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The Proton Microprobe The announcement of the proton microprobe [Science, 199, 765 (1978)l by two groups in Heidelberg marks the addition of still another approach to sensitive elemental analysis with high spatial resolution. As with every new analytical development, the method needs evaluation as to its place in the framework of modern methodology. It is essentially a refinement of particle induced x-ray emission (PIXE) by focusing a proton beam to a spot size of 2 X 2 micrometers. As such, it is a potential competitor to other probe methods such as the electron microprobe, ion microprobe mass spectrometer, and laser mass spectrometer. In comparison with the latter two methods, the proton microprobe offers the advantage of being a nondestructive method. In comparison with the electron microprobe, it is one to two orders of magnitude higher in sensitivity for trace elements. This increased sensitivity was demonstrated by analyzing grains of lunar minerals for trace elements. Several trace elements undetected by the electron microprobe were proved to be present. As always, the advantages of a new method need to be weighed against its disadvantages. Apart from the obvious problems of instrumental complexity and expense, there are inherent differences in principle that need consideration. The relatively large penetration depths (20 to 70 micrometers) of the 0.5 to 4 MeV proton beam increase the background due to bremsstrahlung and characteristic x-rays from the sample backing. What is more important, the analytical sample is defined by the cross section of the beam and its penetration depth. The response from different regions of the sample varies with depth because of loss of proton energy with consequent change in x-ray yield. Thus, for quantitative results to be obtained for trace elements, the nature of the matrix and its physical form must be taken into account. There are, of course, competing trace analytical methods for auerage composition (e.g., neutron activation analysis), so the whole point of considering a probe approach is the need for spatial resolution. In summary, the proton microprobe serves as an excellent example of a new approach which offers advantages for specific applications at the expense of disadvantages that need to be considered by the analytical chemist in deciding whether it should be added to the existing capability of an analytical facility.
Manuscript requirements are published in the January 1978 issue, page 189. Manuscripts for publication (4 copies) should be submitted to ANALYTICAL CHEMiSTRY at the ACS Washington address. The American Chemical Society and its editors assume no responsibility for the statements and opinions advanced by contributors. Views expressed in the editorials are those of the editors and do not necessarily represent the official position of the American Chemical Society.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
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