2.4.2. Preparation and injection of coffee extracts
The extraction protocol was based on the conditions previously
optimized by Rostagno et al. [24]. In short, 0.5 g of each coffee
sample were extracted with 15 ml of 50% methanol/water,
followed by 15 ml of 75% methanol/water, and finally with
15 ml of 100% methanol. Extraction was performed on a multifrequency
ultrasound bath [10,25,26], operating at 25 kHz for
20 min at 60 8C. After each extraction step, the sample is
centrifuged for 10 min at 10 8C and a velocity of 4000 rpm using
a universal centrifuge. The supernatant was collected and the solid
residue was subjected to the next extraction step. After the final
extraction, the supernatants were combined, and water was added
to obtain a final volume of 100 ml. 20 ml of the obtained extract
was filtered and injected into the HPLC apparatus. Analyses were
performed in triplicate.
2.4.3. HPLC gradient
A binary gradient elution (Table 1) was used. The mobile phase
was as follows: 1% phosphoric acid in water (solvent A) and 1%
phosphoric acid in acetonitrile (solvent B). The flow rate was
systematically controlled and set at 1 ml/min.
2.4.4. Analytical method validation
The limit of detection (LOD) was determined considering a
value three times the standard deviation of the background noise
obtained from blank samples (solvent; methanol), divided by the
slope of the calibration curve. The limit of quantification (LOQ) was
determined considering a value 10 times the standard deviation of
the background noise, obtained from noise samples divided by the
slope of the calibration curve. The limit of detection (LOD) ranged
from 0.75 to 14.79 mg/l, while the limit of quantification (LOQ)
ranged from 2.26 to 44.44 mg/l (Table 2a).
The recovery capacities of the method were assessed in
quadruplicate by spiking coffee extracts with three different
concentrations of pure standards (phenolic compounds and
caffeine) (0.5, 0.7 and 1.0 mg/100 g). The average recovery was
greater than 98% for all molecules (Table 2b).