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Simulated distillation (SimDist) is a gas chromatography (GC) technique which separates individual hydrocarbon components in the order of their boiling points, and is used to simulate the time‐consuming laboratory‐scale physical distillation procedure known as true boiling point (TBP) distillation. The separation is accomplished with a nonpolar chromatography column using a gas chromatograph equipped with an oven and injector that can be temperature programmed. A flame ionization detector (FID) is used for detection and measurement of the hydrocarbon analytes. The result of SimDist analysis provides a quantitative percent mass yield as a function of boiling point of the hydrocarbon components of the sample. The chromatographic elution times of the hydrocarbons are calibrated to the atmospheric equivalent boiling point (AEBP) of the paraffins reference material. The SimDist method ASTM (American Society for Testing and Materials) D2887 covers the boiling range 55–538 °C (100–1000 °F) which covers the n‐alkanes (n‐paraffins) of chain length about C 5 –C 44 . The high‐temperature simulated distillation (HTSD) method covers the boiling range 36–750 °C (97–1382 °F) which covers the n‐alkane range of about C 5 –C 120 . A key difference between ASTM D2887 and HTSD is the ability of the latter technique to handle residue‐containing samples (i.e. material boiling > 538 °C, 1000 °F). SimDist and laboratory‐scale physical distillation procedures are routinely used for determining boiling ranges of petroleum crude oils and refined products, which include crude oil bottoms and residue processing characterization. The boiling point with yield profile data of these materials are used in operational decisions made by refinery engineers to improve product yields and product quality. Data from SimDists are valuable for computer modeling of refining processes for improvements in design and process optimization. Precise yield correlations between HTSD and crude assay distillation (methods ASTM D2892 and D5236) have allowed HTSD to be successfully used in place of physical distillation procedures. This has given the refiner the ability to rapidly evaluate crude oils for selection of those with economic advantages and more favorable refining margins. SimDist methods are becoming more widely used in environmental applications. HTSD is useful for characterizing hydrocarbons which can be present as soil and water contaminants; for example, to map and follow hydrocarbon removal processes.

The value of hexavalent chromium as a reference toxicant was investigated by comparing the precision of two laboratories in preparing test solutions, examining the consistency of chromium exposure in the test chambers during experiments and determining the effect of nominal versus measured test concentrations on the calculated toxicity (EC50 or LC50). The sensitivities of the test animals to chromium were also determined. The coefficients of variation associated with preparing chromium test concentrations were 51 and 63.8% in salt and fresh water, respectively, at one laboratory, and 136 and 14.8%, respectively, at the other laboratory. Hexavalent chromium remained stable in the hexavalent (as prepared) form during the toxicity test, with recoveries ranging from 77 to 114%. The precision of the analytical laboratory in measuring chromium‐spiked fresh and salt waters ranged from 0.2 to 9.1%. The impact on the calculated EC50s or LC50s of using analytically measured versus nominal concentrations in the analyses was negligible. A comparison of seven species in 27 tests showed organism sensitivity (mean EC50 or LC50 as mg/L Cr6+) to chromium to be, in decreasing order of sensitivity: Ceriodaphnia sp. (0.031), Daphnia pulex (0.086), Mysidopsis almyra (5.1), Mysidopsis bahia (6.03), Cyprinodon variegatus (21.4), Pimephales promelas (26.1) and Lepomis macrochirus (182.9). The 48‐h LC50s for M. bahia were not significantly different (p < 0.05) either within or between laboratories during a 3‐week study; values ranged from 5.49 to 7.72 mg/L Cr6+ in both laboratories (nominally) and from 4.21 to 7.23 mg/L Cr6+ when analytically verified by an independent laboratory. The acute toxicities of chromium to D. pulex differed significantly between laboratories, by almost one order of magnitude. Interlaboratory variability observed in D. pulex tests was attributed to differences in the test organisms' food. As reference toxicant tests, chromium tests are valuable benchmark indices of the relative health of test organisms over time or among laboratories. When toxicity test information is used in critical decision making, such as compliance with effluent permit limitations, a reference toxicant such as chromium contributes to quality control and assurance.

A summary is presented of a study conducted to characterize tar samples recovered from the marine environment. The samples were collected by the U.S. Coast Guard primarily from the northwestern Atlantic Ocean and along the eastern coast of the United States. A multiparameter analytical approach was applied which involved microscopy, chromatography, infrared, and other analytical methods. Patterns were recognized which allowed a classification of the samples into distinct groups and which suggested possible origins. Contract DOT-CG-23,379A, U.S. Coast Guard Headquarters, Washington, D.C. A complete report of the investigation covered under this contract (DOT-CG-23,379A) is available through the National Technical Information Service, Springfield, Va. 22151.

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