Environmental Engineering Reference
In-Depth Information
Identii cation and quantii cation of contaminants in soil, water, air, food, and other media involve
isolating the contaminants of interest from the sample by
1. Sample extraction and concentration
2. Separating the contaminants on the basis of their rate of passage through a chromato-
graphic column designed with specii c properties to retain the contaminants according to
their vapor pressure, boiling point, and other properties
3. Detecting and quantifying the contaminants by ionization of the analyte followed by mass
separation and detection
4. Identifying contaminants according to their mass spectra by using a computerized library
of reference spectra and quantii cation via calibration standards
The primary approach for analysis of soil and water samples for VOCs has involved PT/GC-MS
analytical methods (USEPA, 1986, 1990; Lesage and Jackson, 1992). PT methods are designed to
measure as many compounds as possible with a single procedure. Such methods are favored for
those compounds that are relatively insoluble in water and that have boiling points below 200°C
(USEPA, 1986, 1990) or below 150°C (Lesage and Jackson, 1992). The PT preparation techniques
promote low detection limits (ppb range), and the MS detector allows positive compound identii -
cation. Compounds that contain polar functional groups such as low molecular weight ketones,
alcohols, aldehydes, nitriles, and ethers (i.e., 1,4-dioxane) are generally soluble in water, do not
purge well, and produce broad, tailing GC peaks that give poor quantitative estimates and are often
difi cu lt to ident i f y by MS (Swa l low, 1992). Some pola r compou nds a re i nclude d i n SW-84 6 Met ho ds
8240/8260 (USEPA, 1986, 1990), but the recovery of polar compounds is often less than 20%
(Swallow, 1992). Azeotropic distillation (SW-846 Method 5031) and closed-system vacuum distilla-
tion with cryogenic condensation (SW-846 Method 5032) were introduced as preparation methods
for analysis of hydrophilic analytes such as alcohols and ethers in the third update to SW-846
(Lesnik, 1993; USEPA, 1993).
4.4.1 G AS C HROMATOGRAPHY
GC involves injecting the mixture to be analyzed into an inert gas stream that sweeps the sample
into an open capillary column coated with a thin i lm or a column coated with a resolving station-
ary phase. The components in the gas stream absorb and interact with the thin i lm or stationary
coating to varying degrees, which leads to differential separation, causing the components in the
gas stream to be swept through the column in a sequence based upon their vapor pressure, boiling
points, and other physical properties. As individual components of the mixture elute from the
chromatographic column, they are swept by the carrier gas to a detector. The detector generates
measurable electrical signals, referred to as peaks, which are proportional to the amount of analyte
present. The detector response is plotted as a function of the time required for the analyte to elute
from the column after it was injected. The resulting plot is called a chromatogram. The position of
the peaks on the time axis may serve to identify the components, and the areas under the peaks
provide a quantitative measure of the amount of each component when using a mass spectrometer.
To identify the compound, the spectra corresponding to the peaks are compared to the spectra of
calibration standards or to a computerized library of spectra. Because several compounds may
possess similar retention times, the nature of the resulting peak may require further identii cation
using MS (McMaster and McMaster, 1998).
Mass spectrometers differentiate compounds based on the mass-to-charge ratio ( m / z ) of their
ionized products. Compounds entering a mass spectrometer are ionized in a vacuum. The resulting
ions are directed through a mass analyzer, which separates the ions by the m / z ratio. MS alone is
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