Cloud Point or Micelle Mediated Extraction

Cloud-point extraction (CPE) and micelle-mediated extraction are methods that utilize aqueous solutions of nonionic or zwitterionic surfactant concentrations above their critical micelle concentrations. The addition of ions or solvents as well as increasing temperature of the solution can cause a reversible phase separation above the cloud-point temperature.112 Also, addition of surfactant disrupts DOM-PAH associations so it will affect the distribution of PAHs in the sample. The surfactant will separate from the aqueous phase taking the associated organic contaminants with it. The surfactant is then removed from solution, usually by centrifugation, before analysis by HPLC-FLD. Triton X-114, a nonionic surfactant, was tested for its efficiency in the extraction of the fifteen fluorescence-active EPA PAHs from aqueous samples. Triton X-114 interferes with the fluorescence signal and retention of PAHs by the HPLC column so it had to be removed before injection of the samples. Removal consisted of dissolution of the surfactant in cyclohexane followed by cleanup on a deactivated silica/sodium sulfate column and elution with cyclohexane:DCM (80:20). The extract was evaporated to near dryness and redissolved in ACN. Even with these conditions, NAP coelutes with the surfactant and so it cannot be determined. Recoveries for the other fourteen PAHs ranged from 35 to 103%. Actual river samples were analyzed and levels of PAH ranged from 26.8 mg 1"1 for FLU to 1.6 ng 1"1 for B[a]P.

Sodium docecane sulfonic acid (SDSA) is an anionic surfactant that does not interfere with the chromatographic analysis and so does not have to be removed thus eliminating the cleanup step.113 SDSA separated into the two isotropic phases upon the addition of hydrochloric acid (HCl) at room temperature. To a 30-ml sample of water, 0.05 g SDSA and 15 ml concentrated HCl were added. After a 24-h equilibration time, the solutions were centrifuged to remove the sorbed analytes. ACN was added to the surfactant-rich phase to reduce its viscosity for analysis. FLU, PHN, PYR, B[a]A, and ACE were detected at low ng 1"1 levels in groundwater in Cordoba, Spain. Seven PAHs were detected in river water at low ng 1"1 concentrations. Recoveries of spiked samples ranged from 63% for ACE to 106% for B[b]F. Recoveries for volatile PAHs were all 83% or better.

Results using poly(A^-isopropylacrylamide) (PNIPAAm) for CPE are more encouraging.114 PNIPAAm forms a gummy precipitate at a critical micelle temperature of about 32°C and is nontoxic. Recoveries of eight PAHs ranged from 28% for NAP to 97% for perylene. The polymer was dissolved in ACN prior to HPLC-FLD analysis and no spectroscopic and chromatographic interferences were observed. The viability of the method was presented, but no real samples were analyzed. Another nonionic surfactant with encouraging results is Tergitol 15-S-7,115 which has a cloud-point temperature of about 37°C at 3 weight % and pH 6.8. It is a readily biodegradable secondary ethoxylated alcohol. Cloud-point temperatures were reduced to below room temperature by adding 0.5 M sodium sulfate and allowing for 10-min equilibration times. Centrifugation separated the precipitated analytes and surfactant, and no washing steps were required. Also, there were no spectroscopic interferences and no retention on the analytical column. Polyoxyethylene-10-lauryl ether (POLE) also shows no spectroscopic interferences and low column retention.116 A 1% (w/v) POLE addition to samples at 95°C for 90 min extracted PAHs from artificial seawater.

Was this article helpful?

0 0

Post a comment