Question

In electrospray ionization mass spectrometry, why does S/N depend on capillary voltage? Why?

Answer 1

Electrospray ionization mass spectrometry is basically a desorption ionization technique, which can be performed on solid or liquid samples, and can be nonvolatile or thermally unstable. Therefore, the ionization of samples such as proteins, peptides, and some inorganic molecules can be done. The technique requires that a molecule should be of a fairly larger mass.

The apparatus has an inlet for the liquid sample in the form of capillary needle. When the liquid sample is nebulized and charged inside capillary needle, a large pressure is applied to the capillary needle, which later nebulizes the liquid sample leading to the formation of a mist. The capillary needle is surrounded by an electrode having a voltage of 4000 volts, which helps in placing a charge on the droplets.

The formation of droplet and expulsion into the desolvation region is known as ‘Nebulisation’. The stable nebulisation aerosol formation is depends on various factors such as follows:

1. The inner and outer diameter,
2. The shape of the capillary tip,
3. The applied potential difference (Vc),
4. The flow rate, surface tension and electrolyte concentration of the HPLC eluent, and
5. The flow rate and temperature of any nebulising gas which is applied concentrically to the capillary to assist droplet formation

The most important thing in establishing a stable and productive electrospray is the amount of applied potential difference between the capillary and the sampling plate of the API interface. It is proportional to the magnitude of the applied potential difference across the system. With the increase of potential difference across the system, the size of the droplet gets reduced. Above a certain applied potential difference, the Taylor cone is formed where the small charged droplets are formed from its tip. This is called the Axial Spray mode, which has the optimum voltage for the experiment. On further increasing the applied voltage a sudden transition takes place – the liquid cone disappears and a fine mist of droplets is produced from several points on the edge of the capillary tip. This process is called as Rim Emission mode. At still higher capillary potentials, a second transformation takes place. There is an establishment of corona discharge between the needle tip and the sampling plate. Discharge is not a state of stable or reproducible spray and the noisy baselines is produced resulting into a lowered signal to noise (S/N) ratio, which is defined as the ratio of the amplitude of a signal to the amount of unwanted interference (the noise) that has mixed in with it.