Understanding Oil-in-Water Instrument Specifications


The measurement capabilities of oil-in-water analysis methods are described by several analytical figures-of-merit. Examples are:
Range (Linear and Dynamic) Limit of Detection (LOD) Limit of Quantification (LOQ) Precision (Repeatability and Reproducibility) Sensitivity Accuracy Response Time
Some or all of these are often specified by customers in the bid packages issued when selecting a grab-sample method or a continuous oil-in-water monitor. The specified figures-of-merit are often ill-defined, inconsistent, incomplete or analytically unreasonable. This is probably due to a general lack of understanding of their definitions and how the values relate to the ability of the instrument to perform in a given application.
Instrument manufacturers are often asked to provide a single list of numbers to describe the general capability of their instruments to monitor oil-in-water. This is impossible due to the fact that oil-in-water is not a unique chemical substance with known chemical structure and properties. Instead, it is the fraction of a complex mixture (which changes radically from site-to-site), that is detected by a regulatory test (e.g. EPA Method 1664, Ospar GC-FID Method).
In this paper, I attempt to clarify the definitions of the most frequently specified figures-of-merit. I also present a discussion of how they should be interpreted. Finally, I introduce the concept of Capability Ratio (CR), a well established six-sigma tool, as a measure of the ability of an analysis method to monitor the oil-in-water content of a specific water process.

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