A while back I posted a question regarding upgrade to gradient
capability. There have been many helpful inputs and many people also
indicated interest in getting the summary.
The general feeling is that Gradient is VERY useful, and if money
is available triple axis gradient is much better than Z-axis gradient.
Some advantage of triple axis gradients are: (1) You can do different axis
gradients in different parts of a "hyphenated" pulse sequence (e.g.,
HSQC-DQFCOSY). This gives significantly better results; (2) Even with
simple sequences, triple axis gradient gives significantly better water
suppression; (3) It is a lot easier to do grdient shimming with triple axis
gradient; (4) You can do micro-imaging of small samples such as seeds.
The following are comments that I have obtained. Hope this is helpful.
Thanks again for all the helpful inputs and comments.
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We have z gradient on our AMX500 and do all our COSYs and HMQCs with it.
Water suppression is also superior. It was thrown in the bargain so I don't
know how much it cost. You can do a lot of basic stuff with z only. If
xyz is highly expensive it might not be worth the expense.
We have bought z gradients on our to-be-delivered DPX300 and expect our
routine users to make a lot of use of it for short COSYs and HMQCs.
You will enjoy the improved 2D noise levels and avoiding the phase cycling.
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We have a VXR-500S that we have upgraded with low-power PFG (10G/cm). The
upgrade was painless, the canned Varian experiments all work very well. We
were doing the experiments within 1 day of the upgrade, and use it
constently. We have both 5mm and 3mm micro probes.
10G/cm gradients are adequate for a surprising range of experiments at 500
MHz. 10G/cm is not sufficiently strong to avoid all artifacts, and is not
adequate for most water suppression/protein experiements. At this time, our
500 is primarily used for natural products, small peptides, sugars, etc.
The low power single-channel z-gradient upgrade and probe was a very cost
effective way to buy a couple more years out of our spectrometer.
I would not have tried for triple axis on our older instrument. I really
can't find a coherent need (with the possible exception of labelled
proteins). However, even with labelled proteins, older spectrometers,
including ours, probably have enough collective problems with noise,
stability, old operating systems, etc, to make a triple axis upgrade very
questionable. If there is a clear need for 3-axis, 3 or 4 or more channels,
I would probably work in the direction of a new console--the difference in
cost may not be that substantial, and the performance improvements would
probably be dramatic.
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We upgraded our VXRS-500 to Z axis and have found it very useful for HMQC/HMBC.
We don't do any 3-D so I can't comment.
Upgrade was painless but I'd upgrade to a Sparc at the same time if you are
not there now.
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I have recently purchased a Bruker DRX400 with a triple axis gradient
inverse probe. The gradient make a difference in HSQC data, and a
HUGE difference in HMBC data. I'm not sure that triple axis is that
much better than single, though. If you runthese expts routinely, its
well worth the investment.
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Recently we did a z-axis upgrade on a bruker DPX 400. It was really
quick to add a gradient coil to our probe. Gradient experiment requires
very little operator control, are robust and easy in most standard
cases, and they provide an impressive time gain. I particularly like the
application to inverse (or MQfiltered) spectroscopy, because of the
excellent suppression of the central peak.
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We added gradients to our Bruker AMX500 earlier this year and have had no
problems with them at all. We are a chemistry laboratory and only have single
axis z-gradients but so far I haven't found an experiment that I would like
to do that needs triple axis gradients. Even solvent suppression in high H2O
concentrations seems perfectly feasible with a single gradient.
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We just finished up examining the performance of the single and triple axis
gradients. We ran the DQF-COSY with triple and Z-axis gradients. The triple
axis gradient experiment was _far_ superior to the single axis. There was no
water streak in F1(indirect), and a slight ridge in F2(direct) that was easily
removed with baseline correction. The single axis had a large water streak
which required a much lower gain setting. Take a look at JMR A 113,
265-270(1995). The spectra really do look like that. Most times I tend not to
believe some of the spectra shown in papers(ie how much massaging they had to
do to get it to look that good). If you run the DQF often it would be worth
getting the triple axis gradients.
The other use of the triple axis gradients is for gradient shimming the
non-spins as well as the spins. Currently Bruker is the only vendor that has
triple axis gradient shimming, and Varian has only Z-axis gradient
shimming(they do have triple axis gradients though). I think that you don't
really need X and Y gradient shimming although it is nice for touching up the
lower order non-spins. There is pretty much no need to adjust the higher order
non-spins once they're dialed in during the mapping.
I don't think there's any additional experiments(other than some diffusion
expts) that require XYZ gradients, I'm not 100% sure of this though. I do
remember seeing a couple of talks by A.J. Shaka, in which he outlined the use
of gradients in his selective excitation sequences. The sequences used many
gradients of different strengths and that having orthogonal gradients would
help in implementing these experiments.
Finally, as you know cost is always an issue, from gradient amps to the
probes. Do you want triple gradients or perhaps another probe etc etc....
I hope I've been some help. I wouldn't mind seeing what other people's
comments are. By the way we are going to get triple axis gradients.
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T. Chyau Liang, Ph.D. Biochemistry & Molecular Biology
Assist. Prof. & Director, NMR facility UT-Houston Medical School
(713) 792-5265 P.O.Box 20708, Houston, TX 77225
(713) 794-4150 (fax) tliang@utmmg.med.uth.tmc.edu
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