Sediment intercalibration

The sediment samples used in the present study were obtained from Narragansett Bay. The sediment sample used for the PAH extraction study and in Experiments 1 and 4 was obtained adjacent to a pier in Narragansett, Rhode Island. The intercalibration sediment (ICS) was obtained near the north end of Jamestown Island (14). Recently contaminated sediments were produced by dispersing sediment obtained near the south end of Jamestown Island in seawater, dosing it with No. 2 fuel oil in a continuous flow oil-dosing system (15), and collecting it after it had settled to the bottom of the tanks. 

To ensure uniformity, sediments were thoroughly mixed before use. The ICS was wet-sieved through a 1.0-mm sieve, followed by resuspension in distilled water and mixing on a ball mill for 2 hr. The intercalibration sediment was then filtered to remove most of the water. Other sediments were thoroughly mixed by hand. Replicate analysis of aliquots of these sediments showed subsampling variability was small (Table II). 

The results from the intercalibration study (Experiment 1) between the EPA Environmental Research Laboratory—Narragansett and the laboratory of James Quinn at the U.R.I. Graduate School of Oceanography agreed closely (Table II). This agreement also indicates that subsampling variability of the intercalibration sediments was quite low. The amount of material recovered from sediments by three extraction methods (Table III) was compared in Experiment 2. A considerable increase in the amount of hydrocarbon material returned from sediment samples was observed as the method of extraction became more vigorous. 

Petroleum pollution monitoring laboratories in the Mediterranean region participated (1984-1986) in two intercalibration exercises (MEDCALI and II) to evaluate the International Oceanographic Commission (IOC) Manual for petroleum hydrocarbon determination in sediment (IOC, Manuals and Guides, No. 11). The main source of error in the analysis was the extraction/ partition step. When the results were corrected for recoveries, relative standard deviations for w-alkancs, UCM (unresolved complex mixture) and total aromatics, which had previously been 60, 56 and 49%, respectively, were reduced to 17, 30 and 6%, respectively. 

Zink-Nielsen, I. 1977. Intercalibration of methods for chemical analysis of sediments results from intercalibrations of methods for determining loss on ignition, COD, total nitrogen, total phosphorus, and heavy metals in sediments. Vatten 33 14—20. 

For physical and chemical measurements it is essential that measurements are referenced to standards accepted by all the laboratories undertaking a particular type of measurement. In particular, the use of chemical analytical procedures validated through their application to certified reference materials (CRM) (26) is highly recommended. It is to be noted that a number of CRMs prepared with Antarctic matrices are already available, i.e., marine sediment and krill, or in preparation (26-29). These and other CRMs should be also used routinely by the participating laboratories to assure a periodical assessment of accuracy and repeatability of measurements. These laboratories should also undertake regular intercalibration studies. It is possible that in the first application of these exercises systematic errors will be found, but better results are expected in subsequent rounds together with a general improvement in the performance of laboratories and data quality. 

Intercalibration and Quality Control. The large increase in the number of laboratories measuring or attempting to measure fossil fuel compounds in environmental samples raises a very serious question that has been posed many times previously, How comparable are analyses from different laboratories The validity of data on incorporation of hydrocarbons into surface sediments at the concentration level of 1 mg/g is of prime importance to fate-and-effect studies in coastal and continental shelf areas. It is not difficult to see how this could also be of importance to legal considerations. The need for intercalibration seemed obvious and a few efforts to establish this practice were completed (15). 

Table III. Summary of Selected Alkane Data from BLM Intercalibration Program with Santa Barbara Sediment Spiked with South Louisiana Crude Oil (/ g/g dry weight)...

The analytical methodology currently available for fossil fuel hydrocarbon analyses is sensitive and selective to the point that individual aromatic hydrocarbons can be measured in sediment and tissue samples at 10 9 g/g dry weight concentration levels. We should all be concerned that intercalibration exercises reveal wide differences, often a factor of 10 or more in values reported by different laboratories. We cannot ignore this and we should strive to calibrate and refine the methodology to the point where we and others can be more confident of the data comparisons from area to area and from one laboratory study to another. A factor of 10 in concentration of toxic compounds can easily be the difference between a healthy and a severely impacted ecosystem. 

Experiment 1 (Table II) was an intercalibration between this laboratory (EPA Environmental Research Laboratory—Narragansett) and the laboratory of James Quinn (University of Rhode Island, Graduate School of Oceanography) using aliquots of the ICS and the pier sediment samples. The amounts of hydrocarbon material extracted from both sediment samples were very close when similar extraction methods (V and VII) were used. 

A priori, it would have been expected that the greatest drive towards intercalibration should have occurred in those areas most important to industry, such as petroleum hydrocarbon and organochlorine compounds in seawater, sediments, and marine organisms. Much has been done in this area (see, e.g. [5-15]), but the topic is covered in greater detail in the chapter by Zitko in this volume. The combination of the availability of reference materials and the participation in intercalibration exercises has resulted in a great improvement in both the accuracy and precision of these analyses. 

The analysis of trace metals in seawater, sediments, and marine organisms has always been a major preoccupation of marine chemists the analytical literature contains methods for almost every element in the periodic table. Most of these methods have never been adopted by anyone other than the original author, as improvements in machine methods overtook methods based on chemical manipulations. In my own case, my Ph.D. thesis [16] described a number of novel methods for trace-metal analysis in sediments, all of which became irrelevant with the advent of atomic absorption analysis. The newer machine methods pin-pointed the wide range of values found in the literature for many trace metals, and by the late 1960s it was obvious that some sort of intercomparison or intercalibration was needed for the trace metals most commonly measured. 

---