After selecting the best conditions for the US-assisted extraction of
Hg, the optimized method was applied for all samples of inorganic fertilizers.
The extracts were analyzed by CV AAS and MIP OES. The results
were compared with those obtained by the officia lmethod based on hot
plate digestion (procedure described in the Experimental section).
Table 7 summaries all analytical results with their respective standard
deviations (n= 5) and recovery values calculated considering as 100%
the concentration values obtained by the official method or in accordance
with the reference values.
The RSD values for the analyses using CV AAS or MP OES were below
10% and recovery values were higher than 92% in all cases. This result indicates
that the US-assisted extraction using 30% (v/v) HCl for 5 min
was effective to extract mercury from inorganic fertilizers prior CV
AAS or MIP OES determinations.
The application of the Student's t-test at the 95% confidence level for
comparison of the analytical results between official and proposed
methods (using either CV AAS or MIP OES) revealed that three results
were not statistically in agreement including the analysis of 05A
(2013) sample by both CV AAS and MIP OES and the analysis of NPK
(5.25.20) by MIP OES. However, recovery values for these samples can
be considered acceptable (N90%) and then the proposed US-assisted extraction
is simple and an effective alternative procedure of sample preparation
for Hg determination in inorganic fertilizers.
The stability of acid leachates was evaluated under optimized conditions
within 10 days using MRR1 sample. Hg determinations were carried
out for the same leachate in six different times within 10 days.
The RSD value for these six measurements was 3.8%. The paired
Student's t-test (95% confidence level) was also applied to compare
the Hg concentrations obtained in these six measurements for the
same leachate with the Hg concentration obtained by the proposed
MIP OES method for MRR1 sample (Table 7) and no significant difference
was verified (tcalculated=0.68 b tcritical= 2.26).