Mass Spectrometry Comes to the Aid of Dendrochronology

Mass Spectrometry Comes to the Aid of Dendrochronology

Friday, April 4, 2008

Keywords: dendrochronology, mass spectrometry, molecular ion analysis
Dendrochronology, or tree-ring dating, is a well-known method for dating wood found at archaeological sites, and probably every student in 'Introduction to Archaeology' learns about it. So what is news-worthy about this article? Dendrochronology relies on our ability to distinguish among the rings of sapwood and heartwood, but this can be difficult, even nearly impossible, in very old pieces of wood. Researchers, however, have used various analytical techniques to distinguish chemically the rings. This article considers the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to look for molecular differences between the sapwood and heartwood. This approach was used to date the oldest beams from the Buddhist Horyuji Temple in Japan.
Above: Close-up photo of Bald Cyprus tree rings. Credit: Louisiana State University Agricultural Center.
Excerpts from Spectrometry Now:
Ringing the Changes in Ancient Trees
By Steve Down
Everyone knows that you can tell the age of a tree by counting the tree rings over its cross section. . .
Trees in one area will tend to build up the same ring pattern over the same period, so if you can collect a series of tree cross sections with overlapping dates going back in time, you can set up a master chronology for that region. A tree of unknown date can be compared with this master set to match up the ring patterns and establish the date when the tree was cut down. This technique is used by archaeologists to verify the construction dates of old wooden buildings, or of structures containing wood sections.
However, this dating technique relies on the ability to differentiate between the outer sapwood of the tree, which grows just beneath the bark, and the inner heartwood. As the tree grows, the sapwood is converted into heartwood and new sapwood layers are grown. Usually, the two areas have different colours and, if no bark is present or the sapwood is incomplete, the felling date can be estimated using a statistical average of the number of sapwood rings.
The problem arises with very old woods, in which discoloration can occur to make the sapwood and heartwood indistinguishable. In these cases, the felling date of the tree can only be estimated and inaccurate dates can result. Now, scientists in Japan have arrived at a solution, developing a mass spectrometric method to discriminate between the sapwood and heartwood and locate the sapwood/heartwood boundary with precision.
Kazuhiko Fukushima and co-researchers from Nagoya University and the Nara National Research Institute for Cultural Properties employed time-of-flight secondary ion mass spectrometry (TOF-SIMS) for this purpose. This is a surface analysis technique in which an ion beam is used to bombard the surface and secondary ions produced from compounds on or near the surface are collected for analysis in the TOF analyser. The technique can be tuned to detect molecular ions only without the need for extraction from the sample. In addition the bombarding beam can be rastered across the sample surface to gain a cross-sectional view. . .

To read the rest of the article, visit the Spectrometry Now website:
http://www.spectroscopynow.com/coi/cda/detail.cda?id=18218&type=Feature&chId=4&page=1

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