ЖУРНАЛ АНАЛИТИЧЕСКОМ ХИМИИ, 2015, том 70, № 3, с. 279-285
ОРИГИНАЛЬНЫЕ СТАТЬИ =
APPLICATION OF CINNAMOYL DERIVATIVE AS A NEW LIGAND FOR DISPERSIVE LIQUID-LIQUID MICROEXTRACTION AND SPECTROPHOTOMETRIC DETERMINATION OF COBALT © 2015 L. EleCkova*, S I. S. Balogh**, J. Imrich***, V. Andruch*
*Department of Analytical Chemistry, P. J. Safarik University SK-04154 Kosice, Slovakia 1E-mail: firstname.lastname@example.org **Department of Chemistry, College of Nyiregyhaza HU-4400 Nyiregyhaza, Hungary ***Laboratory of NMR Spectroscopy, P.J. Safarik University SK-04154 Kosice, Slovakia Received 30.05.2013; in final form 06.05.2014
The method is based on the reaction of Co(II) with 3-[4-(dimethylamino)cinnamoyl]-4-hydroxy-6-methyl-2H-pyran-2-one (ligand) and a dimethylindocarbocyanine dye, followed by dispersive liquid—liquid microextraction of the ion associate formed and subsequent spectrophotometric detection. The appropriate experimental conditions were found to be: pH 9, 0.14 mM dye, toluene (extraction solvent) containing 2.25 mM of ligand, acetonitrile (dispersive solvent) and carbon tetrachloride (auxiliary solvent). Beer's law is obeyed in the range 0.06—0.42 mg/L of Co(II) at a wavelength of 558 nm. The limit of detection, calculated as three times the standard deviation of the blank test (n = 10), was found to be 9 p.g/L for Co(II). The method was applied to the determination of cobalt in spiked water samples and a vitamin B12 pharmaceutical injection.
Keywords: cinnamoyl pyrones, cobalt determination, dispersive liquid—liquid microextraction, UV-Vis detection, vitamin B12.
Cobalt is an essential trace element for humans, but at high levels it can be toxic to human health. Cobalt toxicology is discussed in the publication by Domingo . Cobalt occurs naturally in minerals . It is also an integral part ofwater-soluble vitamin B12 and is present in some foods, nutritional supplements and medications [3, 4].
In recent years, miniaturization of procedures for the determination of analytes has greatly developed and is now very much in use. There are several articles on the determination of cobalt in environmental and pharmaceutical samples using the liquid-phase microextraction (LPME) technique [5, 6]. However, the methods that have been reported employ relatively expensive and sophisticated instrumentation, such as atomic absorption spectrometry [7—12], inductively coupled plasma in combination with optical emission spectrometry or mass spectrometry [13—15], X-ray fluorescence spectrometry  and high-performance liquid chromatography . However, articles in which LPME techniques are combined with ultraviolet-visible (UV-Vis) spectrophotometry are published much less frequently . We found only two papers devoted
to the determination of cobalt using LPME-UV-Vis [19, 20].
Recently, several LPME techniques have been developed, and one very popular technique among researchers has been termed dispersive liquid—liquid microextraction (DLLME). The DLLME technique was introduced in 2006 by Rezaee et al.  and used for the determination of organic compounds ; however, it is nowadays also widely used for the determination of metal ions [22—24]. DLLME procedures are frequently coupled with GC, HPLC [25, 26] and atomic spectroscopy .
The aim of this work was to develop a simple procedure using a cinnamoyl derivative as a new ligand for the analytical determination of cobalt. The ligand 3-[4-(dimethylamino)cinnamoyl]-4-hydroxy-6-methyl-3,4-2H-pyran-2-one (Scheme) belongs to the "family" of cinnamoyl pyrones. Cinnamoyl pyrones are known as medications and also as intermediate products of medication syntheses . The structure, spectral properties, photophysical and photochemical processes of cin-namoyl pyrones and their derivatives are discussed by Tykhanov et al. [28—30]. Although this ligand was first
synthesised in 1932 by Rupe et al. , we are unaware of it being used for complex formation and subsequent spectrophotometric determination of analytes. The present work employs the reaction of Co(II) with a cinnamoyl derivative as ligand and dimethylindocar-bocyanine as dye reagent, followed by DLLME and subsequent UV-Vis spectrophotometric detection.
4| , U 9 11
Structure of 3-[4-(dimethylamino)cinnamoyl]-4-hydroxy-6-methyl-2H-pyran-2-one.
Reagents and chemicals. All chemicals and solvents used were of analytical grade. Distilled water was used throughout the experiment. A stock solution containing 10 mM of cobalt was prepared by dissolving of CoSO4 • 7H2O in water. The working standard solution of Co(II) was prepared by stepwise dilution of the stock solution to 0.1 mM. The ligand 3-[4-(dimethylami-no)cinnamoyl]-4-hydroxy-6-methyl-2H-pyran-2-one (L) is poorly soluble in water; therefore, it was instead dissolved in an extraction solvent. The 2.5 mM stock solution of L was prepared by dissolving L in toluene (^max = 460 nm), and the 1 mM aqueous solution of the dye reagent was prepared by dissolving dimethylin-docarbocyanine dye (DIC) in several droplets of methanol and subsequent dilution with water. The acidity of the aqueous phase was adjusted by the addition of acetic acid—ammonium hydroxide buffer solution.
Apparatus. A Lightwave II UV-Vis spectrophotometer (Biochrom Ltd., United Kingdom) equipped with 1 mm path length microvolume cell (5 ^L) was used for UV-Vis spectrophotometric measurements. The pH values of the solutions were measured using an ORION 720A+ pH meter with a glass electrode. Cen-trifugation was performed using a CN-2060 LED & Multifunctional centrifuge (MRC Ltd., Israel). NMR spectra were recorded using a Varian VNMRS NMR spectrometer (Varian, Palo Alto, USA) operating at 599.871 MHz for 1H and 150.838 MHz for 13C in ace-
the latter was also used
tone-D6 (EurisoTop, 99. as a reference signal (2.05 ppm for XH, 29.92 ppm for 13C) at 25°C. The full assigning of proton and carbon signals was achieved by concerted application of 2D NMR spectra (COSY, TOCSY, NOESY, HSQC, HMBC).
Calibration of the method. The aqueous phase standards in a final amount of 5 mL and containing cobalt in a range of 0.06—0.42 mg/L were put into 10 mL conical centrifuge tubes along with 1 mL of buffer solution of pH 9 and 0.14 mM of DIC. Afterwards, 500 |L of a
mixture of toluene containing 2.25 mM of L, carbon tetrachloride and acetonitrile was vigorously injected using an automatic pipette. In order to achieve sufficient mixing of the aqueous and organic phases, the conical centrifuge tubes were gently shaken three times by hand. Afterwards, the mixture was centrifuged at 3000 rpm for 2 min to accelerate the separation process. In the final step, the sedimented phase was removed by an automatic pipette and inserted into a microvolume cell (1 mm) for UV-Vis spectrophotomet-ric determination.
Procedure for analysis of vitamin B12. A 1 mL
pharmaceutical injection preparation containing 13.04 mg/L of cobalt (300 mg/L of vitamin B12) was dried over a flame. Then 2 mL of conc. H2SO4 (96%) was added. The solution was boiled until fumes of SO3 were evolved (according to Medina—Escriche et al. ). After dilution with water, the pH was changed from acidic to basic by using concentrated ammonia and diluted to final volume 20 mL. Aliquots of 1.5 mL were introduced into a conical centrifugal tube, and all of the necessary reagents were added. The determination was performed as described above in "Calibration of the method". Results were calculated as cobalt content in the pharmaceutical preparation.
RESULTS AND DISCUSSION
Reaction chemistry. The reaction chemistry of complex formation of Co(II) with L and subsequently the Co—L—DIC ion associate in organic phase can be expressed by the following scheme:
Co(a+q) + 3L(org) = [CoL](org) ,
[CoL3 j^ + DIC+aq) = [CoL3]-DIC+org)
Co(a+q) + DIC(aq) + 3L(org) — [C0L3] DIC(org),
where L- is the anion of the ligand, DIC+ is the cation of the dye reagent.
The coupling of L with Co(II) was also studied using high-resolution NMR spectroscopy in acetone-D6, which is a suitable solvent for both L and Co(II). A sample of L was diluted in 0.6 mL of the solvent in a 5 mm sample tube and the XH and 13C NMR spectra were recorded under neutral conditions.
1H NMR: 8 — 8.13 (d, 1H, 3J — 15.0 Hz, H-8), 7.98 (d, 1H, 3J — 15.0 Hz, H-9), 7.62 (d, 2H, 3J — 8.7 Hz, H-11,15), 6.82 (d, 2H, 3J — 8.7 Hz, H-12,14), 6.08 (s, 1H, H-5), 3.10 (s, 6H, 2 x NCH3), 2.28 (s, 3H, 1 x CH3) ppm; 13C NMR: 8 — 191.4 (C-7), 184.0 (C-4), 168.7 (C-6), 160.7 (C-2), 153.1 (C-13), 147.8 (C-9), 131.4 (C-11,15), 122.3 (C-10), 115.9 (C-8), 111.9 (C-12,14), 102.3 (C-5), 98.7 (C-3), 39.2 (2 x NCH3), 19.5 (1 x x CH3) ppm (Scheme).
0.5 1.0 1.5 2.0 2.5
0.05 0.08 0.11 0.14 0.17
Fig. 1. Effect of pH (1), concentration of L (2), concentration of DIC (3) on the formation and the extraction of the ion associate: 0.24 mg/L Co(II), 500 ^L mixture of solvents in a 1 : 1 : 8 (v/v/v) ratio, X = 558 nm, l = 1 mm, (1) - 0.14 mM of DIC and 2.25 mM ofL in toluene, (2) - pH 9 and 0.14 mM of DIC, (3) - pH 9 and 2.25 mM of L in toluene.
To check for the possibility of complexation in the organic phase, 1.22 mg of Co(NO3)2 ■ 6H2O, which is well soluble in acetone-D6, was added to 2 mg of L dissolved in acetone-D6 (0.6 mL). The low concentration of L resulting from its poor solubility in this solvent allowed only for XH NMR, not 13C NMR, monitoring of complexation. A comparison of the XH NMR spectra of the L and Co(II) mixture with the pure L solution showed practically no change in the proton chemical shifts in this neutral medium. We therefore modified the pH of L and also the pH of the L and Co(II) mixture to a value of 9.0 — the optimal value estimated from UV-Vis experiments — by addition of a calculated amount of NaOH in D2O. Afterwards, the spectrum of L showed signals from the main
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