Chapter 5 - Ionization methods in LC-MS
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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.