Page 367 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN0011A-541 July 25, 2001 17:27

470 Organic Chemistry, Compound Detection

acids as been achieved with diastereomers, as well as with the identification of organic compounds than either of the
chiral stationary phases. two individual methods alone.

7. Identification Methods Based on Comparison 1. Esterification in Situ
of Retention Data
Since organic acids are polar, they are converted into esters
Themostfrequentlyusedmethodforestablishingtheiden- prior to analysis. The esterification is carried out either in
tity of an eluting component is comparison of the retention front of the gas chromatographic column or directly on
data of an “unknown” peak with the retention of a similar the column.
injection made under identical operating conditions of a
referencesubstance,which,basedonotherconsiderations,
2. In Situ Hydrolysis
e.g., a known synthesis precursor, could probably occur
in the sample. Where possible the injection should be re- Hydrolysis is widely used in reaction GC for the identi-
peated with several chromatographic phase systems which fication of unstable and reactive compounds. A consecu-
exhibit different types of selectivity, i.e., a normal parti- tive chromatographic separation, saponification, and chro-
tion, a reversed-phase system, and a liquid–solid (adsorp- matographic analysis of the resulting products was used
tion) system. A vast amount of information exists relating for the identification of a mixture of high-boiling esters.
to retention characteristics of samples in GC systems with
chemical structure.
3. Dehydration and Decarboxylation

Monobasic organic acids are decarboxylated to hydro-
8. Identification Methods Using On-Line
carbons having one atom of carbon less than the acid.
Selective Detectors
Alcohols are dehydrated and the olefins obtained are
The most common application of this approach is the ten- hydrogenated to the corresponding hydrocarbons. These
tative identification of compounds that contain character- transformations are carried out in a reactor at 250–300 C.
◦
istic UV or visible absorption spectra. The use of more
than one detector, linked in series or parallel after the chro-
4. Carbon Skeleton Determination
matographic columns, can provide comparative informa-
of Organic Compounds
tion which reduces the possibility of incorrent assignment
of the identity of a component. A simple example is the The technique of carbon-skeleton chromatography is
use of a UV absorbance detector in line with a differen- based on removing the functional groups from a com-
tial refractive index (RI) detector. The latter will respond poundandreducingthedoubleandtriplebonds.Ahottube
to most substances, whereas the former detector is quite containing a catalyst is introduced into the GC pathway
selective in its response. and hydrogen is used as the carrier gas. As the injected
compound passes over the hot catalyst, the compound is
chemically degraded to its carbon skeleton. The hydro-
9. Monitoring of Column Effluents by MS
carbon products pass into the GC and are identified by
The combination of GC–MS and computerized data han- their retention times. In this technique hydrogenation, de-
dling systems has proved to be one of the most powerful hydrogenation (hydrogen abstraction), and hydrogenoly-
analytical methods for identifying minute (10 −12 g) com- sis (cleavage of functional groups or heteroatoms) may
ponents which may be present in chemical samples. The occur.
greatest success has been in its application in the fields of Hydrogenation, the saturation of multiple bonds,
forensic science, pollution, and biochemistry. greatelyreducesthenumberofpossibilitiesindetermining
the carbon skeleton. Dehydrogenation, the abstraction of
hydrogen, takes place with cyclohexane derivatives and
B. Reaction Gas Chromatography
forms aromatic compounds at elevated temperatures of
Reaction GC is a variation of GC in which chemical re- about 300–350 C. Hydrogenolysis involves the cleavage
◦
action is coupled with the chromatographic separation. of functional groups from a molecule and the addition of
Chemical transformations in analytical reaction gas chro- a hydrogen atom to each of the cleaved ends.
matography always take place in an integral chromato- In typical reactions (catalyst temperature 300 C, H 2
◦
graphic system, in a reaction syringe, a precolumn reactor, flow) the parent hydrocarbon is obtained from halides,
or the column itself. The combination of the chemical and alcohols, and heterocyclic compounds containing sulfur,
the chromatographic methods is a more efficient tool for oxygen, or nitrogen: