Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p3
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 3/9)
Character Range: 721900–724839

the spike is within the working range of the method. For compliance monitoring, the spike level should be at or near the regulatory limit, or in the range of 15 times the background concentration.

If the background concentration is not known, the spike level may be at the equivalent concentration to the midpoint of the calibration range, or approximately 10 times the LOR in the matrix of interest (US EPA SW-846, Method 3500C).

The longer the spiked analyte can remain in the sample before extraction or digestion, the closer is the simulation to recovering the analyte from the natural sample (except for volatile organics).

Percent recovery is calculated as follows:
Per cent recovery  = c – a x 100
              b
where:
a = measured concentration of the unspiked sample aliquot
    b = nominal (theoretical) concentration increase that results from spiking the sample
c = measured concentration of the spiked sample aliquot

Note: If 'a' is known beforehand, then 'b' should be approximately equal to 'a', and 'c' should be approximately twice that of 'a', for 100% recovery.

In general, at least 70% recovery should be achievable from a reference method; some standard methods state that recoveries for validated methods can be lower.

'Recovery of the analyte need not be 100%, but the extent of the recovery of the analyte and internal standard should be consistent, precise, and reproducible' (FDA 2001).

Further information may be obtained from General requirements for the competence of testing and calibration laboratories (ISO 17025, 2005) and Uncertainty of measurement—Part 3: Guide to the expression of uncertainty in measurement (ISO/IEC Guide 98-3:2008).

    3.2.2         Precision
Precision is a measure of the variation in the method results. It is a combination of two components, repeatability and reproducibility, and is expressed in terms of standard deviation (SD) or relative standard deviation (RSD) of replicate results (APHA 2005).

3.2.2.1         Repeatability
This is a measure of the variation in the method results produced by the same analyst in the same laboratory using the same equipment under similar conditions and within a short time interval (Eaton et al. 2005).

3.2.2.2         Reproducibility
This is a measure of the variation in the method results for the same sample(s) produced by different analysts in different laboratories under different conditions and using different equipment. It measures the 'ruggedness' of the method. Reproducibility data should be obtained as part of the method validation procedure, and are best obtained through inter-laboratory comparisons and proficiency studies.

3.2.2.3         Confidence limit  and confidence interval
When results are qualified with standard deviations (SD) or their multiples (for example, 'x ± SD'), these are taken to be their confidence limits. This means that a result of 10±4 mg/kg would have confidence limits (CLs) of