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
what is tandem mass spectrometry (MS/MS)?
choosing a specific fragment (precursor ion) from a first ionization and fragmenting it one more time
what is Q1 in tandem mass spectrometry?
precursor or parent ion
what is Q3 in tandem MS?
product or daughter ion
what is the parent-daughter transition in tandem MS?
Q1 > Q3
what is MRM in tandem MS?
multiple reaction monitoring
the quantitative technique in which one or several parent-daughter transitions are monitored to quantify analytes
what is a resolution in MS?
ability to distinguish two peaks of a mass spectrum with a difference of m/z ratios (Delta M)
resolution = M/delta M
what is resolution in optical spectroscopy?
wavenumber, wavelength or frequency difference of two still distinguishable lines in a spectrum
related to spectral and width
what is LRMS?
low resolution mass spectrometer
what is HRMS?
high resolution mass spectrometer
what is LRMS and HRMS measured in?
LRMS: molecular wt is determined to the nearest amu
HRMS: molecular wts are determined to several decimal places
which has better mass accuracy?
a) LRMS
b) HRMS
HRMS
