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INSTRUMENTATION
Advisory Panel Jonathan W. Amy Richard A. Durst G. Phillip Hicks
Donald R. Johnson Charles E. Klopfenstein Marvin Margoshes
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Harry L. Pardue Howard J. Sloane Ralph E. Thiers
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Ionization Sources in Mass Spectrometry E. M. CHAIT, Du Pont Instruments 1500 South Shamrock Avenue, Monrovia, Calif. 91016
Traditional electron impact sources are giving way t o alternative means of ionization for many fascinating special applications, Field ion izat ion, c hem ica I ionization, spa r k source ionization, surface ionization, and photo ionization are finding a place in mass spectrometry techniques in such diverse areas as forensics, toxicology, structure determination, polymer analysis, and plasma chromatography
HE MASS SPECTROMETER is a n anaT l y t i c a l laboratory for charged particles. lllass analyzers commonly in w e (magnetic, time of flight, quadrupole, cycloidal, etc.) all derive their utility from their ability to separate ions which reflect the nature of t’he sample introduced into the spectrometer. The source of these ions is of primary importance and must be considered as the “heart of the mass spectrometer.” Often the success of a mass spectrometric analysis depends upon the type of ion source chosen. The mode of production of ions often determines if they mill indeed be represcntntive of the sample in question. .inalyzed ion signals, positive or negative ions, originating from a variety of ion sources may be interpreted by tlic chemist to yield such vital information as molecular structure, analysis of a gas mixture, identity of a gc effluent, trace dopants in a semiconductor, isotopic composition of a lunar rock-in fact, information on any form of matter that can be ionized in the mass spectrometer vacuum.
Ion Source as a Chemical Reactor
I t is attractive for the chemist to think of the mass spectrometer’s ion source as a chemical reactor, for in some way a chemical change occurs in the sample when it becomes a n ionized
plasma prior to injection into the mass analyzer. This analogy is easily understood in the case of the commonly used electron ionization source. Here the reagent is the stream of 70 ex7 electrons creating excited molecular ions of a complex molecule via a Franck-Condon process. The ensuing chemistry of unimolecular ion decompositions produces the fragment ions, rearrangement ions, and metastables we have learned to interpret to obtain molecular structure. I n the chemical ionization source, a different kind of chemistry may be esploited. Ions resulting from the electron bombardment of methane a t high pressures (1 torr) may be used as a reagent for reaction with a neutral sample gas in bimolecular chemistry. The products of these ion-molecule reactions provide additional information on the nature of the sample. Each ion source is a different kind of chemical reaction vessel and must be chosen as appropriate to the sample-be it a steroid or a semiconductor. Comples instrumentation technology has resulted from the development of multifarious ion sources (Figure l ) , while at the same time ionization of materials for mass spectral analysis remains a challenge for future development of instrumentation. This discussion will elaborate on both the present
technology (1, 2 ) and future challenge of ionization sources in mass spect romet ry . Requirements for Ion Sources Monoenergetic Ion Beam. Ion sources not only have the function of producing ions but must also accelerate the ions into the mass analyzer for eventual detection. T o insure maximum resolving power of the spectrometer, precautions are taken that the ions have a small spread of kinetic energies prior to acceleration. -411 ion sources are ion optical devices and contain lens elements which limit the energy spread of the ion beam. Ion ,sources should produce all masses without discrimination so that there is equal probability for both high and low masses being injected into the analyzer. Even so, some ion sources, as in the case of spark sources for analysis of solids, produce a large spread of energies in the ion beam and are usually used with double focusing mass analyzers which filter the ions according to energy prior to mass analysis so that high resolution may still be obtained. Ionization Eficiency. The ionization efficiency of a source must be high so that a large proportion of the neutral sample particles presented mill become ions to be analyzed and detected
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
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Instrumentation
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Gases liquids & solids inorganic which can be made volatile
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Same as for E I solids by fielddesorption
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Instrumentation
Inorganic solids lnon-volatilel
These are related phenomena
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Gases liquids solids non-volatile inorganic and organic solids
Same as for E I non-volatile solids with laser
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Conventional E I source (positive &negative inns)
Thermal ionization source
Dual FI/EI source
Gaseous discharge
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Ionization & appearance poten-
Instruments far study of desorp-
Low voltage arcs (dual plasmatron)
Monochromatorsource
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Chemical loniza-
Vacuum spark
tion
Cold electron sources
Figure 1. Table of ion sources
as a mass spectrum. The importance of high efficiency is accentuated in the everyday requirements of modern mass spectrometry for analysis of nanogram quantities of natural products and ppb impurities in semiconductors. An ion beam current of 1&10 A is a desirable source output, commonly used ion detectors having limits on the order of A for electronic amplifiers and 10-ls -4for electron multipliers. Vacuum. Vacuum pumps of sufficient size must be provided to create a suitable low-pressure environment for ionization to take place, so that ions a r e not neutralized by collisions. The vacuum required is usually 1 0 - 5 to 10-7 torr. I n cases of trace determination such as in the spark source mass spectrometer, much higher vacua are required, and this may require the use of ion pumps or cryogenic pumping methods. I n electron impact mass spectrometers, the vacuum is also required to preserve the hot filament (tungsten at 2300OC). I n the case of sources in which ion 78A
molecule reactions occur in the ionization chamber, as for chemical ionization ( 3 ) ,very high-speed pumping systems on the order of 1000 l./sec are necessary to maintain the requisite low pressure surrounding the source. A good ion source vacuum also ensures that there is no cross contamination between successive samples. This is particularly important in combinations of gas chromatography and mass spectrometry where new samples are continually being introduced to the ion source on a second-to-second basis. T o improve ion source vacuum conditions, such innovations as differential pumping have evolved, permitting a high pressure to exist inside the ionization chamber for maximum ionization efficiency and sensitivity while the surrounding source housing is at lower pressure. Pressure differentials on the order of factors of 10-100 are common. Materials and Construction. The materials and construction for ion sources utilize the same considerations as in any good vacuum system prac-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
tice, most commonly used metals for source construction being nichrome, monel, and stainless steel. Accurate machining and assembly are prerequisite to the ion optical performance required, and a t the same time the metals must not be catalytic or reactive so as t o change the sample prior to the ionization process. Evolution of Modern Ion Source In the pioneering work of Thomson
and Aston at the Cavendish Laboratory in England, the requirements were met in a simple fashion by an electric discharge tube as the source of ions. The voltage of 10-50 kV used to sustain the gaseous discharge between the electrodes in an atmosphere of the sample gas also served to accelerate the ions generated through a collimating tube fashioned from a hypodermic needle. S o t only did the source produce an intense beam of ions, but the useful resolution of the spectrometer was assured by the crude collimating device to compensate the Iarge energy
Instrumentation
spread of the discharge. These simple beginnings had their limit'ations. The discharge was unstable, samples were limited t o gases, and the energy spread limited the spectrometer resolution. Further work by Demster, Bainbridge, and S i e r produced the modern electron impact, source which made mass spectrometry a versatile analytical technique. This source became the standard because of simplicity and ease of operation, Extensive libraries of clectron impact spectra have been recorded. Electron Impact Ionization
The electron impact ion source is the most commonly used and highly developed ionization method. Molecules in the gas phase are ionized by collision with energetic electrons. This results in a Franck-Condon transition producing a molecular ion, a n odd electron ion usually in a high state of electronic and vibrational excitation. This excitation and its distribution over various modes of decomposition of the molecule determine the resultant fragmentation pattern. Samples. The electron impact mass spectrometer is applicable to any volatile sample. Permanent' gases, liquids, and solids all may be analyzed. I n most cases a heated reservoir is sufficient to volatilize gases and liquids. Samples may be the gaseous effluent of a gas chromatograph in a gc/ms syotem. Solid samples may be volatilized from a heated probe (-160 to 900OC) for organic or inorganic solids or from a Knudsen cell ( u p to 23OOOC with electron bombardment heating) for inorganic solids. Gas pressures in the ionizing region may be from 10-2 to 10-7 torr, or lower with high-sensitivity sources. Description and Principle of Operalion. Necessary elements for the construction of the source are the electron producing filament, a tungsten or rhenium wire, and a n electron t r a p or anode so that a beam of electrons may be established through the ionizing region. I n addition, t,here are highyoltage electrodes for accelerating t,he positive ions generated b y electron impact, collimating slits, and other electrodes to focus the ion beam for optimum resolution. The details of a typical source are shown in Figure 2 . Electrons are produced b y thermionic emission from the electrically heated filament and are accelerated a t n selected energy (usually 70 eV) b y a potential drop between the filament and anode. To produce molecular ions, the energy of the electrons must be p r a t e r than the highest ionization potential likely to be experienced for the ,sample gas. For maximum ionization
efficiency it has been experimentally determined for most substances that an electron energy of 70 eV is practical. Not only are molecular ions produced for many compounds a t 70 eV, but various fragment ions as well. Typical currents of electrons produced in the source are 100-200 pA, which allows about one in a million molecules t o become ions. Ion beam currents are on the order of 10-7 to l@14 A. A narrow energy spread of the ions is ensured by maintaining the ionizing region in a relatively low electric field established by the ion repellers (1-10 V/cm). Ions produced have different kinetic energies because the electron beam is not monoenergetic and has a finite width. When the ions are drawn out of the ionization chamber perpendicular to the electron beam into the accelerating region, these differences in position of ionization reflected as differences in kinetic energy will be small compared t o the accelerating voltage of several kilovolts. Segative as well as positive ions can be generated by electron impact. Electron capture processes are the usual mode of formation of negative ions. Many studies of negative ion mass spectrometry use conventional electron impact sources with the acceleration potentials reversed. I n addition to being a very general tool for obtaining mass spectra for a wide variety of samples, the electron impact source is extremely useful in obtaining fundamental physical chemical data. Since the energy of the bombarding electrons may be varied, the ionization potentials of molecules and the appearance potentials of fragments may be determined by producing the ionization efficiency curve. Special types of sources for accurate determination of ionization potentials have been developed in which the electrons are made monoenergetic b y the use of an electrostatic analyzer or a
retarding potential method. The experimental difficulties encountered in this method are great. Spectra obtained a t low ionizing voltages (
