Carbon sensitivity

I am often asked "Which is the best magnet for acquiring ¹³C spectra?"
The short answer is "The magnet with the 5mm TCI probe".

Probe comparison

Since our two magnets are very similar the ¹³C sensitivity is determined mostly by the probe being used. The 5mm probes allow more sample to be placed in the detection coil, so if you are not mass limited the 5mm probes are better than the 1.7mm. The 5mm BBI and the 5mm TXI cryoprobe have similar ¹³C sensitivity because on the 5mm TXI cryoprobe the ¹³C coil and preamp are not cryocooled - only the ¹H coil and preamp are. On the 5mm TCI cryoprobe both the ¹H and ¹³C coils and preamps are cooled giving it the best ¹³C sensitivity. The table below lists the results of a standard ¹³C sensitivity test that uses the ASTM sample (40% C6D6 in 1,4-dioxane). Clearly, the 5mm TCI cryoprobe is the best.

Probe	Sensitivity
1.7mm TXI cryoprobe	65.04
5mm BBI	220.47
5mm TXI cryoprobe	258.31
5mm TCI cryoprobe	1092.45

Sucrose spectra

To produce a more visual comparison of ¹³C sensitivity I used the same parameters to record 1D ¹³C spectra of 2mM sucrose on four different probes. Each spectrum was recorded using a 30^o pulse, a 0.5 second relaxation delay with ¹H decoupling for NOE enhancement and 4500 scans. Each spectrum took 107 minutes to acquire. The sucrose ¹³C peaks appear at 104, 92 and between 81 and 60 ppm. Only the spectrum acquired with the 5mm TCI cryoprobe shows what I would consider believable peaks.

These spectra also serve as a guide for the amount of material required to obtain a 1D ¹³C spectrum. The 2mM sucrose sample in the 5mm tube contains about 342μg of sucrose (MW 342.2965), which is only enough if using the 5mm TCI cryoprobe.