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Department of Chemistry, Tunghai Unive

cwwhang@thu.edu.tw; Fax: +886-4-2350647

Cite this: Anal. Methods, 2013, 5, 6186

Received 27th June 2013Accepted 1st September 2013

DOI: 10.1039/c3ay41059e

www.rsc.org/methods

6186 | Anal. Methods, 2013, 5, 6186

A simple and rapid screening method for glyphosate inwater using flow-injection withelectrochemiluminescence detection

Hao-Yun Chuang, Tsen-Pei Hong and Chen-Wen Whang*

A flow-injection (FI) system coupled with the electrochemiluminescence (ECL) detection method for rapid

screening of glyphosate (GLY) in water has been developed. A wall-jet type ECL detector coupled with an

indium/tin oxide (ITO)-coated glass working electrode was modified from a thin-layer electrochemical cell,

and tris(2,20-bipyridyl)ruthenium(II) (Ru(bpy)32+)-based ECL reaction was used for the sensitive detection of

GLY. Under optimized conditions, the linear range of the method was 0.03–2.81 mg L�1 GLY with R2 ¼0.9998 (n ¼ 7). Repeatability was 5.3% (n ¼ 27) and the detection limit (3s) was 0.03 mg L�1 GLY.

Sample throughput was about 100 injections h�1. Possible interference from some multivalent cations

can be readily eliminated by passing the acidified water sample through a solid-phase extraction

column packed with strong cation-exchange materials before analysis. The method was examined by

application to surface waters including tap water and irrigation water.

1. Introduction

Glyphosate (N-(phosphonomethyl)glycine; GLY) is a broad-spectrum herbicide widely used to kill unwanted plants both inagricultural and in nonagricultural landscapes. Due to its rela-tively low mammal toxicity, GLY has become the most exten-sively used herbicide worldwide. However, GLY resistance inweeds has evolved under current GLY usage, with little or nodiversity in weed management practices.1 Besides, its indis-criminate application also generates some concerns regardingthe possible health hazard and environmental contaminationcaused by GLY.2,3In order to protect human health, variouscountries have set up maximum residual limits of GLY in waterand crops. The US Environmental Protection Agency (EPA),4

Health Canada,5 and China's Ministry of Water Resources6 haveset the maximum acceptable concentration (MAC) of GLY indrinking water at 0.7, 0.28 and 0.7 mg L�1, respectively.In Taiwan, the MAC of GLY in effluent was set at 1.0 mg L�1.7 Inthis regard, there is a growing need to develop a rapid andsensitive method for monitoring GLY at residue levels in envi-ronmental waters.

Various methods have been reported for quantitative anal-ysis of GLY in water, environmental matrices and agriculturalproducts.8 It is known that GLY oen represents an analyticalchallenge because of its high water solubility and organicsolvent insolubility, which make the extraction and pre-concentration of GLY from sample matrices difficult. Moreover,since GLY possesses no chromophore or uorophore in its

rsity, Taichung 40704, Taiwan. E-mail:

3; Tel: +886-4-23590248 ext. 311

–6191

molecular structure, both pre- and post-column derivatizationshave therefore been employed to GC,9,10 HPLC11,12 and CE13,14 ofGLY followed by spectrophotometric detection. However, theprocedure of derivatization is always tedious and time-consuming, and sometimes generates unstable products.Non-derivatization methods, such as indirect UV detection,15

indirect uorescence detection,16 electrospray ionization-MSdetection,17 amperometric detection,18 coulometric detection,19

contactless conductivity detection,20 electrochemiluminescence(ECL) detection,21 and inductively coupled plasma (ICP)-MSdetection22 have also been coupled to HPLC or CE for GLYanalyses. These methods oen suffer from drawbacks such asinadequate sensitivity and the requirement of costly apparatus.In addition, due to the inclusion of a chromatographic or anelectrophoretic separation process in the method, the speed ofanalysis, and therefore sample throughput, is always low. Thecommercial immunoassay kit for GLY can provide fast, selectiveand highly sensitive screening analysis,23 but it is highlyexpensive. As a screening method, it should be able to selectfrom an initial set of samples, those that are above a pre-setconcentration level in a simple, fast, economic and reliable way.Only those samples that provide a positive response need to befurther processed with a more sophisticated and expensiveanalytical method.

Flow injection (FI)24 analysis is a simple, rapid, and versatiletechnique that is now rmly established, with widespreadapplication mainly in environmental and clinical areas. Deter-mination of GLY by the FI method is not common. Adcocket al.25 reported a FI-chemiluminescence (CL) method for theanalysis of GLY in industrial and commercial formulations. Theactive CL reagent, tris(2,20-bipyridyl)ruthenium(III) (Ru(bpy)3

3+),

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was chemically generated using a lead dioxide recirculatingsystem. Colombo and Masini26 developed a sequential injectionmethod for the uorometric determination of GLY based on theadaptation of the post-column derivatization reaction betweenglycine and o-phthaldialdehyde in the presence of 2-mercap-toethanol, applicable to soil and sediment samples. Recently,Silva et al.27 published a screening method for the determina-tion of GLY in water samples using a multi-pumping owsystem. The method was based on the reaction between GLYand p-dimethylaminocinnamaldehyde in an acid mediumwhere the reaction product can be measured spectrophoto-metrically. In comparison with chromatography- or electro-phoresis-based methods, FI is advantageous as a screeningmethod due to its high sample throughput, simple apparatusand versatile detection schemes.

In this paper, we describe a simple, rapid, and sensitivescreening method for GLY in water samples using FI coupledwith Ru(bpy)3

2+-based ECL detection. This detection strategyhas been successfully employed to HPLC28 and CE21,29 of GLY.The applicability of the developed FI-ECLmethod was examinedby analyzing GLY in surface water samples.

Fig. 1 (A) Schematic diagram of the FI-ECL detection system; (B) exploded viewof the ECL detector.

2. Experimental2.1. Apparatus

The FI-ECL detection system was assembled in the laboratory.The FI system consisted of a Rainim model RP-1 peristalticpump (Emeryville, CA, USA), a Rheodyne model 9125 injectionvalve with 10 mL sample loop, an interconnecting Teon tubing(250 mm i.d.) and a wall-jet type ECL detector modied from acommercial Zensormodel SF-100 thin-layer electrochemical cell(Taichung, Taiwan). The schematic diagrams of the FI-ECLsystem and the ECL detector are represented in Fig. 1.

A 1 cm � 3.5 cm indium/tin oxide (ITO)-coated glass plate(Delta Technologies, Stillwater, MN, USA), situated at the bottomblock of the cell, was used as the working electrode for in situgeneration of the active Ru(bpy)3

3+. The potential of the ITOelectrode (vs. Ag/AgCl reference) was controlled with a BAS(Bioanalytical System, Inc., West Lafayette, IN, USA) modelLC-4C potentiostat. The stainless steel outlet tubing also func-tioned as a counter electrode. A small cone-shaped hole wasdrilled on the cell body under the ITO plate, and the emitted ECLwas captured using a Hamamatsu R928 photomultiplier tube(PMT; Hamamatsu city, Japan) biased at �930 V and positionedunder the detection cell. The photocurrent was amplied by aKeithley model 485 picoammeter (Cleveland, OH, USA), con-verted to voltage and recorded using a PC equipped with a dataacquisition interface. The whole ECL detector was held in a light-tight box constructed from black Plexiglass to exclude stray light.

2.2. Chemicals

Glyphosate (GLY, 96%) and tris(2,20-bipyridyl)ruthenium(II)chloride (Ru(bpy)3Cl2, 99.95%) were purchased from Aldrich(Milwaukee, WI, USA). All other chemicals were of analytical-reagent grade. All solutions were ltered through a 0.45 mmpore-size membrane lter before use.

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2.3. Sample preparation

Tap water was acquired from the campus and irrigation waterwas collected from a local rice eld. Waters were collected inclean polypropylene (PP) bottles, acidied to pH 2 with nitricacid, ltered through a 0.45 mmpore-size nylonmembrane lterand stored at 4 �C. Before analysis, 3 mL of water sample waspassed through a Varian Bond Elut SCX column (500 mg/3 mL;H+ form) at a rate of 0.5 mL min�1. A 100 mL working solutionwas prepared by mixing 10 mL of 40 � 10�3 mol L�1 aqueousRu(bpy)3

2+ solution with 90 mL of the above percolate. Thissolution was used for FI-ECL analysis. The GLY-spiked samplewas prepared by adding appropriate amounts of GLY standardsolution into the acidied water sample prior to membraneltration and solid-phase extraction pre-treatment.

3. Results and discussion3.1. Characteristics of the FI-ECL system

It is known that the CL emission can be generated via reactionbetween tertiary or some secondary amines and Ru(bpy)3

3+. The

Anal. Methods, 2013, 5, 6186–6191 | 6187

Table 1 Optimal conditions for FI-ECL analysis of GLY

Parameter Condition

Carrier solution 20 � 10�3 mol L�1 sodiumphosphate buffer

pH of buffer solution 9.0Ru(bpy)3

2+ concentration 4.0 � 10�3 mol L�1

Flow rate 0.26 mL min�1

ITO electrode potential 1.6 V (vs. Ag/AgCl reference)Voltage of PMT �930 V

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latter can be formed by either chemical or electrochemicaloxidation of Ru(bpy)3

2+.30 However, the active Ru(bpy)33+

reagent produced by the chemical method oen suffers fromthe problem of limited temporal stability with respect toreduction. Despite that some on-line chemical generationsystems of Ru(bpy)3

3+ had been developed to overcome theproblem, to date, electrochemical oxidation is still the mostpredominant means of Ru(bpy)3

3+ production which canprovide a reliable and constant source of Ru(bpy)3

3+. Since GLYis structurally a secondary amine, the possible mechanism ofthe CL reaction between GLY and electrochemically generatedRu(bpy)3

3+ can be depicted as follows:31

Ru(bpy)32+ � e� / Ru(bpy)3

3+

GLY � e� / [GLY_ ]+ / GLY_+ H+

Ru(bpy)33+ + GLY_/ Ru(bpy)3

2+* + products

Ru(bpy)32+* / Ru(bpy)3

2+ + hn

In conventional FI with Ru(bpy)32+-based CL25,32 or ECL33

detection, a manifold ow assembly was always used tocontinuously deliver the buffer carrier, Ru(bpy)3

2+ reagent, andcatalyst to the reaction/detection zone. In our system, just asingle ow channel was employed to deliver the buffer solutionwhile the Ru(bpy)3

2+ reagent was concomitantly injected withthe sample solution via a six-port loop injector. With this owscheme, signicant reduction in the consumption of theexpensive Ru(bpy)3

2+ reagent can be achieved. Besides, ow rateadjustment is much easier.

The wall-jet type ECL detector was modied from acommercial thin-layer electrochemical cell specically designedfor use with disposable screen-printed electrodes (SPEs).34 Thecell was made of foldable polyoxymethylene thick platelets withthe bottom portion consisting of a cavity track to drag the SPE inposition (see Fig. 1B). Since the chosen ITO-coated glass platehas the same thickness (0.7 mm) as the disposable SPE, if cut tothe appropriate length and width, it can readily t the tray-cavityarrangement of the cell assembly. The wall-jet design throughthe center micro-hole can guide the inlet eluent to functionproperly on the ITO working electrode. In order to collect theemitted ECL from the back of the transparent ITO glass plate, asmall cone-shaped hole was drilled on the bottom block of thecell assembly precisely opposite to the inlet micro-hole on thetop block. The major advantage of this button-and-lock typeECL detector is its ease of assembling the detection cell andchanging the sensing electrode.

Fig. 2 FI-ECL responses of GLY. Concentrations of GLY (mg L�1): (a) 0; (b) 0.03; (c)0.06; (d) 0.14; (e) 0.28; (f) 0.56; (g) 1.40; (h) 2.80.

3.2. Optimization of experimental conditions

The observed CL intensity in the FI-ECL system was dependenton the operating parameters such as concentration ofRu(bpy)3

2+, detection potential, pH, and ow rate of buffercarrier. During optimization, effects of various parameters wereinvestigated using a univariate approach. All the studies wereperformed with a 2.1 mg L�1 GLY standard solution.

6188 | Anal. Methods, 2013, 5, 6186–6191

The effect of Ru(bpy)32+ concentrations on the ECL intensity

of GLY was rst examined in the concentration range of 0.2–6.0 � 10�3 mol L�1 Ru(bpy)3

2+. The results showed that the ECLintensity increased continuously with increasing Ru(bpy)3

2+

concentration. A relatively constant peak size was observed aerthe Ru(bpy)3

2+ concentration reached 4.0 � 10�3 mol L�1. Aninjected sample solution of GLY concomitantly containing 4.0�10�3 mol L�1 Ru(bpy)3

2+ was chosen as the optimal sample.The optimal detection potential for GLY was investigated in

the range of 1.0–2.0 V (vs. Ag/AgCl reference). The results indi-cated that the largest ECL signal was obtained at 1.6 V. Thispotential was therefore selected as the optimal potential.

It is known that the ECL intensity for the reaction ofRu(bpy)3

2+ with amines is dependent upon pH values. The effectof buffer pH on the ECL of GLY was examined in the pH range of7–10. With a 20 � 10�3 mol L�1 sodium phosphate buffer as thecarrier solution, the largest signal was obtained at pH 9.0.Accordingly, this pH was selected as the optimal.

The effect of ow rate on the ECL intensity of GLY wasinvestigated in the range of 0.10–0.56 mL min�1. It was foundthat the ECL intensity of GLY increased with increasing owrate in the range of 0.10–0.26 mL min�1. When the rate washigher than 0.26 mL min�1, the ECL intensity decreased,probably due to the less contact time between the analyte andthe ITO electrode surface under a high ow rate. Therefore, aow rate of 0.26 mL min�1 was used for all analyses.

The optimal conditions for FI-ECL analysis of GLY are listedin Table 1.

This journal is ª The Royal Society of Chemistry 2013

Table 2 Comparison of flow-injection methods for the analysis of GLY

Detection Linear range (mg L�1) LOD (mg L�1) Sample Reference

Chemiluminescence 0.002–0.034 0.001 Industrial/commercialformulations

25

Fluorescence 0.043–4.3 0.014 Soil and sediment 26Spectrophotometry 0.5–10 0.17 Water 27Electrochemiluminescence 0.03–2.81 0.03 Water This work

Fig. 3 Repeatability of continuous injection of 1.0 mg L�1 GLY (n ¼ 27).

Table 3 Effect of coexisting ions on ECL intensity of GLY at 1.0 mg L�1

SpeciesConcentration(mg L�1)

Change inECL intensity

Na+, K+, Mg2+, Ba2+, Zn2+, Cd2+, Cl�, 200 <5%

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3.3. Analytical performance

Under the optimum conditions, a calibration graph was con-structed in the concentration range of 0.03–2.81 mg L�1 GLY.Aer subtracting the average peak height of the blank, thecorrected peak heights (mV) were plotted against the concen-tration (mg L�1) of GLY analyzed. Using the least-squaremethod, the regression equation was found to be y ¼ 189.3x �1.3 (R2 ¼ 0.9998; n¼ 7). Fig. 2 shows typical FI-ECL responses ofGLY standards. The limit of detection (LOD), based on 3�standard deviation of blank height, was calculated to be0.03 mg L�1 GLY. This detection sensitivity is similar to thatobtained with the HPLC-ECL method,28 but somewhat inferiorto that obtained with the FI-CLmethod using on-line chemicallygenerated Ru(bpy)3

3+.25 Comparison of linear range and LODvalues for GLY among various FI methods is shown in Table 2.Further, a lower LOD value with the present method can beobtained by increasing the volume of sample solution injected;however, the present LOD has been adequate enough forscreening residual GLY in drinking water and surface water atthe MAC levels set by various countries.4–7 Repeatability of themethod was examined by 27 consecutive injections of1.0 mg L�1 GLY into the FI-ECL system, as illustrated in Fig. 3.The RSD of peak height was calculated to be 5.3%. The samplethroughput was about 100 injections h�1.

NO3�, HCO3

�, HSO4�, H2PO4

�,CO3

2�, SO42�, glufosinate, AMPAa

Fe2+ 200 �9%Ca2+ 200 �18%Pb2+ 200 �35%Cu2+ 200 �52%Al3+ 200 �49%

50 <5%Fe3+ 200 �76%

20 <5%Humic acids 10 <5%

a AMPA ¼ aminomethylphosphonic acid.

3.4. Interferences

The possible interference of foreign ions in water was studied byanalyzing a standard solution of 1.0 mg L�1 GLY to whichvarious amounts of salts containing foreign ions had beenadded. The obtained signals were compared with a pure GLYstandard at the same concentration. A species was considerednot to interfere if it caused a relative signal change less than 5%.The effect of possible interference on GLY analysis is shown inTable 3.

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It can be seen that the most commonly found cations andanions in natural waters did not interfere at a 200-foldconcentration ratio of foreign ions to GLY. Glufosinate, anothercommonly used phosphonic and amino acid group-containingpesticide, and aminomethylphosphonic acid (AMPA), the majormetabolite of GLY, also did not interfere. Since natural organicmatter (NOM) concentration in surface water typically rangesfrom 0.1 mg L�1 to 20 mg L�1 and is mainly composed of humicsubstances,35 humic acids at 10 mg L�1 level were examined toassess the possible effect of NOM on the analysis of GLY inwater. No interference due to the co-existence of humic acidswas observed. However, a signicant suppression of ECL signalintensity due to Fe2+, Ca2+, Pb2+, Cu2+, Al3+ and Fe3+ was found,probably due to the formation of complexes between phos-phate-bearing GLY and some multivalent cations.36 Mostinterferences caused by divalent cations can be effectivelyeliminated by rst acidifying the water sample to pH 2 followedby pretreating the sample with a disposable solid-phaseextraction column packed with strong cation-exchange (SCX)materials. Results indicated that interference in the detection of1.0 mg L�1 GLY standard was insignicant (<5%) at a 200-foldconcentration ratio of interfering cations to GLY if the acidiedwater sample was passed through a Bond Elut SCX column(Varian) in the H+ form prior to FI-ECL analysis. Interferencedue to trivalent cations, e.g. Al3+ and Fe3+, was more trouble-some. We found that the tolerance concentration ratios for Al3+/GLY and Fe3+/GLY at 1.0 mg L�1 GLY level were 50 and 20,respectively. Generally, in waters with higher dissolved organiccarbon (DOC), such as surface water, some of these

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Fig. 4 Analysis of GLY in (A) tap water and (B) rice field irrigation water. Signalidentities: (a) original water; (b) water spiked with 1.0 mg L�1 GLY.

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interferences will be reduced due to a partial complexation ofinterfering cations in the organic matter.

3.5. Applications

The applicability of the method was examined by analyzing GLYin two authentic samples, viz., tap water and rice-eld irrigationwater. The results are shown in Fig. 4. No detectable GLY wasfound in either water sample. On the other hand, a signicantECL signal could be observed in waters fortied with 1.0 mg L�1

GLY. Recoveries of spiked GLY were 97% and 95% for tap waterand irrigation water, respectively, as determined with themethod of standard addition.

4. Conclusion

A ow-injection method coupled with Ru(bpy)32+-ECL detection

for analyzing trace GLY in water has been developed. The singleow channel FI system including the wall-jet type ECL detectorwas easily assembled. As a screening method for residual GLYin water, this method has the advantages of simple apparatus,selective and sensitive detection capability, and high samplethroughput. Besides, screening water samples for GLY at theMAC level can be readily performed without any derivatizationor pre-concentration treatment. In comparison with the similarFI-CL method using the chemically generated active reagentRu(bpy)3

3+,25 the main advantages of this FI-ECL methodinclude signicant reduction in consumption of the expensiveRu(bpy)3

2+ reagent and simple ow assembly. This method isapplicable to the rapid screening of residual GLY in drinkingwater and surface water.

Acknowledgements

Financial support from the National Science Council of Taiwanis gratefully acknowledged.

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