Mobile Nuclear Magnetic Resonance! What does "mobile" mean for NMR? When we talk about mobile NMR, we're definitely not considering moving around the large superconducting magnets (weighing several tones) that we use in conventional NMR. In mobile NMR, we use much smaller magnets, enabling researchers to take the experimental setup to remote locations, such as archaeological sites or museums. This allows us to record the NMR experiments on site, without the need to relocate precious cultural heritage objects.
In mobile NMR, instead of using the large superconducting magnets, as we do in conventional NMR, we use permanent magnets to create the magnetic field. The magnetic field created by these magnets is a low magnetic field and it’s also inhomogeneous. One example of mobile NMR systems is the NMR-MOUSE (MObile Universal Surface Explorer).
The magnetic field of the NMR-MOUSE systems is created by a combination of permanent magnets. On top of the magnet, we add the radio-frequency (RF) coil, which sends the radio-frequency pulses into the sample, the same as the RF coil does in high-field NMR. The difference here is that now the coil is a surface coil, instead of a solenoidal coil, like we have in high-field NMR. This magnet system with a surface coil makes it easier to bring the magnet close to the surface of the object we want to examine, without the need to remove a piece of the object to place it inside the coil to run the experiments.
A non-invasive experimental setup is ideal for cultural heritage because it means we can record NMR experiments non-invasively, so we don’t damage precious objects of cultural heritage. This ability of running the NMR experiments on the surface of the object is why mobile NMR is also called unilateral NMR, or single-sided NMR.
When the sensor is close to the object, the region where we acquire the NMR experiments is called the “sensitive volume”. This region is at a certain depth within the sample, and it varies depending on the magnet characteristics. This region can be at a distance somewhere between around 2 mm to around 2.5 cm from the sensor.
NMR Relaxation Experiments
Longitudinal NMR relaxation (or spin-lattice relaxation, T1)
In my blog post on the basic principles of NMR, we’ve seen how the spins align in the presence of an external magnetic field. When we then apply radio frequency pulses to the sample, the spins flip, and after we stopped applying RF pulses, the magnetization will return to equilibrium, moving from a perpendicular alignment to the magnetic field, back to its parallel alignment to the external magnetic field. And this return of magnetization back to its equilibrium state is called T1 relaxation, or longitudinal relaxation, or spin-lattice relaxation.
We can follow the magnetization vector, shown here in orange as it returns to equilibrium in time. This can be a slower or a faster process. How fast this is happening depends on the type of sample we measure. By fitting these curves with exponential functions, we can identify the exact the value of the longitudinal relaxation time in different samples.
Transverse NMR relaxation (or spin-spin relaxation, T2)
The other type of NMR relaxation is called T2 relaxation, or transverse relaxation, or spin-spin relaxation. Right after applying the RF pulses, the spins are in the transverse plane. Just as the spins move to the transverse plane, the spins are in phase, but in time, they start dephasing. And as they dephase, the transverse magnetization decreases. And the more the spins dephase in time, the more the transverse magnetization decreases.
The decrease of the transverse magnetization in time can be slower, or faster, depending on the type of sample we’re measuring. By fitting the T2 decay curves with exponential functions, we can find out the values of the transverse relaxation time in different samples.
Besides fitting the T2 decays with exponential functions to get the T2 values, we can also do a different type of analysis on the T2 decay curves called depth profiling.
NMR depth profiles
We can integrate the first region of the T2 decay, and that will give us the proton density. That’s a value that’s proportional to the amount of hydrogen atoms that are present in that region of the sample where we recorded the T2 experiment. By measuringT2 experiments, and extracting the proton density at different depths within a sample, we can record depth profiles of the sample.
In a depth profile, the signal intensity is shown as a function of the depth in the sample. Each point in a depth profile represents the proton density at a certain depth. By analyzing depth profiles, we can identify different regions in the sample that have different proton densities. (highlighted in blue, orange and green in the above example). The delineation of these regions with different proton densities gives us the stratigraphy of the sample. For example, when analyzing the stratigraphy of an oil painting, we can discriminate between the different paint layers and the canvas.
How to record an NMR depth profile with the NMR-MOUSE
To record an NMR depth profile, we place the magnet in a positioning system that allows us to move the sensor with respect to the sample. By placing the Profile NMR-MOUSE on a moving table, we can move the magnet up or down with respect to the sample, which is fixed on the top table of the positioning system.
Because the sample is fixed, as we move down the table with the magnet, then that also moves down the sensitive volume where we're recording the NMR experiment in the sample. This means that the sensitive volume moves down with every step the NMR-MOUSE moves down. So the sensitive volume where we record the experiments changes as we change the sensor position. This way, we can acquire experiments at different depths within a sample. But the sample doesn’t have to be fixed on top of the magnet. We can rotate the whole positioning system with the magnet, such that if the sample is vertical (i.e. wall paintings), we can bring the magnet close to the sample from a lateral position. In this case, to record experiments at different depths, we just move the magnet left and right with respect to the sample.
Mobile NMR for cultural heritage
Mobile NMR has so many applications in cultural heritage. It can be used to study the stratigraphy of paintings, to study treatments and aging of cultural heritage materials, monitor the water penetration in wall paintings, which we know is a factor that leads to their deterioration. It can also be used to study ancient mummies and bones, old paper and parchments, and even musical instruments.