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  10 Must know points to Understand Electrospray Ionisation  

1.

Eluent pH adjusted to obtain ionised forms of the analyte in solution – retention in reversed phase HPLC may need to be considered and an appropriate stationary phase chosen.  The eluent flow rate is adjusted so as not to ‘swamp’ the source – typically <1 mL/min

 

adjust pH

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2.

A voltage is applied between the capillary and sampling plate to generate an Electrospray  - a special circuit which is completed by the movement of analyte ions through space (the desolvation zone)

 

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3.

This applied voltage causes charge separation at the capillary tip and defines the ‘mode’ of spraying as an excess of +ve or –ve are created.  Neutral analytes may be charged through redox, proton transfer, adduction or electrostatic processes.

     

4.

A droplet forms at the capillary tip containing an excess of charge and which is constrained in size by the use of a nebulising gas – this ensures the right amount of charge to droplet radius

 

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5.

Droplets are sprayed from the end of a canonical cone (the Taylor cone) which forms to relive columbic repulsion at the sprayer tip once the Rayleigh limit is reached  Care needs to be taken in order to apply just the right voltage to the sprayer in order to avoid ‘discharge’ which is an unstable spray mode

     

6.

As the droplet travels across the desolvation zone, hydrated ions migrate from within the bulk of the droplet to the surface, from where they can ultimately be transferred to the gas phase.  The radius of the ion as well as the degree of hydration will govern the ease of movement to the droplet surface – and hence its relatively intensity in the final spectrum.  Small, highly charged, ions carry large spheres of hydration and are restricted in their movement from the droplet bulk to the surface.  The relative ease of movement through from the droplet bulk to the surface and the competition of ions for the decreasing area at the droplet surface gives rise to ion suppression / enhancement effects

     

     

7.

The droplet desolvates under the influence of a drying gas and the surface charges move together causing increasing amounts of repulsion.

     

     

8.

The Rayleigh limit is exceeded and coulombic jet fission occurs – liberating a number of smaller droplets in which the charge density is much higher than in the original droplet – this then sets off a ‘chain reaction’ of further fissions from these smaller droplets

     

     

9.

Once the droplet reaches a certain mass to charge ratio (or diameter) one of two things happens – a point is reached where the repulsion within the droplet it so great that it is thermodynamically favourable for a hydrated ion to be liberated from the droplet into the gas phase (the Iribarne / Thompson ‘Ion Evaporation’ Theory) or the droplets continue to undergo subsequent droplet fissions until they contain only a single ion which is liberated into the gas phase via further desolvation (the Dole ‘Charged Residue’ Theory)

     

     

10.

Once liberated into the gas phase the ions are sampled into the first vacuum region of the mass spectrometer – usually as hydrated clusters.  The position of ion production within the spray, both axially and laterally, will change markedly with different analyte / sample matrix / eluent combinations which may require that the position of the sprayer relative to the sampling cone is adjusted to optimise instrument response for a particular analyte type

 
 
 
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