Research Article

Electrogenerated ferrate(VI) forCE–chemiluminescence detection todetermine benzenediol isomers

Based on the quenched chemiluminescence intensity of the luminol reaction sensitized

by ferrate(VI) in alkaline medium, a novel CE with on-line inhibited method for the

simultaneous analysis of benzenediol isomers was developed. The ferrate(VI) solution was

freshly prepared by electrochemical technique before electrophoresis. The parameters

influencing detection and separation were carefully investigated. Baseline separation of

benzenediols including catechol, resorcinol, and hydroquinone was achieved in less than

8 min with 5.0 mmol/L sodium tetraborate and 2.0 mmol/L luminol at an applied voltage

of 18 kV. The LODs (S/N 5 3) for catechol, resorcinol and hydroquinone were determined

to be 8.5� 10�9, 8.0� 10�7 and 6.5� 10�9 mol/L, respectively. Finally, the proposed

method was applied for phenolic compounds in hair dye.

Keywords: Benzenediol isomers / CE / Chemiluminescence / Ferrate(VI) / Hair dyeDOI 10.1002/jssc.200900511

1 Introduction

CE, with its high separation efficiency, minimal reagent

consumption, short analysis time and low-cost analyses, has

been widely investigated as a powerful and prevailing

analytical separation tool. It is commonly combined with

UV/vis spectrometry, LIF, MS and chemiluminescence (CL)

to determine different kinds of analytes [1–6]. Since there is

no need for an external energy supply, CL can provide

higher sensitivity and simplicity compared with other

detections, and it is uniquely suited to on-line detection

for CE. CE combined CL (CE-CL) has been successfully used

as an attractive scheme for the analysis of metal ions [7–8],

organic substances [9–12], amino acids [13–14], and

antigen–antibody complexes [15–18]. Typically, the CL

systems consist of CL reagent, oxidant and catalyst.

Common oxidants include hydrogen peroxide, potassium

ferricyanide and potassium permanganate, and so on.

Hydrogen peroxide, which is the most frequently utilized,

can be problematic, as it might be unstable due to bubble

formation in CE. It is a great challenge to search for new CL

oxidants to extend the applications of CL reactions.

Hexavalent iron species (ferrate(VI)), which possesses a

strong oxidizing power, relatively high redox potential and

environmentally friendly by-product, is the high-oxidation-

state compound of iron. Over the last two decades, fer-

rate(VI) has been widely investigated as the oxidant in

environmental remediation [19], as catalyst in organic

synthesis [20], and more recently as cathodic charge storage

material [21]. Stupin et al. [22] reported that the mixing of

the solid sodium ferrate(VI) and luminol solution in alka-

line medium can generate a strong, lasting CL signal. They

also found that certain d-metals and surfactants can affect

the CL signal. These imply that ferrate(VI)–luminol system

has a good potential to be applied in analytical chemistry.

Unfortunately, they did not make an intensive study of these

phenomena. To our best knowledge, no reports have been

published on ferrate(VI)–luminol CL system for CE

separation.

At present, the preparation methods of ferrate(VI)

include chemical method, electrochemical method and

thermal method. Because of its simple operation, safety and

free-hypochlorite, the electrochemical method has been

chosen as the most promising technique and it is very

suitable to synthesize ferrate(VI). The basic principle of

production is shown in Eqs. (1) and (2) [23].

Anode reaction:

Feþ 8OH� ! FeO2�4 þ 4H2Oþ 6e ð1Þ

Cathode reaction:

3H2O! 3H2 þ 6OH� � 6e ð2Þ

We found that the stable sodium ferrate(VI) solution

could be electrochemically produced with low concentration

of sodium hydroxide at low temperature. Mixing it

Fang Li1,2

Yonggang Hu1

Huijing Zhang1,2

Jie Zhang3

1State Key Laboratory ofAgricultural Microbiology,College of Life Science andTechnology, HuazhongAgricultural University, Wuhan,P. R. China

2College of EnvironmentalScience and Engineering,Huazhong University of Scienceand Technology, Wuhan,P. R. China

3College of EnvironmentalScience and Technology,Huangshi Institute ofTechnology, Huangshi,P. R. China

Received July 27, 2009Revised November 8, 2009Accepted November 10, 2009

Abbreviations: CE-CL, CE combined CL; CL,

chemiluminescence

Correspondence: Dr. Yonggang Hu, State Key Laboratory ofAgricultural Microbiology, College of Life Science and Technol-ogy, Huazhong Agricultural University, Wuhan 430070,P. R. ChinaE-mail: yongganghu@hzau.edu.cnFax: 186-27-6201-3553

& 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

J. Sep. Sci. 2010, 33, 631–636 631

with luminol solution could give rise to a strong, stable

and homogeneous CL signal. The CL signals could

be inhibited by benzenediol isomers, and the inhibited

CL signals were highly dependent on the concentration

of benzenediols. Based on these, we developed a novel

highly sensitive indirect CL detection for CE analyzing

of benzenediol isomers. This proposed method was

then applied to detect benzenediols in a real practical

sample.

2 Materials and methods

2.1 Materials

All the chemicals were of analytical grade. Luminol was

purchased from Alfa Aesar (USA). Benzenediol isomers

were supplied by the Chemical Reagent Factory of Shanghai

(Shanghai, China). Sodium hydroxide (NaOH) from the

Third Chemical Reagent Factory of Tianjin (Tianjin, China)

was used to produce ferrate(VI). All solutions were prepared

with doubly distilled water and filtered through a 0.22 mm

pore-size membrane filter before use. The water was

produced by doubly distilled water system (SZ 93, Shanghai

Yarong Biochemistry Instrument Factory, Shanghai,

China).

The hair dye sample (claret-red) was purchased from

Yaer Cosmetic of Guangzhou (Guangdong, China). It was

stored in a refrigerator at 4711C, and diluted 100 times by

running buffer before the analysis.

2.2 Apparatus

All the detections were operated on a laboratory-built

CE-CL detection equipment, designed as described

previously [24]. A 0–30 kV high-voltage DC power supply

(Nucleus Institute, Shanghai, China) provided for separa-

tion. A photomultiplier tube (Hamamatsu, Japan) operated

at �843 V was used to detect the photons. The captured

photocurrent was magnified by a signal magnifier (Nanjing

University, Nanjing, China) and recorded by HW-2000

chromatogram station (Qianpu Software, Shanghai,

China). A separation capillary of 55 cm� 50 mm

id, a reagent capillary of 25 cm� 200 mm id and

a reaction capillary of 15 cm� 530 mm id (Yongnian Optical

Fiber Factory, Hebei, China) were used in the CE-CL

system.

The synthesis of ferrate(VI) was carried out in a

homemade two-compartment cell, which was divided by

cationic exchanged membrane (Asahi Kasei, Japan). The

stainless steels were selected to produce ferrate(VI) similarly

as described in [25]. The RS1303DQ DC power source

(Shenzhen RICH Test & Measurement Instrumentation,

Guangdong, China) was used to supply the constant

current. The magnetic bar driven by a standard stirrer was

purchased from Yihua, Ltd. (China).

2.3 Procedure

Two 16 cm2 (apparent area) sheets of stainless steel were

employed as anode and cathode, respectively. The electro-

lytes in both compartments were 0.3 mol/L NaOH solution,

and their volume was 100 mL. The anode was activated by

cathodic polarization for 30 min at a constant current of

1.0 A before the start of electro-synthesis [26]. Cell

temperature was controlled within 711C by ice-water

cooling bath. Electrogenerated ferrate(VI) was added in a

brown bottle and then the system was put into an ice-water

cooling system to maintain the stability of ferrate(VI). The

content of ferrate(VI) was determined by the chromite

method [27].

The capillaries were successively rinsed using a syringe

with 0.1 mol/L NaOH, 0.1 mol/L HCl and doubly distilled

water for 10 min respectively, followed by equilibration with

the running buffer (pH 5 9.2) for 1 h. The whole system

was placed 20 cm above the outlet of reaction capillary so

that the ferrate(VI) solution could be continuously delivered

via gravity. The buffer was introduced into the separation

capillary by electroosmotic flow. The sample solution was

introduced at the positive electrode from a height of 20 cm

by siphoning. All separations were performed at 25711C.

The quantitation of three test benzenediol compounds were

operated by measuring the net CL intensity DI 5 I0–Ii,

where I0 was the CL intensity of blank signal of ferrate-

(VI)–luminol reaction and Ii was the ICL intensity induced

by the test analyte.

3 Results and discussion

3.1 Influence of electrochemical-synthesizing param-

eters on CE-CL system

Anode passivation layer, which is a firm film developed on

the surface of the anode, prevents the pathways of electron

flow and then breaks the electrolytic process [23]. In order to

reactivate the anode and ensure the repeatability of

ferrate(VI) production yield, cathodic polarization was

selected to destroy the passive layer before synthesis.

As previously reported, the internal cell temperature

was a crucial factor for electrochemically synthesizing

ferrate(VI) solution [20]. A high cell temperature generally

speeded up ferrate(VI) synthesis, but it also increased the

rates of ferrate(VI) decomposition and induced baseline

drift. In addition, the storage temperature of ferrate(VI)

solution was also significant for the CL detection. When the

ferrate(VI) solution was synthesized and stored in ice-water

cooling bath, the stable baseline and minimal noise levels

could be obtained.

In the previous reports, high concentration of NaOH

solution is predominantly utilized as an electrolyte to

synthesize the ferrate(VI) solution. However, strong causti-

city of high-concentration NaOH was harmful to the fused-

silica capillary detection window and the subsequent CL

J. Sep. Sci. 2010, 33, 631–636632 F. Li et al.

& 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

detection. This problem could be solved by the use of low

concentration NaOH solution as electrolyte. Influences of

NaOH concentration on ferrate(VI) production yield and CL

detection were carefully investigated from 0.2 to 0.6 mol/L.

The ferrate(VI) production yield continuously rose with the

NaOH concentration (Fig. 1A), and this was similar to that

described in [20]. The CL signal intensities increased with

the ferrate(VI) concentration, and reached a maximum at

0.26 mmol/L (Fig. 1B). Further increasing the ferrate(VI)

concentration produced a decrease of the CL signal inten-

sities, which might be due to the self-absorption of ferra-

te(VI). As a result, 0.3 mol/L NaOH, with which 0.26 mmol/

L ferrate(VI) was produced, was selected.

Electrolytic current played an important role in fer-

rate(VI) production yield and CL signal intensities. The

concentration of ferrate(VI) increased with the electrolytic

current and up to 2.0 A (Fig. 2A). A large electrolytic current

caused the increase of the oxygen evolution rate to be larger

than that of ferrate(VI) yield, led to a rapid inactivation of

the surface of electrode and then a dramatic decline of

ferrate(VI) production yield. As shown in Fig. 2B, the CL

signal intensities rose with the concentration of ferrate(VI),

and the maximum CL signal intensities appeared at

0.32 mmol/L corresponding to the electrolytic current in

1.8 A. Thus, 1.8 A was chosen to be the optimum electrolytic

current.

Subsequently, dependence of ferrate(VI) production

yield and CL intensities on the duration of electrolysis from

10 to 50 min was examined; 30 min was determined to be

the optimum time.

0.2 0.3 0.4 0.5 0.6

0.1

0.2

0.3

0.4

0.5

0.6

Co

nce

ntr

atio

n o

f fe

rrat

e(V

I) (

mm

ol/L

)

Conentration of sodium hydroxide (mol/L)

A

B

0.0 0.1 0.2 0.3 0.4 0.5 0.60

100

200

300

400

500

Rel

ativ

e C

L in

ten

sity

(m

V)

Conentration of ferrate(VI) (mol/L)

catechol resorcinol hydroquinone

Figure 1. Influence of the concentration of NaOH on theferrate(VI) production yield (A); influence of the ferrate(VI)production yield on the CL intensities (B). Conditions: Capillary,55 cm� 50 mm id, fused silica; applied voltage, 15 kV; injection,5 s; running buffer, 10 mmol/L sodium tetraborate–2.0 mmol/Lluminol; electrolyte, NaOH; electrolytic current, 1.5 A; duration ofelectrolysis, 30 min; 1.0� 10�5 mol/L catechol, 1.0� 10�4 mol/Lresorcinol, 1.0�10�6 mol/L hydroquinone.

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.40.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Co

nce

ntr

atio

n o

f fe

rrat

e(V

I) (

mm

ol/L

)

Electrolytic current (A)

A

B

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400

100

200

300

400

500

600

Rel

ativ

e C

L in

ten

sity

(m

V)

Concentration of ferrate(VI) (mmol/L)

catechol resorcinol hydroquinone

Figure 2. Influence of the electrolytic current on the ferrate(VI)production yield (A); influence of the ferrate(VI) production yieldon the CL intensities (B). Conditions: Capillary, 55 cm�50 mm id,fused silica; applied voltage, 15 kV; injection, 5 s; running buffer,10 mmol/L sodium tetraborate–2.0 mmol/L luminol; electrolyte,0.3 mol/L NaOH; duration of electrolysis, 30 min; 1.0� 10�6 mol/Lcatechol,1.0�10�4 mol/L resorcinol, 1.0� 10�6 mol/L hydroqui-none.

J. Sep. Sci. 2010, 33, 631–636 Electrodriven Separations 633

& 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

3.2 Influence of CE separation parameters on CE-CL

system

The sodium tetraborate containing luminol was employed

as the running buffer for CE separation. Both the separation

efficiency and the CL intensities were affected by the

concentration of sodium tetraborate. Effects of the sodium

tetraborate buffer concentration were examined in the range

of 2.5–15.0 mmol/L. The three analytes were baseline

separated under these conditions. As shown in Fig. 3, the

CL signals were enhanced as tetraborate concentration

increased and peaked at 5.0 mmol/L. Further increasing the

tetraborate concentration could produce considerable Joule

heating while the baseline became unstable. Meanwhile, the

electroosmotic flow decreased with an increase in the

concentration of sodium tetraborate buffer and the migra-

tion time also became longer. Thus, 5.0 mmol/L sodium

tetraborate (pH 5 9.2) was chosen as the optimum buffer

concentration for the consequent detection.

The effect of the concentration of luminol in the

running buffer on the CL intensity was investigated over the

range of 0.5–5.0 mmol/L. The results shown in Fig. 4

indicated that the CL signals increased gradually with

increasing luminol concentration and up to 2.0 mmol/L,

where maximum CL intensities were obtained and further

increasing the luminol concentration led to a decrease in CL

intensities. The luminol concentration of 2.0 mmol/L was

therefore selected for subsequent research work.

Dependence of CL intensities on applied voltage was

tested from 9 to 21 kV. Increasing applied voltage could

improve separation and selectivity until it reached 18 kV.

Sample injection time was further optimized and the best

CL signals were acquired when sample injection time

reached 5 s.

3.3 Performance characteristics of the method

Under the optimal conditions, the three benzenediols were

baseline separated and detected in less than 8 min (Fig. 5).

The dynamic ranges, regression equations, recovery rates

and RSDs are listed in Table 1. The correlation coefficients

(r240.99) show that good linearity could be obtained by this

method. The RSDs were investigated by injecting 1.0� 10�6

mol/L catechol, 1.0� 10�4 mol/L resorcinol, 1.0� 10�6 mol/

L hydroquinone standard solution and the results were in

the range of 1.5–4.5% (n 5 5). The recovery rates were

estimated by testing 1.0� 10�7 mol/L catechol, 1.0� 10�5

2 4 6 8 10 12 14 16200

300

400

500

600

700

800

Rel

ativ

e C

L in

ten

sity

(m

V)

Concentration of sodium tetraborate (mmol/L)

catecholresorcinolhydroquinone

Figure 3. Influence of the concentration of sodium tetraborateon the CL intensity. Conditions: Capillary, 55 cm�50 mm id,fused silica; applied voltage, 15 kV; injection, 5 s; running buffer,sodium tetraborate–2.0 mmol/L luminol; electrolyte, 0.3 mol/LNaOH; electrolytic current, 1.8 A; duration of electrolysis, 30 min;1.0�10�6 mol/L catechol, 1.0� 10�4 mol/L resorcinol, 1.0� 10�6

mol/L hydroquinone.

0 1 2 3 4 5

100

200

300

400

500

600

700

800

Rel

ativ

e C

L in

ten

sity

(m

V)

Concentration of luminol (mmol/L)

catecholresorcinolhydroquinone

Figure 4. Influence of the concentration of luminol on the CLintensity. Conditions: Capillary, 55 cm� 50 mm id, fused silica;applied voltage, 15 kV; injection, 5 s; running buffer, 5.0 mmol/Lsodium tetraborate–luminol; electrolyte, 0.3 mol/L NaOH; elec-trolytic current, 1.8 A; duration of electrolysis, 30 min; 1.0� 10�6

mol/L catechol, 1.0� 10�4 mol/L resorcinol, 1.0� 10�6 mol/Lhydroquinone.

Figure 5. Electropherogram of three analytes under optimizedconditions. Conditions: Capillary, 55 cm� 50 mm id, fused silica;applied voltage, 18 kV; injection, 5 s; running buffer, 5.0 mmol/Lsodium tetraborate–2.0 mmol/L luminol; electrolyte, 0.3 mol/LNaOH; electrolytic current, 1.8 A; duration of electrolysis, 30 min;peak 1, 8.0�10�7 mol/L hydroquinone; peak 2, 5.0� 10�5 mol/Lresorcinol; peak 3, 5.0� 10�7 mol/L catechol.

J. Sep. Sci. 2010, 33, 631–636634 F. Li et al.

& 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

mol/L resorcinol, 1.0� 10�7 mol/L hydroquinone standard

solution 5 times and recording the peak height. The results

were between 94.7 and 99.2%, and it indicated that this

method was reliable.

The LODs were considered as concentration that gave a

S/N of 3. The comparison of the proposed method with

other methods is summarized in Table 2. It is well known

that the injected sample volume in HPLC is much larger

than that in CE mode. The similar concentration LODs

between these two modes demonstrated that the mass LODs

of the proposed method were at least 3 orders of magnitude

lower than that of mentioned in [28]. Meanwhile, the

concentration LODs of the proposed method were lower in

comparison with electrochemical methods [29, 30].

Compared with our earlier studies [24, 31], the sensitivities

of hydroquinone and catechol were improved five and ten

times, respectively, and resorcinol could be detected in the

same magnitude level. Moreover, ferrate(VI) was envir-

onmentally friendly in comparison with ferricyanide.

3.4 Application

Benzenediol isomers are considered to be toxic. Exposure to

them can cause many clinical symptoms [28]. Benzenediols,

as couplers, are used in permanent hair dye [32]. Because of

the health risks from exposure to bezenediols in hair dye, it

is vital to develop an efficient method for simultaneously

analyzing the benzenediol isomers. The proposed method

was applied for the analysis of benezenediol isomers in hair

dye. The electropherogram of hair dye in the optimal

conditions was shown in Fig. 6A. Both migration time and

spiked method were applied for the detection of benzenediol

isomers. As shown in Fig. 6B, resorcinol in hair dye was

successfully separated and detected. Peak 1 has longer

retention time and serious peak tailing. When hydroqui-

none was spiked into hair dye, a peak corresponding to

hydroquinone appeared in the tailing region of peak 1.

Thus, it was difficult to identify whether hydroquinone

existed in hair dye. Moreover, catechol was not detected in

hair dye.

From the hair dye analysis (Table 3), the recoveries of

the analytes at three spiked concentration levels were carried

out and varied between 90.8 and 112.5%. The RSDs (n 5 5)

of peak height were less than 7.8%.

Table 1. Parameters of regression equations for the three analytes

Substance

name

Linear range (mol/L) Regression equations Determination

coefficient (r2)

RSD (%)

(n 5 5)

Adding value

(mol/L)

Recovery

rate (%)

Catechol 1.0� 10�6–9.0� 10�9 Y 5 6.8� 108X146.1 0.9983 1.9 1.0� 10�7 97.5

Resorcinol 9.0� 10�4–9.0� 10�7 Y 5 4.8� 106X167.5 0.9915 4.5 1.0� 10�5 94.7

Hydroquinone 1.0� 10�6–9.0� 10�9 Y 5 6.8� 108X1100.61 0.9955 1.5 1.0� 10�7 99.2

Table 2. Comparison of detections of benzenediol isomers with

other methods

Samples LOD (mol/L)

Catechol Resorcinol Hydroquinone

Proposed

method

Hair dye 8.5� 10�9 8.0� 10�7 6.5� 10�9

HPLC-CL [28] River water 4.7� 10�8 4.3� 10�8 4.3� 10�8

Electrochemical

[29]

Wastewater 6.5� 10�7 9.0� 10�7 5.0� 10�7

CE-SWAD [30] Wastewater 1.5� 10�6 1.0� 10�6 1.0� 10�6

CE-CL [31] Wastewater 8.6� 10�8 3.9� 10�7 3.2� 10�8

CE-CL [24] Urine 9.0� 10�8 – –

Figure 6. Electropherogram of hair dye under optimized condi-tions: (A) hair dye sample; (B) hair dye sample spiked with5.0�10�5 mol/L resorcinol. Peaks 1, 2, 4 unknown compounds;peak 3, resorcinol.

J. Sep. Sci. 2010, 33, 631–636 Electrodriven Separations 635

& 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

4 Concluding remarks

A new method of on-line CL detection coupled to CE based

on ferrate(VI)–luminol system offers a sensitive, simple

analysis of benzenediol isomers. The ferrate(VI) solution

can be electrochemically synthesized, and the proposed

method can be applied to the determination of benzenediols

in hair dye. The results indicate that ferrate(VI) is an

alternative promising oxidant for CL detection.

This work was supported by the National Natural ScienceFoundation of China (Grant No. 20005005) and the OpeningFoundation of State Key Laboratory of Agricultural Micro-biology (Grant No. AML-200905).

The authors have declared no conflict of interest.

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Table 3. The determination results of hair dye

Substance

name

Regression

equations

Determination

coefficient (r2)

Test value

(mol/L)

Adding

value

(mol/L)

Recovery

rate (%)

RSD (%)

(n 5 5)

Hair dye Catechol – – – 1.0� 10�4 94.3 6.2

Resorcinol Y 5 5� 106X173.7 0.9976 2.0� 10�3 2.0� 10�3 90.8 7.2

Hydroquinone – – – 1.0� 10�4 112.5 7.8

J. Sep. Sci. 2010, 33, 631–636636 F. Li et al.

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