Principles of Measurement
Measurements usually are made because a quantitative value for some substance or constituent thereof is needed for some purpose. Ordinarily, one speaks of measurement or determination of the parameter of interest, but this is seldom the case. Measurement is basically a comparison of an unknown with a known. In some cases, the comparison is direct, as in the determination of mass using an equal-arm balance. Direct chemical measurements consist of comparisons on a real time basis or intermittent alternations of standard and unknown. The precision, detection level, and sensitivity are easily evaluated from knowledge of the standard deviation as a function of measurement level. Accordingly, short-term standard deviations are usually smaller than long-term standard deviations. That is to say, the measurement system is more precise over short intervals than long intervals of time. The same concept applies to detection levels, limits of quantitation, and sensitivity.