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Tree core in sample mount, which is placed in target chamber of the instrument {see photo on p. 1107A). patterns of physical features in the rings is to determine and measure ele mental and chemical distributions in the ring material. The first dendrochemical analyses (studies of elemental and chemical distributions in annual growth rings) began in the 1960s. Re searchers have used atomic absorption spectroscopy (AAS), neutron activa tion analysis (NAA), and inductively coupled plasma (ICP) techniques. More recently, proton-induced X-ray emission spectroscopy (PIXE) has been helping to put dendrochemistry on the scientific map. Except for PIXE, these techniques require a lot of sample. Researchers pool tissue from 10 or so rings within each of several cores, and the tissue is either ashed or acid-digested before analysis. The resulting data yield infor mation about environmental trends with a resolution in decades, not years. Also, when AAS and ICP techniques are used, the sample is destroyed and some elements such as the halogens, N, C, and Ο cannot be determined. NAA suffers from similar limitations, al though it has a sensitivity for the lanthanide series that surpasses that of PIXE, according to Gene Hall, a dendrochemist at Rutgers University in Piscataway, NJ.
1104 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
PIXE
"You need an analytical technique that will allow you to sample very small areas of wood in a core in a nondestruc tive way," says Yanosky. PIXE is it for a number of dendroscientists. After Yanosky collects his cores, he sends them to Sene Bauman, senior scientist and president of Element Analysis Corporation (EAC), a commercial PIXE lab in Tallahassee, FL. This lab is one of maybe a half-dozen PIXE fa cilities capable of analyzing thick tar gets such as tree cores. Yanosky first mounts the core in a sample vice and scrapes it down with a steel blade until there is a flat surface with clearly discernible rings. Using a ring width measuring device, he speci fies a series of ring coordinates that Bauman later uses for setting up and controlling the PIXE instrument's ex citation beam. Because ring widths vary from year to year, the beam must be shaped differently for each ring to ensure that bits of several adjacent rings are not analyzed together. Yanosky can specify up to four dif ferent beam geometries, each one cor responding to a set of rings whose members have widths complementary to the beam geometry. A computer uses the stored ring coordinates to control a stepping motor that serially guides the
