TABLE OF CONTENTS Some preparation of the CRM is often required before it can be introduced to the instrument and frequently the preparation of a solution of the calibration standard is necessary. Such operations will, in principle, add to the uncertainty of the documented property value, as described in Section 4.4.2 above. The following example shows how the preparation of a solution can affect the uncertainty of the property value.
The CRM LGC 1110 ( p,p ?-DDE) has a certified purity value of 99.6% and an expanded uncertainty (coverage factor, k = 2) of 0.4%. A solution of the material, in hexane, with a nominal concentration of O.1 mg mL ?1 was prepared. Table 4.3 shows the values for the purity of the CRM, the mass of the CRM taken and the volume of hexane solvent used and their estimated standard uncertainties.
| Source of uncertainty | Units | Value | u | ru | ru 2 |
|---|---|---|---|---|---|
| Certified purity | 0.996 | 0.002 | 0.002008 | 4.03E 06 | |
| Mass of CRM taken | g | 0.0114 | 0.0002 | 0.017544 | 0.000308 |
| Volume of hexane | mL | 100 | 0.06 | 0.0006 | 3.6E 07 |
| Concentration | mg mL ?1 | 0.1135 | 0.0020 | 0.017669 | 0.000312 |
| u = standard uncertainty; ru = relative standard uncertainty (= u/value); ru 2 = relative standard uncertainty squared |
The concentration of the prepared solution is calculated to be 0.1135 mg mL ?1. The standard uncertainty of this value,...
