How is GC-IR Used?
Feb 25 2022
Since the advent of hyphenated techniques in the 1980s, an array of techniques have been developed that combine separation techniques with spectroscopic detection. Some of the most commonly used include gas chromatography mass spectrometry (GC-MS) and liquid chromatography mass spectrometry (LC-MS).
But aside from mass spectrometry, chromatography techniques can also be coupled with infrared spectrometry. One such example is with GC-IR, as this post will explore.
The basics of GC-IR
GC-IR stands for gas chromatography infrared spectroscopy. It combines gas chromatography, as a separation technique, with characterisation power of infrared spectroscopy. The technique is also known as GC-IRD (gas chromatography infrared detection) or GC-FTIR (gas chromatography Fourier transform infrared).
Firstly, gas chromatography works by separating a compound’s various components for individual analysis. That’s typically done by analysing their retention time (how long they take to release a solvent) as a liquid stationary element transforms to a gaseous mobile element.
Gas chromatography has a wide range of applications, from metabolomics – the analysis of metabolites within organisms, cells or tissues – to forensics, with the ability to analyse blood samples at crime scenes, human bodies after death, and even fire remains to detect accelerants.
That’s coupled with infrared spectroscopy, which analyses the different frequencies absorbed by molecules to deduce their structure. This allows it to characterise functional groups with a view to identifying compounds – particularly useful for distinguishing between structural isomers.
Combining the two
There are three potential interface techniques for GC-IR:
- Light pipe technique – a flow through gas cell, which delivers vapour-phase IR spectra of eluting solutes.
- Matrix isolation technique – matrix isolation is used to trap the GC effluent before measuring the IR spectra for each component.
- Direct deposition technique – GC eluites are condensed on a cooled moving window without dilution.
The light pipe technique was the most commonly used. However, its relatively low sensitivity has seen it replaced for many applications by the used of matrix isolation or direct deposition.
Given that this hyphenated technique has the ability to separate mixtures and detect different components, it naturally lends itself to several uses.
In 2018, researchers used a combination of GC-MS, GC-IR and other methods to discover three bicyclic lactones emitted by marine Salinispora bacteria. GC-IR is also used in environmental analyses, such as identifying trace amounts of herbicides in water samples, with an on-column interface used for large sample volumes.
When paired with mass spectrometry (GC-IR-MS), the technique can also be used to identify drugs such as cocaine and barbiturates with high confidence. A similar application is discussed in the article ‘GC-IRD Analysis of Regioisomeric Substituted Phenethylamines of Mass Spectral Equivalence‘, which describes the use of GC-IRD to detect of a series of isomeric phenethylamines.
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