Spectroscopy Basics

[Eliz Cisneroz]

Weknow, for example, that 91.2% of the atoms in the Sun are hydrogen. We know there is water vapor in the atmosphere of WASP-39 b, a distant planet 700 light-years away. We know that the gas at the center of galaxy M82 is 80,000,000 degrees Celsius (150 million degrees Fahrenheit). And that the Whirlpool Galaxy is moving away from us at a rate of 460 kilometers (285 miles) every second.

Astronomy articles are filled with exact compositions, temperatures, and speeds of things that are too far to visit, impractical to probe, and in some cases impossible to observe directly. How is it possible to get such detailed measurements like these?

A comparison of the spectrum of the Sun (top half of the graphic) to the spectrum of a fluorescent light bulb (bottom half of the graphic). The two spectra are distinctly different: The spectrum of sunlight is shown as a continuous curve and rainbow. The spectrum of the light bulb consists of a set of discrete sections, shown as peaks on the graph and bands of color in the picture.

On the right is a graph of brightness on the vertical y-axis versus wavelength in nanometers on the horizontal x-axis. The y-axis has an arrow pointing upward to indicate that brightness increases from the bottom to the top of the graph. There are no numbers, units, or tick marks on the y-axis. The x-axis ranges from 350 nanometers at the origin to 750 nanometers at the far-right end, with labeled tick marks every 100 nanometers.

The curve representing the continuous spectrum of sunlight is concave down with a shape resembling the top front part of a whale as seen from the side: The curve begins about halfway up the y-axis and continues upward with a concave down shape to a broad peak at the top of the graph at about 450 nanometers. It then decreases gradually to the right, leveling out at about 700 nanometers. There are no sharp peaks or valleys superimposed on the overall shape of this curve. (The detailed absorption features of the solar spectrum are not shown.)

Below the graph of the solar spectrum is a picture of the spectrum in the form of a long horizontal bar with rainbow coloring aligned with x-axis of the graph above. The rainbow ranges from purple on the far left to red on the far right. The rainbow is continuous, with no black lines or missing segments.

On the left is a graph of brightness versus wavelength in nanometers. The scale and labels are the same as the graph showing the solar spectrum: Brightness increases from the bottom to the top. Wavelength ranges from 350 nanometers at the origin to 750 nanometers at the far right.

Below the graph is a picture of the emission spectrum in the form of a long horizontal bar aligned with the x-axis of the graph. Unlike the continuous rainbow of sunlight, this picture shows a series of discrete bands of color, separated by bands of black. The color bands correspond to the peaks on the graph. From left to right (shortest to longest wavelength) they are: purple, purplish blue, blue, greenish-yellow, yellow, yellow-orange, orange, orange-red, and red.

While it might sound a bit like an unpleasant medical procedure, at its most basic, spectroscopy is simply a scientific method of studying objects and materials based on color. More specifically, spectroscopy involves analyzing spectra: the detailed patterns of colors (wavelengths) that materials emit, absorb, transmit, or reflect.

Along with imaging (i.e., photography), spectroscopy is one of the most common and useful techniques in astronomy. While images provide information about the size, shape, and structure of matter in space, spectra provide key details such as temperature, composition, and motion.

Spectroscopy allows us to identify gases in planetary atmospheres and minerals on planetary surfaces; figure out what stars are made of and how fast they are rotating; detect and characterize planets orbiting distant stars; measure the temperature and speed of gases in the center of an active galaxy; infer the presence of black holes and dark matter; unravel interactions between colliding galaxies; and help calculate the expansion rate and age of the universe.

The site is secure.

The ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

For more than two decades, Raman spectroscopy has found widespread use in biological and medical applications. The instrumentation and the statistical evaluation procedures have matured, enabling the lengthy transition from ex-vivo demonstration to in-vivo examinations. This transition goes hand-in-hand with many technological developments and tightly bound requirements for a successful implementation in a clinical environment, which are often difficult to assess for novice scientists in the field. This review outlines the required instrumentation and instrumentation parameters, designs, and developments of fiber optic probes for the in-vivo applications in a clinical setting. It aims at providing an overview of contemporary technology and clinical trials and attempts to identify future developments necessary to bring the emerging technology to the clinical end users. A comprehensive overview of in-vivo applications of fiber optic Raman probes to characterize different tissue and disease types is also given.

Ultraviolet-visible (UV-visible or UV-Vis) spectroscopy is a key analytical technique in almost every laboratory in the world due to its versatility and simplicity. Learn about the basics of UV-Vis spectroscopy, with frequently asked questions on the principle of UV-Vis, how UV-Vis spectrophotometers work, UV-Vis applications, accessories and measurement techniques, and optimizing parameters for UV-Vis measurement success.

Spectroscopy allows the study of how matter interacts with or emits electromagnetic radiation. There are different types of spectroscopy, depending on the wavelength range that is being measured. UV-Vis spectroscopy, also known as UV-visible or ultraviolet-visible spectroscopy, uses the ultraviolet and visible regions of the electromagnetic spectrum. Infrared spectroscopy uses the lower-energy infrared part of the spectrum. In UV-Vis spectroscopy, wavelength is usually expressed in nanometers (1 nm = 10-9 m). The UV range normally extends from 100 to 400 nm, with the visible range from approximately 400 to 800 nm.

\r\n\tWhen radiation interacts with matter, several processes can occur, including reflection, scattering, absorbance, fluorescence/phosphorescence (absorption and re-emission), and photochemical reactions (absorbance and bond breaking). Typically, when measuring samples to determine their UV-visible spectrum, absorbance is measured.

\r\n\tBecause light is a form of energy, absorption of light by matter causes the energy content of the molecules (or atoms) in the matter to increase. In some molecules and atoms, incident photons of UV and visible light have enough energy to cause transitions between the different electronic energy levels. The wavelength of light absorbed has the energy required to move an electron from a lower energy level to a higher energy level. The figure below shows an example of electronic transitions in formaldehyde and the wavelengths of light that cause them.

\r\n\tElectronic transitions in formaldehyde. UV light at 187 nm causes excitation of an electron in the C-O bond, and light at 285 nm wavelength causes excitation and transfer of an electron from the oxygen atom to the C-O bond.

\r\n\tIncident light of a specific wavelength causes excitation of electrons in an atom. The type of atom or ion and the energy levels the electron is moving between determines the wavelength of the light that is absorbed. Transitions can be between more than one energy level, with more energy (i.e., lower wavelengths of light) required to move the electron further from the nucleus.

\r\n\tHowever, for molecules, vibrational and rotational energy levels are superimposed on the electronic energy levels. Because many transitions with different energies can occur, the bands are broadened. The broadening is even greater in solutions owing to solvent-solute interactions.

\r\n\tWhen light passes through or is reflected from a sample, the amount of light absorbed is the difference between the incident radiation (Io) and the transmitted radiation (I). The amount of light absorbed is expressed as absorbance. Transmittance, or light that passes through a sample, is usually given in terms of a fraction of 1 or as a percentage. For most applications, absorbance values are used since the relationship between absorbance and both concentration and path length is normally linear.

UV-visible (UV-Vis) spectrophotometers use a light source to illuminate a sample with light across the UV to the visible wavelength range (typically 190 to 900 nm). The instruments then measure the light absorbed, transmitted, or reflected by the sample at each wavelength. Some spectrophotometers have an extended wavelength range, into the near-infrared (NIR) (800 to 3200 nm).

Various measurements can be performed by combining different accessories and sample holders with a UV-Vis spectrophotometer. Accessories exist for different measurement capabilities and sample types (e.g., solids versus liquids) and for different measurement conditions (see later question on UV-Vis accessories).

UV-Vis spectrophotometry is a versatile technique and has been used for close to a century in a wide range of fields. UV-Vis spectrophotometers are in common use in material testing/research, chemistry/petrochemistry, and biotechnology/pharmaceuticals laboratories.

\r\n\tSchematic of the internal layout of an Agilent Cary 5000 UV-Vis-NIR spectrophotometer, showing the main components. Note that this is a high-performance instrument. UV-Vis spectrophotometers for routine measurements have a simpler optical design.

\r\n\tA single monochromator spectrophotometer is used for general-purpose spectroscopy and can be integrated into a compact optical system. A double monochromator is typically found in high-performance instruments.

3a8082e126