Wednesday, February 4, 2015

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 30o 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.

No comments:

Post a Comment