Self-test 9.5 – Estimation of measurement uncertainty in chemical analysis

An analyst is using a titration procedure of determination of the net content of basic substances in a biological sample. He carried out sample preparation (several extraction steps), obtained 100 ml of a solution and made 5 replicate titrations of 10 ml aliquots (measured by a volumetric pipette) of this solution with HCl solution. He obtained five values and calculated standard deviation of these values. This standard deviation can be used in measurement uncertainty calculation. Which of the following statements are correct?

This standard uncertainty takes into account the overall repeatability of all steps of the procedure (sample preparation, pipetting, titration).

This standard uncertainty takes into account the repeatability of the pipetting and titration steps of the procedure.

If the indicator is suboptimal and a systematic over-titration occurs as a result then this standard deviation does not take it into account.

If the organic bases that cause basicity are sensitive compounds and partially decompose during sample preparation, preparation of solution or during titration then this standard uncertainty takes this decomposition into account.

In a liquid chromatographic method determination of tartrazine colorant a sample solution with code SK27 was injected five times during the same day. The injection order was random (i.e. the five samples of SK27 were randomly mixed with other sample solutions and calibration solutions). Five chromatograms of SK27 were obtained. The areas of tartrazine peaks on the chromatograms were 1255, 1248, 1262, 1251 and 1257 mAU·s. Different useful information for measurement uncertainty estimation can be extracted from these data. Which of the following statements are true?

The standard deviation of the mean peak area (i.e. the standard deviation of these 5 values divided by square root of 5) is 2.4 mAU·s and it takes into account repeatability of the chromatographic determination in the case of sample SK27.

The standard deviation of the mean peak area (i.e. the standard deviation of these 5 values divided by square root of 5) is 2.4 mAU·s and it takes into account possible drift of the detector’s characteristics as well as possible decomposition of the analyte in the sample during the chromatographic determination in the case of sample SK27.

The standard deviation of these five values 5.4 mAU·s can be used to take into account repeatability uncertainty in the case of similar samples for which only one single injection was made.

The standard deviation of these five values 5.4 mAU·s can be used to take into account the uncertainty due to sample preparation.

The standard deviation of these five values 5.4 mAU·s can be used to take into account the overall uncertainty due to volumetric operations involved in making the sample solution SK27.

The standard deviation of the mean peak area (i.e. the standard deviation of these 5 values divided by square root of 5) 2.4 mAU·s can be used to take into account partial overlap of the analyte peak with an interfering peak on the chromatogram.