NMR Spectrometer Virtual Tour
Brief tour of a Bruker Avance DMX-500 spectrometer
Brought to you by the Boston University School of Medicine NMR Core Facility
Click on the small images for a closer look!
Here is an overview of the whole system. From front to back we have a computer that controls everything, a large console that houses most of the electrical components. The 11.7 Tesla magnet is the large “tin can” in the background. The samples are placed in a probe within the magnet. The probe is attached to console by a number of cables.
Here are the tubes that hold the samples. These 5 mm tubes are 8 inches tall and require a volume of about 0.5 ml. Other sized tubes are used, for example we also use 10 mm tubes. NMR is a relatively insensitive technique so the concentration of the sample needs to be high as possible. One rule of thumb is that your generic protein should be soluble to at least 0.5 mM.
The sample drops into the magnet on a cushion of air through the orange tube at the top. The sample tube drops into the probe which is centered in the coils of superconducting wire that produce the magnetic field. The superconducting wire is kept in a bath of liquid helium (-269 C) so that it has no resistance to current. The liquid helium bath is kept cold by a dewar of liquid nitrogen (-196 C). You can see the ice build up on the rubber tubes at the top of magnet where the liquid nitrogen boils off. The whole magnet is isolated from the building by the three black vibration isolation columns at the bottom.
Here Jon is holding a broad band probe that he is about to install into the bottom of the magnet. The small box on the floor at the left with the green lights is a preamplifier that is also useful for tuning and matching the inductance of the probe with that of the cables and console to ensure that maximum amount of power reaches the sample.
Here is what a probe looks like once it is installed. There is not all that much to see since most of the probe in contained within the magnet. All that is left outside are the cable connections and the tuning and matching devices.
Here is a close up of the external region of our broad band probe. On the left are the bnc cable connectors to attach the probe to the preamplifer and console. On the right is a glass bulb connector that attaches to a hose that carries an air stream cooled by a cooling unit. This allows the temperature of the sample to be carefully regulated. The gold sliders with the numbers are used to tune and match this probe for each sample and each type of nucleus.
This is the gradient controller. In conjunction with an amplifier (out of view on top of the console) the gradient controller allows us to rapidly and reproducibly apply a magnetic gradient to the sample. This is extremely useful for selecting coherences and water suppression. It also makes shimming the magnet (to produce a homogeneous field around the sample) a snap.
This unit controls the shims. The shims are extra coils that surround the probe. By adjusting the current in the shim coils the magnetic field around the sample can be made more homogeneous. Shimming is somewhat analogous to adding wedges (shims) around a door or window to insure it’s level, hence the term shimming. Good shims are essential for narrow lineshapes and good water suppression.
A variable temperature unit is essential to regulate the temperature of the sample in the probe.
Two components here actually. On the left is the unit that modifies the pure frequency from the frequency generator to produce the actual frequencies that will be sent to the transmitters and ultimately the probe. On the right is a digitizer that collects the signal from the probe after a pulse.
Finally we get to the three transmitters on this spectrometer. The center transmitter is dedicated to 1H signals while the top and bottom can be tuned to a variety of nuclei. Typically, we use them for 13C, 15N, and 31P.
Here I am at the computer. From here you can control the whole beast (except for tuning adjustments which must be done at the probe). Experiments can be set up and run and the collected data processed using the Bruker software. However, we do most of our processing and analysis at other workstations so we can keep the spectrometer running full tilt. Note that I’m signing the log book. Always sign the logbook!