Getting Incorrect Lineshape Specifications

One can easily fool themselves by getting line shape specifications (see BLOG entry for Tuesday October 23, 2007) that are better than the truth. Erroneous specifications will be obtained on the standard line shape sample (1% CHCl3 in acetone-d6) if sufficient time is not allowed for complete relaxation to occur and the height of the 13C satellites is used as a guide to measure the line width. This error results because the T1 of the 13CHCl3 isotopomer is shorter than that of the 12CHCl3 isotopomer. Therefore, if the signals are partially saturated, the 13C satellites will be over-represented with respect to the singlet. If the height of the satellites is used as a guide to measure the line width on the singlet, the line shape specifications will be narrower than they would be if complete relaxation was allowed to occur. This is illustrated in the figure below. The trace on the left is a fully relaxed single scan spectrum using a 90 degree pulse with an 8 second acquisition time. The trace on the right was acquired under identical conditions however the spectrum was partially saturated by collecting 15 dummy scans with an inter-pulse spacing of 8 seconds immediately prior the receiver being turned on. The spectra were scaled such that the singlets had the same height. Note the difference in signal-to-noise ratio and the relative heights of the satellites compared to the singlet.

One can still obtain the correct line shape specification in a partially saturated spectrum by measuring the linewidths at 0.55% and 0.11% of the height of the singlet if the satellites are not used as a guide.