Page 282 - Handbook of Instrumental Techniques for Analytical Chemistry

272 Handbook of Instrumental Techniques for Analytical Chemistry

by EPA Methods.

The Environmental Protection Agency (EPA) has established Methods 413.2 and 418.1 for the mea-
surement of fluorocarbon-113 extractable matter from surface and saline waters and industrial and do-
mestic wastes (4). These methods provide semiquantiative determination of petroleum hydrocarbons,
oil, and grease by comparison of the infrared absorption of the sample extract with standards.
Petroleum hydrocarbons, oil, and grease include biodegradable animal greases and vegetable oils
along with the relative nonbiodegradable mineral oils. They all contain carbon–hydrogen bonds, thus
–1
giving rise to C-H stretching absorption in the 3100 to 2700 cm region of the IR spectrum. Fluoro-
carbon-113 (1,1,2-trichloro-1,2,2-trifluoroethane) is one of the chlorofluorocarbons commonly called
freons. It contains no C-H bonds, and thus does not absorb IR radiation in the aforementioned 3100 to
–1
2700 cm region. The quantity of hydrocarbons, oil, and grease in freon extracts can be estimated by
–1
measuring the intensity of C-H absorption band at 2930 cm .
The sample is acidified to a low pH (less than 2) and extracted with fluorocarbon-113. Interfer-
ence is usually removed with silica adsorbent. Depending on the sample concentration, cells of path-
length from 10 to 100 mm can be used to acquire the normal transmission IR spectrum. The
concentration of hydrocarbon, oil, and grease in the extract is determined by comparing the absor-
bance against the calibration plot prepared from standard calibration mixtures. Figure 15.14 shows
the different FTIR profiles of three calibration standards. The contributions from the solvent and cell
are eliminated by subtracting the reference spectrum of freon from the sample spectrum.
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In the standard EPA methods, peak height at a single frequency, 2930 cm , is used as the basis for
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quantification. In the author’s laboratory, peak area integration from 3150 to 2700 cm is used to quan-
tify the contents of hydrocarbons, oil, and grease. The modified methods have been found to provide
significantly improved results in quantitative analysis. Aromatic hydrocarbons have relatively lower
absorption intensity in this C-H stretching region, thus giving lower response factors when compared
to the IR absorption of oil and grease standards. Using an FTIR instrument, oil and grease at low parts-
per-million levels can be readily determined.

2. Quantitative Analysis of Multicomponent Mixtures of Sulfur Oxygen Anions
by Attenuated Total Reflectance Spectroscopy.

Characterization of complex mixtures of sulfur oxygen anions is encountered in studies such as investi-
gating the decomposition of the dithionite anion in acidic aqueous solution. Many techniques such as
conventional IR, UV/visible, Raman spectroscopy, and titrimetric and electrochemical analysis all have
drawbacks that limit their effectiveness in this challenging analysis. The multicomponent analysis by
FTIR attenuated total reflectance (ATR) spectroscopy successfully provides the accurate quantification
of multiple sulfur–oxygen anion concentrations in aqueous solution (5).
Sulfur–oxygen compounds have relatively intense S-O stretching absorption bands in the 1350 to
–1
750 cm region of the IR spectrum. FTIR/ATR spectroscopy is well suited for quantitative determina-
tion of sulfur oxygen anions in strong IR-absorbing aqueous medium. ATR not only uses water-resis-
tant cell material, but also has a very short and reproducible effective path length that goes beyond the
sample/crystal interface and into the sample medium. A micro CIRCLE cell from Spectra-Tech incor-
porating a ZnSe crystal is used in the study. Its basic optics is illustrated in Fig. 15.15. The representa-
tive FTIR/ATR spectra of sulfur oxide and nitrate anions are shown in Fig. 15.16.
Aqueous decomposition of sodium dithionite under anaerobic conditions is investigated. System-
atic baseline error is characterized and taken into account. Computerized data processing involving
least-squares regression is used in the multivariate analysis. Figure 15.17 illustrates the simultaneous
measurements of concentrations of seven anions and continuous monitoring of total sulfur and average