Sampling and Laboratory Analysis for Solvent Stabilizers - Environmental Investigation and Remediation - page 211

solvents are nonpolar whereas many alcohol, epoxide, and ether stabilizers such as 1,4-dioxane
are polar. This challenge can be overcome with a gas chromatograph in which the sample gas
l ow is split between two columns of different materials to address both polar and nonpolar
compounds.
Polysiloxanes, among the most common stationary phases in GC columns, contain various
substituent groups to change the polarity of the stationary phase. The nonpolar end of the spectrum
is polydimethyl siloxane, which is made more polar by increasing the percentage of phenyl groups
on the polymer. For very polar analytes, polyethylene glycol (i.e., Carbowax) is commonly used as
the stationary phase. Some gas chromatographers conclude that the column material that best sepa-
rates 1,4-dioxane from the sample is DB-WAX, a fused silica column bonded with polyethylene
glycol (Kawata et al., 2001). After the polymer coats the column wall or packing material, it is often
cross-linked to increase the thermal stability of the stationary phase and prevent it from gradually
bleeding out of the column.
The stationary phase should be similar in chemical structure to the components of the mixture
targeted for analysis. To match chromatographic column selectivity to target analytes, several clas-
sii cation schemes of analyte polarity have been developed, including McReynold's numbers and
Rohrschneider constants. McReynold's tables predict the chromatographic separation of various
compounds with different stationary phases and provide the best information available for the com-
parison of the selectivity of the stationary phase. In the McReynold's system, 1,4-dioxane is used as
an index analyte for quantifying column selectivity. 1,4-Dioxane is representative of “Class III”
compounds of medium polarity. Stationary phases that will more strongly retain Class III com-
pounds include the following:

• All polyesters

• SAIB tricresyl phosphate

• Benzyl cyanide

• Propylene carbonate

• Polyphenylether

• OV-17

• Dibutyl tetrachloro-phthalate

• STAP

• Lexan

• QF-1

• Dimethylsulfolane

The stationary phases recommended by USEPA for the analysis of ethers and other compounds
of intermediate polarity (“Class III polarity”) include Carbowax 400, Tricresyl phosphate, LAC,
Apiezon L, and

-oxydipropionitrile (USEPA, 2006c). Other listings for stationary phases
suitable for the separation of ethers include Carbowax 20M, Carbowax 1500, and SE-30 (GC Grade)
(Supelco/SigmaAldrich, 1997).

β - β′

4.4.2 D ETECTORS IN G AS C HROMATOGRAPHY
A variety of detectors are used in GC. The thermal conductivity detector (TCD) was the most
widely used detector system during the early years of GC. The FID became more popular for
common hazardous waste analyses. Specialty detectors were subsequently developed, such as the
electron-capture detector (ECD) and the nitrogen-phosphorus detector. All detectors described in
this section are nonspecii c detectors and have largely been replaced by mass spectrometers. MS is
the preferred detection technique because it is specii c, sensitive, and quantitative, and good preci-
sion can be obtained at low concentrations under the right conditions.