Section F Combined Techniques Instant Notes Analytical
Chemistry D. Kealey & P. J Haines
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Contents 1.Gas Chromatography Mass Spectrometer 2.Gas
Chromatography Infrared Spectrometer 3.Liquid Chromatography Mass
Spectrometer
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1.Gas Chromatography Mass Spectrometer
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Principles The use of Chromatographic Techniques to separate
mixtures is one of the most important Analytical Tools. The
separated components may then be identied 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 Identication
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)
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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
(1)
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Gas Chromatography Principles (2)
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Mass Spectrometer Principles (3)
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Gas Chromatographys 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 -8 Nm -2 The flow of carrier gas is small through a
fine capillary GC column directly into Mass Spectrometer
Instrumentation(1)
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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 (2)
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Fig1. (b) Jet separator Instrumentation (3)
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Instrumentation (4) Fig 1. (a) Schematic of a GC-MS system
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Total Ion Current(TIC) Selected Ion Monitoring(SIM) By
selecting a particular m/z ratio Instrumentation (5)
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Applications (1) Fig 2. TIC Chromatography of fragrance mixture
using a BP 1. nonpolar capillary column direct injection and MS A
mixture of six fragrance components, injected as 5 of a solution in
ether.
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Applications (2) Table 1. Mass spectrometric peaks for peak at
160 s
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Applications (3) Fig 3. Mass spectrum for the peak at 119
s
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2.Gas Chromatography infrared Spectrometer
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Principles The use of Chromatographic Techniques to separate
mixtures is one of the most important Analytical Tools. The
separated components may be classied by their Retention Times, but
other Techniques should be used to aid identication. 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)
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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 (1)
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Principles (2)
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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 (1)
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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 (2)
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Instrumentation (3) Fig 1. gas Chromatography-infrared
Spectrometer system
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Application (1) Fig 2. gas Chromatography of an unknown mixture
A gas chromatogram obtained from a few microliters of a 12
component mixture Peak 1
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Application (2) Fig 3. IR spectrum of peak 1.
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3. Liquid Chromatography Mass Spectrometer
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Principles The components of a mixture, after separation by
Liquid Chromatography. may be identied and quantied by Mass
Spectrometry. Instrumentation The removal of the Liquid mobile
phase, while allowing the analytes to be transferred to the Mass
Spectrometer has presented difculties, 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)
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Mobile phase PumpInjector DetectorComputer Principles (1)
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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 Principles (2)
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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 (1)
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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
(2)
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