Quantitation Limit (QL)

Quantitation Limit (QL)  is the estimated minimum concentration (assuming 100% recovery) of analyte in a sample extract or digest which can, not only be detected by the instrument, but can also be quantified with an acceptable accuracy.  At this limit the measured concentration will be expected to be satisfactorily close to the true concentration.  At concentrations above the QL the analytical instrument should be expected to perform linearly.  The QL should be referred to by data users when considering the accuracy of a reported low level concentration in a sample.  QL values will be established for all standard analytes.

For some methods accurate quantitation cannot be guaranteed below the calibrated range of the instrument and therefore the quantitation limit will be the equivalent concentration of the bottom calibration standard.  For other methods accurate quantitation can be made at concentrations between the bottom calibration standard and the MDL or even at the MDL itself.

The QL will be expressed in the appropriate concentration units for the sample matrix of interest and for soil samples may be expressed in terms of dry weight i.e. theoretical 0% moisture.  It should be recognised that where the QL has been extrapolated from calibration standard concentrations, this does  not account for any method bias introduced during sample preparation.  For this reason satisfactory accuracy and precision at or below the estimated QL must be statistically proven.  Accumulated % recovery and % RSD data from MDL or recovery check studies may be used for this purpose.  It will not be sufficient to assume  accuracy at the MDL level based on one MDL data set.

For some methods where a wide variety of analytes are included, for selective individual analytes the method may only provide accurate quantitation at higher concentrations.  For these analytes the reported QL will be the equivalent of a higher calibration standard unless otherwise statistically determined.