Coupling Gas Chromatography to Mass Spectrometry
Flash 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 (FID),
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
and a few more unusual or VERY expensive choices like the atomic emission detector
(AED) and the ozone-
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; howerver, time-of-flight mass analyzers are up and coming.
Flash Animation of GC/MS here.
The movie 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.
Click here to
download this movie.
These notes were
written by Dr. Thomas G. Chasteen; Department
of Chemistry, Sam Houston State University, Huntsville, Texas 77341.
© 2000, 2007, 2009.