Coupling Gas Chromatography to Mass Spectrometry
Flash animation of GC/MS here.
HTML animation of GC/MS here.
The suite of gas chromatographic detectors includes (roughly in
order from most common to the least): the flame ionization detector
thermal conductivity detector (TCD or hot wire detector), electron
capture detector (ECD), photoionization detector (PID), flame
photometric detector (FPD),
thermionic detector, a new variant of the FPD called the pulsed
flame photometric detector (PFPD),
and a few more unusual or VERY expensive choices like the atomic
emission detector (AED) and
the ozone- or fluorine-induced chemiluminescence detectors. All of
these except the AED produce an electrical signal that varies with
the amount of analyte exiting the chromatographic column. The AED
does that AND yields the emission spectrum of selected elements in
the analytes as well. Another GC detector that is also very
expensive but very powerful is
a scaled down version of the mass
spectrometer. When coupled to a GC the detection system itself
is often referred to as the mass selective detector or more simply
the mass detector. This powerful analytical technique belongs to the
class of hyphenated analytical instrumentation (since each part had
a different beginning and can exist independently) and is called gas
chromatograhy/mass spectrometry (GC/MS).
Placed at the end of a chromatographic column in a manner similar
to the other GC detectors, the mass detector is more complicated
than, for instance, the FID because of the mass spectrometer's
complex requirements for the process of creation, separation, and
detection of gas phase ions. A capillary column most often used in
the chromatograph because the entire MS process must be carried
out at very low pressures (~10-5 torr) and in order to
meet this requirement a vacuum is maintained via constant pumping
using a vacuum pump. It is difficult for packed GC columns to be
interfaced to an MS detector because they have carrier gas flow
rates that cannot be as successfully pumped away by normal vacuum
pumps; however, capillary columns' carrier flow is 25 or 30 times
less and therefore easier to "pump down." That said, GC/MS
interfaces have been developed for packed column systems that
allow for analyte molecules to be dynamically extracted from the
carrier gas stream at the end of a packed column and thereby
selectively sucked into the MS for analysis. For one type
interface, using a silicone membrane, the selectivity for organic
molecules (the analyte) over helium (the carrier gas) is 50,000.
The high cost for the pump, ionization source, mass filter or
separator, ion detector, and computer instrumentation and software
has limited the wide application of this system as compared to the
less expensive GC detectors (e.g., FID cost ~$3000; MS cost
~$40,000). However, the power of this technique lies in the
production of mass spectra from each of the analytes detected
instead of merely an electronic signal that varies with the amount
of analyte. These data can be used to determine the identity as
well as the quantity of unknown chromatographic components with an
assuredness simple unavailable by other techniques.
Components of the GC/MS
Leaving the entire capillary GC system aside, the major components
of the mass selective detector itself are: an ionization source,
mass separator, and ion detector. There are a few common mass
analyzers or separators commercially available for GC/MS and they
are, mainly,the quadrapole and the ion trap; however, time-of-flight
mass analyzers are up and coming.
Flash Animation of GC/MS here.
HTML animation of GC/MS here.
The GC-MS animation available at this site is a short series of
steps for the process of a single analyte (already separated from
the other analytes in the chromatographic mixture) denoted as ABC
exiting the chromatographic column and:
In this example, the lightest fragment is B+; the
heaviest A+. The last frame of the movie is a mass spectrum displaying only these three
fragments. Their relative mass to charge ratios are specified by
their relative position on the x axis (low mass/charge to left, high
mass/charge to right). The relative amounts (commonly called peak
intensity) of each of these fragments determined during the mass
analyzer's scan is reflected by the y axis.
- the analyte (A-B-C) undergoing ionization and fragmentation
- the charged fragments (A+ B+ C+)
being separated by mass
- the fragments which are focused on the mass filter's exit
slit passing into the detector
- and the charged ions being detected.
These notes were
written by Dr. Thomas G. Chasteen;
Department of Chemistry, Sam Houston State University,
Huntsville, Texas 77341.
© 2000, 2007,