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The rational strategy in the evaluation of analytical measurement uncertainty is to combine the “whole method” performance data, such as precision and recovery, with the uncertainty contributions from sources not adequately covered by those data. This paper highlights some common mistakes in evaluating the uncertainty when pursuing that strategy, as revealed in current chromatographic literature. The list of the uncertainty components usually taken into account is discussed first and fallacies with the LOD- and recovery uncertainties are noted. Close attention is paid to the uncertainty arising from a linear calibration normally used. It is demonstrated that following a well-known formula for the standard deviation of an analytical result obtained from a straight line calibration leads to double counting the precision contribution to the uncertainty budget. Furthermore, the precision component itself is often estimated improperly, based on the number of replicates taken from the precision assessment experiment. As a result, the relative uncertainty from linear calibration is overestimated in the budget and may become the largest contribution to the combined uncertainty, which is clearly shown with an example calculation based on the literature data.

Establishing a traceability route with all measurement and uncertainty relationships determined is an important aspect of traceability, and seems to be particularly striking in pH measurement. In this paper the issue of evaluation of secondary pH standards measured with reference to a primary standard in a differential cell with free diffusion type liquid junctions is considered. Relatively high uncertainty, U=0.015, has been assigned to such standards in the recent IUPAC Recommendations on pH (2001), because of a specific residual liquid-junction potential treated statistically as a contribution to the combined uncertainty. Close inspection of the problem leads to the conclusion that a correction for the residual liquid-junction potential should be applied to the measured value of a secondary pH standard. This can be considered as a correction for a known systematic effect on the traceability route. With available experimental data it is demonstrated that such a correction can reasonably be made for well-studied standard buffer systems. In this way the uncertainty associated with secondary pH standards is kept to a low level, and, what is more, the problem of two pH scales, a multi-standard scale and a single-standard scale, gains a proper solution. The need for different treatment of residual liquid-junction potentials at different levels in the measurement hierarchy is noted. Much attention is also given to rational categorization of pH standards in the hierarchy.

The residual liquid junction potential (RLJP) needs to be accounted for in pH uncertainty estimation. Attempts to do this and the currently available methods for evaluating the RLJP are critically discussed and their weak sides are pointed out. In this work an empirical approach to the problem is proposed. It is based on the use of the RLJP bias estimated on a variety of measurement conditions for a specific class of analytical objects essentially differing in ionic strength from the pH calibration buffers. The data from five independent studies, including interlaboratory comparisons, on pH measurement in low ionic strength waters were used to find the overall bias observed in the 10(-4) mol dm(-3) strong acid solution. The procedure includes quantifying the uncertainty of bias values from separate studies by combination of the relevant uncertainty components and testing the consistency of the data. The weighted mean bias in pH was found to be 0.043+/-0.007 (k=2). With this estimate, the pH measurement uncertainties calculated according to the previously suggested procedure (I. Leito, L. Strauss, E. Koort, V. Pihl, Accredit. Qual. Assur. 7 (2002) 242-249.) can be enlarged to take the uncorrected bias into account. The resulting uncertainties on the level of 0.10-0.14 (k=2) are obtained in this way for pH measurement in water and poorly buffered aqueous solutions in the range of pH 7.5-3.5.