It is suggested to specify the test methods based on visual evaluation of a useful signal by the probability distribution law and its parameters related to the detection limits. When solving a definite analytical problem, the known distribution parameters allow to calculate a detection probability of any concentration of the determinand, and vice versa, a concentration value corresponding to a preselected probability. Laws of probability distribution, their parameters and determinands detection limits in surface waters#x2014;by indicator test papers, colour reactions on a carrier, in solution and in a thin-layer adsorbent#x2014;have been investigated by the maximum likelihood and #x3C7;2minimum methods. The normal, lognormal, exponential, and Weibull distributions are corresponding to the tested types of methods. Such approach allowed an advantageous implementation of rapid assays for detection and quantitative determination of Na, K, Rb, Cs, Li, Cu(II), NO#xB7;, 3,4-DCA, propanil, sevin, DDT, etc., in surface waters.
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