Simultaneous determination of sinapic acid and tyrosol by flow-injection analysis with multiple-pulse amperometric detection
This work describes a simple, fast (frequency of 170 injections h−1), and low-cost method for the simultaneous determination of two antioxidants, sinapic acid and tyrosol, using multiple-pulse amperometric detection at a glassy carbon electrode incorporated in a flow-injection analysis cell. A sequence of potential pulses was selected to detect sinapic acid and tyrosol separately in the course of a single injection step. During the characterization of electrochemical detection, conditions for the determination of the two antioxidants (such as the injected volume and the flow rate) were studied and the analytical figures of merit were calculated. The repeatability (expressed as %) RSD was < 4.0% (n = 10) and excellent linearity was obtained across two concentration ranges from 1.0 to 100 μM; the limits of detection of sinapic acid and tyrosol were around 1.0 μM.
KeywordsElectrochemistry Flow-injection analysis Glassy carbon electrode Oxidations Voltammetry
Several methods, mainly based on cyclic voltammetry and differential pulse voltammetry, have been reported for the determination of sinapic acid  or tyrosol . However, to the best of our knowledge, only a few analytical methods have been reported for the simultaneous determination of sinapic acid and tyrosol including HPLC with UV or MS detection [8, 9, 10].
Recent publications have demonstrated that flow-injection analysis (FIA) with multiple-pulse amperometric (MPA) detection could be used for simultaneous measurement of two or more electroactive species [11, 12, 13]. An aliquot of sample solution is directly injected into a FIA system and the compounds are selectively detected at a single working electrode by applying two sequential potential pulses. A simple correction factor must be used for the calculation. This approach has some important advantages: it is inexpensive and simple, and has small sample and reagent consumption (with reduction in waste generation) and high sampling rates . This strategy was used for simultaneous amperometric detection of sugars , drugs [16, 17, 18], antioxidants , synthetic colorants , as well as for the use of internal standard method in FIA .
This paper demonstrates that MPA detection in combination with FIA on a glassy carbon electrode (GCE) can be used for the simultaneous determination of sinapic acid and tyrosol. Results obtained with this method were evaluated with respect to recovery, repeatability, linearity, and detection limits.
Results and discussion
In the development of the proposed method, an additional parameter should be considered: the CF value must be constant in the selected concentration interval. In the concentration interval between 10 to 100 μM of sinapic acid, this requirement was fulfilled and the CF value was calculated as 1.10 ± 0.06 (n = 3).
Figures of merit of the proposed method for the simultaneous MPA-FIA determination of sinapic acid and tyrosol
Slope/nA mol−1 dm3
RSD/% for 10 injections (100 μM)
The present work demonstrates the possibility of simultaneous determination of sinapic acid and tyrosol using a flow-injection system with multiple-pulse amperometric detection. The advantages of the technique are short time of analysis (170 injections h−1), low consumption of samples and reagents, high precision (RSD < 4.0%; n = 10), and linear calibration curves (r > 0.99). The limits of quantification were 0.86 and 1.03 μM for sinapic acid and tyrosol, respectively. This method has a good potential to be applied in routine analysis in substitution of expensive chromatographic separation systems.
Sinapic acid (CAS number 530-59-6) and tyrosol (CAS number 501-94-0) were supplied by Sigma-Aldrich. Their individual stock solutions (c = 1.00 mM) were prepared by dissolving the exact amount of the respective substance in methanol (Merck Millipore, Germany) and they were kept at 4 °C. More diluted solutions were prepared by exact dilution of the stock solutions with mixture of methanol and 0.040 M Britton-Robinson (B-R) buffer (1:9, v/v). All electrochemical measurements were carried out in the same solution. The B-R buffer was prepared by mixing 0.20 M sodium hydroxide (Lach-Ner Neratovice, Czech Republic) with acidic solution consisting of 0.040 M boric acid (Lach-Ner Neratovice, Czech Republic), 0.040 M phosphoric acid (Merck Millipore, Germany), and 0.040 M acetic acid (Merck Millipore, Germany). All chemicals used for buffer preparation were of analytical grade purity. Distilled water was provided from a Mega-Pure 3A Liter Automatic Distillation System, USA.
Instrumentation and apparatus
All electrochemical recordings were performed using an Autolab PGSTAT12 potentiostat/galvanostat, controlled by NOVA version 1.11.2 software (Metrohm, Switzerland) working under Windows 7 (Microsoft Corporation). The three-electrode wall-jet configuration included a glassy carbon working electrode (GCE) (Metrohm, Switzerland, diameter of 2 mm and geometric area 3.1 mm2), a platinum wire, 1 cm in length and 0.5 mm in diameter, as a counter electrode, and an Ag/AgCl (3 M KCl) electrode as a reference electrode (MonokrystalyTurnov, Czech Republic) . Flow of the carrier solution was provided by peristaltic pump MINIPULS Evolution (Gilson, USA) and injection of the sample was performed with a six-way injection valve (VICI Valco Instruments, Canada) equipped with a 100 mm3 sample injection loop. An Orion 266S pH meter (Thermo Fisher Scientific, USA) equipped with a combined glass pH electrode was used for pH measurements. The pH meter was calibrated with aqueous standard buffer solutions at ambient temperature.
Pre-treatment of the GCE was done by polishing with alumina powder suspension (0.1 μm) on a damp polishing cloth (Metrohm, Switzerland) before fixing to the flow cell. This procedure was performed at the beginning of the working day.
Hydrodynamic voltammograms of sinapic acid and tyrosol were obtained separately by application of nine sequential potential pulses (from + 0.40 to + 0.80 V for sinapic acid and from + 0.70 to + 1.10 V for tyrosol, pulse width 100 ms) in triplicate injections of standard solutions through the FIA system using the MPA technique. The same technique was used for simultaneous amperometric detection of sinapic acid and tyrosol, applying pulses + 0.75 V for 100 ms (sinapic acid) and + 1.10 V for 100 ms (tyrosol) continuously (total time of the potential waveform was 200 ms).
The peak height (Ip) was evaluated from the amperometric FIA recording. The limit of quantification (LQ) was calculated as the analyte concentration corresponding to a tenfold standard deviation of the respective response from ten consecutive determinations at the lowest measurable concentration .
Financial support of the Czech Science Foundation (Project P206/12/G151) is acknowledged and Erasmus programme.
- 2.Anthony NR, Samuel SM (2008) Research topics in agricultural and applied economics, 1st edn. Sharjah, United Arab EmiratesGoogle Scholar
- 8.Blekas G, Tsimidou M (2005) Curr Top Nutraceutical Res 3:125Google Scholar
- 25.Miller JN, Miller JC (2005) Chemometrics for analytical chemistry, 5th edn. Pearson Education, HarlowGoogle Scholar