Interfacing Gas Chromatography
with Mass Spectroscopy and
Infra Red Spectroscopy
Is this a large expensive detector….
Or a separation prior to analysis….
Early Use of Mass Spectroscopy
• Quantitative methods for determination of
the components in complex hydrocarbon
mixtures
• Later used for the identification and
structural analysis of complex compounds
• Method requires samples that are “clean” or
interpretation is confusing
Principles of measurements
• As an identification method:
– When a given molecular species is impacted
with an electron beam, a family of positive
particles are produced
– The mass distribution of the particles are
characteristic of the parent species
Interfacing GC with
Spectroscopic Methods - Early
• eluates from column collected as separate
fractions after being detected - composition
measured by Mass Spectrometry or IR
• Limitation - small (micromolar)
composition of the solute
• procedure still useful for qualitative analysis
of multi-component
Application of a Selective
Detector - Modern
• The detector monitors the column effluent
continuously
• Need computers to control instruments and
store spectral data for display of spectrum
and chromatograms
Interfacing Gas Chromatography
and Mass Spectroscopy (GC/MS)
GC/ Mass Spectrometry
• GC equipment can be directly interfaced
with rapid-scan Mass Spectrometers
• The flow rate is usually small enough to
feed directly into the ionization chamber of
the Mass Spectrometer
• Packed columns use a jet separator, which
removes the carrier gas for the analyte
GC/ MS
• Increase momentum of heavier analyte
molecules so that 50% or more go into the
skimmer
• Lighter helium molecules are deflected by
vacuum and pumped away
• Use to identify components present in
natural and biological systems
– odor/flavor of foods - pollutants
What is GC/MS?
• Gas chromatography/mass spectrometry
(GC/MS) is the synergistic combination of
two powerful analytic techniques.
• The gas chromatography separates the
components of a mixture in time
• The mass spectrometer provides
information that aids in the structural
identification of each component
What is GC/MS?
What is GC/MS?
The GC/MS Interface
• Transports the effluent from the gas
chromatograph to the mass spectrometer
• Analyte must not condense in the interface
• Analyte may not decompose before entering
the mass spectrometer ion source
• The gas load entering the ion source must
be within pumping capacity of the mass
spectrometer
GC/MS Interfaces
• Capillary Columns
• Macrobore and Packed Columns
Capillary Columns
• Insert exit end of column into ion source
• Under normal operating conditions, the
mass spectrometer can handle the entire
effluent of the column
• Must heat the capillary column to prevent
condensation
• Surface of columns must be inactive
Macrobore and Packed Columns
• Effluent must be reduced before entering
ion source
• Splitting the effluent results in a loss of
sensitivity
• Enrichment devices are used
– Jet Separators are most common
Jet Separator
• Two capillary tubes aligned with a small
space between them. (1 mm)
• A vacuum is created between the two tubes
using a rotary pump
• The GC effluent enters the vacuum region,
those molecules which continue in the same
direction enter the second capillary tube and
continue to the ion source
Jet Separator
• The carrier gas molecules are more easily
diverted from the linear path by collisions
• The analyte molecules are much larger and
carry more momentum
• The surface of the separator must be
inactive and a reasonably even temperature
• Prone to leaks
Resolution and Mass Accuracy
• With a modern mass spectrometer, it is
possible to measure the mass of an ion to
1ppm with a resolution of 100,000 or better
• GC/MS scanning conditions are limited to
5-10 ppm mass accuracy and resolution is
only between 2,000 and 10,000.
• These limitations are usually sufficient to
allow for only a few reasonable and
possible compositions
Resolution and Mass Accuracy
• Resolution can be increased by restricting
the height and the width of the ion beam
• A compromise must be made between
minimizing mass interference and signal
intensity for low levels of material
• Gas chromatograph eliminates most
compounds that cause mass interference.
• Principle cause of peak overlap is the
internal mass standard.
Uses for GC/MS
• May separate, analyze and identify
unknown mixutres
• May separate, and analyze known mixtures
• For sample GC/MS experiments check out:
– http://www.lehigh.edu/~ingcms/ingcms.html
Complex Mass Spectrometer
Detectors
• Display modes - real time or computer
reconstructed
• Each has a choice of total ion current
chromatogram or selected ion current
chromatogram
• Each can be generated on to a computer
screen for print out
Ion Trap Detector
• compact - less expensive than quadropole
• simplest mass detector for use in GC
• ions are created form eluted sample by
electron impact or chemical ionization
• stored in radio-frequency field
• ions injected from the storage area to a
detector
ITD
• Ejection is controlled so the scanning of
mass to charge ratio is possible
Gas Chromatography Infrared
Spectrometry (GC/IR)
•
•
•
•
•
Instrumentation/Interface
Advantages
Problems/Cons
Solutions
Practical Applications
Infrared Spectrometry
• Is especially useful for qualitative analysis
of functional groups and other structural
features
• measuring concentrations is possible
• establish identity of unknown compound
with standard
Instrumentation/Interface
• Infrared Spectrophotometer determines the
relative strengths and positions of the
infrared region and plots the information on
calibrated paper
• Gas Chromatograph partitions the sample as
it passes through the column
• The two can be linked through glass column
or vacuum tubes and other devices on more
expensive equipment
Fourier-Transform Infrared
Spectroscopy (FTIR)
Overcomes the problem
of scanning for a collected sample
or monitoring one wavelength
Fourier Transform IR
• Mechanically
simple
• Fast, sensitive,
accurate
• Internal
calibration
• No tracking
errors or stray
light
FTIR
• Analyze all wavelengths simultaneously
• signal decoded to generate complete
spectrum
• can be done quickly
• better resolution
• more resolution
• However, . . .
Gas Chromatography / Infrared
Spectrometry
• Capillary GC with IR specs can enable the
separation and identifying the compounds
• The interface between the column and the
detector is the main detail
• Small pipe (length 10-40 cm, diameter 1-3
mm) connected to column by narrow tubing
• Transmission of radiation occurs by
multiple reflection off the wall
GC/ IR
• Light pipe is heated in order to rid
condensation and maximize path length for
enhanced sensitivity
• This also minimizes the dead volume to
lessen band broadening
• Detector - highly sensitivity, liquid nitrogen
cooled
• Scanning is started and a brief delay is
needed for compound to travel form the
More on General GCIR
• Very sensitive
• very expensive
• sample recovery
Practical Uses
• Pharmaceutical
• Industrial
• DNA Analysis of blood samples, other
fluids
• many others
INTERFACE to Multiwavelength
UV / VIS Detectors
• Monitor several specific wavelengths set by
colored dyes attached (DNA)
• Flow through a multiwavelength detector
and optical multichannel analyzer
Conclusion
• Gas chromatography can be effectively
coupled with uv/vis detectors for
monitoring dye labels, and infra-red
spectroscopy and mass spectroscopy to
more effectively analyze mixtures.
• This is also true for liquid chromatography,
although the interfaces present different
problems.