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. The uncertainty of an analytical measurement is not limited to the measurement precision of the instrument. Rather, it is a statistical sum of the random and systematic errors that are encountered throughout the entire analytical process. The uncertainty is a combination of errors from sampling, storage, weight and volume manipulations, preparation, calibration, and measurement, during which contamination issues play a major role in trace determinations. The sampling error can be a major source of uncertainty. In many cases, an estimate of the sampling error can be impossible to judge. The initiator should be aware of this fact. In reality, the uncertainty of a trace measurement will not be known until the project is completed. The accuracy of this value will be only as good as the effort made to identify and measure all of the errors encountered during the entire analytical process. If measurements are being made between 3-5 times the detection limit for the less common elements, then an excellent uncertainty would be ± 30-70%.
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