Detecting an Atom of One Element in a Trillion Other Atoms

Scientific Achievement

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A new time-of-flight mass spectrometer (TOF MS) developed at Argonne National Laboratory (ANL) can detect an element of interest at concentrations as low as 1 ppt in very small samples. This advancement was made possible by using the newly designed TOF MS for resonance ionization mass spectrometry (RIMS). In this technique, atoms in a solid are dislodged from its surface by laser light or energetic ions. Once in the gas phase, the element of interest is selectively photoionized by one or more lasers using resonantly enhanced multiphoton ionization (REMPI). The photoions are extracted, from the overwhelming number of neutral atoms of other elements that are not ionized, by an electrostatic field that directs them into the mass spectrometer for detection. Since only atoms of the selected element are photoionized, the separation of one element from all others is relatively easy, leading to the high discrimination required for trace analysis measurements. The key to ANLÕs breakthrough is the efficient extraction of photoions. Atoms ejected from the surface are distributed in time and space over a large volume, making it difficult to collect all of them. Through DOE/BES funded research, a soon to be patented electrostatic optics design for ion extraction from a large volume was developed by modeling the sputtering process and the photoion flight path. The newly invented extraction optics design permits nearly all of the ions in a laser ionization volume as large as 50 mm3 to be guided through the mass spectrometer, yielding an overall detection efficiency (atoms detected/atoms removed) that exceeds 30% for the new instrument. Thus, about one atom in three will be detected, making it possible, for the first time, to detect a few atoms of a particular element in a solid sample. The combination of sensitivity and selectivity permits detection of most elements at concentrations unparalleled by any other instrument in the world.

Significance

The significance of ArgonneÕs new instrument resides in its ability to detect a small number of atoms in a finite sample. Finite sample size may mean limited vertical size (i.e., monolayer depth resolution) or limited lateral size (nanometer-sized particles). Both represent the same technical problem Š a limited number of atoms for analysis. By achieving the unprecedented detection efficiency of 30% while maintaining high selectivity, the new RIMS instrument can obtain meaningful information on samples that heretofore had too few atoms. When sample size does not limit an analysis, the high sensitivity and high discrimination allows measurements at concentration levels never before possible. Both characterization of sample of limited size and detection of element in solids at extremely low concentrations have wide application for solving scientific problems both inside and outside of DOE. For example, advances in semiconductor technology are predicated on fabrication of smaller devices. The ability to characterize sub-micrometer-size devices and interfaces as well as the ability to confirm surface and bulk contamination at sub-ppb levels are requirements for the next generation of integrated circuits. Also, progress in the field of nanometer-scale structures will require the unique analytical capabilities possessed by the new RIMS instrument developed at ANL.

Performers

I. V. Veryovkin, W. F. Calaway, M. J. Pellin, Argonne National Laboratory