10 



below the MDL. Phthalates are common constituents of plastic and may have been 

 introduced in the laboratory sample preparation process. All sample detections of 

 bis(2-ethylhexyl)phthalate were qualified with a "J" because of the laboratory method blank 

 detections. 



Detected phthalates in MBDS samples may have been introduced in the laboratory; in 

 addition, samples may have been contaminated with phthalates from the plastic storage bags. 

 All phthalate detections were qualified due to the uncertainty. There was no apparent 

 difference between replicate samples taken with and without the use of the stainless steel 

 spoon in the field. 



Matrix spike samples are spiked with a known concentration of a particular inorganic 

 or organic analyte. The percent recovery of the spiked compound indicates the efficiency 

 and effectiveness of sample preparation and analysis procedures, and how tightly bound a 

 compound is to the sample matrix. One sample and sample duplicate (10-7) were spiked 

 with six pesticide compounds; percent recoveries were within acceptance ranges (EPA 

 1988a). No samples were spiked with PCBs. Two matrix spike duplicate pairs were 

 analyzed for eleven BNA compounds; all recoveries were within acceptance limits. 



Pesticide, PCB, and BNA samples were also spiked with surrogate compounds, which 

 are artificial compounds not normally found in environmental samples. Surrogate recovery is 

 also used to indicate the efficiency and effectiveness of sample preparation and analysis 

 procedures. Pesticide and PCB samples were spiked with the surrogate TCMX, and BNA 

 samples were spiked with six surrogate compounds. Percentage recoveries of surrogates 

 indicated acceptable laboratory sample extraction. 



Laboratory duplicate samples indicate precision (replicability) of laboratory 

 procedures. Calculation of the relative percent difference (RPD) between the duplicate 

 samples is the common method to compare duplicate results as a measure of laboratory 

 precision. Matrix spike duplicate samples were analyzed for pesticide, PCB, and BNA 

 analyses, and eight duplicate sample pairs were analyzed for all of the metals analyzed in the 

 MBDS samples. RPDs for all duplicate analyses were acceptable within EPA required limits 

 (EPA 1988a, 1988b), indicating acceptable laboratory precision. 



Field replicate analysis is an indicator of field precision as well as degrees of 

 difference between individual samples. The pesticide 4,4'-DDE (DDE) was the only 

 compound detected in the two field replicates collected at Station 18-18. Measured 

 concentrations were 2.81 ppb and 1.16 ppb with a resulting RPD of 83%. Since only one 

 trace compound was detected, this information is not sufficient to comment on the precision 

 of field sampling. 



Chemical Analyses of Sediment Sampling at the Massachusetts Bay Disposal Site, June 1989 



