The optimal conditions for acid hydrolysis are not well established
in the literature. Thus, in this work the most important
variables for this purpose were studied. The optimum values
were: time 90 min, 100–110 ◦C of temperature and 6M of HCl.
3.4. Optimization of variables
The hydrodynamic parameters of the system: injected reagent
volume, reactor length and flow rate, were optimized by univariate
method in order to achieve a compromise between sensitivity,
sample throughput and reproducibility. The studied ranges and
optimum values were showed in Table 2.
The only chemical variable optimized was the enzyme concentration.
This concentration was tested between 300 and
1200 UE ml−1, and the optimum value was 800 UE ml−1.
3.5. Analytical parameters
The calibration graph was linear between 11 and
132 mg l−1 of catechol. The regression line was A = (0.0039±
0.0001)C+ (0.1737±0.0080), where A is the absorbance and C
is the concentration in mg l−1 of cathecol, with a R2 = 0.998. The
detection limit [20] and the sample throughput were 7.7 mg l−1
of catechol and 49 h−1, respectively. The precision of the
method, represented by the repeatability (%R.S.D.) was 0.9%
and it was calculated from 10 independent measurement of a
standard containing 30 mg l−1 of catechol.
3.6. Analytical application to real samples
The proposed method was applied to determine total phenols
in urine samples. The concentrations of total phenols are shown
in Table 3, where these values were also expressed as a function