Precise scientific measurements are difficult to imagine without considering spectroscopy. Spectroscopy holds a significant stature in all branches of chemistry. Whenever a scientist needs to study the composition of chemical compounds or investigate new substances in the lab or in industry, he/she will need one or multiple spectroscopic techniques to do so.
Considering the huge contribution of spectroscopic analysis in science, in this article, we will introduce you to all that you would like to know about spectroscopy as a beginner. So, let’s start reading!
What is spectroscopy
The word spectroscopy is a combination of two words i.e., spectrum and scopy. Spectrum means an image in Latin while scopy is from Greek skopia which means observation. When sunlight is passed through a prism, the light beam splits into a rainbow set. That means the white light that we see is actually a combination of different colors. Each color has a specific wavelength and frequency. All these colors together make the visible region of the electromagnetic spectrum.
Other than visible light, there are several different regions of the electromagnetic spectrum. Electromagnetic radiations consist of an electric and a magnetic field, oscillating perpendicularly to one another and to their direction of propagation. There are seven main regions of the electromagnetic spectrum, each has a specific wavelength and frequency. Frequency is the energy of radiations where as wavelength is the distance measured between successive crests (highest surface) and/or troughs (lowest part) of a wave. Thus, an electromagnetic radiation behaves both as a wave as well as like a particle.
Matter interacts differently with distinct regions of the electromagnetic spectrum. The study of the interaction of matter with electromagnetic radiations is called spectroscopy.
In spectroscopy, electromagnetic radiations are passed through a sample to see how this sample reacts to such a stimulus. The interaction can occur through absorption, transmission, scattering, etc.
All the elements present in the Periodic Table interact differently with electromagnetic spectrum. Through this interaction, scientists determine the chemical composition; atoms, molecules, functional groups, and/or linkages present between these functional groups in the targeted compounds.
Historical perspective of spectroscopy
Issac Newton laid the foundation of spectroscopy in the mid-1600s by passing light through a prism and splitting it into its characteristic components. Newton gave the name ‘spectrum’ to this rainbow of colors. However, proper spectroscopic analysis began in the 19th century, initiated by two German scientists; Gustav Kirchhoff and Robert Bunsen.
20th century saw the rise and boom of spectroscopy. As we talk about spectroscopy in the 21st century today, it is a well-acclaimed scientific invention. Numerous Nobel prizes are awarded in the field of spectroscopy that further highlight its importance.
The difference between spectroscopy, spectrometry and spectrophotometry
It is very important to understand the basic difference between three interrelated terms i.e., spectroscopy, spectrometry, and spectrophotometry.
- Spectroscopy
As we already discussed, spectroscopy is a general term used for the study of the interaction of electromagnetic radiations with matter. It essentially involves the splitting of an electromagnetic spectrum into different radiations possessing a specific wavelength and energy. The chosen wavelength is passed through the sample and allowed to interact with the molecules present in the sample.
- Spectrometry
Spectrometry is a more specific term that is used to define the method of measuring and studying the composition of matter. For instance, mass spectrometry (MS) is a method of measuring the relative masses of the components within a chemical sample by recording their mass-to-charge ratios.
MS is performed by applying an electric and a magnetic field. It does not directly employs electromagnetic radiations. Therefore, for techniques used to study chemical compounds by specifically using electromagnetic radiations, there is a third exclusive scientific term i.e., spectrophotometry.
- Spectrophotometry
Spectrophotometry refers to specifically measuring the interaction of electromagnetic radiations with matter. For instance, in UV-Vis spectrophotometry, the sample components absorb a specific wavelength of light while transmitting the other. The instrument measures the intensity of transmitted light and records the difference between incident and transmitted light intensities. This is called spectrophotometry and the instrument used for performing it is known as a spectrophotometer.
In short, spectroscopy is a theoretical science while spectrometry and spectrophotometry are practical approaches to it. Spectroscopy presents the whole idea to the scientist while spectrophotometry is how the idea is actually executed.
Classification of spectroscopy into different types
Spectroscopic studies can be differentiated based on the type of matter interacting with radiant energy and the nature of these interactions.
Atomic Spectroscopy
Atomic spectroscopy is based on the interaction of ”single atoms” with radiant energy. The energy absorbed by the atoms leads to the excitation of electrons from a low-energy atomic orbital to an orbital of higher energy. This excitation of electrons is followed by their de-excitation. The energy emitted via de-excitation is recorded. Atomic spectroscopy includes both atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES).
Molecular Spectroscopy
If the electrons present in a ‘molecule” absorb energy for electronic excitation followed by de-excitation then such kind of spectroscopy is known as molecular spectroscopy. UV-Vis spectroscopy and IR spectroscopy are both examples of molecular spectroscopy. The electrons of a molecule are present in molecular orbitals. These molecular orbitals lie at different energy levels. Transition of electrons occur from one molecular orbital to another in molecular spectroscopy. Molecular spectroscopy is further divided into sub-types such as electronic spectroscopy and vibrational spectroscopy.
Electronic Spectroscopy
The different energy levels of electrons in a molecule are called electronic energy levels. If the energy absorbed from electromagnetic radiation is equal to the energy difference between a lower and a higher energy level then the electrons will undergo electronic excitation.
Vibrational Spectroscopy
Within the electronic energy levels, there are different vibrational energy levels and within each vibrational energy level, there are distinct rotational energy levels. If the energy of the incident radiation is low such as the energy provided by infrared radiations then electronic transitions cannot occur. Rather, electrons will change their vibrational energy state. This is called vibrational spectroscopy for example infrared (IR) spectroscopy.
Vibrational transitions occur due to the bending and/or stretching of chemical bonds. Thus, IR spectroscopy helps in determining the different types of bonds/ functional groups present in the sample.
What do spectroscopic techniques measure
There is a plethora of useful spectroscopic techniques. All the techniques are predominantly based on measuring analyte response versus the wavelength of radiated light. The analyte response such as the absorbance or transmittance is plotted on the y-axis against irradiated wavelength on x-axis. The graph obtained is called a spectrum. The greater the analyte response at a particular wavelength, the more intense will be the peak at that point on the spectrum.
Different spectroscopic techniques
Some of the most widely used spectroscopic techniques, their working principles, and the scope of applications is as summarized in the table below.
| Spectroscopic technique | Working principle | Applications |
| UV-VIS spectrophotometry | The molecules and atoms present in a sample absorb ultraviolet-visible (200-800) nm radiations and undergo electronic excitation. A part of the incident beam is absorbed while the rest is transmitted. The intensity of transmitted radiations is recorded. A UV-VIS spectrum is obtained as a plot of detector response vs wavelength | UV-VIS spectrophotometry is important for studying the chemical composition of conjugated organic molecules like aldehydes, ketones, carboxylic acids, etc. |
| Infrared (IR) spectroscopy | The electrons present in a molecule undergo vibrational transitions by absorbing infrared (800-103) nm radiations. A specific wavelength is absorbed while the rest are transmitted. The IR spectrum is plotted as a graph of transmittance versus wavenumber cm-1 (reciprocal of wavelength) | IR spectroscopy is important for studying the structure of a chemical compound by determining the different functional groups present in it. New drugs and medicines synthesized in the laboratory are characterized via IR spectroscopy |
| Fluorescence spectroscopy | A beam of light such as ultraviolet radiation is passed through the sample molecules. Electrons absorb energy from light and get excited to reach a high energy level. Some energy is emitted when these electrons return to their low energy state by fluorescence emission. | Fluorescence spectroscopy is important for studying organic and inorganic compounds that emit light naturally on excitation followed by de-excitation. It is specifically useful for immunoassays and in biochemistry |
| Raman spectroscopy | Raman spectroscopy is based on scattering a beam of light (UV, visible or infra-red) that is passed through the sample. The vibrational motion of electrons in the sample molecule controls the extent of light scattering. | Raman spectroscopy is important for studying the chemical structure, morphology, crystallinity, and bonds present in newly synthesized chemical compounds and polymers |
| Nuclear magnetic resonance (NMR) spectroscopy | An electrically charged nucleus such as 1H nucleus is continuously spinning about its axis. When an external magnetic field is applied, some 1H nuclei align with the field while some align against the applied magnetic field. In this way, the different nuclei occupy two different nuclear spin states i.e., a low-energy spin state and a high-energy nuclear spin state. When radiowaves (>103nm) are passed through the sample containing 1H nuclei, these absorb energy and change their spin state. | NMR spectroscopy is an extremely useful and one of the best spectroscopic techniques. It gives detailed information about the structure of a chemical compound by studying its chemical environment. |
| Atomic spectroscopy | Atomic spectroscopy is based on detecting atoms or ions that absorb or emit a specific wavelength of light when the sample mixture is irradiated or heated. | Atomic spectroscopy is important for determining trace metals present in complex organic or inorganic matrices. |
| Mass spectrometry (MS) | Gaseous ions are produced by passing the sample mixture through a vaporization chamber followed by an applied electric field. The charged ions are then accelerated via a magnetic field based on their mass-to-charge ratio until they reach the detector. | Mass spectroscopy or spectrometry is important for determining the relative atomic masses, and relative abundance of different components of an unknown compound. This information in turn can help elucidate the molecular formula and structure of the tested compound. |
Why is spectroscopy important
You must have noticed from the previous section that each spectroscopic technique is dedicated to a specific type of analysis. The spectroscopic technique used may depend on a characteristic property of the analyte molecule such as its UV light absorbing property or the ability to fluoresce.
Additionally, the spectroscopic technique selection may also depend on the end goal of analysis i.e., whether you need a simple functional group identification or a detailed characterization profile. In all cases, whenever you talk about chemical and structural analysis of new and existing compounds, spectroscopy will have an important role to play.
In general, spectroscopy is important for :
- Finding the chemical constituents of a material having an unknown composition.
- Confirming the structural composition of known chemical compounds.
- Structural characterization to check the purity of pharmaceutical drugs, polymers, medicinal materials, etc.
- Biomedical analysis.
- Studying the composition of biomolecules: proteins and nucleic acids.
- Studying reaction kinetics.
- Detecting trace quantities of metals and ions present in salts, and water.
The list can go on and on. Spectroscopy allows both qualitative as well as quantitative chemical analysis. Spectroscopy is a fundamental exploratory tool in science. In conclusion, all matter is made up of atoms and anything containing atoms can be studied through spectroscopy.
For a more in-depth discussion on spectroscopic applications, we recommend our article: What are the uses of analytical spectroscopic techniques in chemistry.
You may also like to read more insightful information on 10 different spectroscopic techniques here.
Let’s watch a video to quickly revise our basic concepts on spectroscopy.
References
1. Hollas, J. M. (2004). Modern Spectroscopy John Wiley and Sons, Ltd.
2. M.Younas (2017). Organic Spectroscopy and Chromatography.
3. Milton, E. J., M. E. Schaepman, K. Anderson, M. Kneubühler and N. Fox (2009). “Progress in field spectroscopy.” Remote Sensing of Environment 113: S92-S109.
4. Pavia, D. L., G. M. Lampman, G. S. Kriz and J. A. Vyvyan (2013). Introduction to spectroscopy, Cengage Learning.
