Analysis (Instrumentation)

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  • Created by: LBCW0502
  • Created on: 27-03-18 16:27
How is a UV spectrometer set up (single beam)?
Light source. Collimator (lens) narrows beam. Prism (monochromator) splits the beam into component wavelengths. Exit slit selects radiation of specific wavelength. Radiation passed through sample (cuvette) and reader
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How is a UV spectrometer set up (dual beam)?
Light source. Two sets of slits used with prism. Beam splitter. Radiation passes through sample and detector. Radiation passes through reference cell and detector (use mirrors) - ratio given
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Outline the experiment used to construct a calibration curve
Produce standard solutions using serial dilution. Carry out UV spectroscopy. Plot graph of absorbance against concentration. Construct calibration curve which can be used to find unknown concentrations of solutions given the absorbance
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Describe the set up for IR spectroscopy
Same as UV spectroscopy (single beam)
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What are thermocouples?
Pair of junctions of different metals. The voltage between the junctions changes based on difference in temperature (requirements for material are that for a very small change in the amount of incident radiation there is a maximum temperature change
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What are photoelectric detectors?
Comprise a film of semi-conducting material deposited on a glass surface. Absorption of IR promotes non-conducting valence electrons to a higher conducting state
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What is a dispersive IR spectrophotometer?
Double beam recording instrument. Radiation from source flicked between reference and sample paths. Detector only responds if intensity of two beams is unequal. When unequal, a light attenuator restores equality by moving in and out of reference beam
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What is a Fourier-transform spectrometer?
Any waveform can be shown in one of two ways - frequency or time domain
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What are the advantages of FTIR compared to Dispersive IR?
Improved frequency resolution. Improved frequency reproducibility (older dispersive instruments must be recalibrated for each session of use). Higher energy throughput. Faster operation
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Give examples of sampling/cells (solids)
KBR pellets (ratio of 0.2 : 1 %), mulls (nujol)
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Describe the instrumentation needed for NMR spectroscopy
Magnet (B0), radio frequencies for excitation of nuclei and computer
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Describe the features of the sample tube for NMR spectroscopy
Sample tube comes in 5 mm (in diameter) but 10 mm and 3 mm are made of borosilicate glass. Sealed using polyethylene caps. Tubes must be transparent to RF radiation, durable and chemically inert
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Describe the probe needed for NMR spectroscopy
Probe used to hold sample. Contains sweep source to ensure reproducible positioning of the sample. Contains air turbine to spin sample holder while spectrum is acquired. Contains RF transmitting and receiving coil
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Describe features of the magnet required for NMR spectroscopy
Magnet must be strong, stable and produce a homogeneous field (field doesn't vary in strength/direction). Magnet is composed of the superconducting coil and shim coils
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Describe features of the superconducting magnet
Made up of superconducting Nb/Sn or Sb/Ti wire. Surrounded by liquid He (sufficient cooling). Encased on liquid N (decrease evaporation of He). Sample probe in bore with room temperature shim coils
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What are shim coils?
Used to adjust current until the magnetic field has required homogenity. Magnetic fields produced by shim coils cancel small residual homogeneities in main magnetic field
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How is RF radiation generated?
By a RF crystal oscillator. Output of oscillator is amplified, mixed and filtered to produce monochromatic RF radiation delivered to the sample
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Describe the instrumentation for NMR involving the magnetic field
Sample placed in magnetic field. RF - excite nuclear spin to higher energy states. When RF signal turned off, spins return to lower states (Larmor radiation associated with field). Emission radiation links to spin relaxation in detector coil (a/d)
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What does it mean when the NMR signal is in the time domain?
The NMR signal is detected at regular time intervals
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What can the NMR signal (in the time domain) be converted to?
Frequency domain - by application of Fourier transformation
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What are the three functions that the simplest MS performs?
Vaporises compound, produces ion resulting gas-phase molecule and separates ions according to their m/z ratio
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What does the ion source do?
Produce gaseous ions from the sample under investigation
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What does the mass analyser do?
Resolve ions according to their m/z ratio
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What does the detector do?
Detect ions and record their relative abundance
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What are the five types of ion source?
Electron impact (EI), chemical ionisation (CI), electron spray ionisation (ESI), matrix assisted laser desorption (MALD), fast atom bombardment (FAB)
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Describe features of electron impact
Beams aimed at molecule to form ionised (cation radical) molecules (due to bond cleavage). Common. Fragmentation patterns are predictable. Extensive fragmentation so M+ is not always visible
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Describe features of chemical ionisation
Involves ionisation of a reactant gas (methane, ammonia, isobutane). Ionising gas is in larger concentration compared to analyte. Soft ionisation technique. Small simple organic molecules. Able to see: M+, M+1, M+-1 (+/- of H)
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Describe features of electron spray ionisation
Liquid phase samples of large size (biomolecules) can be ionised using ESI. Solvated analyte fed through capillary tube with high charge differential. Charged droplet evaporates/ionises analyte
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Describe features of matrix assisted laser desorption
Sample is bombarded with short duration, intense pulses of laser light. UV laser pulse ablates matrix which carries some of the large molecules into gas phase in ion form. Soft method. Used for peptides/proteins/nucleotides
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Describe features of fast atom bombardment
Dissolve sample in glycerol and bombard solution with beam of fast Xe atoms. Beam impinges sample (series of molecular reactions occur). Soft method. Used for carbohydrates/organometallics/peptides
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What are the five types of mass analyser?
Magnetic sector (single/double focusing), time of flight, quadrupole, quadrupole ion trap (dynamic trap), ion cyclotron (static trap)
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Describe features of magnetic sector mass analyser (single focusing)
Molecules ionised by electrons emitted from hot filament. Accelerated towards entrance slit. Ions follow semi-circular trajectory (Lorentz force in uniform magnetic field). Three slits. Detector - Centripetal motion
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What can be deduced from a fixed radius of curvature?
An ion with a particular m/z ratio can be isolated and measured by an appropriate combination of magnetic field B0 and accelerating voltage V
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Describe features of magnetic sector mass analyser (double focusing)?
Similar features for single focusing (better focusing)
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Describe features of time of flight mass analyser
Measures time taken for particles to reach the detector (retention time)
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What are the advantages of the time of flight mass analyser?
Simplicity, ruggedness, ease of accessibility to ion source, unlimited mass range and rapid data acquisition
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What are the disadvantages of the time of flight mass analyser?
Data acquisition must be fast, variations of ion velocities can create peak broadening which can limit resolution
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Describe features of quadrupole mass analyser
Consists of four rods. Arranged parallel (made of Mo alloys). Rods have different applied voltage (2D quadrupole field formed in x-y plane). Ions sorted by stability of trajectories (DC/RF induce in analyser). Change in field strength (filter m/z)
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Describe features of quadrupole ion trap mass analyser
Donut shaped ring electrode between two end-cap electrodes. Ion trap used without applying DC to electrodes link to operation along horizontal axis. separates/detects masses by discharging ions with unstable oscillations from system
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Describe features of ion cyclotron mass analyser
Ions move in magnetic field in circular orbits (based on m/z values). Cyclotron frequency of ion's circular motion is mass dependent. Able to determine ion mass (centripetal force)
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Describe the features of TLC
Stationary phase, mobile phase, plate, solvent, samples, Rf values, use UV or iodine to expose spots
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What are the advantages of TLC
Simple, inexpensive, applicable to wide range of samples e.g. drugs, qualitative/quantitative, small amounts of sample required
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Describe features of column chromatography
Sample passed through column etc.
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Describe features of GC
Gas passed through column in an oven
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Describe features of HPLC
Normal phase (polar stationary phase, non-polar solvent, least polar component elutes from column first). Reverse phase HPLC (non-polar stationary phase, polar solvent, most polar component elutes from column first)
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What are the requirements for a detector?
Sensitive, stable, reproducible, linear over certain concentration range, responds quickly to changes, reliable, non-destructive of sample
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What are the different types of detectors for GC?
UV, fluorescence, flame ionisation, thermal conductivity, electron capture and flame photometric
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What are the types of detectors for HPLC?
Bulk property and solute property
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Describe features of GC-MS
Used to identify and quantify volatile/semi-volatile compounds. GC separates and MS detects
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Give two examples of coupled techniques
LC-MS and LC-NMR
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Other cards in this set

Card 2

Front

How is a UV spectrometer set up (dual beam)?

Back

Light source. Two sets of slits used with prism. Beam splitter. Radiation passes through sample and detector. Radiation passes through reference cell and detector (use mirrors) - ratio given

Card 3

Front

Outline the experiment used to construct a calibration curve

Back

Preview of the front of card 3

Card 4

Front

Describe the set up for IR spectroscopy

Back

Preview of the front of card 4

Card 5

Front

What are thermocouples?

Back

Preview of the front of card 5
View more cards

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