The raising of Reported Detection Limits (RDL) can occur for a variety of reasons.
The most common reasons are:
Concentration exceeds upper calibration limit of the test instrument.
The most common reason to raise a detection limit is due to an adjustment of the concentration of the sample’s test parameter so that it can fall within the calibration limit of the test instrument. This would be necessary if the concentration of the test parameter within the received sample exceeds the upper calibration level of the instrument. In such cases it is necessary to dilute the sample to bring it within the instrument’s calibration range. The dilution value required to bring it within the calibration range (known as the dilution factor) is also applied to the detection limit of the analysis.
Matrix Interference
Within the test sample there is a component which in some fashion interferes with the ability of the analyst to reliably quantify the parameter of interest. Often by diluting the sample, the interference is reduced to a point where an accurate quantification can be made. The dilution required (dilution factor) is subsequently applied to the Reported Detection Limit (RDL).
Limited Sample
If the amount of sample received for analysis is less than the minimum required by the laboratory, this will be reflected in the parameter’s Reported Detection Limit (RDL). Parameter RDLs are all based upon on an often prescribed minimum weight or volume.