Sampling and Laboratory Analysis for Solvent Stabilizers - Environmental Investigation and Remediation - page 203

4.3.6.2 Solid-PhaseExtraction
Sol id-pha se ext r a ct ion (SPE), a lso ca l le d l iqu id - sol id ext r a ct ion i n USE PA t est met ho d s, is desc r ib e d
by EPA Method 3535. SPE involves adsorbing target analytes on a solid sorbent and then desorbing
and concentrating them with a solvent before injection into the GC-MS system. A number of
researchers have developed viable methods with SPE to achieve low MDLs (Fuh et al., 2005;
Isaacson et al., 2006; Shirey and Linton, 2006; USEPA, 2006a). Carbon disks work well for extrac-
tion of highly polar, water-soluble compounds, such as 1,4-dioxane and other ethers. For the SPE
protocol prescribed by EPA Method 3535, a 1 L sample is treated with 5 mL of methanol, internal
standards and surrogates are added, and a pH adjustment is made. An SPE device, usually a coated
disk or i lter cartridge, is then used to extract the organic analytes. The disks are composed of
hydrophobic materials and are often prewetted with a water-miscible solvent such as acetone or
acetonitrile. The sample is i ltered through the SPE cartridge or disk under vacuum. Vacuum is
maintained after the sample has passed through the SPE device to dry the i lter. Earlier SPE
methods used bulk graphitized carbon or powdered carbon black. Disks reduce the i ltering time
and cost, allow increased l ow rates, improve mass transfer, and eliminate bed channeling (Markell
et al., 1991). Dichloromethane or other solvents are used to elute target analytes from disks. The
eluted solvent is next dried with anhydrous sodium sulfate. The sample is then concentrated through
the use of a nitrogen blow-down and hot water method, and the concentrated extract can be exchanged
into a solvent as needed (USEPA, 2006a).
Solid-phase micro extraction (SPME) is a variation of SPE in which a much smaller volume of
sample is sufi cient to complete the analysis. The use of SPME for the analysis of 1,4-dioxane
has many advantages over conventional solvent phase extraction and headspace pretreatments,
including simplicity, speed, precision, lower detection limit, and minimal solvent consumption
(Fuh et al., 2005). In SPME, the solid phase is coated on a fused silica i ber attached to the end
of a wire plunger that is pushed through a syringe needle and immersed in the aqueous liquid
phase or exposed to the headspace above the liquid. Organic compounds present in the vapor
or liquid phase are absorbed into or adsorbed onto the solid phase. After a specii ed sampling
time, the compounds are thermally desorbed from the solid phase in the heated injection port
of a gas chromatograph (Black and Fine, 2001). Variants of SPME have evolved, including stir
bar sorptive extraction (SBSE) and headspace sorptive extraction (HSSE). These adaptations
were developed to increase sorption capacity and to overcome some drawbacks of SPME, such
as i ber fragility (Jochmann et al., 2006). Solid-phase dynamic extraction (SPDE) is a commer-
cial sample extraction method that is growing in popularity. SPDE uses a 2.5 mL headspace
syringe equipped with a needle fabricated with an interior coating similar to a fused-silica GC
column (see Section 4.4.1.1 ) with an immobilized extraction phase (Jochmann et al., 2006).
SPDE needle coatings incorporate 4-6 times the amount of sorbent as SPME i bers. The syringe
is immersed directly into a sample or the headspace above it, and the plunger is exercised sev-
eral times to extract the sample, thereby adsorbing the analytes to the internal coating. After
several cycles of aspirating and dispensing the sample, analytes are thermally desorbed from
the coating inside the needle and purged with nitrogen gas into the GC injector (Jochmann
et al., 2006).
As with LLE, the addition of sodium chloride or sodium sulfate salt to the sample improves the
recovery of 1,4-dioxane in SPME extraction. The salt helps to decrease the solubility of 1,4-dioxane
in water. The amount of 1,4-dioxane extracted from SPME i bers increases linearly up to 15% of
sodium chloride (NaCl) concentration. High salt concentrations may shorten the lifetime of SPME
i ber coatings. Addition of 10% (w/v) NaCl is considered optimal for 1,4-dioxane determination
using SPME while also preserving SPME i ber coatings (Fuh et al., 2005). Increasing the ionic
strength of the sample by adding NaCl also increases recovery of 1,4-dioxane in SPDE needles. 1,4-
Dioxane recovery increased by approximately 60% when salt content was increased from 10% to
25% (w/w ratios) in samples extracted with SPDE needles (Jochmann et al., 2006). The effect of