Sensors and Actuators B 205 (2014) 215218

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Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

Short Communication

Flow-in f ydietary o

Lubomra Institute of An nologSK-812 37, Slovb Institute of Ch atz 1, G

a r t i c l

Article history:Received 29 AReceived in reAccepted 25 AAvailable onlin

Keywords:YohimbineBoron-doped dFlow-injectionDetection limiRecovery

ectionmonoten

Brittnal i) anevia1. The detausin

2014 Elsevier B.V. All rights reserved.

1. Introdu

Yohimbimethyl estacting as mof 2-recepand releasewith anxiebark of PauYOH as maof sexual d[4].

A surveywith varioution of YOHchromatogrliquid chrospectrometmethods ohowever, thple pretrea

CorresponE-mail add

http://dx.doi.o0925-4005/ ction

ne (17-hydroxy-yohimban-16-carboxylic acider, YOH) is a well-known indole alkaloid primarilyonoamine oxidase enzyme inhibitor [1]. It is antagonisttors which increase brain noradrenaline cell ring

[2]. The side effects of YOH are usually accompaniedty, headache and increased urinary output [3]. Thesinystalia yohimbe and Rauvola serpentina containsin species used for clinical and traditional treatmentysfunction and enhancement of sexual satisfaction

in scientic literature reveals some papers dealings analytical methods for the detection and quantica-. Most of them include separation methods such as gasaphy with mass spectrometry [5], high-performancematography (HPLC) with UV detection [6] and massry [7] as well as capillary electrophoresis [8]. Theseffer sensitive and selective determination of YOH,ey require long analysis time and time-consuming sam-tment processes as well as highly sophisticated and

ding author. Tel.: +421 0 2 59325302; fax: +421 0 2 59325590.ress: lubomir.svorc@stuba.sk (L . Svorc).

expensive instrumentation [9]. Prior to separation in chromato-graphic column, the general procedure for extraction and isolationof indole alkaloids including YOH, reserpine, ajmalicine etc. usuallyconsists of following steps: maceration in methanol, evaporation,dissolution in HCl, ltration, basication to the neutral pH, extrac-tion in chloroform, evaporation and nally dissolution in methanolwith subsequent ltration [10]. It was found that extraction andisolation procedure for these alkaloids may be modied depend-ing on the purpose of the particular work [11,12]. The separationmethodology with this procedure enables the studied indole alka-loids to be sensitively and selectively quantied. However, despitesignicant performance of separation methods, the development ofnovel, inexpensive, simple and rapid analytical alternative meth-ods for the sensitive determination of YOH and other alkaloids invarious matrices is still of great interest.

Electroanalytical techniques have conrmed to be excellentalternatives for the determination of various compounds, sincethey are simple, cheap and require relatively short analysis time.Moreover, they yield useful information about the kinetics andcharge transfer mechanisms of electrode reaction. RegardingYOH, there is a short report available on its basic voltammetriccharacterization using platinum and gold rotating disk electrodes[13]. In the aforementioned study, YOH rendered an oxidationpeak at a potential higher than +1 V vs. SCE in 0.1 M sulphuric acidwith signicant adsorption contribution not enabling so sensitive

rg/10.1016/j.snb.2014.08.0712014 Elsevier B.V. All rights reserved.jection amperometric determination o supplements using a boron-doped diam

Svorca,, Kurt Kalcherb

alytical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Techak Republicemistry, Department of Analytical Chemistry, Karl-Franzens University, Universittspl

e i n f o

pril 2014vised form 31 July 2014ugust 2014e 3 September 2014

iamond electrode analysist

a b s t r a c t

A simple, rapid and sensitive ow-injyohimbine using a boron-doped diainjection analysis were a detection pwith an injection volume of 100 L inresponse of yohimbine was proportioear segments (0.310 and 10100 Mgood repeatability (relative standard dwas calculated about 70 injections h

was successfully demonstrated in threcoveries (9498% and 9295%) for Pohimbine alkaloid innd electrode

y in Bratislava, Radlinskho 9, Bratislava

raz A-8010, Austria

method for the amperometric determination of the alkaloidd electrode has been developed. Best conditions for ow-tial of +1.20 V (vs. Ag/AgCl) and a ow rate of 1.4 mL min1

onRobinson buffer solution at pH 7 as a carrier. The currentn the concentration range from 0.3 to 100 M with two lin-d a low detection limit of 0.15 M (0.053 mg L1) as well astion of 4.5% for n = 10) were achieved. The sampling frequencye practical analytical usefulness of the developed procedureermination of yohimbine in dietary supplements with goodystalia yohimbe and Rauvola serpentina, respectively.

216 L . Svorc, K. Kalcher / Sensors and Actuators B 205 (2014) 215218

determination of YOH (also with higher background current). Asto the combination of separation and electrochemical techniques,an assay for the determination of YOH in human plasma utilizingHPLC with glassy (vitreous) carbon electrode as an amperometricdetector walimit for Yoxidation pas an internand very sewith reserppotential fowith very lresults, theand coulomquanticatipharmacok

Boron-dattention asin electroantial range, lstability asrepresents arials such aSome data relectrode hresearch grvoltammetrmetry usingdation peakthe oxidatioelectrons andened peation of the cbuffer soluappeared to0.13 M. Thment was dYOH in extrP. yohimbe a

In order ow-injectiric detectiogood sensittively shortfor the sensrent reportand rapid Fmination ofpapers for tBDD electro

2. Experim

Yohimbifrom Sigmacation. Its it in deionBrittonRobphoric acidthe pH withchemicals wresistivity nused for pre

The owWaters, Mildyne, Cotat

A) HydDD elmL milues o

the chem

owc Ltdopiouser (0.de, ar to de w

piec was

ow 0.5 Met rid

for. Sucthe rdyna

+1.5erfo

e of 1riginta. Thnd t

d by tary ) wef powdered bark was macerated with 20 mL methanol andated to dryness at 70 C. The residue was dissolved in 30 mL%, w/w) and ltered. The pH of the ltrate was adjusted to

0.1 M NaOH; it was then extracted three times with 20 mLform. The chloroform layers were combined and evaporatedness; the residues were subsequently dissolved in 20 mLnol and ltered [10]. The extract was made up to 50 mL withting electrolyte in a volumetric ask. A 100 L aliquot ofple was injected into the FIA system; concentrations were

ted with the calibration curve.

ults and discussion

is a biologically, structurally and electrochemically inter-indole alkaloid capable of transferring electrons from then to the electrode surface. Fig. 1 displays the hydrody-voltammogram (HDV) for the electrochemical oxidation ofs described by Goldberg et al. [14]. The low detectionOH of 0.025 M was obtained by application of theotential of +0.95 V vs. Ag/AgCl electrode and reserpilineal standard. Hariharan et al. [15] developed a simplensitive HPLC method with coulometric detection alsoiline as internal standard. The optimum oxidationr YOH was found to be +0.80 V vs. Ag/AgCl electrodeow detection limit of 0.51 nM. On the basis of these

combination of HPLC technique with amperometricetric detection appeared to be a sensitive platform foron of YOH in biological samples and could be used forinetic study of indole alkaloids in humans.oped diamond (BDD) has recently attracted a great

modern electrode material opening new possibilitiesalysis due to its superior properties such as wide poten-ow background current, high sensitivity and long-term

well as negligible adsorption of polar species [16]. Itn effective alternative to conventional electrode mate-s platinum, graphite, glassy carbon and carbon paste.egarding the electrochemical behavior of YOH on BDDave been reported by Swain and co-workers [17]. Ouroup has recently published the paper regarding theic determination of YOH by differential pulse voltam-

BDD electrode [18]. In this study, two irreversible oxi-s, a distinct (quantication) peak at +0.80 V assigned ton of the hydroxyl group of YOH including the loss of twod two protons to give a ring ketone and a second poorlyk at +1.65 V associated with the oxidative deprotoniza-arbon at position 6, were observed in BrittonRobinsontion at pH 7. The linear concentration range was

be from 0.25 to 90.9 M with the detection limit ofe practical usefulness of this method in batch arrange-emonstrated by the assessment of the total content ofacts of the primary bark of natural aphrodisiacs such asnd R. serpentina with recoveries in the range of 9297%.to facilitate a high throughput of samples to be analyzed,on analysis (FIA) technique coupled with amperomet-n presents favorable characteristics, such as simplicity,ivity, low cost and consumption of reagents, and rela-

analysis time [19]. This system has been widely useditive determination of various species [20,21]. The cur-

shortly describes the development of a novel, simpleIA methodology for the sensitive amperometric deter-

YOH on BDD electrode. In addition, there have been nohe FIA determination of YOH and related alkaloids ondes published in the literature until now.

ental

ne hydrochloride (purity 98%, YOH) was purchasedAldrich (Austria) and used without any further puri-stock standard solution was prepared by dissolvingized water; it was stored in a refrigerator at 46 C.inson buffer solution was prepared by mixing of phos-

, acetic acid and boric acid (all at 40 mM) and adjusting sodium hydroxide (0.2 M) to the desired value. All otherere of analytical reagent grade. Deionized water withot less than 18 M cm (Millipore Milli-Q system) wasparation of all solutions.-injection system consisted of a peristaltic pump (510ford, MA, USA), a sample injection valve (MX7925 Rheo-i, USA), and a home-made amperometric thin-layer ow

Fig. 1. (with a Brate 1.4 mean va

cell inelectrometricScientiboron an in ha spaceelectro

Prioelectrowith asurfacemetric60 s inface (gat 2 Vsurfacevided Hydro+0.6 towere pvolum8.0 (Othe dathree adivide

Diepentinagram oevaporHCl (27 withchloroto drymethasupporthe samevalua

3. Res

YOHesting solutionamic rodynamic voltammogram of 0.1 mM YOH obtained by a FIA systemectrode as detector in BRBS at pH 7; injection volume 100 L, own1; YOH 0.1 mM. (B) The dependence of the signal on pH; all data aref three measurements.

three-electrode conguration in combination with anical workstation (BAS 100B). The thin-layer ampero-

cell was constructed with the BDD electrode (Windsord., UK; inner diameter 3 mm, resistivity of 0.075 cm,ng level 1000 ppm), which was inserted into a whole of-made Teon plate acting as the back part of the cell,15 mm), a miniaturized Ag/AgCl (3 M KCl) as referencend a steel back plate served as the counter electrode.use at the beginning of every work way, the bare BDDas rinsed with deionized water and rubbed very gentlye of damp silk cloth until a mirror-like appearance of

obtained. Subsequently before inserting into ampero- cell, it was anodically pretreated by applying +2 V for

H2SO4 solution in order to clean the electrode sur- of any impurities) followed by cathodic pretreatment

60 s to retain a predominantly hydrogen-terminatedh electrochemically pretreated working electrode pro-eliable current response during the whole work day.mic voltammograms were recorded at potentials from

V with an increment of 0.1 V. Flow-injection analysesrmed at a potential of +1.2 V with constant injection00 L if not stated otherwise. The program OriginPro

Lab Corporation, USA) was used for the evaluation ofe detection and quantication limit were calculated asen times the standard deviation for the blank solutionthe slope of the calibration curve.supplements (primary bark from P. yohimbe and R. ser-re purchased in a local shop in Vienna (Austria). One

L . Svorc, K. Kalcher / Sensors and Actuators B 205 (2014) 215218 217

Fig. 2. Flow-inexperimental injection voluminset (error ba

0.1 mM YO7 in a FIA current (mesponding apelectrode w1.4 mL min

As can bat approximvalue at +1.5+1.5 V yieldlarge curreninterferenceitive potenta detectionin all subseior was alsosimilar charcurrents (rebuffer was the determ

The effestudied by Fa detectionobtained reYOH when increase shdecreased s7 representfor the FIA the ow ratined. Increathickness oincrease of ally, also thstreaming bcarrier. A m1.4 mL min

a detectionconstant inmum expermeasureme

Once themination ofconcentratiperformanc

Table 1Analytical characteristics for FIA amperometric determination of YOH using pro-posed method (n = 3).

Analytical parameters Value

ion poing fre

conceept (nrd de(nA Mrd deient o

iona (Rion limicati

ulated

ed focont). Th

nshipented

evidnts, fty. Tcal etics f

100 detenseq

MpropM YOntervs 4.5strator suncy wnvesor thas p

se ofd aj

ring ometD eleactiv

excidatioood jection amperograms of various concentrations of YOH at optimizedconditions: detection potential of +1.20 V, ow rate of 1.4 mL min1,

e of 100 L and BRBS at pH 7. The calibration curve appears in thers from n = 3).

H in BrittonRobinson buffer solution (BRBS) at pHsystem. The HDV was obtained by plotting the peakan value of three injections) as a function of the corre-plied detection potential (Edet) using BDD as a workingith an injection volume of 100 L and a ow rate of1.e seen, the oxidation current of YOH started to increaseately +0.7 V and leveled off at +1.2 V with maximum

V (330 nA). However, detection potentials from +1.3 toed worse-shaped responses with high backgrounds andt uctuations. Hence, in order to minimize the potentials of other species (alkaloids) oxidizing at higher pos-ials and to maintain a relative low background signal,

potential of +1.2 V was selected as the most suitablequent FIA measurements. The electrochemical behav-

investigated in phosphate and acetate buffers showingacteristics of the HDVs, however, with lower oxidationsults not shown). Therefore, based on this fact, BRBSadopted as an appropriate supporting electrolyte forination of YOH using FIA system.ct of pH on the oxidation current of 0.1 mM YOH wasIA measurements in the pH range of 212 (BRBS) with

potential of +1.2 V and ow rate of 1.4 mL min1. Thesults indicated a slight enhancement of the current ofincreasing the pH from 2 to 5; beyond pH 5 it started toarply and reached a maximum at pH 7 beyond which itharply as depicted in the inset of Fig. 1. Therefore, pHed the most suitable pH of the supporting electrolyte

DetectSamplLinearIntercStandaSlope StandaCoefcPrecisDetectQuant

a Calc

recordlicate) 100 Mrelatiois pres

It issegmelinearistatistiacterisAbove

Theas a co(0.3-10of the of 10 time ilated ademondetectfreque

To iology fstudy wresponpine aninterfeamperthe BDical in20-foldthe oxvides gamperometric determination of YOH. The inuence ofe in the range from 0.6 to 1.4 mL min1 was also exam-sing the ow caused a diminution of the diffusion layern the BDD electrode surface consequently providing anthe diffusion current with narrower peaks. Addition-e dispersion of the analyte, due to its residence in theulk, is less pronounced with higher velocities of theaximum value was obtained above the ow rate of1 that was selected for further. Overall, BRBS at pH 7,

potential of +1.2 V, a ow rate of 1.4 mL min1 with ajection volume of 100 L were considered to be opti-imental conditions for the quantication of YOH in FIAnts.

most suitable experimental conditions for the deter- YOH were established, FIA measurements at differentons of YOH were carried out to examine the analyticale of proposed method. Fig. 2 depicts FIA amperograms

The extryohimbe anvalidity andcedure. Thethe accuracingredientsences. The ranged frompentina, resmatrix intequantitativguarantee oYOH in the

In conclcedure witworking elements. Thetential (V vs. Ag/AgCl) +1.20quency (injections h1) 70ntration range (M) 0.310 10100A) 3.0 40viation of intercept (nA) 0.3 4

1) 6.2 2.1viation of slope (nA M1) 0.2 0.1f determination (R2) 0.995 0.992SD %) 4.5it (M) 0.15

on limit (M) 0.50

for 10 replicate FIA measurements at 10 M YOH.

r injections of 100 L of standard solutions (in trip-aining increasing concentrations of YOH (from 0.3 toe respective calibration curve with the obtained linear

between oxidation current and concentration of YOH in the inset of Fig. 2.ent that the calibration graph consists of two linearrom 0.3 to 10 M and from 10 to 100 M with goodhe mean data of both segments of calibration curve,valuation of the regression lines and the analytical char-or the developed method are summarized in Table 1.M the signal levels off.

ction limit was calculated to be 0.15 M (0.053 mg L1)uence of the high S/N ratio using rst linear segment) of the calibration graph. The precision (repeatability)osed method was evaluated by ten replicate injectionsH under the same operating conditions over the short

al. The relative standard deviation (RSD) was calcu-% conrming a good repeatability of the procedure anding only minimal adsorption of components onto therface present in the analyzed solution. The samplingas calculated as about 70 injections h1.

tigate the possibility of applying proposed FIA method-e amperometric determination of YOH, an interferenceerformed for assessing the matrix effect on the current

YOH. Structurally related indole alkaloids such as reser-malicine were being considered to be most signicantagents in the bark samples. However, a more detailedric study of these alkaloids under FIA conditions withctrode using BRBS at pH 7 revealed their electrochem-ity. The results of interference study showed that aess of reserpine and ajmalicine had negligible effect onn current of YOH. Thus, the proposed procedure pro-

selectivity for the amperometric determination of YOH.acts from dietary supplements (primary bark from P.d R. serpentina) were analyzed in order to evaluate the

the practical applicability of the developed FIA pro- recovery analysis was performed in order to estimatey of the proposed method and to know whether the present in the analyzed extracts show any interfer-results are summarized in Table 2. The recovery values

94 to 98% and from 92 to 95% for P. yohimbe and R. ser-pectively demonstrating that there are no signicantrferences in the analyzed samples. Thus, YOH can beely determined by the proposed method, being thus af the accuracy and suitability of the determination ofsamples of this kind.usion, we successfully developed a sensitive FIA pro-h amperometric detection using a BDD electrode asctrode for the determination of YOH in dietary supple-

proposed method is considerably more cost-effective

218 L . Svorc, K. Kalcher / Sensors and Actuators B 205 (2014) 215218

Table 2Recovery analysis of YOH in the extracts of Pausinystalia yohimbe and Rauvola serpentina using proposed method (n = 3).

Sample Found (M) Added (M) Determineda (M) Recovery (%)

Pausinystalia yohimbe11.5 10 20.2 0.6 94

20 30.9 0.5 9830 40.7 0.7 98

Rauvola serpentina13.5 10 21.9 0.3 93

20 30.8 0.3 9230 41.3 0.5 95

a Condenc

than separaanalytical pwith that rewith the lindetection liposed FIA mwith samplmethod renlimit of 0.1to the HPLdetection [1sideration tdeveloped floids in biolin the quanthis sense, ble applicatand can besupplemen

Acknowled

This worRepublic (gment Agencbilateral proedged.

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tion methods, especially HPLC and GC techniques. Theerformance of herein described method is comparablecently achieved on BDD electrode in batch arrangementear concentration range from 0.25 to 90.9 M and themit of 0.13 M [18]. However, the signicance of pro-ethodology embodies in rapid determination of YOH

ing frequency about 70 injections h1. The developedders the assessment of YOH traces with the detection5 M which is 6 and 300-fold higher when comparedC method with amperometric [14] and coulometric5], respectively. However, it could be taken into con-hat these combined techniques have been explicitlyor pharmacokinetic study of YOH and other indole alka-ogical samples. In addition, our approach primarily liestication of YOH in P. yohimbe and R. serpentine. Inthe BDD electrode fullls the requirements for possi-ions in FIA systems as sensitive amperometric sensor

easily adapted for quality control analysis of dietaryts containing various alkaloids.

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k has been supported by the Grant Agency of the Slovakrant no. 1/0051/13), the Slovak Research and Develop-y under the contract no. APVV-0797-11. Support of thegram Action: Austria-Slovakia is gratefully acknowl-

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s

Svorc is a young researcher under the age of 35. He was graduatedceived his PhD in 2009 in Analytical Chemistry in Slovak University

in Bratislava, Slovak Republic. His interest covers the developmentization of new electrode materials for application of electrochemicalosensors based on boron-doped diamond for solving the tasks of food,vironmental trace analysis. At his young age, he is author and co-

e than 60 publications.

. Dr. Kurt Kalcher is the head of the work group Electroanalysis andhe Institute of Chemistry-Analytical Chemistry of the Karl-FranzensGraz, Austria. His main scientic interest lies in the development ofal sensors and biosensors based on carbon paste and other carbona-

ls. Besides one of another focus is the development of simple analyticaling microprocessor control and data acquisition and handling. He hasuthored around 200 publications.

Flow-injection amperometric determination of yohimbine alkaloid in dietary supplements using a boron-doped diamond electrode1 Introduction2 Experimental3 Results and discussionAcknowledgementsReferences

Biographies