170135_inorganicventures_flipping

2:4 Planning the project

Detection Limits and Uncertainty

The analyst should know the detection limits for all analytes at several possible wavelengths. Typically, these measurements are obtained during the establishment of the analytical instrument's capabilities.

Modern axial view ICP-OES and ICP-MS instruments are likely to have detection limits under normal sample introduction modes that will meet or exceed the requirements. It is best to not rely upon the limits published by the manufacturer. In addition, the detection limits will be a function of the sample matrix, in both a physical and spectral sense. A key point involves the analytical blank. Due to numerous contamination issues, the analytical blank often determines the detection limit capabilities. It is best for the analyst to be conservative when noting the detection limits, making sure not to quote capabilities calculated from published data or determinations made under "ideal" conditions. For the less common elements, an estimate of the "real" detection limit would be a factor several times higher than the limit determined under ideal conditions. Thus, elements like Na, Mg, Ca, Fe, Cr, Cu, Zn, Si, Al, Cl, and S may have a detection limit that is significantly larger than expected, due to the analytical blank.

2:5 Planning the project

Detec%on Limits and Uncertainty (cont.)

The uncertainty of an analy%cal measurement is not limited to the measurement precision of the instrument. Rather, it is a sta%s%cal sum of the random and systema%c errors that are encountered throughout the en%re analy%cal process. The uncertainty is a combina%on of errors from sampling, storage, weight and volume manipula%ons, prepara%on, calibra%on, and measurement, during which contamina%on issues play a major role in trace determina%ons. The sampling error can be a major source of uncertainty. In many cases, an es%mate of the sampling error can be impossible to judge. The ini%ator should be aware of this fact. In reality, the uncertainty of a trace measurement will not be known un%l the project is completed. The accuracy of this value will be only as good as the effort made to iden%fy and measure all of the errors encountered during the en%re analy%cal process. If measurements are being made between 3-5 %mes the detec%on limit for the less common elements, then an excellent uncertainty would be ± 30-70%.