Mass spectrometry

Mass spectrometry is the technique used for determining particle abundances by their mass and charge characteristics in an evacuated electromagnetic field. Its principal uses in the earth sciences are in determining the isotope ratios of light, stable substances and in measuring the isotopic abundances of radioactive and radiogenic substances.

Early Mass Spectrometry

The years 1896 to 1906 are sometimes referred to as a “golden decade” of physics because this time period saw critical discoveries and experiments that resulted in the quantitative analysis of charged particles by mass. One hundred years ago, experiments with cathode rays led to their identification as streams of electrons by German physicists Eugen Goldstein and Wilhelm Wien and British physicist Joseph John Thomson. Using the earliest application of mass analysis, Thomson identified the two isotopes of neon, neon-20 and neon-22. This work was followed in 1918 and 1919 by Canadian-American physicist A. J. Dempster and British chemist F. W. Aston, who designed mass spectrographs that were used in succeeding years to determine most of the naturally occurring isotopes of the periodic table.

In 1896, French physicist Antoine-Henri Becquerel presented his discovery of the phenomenon of radioactivity to the scientific community in Paris. This finding was followed rapidly by the seminal work of French chemist Marie Curie in radioactivity, a term she coined. Her discovery of the intensely radioactive elements radium and plutonium led Ernest Rutherford to delimit three kinds of radioactivity—alpha, beta, and gamma—and, in 1910, led English radiochemist Frederick Soddy to formulate a theory of radioactive decay. Soddy later proposed the probability of isotopes, the existence of which was demonstrated on early mass spectrographs and mass spectrometers.

Geochronologic

Rutherford and Soddy's theory of the time dependence of radioactive decay, followed by breakthroughs in instrumentation for the measurement of these unstable species and their radiogenic “daughter” nuclides, caught the attention of early geochronologists and had a revolutionary effect on the study of geology. In 1904, Rutherford proposed that geologic time might be measured by the breakdown of uranium in uranium-bearing minerals, and a few years later, American radiochemist Bertram Boltwood announced the “absolute” ages of three samples of uranium minerals. The ages, which approximated half a billion years, indicated that at least some earth materials were much older than had been thought—an idea developed by British geologist Arthur Holmes in his classic The Age of the Earth (1913). Holmes's early time scale for the earth and his enthusiasm for the developing study of radioactive decay were not met with instant acceptance by most contemporary geologists, but eventually absolute ages would become the prime quantitative components in the field of geology.

After the early study of the isotopes of uranium came the discovery of other unstable isotopes and the formulation of the radioactive decay schemes that have become the workhorses of geochronology. The theory of the radioactive decay of the parent, or unstable, nuclide (or the growth of the daughter, or stable, nuclide) developed in the early 1900s has not changed; it is still the basis for geochronologists' measurement of time. This field is one of the arenas for the use of mass spectrometry.

Stable Isotope Fractionation

The other use of mass spectrometry in the earth sciences results from isotopes' potential to fractionate, or change their relative abundance proportions, during geological processes, for physicochemical reasons other than radioactive decay and radiogenic buildup. Fractionation not resulting from radioactive decay (stable isotope fractionation) comes about because the thermodynamic properties of molecules depend on the mass of the atoms from which they are made. The total energy of a molecule can be described in terms of the electronic interactions of its atoms and the other energetic components of these atoms, such as their rotation, vibration, and translation. Molecules that contain in their molecular configurations different isotopes will have differing energies, because of the different energy components (usually vibrational and rotational) that are mass-dependent. The total energy of molecules also decreases with decreasing temperature; at zero kelvin, or absolute zero, this energy has a finite value known as its zero-point energy. The vibrational component of energy, the most important factor in fractionation, is inversely proportional to the square root of its mass. A molecule with the heavier of two isotopes will have a lower vibrational energy and thus a lower zero-point energy than a similar but lighter isotope molecule. Other factors being equal, the chemical bonds of a molecule with lighter isotopic composition will be more easily broken than those of the heavier isotope analogue, and the heavier molecule thus will be less reactive chemically.

Geologic processes that result in stable isotopic fractionation are the redistribution of isotopes as a function of isotopic exchange; nonthermodynamic (kinetic) processes that depend on the amounts of the species present during a reaction; and a range of strictly physical processes, including diffusion, evaporation, condensation, adsorption, desorption, crystallization, and melting. Physical conditions such as these undoubtedly were much more intense during preaccretion events, such as star formation, than during more typical “geologic” processes, such as sedimentation or volcanism; consequently, fractionation effects are observable in primitive materials, such as some components of relatively unprocessed meteorites. These materials show extremely interesting stable isotope fractionation effects even among the heaviest elements.

Elements of Mass Spectrometers

As commonly used, the term “mass spectrometer” refers to an instrument in which beams of ionized isotopes are separated magnetically. The earlier, more qualitative mass spectrograph focused ion beams onto a photographic plate. Mass spectrometers have three common elements: a source component, wherein elemental species are ionized so that they can be accelerated electrically; an analyzer section, where isotopic species are separated in a magnetic field by their mass-charge ratio; and a collector assembly, where the ion beams are quantitatively measured.

Magnetic-Sector Mass Spectrometry

The most common instrument in geologic use is the magnetic-sector machine, in which a uniform magnetic field is bound in a region, or sector, commonly by a stainless steel tube that can be evacuated to very low pressures to prevent sample contamination. The source region may consist of a solid source; a purified and spiked sample of a heavy element is introduced in the solid state onto a filament of purified metal such as tantalum or rhenium, and the filament is heated electrically until a sufficient percentage of the element is vaporized and ionized for efficient measurement. Alternatively, the source may be a gas. In this case, the desired (commonly light) elements in a gaseous state are introduced into an evacuated region and bombarded with electrons to produce a sufficient percentage of ionized species for acceleration into the analyzer section of the instrument. The ionized species are accelerated electrically through a series of slits, onto which variable electric potentials can be applied for the purpose of acceleration and focusing, so that a well-defined, focused beam of the element or its gaseous compound is beamed into the analyzer tube.

The analyzer sector, which is commonly constructed so that the lowest pressure possible can be maintained and the least number of contaminant species will be struck by the focused beam, is bent at angles of 90 or 120 degrees as they pass through a magnetic field capable of efficiently separating the ion beams by their mass-to-charge ratios. (Where the charges are uniform, as is usual in earth science research, the separation is, as desired, only by mass.)

The collector assembly commonly consists of a Faraday cup; the separated isotope beams enter, hit the metal cup, and impart unit charges to the cup as the atoms are neutralized. The resulting direct current is exceedingly low and, in many instruments, must be converted to an alternating current so that the intensity of the signal can be increased for measurement with a strip-chart recorder or, more commonly, for digital readout. Accelerating voltage in the source assembly is adjusted with the magnetic field in the analyzer sector (commonly monitored and controlled with a very precise gaussmeter) so that a beam of a unique charge-to-mass ratio (or of a unique mass, for ions of the same charge) passes through a final slit into the collector. Ions entering the collector are neutralized by electrons that flow from the ground to the metal collector cup, across a resistor whose voltage difference is amplified and measured with a digital or analogue voltmeter. These data are exhibited as a strip-chart readout or, more commonly, as digital output that is computer-collected and reduced for analysis. The collection of large numbers of highly precise isotopic ratios in computer-reduced digital form has made possible the modern use of mass spectrometry in the earth sciences and the determination of isotopic parameters that would not otherwise have been obtainable.

Additional Types of Mass Spectrometry

Many advances have been made, so it is now possible to obtain extremely precise ratio measurements of tiny pieces of material in a relatively short time. Ion probe mass spectrometers allow these measurements on in situ samples in thin sections that, concomitantly, can be studied petrologically. Ion probe mass spectrometry involves the combination of a microbeam probe (using ions, rather than the lighter electrons, as “bullets” for ionization) and a magnetic-sector mass spectrometer. Accelerator mass spectrometry employs the use of a particle accelerator or cyclotron as the mass analyzer; it is useful primarily to make high-abundance measurements for cosmogenic nuclides such as carbon-14. Accelerator mass spectrometry makes possible the precise measurement of cosmogenic nuclides on tiny samples.

The developing field of resonance ionization mass spectrometry holds much promise in earth science studies because of its potentially high ionization efficiency and, therefore, sensitivity. Other possible mass spectrometric practices may include high-accuracy isotope dilution analysis utilizing a plasma ion source, and ion cyclotron resonance (Fourier transform) mass spectrometry. The supermachine of the future may combine some or even all of these potential advances.

Principal Mechanics of Spectrometry

Although some modern methods of determining absolute time do not involve isotopes, most do, and the standard method for their quantitative measurement is by mass spectrometry. Because the various radioactive nuclides useful in geochronology are also varied in their chemical characteristics, several instruments and techniques are involved. The principal mechanics of spectrometry, however, are mainly the same. The standard method involves placing the purified samples of the materials in question as solids on purified metal filaments and inserting the loaded filaments into a solid-source mass spectrometer. Evacuated to very low pressures, the spectrometer source regions are made so that the metal filaments can be heated to the point that elements such as rubidium and strontium ionize. The charged, ionized sample is accelerated through a series of collimating slits into the high-vacuum analyzing tube, where it encounters a controlled electromagnetic field. The beams of ions are separated by charge-mass ratios into beams of separated isotopes. As the charge of the elements is the same for each atom, however, the ions in this case are separated on the basis of mass only. Specific isotopic beams, controlled by the magnetic field, are channeled through more collimating slits to the collector part of the spectrometer. Commonly, a Faraday cup is used to analyze the number of atoms of each isotope by conversion of each atomic impact into a unit of charge, which is subsequently amplified, often with a vibrating reed electrometer. A digital readout is then produced. The actual output is isotope ratio measurements, which are converted by a mathematical program to the required parameters for determining time.

Scientists determine the age graphically, with the use of an isochron diagram, in which isotope ratios collected in the spectrometer are used as coordinates. A line known as an isochron connects points representing samples of equal ages. An isochron has an age value indicated by its slope on the figure; a horizontal isochron has a zero-age value, while successively greater positive slopes have increasingly greater ages given in terms of the slope and the half-life of the parent isotope. A single sample from a mineral or rock is represented by only one point in the diagram. Therefore, for an isochron to be drawn, an estimate of the sample's initial isotopic composition would be necessary. Ages calculated this way are termed “model ages.”

Equilibrium and Nonequilibrium Fractionation

Stable isotope fractionation, or the enrichment of one isotope relative to another in a chemical or physical process, also has earth science applications. The two processes of this sort are equilibrium fractionation, which is useful in determining geologic paleotemperatures, and kinetic (nonequilibrium) fractionation. This is useful in establishing biologically mediated geochemical processes, such as the bacterial utilization of sulfur.

Isotopic fractionation in these processes is measured by the fractionation factor α, defined as A/B, where A is the ratio of the heavy to the light isotope in molecule A, and B is that ratio for molecule B. Although α may be calculated theoretically, in geologic use it is derived mainly from empirical data. This factor, which is largely dependent on the vibrational energies of the molecules involved, is a function of temperature; thus, it is a measure of ambient geologic processes.

Establishing Paleotemperatures of Ancient Seawater

Many earth science applications of stable isotope fractionation are in use, but perhaps the best-known example is the use of oxygen isotope ratios to establish paleotemperatures of ancient seawater. Surface seawater, in at least partial equilibrium with the atmosphere, contains oxygen with a characteristic isotopic composition. This composition is provided by the ratio of the most abundant species: oxygen-18 and oxygen-16. Marine plants and animals, such as foraminifera, that build their hard parts out of components dissolved in seawater, such as calcium, carbon, and oxygen (as in calcium carbonate), utilize oxygen that is isotopically characteristic of the seawater. Although this process also depends on other, incompletely understood, factors, it is primarily a function of water temperature. Therefore, the ratio of oxygen-18 to oxygen-16 in the foraminifera is a measure of the water temperature. Because the calcium carbonate does not readily reequilibrate with ambient water after it is precipitated, it retains its characteristic isotopic composition after sedimentary burial for many millions of years. Isotopic data collected from foraminifera recovered from deep-sea cores are therefore used to record water temperatures (and consequently, climate) of the geologic past. More than any other paleothermometry device, this application has been extremely useful in providing a record of global temperature changes, especially of the past glacial periods, for use in constructing and testing quantified models of the causes of climate change.

For this application, the sample is introduced into the source region of the mass spectrometer as a gas, commonly carbon dioxide. Ionization of the gas may be accomplished by bombardment of the molecules with electrons. The positively charged ions created are accelerated through collimating slits into the analyzer section of the spectrometer. In this type of gaseous analysis, use is made of the double-focusing mass spectrometer, in which the isotopic composition of the sample is determined relative to that of the standard in iterative, alternating measurements.

Value to

The revolution in the earth sciences, largely a result of the plate tectonics paradigm that was introduced in the early 1960s, was preceded by an even more important revolution, one that received little fanfare. In the 1940s and 1950s, earth science began to be significantly influenced by quantitative investigations that may be considered to have provided the quantitative foundation for the plate tectonic revolution. Of these studies, none was more significant than the use of mass spectrometry for determining absolute ages of minerals and rocks and, later, for paleothermometry. Absolute-age determinations gave a firm basis for paleontology and established not only the earth's antiquity but also a quantitative sequencing for its rocks and sediments—the geologic time scale.

Principal Terms

absolute date or age: the numerical timing of a geologic event, as contrasted with relative, or stratigraphic, timing

geochronology: the study of the absolute ages of geologic samples and events

half-life: the time required for a radioactive isotope to decay by one-half of its original weight

ions: atoms or molecules that have too few or too many electrons for neutrality and are therefore electrically charged

isochron: a line connecting points representing samples of equal age on a radioactive isotope (parent) versus radiogenic isotope (daughter) diagram

isotope: a species of an element having the same number of protons but a different number of neutrons and therefore a different atomic weight

nuclide: any observable association of protons and neutrons

radioactive decay: a natural process whereby an unstable, or radioactive, isotope transforms into a stable, or radiogenic, isotope

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