178
V. Schünemann
deep knowledge of quantum chemistry and numerical methods. More sophisticated
calculations have to be performed when also the protein environment should be taken
into account.
4.2.1 Sample Requirements for Conventional Mössbauer
Spectroscopy and Synchrotron Based Techniques
The Mössbauer isotope
57 Fe has a natural relative abundance of only 2.2%. Nevertheless, this is enough to obtain Mössbauer spectra in transmission mode with samples
having a total iron content down to 1% by weight. Powder samples of inorganic
iron compounds can be investigated with samples that have a sample thickness of
~0.5 mm and an area of ~1 cm. There are research groups which prefer to measure
pressed pellets, but if the samples are sensitive to pressure, it is better to use small
samples holders made of e.g. Delrin
® . If one measures iron containing metal foils
at room temperature, these can also be taped on the detector. This is a habit in some
laboratories in order to perform calibration of the Mössbauer spectrometer with αFe foils. For any sample preparation it is important taking special care that there are
no holes in the sample. In case there are holes in the sample the γ-rays go through
the sample holder, but do not penetrate sample material. This can lead to a drastic
decrease in absorption of the γ-quanta by the
57 Fe nuclei in the sample specimen
and thus to a decrease in the intensity of the Mössbauer lines. Such a situation can
easily occur with powder samples. A good test is checking whether light is shining
through the sample holder. In case when only a tiny amount of sample is present,
one can put one or several thin sheets of paper in between the sample holder and the
sample. But also in case the sample holder is prepared correctly and there is sufficient
57 Fe in the sample, it is possible that only a very weak noisy Mössbauer spectrum
is obtained at room temperature. In this case, the Lamb-Mößbauer factor f is small
and therefore one should measure at cryogenic temperatures ≤120 K. One should
also be aware of the fact that heavy atoms like e.g. Pt and Au have a high electronic
absorption coefficient for 14.4 keV radiation which results in low count rates during
the measurement.
Compared to optical spectroscopies taking a Mössbauer spectrum requires long
acquisition times. The measurement can take in between several hours (e.g. for a
calibration spectrum with an unenriched α-Fe foil of 25 μm thickness) up to weeks
for samples very dilute in
57 Fe like brain substancia nigra tissue [24]. To give an
example from our laboratory: Collecting a Mössbauer spectrum of a powder sample
of an iron compound with an atomic mass of around 500–1000 amu at 77 K in a
sample cup made of Delrin
® , like those displayed in Fig. 4.4 normally takes 24 h.
There are only a few biological molecules which can be studied with their natural
57 Fe content. For example, the oxygen transporter protein hemoglobin in the blood
can be measured within the red blood cells by Mössbauer spectroscopy after the blood
has been thickened by centrifugation. A way to enhance the
57 Fe concentration in
V. Schünemann
deep knowledge of quantum chemistry and numerical methods. More sophisticated
calculations have to be performed when also the protein environment should be taken
into account.
4.2.1 Sample Requirements for Conventional Mössbauer
Spectroscopy and Synchrotron Based Techniques
The Mössbauer isotope
57 Fe has a natural relative abundance of only 2.2%. Nevertheless, this is enough to obtain Mössbauer spectra in transmission mode with samples
having a total iron content down to 1% by weight. Powder samples of inorganic
iron compounds can be investigated with samples that have a sample thickness of
~0.5 mm and an area of ~1 cm. There are research groups which prefer to measure
pressed pellets, but if the samples are sensitive to pressure, it is better to use small
samples holders made of e.g. Delrin
® . If one measures iron containing metal foils
at room temperature, these can also be taped on the detector. This is a habit in some
laboratories in order to perform calibration of the Mössbauer spectrometer with αFe foils. For any sample preparation it is important taking special care that there are
no holes in the sample. In case there are holes in the sample the γ-rays go through
the sample holder, but do not penetrate sample material. This can lead to a drastic
decrease in absorption of the γ-quanta by the
57 Fe nuclei in the sample specimen
and thus to a decrease in the intensity of the Mössbauer lines. Such a situation can
easily occur with powder samples. A good test is checking whether light is shining
through the sample holder. In case when only a tiny amount of sample is present,
one can put one or several thin sheets of paper in between the sample holder and the
sample. But also in case the sample holder is prepared correctly and there is sufficient
57 Fe in the sample, it is possible that only a very weak noisy Mössbauer spectrum
is obtained at room temperature. In this case, the Lamb-Mößbauer factor f is small
and therefore one should measure at cryogenic temperatures ≤120 K. One should
also be aware of the fact that heavy atoms like e.g. Pt and Au have a high electronic
absorption coefficient for 14.4 keV radiation which results in low count rates during
the measurement.
Compared to optical spectroscopies taking a Mössbauer spectrum requires long
acquisition times. The measurement can take in between several hours (e.g. for a
calibration spectrum with an unenriched α-Fe foil of 25 μm thickness) up to weeks
for samples very dilute in
57 Fe like brain substancia nigra tissue [24]. To give an
example from our laboratory: Collecting a Mössbauer spectrum of a powder sample
of an iron compound with an atomic mass of around 500–1000 amu at 77 K in a
sample cup made of Delrin
® , like those displayed in Fig. 4.4 normally takes 24 h.
There are only a few biological molecules which can be studied with their natural
57 Fe content. For example, the oxygen transporter protein hemoglobin in the blood
can be measured within the red blood cells by Mössbauer spectroscopy after the blood
has been thickened by centrifugation. A way to enhance the
57 Fe concentration in