Analytical Interferences of Mercuric Chloride Preservative in Environmental Water Samples: Determination of Organic Compounds Isolated by Continuous Liquid-Liquid Extraction or Closed-Loop Stripping Wllllam T. Foreman,* Steven D. Zaugg, Lynda M. Falres, Marilyn G. Werner, Thomas J. Lelker, and Peter F. Rogerson Methods Research and Development Program, National Water Quality Laboratory, U.S. Geological Survey, 5293 Ward Road, Arvada, Colorado 80002
Analytical interferences were observed during the determination of organic compounds in groundwater samples preserved with mercuric chloride. The nature of the interference was different depending on the analytical isolation technique employed. (1)Water samples extracted with dichloromethane by continuous liquid-liquid extraction (CLLE) and analyzed by gas chromatography/ mass spectrometry revealed a broad HgCl, “peak” eluting over a 3-5-min span which interfered with the determination of coeluting organic analytes. Substitution of CLLE for separatory funnel extraction in EPA method 508 also resulted in analytical interferences from the use of HgC1, preservative. (2) Mercuric chloride was purged, along with organic contaminants, during closed-loop stripping (CLS) of groundwater samples and absorbed onto the activated charcoal trap. Competitive sorption of the HgC1, by the trap appeared to contribute to the observed poor recoveries for spiked organic contaminants. The HgC12 was not displaced from the charcoal with the dichloromethane elution solvent and required strong nitric acid to achieve rapid, complete displacement. Similar competitive sorption mechanisms might also occur in other purge and trap methods when this preservative is used.
Introduction
As part of a project to compare eight different analytical methods for the analysis of low-level volatile and semivolatile organic compounds in groundwater, the National Water Quality Laboratory recently evaluated two isolation procedures, continuous liquid-liquid extraction (CLLE) and closed-loop stripping (CLS),followed by analysis using gas chromatography/mass spectrometry (GC/MS). CLLE methods are being increasingly used (1-3)as an alternative to conventional separatory funnel liquid-liquid extraction methods due to the enhanced extraction efficiencies, reduced manual sample-handling steps, and the minimization of emulsion problems in CLLE. Since its initial development in the mid-1970s by Grob (4-3,the closed-loop stripping method has experienced increased application for the isolation of intermediatevolatility organic contaminants in water. A strength of the CLS technique is its ability to ultimately provide a con-
centrated isolation of the analytes in a small solvent volume (usually
