pulsed field gradient
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A pulsed field gradient is a short, timed pulse with spatial-dependent field intensity. Any gradient is identified by four characteristics: axis, strength, shape and duration.
Pulsed field gradient (PFG) techniques are key to magnetic resonance imaging, spatially selective spectroscopy and studies of diffusion via diffusion ordered nuclear magnetic resonance spectroscopy (DOSY).{{cite journal|author1=Johnson Jr., C. S. |journal=Progress in Nuclear Magnetic Resonance Spectroscopy|year=1999| volume=34|pages=203–256|title=Diffusion ordered nuclear magnetic resonance spectroscopy: principles and applications |issue=3–4 |doi=10.1016/S0079-6565(99)00003-5}}{{cite journal|author1=Neufeld, R. |author2=Stalke, D. |journal=Chem. Sci.|year=2015| volume=6|pages=3354–3364|title=Accurate Molecular Weight Determination of Small Molecules via DOSY-NMR by Using External Calibration Curves with Normalized Diffusion Coefficients |issue=6 |doi=10.1039/C5SC00670H|url=http://goedoc.uni-goettingen.de/goescholar/bitstream/handle/1/11975/c5sc00670h.pdf?sequence=1 |pmid=29142693 |pmc=5656982}} {{open access}} PFG techniques are widely used as an alternative to phase cycling in modern NMR spectroscopy.
Common field gradients in NMR
The effect of a uniform magnetic field gradient in the z-direction on spin I, is considered to be a rotation around z-axis by an angle = γIGz; where Gz is the gradient magnitude (along the z-direction) and γI is the gyromagnetic ratio of spin I. It introduces a phase factor to the magnetizations:
Φ (z,τ) = (γI)(Gz)(τ)
The time duration τ is in the order of milliseconds.