A review on applications of chemiluminescence detection in food analysis

Analytica Chimica Acta 670 (2010) 1–10

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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Review

A review on applications of chemiluminescence detection in food analysis Meilin Liu a , Zhen Lin b , Jin-Ming Lin b,∗ a b

National Engineering Laboratory of Biohydrometallurgy, General Research Institute for Nonferrous Metals, Beijing 100088, China Department of Chemistry, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 14 October 2009 Received in revised form 21 April 2010 Accepted 22 April 2010 Available online 29 April 2010 Keywords: Chemiluminescence Determination Food Review Applications

a b s t r a c t There is an increasing interest by consumers for high quality food products with a specified composition. Suitable analytical techniques are needed for the quality control. Chemiluminescence (CL) detection has become quite a useful tool in the last years due to its simplicity, low cost and high sensitivity. Moreover, no external light source is needed. CL is often described as a dark-field technique: the absence of strong background light level reduces the background signal and leads to improved detection limits. Due to these advantages, CL methods have been widely applied to food analysis in recent years. Navas and Jiménez [1] had reviewed the CL methods in food analysis in 1996. The present review covers the papers since 1996, including the CL determinations of nitrogen containing components, sugars, chemical preservatives, metals, hormonal anabolics and metabolites, and other compounds in foods. 122 references have been cited. © 2010 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Nitrogen containing compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. N-nitrosoamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Nitrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Determination of sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Preservatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Parabens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Sulfites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Pesticides/herbicides and metabolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Other types of additives/contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1. Organic acids and salt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2. Alcohols and phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.3. Antibiotics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.4. Peroxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.5. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction It is well-known that food safety has become an important issue which challenges the worldwide and attracts the extensive atten-

∗ Corresponding author. Tel.: +86 10 62792343; fax: +86 10 62792343. E-mail address: [email protected] (J.-M. Lin). 0003-2670/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2010.04.039

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tion. The important factor under the name food safety includes environment and products, such as soil, pesticides and additives. Food additives play a vital role in the modern food industry and are generally used for maintaining food quality and characteristics. Recently, food additives/contaminants in consumer-products have received keen attention because of their possible side effects on humans. Over the past decades, the excessive use of food additives is the most outstanding issue all along. Therefore,

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additive/contaminant-assays have become routine practice. The most common additives/contaminants that have extensively found in food are nitrogen containing components, sugars, preservatives, metals, pesticides/herbicides and metabolites, other types additives/contaminants, etc. [2–5]. The majority of chemical contaminants are commonly analyzed using separative techniques coupled to various detectors such as gas chromatography-flow-injection detection (GC-FID), gas chromatography-electrical chemiluminescence detection (GC-ECD), gas chromatography–mass spectrometer (GC–MS), high-performance liquid chromatography-ultra visible (HPLCUV), high-performance liquid chromatography-flow-injection chemiluminescence (HPLC-FICL) [6], high-performance liquid chromatography-mass spectrum (HPLC-MS), capillary electrophoresis-chemiluminescence detection (CE-CL) [7,8] immunosensors, etc. [9]. As a drawback, the current official methods involved a series of time-consuming steps for the detection. It is necessary to develop a rapid, sensitive and accurate method to detect chemical contaminants. In particular, tests can be completed within minutes or hours, which would enable processors to take quick corrective actions when contaminants are detected [10]. Chemiluminescence (CL) is defined as the production of light generation through a chemical reaction that is accompanied by energy release of >45 kcal mol−1 . Radical species are formed that interact and produce unstable intermediates that decompose with the formation of excited species that either deactivate to the ground state or through energy transfer to luminophore molecules of relatively high quantum yield. It has numerous advantages such as superior sensitivity, rapidity, safety and controllable emission rate. In addition, when combined with flow-injection manifold system, CL-detection acquires extra facilities, such as on-line sample processing and in-line multi-detector installment. CL has grown into a well-established luminometry method in analytical chemistry and has been widely applied to liquid phase samples for over 30 years [11–13]. Many oxidation reactions of CL-reagents such as peroxyoxalate derivates, tris(2,2 -bipyridine) ruthenium(II) (Ru(bpy)3 2+ ), luminol, as well as direct oxidation of analytes have been developed. Amongst, peroxyoxalate CL reaction involves the oxidation of an aryl oxalate ester with hydrogen peroxide, which leads to the formation of one or more energy-rich intermediates that is capable of exciting a large number fluorophores through the chemically initiated electron exchange mechanism. Tris(2,2 -bipyridine) ruthenium(III) (Ru(bipy)3 3+ ) has attracted much attention and can be reduced by a large number of potential analyte compounds, or their electrochemical derivatives. The best known example in direct CL reaction is the oxidation of luminol in alkaline medium, to produce the excited 3-aminophthalate anion which emits light when relaxing to the ground state. Several oxidants such as permanganate, periodate, hexacyamoferrate(III) and hydrogen peroxide are commonly used. Strong oxidants, such as MnO4 − (in acidic or alkaline medium), ClO− , Ce(IV), H2 O2 , IO4 − , Br2 , or Nbromosuccinimide have been also extensively used. During the last years, several reviews have been published about the CL analytical applications in the liquid phase [14–18]. There are numerous reports published [19–22] about the importance of CL methods in analytical chemistry and their applications in the determination of a great variety of compounds, including the applications in pharmaceuticals, biomolecules, antioxidants, pesticides [19], arsenate [20], environmental water [21], drugs [22]. In any case, the general strategy for CL construction is to obtain high sensitivity and minimize the time of measurement. High performance liquid chromatography (HPLC) and gas chromatography (GC) coupled to CL detection represents an interface between the selectivity of a powerful separation method and the sensitivity of this detection technique. This combination provides a high efficiency in separation and low detection limits inher-

ent to CL systems. Capillary electrophoresis owns advantages of high separation efficiency, reduced analysis time and low consumption of reagent [23,24]. When combined with CL detection with high sensitivity, CE-CL exhibited significant applications to various areas [25,26]. CE-CL combination can be considered as a powerful modes of detection. The present review covers papers since 1996, and reviews CL-detection applications in food analysis, including simple and coupled with HPLC. The applications are mainly focused on the following types of compounds in food (a) nitrogen containing components, (b) sugars, (c) preservatives, (d) metals, (e) pesticides/herbicides and metabolites, (f) other types of additives/contaminants. Each group of applications was introduced in detail. An extensive table (Table 1) with a summary of the relevant aspects of the included papers has been included. 122 references have been cited. 2. Applications in food analysis 2.1. Nitrogen containing compounds Nitrogen compounds exist in a wide variety of analytical samples, from harmful contaminants in refinery streams to important ingredients in certain beverages, spices and condiments [27,28]. Accurate determination of nitrogen concentrations in these diverse samples is very important for process monitoring, quality control, product development, as well as basic research in these different industries. Due to the complex nature of the matrices, and the fact that the nitrogen analytes usually exist at low concentrations, GC analysis of these samples was presented by Yan [27] using a nitrogen CL detector. For nitrogen CL detection, the chemiluminescent nitrogen species is nitric oxide and the CL is derived from the well-known NO + O3 reaction. The authors pointed out the advantage of the ability to peer through complex sample matrices. This simplified the detection, identification and quantification of nitrogen-containing compounds. 2.1.1. N-nitrosoamines Due to the potent carcinogens, their detection of N-nitroso compounds in human food is of great interest. The existence of these compounds is so complicated in food samples that the determination of each individual N-nitroso compound would be impossible for a large number of samples [29,30]. Therefore, studies have been mainly focused on the examination of N-nitroso compounds as a group. N-nitrosamines are usually differentiated into volatile and non-volatile nitrosamines [31]. Volatile N-nitrosamines can be removed easily from a food matrix by distillation and separated by GC [32]. Nonvolatile nitrosamines can be determined following extraction and derivatization as methyl esters [33]. The mechanism to determine N-nitroso compounds as a group have been on the ability to make a complete breakdown of the bond between the nitroso compounds and the amino part of the molecule. Total N-nitroso compounds have been determined successfully in meat and sauces based on some means of chemical denitrosation. The produced NO reacted with ozone to form electronically excited nitrogen dioxide which then decayed back to the ground state with the emission of photon. The CL emission was detected and measured by a photomultiplier tube [34]. R 2 N-NO → R 2 NH + NO

(1)

∗

(2)

NO2 → NO2 + h

(3)

NO + O3 → NO2 + O2 ∗

Perez-Ruiz et al. [35] developed a FI-CL determination of Nnitrosodimethylamine based on photooxidation of Ru(bipy)3 2+

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Table 1 Analytical application of common CL systems in food analysis. Food additives Nitrogen containing components

Analytes Sulfonamides

N-nitroso compounds N-nitrosodimethylamine

N-nitrosamines

CL systems

LOD −1

Samples

Ref.

Milk

[28]

bis[4-Nitro-2-(3,6,9trioxadecyloxycarbonyl)phenyl] oxalate/H2 O2 Peroxyoxalate Ru(bipy)3 2+ /peroxydisulfate Luminol/NO2 − /H+ Luminol/K3 Fe(CN)6 Luminol/peroxynitrite

1 ␮g L

0.04 ␮g mL−1 0.29 ng mL−1 0.06 ␮g L−1 4 ␮g L−1 1.5 ng L−1

Meat, beer, beverage Food Food Drink

[29,30] [35–37] [42] [43] [41]

Sugars

Glucose Lactose

Luminol/glucose oxidise Luminol/[Cu(HIO6 )2 ]5− Luminol/K3 Fe(CN)6

4 ␮mol L−1 20 ␮g mL−1 6 ␮g mL−1

Drink, honey Milk Sugar

[50,51] [52] [54]

“Chemical” perservatives

Parabens Sulfite

Ce(IV)/Rhodamine 6G Rh 6G/Tween 80/SO3 2− Ru(bipy)3 2+ /K2 S2 O8 Ru(bipy)3 2+ /KMnO4 Na2 CO3 /NaHCO3 /Cu2+ KMnO4 /riboflavin

34 ng mL−1 0.03 mg L−1 41 nmol L−1 25 nmol L−1 0.05 nmol L−1 8 ng mL−1

Soy sauces Beverage Sugar Sugar Wine Beer, wine

[58,59] [61] [62] [63] [64] [67]

Some metals

Cr(III) Co(II)

Luminol/H2 O2 /OH− Luminol/O2

1.6 × 10−7 nmol L−1 4 fg mL−1

Drinking Egg yolk

[69] [75]

Others

DDT NMCs Carbofurn DDVP Propanil Carbaryl

0.06 ␮g L−1 4 ␮g L−1 0.02 ␮g mL−1 8 ng mL−1 8 ␮g L−1 45.6 ng mL−1 4.9 ng mL−1 4.9 ng mL−1 5.6 pmol 4 ␮mol L−1 0.2 ␮mol L−1 2.8 nmol L−1 0.01 ␮mol L−1 6.2 ␮mol L−1 1.0 ppb 25 pmol L−1 1.40 ng mL−1 0.166 ␮g L−1 10 mg L−1

Fish meat Fruit juice Lettuce Vegetable Drinking Grain Cucumber Cucumber

Phenolic compounds trans-Resveratrol Maltol

Tween 20/BSA/MAbs Peroxyoxalate/CTMAB Luminol/KMnO4 /OH− Luminol/H2 O2 /CTMAB KMnO4 /H+ Ce(IV)/Rh6G/H+ Luminol/KMnO4 /OH− Luminol/KMnO4 /OH− Nitric oxide/O3 Luminol/H2 O2 Luminol/H2 O2 /O2 /K3 Fe(CN)6 Luminol/Fe2+ Ru(phen)3 2+ /OH− Ru(phen)3 2+ /Ce(IV) Luminol/H2 O2 /O2 /K3 Fe(CN)6 Luminol/H2 O2 Ce(IV)/Tween 20 Luminol/K3 Fe(CN)6 KMnO4 /HCOOH/H+

[65] [82] [83] [87] [88] [89] [90] [103] [91] [94] [99] [100] [101] [102] [104] [105] [107] [108] [111]

Tetracycline antibiotic Benzoyl peroxide Peroxidase Olive oil

KMnO4 /SO3 2− /␤-CD Luminol/OH− Luminol/H2 O2 /peroxidase KO2 /dimethoxyethylene

0.9 ng mL−1 0.14 ng mL−1 1.0 ng mL−1 0.8 ␮mol L−1

Imidacloprid Malic acid Lactic acid Citric acid Proline Oxalic acid H2 O2

with K2 S2 O8 and on-line ultraviolet irradiation cleavage of the N–NO bond, followed by dimethylamine-radical reaction with Ru(bipy)3 3+ . The emitted light showed a linear relationship with the concentration of N-nitrosodimethylamine between 1.5 and 148 ng mL−1 , with a detection limit of 0.29 ng mL−1 . The method was applied to studying the recoveries of N-nitrosodimethylamine in different cured meat products. N-nitrosamines [36] were analyzed by CL detection on a thermal energy analyzer following GC separation. Only N-nitrosodimethylamine in the range of 1.2–3.4 ␮g kg−1 was detected in four out of the 29 sample preparations. 138 beer samples from 42 countries were analyzed for the presence of N-nitrosodimethylamine using a CL technique in which the chemical denitrosation product NO reacts with ozone to give NO2 in an excited state, which emits infrared light. The limit of quantification was 0.1 ␮g L−1 [37]. Only three samples exceeded 0.5 ␮g L−1 . No association was found between N-nitrosodimethylamine content and beer strength, type or geographical origin. The authors thought that water can be a potential source of N-nitrosodimethylamine in beverages. N-nitroso compounds (NOC) comprise nitrosamines (R1R2NNO) and nitrosamides [R1CON(NO)R2], produced by the nitrosation of secondary amines and N-alkyl amides, respectively. Haorah et al. [38] determined the total N-nitroso compounds and N-

Wine Milk Milk, fruit juice Wine Spinach Beverage Tea Apple juice Red wine Additives spiked in bread water samples Honey Wheat flour Vegetable Olive oils

[112] [116] [117] [118]

nitroso precursors (NOCP) in frankfurters, fresh meat, dried salted fish, sauces, tobacco, and tobacco smoke particulates. NOC were determined after sulfamic acid treatment to destroy nitrite, and NOCP were determined after treatment with 110 mM nitrite and then sulfamic acid. Then both the sulfamic acid treatment products for NOC and NOCP were decomposed to NO with refluxing HBr/HCl/HOAc/EtOAc and NO was measured by CL [39]. Total Nnitroso compounds were determined by Bouchikhi et al. [40] using the method based on vacuum distillation of nitrite and CL detection. Effect of the addition of nitrate to milk on the formation of volatile N-nitrosamines was studied. The possible mechanism is that some species possessing an enzymatic activity (␣-hydroxylase activity) may catalyse the hydroxylation of the ␣-C to form ␣-hydroxylated N-nitrosamine, which can undergo dealkylation yielding an alkylation agent thought to be an alkyldiazonium hydroxide, or the corresponding carbonium ion. The sensitivity and selectivity of the CL detection system, particularly combination with GC/HPLC has been discussed [36] in the determination of amines in foods. Kodamatani et al. [41] developed a method for the measurement of N-nitrosamines in part-per-trillion concentrations from water samples without pre-concentration steps. This method was based on on-line UV irradiation after HPLC separation and subsequent luminol CL

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Fig. 1. Schematic diagrams of the HPLC-CL system. P, Pump; I, injector; C, column; PR, photochemical reactor; D, chemiluminescence detector; PMT, photomultiplier; and DP, data processor. Cited from Ref. [41].

good reproducibility with the relative standard deviation 4.1% for 50 ␮g L−1 of nitrite (n = 9) and was very sensitive and simple. It has been successfully applied to the determination of nitrite in food. Williams et al. [44] determined plasma levels of nitrite and nitrate by using CL in early and recent classes of fish. Gao [45] utilized uric acid–ferricyanide–luminol system to determine indirectly the nitrite in sausage. Amini et al. [46] proposed a CL method for the determination of nitrite ion based on the gas-phase CL reaction between ozone and nitric oxide, which was generated from the reduction of nitrite with iodide in sulfuric acid solution. The method was applied to the analysis of nitrite in soil extracts. 2.2. Determination of sugars

detection without addition of an oxidant (see Fig. 1). It was confirmed that N-nitrosamines in basic aqueous solution were transformed to peroxynitrite by UV irradiation. The detection limits for this method were 1.5 ng L−1 , 2.9 ng L−1 , 3.0 ng L−1 , and 2.7 ng L−1 for N-nitrosodimethylamine, N-nitrosomorpholine, Nnitrosomethylethylamine, and N-nitrosopyrrolidine, respectively (S/N = 3). The calibration graphs were linear in the range of 5–1000 ng L−1 for these N-nitrosamines. 2.1.2. Nitrites Nitrite is also a common and toxic pollutant in food. The determination of nitrite have attracted much attention, and a simple, sensitive and specific analytical method is highly desirable. Luminol CL systems are well used for the determination of nitrite in food. A rapid method for the determination of nitrite in tinned beef, sausage and vegetables by CL has been proposed by Yang et al. [42]. The method was based on the strong CL of luminol in alkaline and iodine which was produced on-line by nitrite and iodide in acid. He et al. [43] founded a microflow injection analysis (␮FIA) system on a chip for the determination of nitrite in some vegetables and fruits. The chip (see Fig. 2) was made by using two transparent poly(methylmethacrylate) (PMMA) slices measured 50 mm × 40 mm × 5 mm, and the microchannels etched by CO2 laser were 200 ␮m wide and 100 ␮m deep with the volume of reaction area about 1.8 ␮L. Nitrite was sensed by the CL reaction of luminol with ferricyanide that was the product of the reaction of ferrocyanide and nitrite in acidic medium. The linear range of the nitrite concentration was 8–100 ␮g L−1 and the detection limit was 4 ␮g L−1 (S/N = 3). The proposed method had

The sugars are important ingredient of some food and the most commonly found sugars are glucose, sucrose, maltose, lactose and fructose. Excessive consumption sugars has been associated with increased incidence of obesity and decay. Hence, determination of sugars in food products is very necessary. There has been considerable interest in combining the advantages of CL detection with the specificity of enzyme reactions. The glucose reacts with glucose oxidise in the presence of molecular oxygen to produce hydrogen peroxide which was quantified by the reaction with luminol on a glassy carbon electrode [47]. An interesting FI-CL method that exploits soluble glucose oxidase (GOD) for the determination of glucose is based on multi-syringe FIA with an additional reagent (i.e. a Co(II) solution as catalyst) [48]. The proposed approach was applied to the analysis of ultralow glucose content soft drinks as well as fruit juices suitable for diabetic consumers. Besides, an electrochemiluminescent method has been reported for the determination of glucose in situ [49]. The hydrogen peroxide generated reacts with the radical obtained by electrochemical oxidation of luminol on a glassy carbon electrode. The proposed system was applied to the determination of glucose in different types of fruit juice. Panoutsou and Economou [50] proposed a new simplified procedure for the rapid enzymatic CL assay of glucose by means of a hybrid flow-injection/sequential-injection (FIA/SIA) method with soluble enzyme. The method is based on a sequential injection of the sample and enzyme by means of a multi-port selection valve in a flowing water stream. As the two zones were swept downstream, they overlapped and merged so that the glucose in the sample was enzymatically oxidized. The stream with the produced hydrogen peroxide was merged with

Fig. 2. Schematic diagram of the micro-FIA on-chip system used for the CL-determination of nitrite: (a) the features on the plates of chip system, (b) dimensions of the micro-flow channel. Cited from Ref. [43].

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an alkaline luminol solution stream. The produced CL was then monitored. The linear range of the method for glucose determination was 0.01–1 mmol L−1 . The relative standard deviation was 3.9% at the 0.08 mmol L−1 level (n = 8) and the limit of detection at the 2 level was 4 ␮mol L−1 glucose. The method had been applied to the analysis of glucose in energy drinks and honey. Evmiridis et al. [51] described a CL method for the indirect determination of glucose and fructose in mixture based on the difference of their kinetic rates in the periodate oxidation reaction. The periodate consumption at reaction-equilibrium for different glucose/fructose ratios was determined by the CL-signal generated during the oxidation of pyrogallol with the residual periodate in the reaction system. The relative standard deviation of three independent kinetic experiments with the same ratio of concentrations was found to be within 2% and the standard error from the fitting treatment was found to be less than 10% in all samples of different concentration ratios between glucose and fructose. Based on the enhancement effect of some sugars on the CL intensity between luminol and [Cu(HIO6 )2 ]5− , a CL method was developed for the determination of sugars such as glucose, fructose and lactose [52]. The unstable reagent [Cu(HIO6 )2 ]5− can be electrogenerated on the surface of platinum electrodes by constant current following oxidation of Cu(II) in KIO4 –KOH medium. The mixed sugars were separated by HPLC and detected by CL detection. The baseline and peaks shown in the chromatograms were very nice. The detection limits for glucose, fructose and lactose were 4, 3 and 20 ␮g mL−1 , respectively. This method has been successfully applied to the determination of glucose and fructose in grape samples and lactose in milk samples. Saito et al. [53] constructed a FI-CL system for glucose measurement in tomato by using a GOD immobilized reactor. The GOD was immobilized on polytetrafluoroethylene membrane with glutaraldehyde. Hydrogen peroxide generated by the GOD reaction oxidized luminol to produce CL in the presence of horseradish peroxide. Li and He [54] presented a simple continuous-flow CL system consisting of luminol and K3 Fe(CN)6 for the simultaneous determination of glucose, fructose and lactose in ternary mixtures. This method was based on the different kinetics of the individual sugars in the oxidation reaction with potassium ferricyanide. The known luminol–K3 Fe(CN)6 –CL system was used to measure the kinetic data of the system. The CL intensity was measured and recorded every second from 1 to 300 s. The data obtained were processed chemometrically using an artificial neural network. So the different sugar in mixing solution can be detected by CL intensity without previous separation. The proposed method was successfully applied to the simultaneous determination of the three sugars in some food samples, which offered the potential advantages of high reproducibility, simplicity and rapidity for the determinations of glucose, fructose and lactose. 2.3. Preservatives From ancient times, certain methods and materials have been used in the treatment of foods to prevent fermentation and decay. Such preservation is very necessary for future use when food is produced in the greatest abundance. When some of these older methods, such as smoking, salting, or pickling was used, the preserved food had some additional taste and odor. Chemical materials such as sodium benzoate, salicylic acid, and the like were tasteless and odorless. There are no evident for the consumer to identify the preserved food by the taste or order when chemical materials were used. Hence, the detection of preservatives in food has been a matter of great importance, although their use has decreased much in recent years owing to strict enforcement of food legislation. The most common of the preservatives found extensively in food in the past are formaldehyde, formic acid, salicylic acid, sodium

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benzoate, sulfurous acid and sulfites, and so on. The alkyl esters of p-hydroxybenzoic acid (parabens) and sulfites are the two main preservatives discussed in the review. 2.3.1. Parabens Parabens include methylparaben (MP), ethylparaben (EP), propylparaben (PP), and butylparaben (BP) [55,3,4], which are used as preservatives to prevent foods from microbial and fungal attack. Their toxicity is generally low due to rapid hydrolysis in vivo to parent acid, which is rapidly conjugated and excreted [56]. The antimicrobial activity of parabens increases with increasing length of the alkyl chain of ester group, but in practice shorter esters are commonly used because of their high solubility in water [57]. The maximum permitted concentration of parabens in foods is 0.1%. Recently, preservatives in consumer products have received keen attention because of their possible side-effects on humans. As a result, a fast, simple and accurate method of analysis was necessary. Myint et al. [58] reported firstly the CL method for the determination of parabens in food matrices, which is based on the enhancement by parabens of Ce(IV)–rhodamine 6G CL reaction in sulfuric acid medium. The method shows higher sensitivity than most of the reported methods, which also require pre-concentration or derivatization. It was successfully applied to the determination of ethylparaben in soy sauces without tedious pretreatment. The possible mechanism of this CL reaction has been proposed. Ce(IV) was reduced by the products hydrolyzed by parabens to the excited-state Ce(III) in sulfuric acid medium and then energy was transferred from the Ce(III)* to rhodamine 6G to form the excited-state rhodamine 6G, emitting light. Zhang et al. [59] also determined the parabens including methylparaben, ethylparaben, propylparaben, and butylparaben using HPLC-CL technique which was based on the CL enhancement by parabens of the Ce(IV)–rhodamine 6G system in the strong sulfuric acid medium. The proposed method was used to the foods, including orange juice, soy sauce, vinegar and cola soda. 2.3.2. Sulfites The application of sulfites is of importance in the food chemistry. Sulfites are used in a variety of food products because of their dehydrating and antimicrobial activity and other desirable preservative effects [60]. Sulfites are commonly added to wines, fruits and vegetables. Despite the fact that an adequate amount of sulfite prevents browning and ensures that the wine tastes good, its amount must satisfy prevailing legislation. Several CL methods were developed for the determination. Huang et al. [61] described a novel FI-CL system for sulfite based on auto-oxidation sensitized by rhodamine 6G in the presence of Tween 80 surfactant micelles. When sulfite was injected into a mixture of acidic Rh6G and Tween 80, strong CL occurs. The method has been successfully applied to the determination of total sulfite in beverages. He et al. [62] and Meng et al. [63] determined the sulfite in sugar using Ru(bipy)3 2+ –K2 S2 O8 system and Ru(bipy)3 2+ –KMnO4 system, respectively. Lin and Hobo [64] developed the method for the determination of sulfite using Na2 CO3 –NaHCO3 –Cu2+ system. The concentration of sulfite was proportional to the CL intensity in the range of 1.0 × 10−6 to 5 × 10−4 mol L−1 . The limit of detection was 5 × 10−7 mol L−1 and the relative standard deviation is 4.6% for the 5 × 10−6 mol L−1 sulphite solution in nine repeated measurements. This method has been successfully applied to the determination of sulfite in wines. The emission produced by sulfite induced autoxidation of Ni(II)/tetraglycine complex in the presence of luminol can be used to determine sulfite. Bonifácio and Coichev determined the sulfite in wine and juices by this method [65]. In recent years, CL flow sensor systems with immobilized or solid-state reagents have received much attention and many ana-

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lytical applications have appeared in the literature [66]. In most of these systems, it is required to use appropriate eluents to release the analytical reagents from the immobilized substrates, or the solid forms, so that analytes are sensed by the CL reactions with the dissolved reagents. Qin et al. [67] proposed a new CL flow sensor for the determination of sulfite based on the CL-emission generated during oxidation of sulfite by permanganate in the presence of riboflavin phosphate. In this sensor, both the oxidation and the subsequent energy transfer process proceeded directly with the immobilized reagents and there was no need to supply an eluent, thus permitting the flow sensor to operate in a reagentless way. The method was applied for the determination of sulfite in beer and wine. 2.4. Metals FI-CL has been applied to the determination of Cr(III) in food samples [68–71]. The method is based on the measurement of light emitted from the Cr(III)-catalyzed oxidation of luminol by H2 O2 . The luminol–H2 O2 CL reaction was catalyzed by Cr(III) and other metals, and the addition of ethylenediamine tetraacetic acid (EDTA) greatly reduced the CL intensity because of the formation of metal–EDTA complexes. The detection limit was 0.01 ppb and the linear range of extended up to 6 ppb. The system was shown to possess a high selectivity. The proposed method was applied to food (brown bread, shrimp, bovine muscle and brewer’s yeast). The data obtained for the analysis of different types of food samples coincided with the values obtained for Cr(III) in this type of food reference material. Co is a natural earth element present in trace amount in soil, plants and our diets, which is an essential mineral, although the body only needs a small amount. People are commonly exposed to small amounts of Co in the air they breathe, the water they drink, and the food they eat. Very small amounts of Co in people’s diets are necessary for good health. As an essential biochemical element, Co is mainly stored in red blood cells with small amounts in kidney, liver pancreas and spleen [72]. Research indicates that Co helps with the repair of the myelin sheath, increases the effectiveness of glucose transport from the blood into body cells, and increases the assimilation of iron and the building of red blood cells [73]. Co is also an important agent of Vitamin B12 . It increases the body’s ability to absorb it. And Co can stimulate many enzymes of the body and normalize the performance of other body cells. Because of its low adsorption rate and high excretion rate, Co toxicity is not common, but an excess can lead to enlargement of the thyroid gland [74]. Recently, Song et al. [75] reported a CL method for the determination of Co in egg yolk, fish tissue and human serum. In their work, it was found that Co could greatly catalyze the CL reaction between luminol and dissolved oxygen, and the increment of CL was linear over the Co concentration ranging from 10 fg mL−1 to 50 pg mL−1 with a detection limit of 4 fg mL−1 (3) and relative standard deviation of less than 3.0%. And the proposed method has been successfully applied for the determination of Co in egg yolk, fish tissue and human serum. Based on the separation ability of CE, several metal ions, Co(II), Cr(III), Cu(II) and Ni(II) were detected by CE-CL combination due to their influences on the CL system of luminol and H2 O2 [76]. 2.5. Pesticides/herbicides and metabolites The laws in some countries require the evidence of the presence of residual pesticides. Among the several kinds of pesticides employed in agriculture and vector control applications, dichlorodiphenyl-trichloroethane (DDT) displays a recognized toxicity [77] and a long persistence in the environment. Although its use was banned in developed nations after 1970, there is still an important

presence in the food chain. In addition, dichloro-diphenyl-dichloroethylene (DDE) and dichloro-diphenyl-dichloro-ethane (DDD), the two major metabolites of DDT, and other DDT-related compounds are usually present together with the parent compound in DDT-contaminated environmental matrices. Botchkareva et al. [78] reported the application of the developed CL-enzymelinked immunosorbent assays (ELISAs) to the analysis of residues in food samples (fish meat). Two ELISA with CL detection for the insecticide DDT and the group of DDT-related compounds were characterized. Effects of several physicochemical factors (ionic strength, pH, Tween-20 and Bovine serum albumin (BSA) concentrations) and solvents (methanol, ethanol, acetone and N,N -dimethylformamide) on the performance of the assays were studied and optimized. For the DDT-selective assay, the sensitivity was 0.6 ␮g L−1 , with a linear working range between 0.1 and 2 ␮g L−1 and a limit of detection of 0.06 ␮g L−1 . For the DDT groupselective assay, the sensitivity was 0.2 ␮g L−1 , with a linear working range 0.07–1 ␮g L−1 and a limit of detection of 0.04 ␮g L−1 . The method was four times more sensitive compared to colorimetric ELISAs. N-methylcarbamates (NMCs) comprise an important class of pesticides widely used in agriculture for crop protection [79,80] and, therefore, their residues may be encountered in fruits and vegetables, which poses a potential hazard for consumers. To watch over the safety of our food supply, international organizations regulate their maximum residue levels (MRLs) on crops, which are in the 0.02–7 mg kg−1 range depending on the particular pesticide/commodity combination [81]. Numerous analytical procedures have been developed for the determination of NMCs and metabolites in various matrices, including water, soil, fruits, vegetables and other crops. Orejuela and Silva [82] determined NMCs pesticide residues in fruit juices using HPLC with peroxyoxalate-CL detection. The required pre-column hydrolysis of pesticides and derivatization of their hydrolytic metabolites with dansyl chloride were simultaneously carried out in a short time thanks to the micellar catalytic effect provided by cetyltrimethyl ammonium bromide micelles on the hydrolysis step. The LC separation of the dansylated phenols was performed on a reversed-phase C18 column with isocratic elution. The analyzes were detected using an integrated derivatization CL detection unit based on the bis(2,4,6-trichlorophenyl)oxalate–hydrogen peroxide system. Fruit juice samples containing 4.0–1500 mg L−1 pesticides were analyzed with a precision of 6.5%. After contamination of the fruit juice samples, average recovery 93% at fortification levels of 10–100 mg L−1 was obtained. Huertas-Perez et al. [83] found that the pesticide carbofuran produced a great enhancement on the CL emission from the luminol oxidation by potassium permanganate in alkaline medium without catalyst. This enhancement in the CL emission was proportional to the concentration of the studied compound, which can be determined by measuring the increase in the CL intensity. Based on these findings, a simple and fast new direct FICL method had been developed for the determination of carbofuran, which has been satisfactorily applied in vegetal food. During recent years, organophosphorus pesticides have been widely used in agriculture because of their low environmental persistence and high effectiveness [84]. Dichlorvos pesticide (DDVP), an organophosphorus systemic insecticide and acaricide, is often used for crop protection. However, it does have a high acute toxicity. Trace amounts of this insecticide may remain in foodstuffs or drinking water that threatens human health. Therefore, simple, fast and accurate methods are needed to monitor the pesticides residue in food so as to minimize the possible health risk [85,86]. Wang et al. [87] described a simple, fast FI-CL method based on the reaction of luminol with H2 O2 in the presence of a cationic surfactant (cetyltrimethylammonium bromide, CTMAB) for the direct determination of dichlorvos pesticide (DDVP). The

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CL intensity was linear to the DDVP concentration in the range of 0.02–3.1 ␮g mL−1 . The relative standard deviation was 3.4% at 0.35 ␮g mL−1 with a detection limit of 0.008 ␮g mL−1 DDVP. The method has been successfully applied to the determination of trace DDVP residue in vegetable samples. Based on coupling photo degradation, CL and multicommutation continuous-flow methodology [88] for the determination of the pesticide propanil were studied. A photo-CL system for the determination of propanil was presented. The light from a low-pressure mercury lamp is used as a clean, reproducible and inexpensive “reagent” for the derivation of the pesticide, performed in acetic–acetate buffer at pH 4.8. Then, the photo-products from irradiation are oxidized by permanganate in sulphuric acid solution. On the other hand, the use of solenoid valves allows the easy, complete automation of the process with low sample and reagent consumption. The pesticide solution was inserted as small segments sequentially alternated for adjusting the suitable medium. Flow rates are adjusted to the required time for photo degradation. The method was valid for the determination of other pesticides from the same chemical family, namely: alachlor, flumetsulam, furalaxyl and ofurace. Carbaryl, a modern pesticide widely used for both agricultural and non-agricultural purposes, was determined from the CL produced in its reaction with Ce(IV) in a nitric acid medium containing rhodamine 6G as sensitizer, using FI techniques [89]. Calibration graphs were linear over the concentration range from 50 to 2000 ng mL−1 . The limit of detection, as determined according to Clayton, was 45.6 and 28.7 ng mL−1 for peak height and peak area measurements, respectively. The relative standard deviation for 10 samples was less than 1.4% with both types of measurements. Solid-phase extraction was used to concentrate and separate the analyte from the matrix. The method was successfully applied to the analysis of grain samples. The proposed method exhibited a high selectivity no other pesticide containing the naphthalene group such as antu, napropamide or naftalam, etc., was found to interfere with the determination of carbaryl. Huertas-Perez et al. [90] proposed a direct FI-CL method for the determination of carbaryl. The method is based on the enhancing effect of carbaryl on the CL emission generated by the oxidation of luminol with potassium permanganate in an alkaline medium. Under the optimal conditions, the CL intensity was linear for a carbaryl concentration over the range of 5–100 ng mL−1 with a 3 detection limit of 4.9 ng mL−1 . It has been successfully applied to the determination of carbaryl residues in cucumber samples. A FI-CL reaction was investigated [91] to quantify imidacloprid, based upon (1) ultraviolet (254 nm) photochemical dissociation of imidacloprid to produce nitrite, (2) chemical reduction of the nitrite to nitric oxide by iodide in acid, (3) removal of gas-phase nitric oxide from the aqueous stream using a membrane separator, and (4) detection of the nitric oxide by CL reaction with ozone. The cross-reactivity of imidacloprid with eight metabolites of imidacloprid was determined using a commercial ELISA kit and the FIA method. While the ELISA kit demonstrated varying degrees of cross-reactivity, cross-reactivity in the FIA method was observed for only the N-nitro and N-nitroso metabolites. The optimized analytical FIA method, FIA provided a linear response in imidacloprid concentration over four orders of magnitude, with a limit of detection of 5.6 pmol (1.5 ng) of imidacloprid. Spike-recoveries by FIA demonstrated excellent recovery of imidacloprid in natural waters, hemlock xylem fluid, honey, and grapes with little to no interference from the matrix. 2.6. Other types of additives/contaminants 2.6.1. Organic acids and salt Luminol CL has been used for a variety of other analytes in food and beverages, including 3,4-dehydroxybenzoic acid [92], cit-

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ric acid [93], malic acid [94] and l-lactate [95]. Citric acid in orange drinks [96] has also been determined using the luminol reaction. Iron (III) was reduced by citric acid to iron (II), which can then be detected with luminol, but ascorbic acid interfered. l-cysteine interference was removed by prior complexation with copper (II). l-malate [97] and l-lactate [98] have been indirectly detected using immobilized enzyme reactors to produce hydrogen peroxide, followed by luminol detection. Wu et al. [99] proposed a method based on natural animal tissue porcine kidney as recognition element for CL sensing of lactic acid. The principle for lactic acid sensing was that lactic acid was oxidized by oxygen under the catalysis of ␣-hydroxy acid oxidase in the tissue column to produce hydrogen peroxide, which can react with luminol in the presence of potassium ferricyanide to generate a CL signal. The CL emission intensity was linear with lactic acid concentration in the range of 1–1000 ␮mol L−1 and the detection limit for lactic acid was 0.2 ␮mol L−1 . The biosensor could be used continuously for 6 h with no significant changes in the response. A complete analysis, including sampling and washing, could be performed in 1.5 min with a relative standard deviation of 1.12% for 100 ␮mol L−1 lactic acid. The reproducibility among tissue columns was satisfactory. The biosensor has been applied successfully to the analysis of lactic acid in plasma and milk samples. A HPLC method was developed [100] for the determination of citric, lactic, malic, oxalic and tartaric acids by CL detection following online irradiation with visible light. The organic acids were irradiated with visible light in the presence of Fe3+ and UO2 2+ to generate Fe2+ , which was determined by measuring the CL intensity in a luminol system without added oxidant. Factors affecting the photochemical and CL reactions were optimized so that their contribution to the total band-broadening was negligible. The chromatographic separation was performed on a C18 column under isocratic reversed-phase conditions using 0.005 mol L−1 H2 SO4 as the mobile phase. The optimized method was validated with respect to linearity, precision, limits of detection and quantification, accuracy specificity and robustness. The applicability of the assay was demonstrated by analysing these compounds in real samples such as milk, fruit juices, soft drinks, wine and beer. Proline is the predominant amino acid in the grape berry (up to 2 g L−1 ) and accounts for 30–80% of the total nitrogen content. The level of proline has been related to the total nitrogen content, cultivar type and wine quality. FI methodology was described for the determination of proline in red and white wines using Ru(phen)3 2+ CL detection [101]. Selective conditions were achieved for proline at pH 10, while other amino acids and wine components did not interfere. The precision of the method was less than 1.00% for five replicates of a standard (4 × 10−6 mol L−1 ) and the detection limit was 1 × 10−8 mol L−1 . The present method was applied to the determination of proline in various wines. The analysis of oxalate is of great importance in food because of its effect on the human body. High oxalate concentration in the blood or urine accompanies a number of maladies including renal failure, vitamin deficiencies. It has also been implicated in the formation of kidney stones. In this case the precipitation of calcium oxalate within kidney occurs and this can cause renal tissue damage. Vegetables generally contain wide variations in oxalic acid, so that selective and precise methods for the determination of oxalic acid are very important. Wu et al. [102] applied HPLC with post column CL detection as a technique for the determination of oxalic acid in spinach. The method was based on the reaction of Ce(IV) oxidized tris(2,2 -bipyridyl)ruthenium(II) (Ru(phen)3 2+ ) and oxalic acid emitting light. It has higher sensitivity. The method was successfully applied to determination of oxalic acid in spinach. The pesticide carbaryl was directly determined by FI-CL method [103]. The method was based on the enhancing effect of carbaryl on the CL emission generated by the oxidation of luminol with potassium

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permanganate in an alkaline medium. The suggested method was rapid with low detection limits and efficient precision for routine analysis in cucumber sample. 2.6.2. Alcohols and phenols The determination of alcohols involved treating test alcohol in a sample with alcohol oxidase and measuring H2 O2 formed with CL assay. This method was proposed for Fukuoka et al. [104] to determine the H2 O2 treating the sample with luminol and K3 Fe(CN)6 in a vial and counting the CL. Hydrogen peroxide (H2 O2 ) [105] was one of the important by-products produced by plant and fruit tissues during normal metabolism as well as under stress conditions. Evidence suggested that it was actively involved in many physiological activities in plants, including ripening, senescence and the development of disorders. Quantitative measurement of H2 O2 in fruit has been a challenge due to variations in methodologies, and their sensitivities and interferences present in plant samples. This modified protocol can measure H2 O2 content in apple peel and flesh tissues. ‘Red Delicious’ apple peel and flesh tissues were measured with amount of 1.48 and 1.03 ␮mol g−1 , respectively. The established protocol can also be used for a wide variety of tissues in addition to apple fruit, including strawberry tissues (fruit, calyx and leaves) and spinach leaves. This protocol was applied to determine the H2 O2 concentration in monocyte chemoattractant-1 (1-MCP) and diphenylamine (DPA) treated apples after 5 months of storage. Fernandes and Reis [106] described an automatic procedure for the determination of ethanol in wines using a flow system based on multicommutation and enzymatic reaction. Alcohol oxidase was immobilized on aminopropyl glass beads and packed in an acrylic column. The peroxide due to enzymatic reaction with ethanol reacted with luminol and generated the CL radiation that was monitored by a laboratory-made detector based on photodiodes. The system manifold comprised a set of 3-way solenoid valves controlled by a microcomputer furnished with electronic interfaces, which ran on software written in Quick BASIC 4.5 to provide facilities to perform on-line sample dilution, reagent addition, and data acquisition. After system parameters optimization, ethanol samples were processed without prior pretreatment. Salicylic acid, resorcinol, phloroglucinol, p-hydroxybenzoic acid, 2,4-dihydroxybenzoic acid, and m-nitrophenol are biologically and environmentally important phenolic compounds. Thus, highly sensitive methods are still needed for the detection of such compounds at trace level in complex matrices. In recent years, HPLC-CL detection has become more and more attractive for the determination of compounds at trace level in complex matrices because of its high sensitivity, high selectivity and wide dynamic range. Cui et al. [107] developed a novel method for the simultaneous determination of phenolic or hydro-benzoic acid compounds, such as salicylic acid, resorcinol, phloroglucinol, p-hydroxybenzoic acid, 2,4-dihydroxybenzoic acid, and m-nitrophenol by HPLC coupled with CL detection. The procedure was based on the CL enhancement by phenolic compounds of the Ce(IV)-Tween 20 system in a sulfuric acid medium. The method has been successfully applied to the determination of p-hydroxybenzoic acid in apple juices. trans-Resveratrol (3,4 ,5-trihydroxystilbene) is a stilbene produced by plant in response to fungal infection or abiotic stresses such as heavy metal ion or UV light exposure. It occurs in mulberries, peanuts, and grapes as well as in wines. There are increasing interests in resveratrol research owing to its pharmacological activity. Zhou et al. [108] found that trans-resveratrol could enhance strongly the CL of luminol–ferricyanide system. A highly sensitive method was developed for the determination of trans-resveratrol in red wines by using HPLC coupled with CL detection for the first time. It allowed for the determination of trans-resveratrol in the range 0.5–750 ␮g L−1 with a detection limit of 0.166 ␮g L−1 . The relative

standard deviation is 1.16% for 7.5 ␮g L−1 trans-resveratrol. transResveratrol was detected in chinese red wines with the recoveries of 92.2–114.7%. Ren et al. [109] determined resveratrol in red wine by solid phase extraction-FI-CL method. Zhang et al. [110] proposed a sensitive determination of phenolic compounds using HPLC with Ce(IV)–rhodamine 6G–phenolic compound CL detection. The method is based on the chemiluminescent enhancement by phenolic compound of the Ce(IV)–rhodamine 6G system in sulfuric acid medium. Twenty phenolic compounds were separated on a XDB-C8 column with a gradient elution using a mixture of methanol. The proposed method had been applied to the assay of phenolic compounds in red wine without any pretreatment. Sanfeliu Alonso et al. [111] founded a FI-CL method for the determination of maltol, based upon the oxidation of the food additive by KMnO4 in sulfuric acid medium at 80 ◦ C enhanced by hexadecylpyridinium chloride and formic acid. The calibration graph was linear over the range 0.5–4.0 mg L−1 of maltol, with a RSD (n = 50, 0.5 mg L−1 ) of 2.9%. 2.6.3. Antibiotics Wan et al. [112] developed a method for the simultaneous determination of tetracycline antibiotic (TCA) residues such as oxytetracycline (OTC), tetracycline (TC), and metacycline (MTC) by HPLC coupled with CL detection, which was based on the CL enhancement by TCAs of the potassium permanganate–sodium sulfite–␤-cyclodextrin system in a phosphoric acid medium. The separation was carried out with an isocratic elution using a mixture of acetonitrile and 0.001 mol L−1 phosphoric acid. The method was applied to the determination of TCA residues in honey samples. Kaczmarek and Lis [113] proposed a simple CL method for the determination of chlortetracycline (Chlor-TC), oxytetracycline (Oxy-TC) and doxycycline (Doxy-TC). This method is based on the europium(III) emission as a result of the energy transfer process from the excited product of the tetracyclines oxidation to the uncomplexed Eu(III). The method was successfully applied to the determination of honey. 2.6.4. Peroxides Peroxides are widely used in the food industry. For example, benzoyl peroxide (BP) is allowed as a food additive in many countries due to its bleaching and sanitizing properties and BP is used as a flour bleacher [114]. Yang et al. [115] proposed a novel capillary microliter order droplet injection–CL system. In this system, the liquid sample microliter size droplet, automatically formed at the end of a capillary tip by the effect of the gravity and the gas pressure, repeatedly dropped into the miniature reaction cell and reacted with CL reagent (based on the directly oxidation of luminol by benzoyl peroxide in a basic solution) to generate CL signal. The phenomena of sample zone dilution and spreading were eliminated since the capillary was used as the sample channel and gas pressure was applied as driving force in the absence of liquid carrier stream. Therefore, a high sensitivity was obtained. To evaluate the applicability of the proposed method, a determination of benzoyl peroxide (BP) was examined. The system showed that the benzoyl peroxide was detected linearly in the concentration range from 5 × 10−10 to 1 × 10−6 g mL−1 . The detection limit was 1.4 × 10−10 g mL−1 for benzoyl peroxide, which was the best result reported so far. The relative standard deviation was 1.5% for 2.0 × 10−8 g mL−1 BP. The proposed detector for the detection of benzoyl peroxide offered the advantages of sensitivity, simplicity, rapidity, automation and miniaturization. The proposed method has been applied satisfactorily to the determination of benzoyl peroxide in wheat flour. Liu et al. [116] provided a sensitive CL system on the microfluidic chip for determination of BP. In this method, no separation or precon-

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centration steps were needed and the reagent consumption was low. Puchades et al. [117] had determined peroxidases and lactoperoxidase in vegetables and diary products. The CL method involved the peroxidase catalyzed reaction of H2 O2 with luminol. 2.6.5. Others The determination of food authenticity and the detection of adulteration are of increasing importance in the food industry. Virgin olive oils are frequently adulterated with other vegetable oils of lower commercial value. A weak CL emission was observed in commercial Greek extra virgin olive oils (Knossos, Spitiko, Ananias, Altis, Minerva, Xenia) and in refined seed oils such as sunflower oils (Marata, Sanola, Sun, Mana, Sol, Minerva) as well as in corn oils (Flora, Minerva, Marata Sun and Sol) with potassium superoxide in the aprotic solvent dimethoxyethylene [118]. On measuring the CL of mixtures with the cheaper refined seed oils, calibrations were produced which can be used for the determination of the adulteration of olive oils with seed oils down to 3%. Furthermore, depending on the kind of oils, “low” authenticity-CL-factors for olive oils (0.8–2.15 ␮mol L−1 gallic acid) and “high” for seed oils (4.5–11.2 ␮mol L−1 gallic acid) were calculated. Food-borne microbial diseases affect a large number of people each year. One of the most frequent diseases is gastroenteritis, which is caused by the ingestion of food contaminated with staphylococcal enterotoxins (SEs). SEs is a family of major serological types of heat stable enterotoxins. These toxins cause toxic shock-like syndromes and have been implicated in food poisoning and several allergic or autoimmune diseases. SEs are a group of highly conserved proteins with significant immunological cross-reactivity. The three anti-genically distinct SEs subtypes are type C staphylococcal enterotoxins (SEC1 ), SEC2 , and SEC3 . As low as 100 ng SEs are sufficient to cause symptoms of intoxication in humans [119]. These biological effects make the detection of these toxins very important from the standpoint of public health. Luo et al. [120] successfully coupled traditional enzyme-linked immunosorbent assay (ELISA) to CL imaging system for the determination of SEC1 . The proposed method offered not only the advantages of ELISA (non-CL assay) but also the advantages of CL imaging. It has been successfully applied to the determination of SEC1 in milk and water samples. Meanwhile, March et al. [121] applied the ELISA-CL technique to determine the viable beer-spoilage lactic acid bacteria. Yang et al. [122] proposed the detection for SEB in food with carbon nanotubes (CNTs) by enhanced ELISA-CL. They utilized a simple cooled charge-coupled device (CCD) detector which combined with CNTs to develop a simple and portable point-of-care immunosensor. This combination of ECL, CNT, and CCD detector technologies is used to improve the detection limit of SEB in food. Anti-SEB primary antibodies were immobilized onto the CNT surface, and the antibody–nanotube mixture was immobilized onto a polycarbonate surface. SEB was then detected by an ELISA assay on the CNT–polycarbonate surface with an ECL assay. SEB in buffer, soy milk, apple juice, and meat baby food was assayed using CCD detector. Meanwhile, the comparisions were done with a fluorometric detector when using the CNTs. This level is far more sensitive than the conventional ELISA. 3. Conclusions CL reactions have considerable analytical potential because they have numerous advantages: high sensitivity (for many compounds detection limits of 1 ␮g kg−1 or lower have been reported), wide linear range and the use of simple and inexpensive instrumentation for monitoring emission, where the absence of a light source reduces the background noise. All these advantages have allowed

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the method that has been conveniently used in the determination of many inorganic and organic compounds in food samples. There is ample documentation that CL detection in combination with chromatographic techniques becomes highly selective, i.e. determination and identification of volatile nitrosamines or lipid hydroperoxides. In the way, FI-CL in combination with HPLC has demonstrated to be an excellent and fast screening method for detection of hormonal anabolics. The combination of sensitivity of CL with rapidity of FI, together with low cost and simplicity, make the FI-CL system extremely attractive, especially to determine sugars in food. In conclusion, the CL techniques can be used for the routine analysis of complex food samples in many laboratories. Acknowledgements The authors thank the Natural Science Foundation of China (No. 20775042) and National Basic Research Program of China (973 Program, No. 2007CB714507) for the financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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