Ionization Sources in Mass Spec (6) Flashcards
What are the two key functions of ionization sources?
produce and remove ions
Gas phase vs desorption
gas phase: sample volatilized then ionized
desorption: sample probe ionizes sample directly into gaseous ionic state
hard vs soft
hard: ionization imparts sufficient energy to rupture bonds, producing a significant number of fragment ions
soft: ionization not as energetic, resulting mass spectrum consists mostly of molecular ion and only a few other peaks
What are the physical principles behind electron ionization?
gas phase hard source; electrons emitted from a heated filament then accelerated. electron path intersects gas sample at right angles. ionization occurs due to electrostatic repulsion
What are the primary products in electron ionization?
single charged positive ions
What is the efficiency of electron ionization?
not very efficient; 1 in 10e6
What is the molecular ion?
radical ion with the same MW as the molecule; ion peak that corresponds to the same MW as the parent
What is the base peak?
largest abundance peak or the one with the highest response
What are daughter ions?
large number of positive ions of varying masses less than that of the molecular ion
Why do peaks appear that are higher in m/z than the M+ peak?
isotope peaks and collisional product peaks
What is the most common product in electron ionization?
(M+1)+
What are the advantages of electron ionization sources?
convenient, produce large ion currents, good sensitivity, extensive fragmentation allowing for good compound ID
What are the disadvantages of electron ionization sources?
extensive fragmentation can lead to disappearance of molecular ion peak, unable to establish MW, must volatilize sample so thermal degradation possible, only applicable when MW < 1000
What is the physical principle behind chemical ionization?
gas phase soft source; based on gas phase ion molecule reactions. ions produced by bombardment of a reagent gas with electrons from a filament source
What are the most widely used reagent gases?
methane, isobutane, ammonia
What are the 3 main ionization reactions that occur in chemical ionization?
proton transfer, adduct formation, charge transfer
What is the most common ionization reaction in chemical ionization?
proton transfer
How are reagent ions produced when methane is the reagent gas?
Reactions with high energy electrons
What is the most widely used reagent ions when methane is used?
CH5+ and C2H5+
How are reagent ions produced when isobutane is the reagent gas?
ions react through proton transfer to analyte molecules
What are the most widely used reagent ions when isobutane is the reagent gas?
Which types of molecules are useful to study with isobutane?
Polar
Which types of molecules are inefficient to study with isobutane?
How are reagent ions produced when ammonia is the reagent gas?
Proton transfer
What are the primary reactions when ammonia is the reagent gas?
Ammonium ion and a radical
What are the most widely used reagent ions when ammonia is the reagent gas?
most basic molecules, like amines
What types of molecules are useful to study with ammonia?
polar molecules and hydrogen bonding molecules with no basic character
What type of molecules are inefficient to study with ammonia?
alkanes, ethers, aromatics
Which is the more common chemical ionization reaction, proton or hydride transfer?
proton
What is the major result with proton transfer?
(M + 1)+
What is the major result with hydride transfer?
(M - 1)+
How does chemical ionization differ from electron ionization?
less fragmentation; simpler spectra; stronger molecular ion peak; collisions remove excess energy, stabilize parent ion
What are the special types of emitters used in field ionization?
fine tungsten wire, dendrites
What is the sensitivity of field ionization?
10x less than that of EI
How is the sample solution ionized in electrospray ionization?
resonance electron tunneling forming radical cation
How does the solution droplet ionization get transferred to the sample molecules?
High electric field accumulates drops. Charged spray passes through capillary where solvent is evaporated and ions convert to gas
What atmospheric temperatures and pressures are needed for electrospray ionization?
288.15 K and 1 atm
What is the charge state of the molecules in electrospray ionization?
multiply charged that increases linearly with MW
for what types of sample is electrospray ionization useful?
biomolecules like proteins and polymers with high MW
Why can small analyzers having R = 1500 be used to analyze large molecules in electrospray ionization?
m/z values small enough to detect
Why is electrospray ionization particularly useful for chromatographic interfaces?
Readily adaptable to direct sample introduction
What are the advantages of electrospray ionization?
takes place under atmospheric pressures and temperatures; important fo analyzing biomolecules such as proteins and polymers having MW > 10,000; readily adaptable to direct sample introduction; multiple charging allows use of moderate resolution analyzers
What are the disadvantages of electrospray ionization?
electrospray ionization procedure not identical for all compounds in a multi component mix; selective suppression or enhancement of peaks is observed; formation of ions is the result of an electrochemical process and of charge accumulation; electrospray current limited and sensitive to concentration rather than total amount of analyte
how is field desorption different from field ionization?
it is applicable to other than gas phase samples
What type of sample is best suited for field desorption?
high MW nonpolar species
What is the major drawback of field desorption?
thermal degradation
What type of ionization source is fast atom bombardment?
soft
How are Ar atoms generated in fast atom bombardment?
Collision?
In what form is the sample usually introduced for fast atom bombardment?
liquid, glycerol mull matrix
For what types of samples is fast atom bombardment suited?
high MW polar
What is continuous flow fast atom bombardment?
flow solution continuously into probe
What are the physical principles of matrix assisted laser desorption?
laser pulses produce gas ions
What is the mechanism involved in matrix assisted laser desorption?
laser ablates material from surface, creates microplasm of ions and neutral molecules; vaporizes and ionizes sample; sample mixed in alcohol solution with matrix specifically chosen to absorb UV radiation
What matrices are commonly used in matrix assisted laser desorption?
sinapinic acids, nicotinic acid
What laser wavelengths are commonly used in matrix assisted laser desorption and why?
250 to 350 nm
For what types of samples is matrix assisted laser desorption useful?
biopolymers in MW range of 1000 to 10,000 Da
What are the advantages of matrix assisted laser desorption?
very little fragmentation of analyte ion occurs; very large analyte parent ions can be desorbed; especially useful for polymers and biomolecules
What are the disadvantages of matrix assisted laser desorption?
Mechanism of desorption process not completely understood; ionization reactions not fully understood; most widely accepted explanation for ion formation is gas phase proton transfer in the expanding plume with photoionized matrix molecules; sample prep somewhat empirical
How does secondary ion mass spectrometry differ from other desorption techniques?
Determine both atomic and molecular surface compositions, high vacuum surface analysis, energetic ion beam, used secondary ions; complex spectra and destructive
What is the primary use for secondary ion mass spectrometry?
determining atomic and molecular surface composition
