The lower limit of quantification (LLOQ) is the lowest concentration of analyte in a sample which can be quantified reliably, with an acceptable accuracy and precision. In addition, the analyte signal of the LLOQ sample should be at least 5 times the signal of a blank sample. The relative standard deviation (RSD) of the LLOQ sample within 20% and the accuracy was in the range of 80% to 120% according to the USFDA guidelines.