This document discusses mass spectrometry and chemical ionization. It explains that mass spectrometry can provide molecular structure information by determining molecular weight. Chemical ionization is a softer ionization technique than electron ionization, which causes less fragmentation and makes it easier to detect molecular ions. The document describes the instrumentation for chemical ionization, where a reagent gas like methane is ionized by electrons and the resulting ions collide with analyte molecules to form positively or negatively charged molecular ions depending on the mode used. The advantages of chemical ionization are also summarized.

1. 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):CH4 + e- CH4
+ +2e-
CH4 + CH4
+ CH3 + C H5
+
NCI : CH4 + e- CH4
-
• The ionized gas collide with the sample molecules
generating a [M+H]+ or [M+H]- ion that is detected:
PCI: CH5
+ + M  [M+H]+ + C H4
NCI: CH4
- + M  [M+H]- + C H4

11. PCI Spectra:

13. • 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).