Techniques {actinometry} can measure incident radiation using a thermopile.
For solution light absorbance, molecule and solvent extinction coefficient A times solution light-path length L times molecule concentration c equals logarithm of transmitted-light percent T {Beers law} {Beer-Lambert law}: A*L*c = log(T). Molecule and solvent extinction coefficient is molar absorptivity. Outer-shell electron transitions are at ultraviolet and visible wavelengths. People know substance extinction coefficients and molar absorptivities, which depend on outer-shell electron-transition energies and probabilities. If molar absorptivity is high, sensitivity is high. Method can also use integrated absorption coefficient or oscillator strength.
Reaction vibrations and rotations can make light {chemiluminescence}.
Molecule structures {chromophore} determine color. For example, retinal visual pigment molecules have chromophore group. Hydroxyl, amine, and halogen groups do not make UV or visible light, but they can affect nearby-chromophore intensity or wavelength. However, groups separated by two single bonds do not affect each other.
Techniques {densitometry} can use UV or visible light to measure absorbance. Absorbance is linear with concentration. Densitometry measures staining in gels.
Absorbed light can re-emit at lower frequency {fluorescence}|.
purpose
Re-emission can measure concentration. Fluorescent dyes can bind to molecules to trace them. Fluorescence is 100 times more accurate than UV-visible methods and is more sensitive. Fluorescence has no interference, because instrument can choose band.
cause
Molecules with rigid co-planar structures, like anthracene and naphthalene, have fluorescence. Electrons can jump to high orbital, fall back to lower excited orbital by short vibration-induced jumps, and then fall to lowest band by spontaneous emission, giving visible light. Lowest band can have vibrational levels. Fluorescence is fast.
compounds
Substances, such as amino acids tyrosine, phenylalanine, and tryptophan, can absorb ultraviolet light and emit visible light.
concentration
If concentration is less than 0.01 M, Beer's law applies.
factors
Solvent, pH, molecule interactions, and temperature affect fluorescence. Higher temperatures cause less intensity, because more jump types are possible. Nitrates quench fluorescence.
phosphorescence
Excited electrons can fall from excited singlet to triplet and then from triplet to singlet ground state if heavy atom collision is available to change angular momentum. In solids, process is slow, so phosphorescence lasts several seconds.
Visible-light detectors {fluorimetry} can be at right angles to exciting radiation from mercury or xenon arc lamps.
Techniques {nephelometry} can measure light scattered at right angles to light path. At dilute concentrations, absorbance is directly proportional to scattering coefficient, path length, and concentration, as in Beer's law. Particle sizes, particle shapes, solution pH, temperature, and mixing can change scattering. Scattering is more if wavelength is less, so UV light causes more scattering. Nephelometry is 10 times more sensitive than turbidimetry.
Techniques {phosphorescence}| can use absorbed-light re-emission. Orbital ground state is singlet with spins paired. Lower excited orbital state is triplet with spins parallel. Higher excited orbital state is singlet with spins paired. Electron can fall from excited singlet to triplet if heavy atom is available to change angular momentum. Electron can fall from triplet to singlet ground state if heavy atom is available to change angular momentum. Electron fall requires heavy-atom collision. In gas, process is fast. In liquids, process is moderate. In solids, process is slow, so phosphorescence lasts several seconds. Electron-gun TV screens and fluorescent lights have phosphor coatings.
fluorescence
Fluorescence uses ultraviolet light to put electrons in high orbitals, from which they fall back to lower excited orbitals, by short vibration-induced jumps, from which they fall to lowest band by spontaneous emission, giving visible light. Fluorescence does not require collisions because momentum does not change, so fluorescence is fast.
Techniques {reflectance analysis} can compare surface refractive-index differences. Reflectance tests paint, fabric, paper, plaster, drugs, glass, food, and ink. Integrating sphere or ellipsoid mirror receives reflected light.
Reflectance {specular reflectance} can be mirror-like. Reflectance {diffuse reflectance} can be from rough surfaces, which have absorption and scattering.
Methods {turbidimetry} can measure light that passes through suspension compared to total light. Suspensions scatter light. Turbidimetry measures air and water particle pollution, finds wine and drug clarity, measures clear-fluid protein, measures bacteria counts, finds sulfur and sulfate levels, and measures plastic polymerization amount.
Methods {x-ray fluorescence} can detect all elements with mass greater than atomic number 12, by bombardment with x-rays. It can find trace elements in ore, blood, art, alloys, soils, and cultural artifacts. Secondary x-rays come from lithium chloride, sodium chloride, or ammonium dihydrogen phosphate. X-rays can excite inner electrons. Proportional counter with pulse-rate analyzer counts only pulses of specific energy ranges. Spectrum identifies atom. Voltage, matrix, wavelength, time, and absorption affect non-linear measurement.
5-Chemistry-Analytical Chemistry
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Date Modified: 2022.0225