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Interfacing Gas Chromatography with Mass Spectroscopy and Infra Red Spectroscopy

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Title: Interfacing Gas Chromatography with Mass Spectroscopy and Infra Red Spectroscopy


1
Interfacing Gas Chromatography with Mass
Spectroscopy and Infra Red Spectroscopy
  • Is this a large expensive detector.
  • Or a separation prior to analysis.

2
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

3
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

4
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

5
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

6
Interfacing Gas Chromatography and Mass
Spectroscopy (GC/MS)
7
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

8
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

9
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

10
What is GC/MS?
11
What is GC/MS?
12
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

13
GC/MS Interfaces
  • Capillary Columns
  • Macrobore and Packed Columns

14
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

15
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

16
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

17
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

18
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

19
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.

20
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

21
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

22
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

23
ITD
  • Ejection is controlled so the scanning of mass to
    charge ratio is possible

24
Gas Chromatography Infrared Spectrometry (GC/IR)
  • Instrumentation/Interface
  • Advantages
  • Problems/Cons
  • Solutions
  • Practical Applications

25
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

26
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

27
Fourier-Transform Infrared Spectroscopy (FTIR)
  • Overcomes the problem
  • of scanning for a collected sample
  • or monitoring one wavelength

28
Fourier Transform IR
  • Mechanically simple
  • Fast, sensitive, accurate
  • Internal calibration
  • No tracking errors or stray light

29
FTIR
  • Analyze all wavelengths simultaneously
  • signal decoded to generate complete spectrum
  • can be done quickly
  • better resolution
  • more resolution
  • However, . . .

30
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

31
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 detector region
    to the IR cell

32
More on General GCIR
  • Very sensitive
  • very expensive
  • sample recovery

33
Practical Uses
  • Pharmaceutical
  • Industrial
  • DNA Analysis of blood samples, other fluids
  • many others

34
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

35
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.

36
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