Valentin Ananikov
HomoSpoil Experiments on AVANCE




    Contents


Introduction

    This article describes five attempts of HomoSpoil usage on AVANCE DRX spectrometer. Although all the attempts were successful by no means HomoSpoil experiments might claim to the same quality level as performed with the Gradient unit. Whenever possible the standard Z-Gradient selected experiments should be used, since they are much simpler to calibrate and setup.


Warning!

    The HomoSpoil experiments require low level BSMS setup, if you are not well familiar with the hardware ask your "nmrsu" person to make setup and tests. The author does not accept any responsibility for the actions taken as a result of the use of this article . And do not say that you have not been warned!!

HomoSpoil Syntax and Setup

    To apply HomoSpoil Pulse use the following syntax:

        3u HS_ON
        150u     ;fixed trigger delay
        3u HS_OFF

    Note, that HS_OFF command does not stop the HomoSpoil pulse, it just resets the trigger for the next HS_ON step. The actual gradient pulse duration should be set in the BSMS. Therefore, the HomoSpoil pulse duration, compensation pulse duration and recovery time must be defined as a delay (i.e. in "d16").
Also "UNBLKGRAD" and "BLKGRAD" commands in the pulse programs should be changed to "LOCKH_ON" and "LOCKH_OFF" respectively.

    The next step is to setup BSMS. The following parameters should be defined:
        1) Gradient Channel (SCB-R for Z)
        2) Pulse Shape
        3) Sequence
        4) Pulse Amplitude
        5) Pulse Duration
        6) Compensation Duration
        7) Compensation Decays (at least one exponential function)

The way how to input these values using BSMS keyboard is described in the manual[1].

    Setup examples for COSY and WATERGATE experiments are given below, and the appropriate pulse programs are available in the Download section in the bottom of the document.



Calibration Hints

    There are two main disadvantages of HomoSpoil gradients:
            1) 'eddy-currents' and the absence of active-shielding
            2) large recovery time

    Due to second limitation it is difficult to perform Heteronuclear experiments, since they usually contain 1/(nJ) delays which are shorter than full recovery time.
    To minimize influence from the 'eddy-currents' HomoSpoil pulse should be followed by the Compensation Pulse. In most cases finding optimal values for the Compensation Pulses is a key-step in the whole experiment (for another solution of the problem see also COSYDQF section).

The following calibration procedure might be recommended:

    1) Set all compensation parameters to zero and adjust HomoSpoil Amplitude. Criterion: RG.
I.e.: start from the minimal amplitude(about 5%) and increase the value by the small step(1-5%), running "RGA" after each change. A reasonable RG value should be obtained.

    2) Setup one exponential decay function: Compensation Amplitude about the same as for HomoSpoil, Tau - 80-90% of Compensation Duration. Vary decay parameters until you get 'uncorrupted' spectrum (first slice in 2D).

    3) Optimize d16(better with "paropt"). Criterion: lineshape.

    4) Add further exponential decays and repeat steps 2 and 3, try to obtain d16 delay as short as possible.

Note: It is also possible to fit compensation parameters in "GS" mode, maximizing FID or the signals of interest.

A good starting point is parameters listed below, but do not just copy all of them! Very likely you will get garbage instead of FID.



HomoSpoil COSY experiment

Table 1. COSYHS BSMS parameters ("cosyhs.val" pulse program in the download section)
Output: SCB-R
Shape:sine
Sequence:11
Pulse1Amplitude*: 30000Duration: 1000usCompensation Duration: 6000us
Compensation1 Amplitude: 20000Tau: 5000us
d16**=9ms

*Maximum Amplitude range +-100000
**Look to the Pulse Program Text (Download Section)

HomoSpoil COSY spectrum
Fig.1. HomoSpoil COSY90 Spectrum of small peptide in DMSO-d6.
1024x256 points, NS=2, 308K.


HomoSpoil 1D WATERGATE experiment

Table 2. 1D WATERGATE BSMS parameters ("p3919hs.val" pulse program in the download section)
Output: SCB-R
Shape:sine
Sequence:11
Pulse1Amplitude*: 20000Duration: 1000usCompensation Duration: 5000us
Compensation1 Amplitude: 20000Tau: 3000us
d16**=6ms

*Maximum Amplitude range +-100000
**Look to the Pulse Program Text (Download Section)


HomoSpoil WATERGATE spectrum
Fig.2. HomoSpoil 1D WATERGATE Spectrum of small peptide in H2O:D2O=90%:10%; (the arrow indicates residual water signal).


HomoSpoil COSYDQF experiment

    The information concerning COSYDQF experiment has been kindly provided by Detlef Moskau.

    The 'eddy currents' effect from the first HomoSpoil pulse can be minimized by the second one if it is of the opposite sign (bipolar gradients), but in this case the second pulse has to be tuned. Probably, the approach might help in experiments with the even number of gradient pulses and alternated signs .

    This might be even simpler than the method described above, since one can omit compensation parameters. The only the amplitude of the second pulse has to be optimized: switch to "GS" mode and maximize the FID or the signals in the spectrum changing the amplitude value interactively via BSMS keyboard.

Table 3. COSYDQF BSMS parameters ("cosydfhstp.dmo" pulse program in the download section)
Output: SCB-R
Shape:sine
Sequence:12
Pulse1Amplitude*: 30000Duration: 2000us
Pulse2Amplitude*: -60000Duration: 2000us
Pulse2
(after fine adjustment)		Amplitude*: -51214Duration: 2000us
Compensation:not used
d16**=5-10ms

*Maximum Amplitude range +-100000
**Look to the Pulse Program Text (Download Section)

HomoSpoil COSYDQF spectrum HomoSpoil COSYDQF expansion
Fig.3. HomoSpoil COSYDQF Spectrum of small peptide in DMSO-d6 (left) and an expansion (right).
1024x512 points, NS=2, 308K.


HomoSpoil NOESY experiment

    The idea of homospoil/gradient enhancement of the NOESY experiment is more or less easy, and there are two different implementations already available. At the end of the evolution time the second 90o pulse produces "frequency-labeled" longitudinal z-magnetization as well as zero-, double-quantum and antiphase COSY-like coherences in the xy-plain (see [2]). Gradient pulse applied during the mixing time can effectively suppress all unwanted coherences leaving z-magnetization untouched. This can be achieved by the weak gradient pulse applied during the entire mixing time [3] fig.4.


NOESY sequence #1
Fig.4. Gradient NOESY pulse sequence [3] ("noesygptp.val" pulse program in the download section).
    More efficient defocusing of the xy-plain coherences can be achieved if the weak gradient pulse is replaced by two strong opposite sign gradient pulses with the 180o hard pulse between them fig.5.

NOESY sequence #2
Fig.5. Another Gradient NOESY pulse sequence ("noesygptp" pulse program in the standard library).
    Both of the above-mentioned methods can be implemented using the HomoSpoil pulses. Since the HomoSpoil pulses during the mixing period are applied only for defocusing, the experiment itself is not sensitive to the pulses quality and usually is not effected by "eddy-currents". In distinction to the other examples described in this article, HomoSpoil NOESY setup does not require any optimization and the spectra obtained are about the same quality as recorded with the standard gradient unit. A nice surprise!
    The parameters for the HomoSpoil NOESY experiments are given in the tab.4 and tab.5. An example spectrum is shown in fig.6.
Note, a single HomoSpoil pulse can not be longer than 100ms, therefore for the large mixing time values in the pulse sequence shown in fig.4 a series of the sine shaped pulses should be used instead.

Table 4. NOESY BSMS parameters (for the pulse sequence shown in fig.4, "noesyhsl1tp.val" pulse program; mixing time = 7x50m = 350m)
Output: SCB-R
Shape:sine
Sequence:1234567
Pulse1Amplitude*: 2000Duration: 50000us
Pulse2Amplitude*: 2000Duration: 50000us
Pulse3Amplitude*: 2000Duration: 50000us
Pulse4Amplitude*: 2000Duration: 50000us
Pulse5Amplitude*: 2000Duration: 50000us
Pulse6Amplitude*: 2000Duration: 50000us
Pulse7Amplitude*: 2000Duration: 50000us
Compensation:not used
d16**=1-5ms

*Maximum Amplitude range +-100000
**Look to the Pulse Program Text (Download Section)


Table 5. NOESY BSMS parameters (for the pulse sequence shown in fig.5, "noesyhstp.val" pulse program;)
Output: SCB-R
Shape:sine
Sequence:12
Pulse1Amplitude*: 20000Duration: 1000us
Pulse2Amplitude*: -20000Duration: 1000us
Compensation:not used
d16**=1-5ms

*Maximum Amplitude range +-100000
**Look to the Pulse Program Text (Download Section)

HomoSpoil NOESY spectrum
Fig.6. HomoSpoil NOESY Spectrum of small peptide in DMSO-d6.
1024x256 points, NS=2, 303K, mixing time=350ms.



Troubleshooting

    AVANCE HomoSpoil Mini-FAQ


Download

    Tested with XWINNMR 2.0/2.1 on AVANCE DRX500 spectrometer.

 -rw-r--r--  1  valentin user    plain text file    p3919hs.val
 -rw-r--r--  1  valentin user    plain text file    cosyhs.val
 -rw-r--r--  1  valentin user    plain text file    cosydfhstp.dmo
 -rw-r--r--  1  valentin user    plain text file    noesygptp.val
 -rw-r--r--  1  valentin user    plain text file    noesyhstp.val
 -rw-r--r--  1  valentin user    plain text file    noesyhsl1tp.val


acknowledgments
I thank Detlef Moskau for the helpful information concerning BSMS' internal life.


References


1. "BSMS User's Manual" (c) Spectrospin AG Fallanden, Switzerland ("GRASP" chapter).
2. H.Gunther "NMR Spectroscopy: basic principles, concepts, and applications in chemistry", 2nd ed., John-Wiley & Sons, 1996, 581pp.
3. R.Wagner, and S.Berger, J.Magn.Res., Ser.A, 123, 119-121 (1996).


Publications that cite this web article



The appropriate reference to this article:
Ananikov V.P., "HomoSpoil Experiments on AVANCE", 1998; http://nmr.ioc.ac.ru/val/hs/hs.htm

(c) Ananikov VP 1998

Valentin P. Ananikov
NMR Lab
N.D. Zelinsky Institute of Organic Chemistry
http://nmr.ioc.ac.ru/Staff/AnanikovVP/