Mass spectrometers consist of a source where the sample is ionized and accelerated, a mass analyser that produces mass separation, and a collector for measuring the abundances of the ion beams. The basic construct has changed little from Dempster's 3 first thermal-ionization mass spectrometer designed for isotope abundance measurements as illustrated here: (A) analyzing chamber, (B) brass tube fixed with iron plates, (C) brass plate seal with soldered tubes for three openings to the chamber, (D) defining aperture, (E) ebonite plug with electrometer, (F) electron-emitting filament, (G) glass tubing, (H) heating filament, (S1) entrance slit, and (S2) exit slit. The magnetic field covers the analyzer tube and is normal to this section. The source allows thermal ionization from salts placed on the heating filament (H), or through bombarding salts on the heating filament with electrons from the electron filament (F).
Magnetic sector mass spectrometers are based on dispersion according to F = q(v × B). (a) The Lorentz force operates at right angles to both the velocity of the ion, and to the magnetic field, (b) ions move along circular paths with radius proportional to mass.
A double-focusing sector mass spectrometer uses an electrostatic sector to compensate for the energy (velocity) dispersion that may exist in the ion beam. Shown are (a) a forward geometry mass analyser with electrostatic sector before the magnet, and (b) a reverse geometry mass analyser, where the magnet precedes the electrostatic sector. These designs 8,9 are used for the SHRIMP II and SHRIMP RG ion microprobes.
In the quadrupole mass spectrometer, opposite elements of the quadrupole lens are provided a voltage that has a dc component (U), and a high frequency component (Vcosωt). Ions in the center of the quadrupole experience a resultant force such that only ions of a particular m/q are stable and can pass through the mass spectrometer.
Velocity-based mass spectrometers include time-of-flight mass spectrometers and the Wien filter. The ion is accelerated through an electric potential and the velocity of the ion is governed by E = ½mv2. (a) A time-of-flight mass spectrometer simply uses drift to separate ions of different velocity, with time of arrival at the detector being proportional to (the square root of) mass. (b) The Wien filter utilizes crossed electrostatic and magnetic sectors such that the electrostatic force can operate against the magnetic force. The Wien filter allows only one velocity (v = E × B) to pass through undeflected.
For a trapezoidal peak shape, generally produced in a magnetic sector mass spectrometer, the side-of-peak slope is the demagnified source slit width (s), and the 50% height is the collector slit width (c). Hence, the base width is the sum of the collector slit and (de)magnified source slit widths. The mass resolution (m/Δm) is shown for the 10% level of the peak height.
Mass resolution is a function of the mass spectrometer geometry but is also a function of abundance. (a) An example is shown based for the mass spectrum of 48Ca-48Ti that are fully separated at 10 500, but which is often analyzed at a lower operating mass resolution of 7000 m/Δm to increase sensitivity. (b) It can be seen that 48Ca does not contribute to the 48Ti signal (<0.01%), based on the overlay of the 40Ca mass spectrum, however, the converse is not true with the higher abundance 48Ti contributing significantly to the 48Ca signal (∼0.2%). (c) A Ca isotopic measurement requires higher mass resolution than a Ti isotopic measurement of the same sample if 48Ti/48Ca > 1.
Difference between mass resolving power and mass resolution as illustrated by an image of 29Si and 28SiH peaks shown in (a). These peaks are completely resolved in the image at the collector slit, which shows the mass resolving power is adequate to separate them. However, when a collector slit of the equivalent size of the peak width is convolved with this distribution, the mass spectrum shows triangular peaks (b), which are very difficult to analyze for abundance if there is any drift or noise in the mass analyser (equivalent Δm). (c) A wider collector slit must be used to maintain “flat top,” but at the expense of peak overlap.
Definition of mass resolving power. For geoscience applications, mass resolving power is generally defined as m/Δm, where Δm is defined as the equivalent mass width defined between the 10%–90% peak height levels. Δm is effectively the demagnified source slit width, and the collector slit width is ignored.
Illustration of abundance sensitivity from mass 248 (ThO+) from a monazite target. Gas scatter and lens aberrations cause tailing of intense peaks into adjacent peaks. In this example the abundance of the tail is around 10−6 at a mass offset of 1 amu. The abundance sensitivity is generally defined at a mass offset of 1 amu relative to the designated peak, but the fractional mass difference is not the same for all elements and hence the abundance sensitivity is mass dependent (and must be specified for a particular mass).
Schematic isotope mass fractionation measurement and correction. (a) Spectrum with three isotopes at masses m, m+1, and m+2, whose abundances have been normalized to terrestrial and are not mass fractionated with respect to terrestrial. (b) Mass-fractionated abundance pattern where the abundance is linearly proportional to mass, i.e., 1+α for mass m+1, and 1+2α for mass m+2. Measurement of one isotope ratio can be used to quantify mass dependent fractionation. (c) In a three isotope system, one ratio can be used to quantify mass fractionation so that a correction can be applied, and the abundance of that isotope expressed as a residual, δ.
Measurement errors from background and counting statistics for electron multiplier (EM), and Faraday cup with resistive (1010 Ω, 1011 Ω, 1012 Ω) feed back and capacitative feedback (10 pF). Shown for the Faraday cup measurements are three signal levels of 0.16, 1.6, and 16 V.
The RELAX mass spectrometer system. 21 Xenon is collected on to the cold finger by releasing the Xe into the mass spectrometer. A pulsed infrared laser is used to locally heat and release the Xe, and a second pulsed laser of ultraviolet wavelength causes ionization of the Xe. Ions are accelerated from the source and detected with a time-of-flight mass spectrometer. A feature of this system is the extremely low Xe blank level and small samples that can be analyzed (several thousand atoms) Like all static mass spectrometers, the Xe gas that is not ionized may recondense on to the cold finger where it can be processed again.
Inductively coupled-plasma mass spectrometry. (a) Upper: ICP with quadrupole mass spectrometer. An argon plasma is generated through an RF discharge. The sample (gas, liquid) is introduced to the plasma with consequent ionization of all atoms. The mass spectrometer requires an efficient pumping system to allow the ions to be analyzed in the quadrupole mass spectrometer. In the example shown, two stages of differential pumping occur through the sample cone and skimmer cone. Operation of the source at atmospheric pressure requires that the source is kept at ground potential with the detector system at high potential. (b) Lower: ICP with sector mass spectrometer (Thermo Neptune). The setup is the same as the quadrupole mass spectrometer, but is complicated simply through the size of the mass analyzer that must be floated.
A SIMS instrument (Cameca ims 3f). Primary ions (typically O− or Cs+) are accelerated (10–20 keV) and focused on to a polished sample surface (spot order of 10 μm). Secondary ions are produced by the impact of the high energy ions and are normally accelerated back to ground potential. The secondary ions pass through transfer lenses, which shape the secondary ion beam. The mass analyzer is a double-focusing sector magnet. The detector system has two alternate paths. The channel plate allows the pot to be directly imaged in any desired species. A counting system for Faraday and ion counting allows isotope abundance measurements.
SHRIMP uses a double-focusing sector mass spectrometer with only one additional lens element, a quadrupole lens between magnet and electrostatic analyser to correct second-order aberrations. 8 The key development of SHRIMP was making it large (magnet turning radius of 1.000 m) thus allowing high mass resolution and sensitivity. Illustrated is a single collector version of SHRIMP II.
The Cameca nanoSIMS uses a double-focusing mass analyzer that places the focal plane of the magnet very close to the magnet itself, and hence the collector is integrated with the magnet. The nanoSIMS uses normal incidence of the primary beam and a very shallow extraction field to produce extremely small primary ion spots on the target (down to 50 nm).
MegaSIMS consists of a combined SIMS source, and an accelerator-based mass spectrometer. A modified Cameca ims-6f source allows selection of appropriate regions of the sample surface. Initial magnetic separation allows a specific mass range of ions to be selected (e.g., oxygen isotopes) and these are then recombined into a single ion beam before passing in to the accelerator. Ions are refocused in a high energy double-focusing mass spectrometer, with a multiple collector allowing simultaneous collection of the three stable oxygen isotopes.
The MULTUM II mass spectrometer is a time-of-flight mass spectrometer with four toroidal electric sectors. Ions can be cycled repetitively through the mass analyzer for a specified duration (number of cycles) before collection with the MCP detector. The toroidal sector allows the mass spectrometer to have a drift length that is simply proportional to the number of cycles, and hence can operate with extremely high mass resolution.
Isotopologues of CO2 and mass spectrometric requirements.
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