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Quantitative EPR A Practitioners Guide

:	Gareth R. Eaton, Sandra S. Eaton, David P. Barr, Ralph T. Weber
:	Quantitative EPR A Practitioners Guide
:	Springer-Verlag
:	9783211929483
:	1
:	CHF 124.20
:

:	Atomphysik, Kernphysik
:	English

:	185
:	Wasserzeichen/DRM
:	PC/MAC/eReader/Tablet
:	PDF

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.

"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."

	Foreword	6
	Acknowledgments	8
	Contents	10
	Chapter 1: Basics of Continuous Wave EPR	14
	The Zeeman EffectThe Zeeman Effect	14
	Hyperfine Interactions	16
	Signal Intensity	18
	Introduction to Typical CW EPRCW EPRintroduction to Spectrometers	18
	The Microwave BridgeMicrowave Bridge	19
	The EPR Cavity	21
	The Signal Channel	23
	The Magnetic Field Controller	26
	The Spectrum	27
	Chapter 2: Why Should Measurements Be Quantitative?	28
	Examples of Applications of Quantitative EPR	29
	Measuring Unstable Radicals by Spin Trapping: Effect of Resonator Q	31
	Measuring Weak Signals in the Presence of Strong Ones: Dynamic RangeDynamic Range Issues	31
	Signals in Mixtures	32
	Radiation DosimetryRadiation Dosimetry	32
	Use of Accurate Line Width Information	34
	Catalysis and Mineralogy	35
	Free Radical Content in Commercial Materials	35
	Feasibility of Quantitative EPR	36
	Further Reading	37
	Chapter 3: Important Principles for Quantitative EPR	38
	The EPR TransitionEPR Transition and Resulting Signal	38
	Relaxation and Saturation	39
	Why Are EPR Spectra Displayed as the Derivative?	41
	Some Caveats About Modulation and First Derivative Displays	41
	Finding the Signal Area Requires a Double Integration	43
	The CW EPR Line Width	44
	Transition Metal EPR	45
	Spectrometer Field and Frequency May Determine Which Transitions Are Observed	45
	Parallel and Perpendicular Transitions	47
	Chapter 4: A More in Depth Look at the EPR Signal Response	50
	Sample Preparation	50
	Capillary Tube SealantCapillary tube sealant	50
	Searching for a Signal (Also See Appendix A)	51
	Detector Current	51
	Optimize the Receiver Gain	52
	Be Aware of Noise Sources	52
	Number of Data Points	53
	Optimize the Sweep Timesweep time and Conversion Time	54
	Optimize the Time Constant for the Selected Sweep Time and Conversion Time	55
	Background Signals	56
	Integration	57
	Microwave Power	58
	Modulation AmplitudeModulation Amplitude - definition (Also See Appendix B for More Details on This Topic)	61
	Modulation Amplitude CalibrationModulation Amplitude Calibration	64
	How to Select Modulation Frequency	67
	Passage EffectsPassage Effects	68
	Illustration of the Effect of Modulation Amplitude, Modulation Frequency, and Microwave Power on the Spectra of Free Radicals	68
	Phase	69
	Automatic Frequency Control and Microwave Phase	71
	Resonator Design for Specific Samples	72
	Software	72
	Scaling Results for Quantitative Comparisons	72
	Signal AveragingSignal Averaging	73
	Cleanliness	74
	Chapter 5: Practical Advice About Crucial Parameters	75
	Crucial Parameters and How They Affect EPR Signal Intensity	75
	What Accuracy Is Achievable?	77
	A More In-Depth Look at Adjusting the Coupling to the Resonator in the ``Tuning´´ Procedure	78
	Chapter 6: A Deeper Look at B1B1 and Modulation Field Distribution in a Resonator	80
	Separation of B1B1 and E1	80
	Inhomogeneity of B1 and Modulation Amplitude	81
	Sample Size	84
	AFC Considerations	84
	Flat Cells	86
	Double-Cavity Simultaneous Reference and Unknown	87
	Summary	87
	Chapter 7: Resonator Q	90
	Conversion Efficiency, C	91
	Loaded Q and Unloaded Q	92
	Relation of Q to the EPR Signal	94
	Contributions to Q	94
	Measurement of Resonator Q	95
	Estimate Q Using the Bruker Software	96
	Q Measurement Using a Network AnalyzerNetwork Analyzer: By George A. Rinard	96
	Q by Ring DownRing Down Following a Pulse	97
	Chapter 8: Filling Factor	99
	General Definition	99
	Calculation of Filling Factor	99
	Chapter 9: Temperature	101
	Temperature Dependence of Signal Intensity	101
	Sample Preparation for CryogenicCryogenic Temperatures	102
	Selection of Solvent	102
	Sealed Samples	102
	Practical Aspects of Controlling and Measuring Sample Temperature	103
	Cavity Resonators	104
	Flexline Resonators	105
	Other Components of the Cooling Systems	107
	Operation Above Room Temperature	108
	Example for S	108
	108	108
	Chapter 10: Magnetic Field and Microwave Frequency	110
	g-Factorsg-factors	110
	Measurement of Microwave Frequency	110
	Magnetic Field	111
	Magnetic Field Homogeneity	112
	Coupling Constants Vs. Hyperfine Splittings	113
	Achievable Accuracy and Precision: g Value and Hyperfine Splitting	113
	Chapter 11: Standard Samples	116
	Comparison with a Standard Sample	116
	Spin Quantitation with a Calibrated Spectrometer	118
	Appendix	123
	Appendix A: Acquiring EPR Spectra and Optimizing Parameters	123
	Measure the Spectrum with Nominal Settings	123
	Optimize the Microwave Power	123
	Optimize the Modulation AmplitudeModulation Ampl