Page 177 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN006F-275 June 29, 2001 21:12

468 Gas Chromatography

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spectrometer “sees” the proton ( H) as the mass 1 and graphs quite sophisticated instruments with precise elec-
16
oxygen ( O) as 16; a high-resolution instrument can mea- tronic and pneumatic controls.
sure the same species as 1.0078 and 15.9949, respectively. The carrier gas and the auxiliary gases for detectors are
Consequently, the high-resolution instruments are capable controlled by a set of pneumatic devices (pressure regula-
of providing measurements of exact elemental composi- tors, flow-controllers, and restrictors) to assure (a) repro-
tion for various compounds. Different physical principles ducibilityofthecolumnflowrate,andthusretentiontimes,
of mass separation are involved with these instruments. inmultipleanalyses;(b)adjustmentofthegaslinearveloc-
Importantly, both the low- and high-resolution mass spec- ity for optimal column efficiencies; and (c) reproducibil-
trometers can be combined with GC. The methods also ity of detector response for reliable quantitative measure-
strongly overlap with respect to the amounts necessary ments. In addition, filtering devices are inserted in the gas
for analysis. lines to purify all gases mechanically and chemically.
At first, a coupling of GC and mass spectrometry en- Type and design of the injection port are crucial to
countered technological difficulties because the gas chro- performing separations with different types of chromato-
matograph operates at gas pressures above atmospheric graphic columns. Different physical dimensions of the
pressure, while most mass spectrometers operate at high packed and capillary columns cause substantial differ-
vacuum. To overcome these difficulties, molecule separa- ences in the optimum volumetric flow rates. While typical
tors were developed. These devices, working on princi- values for conventional capillary columns range around 1
ples such as molecular effusion, the jet separation effect, ml/min, various packed columns pass one to two orders
and preferential adsorption on a membrane, selectively of magnitude greater gas flows. The volumes of injected
remove most carrier gas, reduce pressure in the interface, samples must be adjusted accordingly. In a typical sam-
and allow most sample molecules to pass into an evacuated pling procedure with a packed column, liquid samples of
mass spectrometer. The process of coupling GC to mass up to a few microliters are injected by a miniature syringe,
spectrometry is further aided by modern pumping tech- through a rubber septum, into the hot zone of the injection
nology. In fact, modern combination instruments need no port. Rapid sample evaporation and transfer into the first
molecule separators for capillary columns (typical flow section of the column are feasible because of a sufficiently
rates around 1 ml/min). high flow rate of the carrier gas.
Contemporary GC/mass spectrometry instruments are Considerably smaller samples are necessary for the
greatly aided by computers, which can control various in- much narrower capillary columns. Since small fractions
strumental parameters, provide data reduction, and com- of a microliter can be neither reproducibly measured nor
pare acquired mass spectra with the extensive libraries of easily introduced into the capillary GC system, indirect
many thousands of previously recorded spectra. sampling techniques are employed. In a commonly used
sampling method, a sample volume of approximately 1µl,
3. GC/Infrared Spectroscopy or slightly less, is injected into a heated T-piece, where an
uneven separation of the vaporized sample stream occurs.
Infrared (IR) spectra of organic compounds are charac-
While the major part of the sample is allowed to escape
teristic of various functional groups in the molecules. IR
from the system, a small fraction (typically, less than 1%)
spectral information is somewhat complementary to mass
enters the first section of a capillary column. Sampling
spectral information. Therefore, the combination of GC
devices based on this principle are called splitting injec-
with IR spectroscopy is, after GC/mass spectrometry, the
tors or splitters. They are generally adequate in situations
second most important structural identification tool. Since
where samples with high concentrations of the analyzed
conventional IR spectroscopy is less sensitive than most
substances are encountered.
GC detectors, the necessary sensitivity enhancement is
Other ways of indirect sampling onto a capillary column
achieved through the use of Fourier transform techniques.
involve the injections of (relatively nonvolatile) samples
With the advent of refined optical systems and fast compu-
diluted in a sufficiently large (measurable) volume of a
tational techniques, the combination of GC with Fourier-
volatile solvent (which serves as a sample “vehicle”). With
transform IR spectrometry is becoming widely used, al-
the column inlet kept at a sufficiently low temperature, the
though its sensitivity is currently less than that of mass
nonvolatile sample trace is trapped at the inlet and focused
spectrometry. Special optical cells were designed for the
into a narrow zone, while the volatile solvent is allowed to
purposes of this combination.
pass through the column and widely separate from the
sample. A subsequent increase of temperature permits
V. INSTRUMENTATION the sample zone to desorb from its inlet position and enter
the usual separation process.
The variety of GC analytical applications, columns, and Most sample introduction techniques in GC have now
specialized techniques make the modern gas chromato- been automated. Process automation permits repeatable