Ultraviolet, visible, or infrared absorption can determine chemical-bond concentrations {spectroscopy}|.
dipole
Molecules with permanent electric dipole moment have rotational spectra. Molecules with changing dipole moment have vibrational spectra, unless symmetry cancels dipole-moment change. Electronic transitions always cause vibrations through recoil.
Doppler effects
Spectral lines broaden by Doppler shifts. Shift is more if wavelength is bigger, because relativistic effects are greater. Shift is more if temperature is high, because molecular-speed range is wider. Shift is less if mass is more, because same-temperature velocities are lower.
Spectral lines broaden {lifetime broadening} by time {lifetime}| in excited state, because short lifetime means large energy. Lifetime can shorten by stimulated emission by other radiation or by electric-field vibration. Higher frequencies cause faster emission, because they have more energy available. Lifetime can shorten by collisions, because they carry energy away.
Light from lamp containing heavy metal can pass through heavy metals in solution in flame, so metal atoms absorb light to measure metal concentration {atomic absorption spectroscopy} {absorption flame spectroscopy}. Atomic absorption measures percent absorption. For atomic absorption, concentration can be 10 to 100 parts per million.
Metals in solution in flame emit light at strong emission wavelength lines, which can measure calcium, sodium, potassium, and low-molecular-mass-metal concentration {atomic emission spectroscopy} {emission flame spectroscopy}. Sample can be in graphite furnace, carbon-rod analyzer, boat, cup, or atomizer. Premixing is in mixing chamber or by baffles. Flame temperature is low enough not to ionize atoms. Flame excites electrons. Atomic emission detects emission-line intensity. For atomic emission, concentration must be 1%.
Techniques {emission spectroscopy} can detect emitted light. Emission spectroscopy can measure amounts of 80 metallic and metalloid elements in minerals, paint, air pollution, water pollution, and oil. Emission spectroscopy produces spectra in UV-visible range. AC spark is more intense and quantifiable. Laser can measure tiny impurities. Quartz tube with microwave coil can make inductively coupled plasmas.
problems
Emission spectroscopy has matrix effects and non-linear results. Emission spectroscopy detects the most-intense spectrum lines, which are also the most wandering. Emission spectroscopy is destructive, because DC arc ionizes metal atoms.
Spectroscopy {infrared spectroscopy} (IR) can detect functional groups and chemical-bond types. IR is simple, is cheap, uses infrared wavelengths 1 micrometer to 20 micrometers, and has narrow absorption bands. Infrared light source is Nernst glower or nichrome wire. Infrared light detector is thermocouple, thermister, or bolometer. Sodium-chloride prisms or reflection gratings select wavelength. Double beam allows scanning.
solvent
Solids, liquids, and gases have concentration 0.1% to 10%. Water absorbs infrared light, so it cannot be solvent. Carbon disulfide solvent is for wavelengths 7.5 micrometers to 16 micrometers. Carbon tetrachloride solvent is for wavelengths 2.5 micrometers to 7.5 micrometers.
container
Sodium chloride or potassium chloride windows hold sample. Path length is 0.1 millimeters to 1.0 millimeter.
theory
Organic-molecule functional groups have chemical bonds with rotations and vibrations. Rotations and vibrations have energies in infrared-light range. IR typically measures vibrations, rather than rotations, because vibrations have higher energies and frequencies. Rotations appear superimposed on vibrational spectral lines and make wider vibrational bands.
theory: dipole moment
The greater the dipole moment, the greater the infrared frequency. The longest chemical bonds and most-polarized functional groups have highest frequencies. Chemical-bond frequencies from highest to lowest are N-H, O-H, C-H, C=O, C=C, C-O, C-C, and H-H.
theory: rotations
Molecule rotation states depend on molecular symmetries. Spherical molecules have no rotational states. Linear molecules have one state, if they are symmetric along axis. Linear molecules with asymmetry have two states. Asymmetric molecules have three states, one for each axis.
Massive molecules have long dipole and have close rotational energy levels. Molecules with large bond distance have long dipole.
theory: vibrations
Chemical bond has stretching vibration. Two same-atom chemical bonds have bending vibrations. Vibrations involve simple harmonic motion, along bond axis or around bond angle. Vibrations give information about bond rigidity and bond-breaking energies. Strong bonds and long bond distances have more energy and make close energy levels. The most-polarized bonds have longest wavelength.
Bond bending is easier than bond stretching, because forces are less, so bending-vibration frequencies are less than stretching-vibration frequencies.
One bond has one less vibrational state, because rotation around bond makes vibrations cancel, because they have all directions. Homonuclear diatomic molecules have no stretching or bending vibrations, because they have no dipole. Heteronuclear diatomic molecules have only stretching vibrations along one bond.
Molecules with three atoms can have bending vibrations. n atoms can have 3*n vibrational states {degrees of freedom, atom}: three for translations and three or less for rotations. Pi-bond twisting is vibration.
problems
Vibrational energy levels get closer together nearer bond-dissociation energy. Liquids and solids have more vibration effects, because they have bonding and van der Waals forces.
Spectroscopy {mass spectroscopy} (MS) can find atomic and molecular mass. Mass spectroscopy is fast, reliable, expensive, and delicate.
sensitivity
Mass spectroscopy can detect masses from 1 to 400,000 daltons, at concentrations as low as 10^-12 M.
uses
Mass spectroscopy can find molecule functional groups by fragment mass differences. For example, methyl groups have mass 15, CO groups have mass 28, water has mass 18, ammonia has mass 17, and phenyl groups have mass 77.
Mass spectroscopy measures leak detection, blood gases, and tracers and analyzes petroleum, plastics, fertilizers, and insecticides. Mass spectroscopy can detect illegal drugs, impurities, pollutants, reaction products, and toxins in gases, liquids, and solids. Mass spectroscopy determines age, quantifies chemical composition, studies metabolism, detects molecular changes, and monitors chemicals. Mass spectroscopy can sequence peptides.
process
Mass spectroscopy measures mass-to-charge ratio of ionized atoms or molecules.
process: vaporization
A 350-C vacuum chamber heats sample to make gas, which expands into ionization chamber through pinhole.
process: ionization
70-eV electron beams ionize sample molecules to make ions with charge +1. Ionization can use proton transfer from ionized methane. Vaporized molecules can ionize {desorption, mass spectroscopy} by californium-252, secondary ions, lasers, high electric field on thin film, or electrospray from high-voltage needle.
process: electric field
Source positive charge repels positively charged ions into analyzer. Ions enter strong electric field and accelerate through slits to collimate.
process: magnetic field
Ions enter magnetic field and arc in semicircle. Typically, magnetic field or voltage sweeps. Big ions move slower and have big radius. Small ions move faster and have small radius.
process: detector
Ions hit surface with applied voltage, causing charge cascade {electron multiplier}. Ions make current, which can be as small as 10^-15 A. Detector measures ion energy and location. Location indicates ion mass. Energy, converted to electric current or light, indicates ion number.
process: types
Double-focusing mass spectroscopy separates masses first by radial electric field and then by radial magnetic field.
Sector detector uses electric field to focus ions, then magnetic field to spread ions. Larger masses need stronger fields to focus them on detector. Smaller masses need smaller field to bring them to detector. Detector plate converts collision energy into ions or electrons, detected as current, or photons, detected by photomultiplier.
Quadrupole mass filter uses four parallel plates or rods with constant direct-current electric field between two plates and varying radio-frequency electric field between two plates.
Ion traps use two parallel plates and ring electrode to trap ions that have mass range. Electric field increases to eject ions toward detector.
High-resolution detectors can measure dalton fractions. Fourier-transform ion-cyclotron resonance (FT-ICR) can trap ions between electrodes in a magnetic field. Radio-frequency electric field makes ions orbit. Orbiting ions create electromagnetic frequency measured by detector plates.
Time-of-flight (TOF) methods send ions accelerated to constant velocity across distance. Detector measures time and so mass.
Two-stage mass spectroscopy (MS/MS) can separate compound from mixture or separate compound constituents to analyze compound structure.
process: results
If electron-beam energy is high enough, sample has unique ion-fragment pattern {cracking, mass spectroscopy}. Highest-mass peak is original molecule with one electron missing. Pattern depends on chemical-bond strength, atoms, total molecular mass, and ionization potential. Chemical bonds break most easily where molecules branch. Double bonds can break. Saturated ring compounds break at side-chain alpha carbon. Carbonyls break so carbonyl ionizes. Aromatic compounds do not break.
process: isotopes
Isotopes cause peak doubling or tripling. Elements have definite peak-doubling or peak-tripling ratios. F, P, and I have no isotopes. 2H, 15N, and 18O [2, 15, and 18 are superscripts] have negligible amounts.
process: isotope ratios
First, find carbon number. 13C [13 is superscript] is 1.11% of total carbon, so first peak to second peak ratio shows carbon number. Next, find oxygen, sulfur, chlorine, and bromine numbers by isotope ratios. Find nitrogen number. If molecular mass is even, nitrogen number is 0 or even. If molecular mass is odd, nitrogen number is odd. You can also use compound spectra tables.
Spectroscopy {Mössbauer spectroscopy} can study number of s orbitals involved in bonding and study valences. Atomic nuclei absorb gamma rays and then emit them, causing crystal recoil and vibration, which causes Doppler shift in electronic-transition wavelengths. Holding crystal in crystal lattice can minimize recoil. Decay is slow. Line width is small.
Spectroscopy {Near Infrared Spectroscopy} (NIRS) can detect glucose and oxygen by infrared reflection.
Spectroscopy {Raman spectroscopy} can study plastics, waxes, pure organic molecules, complex ions, or non-spherical molecules, by polarizing non-polar bonds with UV or visible light and analyzing infrared radiation.
scattering
UV or visible light scatters from molecules to polarize them, with +2 or -2 total angular momentum. Rayleigh scattering is elastic, with unchanged wavelength. Raman scattering is inelastic, because it makes dipoles, so vibration and rotation energy levels affect it.
polarization
Polarization causes infrared-light emission or absorption at vibration or rotation energies. Raman-scattering vibrations are not infrared. Raman-scattering rotations are infrared. Raman lines can have lower frequency {Stokes line} or higher frequency {anti-Stokes line}.
Spectroscopy {UV-visible spectroscopy} can find solution substance concentrations by measuring visible or ultraviolet light absorbance.
purposes
UV-visible spectroscopy can detect functional groups, bonds, and spatial configurations and so identify molecules. It can find equilibrium point, pH, or pK by comparing absorbance at two wavelengths. UV-visible spectroscopy is simple and cheap.
purposes: visible
Visible light can detect colored molecules, in concentrations down to 0.01 M, using standard curve for calibration.
technique
Extinction coefficient or molar absorptivity depends on outer-shell-electron transition energies and probabilities. Tables show values for most substances. Method can also use integrated absorption coefficient or oscillator strength. If molar absorptivity is high, sensitivity is high. Wavelength is maximum-absorbance wavelength. Bandwidth is broad, because energy is low. Path length through solution is typically one centimeter. Air absorbs light of less than 200-nanometer wavelength, so UV light path has vacuum. Glass absorbs all UV light, so containers are quartz.
theory: transition metal
Transition-metal ions have incomplete d orbitals, with three at lower energy and two at higher energy, which have electronic transitions in visible light. Central transition-metal ions can have two sets of low-energy d orbitals, with no symmetry center from metal-ion bonding orbitals to ligand antibonding orbitals or from metal-ion antibonding orbitals to ligand antibonding orbitals. Then intensity is low, because vibrations can also cause such transitions. High intensity is if charges transfer from ligand to ion, or vice versa.
theory: double bonds
Ultraviolet light can detect molecules with carbon-oxygen double or triple bonds or with carbon-carbon conjugated bonds. Unconjugated double bonds involve UV light. Pi-bond electrons jump to antibonding pi orbital to make UV light.
If double bond conjugates, electron delocalization causes small jump and visible light. Intensity is high.
Indicator color changes are large, because proton gain or loss changes conjugation. More conjugation makes longer wavelengths.
Molecules with lone electron pairs can make UV or visible light by jumping to antibonding pi orbitals, typically forbidden visible-light transitions.
theory: no light
Closed-shell electrons and sigma-bond electrons do not give UV or visible light. Hydroxyl, amine, and halogen groups do not give UV or visible light, but they can affect intensity or shift chromophore wavelengths.
Techniques {x-ray absorption} can find heavy atoms among lighter atoms, such as crystal impurities, gasoline lead, broken bones, barium enemas, body iodide, and steel and plastic faults. Absorption depends on molecular mass, density, and thickness. X-rays can come from electrons that hit metal anode at 80,000 V. X-rays hit target, whose ionization energy allows high primary x-ray absorption {absorption edge} and which emits lower-energy x-rays {secondary x-ray}. Detection is by film.
Electronic transitions cause spectra if asymmetric electron-distribution changes cause transient dipole moments {allowed transition}. Then angular momentum change is +1 or -1. Radiation intensity depends on electronic transition probability. Electric dipoles give highest intensity, because they have allowed transitions.
Symmetric electron-distribution changes {forbidden transition} do not change angular momentum. Electric quadrupoles and magnetic dipoles give low intensity, because they have forbidden transitions.
5-Chemistry-Analytical Chemistry
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Date Modified: 2022.0225