Why the Limit of Detection (LOD) Value is Not an Appropriate Specification for Automotive Emissions Analyzers

With the need for emission measurements of super ultra low emission vehicles (SULEV), analyzer manufacturers have been required to produce more precise and accurate analyzers. In order to compare analyzers, the customer must understand the different specifications used by the analyzer manufacturers. One specification that some manufacturers have used is the limit of detection (LOD) to indicate the reliability of the analyzer output at low concentrations.

There are various methods for determining the LOD for a given analyzer. The authors will demonstrate how variations in methodology can produce different LOD values for a specific analyzer and what it means for the automotive emission analyzers. It is also demonstrated that the standard deviations of a zero signal, which is related to LOD, can be heavily influenced by data processing, such as data length in use and/or data smoothing. The LOD values obtained will be compared to the limit of quantification (LOQ) for that analyzer. In order to evaluate the utility of LOD measurements, various analyzer performances will be modeled and evaluated.

While the LOD is a valid statistical approach to determining the presence of a component in a sample matrix, it is not directly applicable to emissions measurements. A discussion about the emission measurement requirements will demonstrate the limited applicability of this analyzer specification. Alternative methods for comparing analyzer specifications will be presented and justified.