The results from laboratory reagent blank (LRB) are close to zero,
indicating no interference or contamination from reagents or materials
used and from the sample preparation processes. The analyte
recoveries from laboratory fortified blank (LFB) samples are close
to 100% except for hydroquinone, demonstrated that no analyte
loss or contamination for most of the analyte (except for hydroquinone)
during sample preparation. Hydroquinone gave 75–90%
recoveries depending on the concentrations spiked and how long
the sample was stored before analysis. Low recoveries for hydroquinone
from LFB samples, especially at low concentration levels
and at extended period of time before analysis indicate that this
analyte is decomposed with time. To obtain accurate results for all
the phenolic compounds, samples should be analyzed as soon as
possible after sample preparation (within 48 h as recommended by
HC methods). Primary standard solutions are stable for two weeks
if kept in darker in a refrigerator. Calibration standards are stable
for a week if kept in darker in a refrigerator after preparation. Sample
tray of the autosampler should be chilled (at 4 ◦C in this study)
to reduce the loss of analytes, especially for hydroquinone in the
samples.