SPME is performed by a commercially available device consisting of a glass fiber with a sorptive coating for direct sampling of air and water.126 Since SPME methods do not require exhaustive extraction of analyte, it was determined that SPME can measure analyte levels without significantly affecting the bulk concentration in water samples in equilibrium with sediments and DOM. SPME with external calibration measures freely dissolved PAHs, while SPME with internal calibration measures total concentration of PAHs.127 Aqueous distribution measurements of freely dissolved vs. reversibly-bound PAHs were performed without doing a physical separation of the solid and liquid phases of aquatic samples, which can affect equilibrium. External standards without DOM are used to calibrate for SPME extractions to determine freely soluble PAH levels. For internal calibration nondeuterated analytes and deuterated standards are assumed to have the same partition coefficients for polydimethylsiloxane (PDMS) coated fibers. The response of nonlabeled to labeled standard multiplied by the labeled standard concentration gives the total concentration of individual PAHs. SPME is limited to low extraction efficiencies because of the very small amount of PDMS (< 0.5 /a1) that is available for partitioning of analytes. The amount extracted by PDMS depends on the phase ratio of water to PDMS volumes. The Kow of PAHs at equilibrium affects the recovery of PAHs because the sorption process is that of partitioning from the water into the PDMS phase.
PAH concentrations in coal wastewater were determined using internal and external calibration.128 Distribution coefficients were determined for the EPA16 PAHs using 100 fim PDMS and 85 fim PA (polyacrylate) fibers. A linear correlation was found for log Kow vs. log iTspme from NAP to CHR. As molecular weight increased, extraction efficiency increased up to PYR and decreased from B[a]A to B[ghi]P on the PA fiber. It was surmised that the highest molecular weight compounds were sorbing on to the surface of the fiber coating rather than absorbing into the polymeric phase allowing for two mechanisms of sorbtion to compete.
SPME with PDMS fibers was employed to measure PAHs and alkylated-PAHs in groundwater samples collected over two years from sites contaminated with coal tar and refinery wastes.129 NAP was the predominant PAH detected. Heavier PAH concentrations were much lower due to partitioning onto soils and sediments. Source determination by examining isomer ratios was difficult due to the very low concentrations measured. Very low detection limits were achieved, 0.07 ng ml"1.
Certified marine sediment SRM 1941a from NIST was analyzed using micellar microwave-assisted extraction (MAE) with SPME.130 Polyoxyethane-10-lauryl ether (POLE) was added and microwave energy used to facilitate desorption of PAHs from the sediment prior to extraction by SPME. Desorption of five- to six-ring PAHs was adequate enough to allow for their quantification. For ten of the EPA16, recoveries ranged from 59% for PHN to 112% for B[k]F. Equilibration times were over 80 h for higher molecular weight PAHs.
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