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Dushyant Patel LC-MS/MS Method

S.K.P.C.P.E.R., Kherva 22 M .Pharm. Thesis

¾ The information obtained from a positive-ion mass spectrum is different from and

complementary to that obtained from a negative-ion spectrum.

¾ Thus, the ability to obtain both positive-ion and negative-ion mass spectra aids you in the

qualitative analysis of your sample. You can choose the ion polarity mode and ionization

mode to obtain maximum sensitivity for the particular analysis of interest.

Ionization modes:

¾ Two ionization modes:

• Electrospray ionization (ESI)

• Atmospheric pressure chemical ionization (APCI)

Electrospray ionization:

¾ The electrospray ionization (ESI) mode transforms ions in solution into ions in the gas

phase. Many samples that previously were not suitable for mass analysis (for example,

heat-labile compounds or high molecular weight compounds) can be analyzed by the use

of ESI. ESI can be used to analyze any polar compound that makes a preformed ion in

solution.

¾ With ESI, the range of molecular weights that can be analyzed by the mass spectrometer

is greater than 100,000 U, due to multiple charging. ESI is especially useful for the mass

analysis of polar compounds, which include biological polymers (for example, proteins,

peptides, glycoproteins, and nucleotides), pharmaceuticals & their metabolites and

industrial polymers (for example, polyethylene glycols).

In ESI, ions are produced and analyzed as follows:

• The sample solution enters the ESI needle, to which a high voltage is applied.

• The ESI needle sprays the sample solution into a fine mist of droplets that are

electrically charged at their surface.

• The electrical charge density at the surface of the droplets increases as solvent

evaporates from the droplets.

• The electrical charge density at the surface of the droplets increases to a critical point

known as the Rayleigh stability limit. At this critical point, the droplets divide into

smaller droplets because the electrostatic repulsion is greater than the surface tension.

The process is repeated many times to form very small droplets.

• From the very small, highly charged droplets, sample ions are ejected into the gas

phase by electrostatic repulsion.

• The sample ions enter the mass spectrometer and are analyzed.

¾ The ESI mode use in either positive or negative ion polarity mode. The ion polarity

mode of choice is determined by the polarity of the preformed ions in solution: Acidic

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Samsung RT34M Manuale Utente Pagina 34