Etiket Arşivleri: Spectroscopy
Atomic spectroscopy requires that atoms of the element of interest be in the atomic state (not combined with other elements in a compound) and that they be well separated in space. In foods, virtually all elements are present as compounds or complexes and, therefore, must be converted to neutral atoms (atomized) before atomic absorption or emission measurements can be made. Atomization involves separating particles into individual molecules (vaporization) and breaking molecules into atoms. It is usually accomplished by exposing the analyte (the substance being measured) to high temperatures in a flame or plasma although other methods may be used. A solution containing the analyte is introduced into the flame or plasma as a fine mist. The solvent quickly evaporates, leaving solid particles of the analyte that vaporize and decompose to atoms that may absorb radiation (atomic absorption) or become excited and subsequently emit radiation (atomic emission).
Electromagnetic (EM) radiation is a periodically changing or oscillating electric field propagating in a certain direction with a magnetic field oscillating at the same frequency but perpendicular to the electric field.
Figure 1. A schematic representation of EM radiation. The wavelength is represented by λ.
EM radiation may be considered as a traveling wave or as a stream of massless elementary particles, often called photons. As a wave, it can be characterized by its wavelength λ (the length of one wave), its frequency ν (the number of vibrations per unit time) and its wavenumber k (the number of waves per unit length
Interaction of Light and Molecules
Roughly, there are 3 possible effects of interaction between radiation and molecules. These are scattering, absorption, and emission. Absorption is the process by which the energy of a photon is taken up by the matter, and this process plays a key role in IR spectroscopy. There are several types of physical processes that could lie behind absorption, depending on the quantum energy of the particular frequency of EM radiation. For example, high energetic ultraviolet (UV) radiation can cause ionization and visible light usually causes electron transitions. As told before, the energy levels for all physical processes at the atomic and molecular levels are quantized, and if there are no available quantized energy levels with spacings which match the quantum energy of the incident radiation, then the material will be transparent to that radiation. In view of IR spectroscopy, I will now focus on the absorption of IR radiation by matter.
ATOMIC ABSORPTION SPECTROSCOPY and PROTEIN DETERMINATION with BIURET METHOD THEORY
ATOMIC ABSORPTION SPECTROSCOPY
Spectroscopy is the use of the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules (or atomic or molecular ions) to qualitatively or quantitatively study the atoms or molecules, or to study physical processes. The interaction of radiation with matter can cause redirection of the radiation and/or transitions between the energy levels of the atoms or molecules. A transition from a lower level to a higher level with transfer of energy from the radiation field to the atom or molecule is called absorption. A transition from a higher level to a lower level is called emission if energy is transfered to the radiation field, or nonradiative decay if no radiation is emitted. Redirection of light due to its interaction with matter is called scattering, and may or may not occur with transfer of energy, i.e., the scattered radiation has a slightly different or the same wavelength. When atoms or molecules absorb light, the incoming energy excites a quantized structure to a higher energy level. The type of excitation depends on the wavelength of the light. Electrons are promoted to higher orbitals by ultraviolet or visible light, vibrations are excited by infrared light, and rotations are excited by microwaves.Atomic-absorption (AA) spectroscopy uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Applying the Beer-Lambert law directly in AA spectroscopy is difficult due to variations in the atomization efficiency from the sample matrix, and nonuniformity of concentration and path length of analyte atoms (in graphite furnace AA). Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.