Environmental Engineering Reference
In-Depth Information
useful for resolving the identity of individual compounds, but is less efi cient for resolving com-
pound identii cation in mixtures. The combination of GC and MS, GC-MS, forms a system that is
effective at separating mixtures into their individual compounds and determining the amount and
structure of the compounds. GC-MS has become the mainstay of environmental chemistry for
volatile analytes such as solvents and related organic compounds in soil, air, and water; however, it
is not without its limitations (McMaster and McMaster, 1998).
4.4.1.1 Methods in Gas Chromatography
The laboratory analyst must choose operating parameters for the analysis of each suite of analytes
by GC. These include selection of carrier gas, injector type, column type, oven temperature, and
temperature gradient, that is, the rate at which column temperature is increased and the duration it
is held at a given temperature (Rödel and Wölm, 1987). The combinations of sample-preparation
methods described in Section 4.3 with the detectors described in Section 4.4.2 differentiate the
individual analytical methods for 1,4-dioxane analysis detailed in Section 4.5 . Although there is a
seemingly ini nite number of permutations of instrumental coni gurations, extraction techniques,
operating parameters, chromatographic columns, and detectors, seven methods for 1,4-dioxane
analysis are published, and most are commercially available.
4.4.1.2 Selecting Chromatographic Columns for 1,4-Dioxane Analysis
The chromatographic column separates mixed analytes in the sample. Columns are classii ed
according to their diameter, type, and length. Conventional columns range from 6 to 20 feet or
more in length and may be of either the packed or open-tube (capillary) design. Packed columns
are i lled with a porous granular or beaded material, such as a specialized variety of diatomaceous
earth, but more often a fused silica material. Capillary columns are longer than conventional col-
umns, 5-100 m long with 30 m being most common, but they have a similar porous or liquid
interior coating. Fused silica capillary columns are common for gas chromatographs used for envi-
ronmental analysis, although a wide variety of columns are available. Capillary columns provide
greater resolution of analytes than packed columns when handling small-volume samples. Most
capillary columns are designed with a fused silica open tube, to which a liquid stationary phase is
chemically bonded.
The liquid stationary phase is selected for its stability at high temperatures and for its low volatil-
ity. Polysiloxanes (i.e., “OV-1”) and polyethylene glycol (i.e., “CarboWax 20M”) are two examples of
liquid stationary phases used in GC columns. For a given analyte, the thicker the liquid stationary-
phase i lm in the column, the longer the retention time, and thicker i lms are used for more volatile
analytes.
Oven temperature is the primary factor in determining retention time of an analyte. Temperature
is used to separate analytes by programming the GC oven to increase temperature at a specii ed
rate. Increasing the temperature and the rate of heating can cause analytes with higher boiling
points to elute earlier from the column to the detector. Temperature programs are recommended for
each column type by the EPA method for particular groups of target analytes (USEPA, 2006a).
Optimum temperature gradients must be independently determined for each analyte and column
combination; however, lowered temperature means an increase in the elution time and the time
required to complete the analysis.
The polarity of the stationary phase should be matched to the group of analytes being tested.
Components of the mixture with similar polarity and therefore a high degree of afi nity for the
stationary phase are strongly retained, whereas components with dissimilar polarity will have low
afi nity for the stationary phase and migrate rapidly through the column. These differing behav-
iors present a challenge for analyzing samples for both solvents and stabilizers, because many
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