Chemical ionization

understanding Chemical ionization with it's mechanism and advantages, disadvantages

1. CHEMICAL IONIZATION By: Aarif khan M. pharm Pharmaceutics dept. YBCCP,AURANGABAD

2. MASS Mass spectrum is analytical technique which can provide information about the molecular structure of organic & inorganic compounds. It can be used to determine the molecular weight. This can be done by Mass sprectrometer .

3. General principles on mass spectrometry Sample introduction Ionization of sample in the ion source Separation of ions by mass analyzer Detector Computer system Resulting chromatogram

4. Ionization techniques The ionization takes place in the ion source. Samples from GC interface are in gas-phase when introduced into the MS. The MS is operated in vacuum to make sure analytes are evaporized and to avoid collisions between analytes and other compounds. Two different ionization methods Electron Impact Ionization (EI) Chemical Ionization (CI)

5. Chemical Ionization (CI) Chemical ionization is a lower energy process than electron ionization (EI). The lower energy yields less or sometimes no fragmentation, and usually a simpler spectrum. Softer ionization technique Less fragmentation  Easier to find molecular ions. Two different modes: Negative chemical ionization (NCI) and Positive Chemical Ionization (PCI) .

6. NCI is used for analytes that are able to form stable negative ions, for example samples containing acidic groups or halogens. NCI is often used to analyze pesticides (contains Cl or Br). PCI is used for samples that can form positive ions (most compounds).

7. Instrumentation:

8. Instrument for chemical ionization

9. ions are produced through the collision of the analyte with ions of a reagent gas that are present in the ion source. Some common reagent gases include : methane , ammonia , and isobutane . Inside the ion source, the reagent gas is present in large excess compared to the analyte . Electrons entering the source will preferentially ionize the reagent gas. The resultant collisions with other reagent gas molecules will create an ionization plasma. Positive and negative ions of the analyte are formed by reactions with this plasma. Mechanism:

10. PCI (simplified):CH 4 + e - CH 4 + +2e - CH 4 + CH 4 +  CH 3 + C H 5 + NCI : CH 4 + e - CH 4 - The ionized gas collide with the sample molecules generating a [M+H] + or [M+H] - ion that is detected: PCI: CH 5 + + M  [M+H] + + C H 4 NCI: CH 4 - + M  [M+H] - + C H 4

11. PCI Spectra:

12. Chemical Ionization Advantages: Milder ionization than EI ; reduces fragmentation. Requires lower energy than EI. Often gives molecular weight information through molecular-like ions such as [M+H] + , even when EI would not produce a molecular ion. Requires little to no additional equipment over EI. Chemical Ionization disadvantages: Molecule must have a Lewis basic or -acidic functional group. Sample must be thermally volatile and stable More external upkeep and Cost (gas cylinders).

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