Samples and Sample Holders

For a sample to be EPR/ESR active, it must have one or more unpaired electrons. Stable free radicals, paramagnetic metal ions, and irradiated materials are some examples of such materials. The amount of sample required depends on the type of spectrometer (what band is used) and on the type of experiment—CW, pulsed, and double-resonance experiments like electron nuclear double resonance (ENDOR) (discussed in the following)—but in general, liquid and solid samples can be measured. Volumes of sample required range from 20 pL to 1 mL at concentrations of 10 nM to 20 mM for most experiments. 

Organic nonpolar solvents can normally be used at all temperatures. Aqueous samples present difficulties at temperatures above 0°C, due to the interaction of the large electric dipole of water with the standing wave in the resonator cavity. To minimize this interaction, thin, flat cells are used for aqueous samples in X-band systems and very narrow quartz capillaries are used in Q-band systems. 

Organic radicals can often be measnred at room temperature. Eor many metals, the spin-lattice relaxation time broadens their EPR signals significantly at room temperature. In order to narrow the EPR linewidth, such samples are often rnn at cryogenic temperatures (liquid N2,77 K, or liquid He, 4 K). The mechanism for the relaxation time is interaction of the orbital angular momentum with local vibrational modes. Metal ions with little orbital angular momentum, such as Cu + and VO can usually be observed at room temperature. 

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Very easy to change samples and sample holders and to carry out other operations simple compatibility with other apparatus for simultaneous measurements convection goes the natural way, no additional shielding is required for protection of the balance against radiation from the furnace. The other two arrangements 15 B and 15 C which are used besides the first variant are equal in the distribution of use. The advantages of variant C are similar to that of A ... 

Fig. 12.3. STM with a double-action lever. Various parts are shown in (a) (A) The macor block onto which the x, y piezo bars (horizontal) and the z piezo bar (vertical) are mounted, (B) the microscope base plate, (C) carriage rod, actuated by a linear feedthrough and a lead screw, (D) stop, (E) ball bushing assembly, (F) lever, (G) sample and sample holder, (H) catch, the pivot point for coarse motion, (I) foot, the pivot point for fine motion, and (J) the probe tip assembly, (b) Shows the STM in coarse motion. Using the catch as the pivot point, the sample can be removed away from the tip. (c) Shows the STM in fine motion. Using the foot as the pivot point, the linear motion of the carriage rod is reduced by a large factor. (After Demuth et al., 1986a.)...

Figure 16.6—Linear time of flight (TOF) and principle of the reflectron. 1) Sample and sample holder 2) MALDI ionisation device 3 and 3 ) extraction and acceleration grid (5 000 V potential drop) 4) control grid 5) multichannel collector plate 6) electron multiplier 7) signal output. The bottom figure shows a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass, but which have different initial energies. This device increases resolution, which can attain several thousand.

Fig. 4. Scan of cavity modes in the presence of a sample and sample holder. The most intense resonances are the longitudinal resonances, which are the resonances of the fundamental Gaussian beam. Radial and azimuthal modes are also present and appear as shoulders on the longitudinal resonances. Note that the higher order radial and azimuthal modes are slightly dispersive.

In any practical system, there will be at least three layers arranged as follows sample holder, sample, sample holder. If the sample and sample holder layers are an integral number of half-wavelengths thick, we may think of the ensemble as a set of three absentee layers in series and the calcultion carries through as before. These qualitative arguments are put on a firmer footing in Section X, where we calculate the transmission and reflection coefficients for a compound Fabiy-Perot resonator with more than two reflecting surfaces. 

Figure 16.6 A simplified schematic of a time of flight spectrometer and the principle of the ion reflector (reflectron). (1) sample and sample holder (2) MALDI ionization device by pulsed laser bombardment (3 and (3 ) ions are formed between a repeUer plate and an extraction grid (PD 5000V) then accelerated by an other grid (4) control grid (5) microchannel collector plate (6) signal output. Below, a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass but which have initially different energies. The widths of the peaks are of the order of 10 and the resolution ranges between 15 to 20 000.

Sturm (115) has described a systematic error in quantitative DTA that is caused by the change in the apparent heat transfer coefficient and the apparent heat capacity of the sample and sample holder. The logarithm of the peak area furnished an approximate measure of these changes the ratio of the logarithms of the areas of the standard and sample provided a correction factor for K. 

The slightly improved repeatability of the He- and Hf2-values in comparison with that of the Hfl-value might be caused by the improved thermal contact between sample and sample holder after the fusion process. Nakamura [5] reports a reproducibility of 3°C for the Tm/Tc determination. The difference between the repeatability and the reproducibility values of the Tm/Tc determinations is thus considerably higher than those found for the Tg(onset)-value determination. 

The magnitude of the difference in temperature between the exterior and the interior of the sample depends upon two factors the rate of heating (as in TG), and the thermal conductivity of the sample and sample holder. Thus, a metal sample (which has a high thermal conductivity) is close to isothermal, even at high rates of heating. An apparent solution to the problem of thermal equilibrium is to increase the thermal conductivity of the sample appreciably (for instance, by mixing it with a diluent of high conductivity). This approach has limitations, however Equation... 

The furnace surrounds the sample and sample holder. It must be capable of being programmed for a linear heating rate. Modern instruments can be heated and cooled rapidly, which increases sample throughput. Instruments that heat at rates of up to 1000°C/min are available. One commercial instrument can heat at rates up to 200°C/min from room temperature to about 1200°C cooling by forced air can be done at 50°C/min. There are furnaces available with upper temperatures of 1500°C,... 

The first term in Eq. (14) is only the approximate heat capacity in a differential thermal analysis experiment. It is derived already in part in Fig. 4.16 as the steady-state difference between block and sample temperatures. The second term is made up of two factors. The factor in the first set of parentheses represents (close to) the overall sample and sample holder heat capacity. The factor in the second set of parentheses contains a correction factor accounting for the different heating rates of reference and sample. For steady-state a horizontal base line is expected, or dAT/dTj. = 0 the heat capacity of the sample is simply represented by the first term, as suggested in Fig. 4.16. When, however, AT is not constant, the first term must be corrected by the second. 

Much of the intrinsic difficulties with DSC measurements and the tedious data collecting process of traditional adiabatic calorimetry can be avoided by adiabatic scanning calorimetry. In this technique a measured heating power is continuously applied to (or extracted from) the sample and sample holder. It was used in the 1970s for the study of liquid-gas [17] and liquid-liquid critical points [18] and first applied to liquid crystals by us [19] and later also by Anisimov et al. [20]. In the dynamic modes the total heat capacity Cj is now given by ... 

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