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Instant Notes Analytical Chemistry
Section F
Combined Techniques
D. Kealey & P. J Haines
Contents
1. Gas Chromatography – Mass Spectrometer
2. Gas Chromatography – Infrared Spectrometer
3. Liquid Chromatography – Mass Spectrometer
1.Gas Chromatography – Mass
Spectrometer
Principles
The use of Chromatographic Techniques to separate mixtures is one of the most
important Analytical Tools. The separated components may then be identified by other
techniques. Mass Spectrometry is the most important of these.
Instrumentation
Combining Gas Chromatography with Mass Spectrometry requires special methods of
Interfacing since the Two Techniques Operate under Different Conditions.
Applications
Gas Chromatography-Mass Spectrometry has been used to study the Separation and
Identification of volatile mixtures such as Natural Products, Crude Oils and
Environmental Samples.
Related topics
Gas Chromatography: Principles Mass Spectrometry (E14) Instrumentation (D4)
Gas Chromatography: Procedure Sand Applications (D5)
Principles (1)
The operating temperature range of instrument
☞ Room Temperature ~ 400℃
The separated components may be classified according to
their Retention times or by Chromatography spiked
sample.
Solutes may be ionized by Electron Impact or
Chemical Ionization
Principles (2)
Gas Chromatography
Principles (3)
Mass Spectrometer
Instrumentation(1)
Gas Chromatography’s detection by Flame
Ionization Detector(FID) or one of the other GC detector
is possible.
GC/MS is then necessary to reduce the pressure
to operating pressure of Mass Spectrometer ☞ 10-8Nm-2
The flow of carrier gas is small
☞ through a fine capillary GC column directly into
Mass Spectrometer
Instrumentation (2)
For packed column, an Interface between the GC and MS is
required.
☞ Porous Tube Separator or Jet Separator
The interface should be maintained at the temperature of
GC outlet
A Quadruple Analyzer are very often used because of their
ability to scan rapidly
Instrumentation (3)
Fig1. (b) Jet separator
Instrumentation (4)
Fig 1. (a) Schematic of a GC-MS system
Instrumentation (5)
Total Ion Current(TIC)
Selected Ion Monitoring(SIM)
☞ By selecting a particular m/z ratio
Applications (1)
A mixture of six fragrance components, injected as 5 ㎕ of a
solution in ether.
Fig 2.TIC Chromatography of fragrance mixture using a BP – 1. nonpolar capillary
column direct injection and MS
Applications (2)
Table 1. Mass spectrometric peaks for peak at 160 s
Applications (3)
Fig 3. Mass spectrum for the peak at 119 s
2.Gas Chromatography – infrared
Spectrometer
Principles
The use of Chromatographic Techniques to separate mixtures is one of the most
important Analytical Tools. The separated components may be classified by their
Retention Times, but other Techniques should be used to aid identification. Infrared
Spectrometry is capable of establishing which Functional Groups are present in the
separated components
Instrumentation
Combining Gas Chromatography with Infrared Spectrometry involves passing the
solutes in the Carrier Gas Stream through a heated Infrared Gas Cell positioned in a
rapid scanning Fourier Transform Spectrometer.
Applications
Gas chromatography-Fourier transform infrared spectrometry has been employed in
the analysis of Biological Materials such as Fragrances, to determine the proportions and
nature of each component, of Solvents to determine their purity and composition, and
to Identify the evolved products when substances are degraded by heating.
Related topics
Gas chromatography : principles and Infrared and Raman spectrometry :
instrumentation (D4)
Principles and Instrumentation Gas Chromatography : procedures and (E10)
applications (D5)
Infrared and Raman Spectrometry : applications (E11)
Principles (1)
Identification of the separated solutes by their Retention
Time alone is often ambiguous.
Infrared Spectrometry is a very powerful and versatile
technique for the identification of Function group.
Using computerized Fourier transform processing of
Spectral Information.
Principles (2)
Instrumentation (1)
The greater sensitivity of modern FTIR have allowed
the use of Capillary Columns.
Typically, a 30m long, 0.3mm diameter fused
Silica Column coated with a 1－㎛ thickness of
Stationary Phase may be employed.
The Gas Stream is therefore usually split at the Column
exit so that 90% goes onto the Spectrometer and
only 10% to the FID
Instrumentation (2)
To obtain the maximum response it is necessary to do
one of two thing
1. The sample must be concentrated by
condensation onto a cooled surface or
by absorption
2. If the sample is to remain gaseous, it must be
passed through a small volume cell, or light pipe.
Instrumentation (3)
Fig 1. gas Chromatography-infrared Spectrometer system
Application (1)
A gas chromatogram obtained from a few microliters of
a 12 – component mixture
Peak 1
Fig 2. gas Chromatography of an unknown mixture
Application (2)
Fig 3. IR spectrum of peak 1.
3. Liquid Chromatography – Mass
Spectrometer
Principles
The components of a mixture, after separation by Liquid Chromatography. may be
identified and quantified by Mass Spectrometry.
Instrumentation
The removal of the Liquid mobile phase, while allowing the analytes to be transferred
to the Mass Spectrometer has presented difficulties, and the design of the Interface is
critical.
Applications
The analysis of mixtures of Pharmaceuticals and Drugs, the detection of degradation pathways using Isotopic
Labeling, and the separation and analysis of Peptides using Soft Ionization Methods are typical of the application
of LC-MS.
Related topics
High-performance liquid Mass spectrometry (E14)
chromatography: modes, procedures and applications (D7)
Principles (1)
Pump
Mobile phase
Detector
Computer
Injector
Principles (2)
Since these employ Liquid mobile phases, sometimes
containing Inorganic salts, the most difficult problem
☞ How to transfer separated component to
Mass Spectrometer without interference
from the Solvent
Instrumentation (1)
The Interface between the Liquid Chromatography and
Mass Spectrometer is the most vital part of the
Combined Instrument.
Atmospheric Pressure Chemical Ionization (APCI)
☞ Nitrogen is introduced to nebulize the Mobile phase
producing an Aerosol of Nitrogen and Solvent Droplets
which are passed into a heated region
Desolvation occurs, and Ionization is achieved by Gas phase
Ion-molecule reaction at Atmospheric, Electrons and the
primary Ions being produced by Corona Discharge
Instrumentation (2)
The pressure is close to Atmospheric, the Collision frequency
is high and Pseudomolecular Ions are formed with
high efficiency by Chemical Ionization.
Electrospray (ES)
☞ Operating at Atmospheric, the Liquid mobile phase is ejected
from a Metal Capillary Tube into an Electric Filed obtained
by applying a potential difference of 3 ~ 6 ㎸
between the Tube and a counter Electrode.
Instrumentation (3)
Fig 1. Electrospray (ES) interface
Applications (1)
The detection of impurities in synthesized drugs
Fig 2. (a) TIC chromatogram of dug sample
Applications (2)
Fig 2. (b) Mass spectrum of dug impurity