Vapour-Phase IR Spectrum
Definition
The infrared spectrum recorded as a GC analyte elutes into a heated light pipe inside the FTIR cell, with no solvent or KBr matrix. Vapour-phase bands are slightly shifted from condensed-phase reference spectra, which is why GC-FTIR work uses dedicated vapour-phase libraries rather than the standard Aldrich or Sadtler condensed-phase collections.
- Technique
- GC-FTIR with a heated light pipe interface
- Sample state
- Gas phase, no solvent or KBr matrix
- Library needed
- Dedicated vapour-phase spectral libraries
- Complements
- GC-MS, providing structural confirmation for coeluting or isomeric compounds
Common questions
Why can't an analyst just search a vapour-phase spectrum against a standard condensed-phase library?+
Molecules in the gas phase are not constrained by intermolecular forces the way they are in a liquid film or KBr pellet, so band positions and relative intensities shift, sometimes by tens of wavenumbers. A vapour-phase spectrum searched against a condensed-phase library like Sadtler can produce poor or wrong matches even for the correct compound.
When is GC-FTIR chosen over GC-MS alone for a compound?+
GC-FTIR is most valuable for distinguishing isomers or structurally similar compounds that give near-identical mass spectra but different functional-group absorption patterns, such as differentiating positional isomers. It is typically run as a complement to GC-MS rather than a replacement, since IR is less sensitive.
Related terms
- Atmospheric Pressure Chemical Ionisation (APCI)
- A complementary atmospheric-pressure source that vaporises the LC eluent and uses a corona discharge to ionise a reagent gas, which transfers charge...
- Electrospray Ionisation (ESI)
- A soft atmospheric-pressure ionisation source that sprays the LC eluent through a charged capillary into a heated counter-current gas, producing protonated or...
- GC-MS Interface
- The physical link between a gas chromatograph and a mass spectrometer. Older designs used jet separators and open splits to manage the...
- Hyphenated Technique
- A single workflow that couples a separation method to a spectroscopic detector, so a complex mixture is resolved and each component identified...
- MRM (Multiple Reaction Monitoring)
- A targeted MS/MS scan mode on a triple quadrupole that monitors specific precursor-to-product ion transitions, with two transitions per analyte (quantifier and...