Every measuring device, regardless of the technology used, has detection and quantification limits. The same is true for lead devices.
What does this mean? And what are the limits of lead machines?
Definition of LOD and LOQ
With our devices, we measure lead concentrations (in mg/cm²). We need to know the minimum detectable concentration and the minimum quantifiable concentration.
- Based on the limit of detection or LOD (limit of detection), we can say that lead is present. Below this level, it is not possible to say.
- Based on the limit of quantification or LOQ (limit of quantification), the concentration is defined with reasonable uncertainty. Below this limit, lead can be detected but it will be quantified with insufficient accuracy.
How are LOD and LOQ determined?
The limits can be calculated using different methods:
- For the limit of detection, the preferred method is “blank analysis”;
“Blank analysis” is based on the measurement of a sample that does not contain the element being sought.
- For the limit of quantification, an acceptable level of uncertainty is established.
This definition of the LOQ is based on statistical measurements of a certified sample containing the element being sought and the calculation of the dispersion of these measurements.
Calculation of the LOD
We use the NIST SRM 2570 paint standard, which has a lead concentration of < 0.001 mg/cm². We consider this standard to be our “blank.”
Under strictly identical conditions, we perform numerous measurements with the lead analyzer on this standard and calculate the mean of the measurements (m) and the standard deviation* (s).
The formula for obtaining the detection limit is then as follows:
LOD = m + 3s
This formula can be found in the Metrology Guide and in the ANSES Guide to the Validation of Analytical Methods (see p.59)
* INSEE definition: The standard deviation is used to measure the dispersion, or spread, of a set of values around their mean. The lower the standard deviation, the more homogeneous the population.
Calculation of the LOQ
To verify that our LOQ hypothesis is correct, we use the NIST SRM 2575 standard, which has the lowest concentration of lead available: 0.31 ± 0.02 mg/cm²*
Under strictly identical conditions, we take numerous measurements with the lead analyzer on this standard placed on a plaster substrate and calculate the mean of the measurements (m) and the standard deviation (s).
These results allow us to verify our hypothesis with an acceptable associated uncertainty.
*For the FEnX, the LOQ could be lower, but without a certified sample with a lower lead concentration, it is not possible to demonstrate this.
LOD and LOQ of Fondis lead analyzers
Based on these methods, here are the LOD and LOQ of our devices.
The 10-second measurements were performed with NIST SRM 2575 and SRM 2570 standards placed on a plaster substrate.
