Unit 8 Flashcards
what are general steps in sample preparation?
- homogenization
- drying/weighing
- extraction
- purification and cleanup
- preconcentration
- chemical modification
- analytical separation (chromatography, capillary electrophoresis, fractionation)
- detection (spectrophotometry, mass spectrometry, conductivity, refractive index, specific detectors)
what is m/z?
mass-to-charge ratio
mass number of an ion / charge number
what is the general design of MS
- inlet
- ion source: production of gas-phase ions
- mass analyzer: where ions are separated according to their m/z values
- detector: collect and convert ions (w/ a particular m/z) into an electrical signal by an ion transducer
- computer collection of data for analysis
what are 2 types of ion sources of MS?
what are types of each?
- gas phase:
- EI: electron impact
- CI: chemical ionization - desorption
- FAB: fast atom bombardment
- MALDI: matrix assisted laser desorption ionization
- ESI: electrospray ionization
how does electron impact GC function?
molecular ion relaxes, resulting in fragmentation
what are two modes of electrospray ionization?
ESI positive mode: if [M+] exists. Production of [M+H]+
ESI neg mode: deprotonation [M-H]-
what does M.+ stand for?
molecular ion
describe the principle of magnetic sector mass spectrometry
deflection of ions in a magnetic field
ion trajectories depend on m/z value
describe the principle of double focusing mass spectrometry
electrostatic focusing followed by magnetic field deflection
trajectories depend on m/z values
describe the principle of quadrupole mass spectrometry
ion motion in DC and radio-frequency field
only certain m/z values are passed
describe the principle of ion trap mass spectrometry
storage of ions in space are defined by ring and end cap electrodes
electric field sequentially ejects ions of increasing m/z values
describe the principle of ion cyclotron resonance mass spectrometry
trapping ions in cubic cell under influence of trapping voltage and magnetic field
orbital frequency related inversely to m/z value
describe the principle of time of flight mass spectrometry
equal kinetic energy ions enter drift tube
drift velocity and arrival time at detector depend on mass
how do mass analyzers perform selected ion monitoring?
- continuously scan all masses of a mass range
2. monitor specific m/z
describe what happens in LC-MS
- electrically neutral voltage from HPLC unit. Sample and mobile phase enters
- nebulizer gas and unit enters
- high voltage is applied
- charges move to surface to minimize Coulomb repulsion
- vaporization of ions. Ions can;t go anywhere within droplets, thus are liberated from droplets into MS
