Introduction to Spectroscopy

Spectroscopy is a type of chemical analysis done by shining light on a sample to determine what it contains. Chemists commonly measure either the absorbance (how much light is absorbed by the sample, or the transmittance (how much light passes through the sample).

Because spectroscopy has to do with light, in order to understand it we need to know a little bit about the nature of light, which may seem like a simple undertaking but has presented some of the most challenging aspects in the long history of physics.

Light acts like a wave, but light has particle-like properties too, which is why it has taken so many years to figure out. For purposes of this discussion, let’s deal with light as a wave.

Picture yourself wading around on an ocean beach for a moment, and watch the many water waves sweeping past you. Waves are disturbances, ripples on the water, and they possess a certain height (amplitude), with a certain number of waves rushing past you every minute (the frequency) and all moving at a characteristic speed across the water (the wave speed). Notice the distance between successive waves? That's called the wavelength.

In spectroscopy, then, the wave speed of a light wave is simply the speed of light, and different wavelengths of light manifest themselves as different colors! The energy of a light wave is inversely-proportional to its wavelength; in other words, low-energy waves have long wavelengths, and high-energy waves have short wavelengths.

Imagine that light is food and the sample is a room full of people, a complete spectrum of light would be like giving all the food in a grocery store to the people in the room. As you might imagine, almost all of the food would be wasted (transmitted). But some of the food would be eaten (absorbed). Now imagine that you knew there was only one person who would always eat broccoli. If all of the broccoli came through the room uneaten, that person must not be in the room, but if some were missing, you would know that person was in the room. Furthermore, no matter how much asparagus you put in the room, the broccoli-lover would never eat any, and so adding asparagus would never tell you if the broccoli-lover was in the room or not.

In a chemical analysis, many different kinds (wavelengths or energies) of light (a spectrum) are shone through a sample. Some of the light is absorbed. By knowing what wavelengths of light are absorbed (or "eaten") by the sample, we know what is inside. But, if we are looking for a specific molecule or characteristic and shine the wrong wavelengths of light through a sample, no matter how much light we put through, we will never learn anything about the sample. Tiny changes of energy in an atom generate photons with small energies and long wavelengths, such as radio waves! Similarly, large changes of energy in an atom will mean that high-energy, short-wavelength photons (UV, x-ray, gamma-rays) are emitted.

Because different molecules and characteristics of molecules absorb at different energies of light, there is a need for different forms of spectroscopy. The types are:

• Absorption

Absorption spectroscopy is a technique in which the power of a beam of light measured before and after interaction with a sample is compared.

• Fluorescence

Fluorescence spectroscopy uses higher energy photons to excite a sample, which will then emit lower energy photons.

• X-Ray

When X-rays of sufficient frequency (energy) interact with a substance, inner shell electrons in the atom are excited to outer empty orbitals, or they may be removed completely, ionizing the atom. The inner shell "hole" will then be filled by electrons from outer orbitals. The energy available in this de-excitation process is emitted as radiation.

• Flame

Liquid solution samples are aspirated into a burner or nebulizer/burner combination, dissolved, atomized, and sometimes excited to a higher energy electronic state. This change in state will be measured as either emission or absorption.

• Plasma Emission Spectroscopy

Similar to flame atomic emission spectroscopy, plasma emission spectroscopy uses a plasma flame and has largely replaced it.

• Visible

Many atoms emit or absorb visible light. In order to obtain a fine line spectrum, the atoms must be in a gas phase. This means that the substance has to be vaporised. The spectrum is studied in absorption or emission. Visible absorption spectroscopy is often combined with UV absorption spectroscopy.

• Ultraviolet (UV)

All atoms absorb in the ultraviolet region, because these photons are energetic enough to excite outer electrons. This is measured in UV spectroscopy.

• Infrared (IR)

Infrared spectroscopy offers the possibility to measure different types of inter atomic bond vibrations at different frequencies.

• Raman

Raman spectroscopy uses the inelastic scattering of light to analyse vibrational and rotational modes of molecules. The resulting 'fingerprints' are an aid to analysis.

• Nuclear Magnetic Resonance (NMR)

Nuclear magnetic resonance spectroscopy analyzes the magnetic properties of certain atomic nuclei to determine different electronic environments of hydrogen, carbon, or other atoms in an organic compound. This is used to help determine the structure of the compound.

• Photoemission

Photoemission spectroscopy measures the electron emission from matter after the absorption of energetic photons.

• Mossbauer

Used to examine the properties of specific isotope nuclei in different atomic environments, Mossbauer spectroscopy is named after the Mossbauer effect, which is based on the resonant absorption of characteristic gamma rays.

Spectroscopy is used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. It is also heavily used in astronomy. Most large telescopes have spectrometers, which are then used to measure the chemical composition and physical properties of astronomical objects or to measure their velocities via the Doppler shift of their spectral lines. Spectroscopy is even used in the biological sciences or biochemistry.