of the background organic vapors al- envelope that maintains the inert at- ation of the source, including the fila- ways present in the instrument. Sam- mosphere is breached. The equation ment, to accommodate this higher ple cross talk occurs when the sample that describes thermionic emission as- operating pressure are discussed in an molecules adsorb on the surface, re- sumes emission into a vacuum, and so upcoming parallel series of columns main there for some time that be- the fit of theoretical equation to ex- on the design and operation of the comes similar to the time between the perimental results is good (subject to CI source. introduction of the subsequent sam- corrections such as the Schottky ef- Stable electron emission from a ple, and then desorb from the surface. fect). Remember that the pressure of metal filament (as clear from the The ideal situation is that neutral gas- sample molecules in the EI source will equation above) requires a few min- phase sample molecules are intro- be about 10 6 torr, or about the same utes of operation to reach a thermal duced into the source and are as the base pressure of the instrument “equilibrium,”or more accurately, a promptly ionized or pumped away out itself. In a chemical ionization (CI) constant temperature at which the flux of the source, with no residue. When source, as you recall from previous of emitted electrons becomes con- sample molecule introduction is via a www.spectroscopyonline.com July 2006 21(7) Spectroscopy 17 gas chromatograph, and peaks are potentials of the surrounding block. texts might indicate. The filament can only a few seconds wide and often Because the electrons are accelerated be inside the source block, or, to facil- closely spaced, sample cross talk can through a potential of 70 V, their ve- itate replacement when the filament decrease the apparent separation res- locity can be calculated. As described eventually fails, the filament is usually olution of the chromatography. previously, a 70 V kinetic energy for physically located outside the source After electrons are emitted from the electrons represents a plateau re- block, attached to it by insulating the filament, they must pass through stand-offs. Small holes are drilled in the internal volume of the ion source, the metal of the ion source block be- (hopefully) ionizing any gas-phase Designers of tween the filament and the interior neutral molecules found there. Ide- (sometimes called the ion volume). ally, the electron beam should uni- instruments and ion From an inside-out perspective, the formly illuminate the entire source sources want to electron entrance holes are uniformly volume. Thus, the bigger the emitting illuminated with electrons, which surface, the more likely that this ideal increase the chances enter the interior volume of the ion situation can be realized. In classical of electron–molecule source with 70 V kinetic energy. In electron ionization, the source fila- some designs, a V-shaped repeller ment (and its power supply) is held at interaction. plate located behind the filament fur- 70 V relative to the ground poten- ther increases the flux of electrons tial (zero volts) of the metal block of emitted from the filament that enter the ion source. An imposed accelera- the source. An independent variable tion potential between the ion source gion in which small changes in the voltage is determined empirically to and the mass analyzer only shifts the velocity do not lead to large changes maximize the production of ions frame of reference, but does not shift in the ionization efficiency for from the source under the usual oper- the filament–source potential differ- organic molecules. ating conditions. ence. Simplistically, electrons released Of course, the design of the source The filament current is measured from the filament are repelled from is usually a bit more complicated than at a collector located on the side of the wire and attracted to the ground the simple diagrams in introductory the ion source opposite from the elec-
Circle 16 18 Spectroscopy 21(7) July 2006 www.spectroscopyonline.com tron entrance holes. The collector can forward first approach that simply ad- headspace application, longer times be biased at a slight positive potential vocates more electrons. might be appropriate. Further, we or can itself operate at ground poten- Optical spectroscopy will recognize would need to know whether the ion tial. The collector does not intercept the third approach as the EI source current is specified as measured for all electrons that enter the source, and equivalent of increasing the optical the sum over a mass range, or even for the number of electrons collected will path length in an absorbance cell. a particular ion. For methyl stearate also reflect the ionization process Electron mirrors can be configured, and EI, the molecular ion of the com- (equation 1). But because the geome- and multipass electron paths can be pound at m/z 298 usually is specified. try is constant, the fraction of elec- used. But a simpler and more elegant Given the molecular weight and Avo- trons collected is fixed, and the proto- approach is to use a source magnet. gadro’s number, the actual fraction of col is such that for standard EI This is not the magnet of the mass an- neutral sample molecules converted operations, a collector value of about alyzer in a sector mass spectrometer. into molecular ions can be calculated. 150 A is set. Usually this set value is The source magnet is a small, perma- We will start with that answer at the part of a feedback circuit that changes nent magnet fitted to the source that beginning of the next column. the current driving the filament to causes the electrons to follow a spiral maintain a constant collector current. References Designers of instruments and ion (1) J.J. Manura, The Mass Spec Source sources want to increase the chances But a simpler and 27(1), 3–4 (2005). of electron–molecule interaction. We more elegant (2) http://www.sisweb.com/portal/ms-fil- can accomplish this by increasing the ament-wire.htm number of electrons, the illuminated approach is to use a (3) S. Dushman, Rev. Mod. Phys. 2(4), volume, or the electron path length source magnet. This 381–476 (1930). inside the ion source. These possible (4) P.A. Redhead, J. Vac. Sci. Technol., A approaches must be balanced against is not the magnet of 16(3), 1394–1401 (1998). the general rule that the EI source the mass analyzer in (5) A.L. Smith and R. Breitwieser, J. Appl. should be compact. In a compact Phys. 41(1), 463–467 (1970). source, gas-phase samples are intro- a sector mass (6) Owen Willans Richardson was duced and removed in a short time, spectrometer. awarded the Nobel prize in Physics in concordant with sequential sample 1928 for his work on thermionic peaks introduced from a gas chro- emission. Saul Dushman was a re- matograph in which the peaks may be path within the ion volume, effectively search scientist at the General Electric only a few seconds wide and a few sec- increasing the path length and increas- Company. The Nobel prize lecture of onds apart. A compact source is easier ing the chances for sample Richardson is found here: http://no- to pump and easier to design as a molecule–electron interaction. The belprize.org/physics/laureates/1928/r point source for extraction of ions strength of the magnet is such that ichardson-lecture.pdf. into the mass analyzer. Finally, and not electron trajectories are suitably to put too fine a point on it, a compact changed but any of the more massive source is smaller. In some applications, molecular ions formed (remember size is critical, but in others, it is sim- that the mass of an electron is much ply the associated scaling with materi- smaller than the mass of a proton) are als and power supplies that becomes not perturbed, and sample ion extrac- the balancing factor. With any given tion is not affected. size of ion source, only a certain flux We end this column with a funda- of electrons can be achieved based on mental consideration of the efficiency filament design and ion optics (includ- of an EI source. We consider an EI ing space charge, a topic covered in a source sensitivity specified as 1 10 7 previous column). So while electron C/ g of methyl stearate introduced Kenneth L. Busch learned to spot-weld fila- flux can be increased, the flux that into the source (this is a reasonable ments onto support posts leads to a collector current of 150 A number for instruments of moderate at an early age, and remembers why the represents a balance of many factors. performance). Detail-oriented readers chemical symbol for The second approach, to increase immediately ask about the time asso- tungsten is W, and the associated debate about the illuminated volume inside the ciated with that sample introduction. the name and the symbol. source, has been implemented using a Is it 1 s? 10 s? 100 s? These times repre- He also remembers his first musings about the chemical composition of the ion higher number of electron entrance sent the period over which the ion burns found in an electron ionization apertures or using multiple filaments. current produced is integrated. In gas source; those secrets remain unrevealed until a future column. He can be reached at But this approach ultimately reaches chromatography–MS, a 1-s period [email protected]. the same realistic limits as the straight- would be desirable, but for a static