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We present high-precision measurements of pure element stable isotope pairs that demonstrate mass has no influence on the backscattering of electrons at typical electron microprobe energies. The traditional prediction of average backscatter intensities in compounds was pragmatically based on elemental mass fractions. Our isotopic measurements establish that this approximation has no physical basis. We propose an alternative model to mass fraction averaging, based on the number of electrons or protons, termed “electron fraction,” which predicts backscatter yield better than mass fraction averaging. We also present an improved backscatter (electron loss) factor based on a modified electron fraction average for the ZAF atomic number correction that provides a significant analytical improvement, especially where large atomic number corrections are required.

We find no physical basis for traditional mass-fraction weighted averaging of X-ray continuum measured in pure elements to estimate continuum production from compounds. Measurements of continuum from pairs of stable isotopes demonstrate that mass alone (the presence of more or fewer neutrons) has no effect on continuum production in electron probe microanalysis. Electron-fraction weighted averaging, which is based on the relative contribution of each constituent element to the electron (or proton) composition of a compound, proved superior to mass-fraction weighted averaging, in predicting the continuum of a compound from the measured continuum of its elements.

Castaing’s first approximation assumes that the characteristic radiation emitted by each element in a compound is unaffected by the presence of the other elements, and that the intensity of the radiation emitted by each element in the compound is proportional to its mass concentration. From Castaing’s first approximation based on mass fractions, the initial estimate of the concentration of an element in a compound iswhere Cand C°are mass fractions in the sample and standard, and Iand I°are x-ray emission intensities of a specific transition measured in the sample and the standard. Surprisingly, the mass fraction approach works well in practice, even though it lacks any obvious physical basis. This can be understood by considering that for typical energies involved in EPMA (50 keV or less), the size of the nucleus is several orders of magnitude smaller than the average incident beam electron wavelength.

X-ray microfluorescence (XRMF) is spatially resolved x-ray fluorescence (XRF) analysis of elemental composition. Formerly available only at synchrotron sources, recent development of commercial instruments has brought a tamer version of this technique into the size and price range of the academic laboratory. Our Kevex Omicron system features a primary beam diameter as small as 30 μm, with an energy dispersive spectrometer collecting the x-ray spectrum from Na to U. The micro-positioning stage and optical microscope permit selective spatially resolved analysis on the surfaces of objects as large as a desktop dictionary. The large sample stage, functional spatial resolution, and non-destructive analysis present significant opportunities for elemental analysis of a variety of archaeological materials. In particular, objects which for aesthetic, religious, regulatory, or economic reasons cannot be subjected to any alteration in sample preparation may still be analyzed by XRMF. Our laboratory has been conducting a number of investigations which illustrate the potential impact of XRMF technology on archaeometry.

Is average atomic mass or average atomic number more appropriate to describe backscattered electron (BSE) effects in electron-solid interactions? In electron microprobe literature, either one or both of the terms A and Z may be found utilized in various theoretical models to describe electron backscattering. Physical theory would seem to favor atomic number averaging, because backscattering ultimately represents an electromagnetic interaction between the electron and the nucleus. At this scale, mass and its associated property of gravity, is 10 times less powerful than the electromagnetic force. Nonetheless, some empirical results show a strong relation between atomic mass and electron scattering, thus endorsing mass averaging.The periodic table presents an opportunity for distinguishing between these two influences: those pairs of elements for which an increase in Z yields a decrease in average A (e.g., Ar-39.95 and K-39.10; Co-58.93 and Ni-58.71; Te-127.60 and 1-126.90) due to natural isotope abundances.

Coral aragonite encodes a rich, complex record of ambient environmental conditions (including temperature, salinity, pollutants) during skeletogenesis. Annual growth banding in appropriate species provides an internal calendar, and the rapidity of skeletal growth permits temporal sampling to at least a sub-monthly basis. The longevity of reef coral colonies can provide continuous centuries-long environmental records. The presence of well-dated Pleistocene coral reefs at sites across the world ocean extends this record into the more distant past. Thus corals are a remarkable resource for the investigation of environmental conditions over perhaps 7 orders of magnitude of time.The past few years have seen considerable use of Sr/Ca ratios in scleractinian corals to reconstruct sea surface temperatures (SSTs) to document global change. The detailed correlation between Sr/Ca ratios in living corals and measured ambient water temperature is often remarkable, as is the match in older corals between Sr/Ca ratios and such other SST temperature proxies as 18O /16O ratios.

X-ray microfluorescence (XRMF) is spatially resolved x-ray fluorescence (XRF) analysis of elemental composition. Formerly available only at synchrotron sources, recent development of commercial instruments has brought a tamer version of this technique into the size and price range of the academic laboratory. Our Kevex Omicron system features a primary beam diameter as small as 30 μm, with an energy dispersive spectrometer collecting the x-ray spectrum from Na to U. The micro-positioning stage and optical microscope permit selective spatially resolved analysis on the surfaces of objects as large as a desktop dictionary. The large sample stage, functional spatial resolution, and non-destructive analysis present significant opportunities for elemental analysis of a variety of archaeological materials. In particular, objects which for aesthetic, religious, regulatory, or economic reasons cannot be subjected to any alteration in sample preparation may still be analyzed by XRMF. Our laboratory has been conducting a number of investigations which illustrate the potential impact of XRMF technology on archaeometry.

Coral aragonite encodes a rich, complex record of ambient environmental conditions (including temperature, salinity, pollutants) during skeletogenesis. Annual growth banding in appropriate species provides an internal calendar, and the rapidity of skeletal growth permits temporal sampling to at least a sub-monthly basis. The longevity of reef coral colonies can provide continuous centuries-long environmental records. The presence of well-dated Pleistocene coral reefs at sites across the world ocean extends this record into the more distant past. Thus corals are a remarkable resource for the investigation of environmental conditions over perhaps 7 orders of magnitude of time.The past few years have seen considerable use of Sr/Ca ratios in scleractinian corals to reconstruct sea surface temperatures (SSTs) to document global change. The detailed correlation between Sr/Ca ratios in living corals and measured ambient water temperature is often remarkable, as is the match in older corals between Sr/Ca ratios and such other SST temperature proxies as 18O /16O ratios.

Is average atomic mass or average atomic number more appropriate to describe backscattered electron (BSE) effects in electron-solid interactions? In electron microprobe literature, either one or both of the terms A and Z may be found utilized in various theoretical models to describe electron backscattering. Physical theory would seem to favor atomic number averaging, because backscattering ultimately represents an electromagnetic interaction between the electron and the nucleus. At this scale, mass and its associated property of gravity, is 10 times less powerful than the electromagnetic force. Nonetheless, some empirical results show a strong relation between atomic mass and electron scattering, thus endorsing mass averaging.The periodic table presents an opportunity for distinguishing between these two influences: those pairs of elements for which an increase in Z yields a decrease in average A (e.g., Ar-39.95 and K-39.10; Co-58.93 and Ni-58.71; Te-127.60 and 1-126.90) due to natural isotope abundances.