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I recently read an article about executing a WET sequence for water suppression in urine samples. The 90°excitation pulse should be calibrated at the beginning of the data acquisition for each sample. what causes the changes of pulse length between different samples? I've read the book "200 and more nmr experiments" and other manuals but I couldn't find the answer. I'm confused about how to selected a proper resonance in mixture for calibration. Should I choose the water resonance in urine spectra for calibraion? Because other signals were covered by this huge resonance. Thanks a lot! |
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ok? I will answer my own question now~~ Since each nucleus in compounds has a different chemical environment, each has a specific 90°pulse length. This experiment should be carried out using a presaturation pulse sequence (zgpr). an estimated value of 90°pulse length was used in data acquisition. then we look at the 360°pulse (keep the same phase correction). The 360° pulse corresponds to a 'null'. Searching for null by changing 360° pulse length slightly. Reference: How to calibrate the 1H 90° pulse on the Bruker NMRs. http://web.mit.edu |
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Hi, The changes in 90 degree pulse length are due to variations in salt concetration in the samples, which can be a particular issue in urine. Assuming you're using a Bruker system, you can use the "pulsecal" AU program to calibrate the 90 degree pulse. This uses the stroboscopic nutation method of Wu and Otting: Journal of Magnetic Resonance 176 (2005) 115–119 and is rather quick. It uses the water signal for calibration, but that doesn't matter - the 90 degree pulse is the same (at least to meaningful levels of accuracy). As a more general comment we (at Bruker) don't use WET at all for urine samples, only NOESY-presat with gradients (pulse sequence noesygppr1d). There is a standard parameter set in TopSpin, PROF_1H which is set up for generic profiling experiments and has pulsecal in the acquisition AU program. But there is a lot more to this in terms of setup and you should contact your local Bruker representative for more details of how we set things up for metabonomics. Significant optimisation is needed and it is well worth getting to grips with what's required before you get involved in running real samples! A significant problem we see is people starting off down the wrong path with metabonomics, and having to backtrack significantly to get to doing things in the right way. A lot of time can be wasted in this way, and potentially a lot of samples... Thank you for your answer,It's very useful.I still have a question. I used "pulsecal" to calibrate the 90°pulse of the water signal.The result is 24 microseconds. It's twice longer than the real signals of this sample.Is it appropriate to use water signal for optimisation? Thanks a lot^_^~~ - brighman (Dec 26 '13 at 18:10) |
I found some information from the book ?Structural Biology Practical NMR Applications? "An alternative way to calibrate the 90 pulse is to observe real signals of an aqueous sample rather than the water signal, using a PRESAT sequence",I'll make an attempt.I'm still finding the stepbystep tutoria - brighman (Dec 19 '13 at 04:24)