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Sample Considerations


     The 57Fe isotope occurs in 2.2 % natural abundance. A thorough study, involving about 10 spectra taken in various applied magnetic fields over a wide temperature range, requires either a frozen solution sample containing ca. 1 mM 57Fe in ca. 0.5 mL volume or a solid sample containing ca. 40 μg/ cm2 57-Fe. Using natural abundance iron corresponds to ca. 50 fold increase in these numbers.

     Especially in solvents with low melting points, 50 mM solution samples pose added solubility problems. For instance, a compound that is well dissolved at room temperature may precipitate before the sample freezes, leading to an inhomogeneous sample, which (to make things more complicated) possibly contains a mixture of molecules with slow, medium and fast relaxing electronic spins. To guard against this problem, the sample can be cooled to just above the solvent freezing point. At this temperature the supernatant, along with slightly more solvent for good measure, should be transferred to a Mössbauer cell.
     If the species is paramagnetic, solubility is not the only consideration. For molecules with large spins, a sample concentration exceeding 5 mM can lead to intermolecular spin-spin interactions (In a non-glassforming solvent this can happen even at 100 mM). Such interactions then have to be overcome by a combination of strong applied fields (B > 6.0 T) and very low temperature (1.5 K). Similar problems are often encountered in polycrystalline samples, which initially seem attractive due to the advantage of high purity and frequently added structural information. Additional problems include alignment of the crystallites by the applied magnetic field (This is likely if the electronic ground state has a very anisotropic magnetic moment, which is the case for many paramagnetic compounds). Grinding the crystallites in a mortar generally does not solve the problem; however, grinding the sample in a powder mill with a corundum ball often reduces the size of the crystallites sufficiently to prevent texture. Orientation of the resulting crystallites by the applied field is best prevented by embedding them in a non-dissolving solvent or mineral oil such as nujol; the powder should be WELL stirred to provide an absorber with uniform mass density.


     Finally, it should be noted that solvents such as CH2Cl2 or DMSO have very high electronic extinction coefficients for the 14.4 KeV Mössbauer radiation. It is therefore advisable to use solvents containing elements with atomic number Z < 10. (The extinction coefficient at 14.4 KeV is 2.2 cm2/g for oxygen and 17.0 cm2/g for sulfur; the 14.4 KeV γ-beam will be attenuated by about I = I0 e-2.2 x 0.5 = 0.33 I0 in passing through a 0.5 cm water sample.) Buffers containing 50 mM phosphate, chloride or sulfur groups do not pose any problems; it is the high molar concentrations of the pure liquids that cause trouble. In principle, a solvent such as CH2Cl2 could be used, provided the sample cup was filled only to ca. 1 mm height. This, however, leads to a serious meniscus problem upon freezing, with the result being that the central portion of the holder contains no sample material.

Spectrometer Dependent Considerations

     In our laboratory we typically use cylindrical (12.3 mm OD, 10 mm height) sample holders made of delrin or nylon, for which the optimal filling volume is 0.5 mL (6 mm high in the cup). If you fill in more, we loose counts by electronic absorption; if you fill in less, we will loose resonance absorption (less "effect"). For a very scarce protein sample, the optimal cup above is exchanged for one which is tapered on the inside. This cup requires only 0.3 mL of material and yields data as good as the standard cup when analyzed in zero or small applied fields (i.e. the "little" dewar). This cup is far from optimal for high field studies.
     We have also frequently designed alternative cups for specialized experiments. For example, using lucite holders we can perform flash-photolysis studies in the Mössbauer dewar. For trouble makers like Miquel we have made more drastic alterations to either simplify sample preparation or accommodate subsequent spectroscopies. For example, we have recently used a holder of 12.3 mm diameter and 10 cm height. This unusual sample holder allowed us to bubble oxygen (in a Schlenk line) through a 0.5 mL acetonitile solution at -60 °C to isolate an Fe(III)-peroxo complex without spilling sample over the rim.
     The main point is when considering spectrometer dependent variables, it is always advisable for the chemists to discuss preparation details with the Mössbauer spectroscopist. Mössbauer spectrometers can accommodate a variety of sample holders, and we have always found a way to get around the chemist's problems.


Sample Labels

     As to avoid confusion, samples are to be labeled with a 6-character code such as EMAS01, where "EM" are the initials of the P.I., "AS" is the researcher who has made the sample. No two samples should have the same codes. Send ALWAYS an email describing the samples.

If you are confused or are missing information, please call us!
It could save everyone involved time and money.


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updated 2/2011