SARISA

Surface Analysis by Resonance Ionization of Sputtered Atoms (SARISA), developed and built at Argonne, uses lasers to ionize material desorbed by sputtering from the surface of a specimen and analyzes it with a mass spectrometer. The purity standards required the Genesis project pushed even this device to its limits.

Analysis of the solar wind samples will require even better detection methods. The SARISA Team is currently writing a proposal to develop a super-SARISA with further developed detection methods in place.

  Mass Spectrometry — A Closer Look

The analytical instrument called a mass spectrometer was developed in 1919 by Francis Aston in Cambridge, England. A spectrometer is a spectrograph with measurement capabilities. The importance of this technology was immediately recognized, and Aston was awarded the Nobel Prize for his work in 1922. So, what does a mass spectrometer do and how does it work?

Mass spectrometers permit the experimental determination of atomic and molecular masses with great accuracy. Aston's mass spectrometer had a precision of one part in 10,000, which was sufficient for him to discover the isotopes of many elements. Modern instruments are even more precise.

Ionization of sample Courtesy: McREL

Mass spectrometers operate under conditions of high vacuum, typically 10-8 torr. (A torr is a unit of pressure, equal to 1.316 x 10-3 atmosphere. In comparison the pressure in outer space may be in the order of 10-12 torr.) Low-pressure samples in the spectrometer's ionization chamber are exposed to a beam of rapidly moving, energetic electrons shot out of an electron gun. The samples can be a gaseous element such as neon, the vapor of a solid or liquid element such as mercury, or even the vapor of a molecule such as water or methane. With modern technology it is possible to introduce a wide variety of materials, including mixtures, into a mass spectrometer. In the ionization chamber, the atom or molecule hits or is hit by the accelerated electrons. During the collision an electron is knocked out of the sample atom or molecule, leaving it with a positive charge. In other words, positively-charged gaseous ions are formed. Ion accelerates in spectrometer chamber

Ion accelerates in spectrometer chamber Courtesy: McREL

These newly-formed ions are then pushed out of the ionization chamber by an electric field applied between two metal grids. This is an application of Coul.omb's law: the positively-charged grid repels the positive ions and the negatively-charged grid attracts the positive ions. This attraction and repulsion both act in the same direction to give the ions a nudge (net acceleration) toward the negatively-charged grid. The negative grid, which is full of holes, allows the accelerated ions to pass through it and leave the ionization chamber. The speeds to which the ions can be accelerated by the electric field are determined by their masses. Lighter ions reach higher speeds than do the heavier ones. Moving ion creates magnetic field

Moving ion creates magnetic field Courtesy: McREL

The accelerated beam of positively-charged ions generates a magnetic field of its own, as do all moving electrically-charged particles. The ion beam passes through an externally-applied magnetic field. The magnetic field created by the beam of moving charged particles interacts with the external magnetic field. The net result is that the trajectory of each charged particle is bent in a curve to an extent that depends on its speed (and therefore its mass). If the beam of a mixture of particles of different masses is allowed to hit a photographic plate, the particles converge at different points, corresponding to the different radii of their semicircular paths. Modern mass spectrometers feed their results directly to computers that do the analysis and produce a graph (spectrum). To learn more about the mathematics behind mass spectrometry, read "The Mathematics of Mass Spectrometry."

Diagram of Mass Spectrometer Courtesy: McREL Note. From Chemistry, Molecules, Matter and Change, 3rd Ed., (p.9), by P. Atkins and L. Jones, 1997, New York: W. H. Freeman.

There are many variations in the process of mass spectrometry, but all of them are based on the principles outlined above. Mass spectrometric techniques have played an important role in science (particularly in chemistry). This historically important technology is likely to play a major role in the research phase of the Genesis project when the solar wind samples are returned to Earth for analysis. By that time the technology may be significantly improved over what is now available

  Types of Mass Spectrometry

Mass Spectrometer at Arizona State University (ASU)

   Secondary Ion Mass Spectrometry (SIMS)

SIMS is a technique for the characterization of solid surfaces and thin films. It uses the process of ion formation by bombarding the surface to be tested with a highly collimated beam of primary ions. The surface then emits material through a sputtering process - only a fraction of these emitted particles is ionized. These secondary ions are measured with a mass spectrometer to determine the quantitative elemental, isotopic or molecular composition of the surface. SIMS is the most sensitive surface analysis technique, but it is more difficult to obtain quantitative results compared to other techniques.

Take a 360-degree tour of the MegaSIMS Laboratory (High Res QT, 4.1 MB) at UCLA. Click, hold, and drag your mouse slowly left, right, up, or down on the image.

    Gas Source Mass Spectrometry

Gas source mass spectrometers are used for measuring isotopic ratios of light elements, which includes hydrogen, carbon, nitrogen, and oxygen. Samples are prepared in gaseous form, often hydrogen, nitrogen, or carbon dioxide, and inlet into the mass spectrometer for analysis.

    Resonance Ionization Mass Spectrometry (RIMS)

RIMS is designed to determine the relative weights of an atomic nuclei.

    Total Reflection X-Ray Florescence

In-Situ analysis used the Genesis Science team members, which is unique in the fact that it does not require extraction allowing the solar wind regimes to remain intact during analysis and non-destructive to particle.

    Plasma Mass Spectrometry

A non-invasive method of extraction, allows the science team members to study particles by using differential chemical etching.

    Accelerator Mass Spectrometry

Accelerator mass spectrometry (AMS) differs from other forms of mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass analysis. AMS is exceptional in its ability to sensitively and accurately analyze elemental and isotopic compositions

    How it all works

Generally negative ions are created (atoms are ionized) in an ion source. It is preferable, but not necessary that the charges be the same for each atom. These ions are introduced to the gas phase and they enter an electrostatic accelerator that accelerates them to very high kinetic energy by presenting ever more positive electrical potentials. Half-way through the accelerator they impact a sheet of carbon. The impact strips off many of the ion's electrons, converting it into a positively charged ion. In the second half of the accelerator the now positively charged ion is accelerated away from the highly positive center of the electrostatic accelerator, which previously attracted the negative ion. When the ions leave the accelerator they are positively charged and are moving very fast. Next, the exact ion velocities must be filtered such that only a narrow selection of ion velocities is allowed to pass to allow for proper mass analysis. A device called a velocity selector, which utilizes both electric fields and magnetic fields to allow only ions of a specific charge and kinetic energy to pass, most frequently accomplishes this. The ions then pass through at least one mass analyzer, most often a magnetic or electric sector. For example with a magnetic sector, the atom, at its known velocity (relative to mass) and charge is released into a magnetic field perpendicular to it velocity. This field causes the particle's path to curve in a circular arc. The radius of this circular arc is related to the mass-to-charge ratio of the particle. Dedicated detectors for each isotope or element then detect the ions.

  Radiochemical Neutron Activation Analysis (NAA)

The Neutron Activation Analysis is a nuclear process used for determining certain concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. This method requires a source of neutrons, a range of different sources can be used.