BY- K. Sai Manogna (MSIWM014)
Absorption spectroscopy is a technique that compares the power of a beam of light determined before and after a sample contact. It is also referred to as Tunable Diode Laser Absorption Spectroscopy (TDLAS) when done with a tunable diode laser. To decrease the device’s noise, it is most often paired with a modulation technique, most often wavelength modulation spectrometry (WMS) and sometimes frequency modulation spectrometry (FMS).
To excite a sample, fluorescence spectroscopy uses higher-energy photons, which will then release lower energy photons. This method is known for its biochemical and medical applications and can be used for confocal microscopy, energy transfer of fluorescence resonance, and lifetime imaging of fluorescence.
When X-rays with appropriate frequency interact with a material, the atom’s inner shell electrons are excited into empty outer orbitals, or they can be entirely expelled, ionizing the atom. Then electrons from the outer orbitals would fill the inner shell “hole.” In this de-excitation process, the energy available is released as radiation (fluorescence), or other less-bound electrons are extracted from the atom (known as Auger effect). The frequencies (energies) of absorption or emission are characteristic of the individual atom. Moreover, there are minor frequency variations for a single atom that is typical of chemical bonding. These specific X-ray frequencies or Auger electron energies can be determined with an appropriate instrument. In chemistry and material sciences, X-ray absorption and emission spectroscopy are used for determining the elemental composition and chemical bonding. X-ray crystallography is a method of scattering; X-rays are dispersed at well-defined angles by crystalline materials. If the incident X-ray wavelength is known, the distances between the atoms’ planes inside the crystal can be measured. The scattered X-ray intensities provide information about the atomic positions and measure the atoms’ arrangement within the crystal structure.
Samples of liquid solution are aspirated into a combination of a burner or nebulizer/burner, dissolved, atomized, and often excited to a higher electronic state of energy. During analysis, the use of a flame includes fuel and oxidant, usually in gases. Gases such as acetylene (ethyne) or hydrogen are used as typical fuel gases. Oxygen, air, or nitrous oxide are common oxidant gases used. These methods can also analyze metallic element analytes in the concentration ranges of part per million, billion, or probably lower. In order to identify light with the analysis data coming from the flame, light detectors are required.
Atomic Emission Spectroscopy: This technique uses the flame’s excitation; atoms are excited to emit light from the flame’s heat. The total consumption burner with a round burning outlet is usually used in this technique. A more significant temperature flame is usually used to induce analyte atoms’ excitation than atomic absorption spectroscopy (AA). Since the flame’s heat excites the analyte atoms, no particular elemental lamps must shine into the flame. A high-resolution polychromator can be used to generate an emission intensity vs. wavelength spectrum over a range of wavelengths exhibiting multiple-element excitation lines, meaning multiple elements can be detected in one run. Alternatively, a single wavelength monochromator may be set to focus on studying a single element at a specific emission line. A more advanced variant of this process is plasma emission spectroscopy.
Atomic absorption spectroscopy (often referred to as AA) – A pre-burner nebulizer (or nebulizing chamber) is widely used to produce a sample mist and a slot-shaped burner that gives a longer flame pathlength. The flame temperature is low enough that sample atoms are not excited from their ground state by the flame itself. The nebulizer and flame are used to dissolve and atomize the sample, but for each type of analyte, the analyte atoms’ excitation is achieved by using lamps that glow through the flame at different wavelengths. The amount of light absorbed after passing through the flame defines the analyte quantity in the sample in AA. For greater sensitivity, a graphite furnace is typically used to heat the sample for desolvation and atomization. The graphite furnace process can also analyze any substantial or slurry samples. It is still a widely used analysis method for some trace elements in aqueous (and other liquid samples, due to its strong sensitivity and selectivity.
Atomic Fluorescence Spectroscopy: A burner with a circular burning outlet is widely used in this technique. To solve and atomize the sample, the flame is used. However, a lamp shines a light into the flame at a particular wavelength to excite its analyte atoms. Then the atoms of some components will fluoresce, emitting light in another direction. For quantifying the amount of analyte component in the sample, this fluorescent light’s strength is used. A graphite furnace is also used for atomic fluorescence spectroscopy. This technique is not as widely used as spectroscopy of atomic absorption or plasma emission.
Plasma Emission Spectroscopy:
It has virtually replaced in several respects similar to flame atomic emission spectroscopy.
- Direct-current plasma (DCP) An electrical discharge between two electrodes creates a direct-current plasma (DCP). It needs a plasma support gas, and Ar is standard. Samples could be deposited on one of the electrodes, or one electrode can be built up by conducting them.
- Glow discharge-spectrometry of optical pollutants (GD-OES)
- Plasma-atomic emission spectrometry, inductively coupled (ICP-AES)
- Laser-Induced Breakdown Spectroscopy (LIBS) (LIBS), also called plasma spectrometry induced by laser (LIPS)
- Plasma caused by microwave (MIP)
Spark or arc/emission spectroscopy – used in solid samples for the study of metallic elements. In order to make it conductive, a sample is ground with graphite powder for non-conductive materials. A sample of the solid was usually ground up and damaged during research in conventional arc spectroscopy methods. The spark or electric arc is passed through the sample to excite the atoms, heating the sample to a high temperature. The excited analyte atoms glow at different wavelengths, producing light that can be detected by standard spectroscopic methods. Since the conditions generating the arc emission are usually not quantitatively regulated, the study is qualitative for the components. Nowadays, under an argon atmosphere, spark sources with controlled discharges allow this method to be considered eminently quantitative, and its use is widely applied worldwide through the production control laboratories of foundries and steel mills.
Many atoms emit visible light or absorb it. In order to achieve a continuum of fine lines, the atoms must be in the gas phase. It suggests the material has to be vaporized. In absorption or emission, the spectrum is studied. In UV/Vis spectroscopy, visible absorption spectroscopy is mostly paired with UV absorption spectroscopy. While this type may be unusual as a similar indicator is a human eye, it still helps identify colors.
In the Ultraviolet (UV) field, all atoms are absorbed because these photons are energetic enough to excite outer electrons. Photoionization takes place if the frequency is high enough. In quantifying protein and DNA concentration and protein ratio to DNA concentration in a solution, UV spectroscopy is also used. Several amino acids, such as tryptophan, usually present in proteins, absorb light in the range of 280 nm, and DNA absorbs light in the 260 nm range. For this reason, in terms of these two macromolecules, the 260/280 nm absorption ratio is a good general measure of the relative purity of a solution. It is also possible to make fair estimates of protein or DNA concentration using Beer’s law.
The IR absorption spectrum analysis shows what kind of bonds are present in the sample, especially in organic chemistry. The study of polymers and components such as fillers, pigments, and plasticizers is also necessary.
To study the vibrational and rotational modes of molecules, Raman spectroscopy uses the inelastic scattering of light. An interpretation help is the resulting ‘fingerprints.’
Coherent anti-Stokes Raman spectroscopy (CARS) is a recent technique for in vivo spectroscopy and imaging with high sensitivity and robust applications.
Nuclear Magnetic Resonance Spectroscopy (NMR):
To determine the various electronic local environments of hydrogen, carbon, or other atoms in an organic compound or other compounds, nuclear magnetic resonance spectroscopy analyses such as atomic nuclei’s magnetic properties. This is used to assist in assessing the compound structure.
Mossbauer spectroscopy modes of transmission or conversion-electron (CEMS) probe individual isotope nuclei’s properties in various atomic environments by studying the resonant absorption of characteristic gamma-ray energy as the Mossbauer effect.
In the next chapter we will discuss in detail about each spectroscopic methods.