Protocol: Br24q  Sequence: br24q.c  Apptype: solidseq1d

Description: 

Quadrature BR24 multiple-pulse acquisition for high resolution proton 
NMR. This sequence uses a variable-length preparation pulse and fine 
phase array to prepare the magnetization in quadrature perpendicular to 
the multiple-pulse axis of precession (1,1,1). The four variable length 
pulses are calculated assuming that pwXprep has a 90-degree flip angle. 
BR24 has a cycle time of 36*tau with 24 pulses per cycle.  Set phXprep 
= -45.0 degrees.  Br24q is used with slow spinning (< 2500 Hz). 

Additional Software: 

reset_br24q - This macro uses the calibrated values of pwX90 and aX90 
to calculate the initial parameters for br24X.  This macro is run from 
the protocol. 

Setup: 

Load a calibrated data set and select the protocol Br24q.  For a new 
calibration set the observe nucleus to proton and calibrate pw90X with 
a sample of Benzene doped with Chromium AcAc using Onepul, Xx and Xmx. 
An alternate sample is "RTV" silicon rubber spinning at about 2.5 kHz. 

An alternate calibration is to set the proton pulse width pwH90 using 
Tancpx, a sample of Glycine and 13C observe. After calibration set the 
nucleus to proton and set tpwr, aX90 and pwX90 (of protons) equal to 
the calibrated values of dpwr, aH90 and pwH90 of Tancpx.  Set the 
sequence to Onepul and turn off the decoupler.  Observe a proton 
wideline spectrum with a 200 kHz spectral width and gain < 10. A good 
spin rate for this measurement is 10 kHz.  

Load the data set obtained with Onepul of either calibration and select 
Br24q.    

Set aXprep = aX90, set pwXprep = pwX90 and phXprep = -45.0. Set aXbr24 
= aX90 and set pwXbr24 = pwX90.

Optimize the sample window.  For defaults set tauXbr24 = 3.5 us. Set 
npaXbr24 = 10. Set r1Xbr24 = 1.0uus, r2Xbr24 = 0.5uus and r3Xbr24 = 
1.0uus.  Set rof3 = r3Xbr24 on the Receiver page (important!!).  Set 
npaXbr24 = 10 for a 1.0 us reciever on-time. 

Set sw = 1/(36.0*tauXbr24) to obtain one point per cycle. Obtain a 
spectrum of the glycine (spin rate 2200 Hz) or doped benzene with a 
minimum of 4 scans.  The quadrature phase cycle must be used to avoid 
artifacts.  See the discussion below to optimize the spectrum. 

Optimizing the Spectrum: 

All multiple-pulse spectra are strongly affected by phase transient of 
the probe.  The effect of phase transient is to shift the frequencyof 
the entire spectrum and increase the individual linewidths. In an 
optimized setup, a Benzene (RTV) line should be on resonance with both 
the sequences Onepul and Br24q. Likewise a good spectrum of glycine 
obtained with tof = 0.0 and proper referencing with setref should be 
centered around 5 ppm and have the minimum linewidths.  

A spectrum that is strongly affected by phase transient will be shifted 
to lower frequencies and it will be necessary to set an even lower 
frequency offset to get the best linewidths.  A common practice in the 
presence of phase transient is to shift the carrier to about -5 ppm and 
obtain the entire spectrum at higher frequencies. Good spectra can be 
obtained this way and historically a low frequency carrier was needed 
anyway for non-quadrature experiments. 

Phase transient can be minimized through the adjustment of the cable 
length between the preamp and the probe.  A small further adjustment 
can be made by moving the center of the tuning dip off of the carrier 
frequency. The manual file tunerp has the procedures for minimization 
of phase transient. 

Spectra obtained with Br24q also can show a strong center glitch or 
"pedestal". The pedestal is a real signal that results from spins that 
have been spin-locked along the axis of precession of BR24. Spins from 
the sample end up in pedestal if the preparation pulse pwXprep is 
adjusted incorrectly.  Also, proton background spins from the module 
end up along the precession axis because they have a low flip-angle and 
are not refocused by BR24. Vespel modules have a strong proton signal 
that may be larger than the signal of the sample so pedestal removal 
involves an an exact subtraction of two large numbers. Kel-f modules 
have fewer protons but have a 19F background.  Also, spectra that are 
not obtained with a multiple of 4 scans will have a pedestal. 

The pedestal can be removed completely, even in the presence of high 
background, by adjustment of pwXprep if the pulses are close to 
resonance and phase transient has been minimized. Array pwXprep over 
several microseconds around the 90-degree pulse. If one would rather 
not make the effort, move the carrier and pedestal to lower a frequency 
where it is out of the spectrum. 

BR24 spectra are best obtained with a low spinning frequency, in the 
range 1500 to 2500 Hz. At faster speeds the rotor cycle interferes with 
the multiple pulse cycle and lowers performance. 

Referencing and Scaling Br24q:   

Select the optimum offset and obtain data with a multiple of 4 scans. 
Optionally fine-adjust the scale factor with two lines of known ppm.  
Put a cursor on each line and set scalesw = (ppm2 - ppm1)*sfrq/delta. 
To reference a scaled spectrum put a cursor on the line of interest and 
type rl(ppm*sfrq/scalesw).  scalesw should be about 2.5 for a good BR24 
spectrum.

Parameter Groups:  

prepX:  Module:  no 
Sequence:  br24q.c      
Description:  Implements a variable length pulse to provide quadrature 
              detection about a tilted precession axis. Typically followed 
              by a windowed multiple-pulse acquisition.
Parameters:  Sequence Page
       aXprep - the amplitude of the obs pulses. 
       pwXprep - 90-degree flip on the obs channel. 
       phXprep - small-angle phase offset of the pulse. The phase must 
              be reset to zero after the pulse. 

scaleX: (processing)  Module:  no 
Sequence:  br24.c    
Description:  Shows the F2 scale factor, scalesw, for evolution of the 
              obs nuclei under a multiple-pulse waveform. 
Parameters:  Sequence Page
       scalesw - the F2 scale factor. Typically scalesw is the inverse 
              of the mutiple-pulse scale factor.         

br24X: Module:  yes
Sequence:  br24.c 
Description:  Implements BR24 on obs with acquisition in the first 
              window.
Parameters:  Sequence Page
       aXbr24 - amplitude of obs pulses.
       pwXbr24 - 90-degree flip on obs.
       r1Xbr24 - pre-pulse delay (similar to rof1). 
       r2Xbr24 - post_pulse delay (similar to rd or rof2).
       r3Xbr24 - T/R-to-rcvron delay (must equal rof3 or 2.0 us if rof3 
              is not defined). 
       tauXbr24 - time for one "one-tau" window and pulse. 
       npaXbr24 - number of 100 ns ticks in the detection window. 
       chXbr24 = 'obs' must be set (not shown).
 Implementation:  WMPA br24 = getbr24("br24X");
 Underscore function: _br24(br24,phXbr24);  phXbr24 is the overall 
              phase table.  
