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42 Cards in this Set
- Front
- Back
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What methods of ionization are discussed for MS?
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REMPI (for SIMS)
Electron ionization Chemical ionization (and APCI) Photoionization Laser desorption Electrospray ionization (and DESI) DART |
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How does chemical ionization work, and under what circumstances might it be favored over EI?
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A reagent gas at high pressure (mixed with gaseous sample at low pressure) is ionized. Collisions of the reagent gas with the sample result in the formation of a pseudo-molecular ion, a protonated form of the sample molecule, which can be detected by MS. Selectivity is achieved by choosing the reagent gas. The proton affinity of that gas will affect which compounds it can protonate (e.g. Methane has very low PA, so essentially universal, but ammonia has high PA, so selective for amines)
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How do gas-phase sources differ from desorption sources?
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Gas-phase sources require samples to be vaporized prior to ionization, whereas desorption sources directly convert sample to gas-phase ions
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In what cases are desorption sources favored over gas-phase sources?
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When sample is thermally-labile or non-volatile
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What sorts of compounds may be ionized by gas-phase samples?
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-Compounds with boiling points < 500C
-Generally compounds <1000 Da -Compounds that are not thermally labile |
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What is the cost range for an ms machine?
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$1000 to $10,000,000
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On what properties does the charge transferred in ESI depend?
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1) Size of analyte
2) Shape of analyte 3) GB of analyte 4) Properties of the solvent (surface tension, GB) 5) Nature of the experiment |
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Under what circumstances is the charge on a molecule proportional to its size?
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If the molecule s unstructured (as a denatured protein, etc)
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Who won the Nobel Prize in Chemistry 2002 and for what?
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Fenn, for developing ESI for biomolecules
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Draw the basic schematic for an EI source
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See notes: 10/27
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What are the major advantages of ESI?
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-Atmospheric pressures OK
-Can prepare biomolecules larger than (10^5) Da -Easily adapted to use with LC -High efficiency => micromolar concentrations OK |
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How are droplets charged in electrospray?
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Induce several kilovolt potential between the capillary and a cylindrical elecrode surrounding it
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What is the major drawback of ESI, and how is this usually corrected?
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No fragmentation => little structural information. Generally tandem MS/MS used, having isolated the compound of interest
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What are the disadvantages of atmospheric MALDI?
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Decreased sensitivity, mass range
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What are the major factors affecting analyte binding affinities for ion-exchange resins?
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1. Increasing ionic charge increases affinity
2. Decreasing hydrated radius increases affinity 3. Increasing polarizability of a molecule increases its binding affinity |
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How are gradient elutions used with ion chromatography?
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By adjusting pH (generally lowering pH)
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What is the general form of ESI-generated quasi-molecular ions?
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[M+nH] or [M-nH], which is advantageous because the smaller m/z allows detection on limited m/z instruments, such as the quadrupole
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What analyte is best ionized by EI?
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Small, volatile organic compounds
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Which MS ionization method is most commonly used with LC-MS?
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ESI, because volatile solvents are employed for this method
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Why is the MS maintained under high vacuum?
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Because this ensures that ion trajectories are not altered by collision, which would skew signal
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What is a typical ionization energy of a molecule studied using EI?
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8-12 eV
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What is the average energy deposited in a molecule in EI-MS?
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2-3 eV, which is enough to cause fragmentation
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Why are electrons usually accelerated to 70 eV for EIMS?
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Because this gives reproducible fragmentation that may be compared to library data
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What determines the degree of fragmentation in CI?
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The proton affinity of the analyte. A very exothermic reaction will result in fractionation.
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What is the advantage of the quadrupole over the magnetic sector mass analyzer?
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No magnets means no hystoresis effect, so can scan very quickly => can be coupled to chromatographic separators
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What disadvantage tends to balance increased resolution?
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Decreased sensitivity
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In general, which variable is scanned in magnetic sector MS and why?
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The accelerated potential, because the magnetic field scans too slowly to be practical due to the hystoresis effect
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What are the biggest handicaps of the quadrupole mass filter?
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-Poor mass-measuring accuracy
-Poor resolution -Small m/z range (though this is fine for GC-MS, as GC simlarly handicapped) |
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What major advantage is conferred to MS measurements by ion traps?
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Effectively multichannel detection in inherently scanning devices
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What is the major factor inhibiting the performance of a quadrupole ion trap?
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Ion interactions that decrease dynamic range
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What is the relationship between cyclotron frequency and m/z? How is this used practically?
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Cyclotron frequency is inversely proportional to m/z (through the multiplicative constant = B = strength of magnetic field). This is used in FT-ICR to identify m/z from I (induced).
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What is the major detection advantage of FT-ICR methods?
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Ultra high resolution, though at the cost of longer sampling time => not super practical in tandem with chromatography unless ion trapping is possible
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Define: cyclotron frequency
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The angular frequency with which an ion rotates in the plane perpendicular to an applied magnetic field. This value is inversely proportional to the ion's m/z
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What is the effect of an AC voltage on an ion in a magnetic field?
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An AC voltage on-resonant with the ion's cyclotron velocity will increase the velocity of the ion (therefore increasing the radius of its rotation) without altering its frequency
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What is the effect of an off-resonant AC voltage on an ion rotating in a magnetic field?
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None. This energy will not be absorbed
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Define: Image current
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Induced current in detector plates from motion of charged particles nearby. Used in FT-ICR and other non-destructive trap-based MS methods as a detection mechanism
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How is FT-ICR used in proteomics?
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In the identification of AAs in a peptide through their masses, for example
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What is the major factor decreasing resolution in a simple TOF mass analyzer?
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That ions have different KE depending on their distribution in the electrostatic acceleration region. Those closer to the backplate of the accelerator will enter the drift tube with great KE, altering.
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What is the term for the plane at which ions of the same mass emitted at different kinetic energies will meet in a TOF mass analyzer?
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Spatial focus plane
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At about what degree do the channels of a microchannel plate detector enter the instrument?
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About 6-10 degrees from the normal
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What is the approximate pressure in an EI source?
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~10^(-5), 10^(-6) Torr
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What is the proton affinity of methane?
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about 5.5 eV
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