On the sampling variance of ultra-dilute solutions

Contemporary analytical methodology allows the determination of substances in ultra-dilute solutions, i.e. picomol per liter (pM) and lower, by employing a wide spectrum of techniques. The reduction of both sample volume and quantification limits made possible the determination of zeptomol amounts of analytes. Among the many problems associated with the analysis of this kind of sample is the relatively high intrinsic sampling variance which restricts the attainable overall analytical precision. This sampling variance is due to the quantized nature of matter. The expected sampling variance can be calculated considering that the limited number of analyte molecules per test-portion is subjected to Poisson distribution.