Application of LOD/LOQ to purity methods presents specific challenges that deserve
additional consideration. The reporting unit for purity methods is percent (%) purity, a
unit that is not compatible with the unit in which LOD or LOQ are typically expressed
(units of concentration or amount). The signal created by the analyte may vary with the
load, while the relative percentage of the analyte does not change. This creates a
situation where the analyte of interest can be hidden within the noise or, alternatively,
can be significantly above the noise for the same sample analyzed at two different load
levels within the range allowed by the method. This has been addressed by the concept
of “dynamic LOQ” by combining statistical and S/N approaches (Apostol, Miller et al.
2009).