Page 162 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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404 Elemental Analysis, Organic Compounds

The reference standard is optional. The apparatus com-
prises three parts: gas chromatograph, microwave genera-
torandplasmadischargetube,andpolychromatoranddata
recording. The gas chromatograph serves to separate mix-
tures of organic compounds and submit pure samples for
decomposition. The spectrometer has channels for carbon,
hydrogen, oxygen, nitrogen, fluorine, chlorine, bromine,
iodine, phosphorus, sulfur, and mercury. This method is
particularly suited for the analysis of petroleum products
and halogenated hydrocarbons.
Hughes, Brown, and Fry described the photodiode ar-
FIGURE 12 Equipment for the determination of carbon, hydro- ray application of the red and near-infrared region of the
gen, oxygen, and nitrogen ratios by the sulfurization technique. argon inductively coupled plasma atomic emission spec-
[Courtesy of Prof. T. Hara.] trum from 650 to 900 nm for nonmetallic elements deriv-
ing from the atomization of pure organic compounds. It
was found that C, H, N, and O lines of reasonable inten-
placed at the bottom of the ampoule, followed by the or-
sity can be obtained with short exposure times, and the
ganic sample. Air inside the ampoule is displaced by a
red and near-infrared spectra of these elements are unusu-
stream of helium with a special displacement device (see
ally simple, with no apparent line interferences and very
Fig. 12), and the ampoule is quickly sealed. The ampoule
low levels of plasma continuum background. Hence it is
◦
is then heated at 1500 C in the high-frequency induction
feasible to use this technique for the simultaneous deter-
furnace for 1 min and left there for 3 min. Subsequently,
mination of carbon, hydrogen, nitrogen, and oxygen in the
the ampoule containing the reaction products is inserted
organic sample.
into a Teflon sampler, which is connected to the gas chro-
When the atomic ratios of all elements present in an
matograph. The ampoule is broken by driving a jack across
organic compound have been determined, the empirical
the Teflon sampler, and the mixture of gaseous products
formula of the compound can be calculated. The empiri-
is introduced into the gas chromatograph, where N 2 ,CO 2 ,
cal formula can be checked against the molecular weight
H 2 S, COS, H 2 O, SO 2 , and CS 2 are separated and their
found by a suitable method. Alternatively, the chemical
peak areas are recorded. Correction factors are obtained
formula of an organic compound can be obtained with the
by analyzing five standard compounds. The atomic ratios
aid of high-resolution mass spectrometry. Since the latter
of carbon, hydrogen, oxygen, and nitrogen in an unknown
technique can measure molecular species to a precision of
compound are determined by using these correction fac-
seven digits, it is possible to deduce the chemical formula
tors. Figure 12 shows the complete equipment: the air-
by matching the molecular weight (mass) with the vari-
displacement device is on the left side; the Teflon sampler
ous combinations of atoms of the elements expected to be
and crushing device are at the center, with the connecting
present in the compound. It is prudent, however, to verify
tubes to the gas chromatograph; and the high-frequency
the chemical formula by determining one or two elements
induction furnace is on the right side of the assembly.
by chemical methods. Obviously, mass spectrometry is
It is obvious that the technique of Hara et al. described
not applicable to the determination of any element in a
above is not applicable to the analysis of gaseous samples
mixture of organic compounds.
and low-boiling liquids. For volatile organic compounds,
Yu et al. put forth a device that uses high-resolution glass
capillary gas chromatography in combination with mi-
crowave plasma emission spectrometry. The method is SEE ALSO THE FOLLOWING ARTICLES
based on the complete destruction of a compound into
its constituent atoms by means of the helium plasma, fol- COMBUSTION • GAS CHROMATOGRAPHY • ORGANIC
CHEMISTRY,COMPOUND DETECTION • ORGANIC CHEM-
lowed by measurement of each atomic species in the spec-
trometer. The signal obtained for a specific element is ISTRY,SYNTHESIS
nearly proportional to the quantity of the element; thus it
is possible to calculate the elemental ratios of the com-
pound according to the formula BIBLIOGRAPHY
elemental ratio of standard× Belcher, R., ed. (1977). “Instrumental Organic Elemental Analysis,”
element signal ratio of unknown Academic Press, London.
Elemental ratio = .
element signal ratio of standard Collins, L. W., Chalk, S. J., and Kingston, H. M. S. (1996). “Atmospheric
(36) pressure microwave sample preparation procedure for the combined