MAKING PERFORMANCE-BASED CHEMISTRY WORK: HOW WE CREATED COMPARABLE DATA AMONG LABORATORIES AS PART OF A SOUTHERN CALIFORNIA MARINE REGIONAL ASSESSMENT RICHARD GOSSETT 1* , RODGER BAIRD 2 , KIMBERLY CHRISTENSEN 3 , AND STEPHEN B. WEISBERG 4 1 CRG Marine Laboratories, Inc., 2020 Del Amo Blvd., Suite 200, Torrance, CA 90501 USA; 2 Los Angeles County Sanitation Districts, 1965 S. Workman Mill Rd, Whittier, CA 90601 USA; 3 Orange County Sanitation Districts, P.O. Box 8127, Fountain Valley, CA 92728 USA; 4 Southern California Coastal Water Research Project, 7171 Fenwick Lane, Westminster, CA 92683 USA * (author for correspondence, e-mail: crglabs@sbcglobal.net) Abstract. Quality assurance procedures to ensure consistency among chemistry laboratories typi- cally involves the use of standard methods and state certification programs that require laboratories to demonstrate their ability to attain generic performance criteria. To assess whether these proce- dures are effective for ensuring comparability when processing local samples with potentially com- plex matrices, seven experienced, state-certified laboratories participated in an intercalibration exer- cise. Each laboratory was permitted to use their typical methodology for quantifying PAH, PCB and DDT on shared samples collected from Santa Monica Bay and the Palos Verdes Shelf, two sites with a complex mix of constituents. In the initial intercalibration exercise, results from these laboratories differed by as much as an order of magnitude for all three chemical groups. Much, but not all, of the difference was attributable to differences in detection capability. A series of studies was conducted to identify the reasons for the observed differences, which varied among laboratories and included methodological differences, instrument sensitivity differences, and differing interpretations of chro- matograms. Following these investigations and resulting modifications to laboratory procedures, the exercise was repeated. The average coefficient of variation among laboratories across all chemical parameters was reduced to less than 30%. Our results suggest that performance-based chemistry can produce comparable results, but the certification processes presently in place that focus on general laboratory procedures and simple matrices are insufficient to achieve comparability. Keywords: Intercalibration exercise, DDT, PCB, PAH, Performance-based chemistry 1. Introduction Many environmental assessments require measurement and compilation of sedi- ment chemistry data from multiple laboratories, either to extend temporal records for trends assessment or to extend geographic scale for spatial assessment. Such data compilations assume a degree of comparability among laboratories, even though analytical personnel, methods, and instrumentation may vary. Dissimilar- ity in sampling and sub-sampling techniques, sample preparation, clean-up proce- dures, detection capabilities, and instrumental techniques can all lead to differ- ences in analytical results. Data comparability is enhanced when the chemical analyses are conducted by State-certified laboratories that use standardized methods. The certification pro- cess requires laboratories to demonstrate their ability to attain generic performance criteria, but only partially satisfies the presumption of data comparability in real world environmental assessments. Reliance on a priori performance demonstra- Environmental Monitoring and Assessment 81: 269–287, 2003. c 2003 Kluwer Academic Publishers. Printed in the Netherlands.