A STATISTICAL OVERVIEW OF STANDARD (IUPAC AND ACS) AND NEW PROCEDURESFOR DETERMINING THE LIMITS OF DETECTION AND QUANTIFICATION - APPLICATION TO VOLTAMMETRIC AND STRIPPING TECHNIQUES (TECHNICAL REPORT)

Traditional methods for determining the limit of detection (LOD) and t he limit of quantification (LOQ), based on the IUPAC and ACS definitio ns, often are unsatisfactory. Consequently, a new, simple and statisti cally correct way of obtaining both the LOD and LOQ values has been de rived and compared to commonly used methods. The new Upper Limit Appro ach, ULA, calculates the upper confidence limit of an individual blank signal using a critical value of the t- distribution and standard err or of estimate (residual standard deviation) of regression. The uncert ainty of the calibration plot position, its intercept and the mean bla nk signal are taken also into consideration. The proper choice of cali bration model and calibration design also are discussed in detail. An improved derivation of the signal value relevant to the LOQ is based o n the use of the same significance level needed for defining the LOD. The concepts developed in this paper have been applied to the determin ation of cadmium by five common techniques of electrochemical trace an alysis (d.c. tast polarography, differential pulse polarography, linea r sweep stripping voltammetry, differential pulse stripping voltammetr y, and potentiometric stripping analysis) under equivalent (as possibl e) conditions. The lowest LOD and LOQ values, obtained by the new reli able ULA method, were 1.0x10(-9) mol/L and 3.1x10(-9) mol/L Cd, resp., for differential pulse stripping voltammetry and significance level a lpha=0.01. The relevant LOD and LOQ values for alpha=0.05 are 6.6x10(- 10) mol/L and 2.0x10(-9) mol/L Cd, resp. Potentiometric stripping anal ysis provided similar results.