Spectroscopy, MRI and Other Technologies: Benefits, Costs and Canadian Contributions

From theory to the hospital

Everything in this unit so far may seem abstract: orbitals, spins, energy levels. Yet the same ideas run machines that diagnose disease, catch impaired drivers, analyse drinking water and study distant stars. In this lesson you will connect the theory to real technologies, weigh their benefits and costs to society, and meet some of the Canadians who helped build the science.

Spectroscopy: reading the light

Spectroscopy is the study of how matter absorbs or emits electromagnetic radiation. Because energy levels are quantized, every element and molecule has its own fingerprint.

• Atomic absorption and emission spectroscopy (from the Bohr model and electron transitions): labs measure metals such as lead in drinking water or in children's toys by how strongly a sample absorbs the element's particular wavelengths. Flame colours and fireworks use emission. Astronomers find the composition and temperature of stars from their spectra.
• Infrared (IR) spectroscopy (from molecular vibrations): bonds stretch and bend like springs, and each type absorbs IR light of particular energies. An O–H bond absorbs around 3200 to 3600 cm⁻¹, C–H around 2850 to 3000 cm⁻¹, and C=O around 1700 cm⁻¹. Chemists use IR to identify functional groups (very useful in the Organic Chemistry unit), forensic labs use it to identify drugs and fibres, and some evidential breath testers measure ethanol by its IR absorption. Carbon dioxide and water vapour absorb IR from Earth's surface, which is the basis of the greenhouse effect.
• Mass spectrometry sorts ions by mass-to-charge ratio. It measures isotope abundances (as in Grade 11), detects banned substances in athletes, and identifies pollutants.
• X-ray crystallography shines X-rays through crystals. The diffraction pattern reveals where the atoms are. Rosalind Franklin's X-ray images were key evidence for the double helix of DNA, and Dorothy Hodgkin used the method to find the structures of penicillin, vitamin B₁₂ and insulin.

Magnetic resonance imaging (MRI)

Protons have spin (like electrons), which makes hydrogen nuclei behave like tiny magnets. In an MRI scanner:

1. A very strong magnetic field (commonly 1.5 or 3 tesla, tens of thousands of times Earth's field) lines up the hydrogen nuclei in the water and fat of the body.
2. Pulses of radio waves at just the right frequency tip the nuclei out of line (a quantized energy absorption, like an electron absorbing a photon).
3. As the nuclei relax back, they emit radio signals. Different tissues relax at different rates, so a computer can build detailed images of soft tissues such as the brain, spinal cord, joints and organs.

The magnets in most MRI scanners are superconducting coils cooled with liquid helium to about −269 °C (see the next lesson).

Benefits: MRI uses no ionizing radiation (unlike X-rays and CT scans) and shows soft tissue in great detail. Costs and issues: scanners are expensive to buy and run, so wait times for non-urgent scans can be long; patients with some metal implants or devices cannot be scanned; the scanners need helium, a non-renewable resource; and the strong magnet means that metal objects must be kept out of the room for safety.

The related technique, NMR spectroscopy, uses the same physics to work out the structures of molecules and is one of the most important tools of modern chemistry.

Lasers and nuclear energy

Lasers depend on electrons in excited states being stimulated to drop to lower levels all together, giving light of one wavelength travelling in step. Lasers read barcodes, carry internet data along fibre-optic cables, cut metal and perform eye surgery.

Nuclear energy comes from the structure of the nucleus that Rutherford discovered. Ontario generates a large share of its electricity in CANDU nuclear reactors, which were designed in Canada. The benefit is electricity without greenhouse-gas emissions during operation; the challenges include the long-term storage of radioactive waste and the cost of building and refurbishing plants.

Canadian contributions

Canadian scientists have made important contributions to the science in this unit:

• Ernest Rutherford did his Nobel-winning research on radioactivity at McGill University (1898 to 1907). Harriet Brooks, who worked with him there, is often described as Canada's first female nuclear physicist; her research helped show that radioactive elements give off a radioactive gas (now known to be an isotope of radon).
• Gerhard Herzberg, who worked at the University of Saskatchewan and later at the National Research Council in Ottawa, won the 1971 Nobel Prize in Chemistry for his contributions to knowledge of the electronic structure and geometry of molecules, especially free radicals, using spectroscopy.
• Ronald Gillespie of McMaster University developed VSEPR theory with Ronald Nyholm (1957), the theory you used to predict molecular shapes.
• Richard Bader, also at McMaster, developed the quantum theory of atoms in molecules, a way of identifying atoms and bonds from the electron density calculated by quantum mechanics.
• John Polanyi of the University of Toronto shared the 1986 Nobel Prize in Chemistry (with Dudley Herschbach and Yuan T. Lee) for work on the dynamics of chemical reactions; his study of the infrared light given off by newly formed molecules contributed to the development of chemical lasers.
• Bertram Brockhouse, who did his prize-winning research at the Chalk River Laboratories in Ontario and later taught at McMaster, shared the 1994 Nobel Prize in Physics (with Clifford Shull) for developing neutron-scattering techniques for studying solids; Brockhouse's part, neutron spectroscopy, shows how the atoms in a solid move and vibrate.
• Donna Strickland of the University of Waterloo shared half of the 2018 Nobel Prize in Physics with Gérard Mourou for a method of making extremely short, intense laser pulses, now used in eye surgery and precision machining. (The other half went to Arthur Ashkin for separate work on optical tweezers.)
• The Canadian Light Source in Saskatoon is a national synchrotron facility where researchers use intense X-ray and infrared light to study the structure of materials, from new batteries to proteins.