Sampling and Laboratory Analysis for Solvent Stabilizers - Environmental Investigation and Remediation - page 212

TCDs were widely used in the early years of GC because of their simplicity, universal applica-
bility, and low cost. Analyte detection is based on changes in the conductivity of the column efl u-
ent. The TCD is a destructive detector that can be used in series only after nondestructive detectors.
The TCD detects gaseous compounds in the ppm range. TCDs are not generally used for analysis
of low-concentration samples as the possibility of false identii cation of analytes is a large problem.
Larger sample volumes are required to achieve increased sensitivity, which in turn requires using a
large-diameter chromatographic column (USEPA, 2006b). TCDs are not well suited for detection of
1,4-dioxane and are no longer widely used for environmental sample analysis.
The FID is sensitive for most organic compounds containing oxidizable carbon such as aromatic
and chlorinated aliphatics, petroleum compounds, semivolatile compounds, and polychlorinated
biphenyls (PCBs). The FID is a destructive detector that can be used in series only after nondestruc-
tive detectors. The FID is a nonspecii c detector, as most carbon-containing compounds are detect-
able by the FID.
An FID uses a small stainless steel jet positioned at the end of the chromatographic column. As
carrier gas exits the column and l ows through the jet, it mixes with hydrogen supplied in the jet
and burns at the tip of the jet. Hydrocarbons and other molecules in the sample are ionized in the
l ame and attracted to a metal collector electrode located just to the side of the l ame. The resulting
electron current is amplii ed to convert very small currents to voltages that are recorded as the
sample signal (USEPA, 2006b). An FID has been used to detect 1,4-dioxane vapors in occupa-
tional exposure testing; the detection limits are 5-190 ppmv (20-700 mg/m 3 ) (Cooper et al., 1971,
in NICNAS 1998).
An ECD consists of a sealed stainless steel cylinder that typically contains radioactive nickel-63.
The nickel-63 emits beta particles that collide with and ionize carrier-gas molecules; in the process, a
stable cloud of free electrons forms in the ECD cell. When a halogenated or other electronegative
molecule enters the cell, it is immediately combined with one of the free electrons, causing a tempo-
rary but measurable reduction in the number of free electrons in the cell. The ECD is a nondestructive
detector that can be used in series before other detectors (USEPA, 2006b). Compared to TCD or FID,
the ECD is a more specii c detector for halogenated or other electronegative compounds, but it is rela-
tively insensitive for hydrocarbons, alcohols, ketones, and ethers such as 1,4-dioxane.
4.4.2.1 MassSpectroscopy
A mass spectrometer is a detector used in GC. Mass spectrometers ionize gaseous molecules,
separate the ions produced on the basis of the mass-to-charge ratio ( m / z ), and then record the rela-
tive number of different ions produced. The m / z ratio is then plotted as the abscissa with relative
intensity as the ordinate. This plot is referred to as a “mass spectrum.” The mass spectrum of a
compound can be considered its “i ngerprint” and can be used to identify a compound through
comparison with published reference spectra. MS systems interface with computers that compare
experimental spectra to standard spectra and perform the identii cation automatically. Spectra that
do not match calibration standard spectra can be compared with a library of spectra, and a tenta-
tive identii cation can be made.
Analytes can be fragmented by using either electron or chemical ionization. In electron-impact
ionization, a 70 eV beam of electrons ionizes the analyte. Many analytes will fragment when
exposed to the electron beam. The fragmentation pattern is characteristic of the ionized analyte and
produces the spectrum from which the compound is identii ed. This experimental spectrum is then
compared with spectra of compounds of the same molecular weight in a computerized spectra data-
base or a published library of spectra (McMaster and McMaster, 1998).
Chemical ionization uses an ionization gas such as methane, butane, or carbon dioxide mixed
with the sample stream. The ionization gas absorbs the initial ionizing electron and transfers the
energy to the sample molecule. Chemical ionization creates a lower energy state than electron-
impact ionization and thus provides less fragmentation and a larger abundance of molecular ions
(McMaster and McMaster, 1998).