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Sep. & Spec.
| Question | Answer |
|---|---|
| Types of Chromatography | Adsorption (stick to surface), Partition (liquid phase bound to solid), Ion exchange, Molecular Exclusion, Affinity |
| Chromatogram | plot of detector signal vs. time |
| Retention time | How long a compound is retained in the solid/stat. phase. tr= tm + ts |
| Resolution | in chromatograph good resolution means good sep. of peaks (no overlap) and sharp |
| Dead Time | The time it takes for a non-retained analyte to move through |
| Avg. linear velocity | L/tm = u |
| Selectivity Factor | Way to measure peak sep. = Tra - Tm/Trb - Tm |
| Peak Broadening | Longitudinal Diffusion, mass transfer, diff. paths, fronting or tailing |
| Column Efficiency | Theoretical plates (N) = 16(tr/w)2 |
| Factors affecting column efficiency | Linear velocity of mobile phase, diffusion coeffiecient in mobile phase, diffusion coefficient in stat. phase, retention factor, diameter of solid particles, capillary columns (thinner is usually better) |
| Increase efficiency | Change solvent system, faster push through, column length longer |
| Gas Chromatography | Injector, column, detector. GC separates based on 2 things - boiling point and polarity |
| Flame Ionization GC detector (FID) | read current flow as analyte hits flame burning gas and air, very sensitive, most organics |
| Thermal Conductivity Detector (TCD) | universal and good for inert gases, wire is sensitive to temperature changes, measure current flow through the wire, resistance changes as temp changes |
| Electron capture detector (ECD) | very sensitive, very specific for halogens, nitrates, nitirles, peroxides, some organometallic compounds. Creates ions and measures electricity flow |
| (PID) - Photoionization detector | Light causes electrons to be removed, creates ions; current flows between electrodes, good for volatile organic cmpds |
| GC/MS | Mass spectrometer can give definite identification of a molecule; lines correspond to mass |
| HPLC | High - Performance liquid chromatography - non - destructive and handles high boiling point analytes (large organics, peptides, etc) |
| HPLC system | pump, injector, precolumn, detector; thin tubing affects retention times and minimizes diffusion |
| Common HPLC Detectors | Uv-vis, diode array, fluorescence, mass spec, refractive index |
| Solvents for HPLC | Water, CH3OH, acetonitrile, 1%Ch3COOH or salts; separation is very sensitive to solvent polarity |
| Isocratic | solvent composition remains constant for entire separation |
| Gradient | solvent composition during the separation cahnges |
| Reverse Phase HPLC | column is less polar C18; solvent = mobile phase is more polar; analyte movement is based on polarity, polar solvents tend to increase pressure |
| Spectroscopy | using light to study chemicals; light absorbance or emission |
| Light regions | 400-700 (visible), 50-400 (UV), 7000-50,000 (IR), microwaves - longer wavelengths, xray (<50) |
| UV-Vis instrument | Lamp, monochromator, beam splitter sends light to sample and reference, chopper makes sure both don't hit the detector at once, PMT, amplifier, display |
| Limitations to UV-vis | Analyte must absorb in 190-800 nm range, can't be too concentrated, no structural info., slow if scanning samples |
| Speed up UV-Vis | Diode array detector - move the monochromater into the detector use photodiodes to detect all the wavelengths |
| Bandwidth | wider slits give more light on the sample but allows a wider range of wavelengths to reach the detector (lose resolution); higher intensity = less resolving power; narrower slits give lower intensity and better resolution |
| PMTs | photo multiplier tubes - light hits surface and wire is at +80 volts, when photons hit the electrons willb e ejected and this creates a current |
| Kinetics | zero order - reactant conc. has no effect on the rxn rate; 1st order - 1 reactant conc. controls the rxn rate; 2nd order - 2nd order in 1 reactant, relies on conc. squared or 1st order with 2 reactants, hold one constant and change other |
| Fluorescence | photon emission in the singlet state |
| phosphorescence | relatively long life time, flips spin and then comes back down to ground state longer than fluorescence |
| vibrational relaxation | dropping down the vibrational states |
| advantages to fluor/phos | very sensitive, very selective (also a disadvantage since only some molecules do this) |
| Fluor/Phos instrument | xeon arc lamp, excitation mono to ref. and sample, then a 90 degree turn in the sample to an emission mono, then to PMT |
| NMR | a form of absorption spectrometry under certain conditions in a mag. field; a sample can absorb rf radiation which depends on the nuclei in the molecule |
| NMR instrument | something emits rf and something detects it; plot intensity vs. ppm shift; res. increases as magnetic strength increases |
| Chemical shift NMR | a completely shielded nucleus, TMS used to 0 the instrument; shift depends on the electron environment around the proton |
| Beer's law | A=Ebc E= molar absortivity, b= path length, c = concentration |
| Relation between wavelength and frequency | frequency x wavelength = speed of light |
| Transmittance | A = -logT |
Created by:
eannl25
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