Tuning of the instrument

In this chapter, we present the principles of conventional Mossbauer spectrometers with radioactive isotopes as the light source Mossbauer experiments with synchrotron radiation are discussed in Chap. 9 including technical principles. Since complete spectrometers, suitable for virtually all the common isotopes, have been commercially available for many years, we refrain from presenting technical details like electronic circuits. We are concerned here with the functional components of a spectrometer, their interaction and synchronization, the different operation modes and proper tuning of the instrument. We discuss the properties of radioactive y-sources to understand the requirements of an efficient y-counting system, and finally we deal with sample preparation and the optimization of Mossbauer absorbers. For further reading on spectrometers and their technical details, we refer to the review articles [1-3]. 

However, each of these requires a different hardware configuration, and therefore specific tuning of the instrument, before use. It is not possible to switch between these modes during the same experimental run. 

Without changing the tuning of the instrument, a compound X is now injected onto the column. The new chromatogram obtained allows the calculation of the Kovats retention index of X on the specific column used. This index is obtained by multiplying the equivalent alkane carbon number that has the same retention time as X by 100. 

For a given tuning of the instrument, it is assumed that there is a linear relationship between the area of each peak in the chromatogram and the quantity of the compound in the sample. This applies for a given concentration range, depending on the detector. It translates into the following equation. 

A compound (X) is now injected onto the column without changing the tuning of the instrument. The resulting chromatogram will enable 4, the Kovats retention index, to be calculated for X and the specific column employed this is equal to 100 times the equivalent number of carbon atoms of the theoretical alkane having the same adjusted retention time than X. Two methods can be used to find... 

A variable UV spectrophotometer is to be given preference over a fixed-wavelength detector because it allows de-tuning of the instrument in order to reduce the detector sensitivity for preparative separations and thereby to... 

Place a test cell containing sample in the coil and adjust the tuning of the instrument until the two resonance curves on the oscilloscope are coincident. This setting may need to be readjusted during determinations. 

Analysis of terrestrial samples demonstrated that the tuning of the ion extraction system, and consequently the amount of instrumental mass fractionation, was very sensitive to charge build-up on the sample, the position of the primary beam relative to the spectrometer optic axis, and the position of the sample relative to the extraction lens. To optimize reproducible tuning of the extraction system from sample to sample, we developed the following criteria (1) resistance of the sample (Au-coated) to ground less than 106 fi (2) alignment of the primary beam to within 10 pm using... 

Fig. 6.13. Data obtained by the phase-modulation technique with a Fluorolog tau-3 instrument (Jobin Yvon-Spex) operating with a xenon lamp and a Pockel s cell. Note that because the fluorescence decay is a single exponential, a single appropriate modulation frequency suffices for the lifetime determination. The broad set of frequencies permits control of the proper tuning of the...

The instmment shown in Figure 2.4 is a double-axis instrument. The first axis is the adjustment of the beam conditioner, the second is the scan of the specimen through the Bragg angle. It is irrelevant to this definition that a practical diffractometer may contain a dozen or more controlled axes , for example, to tune and to align the beam conditioner, to locate the specimen in the beam, to align and to scan the specimen and to control shts. It is the differential movement of the two main axes that make the measnrement and determine the precision and accuracy of the instrument. This is the basic high resolntion diffractometer, which is now widely nsed for measniements of crystal perfection, epilayer composition and thickness. 

The constants Kp, Kt, and Kd are settings of the instrument. When the controller is hooked up to the process, the settings appropriate to a desired quality of control depend on the inertia (capacitance) and various response times of the system, and they can be determined by field tests. The method of Ziegler and Nichols used in Example 3.1 is based on step response of a damped system and provides at least approximate values of instrument settings which can be further fine-tuned in the field. 

It takes years to develop and fine-tune a prooftest program. Time is required to identify all of the instrument loops that need to be included, and more time is needed to systematically collect or develop data sheets containing basic operational information for each device or loop. During the development stage, test methods need to be defined, and the appropriate personnel responsible for testing must be identified and trained. Finally, test frequencies must be decided. 

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