How Laser Analysis Works

        Science | Optics

Among their many uses, lasers can even help ex-Rolling Stones fans rid themselves of their rock 'n' roll roots.
Among their many uses, lasers can even help ex-Rolling Stones fans rid themselves of their rock 'n' roll roots.
Fredrik Skold/­Getty Images

­When Theodore Maiman fired the first laser pulse in 1960, some described the new technology as a solution in need of a problem­. But scientists quickly discovered that lasers were not novelties and began developing practical applications for them. Today, physicians use lasers to repair damaged retinas, bleach birthmarks, remove regrettable tattoos and make delicate surgical cuts. T­he electronics industry incorporates lasers into a variety of components, including bar-code scanners, optical-storage systems and computer printers. And manufacturers harness the energy of lasers to drill holes in diamonds and cut materials ranging from titanium to plastic.

Lasers are particularly important to the field of analytical chemistry. Experts in analytical chemistry develop techniques to determine the chemical composition of substances. Some of these techniques measure physical properties, such as mass, refractive index or thermal conductivity. Other techniques rely on electric charges or current to help identify constituent parts of a substance. And still other methods measure the absorption, emission or scattering of electromagnetic radiation. This latter category is known as spectroscopy.

Laser-based spectroscopy is becoming an increasingly important analytical tool. Imagine a laser system mounted on a Mars-based rover. When it fires a laser pulse at Martian dirt, instrumentation on the rover detects the reflected light and determines the chemical makeup of the soil. Now imagine a soldier bearing a laser system mounted on his back. Using a hand-held probe that contains both laser and optics, the soldier analyzes a suspicious roadside package and determines it contains explosive material.

­This­ kind of laser analysis seems like science fiction, but it's not. Scientists today have at their disposal many different types of laser-based analytical techniques. We'll explore some of those techniques in this article and examine one -- laser-induced breakdown spectroscopy, or LIBS -- in detail to illustrate the fundamentals of the technology. As we do, you'll learn how laser analysis is helping to advance everything from homeland security, forensics and medical diagnostics to health care, archeology and art history.

­First, let's dive deeper into the basics of analytical chemistry to understand how laser technology fits into an arsenal o­f tools and techniques that can be used to determine the elemental or molecular building blocks of substances.

The Laser as Analytical Tool

A researcher prepares samples for mass spectrometry research.
A researcher prepares samples for mass spectrometry research.

Take a break from reading for a moment to survey your immediate surroundings. You can clearly see solid objects, such as your computer, desk and printer. Liquids -- the soda in your glass and the water in your aquarium -- are just as clearly visible. Even materials that seem invisible, such as odors and air currents, can be detected by other senses. All of this "stuff" -- what scientists call matter -- is composed of molecules, or combinations of atoms. Analytical chemists like to break apart molecules into their constituent atoms or just know what molecules or atoms make up a particular substance.

­Over the years, analytical chemistry has yielded several tools and techniques. Some of these tools and techniques are qualitative in nature: They identify the elements or compounds present in a substance, what chemists call analytes. Other techniques are quantitative: They actually measure the amounts of some or all of the analytes. In either case, the chemical analysis involves stimulating a sample with light, electricity or a strong magnet to cause a change in the sample that will reveal its chemical makeup.

Take mass spectrometry, a tried-and-true analytical technique. Suppose a biologist wants to know what toxins are present in contaminated fish. She could take a very small piece of muscle tissue from the fish and dissolve it in a liqui­d solvent. Then she could place the liquid in the reservoir, or inlet, of the mass spectrometer. From there, the liquid leaks into an ion chamber, where it's bombarded with a beam of electrons. This bombardment converts the atoms and molecules in the sample into electrically charged particles known as ions. The biologist then uses electric or magnetic fields to separate the various ions according to their mass or electric charge, thus revealing the specific toxins, such as DDT, present in the fish.

In recent years, the laser, which is used as the stimulating agent, has become a valuable tool in chemical analysis. The various laser-based techniques used to analyze substances fall roughly into two categories: optical and nonoptical detection methods.

For instance, one nonoptical laser analysis technique actually lets scientists "hear" different elements. It's known as pulsed-laser photoacoustics, and it involves directing a laser onto a sample. As the sample absorbs energy from the laser, it heats up and expands, creating an acoustic pressure wave. A piezoelectric transducer, which converts mechanical vibrations into electrical pulses, listens to the waves and helps chemists identify molecules in the sample.

Ion mobility spectrometry, or IMS, is another nonoptical method. In IMS, a laser first ablates, or cuts, minute particles from the sample surface before ionizing the material. Ions created by laser-blasting the sample are introduced into a fast-moving stream of gas. Scientists measure how quickly the ions move through the gas stream, which is affected by the size and shape of the ions.

­Laser analysis based on optical detection methods is called laser spectroscopy. Spectroscopy involves stimulating a sample and then analyzing the resultant spectrum -- the range of electromagnetic radiation emitted or absorbed. Spectroscopy is so vital as an analytical tool that it warrants a closer look. On the next page, we'll get into the basics of spectroscopy to understand how the electromagnetic signature of every element can act like a fingerprint.

Spectroscopy Basics

In this simple image of an atom, you can see the electrons existing in separate orbits as Bohr envisioned.
In this simple image of an atom, you can see the electrons existing in separate orbits as Bohr envisioned.

Spectroscopy takes advantage of the fact that all atoms and molecules absorb and emit light at certain wavelengths. To understand why, you must understand how atoms are structured. You can read about atomic structure in How Atoms Work, but a quick recap here will be helpful. In 1913, a Danish scientist by the name of Niels Bohr took Ernest Rutherford's model of the atom -- a dense nucleus surrounded by a cloud of electrons -- and made some slight improvements that better fit with experimental data. In Bohr's model, the electrons surrounding the nucleus existed in discrete orbits, much like planets orbiting the sun. In fact, the classic visual image we all have of atoms, such as the one on the right, is modeled after Bohr's concept. (Scientists have since moved away from some of Bohr's conclusions, including the idea of electrons moving around the nucleus in fixed paths, instead envisioning electrons congregating around the nucleus in a cloud.)

In the Bohr atom, an electron in a particular orbit is associated with a specific amount of energy. Unlike planets, which remain fixed in their orbits, electrons can hop from one orbit to another. An electron in its default orbit is in its ground state. To move from the ground state to an orbit farther away from the nucleus, an electron must absorb energy. When this happens, chemists say the electron is in an excited state. Electrons generally can't remain in an excited state indefinitely. Instead, they jump back down to the ground state, a move that requires the release of the same energy that enabled them to become excited in the first place. This energy takes the form of a photon -- the tiniest particle of light -- at a certain wavelength and, because wavelength and color are related, at a certain color.

An atom absorbs energy in the form of heat, light or electricity. Electrons may move from a lower-energy orbit to a higher-energy orbit.
An atom absorbs energy in the form of heat, light or electricity. Electrons may move from a lower-energy orbit to a higher-energy orbit.

­Each element on the periodic table has a unique set of Bohr orbits that no other element shares. In other words, the electrons of one element exist in slightly different orbits than the electrons of another element. Because the internal structures of the elements are unique, they emit different wavelengths of light when their electrons get excited. In essence, every element has a unique atomic "fingerprint" that takes the form of a set of wavelengths, or a spectrum.

William Wollaston and Joseph von Fraunhofer developed the first spectrometer to see the spectral fingerprints of elements. A spectrometer is an instrument that both spreads out light and displays it for study. Light enters a narrow slit and passes through a lens that creates a beam of parallel rays. These rays travel through a prism, which bends the light. Each wavelength is bent a slightly different amount, so a series of colored bands is produced. A second lens focuses the light on an exit slit, which allows one color of light to pass through at a time. Scientists often use a small telescope, mounted on a turntable, to observe the color exiting through the slit more easily. Then, the scientist rotates either the telescope or the prism to bring another color into view. By noting the angle of the prism or the telescope, the wavelength of the exiting light can be determined. Using a spectroscope to analyze a sample may take several minutes, but it can reveal much about the light source. Some spectrometers, known as spectrographs, are set up to photograph the spectrum.

­As you'd expect,­ th­e spectrometer is an essential tool for chemists conducting laser spectroscopy. Next, we'll briefly look at some of the most important types of laser spectroscopy.

Overview of Laser Spectroscopy

An ultraviolet imaging spectrograph took this picture of Saturn's C rings (left) and B rings (right). The red bands indicate "dirty" particles while cleaner ice particles are shown as turquoise in the outer parts of the rings.
An ultraviolet imaging spectrograph took this picture of Saturn's C rings (left) and B rings (right). The red bands indicate "dirty" particles while cleaner ice particles are shown as turquoise in the outer parts of the rings.
NASA/JPL/University of Colorado

In laser spectroscopy, chemists train a laser beam on a sample, yielding a characteristic light source that can be analyzed by a spectrometer. But laser spectroscopy falls into several different schools, depending on what kind of laser chemists favor and which aspect of an atom's excited response they study. Let's look at some of these more closely.

Name­d after the Indian scientist who discovered it, C.V. Raman, Raman spectroscopy measures the scattering of monochromatic light caused by a sample. The beam from an argon-ion laser is directed by a system of mirrors to a lens, which focuses monochromatic light onto the sample. Most of the light bouncing off the sample scatters at the same wavelength as the incoming light, but some of the light does scatter at different wavelengths. This happens because the laser light interacts with phonons, or naturally occurring vibrations present in the molecules of most solid and liquid samples. These vibrations cause the photons of the laser beam to gain or lose energy. The shift in energy gives information about the phonon modes in the system and ultimately about the molecules present in the sample.

Fluorescence refers to the visible radiation emitted by certain substances because of incident radiation at a shorter wavelength. In laser-induced fluorescence (LIF), a chemist activates a sample usually with a nitrogen laser alone or a nitrogen laser in combination with a dye laser. The sample's electrons become excited and jump up to higher energy levels. This excitation lasts for a few nanoseconds before the electrons return to their ground state. As they lose energy, the electrons emit light, or fluoresce, at a wavelength longer than the laser wavelength. Because the energy states are unique for each atom and molecule, the fluorescence emissions are discrete and can be used for identification. ­

­LIF is a widely used analytical tool with many applications. For instance, some countries have adopted LIF to protect consumers from pesticide-tainted vegetables. The tool itself consists of a nitrogen laser, a sensor head and a spectrometer, all packaged in a small, portable system. An agricultural inspector directs the laser­ on a vegetable -- lettuce leaves, let's say -- and then analyzes the resulting fluorescence. In some cases, the pesticides can be identified directly. In other cases, they must be identified based on how they interact with chlorophyll, the green pigment present in all leaves. ­

Laser ablation inductively coupled plasma optical emission spectroscopy (LA-ICP-OES) has a ridiculously complicated name, so let's start with ICP, which is the heart of the analytical technique. The "P" in ICP stands for plasma, an ionized gas consisting of positive ions and free electrons. In nature, plasmas usually form only in stars, where the temperatures are high enough to ionize the gas. But scientists can create plasmas in the lab using something known as a plasma torch. The torch consists of three concentric tubes of silica surrounded by a metal coil. When an electric current passes through the coil, a magnetic field is created, which in turn induces electric currents in a gas, usually argon, allowed to pass through the silica tubes. This excites the argon gas and creates the plasma. A nozzle at the end of the torch acts as an exit for the plasma.

Now the instrument is ready to analyze a sample. In the laser-based version of ICP-OES, a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser is used to cut, or ablate, a few microscopic particles from the sample's surface. That means analysis isn't limited to liquids -- solids are fair game, as well. The ablated particles are then carried to the pl­asma torch, where they become excited and emit light.

­Laser-induced breakdown spectroscopy (LIBS) is similar to LA-ICP-OES, except that the laser both ablates the sample and creates the plasma. Because LIBS has become increasingly popular in recent years, we're going to give it more attention next.

A Closer Look at Laser-induced Breakdown Spectroscopy

The set-up for laser-induced breakdown spectroscopy
The set-up for laser-induced breakdown spectroscopy

­Laser-induced breakdown spectroscopy, or LIBS, has been advanc­ing significantly over the last decade. It can analyze solids, liquids and gases and can return results rapidly, with very little damage to the sample. Not only that, it can do its work from a distance, unlike some analytical tools that require samples being brought to a lab. For example, LIBS is being used to detect surface contaminants in a few nuclear reactors around the world. The laser in these systems is located several meters from the reactor surface and yet is still able to function effectively. These systems keep most of the instrumentation behind shielding material, with only a mirror and a lens (which are used to steer and focus the laser beam respectively) exposed to the nuclear radiation.

We'll consider other practical applications of LIBS in a moment, but how exactly does it work? Like LA-ICP-OES, LIBS uses a laser to cut tiny particles from the surface of a sample. But in LIBS, the laser itself creates the plasma instead of a plasma torch. Let's take a look at the four major parts of a typical LIBS system and how they work. The diagram above shows a schematic of the setup.

  1. The laser, of course, is the business end of the instrument. Generally, LIBS systems use a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at its fundamental wavelength of 1,064 nanometers, but many different lasers have been used. The laser doesn't blast the sample with a nonstop beam. Instead, it shoots pulses, with each pulse lasting about 5 to 20 nanoseconds.
  2. The laser light passes through a lens, which focuses the energy onto the sample. Some systems work on the laboratory bench and accommodate small samples, maybe a few centimeters thick, placed inside a chamber. Other systems can be carried to a remote site and used to analyze larger objects. In either case, the more tightly focused the laser, the less energy is required to break down the sample. In fact, the laser pulses in LIBS typically carry energies of only 10 to 100 millijoules. To put that into context, think of the energy required to lift an apple one meter straight up. That's equivalent to a joule. One millijoule is 0.001 joules -- considerably less energy. And yet that's still enough to ablate some of the sample material. As the particles are removed from the sample surface, they are ionized to form a small plume of plasma, what chemists call a "laser spark."
  3. As the plasma plume expands, constituent atoms in the ionized gas become excited. Over just a few microseconds, the excited atoms began to relax, resulting in characteristic spectral emissions. The emitted light travels through a series of collecting lenses, which focus the light and deliver it to a fiber-optic system. The fiber-optic system carries the light to a spectrometer.

­LIBS has several benefits. Because the sample requires no special preparation, the process is relatively simple and inexpensive. Not only that, LIBS can be used to determine the elemental composition of any sample, unlike certain techniques that are great at analyzing solids, but not liquids and gases. Even very hard materials are fair game because the lasers carry so much energy. But one of the greatest benefits of LIBS is its ability to provide information without destroying the sample. The laser removes less than a milligram of material, which is practically invisible. As we'll see on the next page, this makes LIBS an ideal solution for analyzing valuable items, such as paintings or archaeological artifacts.

A Case Study: Using Laser Analysis to Study a Painting

Works of art don't necessarily age gracefully, so conservators may be enlisted to painstakingly restore paintings, such as "Pieto" by Jose Ribera. Laser analysis, can eliminate some of the guesswork in art restoration.
Works of art don't necessarily age gracefully, so conservators may be enlisted to painstakingly restore paintings, such as "Pieto" by Jose Ribera. Laser analysis, can eliminate some of the guesswork in art restoration.
Robert Frerck/­Getty Images

To understand how laser analysis can be used in a very practical way, consider a museum that owns a valuable, 18th-century, oil-on-canvas painting. Over the years, several well-intentioned conservators and patrons have made restoration attempts, adding new layers of paint to the artist's original work. In addition, dirt and smoke have adhered to the painting surface, which has an overall darkening effect. Now the masterpiece looks dull and lifeless. The museum decides to conduct an analysis of the painting to both understand its restoration history and to return it to its former glory.

­In a normal cleaning process, various cleaners and varnish removers are applied to a painting to strip away everything above the original artwork. Conservators use cotton swabs to apply these solvents, working slowly and with great care to make sure they don't remove too much material. But because it's difficult to tell one layer from the next, some of the original pigment is inevitably lost. Our museum owners would like to avoid this problem if they can. They've heard about a revolutionary new technique -- laser-induced breakdown spectroscopy -- and decide to try it.

The painting is taken to an off-site conservation facility that includes LIBS hardware and instrumentation. Inch by inch, the painting is analyzed. As the laser ablates some of the surface material and the spectrometer studies the emissions produced by the plasma plume, the chemists working in the lab can determine exactly what molecules are present. For example, when they analyze a section with white paint, they learn that two different pigments are present. One contains lead, while the other contains titanium. Titanium white wasn't commercially available until after 1920, so they know that the titanium application came later as part of a restoration. Not only that, the lab workers can tell precisely where one layer ends and the next begins simply by noting the change in the spectral emissions.

In reality, museums are just beginning to experiment with LIBS and usually on small, hidden sections of a canvas. But in the near future, they will be able to use the technology to both analyze the painting and restore it completely. In such a situation, a conservator will remove paint and dirt layer by layer until he or she reaches the artist's original artwork.

­Dentists are also beginning to experiment with LIBS, using laser analysis to determine exactly where a ­cavity ends and healthy tooth enamel begins. And quality control engineers in aluminum manufacturing plants are adopting laser analysis techniques to ensure alloys have precisely the right proportion of constituent metals. Archaeologists and forensic scientists are finding the technology invaluable, as well. In fact, laser analysis is proving that the laser, almost 50 years old, is not a solution in search of a problem, but a powerful tool that can help answer a number of questions.



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