Molecular Spectroscopy

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Molecular spectroscopy

Even though you don't see it, electromagnetic radiation is all around us. And some really interesting interactions occur when electromagnetic waves hit matter. Radio waves change the nuclear spin of individual atoms. Microwaves cause molecules to rotate, and UV radiation kicks valence electrons in atoms up to a higher energy level. Analytic chemistry was early to recognize the many capabilities of spectroscopy, and today it employs spectroscopic methods in countless areas. More than any others, atomic spectroscopy and molecular spectroscopy have stood the test of time.

The main difference between these two methodological approaches lies in the interaction: While in atomic spectroscopy, electron transfers create precise line spectra, in molecular spectroscopy we observe "bands" of spectra with tightly packed lines that overlap in groups. The cause for this lies in the diversity of molecular interactions. In addition to electron transfers, these include atomic vibrations and molecular rotations.

Analyzing molecular spectra gives the user a wealth of information about the sample being tested: Insights range from moments of inertia, rotational frequencies and energy levels to isotope compositions, chemical reaction kinetics and electron structures. The most important molecular spectroscopic methods include UV/Vis spectroscopy, fluorescence spectroscopy, nuclear magnetic resonance spectroscopy and near-infrared spectroscopy. Different applications require different measurement methods. UV/Vis spectroscopy can be employed in many areas, from testing biological samples, to quality assurance in the foodstuffs industry, to wastewater analysis in the chemical industry.

Molecules, vibrations and energy

When electromagnetic radiation strikes molecules, a characteristic and frequency-dependent line or band spectrum is produced. Molecular spectroscopy exploits this phenomenon. This spectrum reveals the interactions between radiation and matter. Analyzing the spectrum allows us to make conclusions about the nature of the molecular structure. The physical interactions are essentially those of absorption, scattering, diffraction and reflection of electromagnetic radiation on or around the molecule.

Absorption provides an intuitive example of the basic principle (also used in UV/Vis spectroscopy). The outer orbitals of atoms contain valence electrons, also commonly known as outer electrons. These electrons belong to the p and d orbitals of atoms, for example, and can be excited by electromagnetic radiation. This means that the electrons transition to a higher-energy state by absorbing energy in a very specific wavelength. However, this only happens if the energy of the absorbed photon is exactly the same as the difference between the two energy levels.

If such an excitation occurs, the magnitude of the absorption (extinction) can be calculated with the Beer-Lambert law. This is precisely the phenomenon that analytic chemists exploit in molecular spectroscopy. By bombarding samples with light targeted to a specific wavelength and then analyzing the ensuing interactions, it is possible to make conclusions about the molecular structure.

Molecular spectroscopy for every application

The various methods of molecular spectroscopy differ mainly in the composition of the electromagnetic spectrum they use: Different wavelength spectra elicit different interactions in the sample. Here is an overview of the key methods of molecular spectroscopy:

UV/Vis spectroscopy

UV/Vis spectroscopy only uses a very specific range of the electromagnetic spectrum that comprises the wavelengths between 200 and 800 nm. This spectrum thus covers a visible range as well as a colorless UV range. Radiation with wavelengths under 400 nm is perceived as colorless by the human eye.

The physical principle of UV/Vis spectroscopy is the same as the absorption effect described above. If the photon energy of the light spectrum exactly matches the transfer energy of the electrons in the sample, valence electrons can move to higher energy states. The magnitude of the absorption effect in the sample can ultimately be read from a spectrophotometer, providing clues about the composition of the molecule being analyzed.

The UV/Vis method has become a mainstay in many areas of analytic chemistry. The SPECORD PLUS Series, for example, is used to test samples in the pharma and life sciences sector, in the foodstuffs industry, and in the chemical industry.

NIR spectroscopy

Near-infrared spectroscopy investigates samples using electromagnetic radiation from the short-wave infrared range (760 to 2,500 nm). The NIR method works by causing covalent molecular bonds in organic compounds to vibrate. The resulting overtone and combination modes can be statistically analyzed, yielding insights about the composition of the sample under analysis.

NIR spectroscopy truly shines when analyzing popular agricultural, pharmaceutical or chemical products. For example, the method allows measurement of the water or moisture content in agricultural samples, providing insights about the quality of the sample being analyzed. In neurology, NIR spectroscopy is also used as an imaging technique for measuring brain activity.

And in case you were wondering: Analytik Jena has combined the advantages of the UV/Vis method and NIR spectroscopy in the powerful SPECORD PLUS Series machines. The precision instruments in this series are capable of imaging part of the NIR spectrum up to 1,200 nm, thereby extending their range of applications.

Fluorescence spectroscopy

Fluorescence spectroscopy does not analyze radiation absorption (as do UV/Vis and near-infrared spectroscopy), rather, it monitors the fluorescent light that is re-emitted by the sample. The method makes use of the fact that different interactions unfold at different times after the electrons are excited. Specifically, it examines the period of time between when a quantum of light is absorbed and when it is later "re-emitted" (fluoresced). Thanks to this method, it is possible to examine properties of biological samples that remain opaque to other methods.

Fluorescence spectroscopy entails a different instrument layout than do absorption-based measurement methods: Fluorescence is typically measured at a 90-degree angle in an instrument known as a fluorometer. Distortion of the results from the incident radiation is thereby avoided.

NMR spectroscopy

NMR spectroscopy, or nuclear magnetic resonance spectroscopy, analyzes interactions between electromagnetic radiation and the atomic nuclei in a sample. This method utilizes the effect of nuclear magnetic resonance: If an atomic nucleus is exposed to an electromagnetic field, the result can be an excitation of the its nuclear spin, i.e. its angular momentum. This excitation is associated with transitions between energy levels, which can be detected in the form of electric current.

Nuclear magnetic resonance spectroscopy is capable of non-destructively testing samples for certain substances. It can also analyze molecular structures and interactions between molecules. This method has not only proven itself in analytic chemistry, but is also used as a medical imaging technique for medical diagnoses (magnetic resonance tomography).

How to build a spectrometer

Modern spectrometers are capable of generating electromagnetic radiation of a very specific wavelength spectrum, thereby testing samples for certain interactions and structural properties. Since the various methods in molecular spectroscopy each rely on different wavelength ranges, the machines are structured differently depending on the measurement method.

The structure of a given spectrophotometer for analyzing molecules may differ in the specifics, but it generally follows the same fundamental plan. A light source generates electromagnetic radiation of a certain wavelength range. This light is then broken down into its spectral components with a light decomposition unit. Finally, the light of a specific wavelength is directed at the sample through an outlet slit. The interaction between radiation and sample can then be measured with a detector: By comparing the light intensities either with or without the sample, it is possible to analyze the absorption intensities, in turn yielding insights about the sample's composition.

The structure of a spectrometer described here is that of a monochromator system, such as the one used in the SPECORD PLUS Series. The alternative layout with a polychromator system pursues a different approach: Here, the full light spectrum hits the sample and is spectrally decomposed only afterwards.

An overview of the key components of a spectrometer:

• Light source: Generates electromagnetic radiation of a certain wavelength range.
• Light decomposition unit: Breaks down the radiation into its spectral components.
• Sample chamber: The sample resides here, usually in a cuvette.
• Detector: Records the intensity of the light after it hits the sample.

Besides the main components described here, modern spectrometers have various optical components that precisely bundle and direct the light through the instrument.

Areas of application for molecular spectroscopy

In addition to the methods of molecular spectroscopy mentioned here, there is a great number of other measurement methods used in analytic chemistry to investigate interactions between molecules and electromagnetic radiation. The range of applications for molecular spectroscopy is equally diverse. These range from quality assurance in the foodstuffs industry, to finding defects in semiconductor manufacturing, to verifying precursor materials in the chemical industry.

The following examples from UV/Vis spectroscopy could just as well have come from the diverse applications of molecular spectroscopy:

• Pharma & life science: Whether blood, serum or plasma: Molecular spectroscopy can yield valuable insights from the absorption spectrum of medical samples. The focus is usually on quantitatively analyzing molecular structures: For example, the purity and concentrations of proteins can be measured with precision.
• Foodstuffs industry and agriculture: The foodstuffs industry depends on precise analytic measurements in order to prove compliance with stringent quality standards. UV/Vis spectroscopy plays a key role in this by analyzing the molecular structures of diary products, beverages, meat products or agricultural products.
• Chemical industry: In the chemical industry, UV/Vis spectroscopy helps document quality characteristics such as color concentrations in liquids and solids. For example, an integration sphere makes it possible to measure the whiteness of teeth or the amount of UV protection in sunscreen.