INSTRUMENTATION damage, whereas for non-noble metals the problems are the presence of patina or thick layers of corrosion. The shallow analytical depth of the nuclear microprobe requires t h e r e moval of any such layers to obtain representative results from t h e bulk material. This may be prohibitive in some cases, but the situation is even worse for m o s t o t h e r a n a l y t i c a l methods. Because very small areas are sufficient for analysis with t h e nuclear microprobe, often t h e damage will be deemed insignificant, even by a conscientious curator. Several large-scale studies have been performed on copper-based alloys, mainly antique bronzes. In such studies, particular care has to be taken to avoid erroneous analytical data because of surface effects caused by corrosion. Results from t h e analysis of the patina and the core of an object may show large deviations. On t h e other hand, t h e nuclear microprobe m a k e s i t possible to characterize bronze objects nondestructively and in much greater detail t h a n previously. Because noble metals are unaffected by corrosion over long time spans, Demortier a n d co-workers (13) a t
Tekmar
Namur in Belgium used a n external nuclear microprobe to perform systematic investigations of gold jewelry and determine the age and manufact u r i n g processes. Since t h e n i n e teenth century, cadmium h a s been used a s a n additive to lower t h e melting point when soldering gold and h a s been regarded as the signature of a modern object. The Belgian group showed t h a t low concentrations of cadmium could be found also in ancient gold objects. However, the correlation between copper and cadmium is reversed in newly manufactured m a t e r i a l . By exploiting t h e m u l t i e l e m e n t a l c h a r a c t e r of t h e PIXE method, forgeries of "ancient" gold items can be revealed. Hidden Greek letters Paper and related materials are sensitive to radiation damage a n d to heating effects during t h e irradiation. However, by using the high lateral resolution of an external nuclear microprobe, it is possible to extract new information on m a t e r i a l a n d printing or writing procedures, identifying faded letters and characterizing inks. In our laboratory we have used a
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Nonanalytical use of the nuclear microprobe: irradiation of single cells Although the nuclear microprobe is used primarily for sample analysis, focused ion beams can be used for other applications. For example, it has been suggested that t h e nuclear microprobe, using a beam collimated
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scanning nuclear microprobe combined with sophisticated multivariate statistical methods to identify faded letters in ancient.Greek handwriting on papyrus (14). The irradiation was performed in an external microprobe, and the sheet of papyrus was mounted on an x-y scanning table. By moving t h e specimen during t h e a n a l y s i s , we p r o d u c e d t w o d i m e n s i o n a l e l e m e n t m a p s . We wanted to identify letters by detecting trace elements from the ink, but this simple approach did not yield good image contrast. After applying chemometric methods and compiling maps of several elements by multivariate statistical methods, we obtained much better contrast in t h e maps and we were able to identify the letters (see Figure 9).
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724 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 14, JULY 15, 1991
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