RESUMEN
A solid-phase microextraction technique using steel fiber coated with 20 microm polypyrrole (Ppy) doped with polyphosphate was developed for the GC determination of a group of organochlorine pesticides (OCPs) in water. The coating was prepared using a three-electrode electrochemical system from a 10% aqueous sodium polyphosphate solution containing 0.05 M pyrrole by applying a constant potential of 1.2 V for 30 min. In order to obtain an adherent, smooth and stable film of polypyrrole, experimental parameters related to the coating process consisting of the type of dopant or counter-ion, deposition potential, concentration of the monomer, concentration of the counter-ion, and deposition time were optimized. The effects of various parameters on the efficiency of SPME process such as extraction time, extraction temperature, ionic strength, desorption time, and desorption temperature were also studied. The coating was highly stable and extremely adherent to the surface of the steel fiber. The method was linear for at least three orders of magnitude with correlation coefficients varying from 0.9818 to 0.9977. The accuracies found through spiking blank samples showed high recoveries between 82 and 110%. Intra- and inter-day precisions of the method were determined from mixed aqueous solutions containing 1.0 ng ml(-1) of each OCP. The intra-day precisions varied from 4.7% for heptachlor to 11.4% for methoxychlor, while the inter-day precisions varied from 6.8% for endosulfan I to 13.0% for p,p'-DDD and o,p-DDD. Limits of detection based on S/N=3 were in the range 0.015-0.66 pg ml(-1). The proposed method was applied to monitor organochlorine pesticides in some well water samples.
RESUMEN
Solid-phase microextraction (SPME) coupled to gas chromatography (GC) was applied to the extraction of phenol and some of its volatile derivatives in water samples. The SPME fiber consisted of a thin layer of polyaniline, which was electrochemically coated on a fine Pt wire. The stability of the coating was such that it could be used at temperatures as high as 325 degrees C, without any deterioration. The effects of various parameters affecting the extraction efficiency were studied, simultaneously. From these, optimization of the extraction temperature, extraction time, coating thickness, sample pH, salt concentration and desorption time was carried out by means of a (2(6-2)) fractional factorial design. It was found that the effects and interactions of five out of six factors were significant. However, the coating thickness showed a large main effect but an insignificant interaction effect, so it was kept constant. Also, the effect of desorption time was insignificant if sufficient time was allowed for desorption to take place. Therefore, a central composite design (CCD) with four remaining factors, i.e., sample pH, salt concentration, extraction time and sample temperature was performed and a response surface equation was derived. The statistical parameters of the derived model were r=0.97 and F=25.3. The optimum conditions were obtained using a grid method. Using the optimum conditions, the method was analytically evaluated. The detection limit, relative standard deviation, linear range and recovery were 1.3-12.8 ng mL(-1), 2.2-5.3%, 0.01-5.0 microg mL(-1), and 88-103%, respectively. The results showed the suitability of polyaniline-coated fiber in analyzing volatile phenolic compounds in water samples.