Eaton / Barr / Weber Quantitative EPR

1. Auflage 2010
ISBN: 978-3-211-92948-3
Verlag: Springer Wien
Format: PDF
Kopierschutz: 1 - PDF Watermark

E-Book, Englisch, 185 Seiten, eBook

There is a growing need in both industrial and academic research to obtain accurate quantitative results from continuous wave (CW) electron paramagnetic resonance (EPR) experiments. This book describes various sample-related, instrument-related and software-related aspects of obtaining quantitative results from EPR expe- ments. Some speci?c items to be discussed include: selection of a reference standard, resonator considerations (Q, B ,B ), power saturation, sample position- 1 m ing, and ?nally, the blending of all the factors together to provide a calculation model for obtaining an accurate spin concentration of a sample. This book might, at ?rst glance, appear to be a step back from some of the more advanced pulsed methods discussed in recent EPR texts, but actually quantitative “routine CW EPR” is a challenging technique, and requires a thorough understa- ing of the spectrometer and the spin system. Quantitation of CW EPR can be subdivided into two main categories: (1) intensity and (2) magnetic ?eld/mic- wave frequency measurement. Intensity is important for spin counting. Both re- tive intensity quantitation of EPR samples and their absolute spin concentration of samples are often of interest. This information is important for kinetics, mechanism elucidation, and commercial applications where EPR serves as a detection system for free radicals produced in an industrial process. It is also important for the study of magnetic properties. Magnetic ?eld/microwave frequency is important for g and nuclear hyper?ne coupling measurements that re?ect the electronic structure of the radicals or metal ions.

Zielgruppe

Research

Autoren/Hrsg.

Eaton, Gareth R.

Eaton, Sandra S.

Barr, David P.

Weber, Ralph T.

Inhaltsverzeichnis

Introduction.- Principles of Quantitative EPR; Why should examples of applications be quantitative?; Examples of applications.- Introduction to Quantitative EPR; General expression for CW EPR signal intensity; The EPR transition; Derivative spectra; The CW EPR line width; Second derivative operation; What transitions can we observe; Features of transition metal EPR; Parallel and perpendicular transitions.- Getting started- some practical matters; Operating the spectrometer-cautionary notes; Sample preparation; Don´t forget the cooling water!; Detector current; Automatic frequency control and microwave phase; Searching for a signal; Gain; Effect of scan rates and time constants on S/N and signal fidelity; bandwidth considerations; scan rate and filter time constant; selecting a non-distorting filter and scan rate; A note about comparing noise in CW and pulsed EPR; Background signals; Integration; Microwave power; Modulation amplitude; Modulation amplitude calibration; How to select modulation frequency; Modulation sidebands;Illustration of the effect of modulation amplitude, Modulation frequency, Microwave power on the spectra of free radicals; Phase; Automatic frequency control and microwave phase;Sample considerations; Passage effects; Software; Summary guidance for the operator; Scaling results for quantitative comparisons; Signal averaging; Number of data points, Cleanliness; Changing samples; NMR gaussmeter interference.- What matters, and what can you control? Crucial parameters and how they affect EPR signal intensity;What accuracy can one aspire to?- A deeper look at B1 and modulation field distribution in a resonator; Inhomogenity of B1 and modulation amplitude; Flat cells; Double-cavity simultaneous reference and unknown.-Resonator Q; Conversion efficiancy, C;Contributions to Q; Measurement of Resonator Q.- Filling factor. Temperature; Intensity vs. temperature; Practical example; Glass-forming solvents; Practical aspects of controlling and measuringsample temperature; Operation above room temperature.-Magnetic field and Microwave frequency;G-values; Microwave frequency; Magnetic field; Magnetic field homogenity; Coupling constants vs. hyperfine splittings.-Standard samples; Comparison with a standard sample; Standard samples for Q-band; Achievable accuracy and precision- g value and hyperfine splitting.- How good can it get?- Absolute EPR signal intensities; The spin magnetization M for an arbitrary spin S; Signal voltage; Calculation of noise; Calculation of S/N; Summary of impact of parameters on S/N; How to improve the spectrometer-the Friis equation.- Less common measurements with EPR spectrometers; Multiple resonance methods; Saturation transfer spectroscopy; Electrical conductivity; Static magnetization.

Leseproben

"Chapter 6 A Deeper Look at B1 and Modulation Field Distribution in a Resonator (p. 69-70)

The EPR signal is proportional to the microwave B1 at the sample, which is proportional to ? p P. Consequently, it is important to carefully examine the distribution of B1 over a sample of finite size, such as a standard liquid or powdered sample in a 4 mm o.d. quartz sample tube. In the typical EPR experiment that uses magnetic field modulation and phase-sensitive detection, the integrated signal intensity is proportional to the modulation amplitude at the sample.

Therefore, it is also important to consider the distribution of modulation amplitude over the sample. The details of these two factors are discussed in this chapter. This chapter also includes discussion of sample size, issues related to automatic frequency control (AFC) for very narrow signals, and cell geometries for aqueous samples.

6.1 Separation of B1 and E1

It is the microwave magnetic field (B1) that induces the EPR transitions that are detected in EPR spectroscopy. Also associated with B1 is the microwave electric field (E1). The E1 can induce rotational transitions in the sample, thereby generating heat. This phenomenon should be familiar to readers from the effects of a microwave oven on food. This microwave absorption contributes to additional energy dissipation and thereby reduces the resonator Q (see Chap. 7).

To avoid excessive interaction of the sample with the E1 field (and resultant Q lowering), it is important to position the sample in a region of the cavity with high B1 and low E1. For cavities, there is a natural separation between B1 and E1 because upon resonance, a standing wave is excited within the cavity. Standing electromagnetic waves have their electric and magnetic field components exactly out of phase, i.e. where the magnetic field is maximum, the electric field is minimum and vice versa.

The spatial distribution of the electric and magnetic field amplitudes in the commonly- used TE102 rectangular mode cavity is shown in Fig. 6.1. The spatial separation of the electric and magnetic fields in a cavity is used to great advantage. When the sample is placed in the electric field minimum and the magnetic field maximum, the biggest signals and the highest Q are obtained. Dielectric properties of the sample can also change the field distribution. Cavities are specifically designed to provide optimal placement of the sample with regard to B1."