Monday, May 4, 2015

Scans vs slices

When running a multi-dimensional NMR experiment you have to decide how many scans and how many slices (or rows, or increments) to use. If your time is limited, as it normally is, increasing one means reducing the other. So which is better more scans or more slices?


To try and answer this question I collected four 1H-13C HSQCs each with a different number of scans (1,2,4,8) and t1 increments (512,256,128,64), but with the same total experimental time (11 minutes 34 seconds). The sample I used was 5% ortho-dichlorobenzene in acetone-d6, which shows two aromatic resonances. Shown below is an expansion of the aromatic resonances in the overlaid HSQCs. The spectra are offset so that all the peaks can be clearly seen. The first thing to notice is that the width of the peaks in the 1H dimension is the same in all the spectra, but the width in the 13C dimension increases as the number of slices collected decreases.


Measuring the signal to noise in a row taken through the left peak we find that increasing the number of scans increases the signal to noise, as you might expect, but a doubling of scans does not double the signal to noise. The increase is in fact much less. Measuring the linewidth of the same peak in the 13C dimension we find that the linewidth scales linearly with the number of slices.

Spectrum Scans Slices 1H S:N 13C linewidth (ppm)
plum 1 512 346.27 0.64
pink 2 256 350.29 1.27
green 4 128 385.25 2.60
blue 8 64 392.42 5.16

Since linewidth, and thus resolution, scales linearly with the number of slices, whereas signal to noise does not scale linearly with the number of scans, increasing the number of slices is a more effective use of time. In practice, you need a minimum number of scans to detect a signal and a minimum number of slices to obtain sufficient resolution for the experiment to produce useful data. If you have more time available than what is required for the bare minimum number of scans and slices then I recommend increasing the number of slices.

2 comments:

  1. Tadeusz MolinskiMay 5, 2015 at 10:14 AM

    Thanks for this edifying blog (I always enjoy reading your blogs). The increasing resolution in F1 with increased number of increments (slices) makes sense, but not the non-linear increase in S/N with increasing scans.

    One possible explanation is sample related: partial saturation of the (very long T1) 1H signals in 1,2-dichlorobenzene. Is it possible that explains both phenomenon? You didn't state what D1 you were using.

    With a typical largish natural product molecule where T1s are diminished to below 1 s (except CH3s), loss of S/N with increasing increments may be a problem with 2D 1H-13C. By the 512th increment, there may not be much 1H signal left in the FID due to the long evolution delay. We never had this problem in the 'old days' running direct detect 13C-1H experiments (e.g. Horst Kessler's COLOC for long-range correlations), I assume, because 13C T1s were longer, anyway.

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    1. You are correct Ted, the slight increase in S/N with increasing scans does not make sense. Theoretically the S/N should increase as the square root of the number of scans. As you suggested, the most likely explanation is that the relaxation time of these resonances is longer than the relaxation delay of one second that I used. But this offers more support for the conclusion that extra slices are more time efficient than more scans. Only rarely will one have a relaxation delay long enough for full recovery of the signal between scans. Most of the time additional scans will only increase S/N at less than the square root of the additional scans.

      It is also true that with larger molecules the signal may have decayed to zero if a large number of increments are collected (this can be a consideration when setting up protein experiments), but I have not seen it be a problem with natural products. In a recent paper from Williamson et al (http://dx.doi.org/10.1021/jo500333u) they reported observing additional resonances in a LR-HSQMBC experiment as they increased the number of increments up to 640.

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