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Hi, I was wondering why so many pulse programs use dummy scans. Correct me if I'm wrong but I think dummy scans are supposed to correct for machine noise, however, I have found that they are somewhat unnecessary. Can someone explain or give me some arguments in favor of dummy scans? |
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Dummy scans also steady state transients are helpful for a stable aquired data sets. These prescans are useful to get the 2D in the homogenious and quilibrated state. You are heating the sample extra with the RF pulses, ... this can not easily compensated by the VT unit. take 32 or 16 "clever and smart" scans first and acquire the rest. This is quite more practible. "dummy" is the wrong name for this here - if you take "zero" scans here, then the name "dummy" is correct ;-) Is heating still an issue in case of pulsed FT NMR ? I've never observed it by myself and I thought that it was just some kind of old timers legend, the ones who worked with CW NMR. I want to check it by measuring OH shift in methanol using short d1 and 90 1H pulses. - Arkadiusz Leniak (Nov 17 '14 at 23:54) Turning off VTU, waiting for temperature stabilization, obtaining reference 1H 1D spectrum of methanol-d4, starting one hour aquisition with rapid pulses (relaxation delay 0,5s, 90 degree 1H pulses), taking spectrum after pulsing, comparing two spectra. Should it be enough to find this heating ? - Arkadiusz Leniak (Nov 18 '14 at 00:03) You would see heating in something like an HSQC, not with a 1D single pulse experiment. HSQC on inverse probes can easily use >5W for decoupling, which even at the usual duty cycles is enough to cause a degree or so of heating if VT not regulated. TOCSY can be even worse, esp. for salty samples. - Pete Gierth (Dec 05 '14 at 10:44) |
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Dummy scans have some very important meaning in case of 2D experiments, especially. During 2D experiments, you are operating on some equilibrium state of spin systems. Some of them are excited, not relaxed, and so on. On the other hand, before any pulses, system is in a steady state. If you will not use dummy scans to build up equilibrium state, you will collect first few 2D incerements on the steady or transitional states. It's important in case when you are running experiments like NOESY or experiments with short relaxation times. Without dummy scans you will obtain additional artifacts on spectrum. |
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When you do many scans to have a spectrum with a good signal to noise, you want to have the same result for every scan. What happens is that in fact the second scan may give a different result with respect to the first scan. The simplest reason may be that you don't give the sample enough time to return to the equilibrium between the first and the second scan, then the starting point of the second scan will be probably different from that of the first scan and so the resulting FID. The idea behind the dummy scans is that after enough scans you reach a stedy state in which the next scan gives the same result as the previous, so you start recording the FID only after some "dummy" scans. This is what I understand. |
