What is GC×GC?
GC×GC (2D GC or multi-dimensional GC)
Comprehensive two-dimensional gas chromatography,
commonly known as GC×GC, separates a sample using two chromatographic columns
with different properties, run in tandem.
Having two dimensions of separation means that
GC×GC provides greater separation capacity than conventional (one-dimensional)
GC, enabling complex mixtures to be resolved, and revealing minor components
that would otherwise be ‘hidden’ under larger peaks.
In GC×GC, the use of two columns with
different properties results in a greater ability to separate complex mixtures.
How does GC×GC work?
GC×GC involves two columns: a primary column that is typically 20–30 m long, and a
short secondary column that is 1–5 m long. The two columns
are connected with a modulator.
As in one-dimensional GC,
the sample is introduced into a heated port and swept through the primary
column by a carrier gas. The peaks that elute are then sampled (ideally three
or four times per peak) and each fraction is re-injected as a narrow chromatographic
band into the secondary column. Passage through the secondary column is quick
to ensure sharp peaks and no band overlap, with elution usually taking place in
under 10 seconds.
Following elution from the
second column, detection takes place, and the signal is processed to generate a
three-dimensional ‘surface plot’ (which can alternatively be viewed as a two-dimensional
The operation of
GC×GC. The sample passes through a primary column, before being split up by a
modulator and passed into a secondary column.
Importantly, the primary and secondary columns
used in GC×GC have different stationary phases, allowing a mixture to be
separated on the basis of two different properties – typically volatility
(molecular weight) and polarity. These can be set up in one of two ways:
- Normal-phase GC×GC uses a non-polar primary column and a polar secondary column, and is
the standard approach for most applications.
- Reverse-phase (or
inverse-phase) GC×GC uses a polar primary column and
a non-polar secondary column. This setup provides better separation of analyte
groupings in certain cases.
What’s the purpose of a GC×GC modulator?
The most critical part of the GC×GC system is
the modulator, which samples the analytes eluting from the primary column and
injects them as narrow bands into the faster-eluting secondary column. This
preserves the separation achieved in the primary column and prevents the short
secondary column from becoming overloaded.
There are two main types of GC×GC modulators:
Thermal modulators use hot and
cold jets to retain or desorb analytes eluting from the primary column.
Flow modulators use precise control
of carrier and auxiliary gas flows to fill and flush a sampling channel or
Thermal modulators provide
sharp peaks to maximise sensitivity and chromatographic
resolution. However, they suffer from the drawback that they cannot trap analytes boiling below C5
(even when they use liquid cryogen in the cold jet), limiting the application scope.
Flow modulators, in contrast, can cope with the most volatile analytes, while
at the same time avoiding the need for expensive cryogen. Modern flow
modulators, such as our INSIGHT® flow modulator, also use ‘reverse fill/flush’
dynamics to overcome the issue of analyte breakthrough that was a problem with
earlier ‘forward fill/flush’ designs.
What are the benefits of GC×GC?
Compared to one-dimensional methods, GC×GC
offers several benefits:
- Increased peak capacity, meaning better ability to analyse complex mixtures and detect
- More effective removal of
background signals, meaning improved
signal-to-noise ratios and better sensitivity.
- Simultaneous monitoring of
multiple compound classes, meaning simplified
sample preparation (and no need for prior fractionation).
- Structured elution of compound groups, meaning easier identification.
Just like one-dimensional methods, GC×GC is
quantitative, highly reproducible, and compatible with a range of detectors,
including FID and TOF MS.
What applications is GC×GC used for?
GC×GC began as a highly specialised tool for
researchers, but as the technology and software have been improved, and the
user experience become more streamlined, applications have expanded.
Today, GC×GC is increasingly being used for many
routine applications that can benefit from the technique’s ability to separate
highly complex mixtures. These include:
- Food and beverage analysis.
- Fragrance profiling.
- Environmental monitoring.
For advice about how GC×GC could transform your
application, talk to one of our specialists today.