Quantitative EPR (eBook)

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> 1/2
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 Amplitude - Optimizing 126
Optimize Magnetic Field Sweep Width and Number of Data PointsNumber of Data Pointsoptimizing 127
Summary 130
Appendix B: Field Modulation and Phase Sensitive Detection 132
Details of Field Modulation and Phase Sensitive Detection 132
Field Modulation and Demodulation 135
A Visual Description of Why the EPR Signal Appears in the First Derivative Form 135
Suppression of 1/f Noise 137
Appendix C: Post Processing for Optimal Quantitative Results 140
Example of Baseline Subtraction to Improve Spectrum for Double Integration 140
Convert the First Derivative EPR Spectrum into an Absorption Spectrumabsorption spectrum 143
Correct the Baselinebaselinecorrection 143
Calculate the Double Integration 145
Obtain the Double Integration Value 145
Use of a Simulation to Improve Peak Intensity Measurements from Noisy Spectranoisy spectra 147
Additional Techniques to Improve Double Integration Results 149
Appendix D: Quantitation of Organic Radicals Using Tempol 149
Determine the Concentration of the Tempol Solution 150
Prepare Several Dilutions of the Stock Tempol Solution 150
Record the EPR Spectra of the Tempol Dilutions 150
Determine the Double Integrals of the EPR Spectra from Each of the Dilutions 151
Make a Standard Curvestandard curve of Double Integrated Intensityintegrated intensity Versus Tempol Concentration 152
Prepare a DMPO/OH Sample 152
Use WIN EPRWIN EPR to Determine the Double Integral of the DMPO/OH Spectrum 152
Summary 154
Appendix E: Using a Reference Standard for Relative Intensity Measurements 154
Why use an EPR Reference Standard? 154
Properties of an Ideal EPR Reference Standard 155
Positioning of the Reference StandardPositioning of the reference marker is Critical 156
Testing the Measurement Reproducibility of an EPR Reference Standard 156
Repeatability TestRepeatability test 157
Reproducibility Testreproducibility test 157
Summary 158
Appendix F: Example Procedure for Measuring Signal-to-Noise Ratio 159
Signal-to-Noise Testing for Spectrometer Maintenance 159
Spectrometer Settings for Signal/Noise Measurements Using the Bruker ER 4119HS Cavity 160
Measuring the Signal to Noise Ratio 160
Appendix G: How Good Can It Get: Absolute EPR Signal Intensity 165
The Spin Magnetization, M, for an Arbitrary Spin, S: Definitions 166
Signal Voltage 168
Calculation of Noise 169
Calculation of S/N for a Nitroxide Sample 170
Calculation of S/N for a Weak Pitch Sample 170
Summary of Impact of Parameters on S/N 171
How to Improve the Spectrometer: The Friis Equation 172
Experimental Comparison 172
References 174
Index 186

"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."