For the Soxhlet extraction-concentrated sulfuric acid method,
samples of 5.00 g were weighed precisely, and the tracing standard
(TCMX) was added. The Soxhlet extraction was performed for 24 h
with the anhydrous Na2SO4 five times the mass of the sample, followed
by addition of an appropriate amount of copper powder and
petroleum ether. The extract was transferred to a separatory funnel,
and 10 mL concentrated H2SO4 was added. The mixture was allowed
to stand, and then the H2SO4 layer was removed. The
operation was repeated once. The mixture was concentrated to
1–2 mL using a rotary evaporator