Steps for ESI Optimization
1 If analyte’s pKa is unknown, evaluate 3 pH regions in positive and negative ion modes.
2 Acids – Negative Ion detection, adjust pH 2 units above pKa: Increase pH with NH4OH, TEA, TMA.
3 Bases – Postive Ion Detection, adjust pH 2 units below pKa* Decrease pH use formic acid , acetic acid, TFA. *In complex molecules, many exceptions to these rules are observed.
4 Remove salts which may cause ion suppression.
5 Adjust source temperature and source voltages to maximize signal
6 In negative ion mode, use lower spray voltage to minimize discharge.
The analyte is introduced to the source in solution either from the eluent flow from liquid chromatography. (Flow rate is 1µl min-1) . The analyte solution flow passes through the electrospray needle that has a high potential difference (with respect to the counter electrode) applied to it (typically in the range from 2.5 to 4 kV). This forces the spraying of charged droplets from the needle with a surface charge of the same polarity to the charge on the needle. The droplets are repelled from the needle towards the source sampling cone on the counter electrode . As the droplets traverse the space between the needle tip and the cone and solvent evaporation occurs.Once the droplet leaves the capillary and enters the nitrogen it continues to lose solvent until the charge density exceeds the surface tension, i.e. the Raleigh constant is exceeded, the droplet explodes resulting in smaller charged droplets. This process continues until the droplets are small enough for ion desorption. The creation of these ions facilitates the transfer of the sample molecules from the source into the MS as the ions are attracted and accelerated into the mass analyser.
Note: The first pioneering experiments on electrospray ionization (ESI) was conducted by Malcom Dole, Not me--- Malcolm. haha...
