BCMB/CHEM 8190 - University of...

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NMR Instrumentation BCMB/CHEM 8190 Biomolecular NMR

Transcript of BCMB/CHEM 8190 - University of...

Page 1: BCMB/CHEM 8190 - University of Georgiatesla.ccrc.uga.edu/courses/BioNMR2010/lectures/pdfs/NMR_Inst_2010… · Instrumental Considerations - Block Diagram of an NMR Spectrometer B

NMR Instrumentation

BCMB/CHEM 8190

Biomolecular NMR

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Instrumental Considerations - Block Diagram of an NMR Spectrometer

B0

Magnet

Probe

Lock

Receiver

Transmit

Computer

Sample

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Modern NMR Magnets are Super Conducting Solenoids

• Materials: NbTi < 10T, NbSn > 10T

• Max Field (2000): 23.5 T (900 MHz)

• Advantages: high field, stability, homogeneity > 1 : 109

B0 i

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Shim Coils

B0

Z

Inherent field profile

B0(z) = B0 + a1 z + a2 z2 + ….

Design coils to produce –a1 z, -a2 z2, etc ai are set by adjusting current in each of 16-40 coil sets – x, y, z, powers and cross terms

i i

z a linear z coil

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Room Temperature Shim Assemblies

From Modern NMR Techniques for Chemistry Research, Andrew E. Derome

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Shimming Origin: old electromagnets used shims under pole pieces

Interactive shimming

B0

Z

ν0

Auto shim: - based on amplitude of lock signal Gradient shim: observing effect of imposed field gradient –

can deconvolute field inhomogeniety

Adjust z2

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A Deuterium Lock Stabilizes the Field

X axis (absorptive)

Y axis (dispersive)

ω0 ω0

Drift of field will produce positive and negative signals Depending on direction when dispersive signal is observed

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Detection of NMR Signals: Probe Coils

E x

y

M

B’

E ∝ dB’/dt ∝ dM/dt ∝ γB0M0 = γ3B02h2I(I+1) / (12π2kT)

S/N ∝ (γC / γH)3 = 64-1

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NMR Probe

x y

M coils: to B0

dual role: -generate B1 -detect dMy/dt

solenoid coil: specialized applications

Helmoltz coil: normal applications

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Coil Design: Needs/Implications Requirements: 1). Maximize B1 from applied rf current

2). Maximize signal from M of sample

r

s

look at 2

So: -minimize coil size to increase S -maximize sample volume to increase M -these demands conflict one another (compromise)

surface area of coil

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Tuned Radiofrequency (RF) Circuits Improve Efficiency

Example: parallel LC (inductance/capacitance) circuit

tune

i R

-the circuit is resonant (tuned) when the impedance is purely resistive (χL= χC)

-the tuned resonance frequency is υ0

-Quality factors (Q) measure how much current is stored vs dissipated

– cryoprobes reduce resistance and loss Q ~ 1000

χL

χC

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What does an NMR Probe Look Like?

This is for a 7T magnet – 13C observe at 75 MHz

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Probes are delicate – glass, teflon, ceramic

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Receiver Functions

1.  Amplify signals: 10 microV -> 1 V Three stages 30dB/stage dB = 10 x log(Pout/Pin) = 20 x log (Vout/Vin)

2. Shift Frequency: 100 MHz -> 1 KHz convenient digitization in the “rotating” frame often uses a “double-balanced mixer”

Signal in

Ref in

Signal out (low pass filtered)

Vout = a0Vin + a1Vin2 + …

2cos(ωs)cos(ωr) = cos(ωs-ωr) + cos(ωs+ωr)

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Quadrature Detection Allows distinction of +/- Frequencies

Sig

Ref + 90

Ref

Real out

Imaginary out

x y

x y = =

My

My

Mx Two detectors

One detector

Actually split signal And use two reference Frequency phases

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Why use Quadrature Detection? To put ωref in middle of spectrum.

ω0

No Quad

ω0

With Quad

Reduced band width of audio filter increases S/N by 1/√2