Probe sensitivity specifications

Despite my best attempts to convince users otherwise, many still want to run 1D ¹³C spectra. Because of the poor sensitivity of ¹³C this is a time consuming, and thus costly, experiment. Sensitivity is mainly determined by the strength of the magnet and the type of probe being used. Since we are currently considering acquiring new equipment, I was curious to see how different probes and fields would impact the time taken to record ¹³C spectra and so collected sensitivity specs for a variety of probes.

The table below lists sensitivity (S/N) specifications for the Skaggs NMR Facility's own probes (the first three listed) and for a variety of other probes that I was able to find on the web. Some of the probes do not give specifications for the ¹³C sensitivity. I also list the age of these specifications, though they do not appear to have improved significantly in the last decade. Clicking on the year will take you to the source documents for the specifications. 

The ¹H sensitivity specification is recorded using a standard sample of 0.1% ethylbenzene in chloroform-d, while the ¹³C sensitivity specification is measured using the standard ASTM sample - 40% 1,4-dioxane in benzene-d₆.

Field	Vendor	Probe type	1H S/N	13C S/N	Year
600	Bruker	1.7mm TXI	130	28	2021
600	Bruker	1.7mm TCI cryoprobe	1000	60	2021
600	Bruker	5mm TCI cryoprobe	6000	750	2005
600	Bruker	5mm TXI	1200	no spec	2021
600	Bruker	5mm BBI	1000	no spec	2005
600	Bruker	5mm BBFO iProbe	900	330	2021
600	Bruker	5mm BBFO	875	330	2011
400	Bruker	5mm BBI	600	no spec	2005
400	Bruker	5mm BBFO	250	160	2011
600	Varian	5mm 1H{15N-31P} PFG Indirect	625	no spec	2005
400	Varian	5mm 1H{15N-31P} PFG Indirect	350	140	2005
400	Varian	5mm 1H/19F/15N-31P switchable	350	140	2005

The 5mm TCI cryoprobe has easily the best sensitivity. Cooling of the preamplifiers and detection coils reduces the thermal noise in the electronic circuits, giving an increase in the signal-to-noise ratio. While the 1.7mm TCI cryoprobe's specs do not look as impressive, remember that they are obtained with less than a tenth of the material used by the other probes. Calculating the sensitivity on a mass basis would give numbers superior to those of the 5mm TCI cryoprobe.

The Facility's BBI probe has the same ¹H sensitivity specification as the 1.7mm TCI cryoprobe. Bruker does not provide a specification for ¹³C sensitivity on this probe, but I have been able to get around 220, making this probe better for recording 1D ¹³C spectra than the 1.7mm cryoprobe.

All of the Facility's probes are inverse probes, which means the inner coil is optimised for ¹H and the outer coil for X nuclei. This maximises ¹H sensitivity at the expense of sensitivity for X nuclei. The BBFO probe has the opposite geometry, where decreased ¹H sensitivity is accepted in exchange for higher X nuclei sensitivity. BBFO probes also allow measurement of ¹⁹F, which is not possible with the other probes.

Comparing the BBI and BBFO probes at 600 and 400 MHz we see that a 50% increase in field gives more than a 50% increase in sensitivity. This is consistent with the theoretical 3/2 power relationship.

The Varian ¹H{¹⁵N-³¹P} PFG Indirect probes should be comparable to the Bruker BBI probes but, surprisingly, they appear to be less sensitive. I expected the two vendors to have very similar specifications. The Varian ¹H/¹⁹F/¹⁵N-³¹P switchable probe has similar capabilities to the Bruker BBFO but the sensitivity values suggest it has an indirect geometry rather than direct.

So what do all these numbers mean for recording 1D ¹³C spectra? Remember, signal to noise increases as the square root of number of scans, so halving sensitivity means a four fold increase in scans (and time and cost) to get the same sensitivity. Comparing our 5mm and 1.7mm TCI cryoprobes the ¹³C sensitivity differs by a factor of 12.5. To get the same quality spectrum on a sample of the same concentration will take 156 times as long on the 1.7mm probe. This is not as bad as it sounds because it is likely that much higher concentrations can be used in the 1.7mm tubes. Looking at the 400 MHz BBI probe, the ¹³C sensitivity is not specified, but extrapolating from the other probes it is likely to be around 125. Compared to the 5mm TCI cryoprobe, comparable 1D ¹³C spectra would take 36 times as long to obtain on a 400 MHz BBI probe, and there is no possibility of compensating by increasing the concentration.