Eaton / Barr / Weber Quantitative EPR

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

E-Book, Englisch, 185 Seiten

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.

Autoren/Hrsg.

Eaton, Gareth R.

Eaton, Sandra S.

Barr, David P.

Weber, Ralph T.

Inhaltsverzeichnis

1;Foreword;6
2;Acknowledgments;8
3;Contents;10
4;Chapter 1: Basics of Continuous Wave EPR;14
4.1;The Zeeman EffectThe Zeeman Effect;14
4.2;Hyperfine Interactions;16
4.3;Signal Intensity;18
4.4;Introduction to Typical CW EPRCW EPRintroduction to Spectrometers;18
4.5;The Microwave BridgeMicrowave Bridge;19
4.6;The EPR Cavity;21
4.7;The Signal Channel;23
4.8;The Magnetic Field Controller;26
4.9;The Spectrum;27
5;Chapter 2: Why Should Measurements Be Quantitative?;28
5.1;Examples of Applications of Quantitative EPR;29
5.2;Measuring Unstable Radicals by Spin Trapping: Effect of Resonator Q;31
5.3;Measuring Weak Signals in the Presence of Strong Ones: Dynamic RangeDynamic Range Issues;31
5.4;Signals in Mixtures;32
5.5;Radiation DosimetryRadiation Dosimetry;32
5.6;Use of Accurate Line Width Information;34
5.7;Catalysis and Mineralogy;35
5.8;Free Radical Content in Commercial Materials;35
5.9;Feasibility of Quantitative EPR;36
5.10;Further Reading;37
6;Chapter 3: Important Principles for Quantitative EPR;38
6.1;The EPR TransitionEPR Transition and Resulting Signal;38
6.2;Relaxation and Saturation;39
6.3;Why Are EPR Spectra Displayed as the Derivative?;41
6.4;Some Caveats About Modulation and First Derivative Displays;41
6.5;Finding the Signal Area Requires a Double Integration;43
6.6;The CW EPR Line Width;44
6.7;Transition Metal EPR;45
6.8;Spectrometer Field and Frequency May Determine Which Transitions Are Observed;45
6.9;Parallel and Perpendicular Transitions;47
7;Chapter 4: A More in Depth Look at the EPR Signal Response;50
7.1;Sample Preparation;50
7.1.1;Capillary Tube SealantCapillary tube sealant;50
7.2;Searching for a Signal (Also See Appendix A);51
7.3;Detector Current;51
7.4;Optimize the Receiver Gain;52
7.5;Be Aware of Noise Sources;52
7.6;Number of Data Points;53
7.7;Optimize the Sweep Timesweep time and Conversion Time;54
7.8;Optimize the Time Constant for the Selected Sweep Time and Conversion Time;55
7.9;Background Signals;56
7.10;Integration;57
7.11;Microwave Power;58
7.12;Modulation AmplitudeModulation Amplitude - definition (Also See Appendix B for More Details on This Topic);61
7.13;Modulation Amplitude CalibrationModulation Amplitude Calibration;64
7.14;How to Select Modulation Frequency;67
7.15;Passage EffectsPassage Effects;68
7.16;Illustration of the Effect of Modulation Amplitude, Modulation Frequency, and Microwave Power on the Spectra of Free Radicals;68
7.17;Phase;69
7.18;Automatic Frequency Control and Microwave Phase;71
7.19;Resonator Design for Specific Samples;72
7.20;Software;72
7.21;Scaling Results for Quantitative Comparisons;72
7.22;Signal AveragingSignal Averaging;73
7.23;Cleanliness;74
8;Chapter 5: Practical Advice About Crucial Parameters;75
8.1;Crucial Parameters and How They Affect EPR Signal Intensity;75
8.2;What Accuracy Is Achievable?;77
8.3;A More In-Depth Look at Adjusting the Coupling to the Resonator in the ``Tuning´´ Procedure;78
9;Chapter 6: A Deeper Look at B1B1 and Modulation Field Distribution in a Resonator;80
9.1;Separation of B1B1 and E1;80
9.2;Inhomogeneity of B1 and Modulation Amplitude;81
9.3;Sample Size;84
9.4;AFC Considerations;84
9.5;Flat Cells;86
9.6;Double-Cavity Simultaneous Reference and Unknown;87
9.7;Summary;87
10;Chapter 7: Resonator Q;90
10.1;Conversion Efficiency, C;91
10.2;Loaded Q and Unloaded Q;92
10.3;Relation of Q to the EPR Signal;94
10.4;Contributions to Q;94
10.5;Measurement of Resonator Q;95
10.5.1;Estimate Q Using the Bruker Software;96
10.5.2;Q Measurement Using a Network AnalyzerNetwork Analyzer: By George A. Rinard;96
10.5.3;Q by Ring DownRing Down Following a Pulse;97
11;Chapter 8: Filling Factor;99
11.1;General Definition;99
11.2;Calculation of Filling Factor;99
12;Chapter 9: Temperature;101
12.1;Temperature Dependence of Signal Intensity;101
12.2;Sample Preparation for CryogenicCryogenic Temperatures;102
12.2.1;Selection of Solvent;102
12.2.2;Sealed Samples;102
12.3;Practical Aspects of Controlling and Measuring Sample Temperature;103
12.3.1;Cavity Resonators;104
12.3.2;Flexline Resonators;105
12.3.3;Other Components of the Cooling Systems;107
12.4;Operation Above Room Temperature;108
12.5;Example for S>1/2;108
13;Chapter 10: Magnetic Field and Microwave Frequency;110
13.1;g-Factorsg-factors;110
13.2;Measurement of Microwave Frequency;110
13.3;Magnetic Field;111
13.4;Magnetic Field Homogeneity;112
13.5;Coupling Constants Vs. Hyperfine Splittings;113
13.6;Achievable Accuracy and Precision: g Value and Hyperfine Splitting;113
14;Chapter 11: Standard Samples;116
14.1;Comparison with a Standard Sample;116
14.2;Spin Quantitation with a Calibrated Spectrometer;118
15;Appendix;123
15.1;Appendix A: Acquiring EPR Spectra and Optimizing Parameters;123
15.1.1;Measure the Spectrum with Nominal Settings;123
15.1.2;Optimize the Microwave Power;123
15.1.3;Optimize the Modulation AmplitudeModulation Amplitude - Optimizing;126
15.1.4;Optimize Magnetic Field Sweep Width and Number of Data PointsNumber of Data Pointsoptimizing;127
15.1.5;Summary;130
15.2;Appendix B: Field Modulation and Phase Sensitive Detection;132
15.2.1;Details of Field Modulation and Phase Sensitive Detection;132
15.2.2;Field Modulation and Demodulation;135
15.2.3;A Visual Description of Why the EPR Signal Appears in the First Derivative Form;135
15.2.4;Suppression of 1/f Noise;137
15.3;Appendix C: Post Processing for Optimal Quantitative Results;140
15.3.1;Example of Baseline Subtraction to Improve Spectrum for Double Integration;140
15.3.2;Convert the First Derivative EPR Spectrum into an Absorption Spectrumabsorption spectrum;143
15.3.3;Correct the Baselinebaselinecorrection;143
15.3.4;Calculate the Double Integration;145
15.3.5;Obtain the Double Integration Value;145
15.3.6;Use of a Simulation to Improve Peak Intensity Measurements from Noisy Spectranoisy spectra;147
15.3.7;Additional Techniques to Improve Double Integration Results;149
15.4;Appendix D: Quantitation of Organic Radicals Using Tempol;149
15.4.1;Determine the Concentration of the Tempol Solution;150
15.4.2;Prepare Several Dilutions of the Stock Tempol Solution;150
15.4.3;Record the EPR Spectra of the Tempol Dilutions;150
15.4.4;Determine the Double Integrals of the EPR Spectra from Each of the Dilutions;151
15.4.5;Make a Standard Curvestandard curve of Double Integrated Intensityintegrated intensity Versus Tempol Concentration;152
15.4.6;Prepare a DMPO/OH Sample;152
15.4.7;Use WIN EPRWIN EPR to Determine the Double Integral of the DMPO/OH Spectrum;152
15.4.8;Summary;154
15.5;Appendix E: Using a Reference Standard for Relative Intensity Measurements;154
15.5.1;Why use an EPR Reference Standard?;154
15.5.2;Properties of an Ideal EPR Reference Standard;155
15.5.3;Positioning of the Reference StandardPositioning of the reference marker is Critical;156
15.5.4;Testing the Measurement Reproducibility of an EPR Reference Standard;156
15.5.5;Repeatability TestRepeatability test;157
15.5.6;Reproducibility Testreproducibility test;157
15.5.7;Summary;158
15.6;Appendix F: Example Procedure for Measuring Signal-to-Noise Ratio;159
15.6.1;Signal-to-Noise Testing for Spectrometer Maintenance;159
15.6.2;Spectrometer Settings for Signal/Noise Measurements Using the Bruker ER 4119HS Cavity;160
15.6.3;Measuring the Signal to Noise Ratio;160
15.7;Appendix G: How Good Can It Get: Absolute EPR Signal Intensity;165
15.7.1;The Spin Magnetization, M, for an Arbitrary Spin, S: Definitions;166
15.7.2;Signal Voltage;168
15.8;Calculation of Noise;169
15.9;Calculation of S/N for a Nitroxide Sample;170
15.10;Calculation of S/N for a Weak Pitch Sample;170
15.11;Summary of Impact of Parameters on S/N;171
15.12;How to Improve the Spectrometer: The Friis Equation;172
15.13;Experimental Comparison;172
16;References;174
17;Index;186

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."