Mass Spectrometry Flashcards
What steps may be required to prepare samples for MS?
Sample prep:
homogenization, dry/weigh, extraction (LLE, LPME, SPE…), purification/clean up, preconcentration, chem. modification (if needed)
separation:
LC, GC, IC, CE, FFF…
*or can be direct analysis
How is exact atomic mass different from the molecular mass?
molecular mass is AVERAGE from relative abundance of various natural isotopes
each natural isotope for a element will have a different exact mass
What are the most abundant forms of H, C, N, O, and S?
1H, 12C, 14N, 16O, 32S
What is the y and x axis in a mass spectrum?
y axis: relative intensity
x axis: m/z
What is m/z?
mass to charge ratio
divide mass number of ion by charge number
What environment is necessary for mass spectrometry?
vacuum (10-5 to 10-8 torr)
What are the general parts of a mass spectrometer? (6)
inlet system (connect to GC, LC….)
vacuum system
ion source (produce gas-phase ions)
mass analyzer (ions separated according to m/z)
detector (collect/convert specific ions into electric signal w/ ion transducer)
computer/signal processor (control + collect/analyze data)
The MS is connected to the LC or GC or CE by the ___ ____
inlet system
Where are the ions separated in the MS?
Mass analyzer (separate according to m/z)
The detector will _____ ions with a certain ____, and convert into an electric signal using a ___ _____
collect
m/z
ion transducer
What are common ion sources for mass spectrometry? (5)
gas phase:
- electron impact (EI)
- Chemical ionization (CI)
Desorption:
- Fast atom bombardment (FAB)
- matrix assisted laser desorption/ionization (MALDI)
- Electrospray ionization (ESI)
What is the method of ionization for EI? what type of spectra does it produce?
energetic electrons
fragmentation patterns
What is the method of ionization for CI? what type of spectra does it produce?
reagent gaseous ions
proton adducts, few fragments
What is the method of ionization for FAB? what type of spectra does it produce?
energetic atom beam
molecular ions and fragments
What ionization type produces proton adducts?
chemical ionization
What method of ionization uses high energy photons?
MALDI
what type of spectra does MALDI produce?
molecular ions, multiply charged ions
What ionization methods can produce multiply charged ions?
MALDI
ESI
What is the method of ionization for ESI? what type of spectra does it produce?
electric field produces charged spray, which desolvates
multiply charged molecular ions
ESI can be in ___ or ___ mode
positive
negative
describe the process of EI ionization:
hit molecular ion with electron (M + e-)
produces molecular ion
M+ + 2e-
*relaxation of M+ will result in some fragmentation
What is EI ionization usually paired with? (separation technique)
GC
What ions can ESI (positive mode) produce? (6)
M+ (if it exists); [M+H]+ [M+2H]2+ [M+3H]3+ [M+NH4]+ [M+Na]+
What ions can ESI (neg mode) produce?
deprotonation [M-H]-
ESI is usually paired with ____ separation
LC
What are common mass analyzers for MS? (6)
Magnetic sector double focusing quadrupole ion trap ion cyclotron resonance time-of-flight
What are ‘mass filters?’
another name for mass analyzers
What is the principle of magnetic sector mass filters?
deflect ions in magnetic field (trajectory depends on m/z value)
What is double focusing?
- electrostatic focusing
2. magnetic field deflection
What is the principle of quadrupole mass filters?
ion motion in dc and radiofrequency fields - only certain m/z ions pass through
How do ion trap mass filters work?
store ions in space (defined by ring + end cap electrodes)
E field will sequentially eject ions (in increasing m/z order)
How do ion cyclotron resonance mass filters work?
trap ions in cubic cell (from trapping voltage + mag field)
orbital frequency related inversely to m/z
how do TOF mass filters work?
long tube with detector at end
equal kinetic energy ions enter drift tube -> different mass = different velocity/arrival time at detector
Describe the general parts of a GC-quadrupole MS system
GC: carrier gas inlet injection port column oven transfer line
MS: ion source (ex: EI) focusing lenses mass analyzer (quadrupole) electron multiplier data system
What are the possible functions (modes) of a mass analyzer?
- SCAN mode
continuously scan all masses of a mass range (complete mass spectrum) - SIM mode
monitor SPECIFIC m/z
(selected ion monitoring to target specific mass fragments)
What mass analyzer mode is selected for quantification of traces? Why?
SIM mode (selected ion monitoring)
more sensitive/selective
How does a electrically neutral mobile phase/sample from the LC become charged for the MS?
ESI:
charged by applying high voltage in the nebulizer unit
(vaporized into ultra-fine charged droplets)
charges move to surface of droplets -> ions liberated from droplets -> go into MS
What causes the droplets formed by ESI to dissipate into ions?
charges will move to surface to minimize coulomb repulsion -> ions can’t go anywhere in droplets, so they break free
What is MS/MS known as?
tandem MS
What is tandem MS?
2 consecutive MS analyzers; monitor parent-daughter transition of ion (second MS will further analyze for a fragment from the first MS)
the selected fragment in the first mass analyzer is called the ___ ion, abbreviated as ____
parent/precursor
Q1
The fragment resulting from fragmentation of the parent ion is called the _____ ion, abbreviated as ______
daughter/product
Q3
Q1 -> Q3 is called a _________
parent-daughter transition
monitoring several parent-daughter transitions to quantify analytes is called ___ ____ ____
multiple reaction monitoring (MRM)
what is ‘resolution’ in MS?
Ability to distinguish 2 peaks of mass spectrum
resolution = M/(difference in m/z)
What is ‘resolution’ in optical spectroscopy?
wavelength, wavenumber, or frequency difference of 2 still distinguishable lines in spectrum (related o spectral band width)
What is the ‘valley’ definition of resolution in MS according to IUPAC?
10% Valley definition: 2 peaks w/ equal height in spectrum are separated by a valley w/ lowest point 10% of height of either peak
