What is Spectroscopy?

Spectroscopy refers to a plethora of different techniques that employ radiation in order to obtain data on the structure and properties of matter, which is used for solving a wide variety of analytical problems. The term is derived from a Latin word “spectron” which means spirit or ghost, and a Greek word “skopein” which means looking onto the world.

In short, spectroscopy deals with measuring and interpreting spectra that arise from the interaction of electromagnetic radiation (a form of energy propagated in the form of electromagnetic waves) with matter. It is concerned with the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules.

Since its inception in the second half of the 19th century, the technique has developed to include every region of electromagnetic spectrum and every attainable atomic or molecular process. Consequently, most engineers and scientists work directly or indirectly with spectroscopy at some point in their career.

Basic Principles of Spectroscopy

Spectroscopy represents a general methodological approach, while the methods can vary with respect to the species analyzed (such as atomic or molecular spectroscopy), the region of the electromagnetic spectrum, and the type of monitored radiation-matter interaction (such as emission, absorption, or diffraction).

Nevertheless, the fundamental principle shared by all the different techniques is shining a beam of electromagnetic radiation onto a desired sample in order to observe how it responds to such stimulus. The response is typically recorded as a function of radiation wavelength, and a plot of such responses represents a spectrum. Any energy of light (from low-energy radio waves to high-energy gamma-rays) can result in producing a spectrum.

General goals of spectroscopy are understanding how exactly light interacts with matter and how that information can be used to quantitatively understand certain sample; however, spectroscopy should also be appreciated as a set of tools that can be employed together to understand different systems and to solve complex chemical problems.

Optical Instruments in Spectroscopy

Several different instruments can be used to perform a spectroscopic analysis, but even the most simple ones entail an energy source (most often a laser, although a radiation or ion source can also be used) and a device to measure the change in the energy source after interaction with the sample.

The light usually passes from the entrance slit through the lens to the prism, which subsequently disperses the light. The eyes see the radiation emerging from the exit slit as a spectral line which is an image of the entrance slit. Ultimately, the resolution is determined by the size of the prism and is proportional to the length of the base of the prism.

If the exit slit is replaced by a photographic plate detector, the instrument is then called a spectrograph (albeit photographic detection is seldom used). Other types of detectors - usually specific electronic devices - which record the intensity of radiation falling on it as a function of wavelength - are more useful and known as spectrometers or spectrophotometers.

The operating region of the source in a certain spectroscopic technique is customarily used to give that technique a name. For example, if an ultraviolet source is used, then the technique can be referred to as ultraviolet spectroscopy. The same principle is used to name other techniques such as infrared, fluorescence, or atomic spectroscopy.