The Gradient-BIRD (G-BIRD) element is an isotope-editing application of the so-called excitation sculpting approach in which the refocusing element is a BIRD sequence ( 95JMRA137-115 ).

It is based in the double pulsed-field gradient spin-echo (DPFGE) sequence, G₁-S-G₁-G₂-S-G₂, where G₁ and G₂ are strong PFGs and S is a BIRD cluster. It can be used to select natural-abundance carbon-13 isotopomers (the satellites) and efficiently remove signal arising from ¹H-¹²C magnetization or viceversa by only changing the phase of a single pulse.

 

Application of the G-BIRD scheme:

• In phase-cycled 2D HMQC and HMQC-TOCSY experiments (  96JMRA247-118 ) affords an useful way to obtain undistortioned cross-peaks from which analysis of the proton multiplets can be carried out with simplicity.
• X(w1) filter in NOESY experiment ( 99JMR37-136).
• Removing direct responses in HMBC spectra ( 99JMR93-137).
• Equivalent G-BIRD scheme using RF gradients instead of PFG ( 97JMR80-127 and 99JMR93-137)

Easy implementation on AVANCE spectrometers equipped with pulsed-field gradients (PFGs).

The standard way to implement the g-BIRD cluster in a pulse program is:

... p1 ph1 50u UNBLKGRAD p16:gp1 d16 p1 ph2 d2 pl2:f2 (p2 ph3) (p4 ph8):f2 d2 p1 ph4 50u p16:gp1 d16 (p4 ph9):f2 50u p16:gp2 d16 p1 ph5 d2 pl2:f2 (p2 ph6) (p4 ph10):f2 d2 p1 ph7 50u p16:gp2 d16 4u BLKGRAD ...

in which:

p1 and p2 are the 90º and 180º ¹H pulses (in microseconds) applied at a power level pl1 from the f1 channel.

p4 is the 180º ¹³C pulse (in microseconds) applied at a power level pl2 from the f2 channel.

d2 is set to 1/(2*¹J_CH).

Gradients are defined by the gp, p16 and d16 commands.

All phases must be specified at the end of pulse program.