Spectroscopic Methods Flashcards
Absorption
Methods that are based upon attenuation ( or weakening) of a beam of light when it interacts with an analyte
Emission
Methods that are based upon the light given off after an analyte has been excited by thermal, electrical, or radiant energy
Methods based on the adsorption of radiation
UV-VIS, IR, Raman, Microwave, NMR, mass spec
Methods based on emission of radiation
Atomic spec, fluorimetry,
UV-VIS
Measures adsorption of ultraviolet and visible light in a species
Transmittance
The fraction of incident radiation transmitted by the sample
T = I/Io
Absorbance (A)
Quantity defined by A = £bc, where £ is the molar absorptivity, b is the length of the measuring cell, and c is the concentration
Beer-Lambert Law
A = -logT = 2 - log(%T) = £bc
IR spectroscopy
Spectroscopic technique that analyzes the vibrations of a molecule due to the adsorption of IR radiation
Modes of vibration
The band shapes and positions depend on the symmetry of the molecule’s vibrations
Modes of vibration
Symmetric stretch, asymmetric stretch, twist, wag, bending, rocking, scissoring
IR
The higher the bond order the higher the IR frequency
The greater the mass difference of the atoms involved in the vibrating bond the greater the frequency
Group frequencies
Are associated with specific groups of atoms producing IR bands that are composite modes of vibration. Several of these groups consist of organic functional groups
Raman spectroscopy
Complements IR, measures the difference in the frequencies of the scattered light after it hits a compound resulting in emergent light that contains frequencies that differ from the original one, used for identification purposes
Microwave spectroscopy
Analyzes the absorption of light in the microwave region which results in changes in the rotational frequency levels of gaseous polar molecules. Used for structural information
NMR
Measures the chemical shift of the protons or atoms in order to identify the compound
Resonance
The energy gap corresponds to the radio-frequency range, and radiation in that range can excite the nuclei, inducing spin flips between the higher and lower energy states
Upfield
Signals of more shielded protons, to the right
Downfield
The signals of less shielded protons, to the left
Spin-spin splitting
Indicative of the magnetic nonequivalence of protons located on adjacent carbon atoms. N+ 1 peaks are observed in the slitting of a given proton signal of N magnetically equivalent protons are found in the adjacent carbon
Atomic spectroscopy
Based on the emission of radiati n after the adsorption of energy by a sample, usually used for elemental trace analysis
Atomization
The sample is volatilizations and decomposed to produce particles, which can either be atoms or ions
Fluorescence
Caused by the absorption of radiant energy and reemission of some of this energy in the form of light, light emitted is almost always a high wavelength then the light absorbed
Phosphorescence
The phenomenon that occurs when an interstate crossing occurs to a state of different multiplicity, the light is emitted with a time delay
Total fluorescence intensity
F = Io • 2.3£cl(omega)
Io = intensity of the incident light l = cell length Omega = quantum yield of fluorescence
