Routine analysis of mercury species using commercially available instrumentation: chemometric optimisation of the instrumental variables

摘要

Mercury speciation analysis (inorganic mercury, Hg~(2+), methylmercury, CH_3Hg~+ and dimethylmercury, (CH_3)_2Hg) by gas chromatography (GC) coupled to atomic emission spectroscopy with microwave induced plasma as excitation source (MIP-AES0, after ethylation of the sample and extraction of the derivatised species into an organic phase, has been optimised using factorial design, analysis of variance and MultiSimplex techniques. Standard conditions were used in the derivatisation step with sodium tetraethylborate (NaB(C_2H_5)_4) and in the extraction step into hexane. Good separation of the species investigated and maximum sensitivity was achieved using an OV-1701 capillary column. The sensitivity was found to be maximum with an helium flow rate (make-up flow) of 100 ml min~(-1). Procedures for a correct cleaning of gall and plastic ware, as well as for the purification of reagents used throughout the analytical process, are also suggested in order to avoid unacceptably high blank signals. The effect that ageing of stock solutions used in calibrations has on the artefact formation of CH_3Hg~+ has been also investigated. Using the optimum conditions found, good quality calibration curves (R~2 > 0.995) for the three mercury species were obtained. Absolute detection limits of 0.5, 3 and 15 pg of (CH_3)_2Hg, CH_3Hg~+ and Hg~(2+), respectively, were estimated. The repeatability of the analysis was found to be better than 5% (n=5) in relative standard deviation (R.S.D.) units. The optimized procedure for the speciation of mercury in standard samples is the fist step in the development of a method for routine analysis of mercury species in aquatic environmental samples.