TABLE OF CONTENTS It will be recalled from Sections 4.3.2 and 4.4.2 that there are additional sources of uncertainty in an analytical determination that are not taken account of in the calculation of s x as described in the preceding Section 4.8. These are:
the uncertainty due to the CRM itself ( e.g. the uncertainty of the certified property value), u(CRM
the uncertainty associated with any preparation of the CRM prior to its introduction to the instrument, u(CRM prep.)
the uncertainty associated with any preparation of the sample prior to its introduction to the instrument, u(sample prep.)
An estimate of the overall standard uncertainty of an analytical result is therefore obtained as follows:
An example of the use of this equation to combine uncertainties is given below. The example is based on the determination of p,p ?-DDE in a sample of animal oil at a nominal level of 1 mg kg ?1. It is assumed that a 10 g sample was taken and a 10 mL extract prepared for instrumental measurement of the p,p ?-DDE concentration by GC-MS or GC-ECD. The instrument calibration data are those given in Table 4.7 and Figure 4.7. The calibration solutions are assumed to have been prepared from the CRM LGC1110 ( p,p ?-DDE with a certified purity of 99.6% and an expanded uncertainty [ k = 2] of 0.4%). The procedures used to prepare the solutions are assumed...
