Methods
To analyse the samples several x-ray based methods, XANES, ESCA and
powder diffraction XRD, have been used. The methods differ in the effects measured.
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(right) X-ray analytical methods used in these investigations
are XANES, ESCA and x-ray powder diffraction XRD. (left) Scheme of electronic energy levels in an atom. The bound electron,
e.g. an 1s electron in the sulfur K-shell, absorbs x-ray radiation
(hν) and is
ejected as a photoelectron.
ESCA: the energy of the photoelectron is measured and gives
the bonding energy of the electrons in the element. XANES: The empty "hole"
created in the energy level A is filled very rapidly and
fluorescence x-ray radiation is emitted and detected to
measure the x-ray absorption.
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XANES
When the X-ray energy is varied the absorption of the X-rays can
increase sharply when a core electron in the
K-shell of the sulfur atom is excited to an unoccupied molecular
orbital. This is called X-ray Absorption Near Edge Structure (XANES)
spectroscopy,
The energy of the absorption edge is sensitive to the sulfur
oxidation state and shifts as much as 13 eV from sulfide, S(-II), to
sulfate, S(VI). The shape, intensity and energy of the XANES features
allow insight
into the chemical state and bonding of the sulfur atom, and are used to
characterize the sulfur compound.
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The principle of the dedicated sulfur XANES instrument at
beamline 6-2, Stanford Synchrotron Radiation
Laboratory (SSRL), which was used to identify sulfur compounds
in core samples. The intense x-rays from the
synchrotron pass through a monochromator that can vary the energy,
before hitting the sample in 1 atm helium pressure.
The Lytle detector measures the x-ray fluorescence
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ESCA
Electron Spectroscopy for Chemical Analysis (ESCA) was performed by exposing slices
from
various depths of a core sample to x-rays of known energy (Al Kα 1487
eV), and measure the kinetic energy of the ejected photoelectrons.from all elements in the sample. Since the kinetic
energy directly depends on the binding energy of the electrons
in different elements, the number of
photoelectrons at different energies provide, with fair
accuracy, concentrations of all elements (except hydrogen)
in the sample. For sulfur, the 2p electrons were used, and 1s for boron.
High vacuum is needed and volatile
compounds, such as water, are pumped away. The vacuum pumping takes several hours and
may affect the composition of the Vasa's core samples to
some extent
(Sandström 2000a).
ESCA is very surface-sensitive and since the samples are not very
homogeneous, also with variation of the composition in
cracks etc, or smearing of the PEG when cutting slices,
may influence the results.
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ESCA principles. X-rays from a rotating anode are focussed on the
sample. Photoelectrons are emitted from all elements and their kinetic energy
measured in a hemi-spherical analyser. |
XRD (X-Ray Powder Diffraction)
X-ray powder diffraction is a useful method for identifying crystalline
substances in a sample. In these investigations we have used a Guinier
camera, which requires a small amount of sample.
The diffracted x-rays in a Guinier camera produce sharp
lines on an X-ray film, while the disordered (amorphous)
substances of the wood, e.g. lignin, do not give sharp lines. A scanner is used to eliminate the
background from the characteristic diffraction pattern (see below) that is obtained
because of the constructive interference between waves from
different planes through the crystal lattice. This pattern can be used
as a fingerprint of the crystalline compound, and the
identification is performed by comparing with such patterns of
known standards in a data base to identify which substances that are
present in a sample.
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If the distance between the atomic planes is d, the path difference between successive
reflections is
2dsin(θ),
where θ is the angle between the
incident beam and the planes (see the figur above). For constructive
interference the path
difference must be an integral number of wavelengths. This gives Bragg's
law for possible reflections: 2d sin(θ) = nλ .
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The diffraction pattern above is from the yellow jarosite salt,
NaFe3(SO4)2(OH)6,
that did not dissolve when a deck plank of the Vasa was treated with
DTPA, see below where Yvonne samples the plank. |
