Question

What are the essential features of the following type of ionization mode used in mass spectrometry? Include whether it is a hard or soft process and what are the main benefits and/or limitations.

a) Ramped or “Smart Fragmentation” (i mostly need this one)

b) chemical ionization

c) Atmospheric pressure chemical ionization

Answer 1

a) Ramped Fragmentation or “Smart Fragmentation” :

This technology have dramatically increased power and conformation of Mass Spectroscopy. It is a type of Computational mass spectrometry.

In this voltage ramping used for obtaining maximum fragmentation patterns.It is a recent advancement a lookup table of common neutral losses during fragmentation has also been published. There exist commercial tools that follow the same line of thought : e Smart Formula three-dimensional (3D) algorithm that includes this information by using a recursive algorithm to exclude unfeasible molecular formulae from lower mass fragments combined matching isotope patterns with filtering based on the molecular formulas of product ions.

Voltage ramping continuously increases the fragmentation energy during a single acquisition .

It is very identified the correct molecular formula for small molecules.

It is a hard process.

b) chemical ionization :

Chemical ionization uses ion-molecule reactions to produce ions from the analyte. The chemical ionization process begins when a reagent gas such as methane, or ammonia is ionized by electron impact. A high reagent gas pressure (or long reaction time) results in ion-molecule reactions between the reagent gas ions and reagent gas neutrals.

It is Soft process (it refers to the processes which result in little fragmentation).

Some of the products of these ion-molecule reactions can react with the analyte molecules to produce analyte ions.

Example (R = reagent, S = sample, e = electron, . = radical electron , H = hydrogen):

R + e ---> R^+.+ 2e

R^+. + RH ---> RH⁺ + R^.

Benefits :

It gives molecular weight information through molecular-like ions such as [M+H]⁺, even when EI would not produce a molecular ion.

Mass spectra is simple, fragmentation reduced compared to EI

Limitations :

Sample must be thermally volatile and stable.

Less fragmentation than EI, fragment pattern less informative or reproducible.

Results depend on reagent gas type, reagent gas pressure or reaction time, and nature of sample.

c) Atmospheric pressure chemical ionization

APCI is a method that is typically done using a similar source as ESI ( voltage is placed on a needle that creates a corona discharge at atmospheric pressures ). In APCI, a corona discharge is used to ionize the analyte in the atmospheric pressure region.

This discharge creates ions, in theory mostly H3O+ or water clusters. The sample is injected into the discharge by a spray created by a flow of liquid combined with a heated gas that volatilizes the sample. The ions are formed by proton transfer from the H3O+ or the water clusters to the sample.

It is Soft process (it refers to the processes which result in little fragmentation).

The gas-phase ionization in APCI is more effective than ESI for analyzing less-polar species.

Benefits :

• Good for less-polar compounds.
• Excellent LC/MS interface.
• Compatible with MS/MS methods.

Limitations : ESI and APCI are complementary methods.