Category: Basic science

Principles of Nuclear Magnetic Resonance (NMR)

Principles of Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance! You’re more familiar with this term than you may think. You’ve probably heard of Magnetic Resonance Imaging, or MRI. This is the technique used in hospitals to acquire images inside the human body. MRI is based on the principles of Nuclear Magnetic Resonance (NMR).

The NMR magnet

This is the most common use of NMR outside of research. In the medical field, MRI is used for acquiring images inside the human body. And to acquire the images we make use of very large superconducting magnets. Similarly, to run NMR experiments we use large superconducting magnets.

Bruker NMR magnet
NMR magnet

The sample, which is either a liquid or a powdered solid, is placed inside the magnetic field created by a superconducting coil, which is what we refer to when we talk about the NMR magnet. A superconducting material is a material that can transport the electrons without any resistance. And the continuous circulation of electrons then creates the magnetic field. To keep the coil superconducting, we have to keep it very cold. We have to keep the superconducting coil in a bath of liquid helium. The temperature of liquid helium is 4.2 Kelvin, which is -269 degrees Celsius, or -452 degrees Fahrenheit. There’s also a liquid nitrogen dewar surrounding the liquid helium dewar. This acts as a buffer between the very cold temperature of the liquid helium and the room temperature where the instrument is kept to slow down the liquid helium from boiling off and having to refill it often, which is a problem because of the high costs of liquid helium and the scarce helium resources. Some of the newer magnets no longer use liquid nitrogen; instead they have a system of liquid helium reliquefaction, thus reducing the need to refill the magnet with liquid helium. 

The sample goes inside a probe head that contains a radio-frequency coil which is used to send radio frequency pulses inside the sample. The probe head then goes inside the bore at the center of the magnet.

The nuclear spin

To understand the basic principles of Nuclear Magnetic Resonance we have to look at the nuclear spin. Let’s take the hydrogen atom for example. We’ve learned previously, when we talked about the atomic structure, that the hydrogen atom has one proton in the nucleus. And since the proton is a positively charged particle, that means that the nucleus is positively charged. The spinning charge then creates a magnetic field. So these nuclear spins act like tiny bar magnets.

nuclear spins in an external magnetic field
nuclear spins in an external magnetic field

In the absence of an external magnetic field, the nuclear spins in a sample have random orientations. When we place the sample in an external magnetic field, such as that created by the superconducting magnets we use in NMR and MRI, the spins align with the external magnetic field, denoted as B0 and shown by the purple arrow. The spins align either parallel or antiparallel with this external magnetic field. The spins that are parallel with the external magnetic field are at a lower energy, and the spin population at the lower energy is slightly higher than half. The spins that are anti-parallel are at a higher energy and the spin population at the higher energy is slightly lower than half. And the higher the magnetic field, the higher the energy separation between the parallel spins and the antiparallel spins. 

Another important thing to know about the nuclear spins in an external magnetic field is that, besides the fact that they are aligned with the magnetic field, they also precess around the direction of the magnetic field with a frequency that is dependent on the magnetic field. This precession frequency is called the Larmor frequency. This is similar to the precession of a spinning top.

The "resonance" in nuclear magnetic resonance

When we place the sample in the magnet, it is also inside another, smaller coil which can be used to send radio-frequency pulses into the sample. The electromagnetic waves in the radio region have the lowest energy and the lowest frequency of the entire electromagnetic spectrum. When we send radio-frequency pulses to the sample, if the frequency of these pulses matches the Larmor frequency, so the energy of those pulses is equal to the energy difference between the parallel and the anti-parallel state, then we have the phenomenon of resonance. Then we’ll have a transition of a spin from the lower energy state to the higher energy state. This spin transition is then detected, and as the magnetization returns to equilibrium, the signal is detected by the coil and it appears as a a free induction decay. This is the NMR signal as function of time and if we apply to this signal a mathematical transformation called a Fourier transform, we can transform the signal from the time domain to the frequency domain. Here we have peaks that appear at different frequencies, and this is called an NMR spectrum.

NMR signal recorded as a free induction decay in nuclear magnetic resonance
NMR signal recorded as a free induction decay in nuclear magnetic resonance

The NMR spectrum

The different peaks that we observe in an NMR spectrum correspond to the resonance frequency of atoms in a sample that have different local environments. These are atoms of the same type, let’s say hydrogen atoms, in a molecule where these atoms have different environments. Since the local environment for each hydrogen atom in a molecule could be different, this will be reflected in the NMR spectrum by the frequency at which the peak corresponding to that atom appears. Atoms in different chemical groups have characteristic resonance frequencies, which can be found in chemical shift tables. All the NMR spectra are referenced to the resonance frequency of the atoms in a reference sample, whose resonance frequency is known. An example of reference sample is tetramethylsilane (TMS), but there are other references used in NMR. 

chemical shifts in an NMR spectrum
chemical shifts in an NMR spectrum

The resonance frequency of an atom in a sample with respect to the reference sample is called a chemical shift, and the units that are used are parts per million, or ppm, of the frequency with respect to the reference. We can record these kinds of spectra for many other nuclei, not just hydrogens. Another one that is often use, especially for studying organic molecules is carbon, but we can use many other nuclei.

Applications of Nuclear Magnetic Resonance

NMR has many applications, varying from pharmaceuticals, to energy materials, to polymers, food, cosmetics, to biological research on proteins, DNA and RNA, and to the MRI applications of scanning the human body to acquire images inside the body.

In cultural heritage, NMR can be used to study the materials that cultural heritage objects are made of and it can be used to look at the degradation of objects of cultural heritage.

X-Ray Fluorescence for the Elemental Analysis of Artwork

X-Ray Fluorescence for the Elemental Analysis of Artwork

Imagine you’re Indiana Jones, and in one of your archaeological adventures, you find a mysterious object. You can learn about the composition of that object by using an elemental analysis method called X-ray fluorescence, or XRF. The advantage of XRF spectroscopy is that it can reveal the elemental composition in a non-invasive way. This way, we can continue to preserve the heritage objects.

How does X-ray fluorescence work?

To analyze an object, we bring the XRF instrument close to the area of the object we want to investigate, and we irradiate that area with X-rays. X-rays are high-energy electromagnetic waves.

schematic representation of the X-ray fluorescence method
Schematic representation of the X-ray fluorescence method.

After we send the beam of X-rays to that area, we then record with the help of a detector the radiation emitted as a response to the irradiation with X-rays. What we’re recording is the X-ray fluorescence. This recorded signal is then analyzed, and after the analysis, we can find out which chemical elements are present in the sample. 

There are different types of XRF instruments – some are larger and are fixed in labs; others are smaller and portable. With the portable XRF instruments, we can record experiments even in more remote locations like archaeological sites. The entire XRF experiment can be as fast as a few seconds or longer, lasting several minutes. 

X-ray fluorescence spectroscopy

When we irradiate a material with X-rays, we can find out which elements are present in that material by analyzing the X-ray fluorescence energy.

At the atomic level, when we irradiate the sample with a beam of X-rays, in response to those X-rays, one electron from the inner shells is removed from the atomWith this electron out of the atom, now there’s a vacancy in one of the inner shells of the atoms. Then, an electron from an outer shell comes to the inner shell and takes the place of the electron that was kicked out by the primary X-ray. When the outer shell electron moves to the inner shell, an energy is released in the form of X-ray fluorescence. This energy is then recoded by the detector. And by analyzing this energy, we can find out which elements are present in the sample. 

X-ray fluorescence spectroscopy
X-ray fluorescence spectroscopy

The X-ray fluorescence associated with each of these transitions has a precise energy value. This energy is given by the energy difference between the inner shell and the outer shell. In X-ray fluorescence spectroscopy, we obtain spectra where peaks are present at the energy values specific to the elements present in the sample. This reveals the elemental composition of the sample.

Applications of XRF in the elemental analysis of artwork

One of the advantages of using XRF in the elemental analysis of artwork is given by its non-invasive nature. This allows for the experiments to be recorded without damaging the investigated objects, which is of great benefit when analyzing precious objects of cultural heritage that need to be kept intact.

We can use XRF spectroscopy to analyze cultural heritage objects like paintings, sculptures, mosaics, coins, wall paintings, etc. The analysis can reveal details about the materials used in the construction of those objects. 

Besides the elemental analysis and getting information on which elements are present in a sample, we can also record XRF mapping of artworks. So if we analyze a painting, we can obtain a full two-dimensional map of that painting showing the distribution of different elements in the painting. Scientists from the Metropolitan Museum of Art in New York recoded XRF maps of Van Gogh’s Irises and Roses and Johannes Vermeer’s Mistress and Maid, revealing which elements are present in which areas of the paintings.

By knowing which is the elemental composition of artwork, scientists can propose the best conservation and restoration methods suitable for those objects.

The electromagnetic spectrum in cultural heritage

The electromagnetic spectrum in cultural heritage

How much of the world do you think we can see in visible light? The answer is: about 0.0035%. That’s just a very, very tiny portion of the electromagnetic spectrum that is the light that’s visible to our eyes! Everything else is energy, or electromagnetic radiation, of wavelengths that are not visible to the human eye. We need special equipment to detect electromagnetic radiation of all these other wavelengths. Let’s have a look at the electromagnetic spectrum, the electromagnetic waves, and the applications of electromagnetic waves in cultural heritage and in our daily lives.

What is the electromagnetic spectrum?

The electromagnetic spectrum consists of all the electromagnetic waves of all possible energies moving through space at the speed of light. The different regions of the electromagnetic spectrum are: gamma rays, X-rays, UV rays, visible light, IR light, microwaves, and radio waves. All of these electromagnetic waves have applications in cultural heritage.

The electromagnetic spectrum - the wavelength of the electromagnetic waves increases from the gamma rays to the radio waves.
The electromagnetic spectrum - the wavelength of the electromagnetic waves increases from the gamma rays to the radio waves.

What are electromagnetic waves?

Electromagnetic waves are composed of oscillating electric and magnetic fields, as shown in the figure below. The electric field (in blue) and magnetic field (in red) are perpendicular to one another and perpendicular to the direction of propagation of the wave. These waves travel in vacuum at the speed of light, and they carry electromagnetic energy. Their wavelength, defined as the distance between two successive peaks of the wave, can be very short or very long, or anything in between. The shorter the wavelength, the higher the frequency of the wave, and the higher the energy of the electromagnetic radiation.

electromagnetic waves composed of oscillating electric and magnetic fields
Electromagnetic waves composed of oscillating electric and magnetic fields.

Gamma rays

The gamma rays are the highest energy waves of the entire electromagnetic spectrum. That means they have the highest frequency and the shortest wavelengths.

They are produced in the radioactive decay of atomic nuclei and in nuclear explosions. Even though we can’t see them with the naked eye, using specialized equipment, we can detect the gamma rays produced by grand events, such as supernova explosions to smaller-scale events, like the radioactive decay of radioisotopes. 

In cultural heritage, the gamma radiation can be used to determine the composition of certain artifacts because of the different energy signature of each chemical element in a gamma-ray spectrum.

X-rays

X-rays are also very high in energy, but not quite as high as the gamma rays.

You may be familiar with the X-rays from the medical field, where X-rays are used to acquire images of your bones. Another familiar application of X-rays is in airport security, where the contents of your luggage are scanned with X-rays.

In cultural heritage, we can use X-rays in X-radiography to obtain images of paintings in X-rays. This can sometimes reveal other paintings beneath the visible paint layer.

X-rays can also be employed in cultural heritage thought a method called X-ray fluorescence, or XRF (my favorite scientific method used in cultural heritage). Using XRF, we can determine the elemental composition of objects of cultural heritage. This can help us identify the materials used in creating those objects. And knowing the chemical composition of heritage objects can help us find the best conservation conditions for those objects.

Ultraviolet rays

The Ultraviolet (or the UV) region is the region between the visible and the X-ray regions of the electromagnetic spectrum.

Probably the most common topic where UV comes up is the UV rays that we receive from the sun. On the one hand, this UV light is beneficial because it helps the organism produce vitamin D. On the other hand, overexposure to the sun’s UV light can lead to sunburn, or more severe consequences, such as skin cancer. So be careful with the exposure to UV light, whether that’s from the sun or from artificial sources like tanning beds. 

In cultural heritage, UV lamps can be used to examine the surface of art objects. For example, they can be used to detect retouches and restorations of art objects, as these are done on the object surface. This way, through the details revealed in UV light, we can learn about the history of a certain object.

Visible light

The visible region of the electromagnetic spectrum is composed of electromagnetic radiation whose wavelengths we can detect with our eyes.

There are many sources of visible light. Some of these include the sun, the aurora borealis and australis, or the fireflies.

In cultural heritage, in visible light, we can observe and admire our favorite works of art. But, visible light can also damage the objects. That’s why you see certain objects, like old books and parchments in museums, kept in dimmer light.

Infrared light

The infrared (or IR) radiation is lower in energy than the visible light.

In the infrared region, the most common application is the TV remote. But it can also be used in thermal imaging, which works by detecting the radiation with infrared wavelengths, which is emitted by hot objects, including human bodies. This is very useful currently, during the COVID-19 pandemic, to detect the high temperature in people.

In cultural heritage, IR spectroscopy can be used to identify the chemical composition of art objects. And infrared images can be used to view the sketch layers of paintings, underneath the painting layers that we can observe with our eyes. This way, we can see what the original sketch looked like, and, by comparing it to the final product, we can see if the artist changed his or her mind while creating the painting.

Microwaves

With lower energy than the IR radiation, this is where our microwave ovens work. But these waves are also used in communications and radars. 

In cultural heritage, microwaves treatment of artwork can help with the disinfestation from various biological agents infesting these artworks.

Radio waves

The radio waves, at the very left of the electromagnetic spectrum,  are the least energetic electromagnetic waves.

Their most common applications are in radios, communications, and air-traffic control. One of my favorite applications of radio waves is in the search for extraterrestrial intelligence (or SETI). This is a project that uses big radio antennas to detect signals from outer space. This signal is then analyzed for any patterns that might indicate intelligent communication from someone from outer space. Let’s keep on searching for messages from E.T.!

In cultural heritage, the radio waves are used through a scientific method called Nuclear Magnetic Resonance, or NMR. Mobile NMR (NMR in low magnetic fields) can reveal the stratigraphy of paintings. It can also be used to study the aging of heritage objects and to monitor the water penetration in wall paintings, which can lead to their deterioration.

The building blocks of tangible heritage

The building blocks of tangible heritage

Tangible heritage is an essential aspect of our world’s cultural heritage. It consists of physical items varying from small artifacts to large buildings and archaeological sites. But what are these heritage objects, such as, Leonardo da Vinci’s Mona Lisa, for example, made of? Atoms, molecules, and crystals!

The atom and the atomic structure

atomic structureThe atom is composed of a central region, called the nucleus, and of electrons, which surround the nucleus. The nucleus is made up of protons, positively charged particles, and neutrons, neutral particles. Because of the positively charged protons in the nucleus, the nucleus itself has a positive charge. 

The electrons, which are outside the nucleus, are negatively charged particles that circle around the nucleus. They are attracted by the positive charge of the nucleus, thus keeping the atom together.

atomic nucleus and electrons

The electrons are organized on different shells surrounding the nucleus, represented here by the blue circle and the orange circle. Each of these shells corresponds to a different energy level. So the shells closer to the nucleus (the blue circle here) have lower energy than the ones that are further away (orange circle). The lower energy shells get populated first, and when they are filled with the maximum number of electrons allowed in that shell, then the outer shells get populated.

The number of electrons around the nucleus is equal to the number of protons inside the nucleus. And this gives us the atomic number (Z), which is the number of protons in the nucleus. This is a number that is specific to each element in the periodic table. The elements in the periodic table are organized function of their increasing atomic number. In the periodic table, we see that the further we go in periods and down the groups, the higher the atomic number. So we’ll have more and more protons in the nucleus and more electrons surrounding it. That means that we need to add more shells in order to accommodate all the electrons of the heavier elements. 

These shells and electrons and how the electrons can move from one shell to another one are very important when discussing different scientific techniques, especially X-ray fluorescence (XRF), my favorite scientific technique that is used to study tangible heritage. Different scientific methods that can be used to study heritage objects made up of all the different elements. Some techniques are better suited than others to study different elements.

From atoms to molecules

Atoms can bond to one another to form molecules. There are different types of bonds that the different elements can form with one another, but what’s important to know here is that they can associate to form molecules with varying degrees of complexity.

the water moleculeA simple example is the water molecule. It is made up of only three atoms – one oxygen atom and two hydrogen atoms. The water molecule is formed by binding the two hydrogen atoms to the oxygen atom. 

Why, you may wonder, do we care about water in cultural heritage?! We care a great deal about water in cultural heritage. That’s because humidity, that is, water, can lead to the aging and, therefore, the deterioration of art objects. 

Similarly to creating a simple molecule, like water, by putting three atoms together, we can do the same with a larger number of atoms and a larger variety of elements. This way, we can create much more complex molecules, each with its own special properties.

From molecules to crystal structures

Atoms and molecules can further associate to form crystals. Starting from one molecule, we can get different types of crystal structures depending on how the molecules are organized with respect to one another in the unit cell, how many molecules of the same kind there are in the crystal unit, how big the unit cell is, etc. These are called polymorphs, and that’s another one of my favorite research topics, besides cultural heritage.

titanium dioxide crystal structure
titanium dioxide crystal structure

In the unit cell, each atom has an exact position, and by translating the unit cell in all the directions of space, we create the material which is based on the composition and structure of that unit cell. An example of crystal structure relevant to cultural heritage is titanium dioxide, the chemical name for the titanium white pigment. This pigment led to the discovery of a series of painting forgeries and to the arrest of the art forger. This forger is Wolfgang Beltracchi, and he was caught because science detected the presence of titanium white in a certain painting where it shouldn’t have been.

Knowledge of the atomic, molecular, and crystal structure of tangible heritage materials can help us learn more about the materials which the artists used in their work. That will help us better conserve and maybe even restore them, and it can also help in catching art forgers.

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