Tuning, Lock and Shimming Claridge- 3.3, 3.4

Our GOAL is to acquire the best NMR spectrum that we can. What does that mean? SIGNAL-TO-NOISE AS HIGH AS POSSIBLE RESOLUTION AS GOOD AS POSSIBLE

HOW DO WE GET HIGH SIGNAL-TO-NOISE AND GOOD RESOLUTION? 1) Make a good sample in the optimal solvent 2) Lock and Shim the sample optimally 3) Tune the probe to our sample

Sample Preparation Samples should be free of dust and other particulate matter. Sample volume. Degassing. Sample concentration.

Solvents 1) Solubility- More soluble, higher concentration, better signal-to-noise. 2) Deuteration- Solvents need to have at least 10% deuterium. 3) Temperature- Solvent that should be liquid at the temperature that you want to work at. 4) Solvent Resonances- Residual protonated solvent and water can overlap with sample. 5) Structural Relevance 6) Standard Sample- Setup (Shimming, tuning) of a spectrometer is set to a specific sample 7) Viscosity- Viscous solvents cause slower tumbling, and thus broader resonances. 8) Exchangeable Protons- Solvents with exchangeable protons will exchange with the sample. 9) Cost

NMR Tubes Standard tubes Frequency factor for quality Diameter correlates to the probe diameter Length 507pp-7 = 300 MHz, 5 mm, 7 inches 2) Shigemi tubes- increase signal-to-noise 2 parts to the tube, solvent specific 3) Quartz tubes- B11 NMR 4) J-Young tubes- NMR tubes with valves/teflon caps

Lock Even the best magnets have slight field drifts (~0.25 Hz/hour). Since chemical shifts are field dependent, the field/frequency ratio of the particular spectrometer must be constant during measurement. => LOCK CHANNEL Heterolock system, almost always deuterium. 2H is spin 1, resonating at ~76 MHz on a 11.7 T When the sample is locked, the spectrometer records the deuterium reference frequency constantly to offset small changes in the magnetic field. Spectra can be acquired without lock but field drifts cannot be accounted for. Without lock, the drift of the magnet will cause the frequency of the resonances to apparently shift over time.

Optimizing the Lock The lock signal is a constant observation of the deuterium signal of the sample in dispersion mode The lock signal has 4 adjustable parameters: 1) Center of the Field (Z0) 2) Lock Gain (receiver) 3) Lock Power (transmitter) 4) Lock Phase

Diagram of Lock Circuit

LOCK DEMO

Shimming Shims are small magnetic fields used to cancel out errors in the static magnetic field. Magnet homogeneity Shims are physically printed coils wrapped around a cylinder inserted into the magnet. The magnetic field is adjusted to be most sensitive around the sample. Optimal shim values lead to sharper and more symmetrical resonances, Lorentzian line shape Difficulty of adjusting the field increases with: 1) increasing magnetic field 2) increasing resonance frequency of a nucleus 3) increasing bore of the magnet

RoomTemp and Cryo Shims 2 Types of shims- Room Temperature and Cryo 3 Axes of shims: 1) Those aligned with the vertical axis of the magnetic field (Z) 2) Those aligned with the horizontal axes of the magnetic field (X and Y)

Parameters Affecting the Optimal Shim Settings 1) Probe 2) Solvent- Dielectric constant, viscosity, etc., have large effects on shimming 3) Temperature 4) Sample- Various reason for sample effects, including concentration, paramagnetic species, residual impurities, especially ions/salts, particulate matter, fuzz, dust, etc.... 5) Sample Height 6) NMR Tube- Different tubes, different thickness of glass, quality of glass, scratches on the glass.

Axial Shims The Z shims are aligned with the Z axis of the magnetic field. Z shims are commonly referred to as the Spinning Shims The number in the shim refers to a power, e.g. Z1 = Z1, Z2 = Z2 ... Thus, each shim has a characteristic shape, Z1 is linear, Z2 is a parabola ... The curves that the shim gradients have can be used to optimize them automatically using gradients The amount of shim coils is different for different instruments.

Radial Shims Spinning: Radial shims are any shim with an X or Y Radial shims can be somewhat averaged out by spinning the sample Spinning: For 1D data, samples are spun in the magnet by use of a small air turbine (the spinner) with compressed air Spinning improves the field homogeneity because the nuclei see an average of the magnetic field, not the static inhomogeneous magnetic field Spin Rate Spinning produces spinning side bands Radial shims are approximately averaged out by spinning, axial shims are not.

Sharper line, better line shape => more intense signal Measuring Shimming What does ideal shimming mean? Perfect line shape and narrow line width, but to acquire a spectrum each time you change a shim is impractical and time consuming. Thus, shimming on the instrument is often accomplished by measuring either 1) Lock signal 2) Free Induction Decay/1H Spectrum The lock signal or lock meter is essentially a measure of the intensity of the deuterium solvent resonance. Sharper line, better line shape => more intense signal The real measure of the quality of the shims is the spectrum, and the best resonance to look at is a sharp singlet.

Adjusting the Shims 1) The lower the number, the more important the shim is for a good spectrum 2) The lower the number the more sensitive it is 3) If changing a shim by a large amount has little effect, then that shim is far from optimal. 4) Each shim is not completely independent of all other shims

Adjusting the Axial Shims In general, odd numbered axial shims affect line width, even numbered axial shims affect line shape

Adjusting the Radial Shims 1) Radial shims can only be adjusted with the sample NOT spinning because they are averaged out by spinning 2) When spinning, poorly adjusted low order radial shims cause spinning sidebands Non-Spinning

3) High order radial shims (X3, Y3 3) High order radial shims (X3, Y3...) cause broadening, even for spinning samples X3 Y3 off non-spinning spinning X3 and Y3 far off, non-spinning spinning (Lock 25% of normal) (Lock ~80% of normal) 4) Low order radial shims (X, Y, XZ, YZ) are sample (solvent) dependent and very temperature dependent. 5) High order radial shims are mostly only probe dependent, should not normally need to be adjusted except when changing probe (which is why there are saved shim files for probes).

SHIM ON LOCK AND FID DEMO

Pulsed Field Gradients Pulsed- A short time period Field- The B0 magnetic field Gradient- A variation in some quantity with respect to another. Pulsed Field Gradient = A brief change in the magnetic field with respect to distance. Normally applied in the z-direction although x and y pulsed field gradients exist as well. PFGs have a time, a power, and a shape. The time is usually 100's of s or a few ms For PFGs to be used, the instrument must have a gradient amplifier that creates the pulses, a connection to the probe, and a probe with gradient coils. Not all probes made for instruments have gradient coils. On recent NMR instruments (post ~1995), this is normal hardware

Gradient Shimming Shims are required to eliminate inhomogeneity of the magnetic field Thus, different parts of the sample will see a slightly different field. The shims of the magnet are supposed to make up for this problem. Gradient shimming uses magnetic resonance imaging to get an image of the magnetic field, then adjust shims according to the image. All spins that contribute to a single line should have the same Larmor frequency except that bad shimming causes slight differences- this is then translated to phase difference. Errors in the local magnetic field cause phase differences between protons with equivalent Larmor frequencies as a function of time in the x-y plane and distance in the sample tube:

2 minutes of gradient shimming is amazing!

Gradient Shimming Demo

Tuning the Probe to the Sample Tune to a nucleus. The detection coil in a probe is a piece of wire that forms a coil around the sample sitting in the probe. The transmitting coil and receiver coil are often the same coil in modern spectrometers. To transmit the full power of the radio frequency field into the sample and to fully amplify the signal of the receiver, the impedance of the coil must be matched with the transmitter and receiver. 2 capacitors- one changes the frequency of the circuit, the other changes the impedance. The object in tuning is to minimize the power reflected back to the radio frequency generator from the probe at the proper frequency The two capacitors are dependent upon each other, like the Z1, Z2 shims.

What Effects does Poor Tuning Have? 1) Primarily loss of sensitivity, signal-noise goes down 2) Longer 90 pulse When is Tuning Necessary? 1) Detect a nucleus other than the standard nucleus, normally that means something other than proton or carbon. 2) When sample conditions don't match the standard sample (not CDCl3). 3) When experimental conditions don't match the standard (e.g. not room temperature). 4) Any time that long experiments are acquired, as a poorly tuned probe will lead to lower signal-noise or a longer time to acquire equivalent signal-to-noise.

What Sample Conditions are Relevant? 1) Primarily solvent. Dielectric constant of solvent has great effect on the inductance of the coil 2) Concentration of sample. High concentration sample are often different as the solvent is no longer the only contributor 3) Ions. Salt concentration has a great effect on tuning What Experimental Conditions are Relevant? 1) Temperature. Probe tuning is very temperature dependent. 2) Nuclei for direct/indirect detection.

Probe Tuning Poor tune and match In tune and well matched

Double Tuned Probe

Measuring Tuning The normal measure of tuning on a Varian spectrometer is measuring the power that is reflected back by the sample. RF power is input to the sample and if the probe is poorly tuned, it will be reflected back to the console Table of Tune Meter Readings on 500 MHz Instrument when Tuned to Standard Solvent 1H 13C CDCl3 1 DMSO 101 576 D2O 95 572 Deutero-Benzene 6 353 Methanol 13 539 Acetone 49 505 30% menthol/70% CDCl3

Effects of Poor Tuning on Spectra 13C Spectrum of Standard (CDCl3 solvent resonance) Well-Tuned (1 scan, 1 second) Probe Tuned to DMSO (1 scan, 1 second) (64 scans, 3 minutes)

Tune demo

Temperature Regulation Temperature regulation has several purposes in NMR: 1) Dynamics- Exchange rates are affected by temperature. As the temperature is lowered, exchange (or rotation) slows. 2) Resonance Dispersion- Chemical shifts, particularly of protons that exchange with solvent, are affected by temperature. 3) Linewidth- Resonances sharpen as temperature is increased for molecules that tumble slowly in solution (mostly macromolecules). 4) Fluctuating temperature in 2D experiments (and some 1Ds) will cause noise, particularly in t1 (the indirect detect dimension). Instrumental effects of changing temperature: 1) Tuning of the probe 2) Shimming

Temperature demo

When should you use gradient shimming vs. manual shimming? When should you tune?