Chapter 5 - Ionization methods in LC-MS Flashcards
Electrospray ionization (ESI) is a method by which
ions present in solution can be transferred to the gas phase.
ESI is highly compatible with analytes possessing
Moderate to high polarity
Up to 100,000 Dalton
Ionize in solution, perhaps with multiple charge
3 important processes to transfer ions from LC eluate to gas phase in ESI.
Production of charged droplets at the capillary tip.
Shrinkage of the charged droplets.
Production of gas phase ions.
The capillary is used to
introduce the LC eluate into the ESI source.
Capillary internal diameter is approximately
0.1 mm
Potential difference between capillary and sampling cone
2-6 kV
Electric field in the air around the capillary tip.
10^6 V/m
In positive ion mode, the capillary is the ______ electrode and the sampling aperture plate is the _______ electrode.
positive, negative
In positive ion mode, _______ ions predominately populate the sprayed droplet.
positive
Rayleigh Instability Limit
The point at which the surface charge repulsion matches the surface tension of the eluate.
Taylor Cone
When Coulombic repulsions overcome the Rayleigh limit.
Nebulization
Droplet formation and expulsion into the desolvation region.
As the potential difference across the system increases, the size of the droplet
decrease.
Axial spray mode
Above a certain applied potential difference, the Taylor cone is formed and small charged droplets are formed from its tip (optimum voltage for the experiment).
Rim emission mode
Further increasing applied potential difference causes the liquid cone to vanish and a fine mist of droplets is produced (poor response and irreproducible).
Corona discharge
Electrical discharge brought on by the ionization of a fluid surrounding a conductor (produces a noisy baseline).
ESI flow rate (when by itself)
10-20 microliters per minute
Pneumatically assisted ESI flow rate
Up to 1 mL/min (optimized around 200 microliters)
Solvents with low surface tension lead to
smaller droplets, an increase in sensitivity and a reduced risk of rim emission and electrical discharge.
The addition of a small amount of _______ or _______ to a highly aqueous LC eluate can often bring about an increase in response.
methanol or acetonitrile
LC grade acetonitrile contains a fairly high background level of
sodium ions.
The sprayer current is dependent upon the
conductivity of the eluate solution.
Low concentration of electrolyte ions in the eluate solution
interferes less with analyte gas phase ion production.
Electrolyte ions with low m/z values in the eluate solution
will interfere less with analyte gas phase ion production due to large sphere of hydration.
As a droplet shrinks due to solvent evaporation, its radius _________ while its charge _________.
decreases, remains constant
Droplet jet fission
When enough solvent has evaporated, electrostatic forces are higher than surface tension, and the droplet undergoes Coulombic Fission.
The whole evaporative usually occurs within
a few hundred microseconds and a few milliseconds.
Radius of typical first generation droplet radius & charge.
0.10 mm
280 charges
Typical 2nd generation droplet radius & charge
0.04 micrometers
10 charges
Typical 3rd generation droplet radius & charge
0.003 micrometers
2 charges
Doel model
Droplet fission proceeds until a single ion is produced.
Iribarne & Thompson model
Below radius 10 nm, the ion is able to ‘evaporate’ from within the droplet (most widely accepted theory).
The closer the ion is to the droplet surface,
the more likely it is to evaporate from the droplet.
Smaller ions with more charges have _______ spheres of hydration.
larger
Larger ions with a single charge will have _______ spheres of hydration.
smaller
The rate of ESI production increases with the increase in _________
ion number.
The rate of ESI ion production decreases with the increase of
droplet radius.
Factors that determine the number of charges in the droplet and the radius.
LC eluate flow rate LC eluate composition Nebulization gas flow rate Capillary voltage Drying gas temperature
Ion suppression
In the presence of a second electrolyte species, the intensity of the analyte can be suppressed.
Ion suppression can lead to
artificial and irreproducible reduction in analyte signal.
