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1) Introduction 2) Ionization Region Anatomy of an IRMS Lecture 5 – Ionization Sources Reading: Nier (1947) 1) Introduction a) Last week we figured out how to get the sample into the vacuum system, now what? In order to manipulate (weigh) molecules, they need to be charged (ie, ions). This is universally true of mass spectrometers. Many different ways of ionizing a gas. b) Jobs of an ion source i) Get sample into gas phase ii) Ionize that gas iii) Collect and collimate ions into a focused beam, accelerated into the analyzer region c) Overview of ionization sources i) Electron impact. Beam of electrons strike gas molecules, knock of another electron to yield + ions. Very stable and highly efficient, but leads to substantial fragmentation. ii) Chemical ionization. Generally use an EI source to ionize a reagent gas (NH4 or CH4), then that can charge-transfer to sample gas. Soft method, limited fragmentation. Mainly used in molecular studies of unknowns iii) Plasma ionization. Generate an external ionized gas (plasma) that is very hot. Spray sample in to produce aerosol particles, molecules thermally loses electrons to become ionized. Most flexible approach for liquids, but downside is that it can be quite difficult to keep it stable. (1) Note conventional source is Ar plasma heated by radio-frequency induction (2) Also possible to generate plasma by microwave discharge; NOSAMS has developed He plasma that is ~20% efficient. iv) Thermal ionization. Sample is isolated in solution, dried onto a conductive filament. When filament is heated in IRMS, sample is thermally desorbed and ionized. The most stable way of continually introducing a solid sample matrix, so used for highest-precision geochronology. Can yield negative or positive ions by changing extraction polarity. v) Secondary ionization. Hit sample with primary ion beam (Cs+ or O-) which knock atoms out of sample matrix and ionize them in process. Need to collect and collimate ions, then transfer to MS. Can provide spatial resolution, such as used in SIMS. Or with a big beam can provide highly efficient ionization (10’s of percent), as in the graphite sputter source of AMS. All are relatively finicky sources. vi) Photoionization. Can use photons of proper wavelength to ionize molecules. Most common is laser desorption, which is effective for large biomolecules in the solid phase. Uses special matrix to assist in ionization of sample (MALDI). Can also use resonant-frequency ionization in gas phase to very selectively ionize certain components. This is how PAH’s are detected at absurdly low levels, for example in ALHXXX meteorite. vii) Electrospray ionization. Spray a conductive solution out of a high-voltage needle into vacuum. As solvent evaporates, charge is deposited on remaining aerosol particles. Very soft ionization technique, yields to multiple charge states. Standard approach for HPLC of large biomolecules. d) Relevant characteristics i) Efficiency (molecules/ion; a part of “useful ion yield”) ii) Selectivity (probability to ionize different kinds of molecules) iii) Fragmentation (hard vs soft), scrambling iv) Adducts (Na in electrospray, H in EI) v) Intensity (beam size) vi) Stability vii) Collimation and energy distribution of ions 2) Ionization Region 1 Anatomy of an IRMS Lecture 5 – Ionization Sources a) Use Nier-type EI source as example b) Ionization energy i) Control electron energy by adjusting potential difference between filament and trap. Energy transfer (thus efficiency) is maximized at around 70 eV, where the de Broglie wavelength of ionizing electrons approximately matches the bond length of molecules. Can adjust higher or lower to provide some selectivity in ionizing certain molecules. Typically adjusted over very narrow range. ii) Path length c) Path length i) The longer the path length of ionizing electrons, the greater the probability of interaction with gas molecules. ii) Physical size of source is limited by ion optics, typically ~1cm3. iii) Placing source in magnetic field causes electrons to follow field lines and be pushed in a spiral pattern. This greatly increases path length and thus efficiency. d) Open vs Closed source i) The longer the lifetime of gas molecules in source, the higher the probability of being ionized. Downside of longer residence time is temporal resolution of dynamic peaks (eg, chromatographic peaks), as well as greater probability of intermolecular reactions. Represents a fundamental tradeoff. IRMS instruments generally use a “tight” source. GC/MS instruments generally use an “open” source. ii) Managed by adjusting the conductance of the source. An “open” source provides short residence times and low sensitivity, “closed” source has higher gas pressure, longer residence time, higher sensitivity. (1) Some older instruments included a VISC (variable ion-source conductance, or ‘sulfur window’); essentially a window that can be manually opened or closed. 3) Extraction and collimation a) Extraction. Two fundamental approaches are to push ions out (repeller lens) versus pull them out (extraction lens). Both rely on putting the ionization region into an electrical gradient, such that positive ions are accelerated “downhill” and out some exit aperture. IRMS sources generally use extraction lens. Note that the extraction lens does not have to be at negative potential relative to ground, just lower than ionization region. Entire source is held at some positive voltage to accelerate ions towards entrance slit which is at ground. b) This process imparts the kinetic energy to the ions, determines the energy of the primary ion beam. i) Higher energy beams provide better characteristics for focusing and transmission, less scattering, as well as better detection. For us main things are higher transmission, better abundance sensitivity. Typical range for IRMS is 3-10 kV. Note that a singly charged ion would then have translational kinetic energy of 3-10 KeV. Many benchtop MS instruments are only 1-2 kV. Accelerators (like AMS) often use energies of MeV. ii) Traditional in physics to talk about instruments in terms of ion energy (as the limiting characteristic). There is still some holdover of this in IRMS, a 10KeV instrument is better than a 3 KeV… For most mass spectrometry, resolution and speed are way more important than energy. iii) Drawback of higher energies is that you have to generate and insulate higher voltages, which increases the size and cost and complexity of everything. This is already a challenge at 10KV. One thing to look at in our ion source is how components are electrically isolated. c) Second important characteristic is energy distribution of ions. Ions forming at different points in the ion source will be accelerated across a different distance in the extraction field, and so will have different kinetic energy. Typically end up with ~Gaussian energy distribution. Since analyzers separate ions based on mass + charge, a broader range of energies will lead to broader 2 Anatomy of an IRMS Lecture 5 – Ionization Sources “peaks” in the analyzer, thus a narrow range is better. Very important to both mass resolution and accuracy. Other tricks to reduce energy spread of ions. d) Extraction produces a cloud of ions moving in same general direction, but with divergent velocities. For the analyzer, we need them to all have same direction. Known as “collimation”. i) Collimation and focusing of ions is accomplished by a series of ion optical elements. Simplest one is an ‘electrostatic lens’, also called ‘Einzel lens’, which is 2 or more thin metal disks with hole in the center. Difference in voltage between lenses causes a shaped electric field that will bend the path of ions without altering their energy. Very much analogous to optical lenses. ii) Other types of lenses exist – cylindrical, quadrupole, magnetic, etc. Some appear in Ultra/Neptune, not in simple IRMS instruments. 4) Ion Beam Intensity a) We now have a collimated and focused ion beam. The flux of ions in this beam is referred to as “intensity”. Jargon is “bright” or “dim”. Since precision of isotope-ratio measurement is a function of how many ions you can count, we generally want more intense ion beams. b) Problem is that in a more intense beam, have a higher density of charged particles. Electrostatic repulsion tends to push them apart, such that there is a practical limit on how dense the ions can be. Leads to general requirement that more intense ion beams are bigger diameter, which means everything downstream must be bigger. Biggest hurdle is the size of the magnet, which gets exponentially more expensive as size increases. c) Ion flux is conveniently measured as charge/time. For singly charged ions, reduces to units of current. i) Typical range is from mA (accelerator) to nA (IRMS) to pA (typical GC/MS). 1 nA corresponds to ~109 ions/second. ii) Note that we typically refer to intensity in units of Volts, but this depends on the (arbitrary) conversion of current to voltage in the detector. Much better approach is to use units of current. Will see how to do this conversion in a later lecture. 3 .
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