Division 2.3.2.2 Precision of quantitative methods
United Kingdom
The inter-laboratory coefficient of variation (CV) for the repeated analysis of a reference or fortified material, under reproducibility conditions, shall not exceed the level calculated by the Horwitz Equation. The equation is:
where C is the mass fraction expressed as a power (exponent) of 10 (e.g. 1 mg/g = 10-3). Examples are shown in the table 3.
Table 3
Examples for reproducibility CVs for quantitative methods at a range of analyte mass fractions
| Mass fraction | Reproducibility CV(%) |
|---|---|
(*) For mass fractions lower than 100 μg/kg the application of the Horwitz Equation gives unacceptable high values. Therefore, the CVs for concentrations lower than 100 μg/kg shall be as low as possible. | |
| 1 μg/kg | f1 |
| 10 μg/kg | |
| 100 μg/kg | 23 |
| 1 000 μg/kg (1 mg/kg) | 16 |
For analyses carried out under repeatability conditions, the intra-laboratory CV would typically be between one half and two thirds of the above values. For analyses carried out under within-laboratory reproducibility conditions, the within-laboratory CV shall not be greater than the reproducibility CV.
In the case of substances with an established permitted limit, the method shall achieve within-laboratory reproducibility not greater than the corresponding reproducibility CV at a concentration of 0,5 × the permitted limit.
Annotations
Footnotes
- f1
(*)
For mass fractions lower than 100 μg/kg the application of the Horwitz Equation gives unacceptable high values. Therefore, the CVs for concentrations lower than 100 μg/kg shall be as low as possible.