Performance improvements in the determination of mercury species in natural gas condensate using an on-line amalgamation trap or solid-phase micro-extraction with capillary gas chromatography–microwave-induced plasma atomic emission spectrometry

Abstract

An on-line amalgamation trap was constructed for the collection of mercury species separated by capillary GC for detection by microwave-induced plasma atomic emission spectrometry. For direct measurement of the column eluate, the detection limits for mercury species in natural gas condensate are elevated because of background interference from carbon compounds passed to the plasma at the same time. Carbon compounds give rise to emission that spectrally interferes with the signal from the mercury detector and can overload the plasma discharge, reducing the excitation capability. With an amalgamation trap, mercury can be selectively collected from the column eluate and subsequently passed to the plasma in a flow of pure helium. By removing the carbon background emission, the trap allows the determination of dimethylmercury in condensate down to a detection limit of 0.24 µg l–1 and derivatized (butylated) monomethyl and inorganic mercury down to a detection limit of 0.56 µg l–1 in natural gas condensate. Use of the trap is compared with some existing determination methods for mercury species in condensate and other samples. Solid-phase micro-extraction is used for dimethylmercury, which gives a poorer detection limit of around 20 µg l–1 because the analyte mass collected is small compared with liquid sample injection. Following derivatization, samples are diluted for direct methylbutylmercury and dibutylmercury determination, corresponding to methylmercury and inorganic mercury. Dilution ensures a reasonably stable background for mercury signals. Again, poorer detection limits of 3.1 and 2.3 µg l–1, respectively, are obtained owing to the reduced analyte mass. The trap permits the injection of untreated condensate, or condensate reacted with butylmagnesium chloride, into the gas chromatograph without the need for dilution or sample clean-up.