On-Line Nonmetal Detection for Argon Supercritical Fluid Extraction Using Inductively Coupled Plasma Optical Emission Spectroscopy

Abstract

This paper describes the development of a solventless instrumental method for determining organic contaminants in soil by coupling argon supercritical fluid extraction (SFE) to inductively coupled plasma atomic emission spectroscopy (ICP-AES). In this method, organic compounds are first extracted by Ar SFE, transferred to the ICP directly in the supercritical (SC) argon, fragmented in the plasma, and then determined via their nonmetal atomic emissions. Supercritical Ar is superior to SC CO(2) for use in ICP-AES because it overcomes the disadvantages of plasma blowout, noisy baselines, and CO(2) interference in the determination of carbon. All instruments employed were commercially available, and the interface between the SFE and the torch of the ICP consisted of a simple glass capillary tube. Four nonmetals, C, S, P, and Si, were selected for this preliminary study. The selectivities obtained for these nonmetals, referenced to carbon, were found to be 345, 38 000, and 1400 for sulfur, phosphorus, and silicon, respectively. With the exception of carbon, the mass detection limits are in the range of 0.06-1.8 μg. For this work, the mass detection limit of carbon is 66 μg and is limited by the smallest aliquot of material that can be injected by syringe. Finally, total petroleum hydrocarbons (TPHs) were determined in two "real world" contaminated soil samples via the carbon emission at 247.9 nm. Dodecane was used as the standard reference compound because its response was found to be similar to other petroleum hydrocarbons (e.g., kerosene, gasoline, no. 1 fuel oil, and no. 2 fuel oil). Additionally, it was found necessary to add a small amount (i.e., 250 mg) of drying agent to the SFE vessel in order to prevent the plasma from extinguishing from the extracted water. The results of the on-line Ar SFE/ICP-AES determinations of TPH in the two samples agreed well with those obtained from conventional off-line CO(2) SFE and off-line Ar SFE.