Advances in the development of organic polymer monolithic columns and their applications in food analysis—A review

Journal of Chromatography A, 1313 (2013) 37–53

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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

Review

Advances in the development of organic polymer monolithic columns and their applications in food analysis—A review Pavel Jandera ∗ Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210 Pardubice, Czech Republic

a r t i c l e

i n f o

Article history: Received 14 May 2013 Received in revised form 29 July 2013 Accepted 3 August 2013 Available online 22 August 2013 Keywords: Food analysis Silica monolithic columns Organic polymer monoliths MIPS Disks

a b s t r a c t Monolithic continuous separation media are gradually finding their way to sample pre-treatment, isolation, enrichment and final analytical separations of a plethora of compounds, occurring as food components, additives or contaminants, including pharmaceuticals, pesticides and toxins, which have traditionally been the domain of particulate chromatographic materials. In the present review, recent advances in the technology of monolithic columns and the applications in food analysis are addressed. Silica-based monoliths are excellent substitutes to conventional particle-packed columns, improving the speed of analysis for low-molecular weight compounds, due to their excellent efficiency and high permeability. These properties have been recently appreciated in two-dimensional HPLC, where the performance in the second dimension is of crucial importance. Organic-polymer monoliths in various formats provide excellent separations of biopolymers. Thin monolithic disks or rod columns are widely employed in isolation, purification and pre-treatment of sample containing proteins, peptides or nucleic acid fragments. Monolithic capillaries were originally intended for use in electrochromatography, but are becoming more frequently used for capillary and micro-HPLC. Monoliths are ideal highly porous support media for immobilization or imprinting template molecules, to provide sorbents for shape-selective isolation of target molecules from various matrices occurring in food analysis. The separation efficiency of organic polymer monoliths for small molecules can be significantly improved by optimization of polymerization approach, or by post-polymerization modification. This will enable full utilization of a large variety of available monomers to prepare monoliths with chemistry matching the needs of selectivity of separations of various food samples containing even very polar or ionized compounds. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silica and organic polymer monoliths – types, morphology, characterization and recent improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reversed-phase separations on monolithic columns in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Silica-based monoliths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Organic polymer monoliths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal-phase and HILIC chromatography on monolithic columns in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ion-exchange separations on monolithic columns in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zwitterionic and mixed-mode separations on monolithic columns in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bioaffinity monolithic materials and sorbents with imprinted molecules in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Materials and techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Applications of MI-SPE in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two-dimensional applications of monolithic columns in food analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions, future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Tel.: +420 406 037023; fax: +420 406 037068. E-mail address: [email protected] 0021-9673/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2013.08.010

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1. Introduction As early as in 1967, Kubín et al. [1] reported on in situ preparation of a porous organic polymer block in a glass column, which however showed poor permeability to be of practical use as a separation medium. Later, Hjertén introduced compressed gels in the ˇ form of continuous beds [2]. Tennikova et al. [3] and Svec and Fréchet [4] developed “continuous separation media” consisting of one piece of porous organic polymer material filling whole volume of a cylindrical column, which were later named “monolithic stationary phases” [5]. Monolithic columns based on inorganic matrices were introduced by the beginning of 1990s [6]; the silica monolith fabrication by a sol–gel approach was later reported by Tanaka group [7]. Since then, two types of monolithic materials have been available for chromatographic separations – monoliths based on inorganic precursors and those based on organic polymers. The structure of monolithic media can be represented as a network of small mesopores, which are responsible for the retention and separation selectivity, interconnected by large flow-through pores. This morphology provides good bed permeability and enables fast separations at high flow of the mobile phase and moderate backpressures, in comparison to particle-packed columns with similar efficiency [8]. Hence monolithic columns show higher permeability and lower flow resistance than the conventional LC columns packed with small particles, providing thus significantly shorter separation times at moderate operation pressures. Due to these favorable properties, monolithic columns have been widely applied almost in all LC application areas, including environmental, pharmaceutical, clinical, forensic, industrial, and food analysis. Silica-based monolithic columns are available commercially in the conventional column format (4 mm i.d. rods, encased in a cladding tube to produce the final column), or larger diameter columns for preparative applications; monoliths prepared in situ in fused silica capillaries < 0.2 mm i.d., and recently in the most difficult narrow bore format (2 mm i.d.). Because of their high efficiency, bare silica or chemically modified commercial monolithic columns have been used routinely as the time-saving substitutions of conventional HPLC columns packed with porous particles for the separations of small molecules and biomolecules in many application areas, including food analysis, with excellent resolution and short times of separation. Recently, preparation of titanium-based monolithic columns was reported, even though the chromatographic applications have been so far relatively rare [9]. Some earlier applications of silica-based monolithic columns in food analysis were covered in the review by Cabrera [10]. More recent specific reviews focused on the applications of monoliths in the analysis of milk and diary products [11] and on the applications in the analysis of proteins in food [12]. Some applications of monolithic columns are included in a recent review on new trends in fast liquid chromatography for food analysis [13]. The present review focuses on the remarkable progress recently achieved in the development of monolithic stationary phases, with attention to the reversed-phase, normal-phase, HILIC and ion-exchange applications of monolithic LC columns in food analysis. The applications of molecularly imprinted monolithic polymer media (MIPs) in sample treatment and monolithic columns in twodimensional separations (2D LC) are also addressed.

2. Silica and organic polymer monoliths – types, morphology, characterization and recent improvements In spite of the internal structure with dominant flow-through pores, the silica-based and the organic polymer monoliths show significant differences in pore morphology. The differences are

Fig. 1. The bimodal pore structure of silica-based monoliths.

apparent from the micrographs in Fig. 1. Silica-based monoliths have a bimodal pore structure with significant representation of 7–12 nm mesopores (∼13%) and relatively high specific surface area of several hundred m2 /g, similar to the specific surface area 150–400 m2 /g, typical for particles packed in conventional columns [7,14]. Hence the silica monoliths are ideal for separations of small molecules, as the mesopore size allows easy penetration to the adsorption sites and fast diffusion, resulting in high efficiency of separation. Like silica gel particles, the monolithic silica rods can be chemically modified by binding a variety of non-polar, weakly or strongly polar functionalities for various LC applications. Chemically bonded silica-based monoliths allow fast separations of low-molecular samples with column efficiencies up to 100,000 theoretical plates/m [15]. Commercial silica-based monolithic columns with chemically bonded alkyls such as Chromolith C18 or Chromolith C8 from Merck (Darmstadt, Germany) have been widely used for separation of various low-molecular compounds, including applications in food analysis [10]. These columns provide lower column pressure and large decrease in separation time in comparison to columns packed with fully porous and even fused-core particles, even though in some cases at the cost of lower chromatographic efficiency. Also the method transfer between the particulate and monolithic columns often requires adjustment of separation conditions [16,17]. Unfortunately, silica monoliths generally show less good performance for separations of macromolecular compounds such as proteins and other biopolymers. The separation of polymers requires different conditions than the separations of small molecules. The penetration of macromolecules requires wide pores (at least 15–100 nm) and relatively low specific surface area (10–150 m2 /g), to allow easy access to the interactive adsorbent surface within the pores. Further, limited stability at temperatures higher than 60 ◦ C and pH > 8.5, similar to silica particles, might limit their performance for the analysis of polar – especially basic – compounds. The polymer-based monoliths have a heterogeneous structure, which resembles rather a net of interconnected non-porous cauliflower-like microglobules with significantly lower surface area (Fig. 2). This morphology is more suitable for separation of polymers requiring large pores (15–100 nm) with relatively low specific surface area (10–150 m2 /g) [18]. The small pores in the microglobules of organic monoliths are generally inaccessible for large molecules of biopolymers, which are therefore not subject to slow diffusion decreasing the separation efficiency, unlike the silica-based monoliths with partially accessible larger mesopores [19]. In the past, organic monoliths have shown low efficiency for

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Fig. 2. The microglobular structure of organic polymer monoliths.

the separation of small molecules [20,21]. Besides the low proportion of small pores and resulting low surface area, inadequate “gel porosity” of the monolith swollen in the mobile phase was blamed for poor separation performance for low-molecular compounds [22]. The mesopores in organic polymer monoliths are often narrow enough to enable penetration of small molecules, but may cause slow diffusion, resulting in relatively low separation efficiency [23]. Organic polymer monoliths were produced in three formats [24]: flat cylindrical molds, forming thin disks (up to 3 mm), cylindrical monolithic rod-like columns prepared by in situ polymerization in glass or stainless steel tubing, 30–50 mm long, 1–8 mm i.d., enabling very fast protein separations [25,26] and, finally, monolithic capillaries, originally intended for capillary electrochromatography [27,28]; later employed also in capillary HPLC [29–31]. Organic polymer monoliths are usually – but not always [32] – prepared by radical polymerization of monovinyl and crosslinking divinyl monomers, in the presence of pore forming solvents, typically alcohols [33]. The polymerization reaction starts by decomposition of a radical initiator, usually 2,2 azobisisobutyronitrile (AIBN), induced by elevated temperature or exposition to UV light. At an early stage of the reaction, polymerization nuclei are formed and (usually) a divinyl crosslinking monomer is incorporated [34,35]. At a later stage, the nuclei grow and a highly interconnected moiety is formed. Finally, a heterogeneous, cauliflower-like, polymeric material fills the whole available (usually tubular) space, such as a fused-silica capillary (Fig. 2). The finished monolithic block is flushed to remove all non-reacted components of the polymerization mixture. A large selection of suitable monomers is available for preparation of polymer monolithic columns with a variety of surface chemistries. The most convenient and simple way to obtain the desired surface chemistry is direct copolymerization with reaction conditions optimized for each monomer mixture to achieve satisfactory performance of the resulting monolith [36,37]. Organic polymer monoliths are based on polystyrene-, polymethacrylate-, polyacrylamide- and other matrices [34,38]. Monolithic stationary phases can be synthesized using only a single monomer with cross-linking functionalities, such as divinylbenzene [39,40], glycerol dimethacrylate [41–45], methylenebis-acrylamide [46], poly(ethylene) diacrylate [47], bisphenol A dimetacrylate, bisphenol A ethoxylate diacrylate and pentaerythritol diacrylate monostearate [48,49]. Poly(styrene-co-divinylbenzene) (PS-DVB) monolithic polymers are prepared by free radical cross-linking copolymerization of styrene and divinylbenzene monomers in the presence of a pore forming agent as a diluent. The PS-DVB rods have

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relatively large flow pores with broad size distribution and irregular shape of the pores. The polymers consist of coalesced spherical globules between 10 and 30 ␮m in diameter. The column efficiency of PS-DVB monoliths is almost independent of the mobile phase velocity and the resolution is similar at the flow rate of 1 mL/min as at 25 mL/min (for the columns with i.d. of 8 mm) [50]. Other polystyrene-based monolithic columns were prepared using p-methylstyrene monomer co-polymerized with 1,2-bis(pvinylphenyl)ethane or with bis(p-vinylbenzyl)dimethylsilane in the presence of tetrahydrofuran and toluene [51], or 2-propanol and toluene [52]. Non-polar norbornene monoliths are prepared by a living polymerization which allows flexible surface grafting of various chromatographic ligands. Capillary monolithic columns with specific surface areas in the range of 60–210 m2 /g and pore size between 2 and 30 ␮m were obtained [53]. The norbornene monoliths functionalized with ␤-cyclodextrine were used for fast separations of proglumide enantiomers [54]. There is a broad range of commercially available moderately polar methacrylate monomers for synthesis poly(methacrylate) monolithic columns. Usually, an alkyl methacrylate functional monomer, such as butyl methacrylate, glycidyl methacrylate, 2hydroxyethyl methacrylate, or other methacrylic acid esters, is co-polymerized with ethylene dimethacrylate (EDMA) or other cross-linking agents, using mixtures of alcohols (1-propanol, 1,4butanediol or dodecanol) or tetrahydrofuran as porogen solvents [3,55,56]. The polarity and the size of the functional monomer and its concentration in the polymerization mixture affect the early stages of the polymerization, when the phase separation and formation of the first crosslinked polymeric nuclei occurs. Replacing a small percentage of butyl acrylate with a longer alkyl chain lauryl acrylate [57], or of butyl methacrylate with lauryl methacrylate monomer [58,59] leads to the formation of slightly smaller microglobules, increases the size of mesopores, improves the separation efficiency for small molecules and leads to stronger retention in the reversed-phase mode. Some combinations of polar and non-polar monomers may improve the pore size distribution and the surface area of monolithic materials and consequently its permeability and efficiency for low-molecular compounds. For example, incorporating methacrylic acid or polar methacrylate monomers into a highly non-polar polymerization mixture of styrene and divinylbenzene obviously contributes to the formation of a significant number of small pores, significantly increasing the surface area of monolithic columns. These materials show efficiency of 28,000 plates/m for the separation of small molecules [60]. On the other hand, addition of 2-hydroxyetlyl methacrylate into the polymerization mixture containing divinylbenzene and ethyl vinylbenzene increased the surface area of the monolith to 500 m2 /g [61]. In a series of poly(divinylbenzene-alkyl methacrylate) monolithic phases, increased chain length of alkyl methacrylates caused decrease in the retention of sulfonamide in capillary electrochromatography (CEC) [62]. By selection of a suitable dimethacrylate cross-linking monomer, the polymerization conditions could be tuned to prepare capillary polymethacrylate monolithic columns suitable for separations of either low-molecular compounds, or of biopolymers. Butyl methacrylate co-polymers with ethylene dimethacrylate (EDMA), and pentaerythritol tetraacrylate (PETA) cross-linkers provided median pore diameters of 0.6–3.8 ␮m in the monolithic structure, which allowed for fast separations of proteins [63]. Stearyl methacrylate-co-2-methyl-1,8-octanediol dimethacrylate monoliths also yielded good efficiency in gradient separations of proteins [64]. The efficiency, the retention and the separation selectivity of capillary (poly)methacrylate monolithic columns for small

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molecules and proteins increases for longer-chain (poly)methylene dimethacrylate cross-linkers, correlating with decreasing proportion of large through pores (>50 nm) and increasing proportion of smaller mesopores (1–50 nm) [65]. Further, the proportion of large through pores decreases and the separation efficiency for small molecules significantly improves when substituting non-polar poly(methylene dimethacrylates) with more polar poly(oxyethylene) dimethacrylate cross-linkers. The columns prepared with tetraoxyethylene dimethacrylate cross-linker show high efficiencies of approximately 70,000 theoretical plates/m, high permeability, excellent reproducibility and long-term stability at elevated temperatures and can be used also in some size-exclusion applications [65]. Post-polymerization chemical modification of reactive monomer groups allows introduction of various functionalities, while preserving the original porous structure of the monolithic materials. Glycidyl methacrylate is widely used functional methacrylate monomer. Its epoxy groups allow chemical conversion to non-polar butyl-, octyl- and phenyl- stationary phases, or to weak or strong anion-exchange or cation-exchangers [66], for example by using sulfonation based on ring opening of epoxides with Na2 SO3 [67]. Polar polyacrylamide monolithic polymers are formed by free-radical chain polymerization of acrlyamide, piperazine diacrylamide, or methacrylamide monomers and cross-linkers such as N,N -methylenebisacrylamide. Either AMPS or vinylsulfonic acid are generally added for the introduction of charge necessary in CEC applications, while stearoyl methacrylate or butyl methacrylate ligands are introduced to provide reversed-phase chromatographic retention and selectivity [68]. Unlike other organic polymer monoliths, polyacrylamide continuous beds synthesized in aqueous polymerization media show larger flow pores at increased polymerization temperature [69]. A broad variety of porous structures of poly-acrylamide gels ranging from closed macropores with microporous walls to open interconnected macropore systems with nonporous walls and uniform (in water) or oriented (in formamide) porosity can be produced by radical polymerization at sub-zero temperatures, adjusting the content of the initiator, freezing temperature and the solvent used [70]. In spite of high bed compression, polyacrylamide monolithic columns exhibit good permeability and, surprisingly enough, good efficiency and mechanical stability. Broad chemical variability and high temperature and chemical stability of organic polymer monoliths stimulated on-going efforts in tailored preparations of polymer-based highly efficient monolithic stationary phase suitable for fast and efficient analysis of low molecular weight compounds. Significant progress has been recently achieved in the preparation of organic polymer monoliths with improved separation efficiency up to 70,000–80,000 plates/m for small molecules, see several recent [20,23,70–73]. Various strategies were adopted to adjust the porosity and mass transfer in the organic polymer monoliths: (1) adjusting initiation conditions and temperature of polymerization, (2) controlling the time of the polymerization reaction, (3) careful optimization of the chemistry and proportions of functional monomers, crosslinkers and porogen solvents in the polymerization mixture, (4) post-polymerization monolith modifications, or (5) incorporating additional structural elements into the monolithic skeleton, such as carbon nanotubes. 1. Increasing polymerization temperature usually enhances formation of smaller pores and higher surface area of monolithic materials, connected with significant decrease in the column permeability and slower pore diffusion, hindering fast efficient separations [74]. Hence, lower polymerization temperatures may improve the column efficiency and permeability. With

monolithic columns prepared by UV initiated polymerization of butyl methacrylate and ethylene dimethacrylate at −15 ◦ C [75], or at 0 ◦ C [76], column efficiencies of approximately 45,000 plates/m were achieved for isocratic separations of lowmolecular samples. 2. The polymerization time significantly affects the pore formation and properties of polymer materials, as the polymer formation is the fastest at the early stage of the polymerization [77]. For example, glycidyl methacrylate-co-ethylene dimethacrylate monoliths after 1 h conversion contain significant amount of small mesopores lower than 50 nm [35]. Monolithic poly(4methylstyrene-co-1,2-bis(4-vinylphenyl)ethane) or poly(butyl methacrylate-co-ethylene dimethacrylate) capillary columns polymerized for less than 60 min instead of the typical 24 h, were reported to provide 70,000 plates/m for the separation of small molecules [78,79]. However, in this way the polymerization is stopped in the steepest part of the conversion reaction, so that the exact timing of the polymerization reaction may not be easy to achieve in a reproducible way. 3. The composition of the polymerization mixture has major effect on the pore distribution, permeability and efficiency of organic polymer monolithc columns. There are almost unlimited possibilities of combinations of functional monomers, crosslinking monomers and porogenic solvents which can potentially improve the chromatographic properties of the resulting monoliths prepared in a single-step polymerization approach. In spite of recent research activities in this direction, there is still a large field to be explored. The porogenic solvent very significantly affects the porosity. By the optimization of the composition and ratio of porogenic solvents, it is possible to tune the average macropore size of polymer monoliths within a range spanning from tens of nanometers to several micrometers [5,80]. The porosity of the poly(methacrylate) monoliths could be adjusted by changing the ratio of 1-propanol to 1,4-butanediol in the porogen system. At increasing proportion of 1-propanol, the size of flow pores increases. Mixtures of alcohols (1-propanol, 1,4-butanediol or dodecanol) and tetrahydrofuran used as porogen solvents significantly affect the pore morphology and the column efficiency [30,31,81–83]. Changing the concentration ratio of monomers to porogen solvents in the polymerization mixture affects the globule size independently of the pore size. By decreasing the monomer content, smaller microglobules are formed. A relatively minor change in the proportion of 1-propanol to 1,4-butanediol from 60:40 to 64:36 in the porogen mixture, resulted in the improvement in efficiency of butyl methacrylateco-ethylene dimethacrylate capillary monolithic columns for low molecular alkylbenzenes from 5000 to 35,000 plates/m [30]. Eeltink et al. [84] improved the efficiency of (poly)methacrylate-based monolithic columns from 13,000 plates/m to 67,000 plates/m by decreasing the concentration of the monomers in the polymerization mixture from 40 to 20%. Some authors claimed that the optimal pore size of the monolith was obtained with 12 wt% 1,4-butanediol in propanol as the porogenic solvent mixture [58,85]. Ring-opening polymerization reaction of polyhedral oligomeric silsesquioxanes (POSSs) was used for preparation of wellcontrolled 3D skeletal hybrid monoliths [86]. 4. Post-polymerization surface modification may provide independent control of the pore size distribution of both large flow-through pores and small mesopores in the organic polymer monoliths. Functional monomers can be grafted within large pores of a generic, well-defined monolith with controlled pore size. The desired surface chemistry is introduced via the photoinitiated polymerization onto the surface of the pores [87].

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Davankov reported on the hypercrosslinking surface modification approach more than 40 years ago [88]. This seemingly forgotten approach has been recently applied to monolithic stationary phases prepared from styrene and chloromethyl styrene monomers with divinylbenzene cross-linker. The procedure principally consists in a free-radical polymerization reaction, followed by Friedel–Crafts alkylation, fixing loose styrene and chloromethyl styrene polymeric chains covering the surface of the generic monolith [18,89]. The hypercrosslinking process results in monoliths showing the primary distribution of large flow-through pores of the generic material and a relatively thin hypercrosslinked layer with small pores on the pore surface, and was reported to dramatically increase the surface area from the original specific surface area of 17–56 m2 /g up to 630 m2 /g [18]. This topic is treated in more detail by Dario Arrua et al. [71,72]. 5. Horvath et al. [90] pioneering work on incorporating carbon nanotubes in the capillary monolithic columns was later elabˇ orated by the Svec’s group [91], who combined the addition of the carbon nanotubes to the polymerization mixture. The surface attachment of the cut carbon nanotubes was reported to provide monolithic capillary columns exhibiting the efficiency of 44,000 plates/m for benzene. The efficiency was further significantly improved by adding C60 fullerene-into the polymerization mixture [92]. 3. Reversed-phase separations on monolithic columns in food analysis 3.1. Silica-based monoliths Reversed-phase (RP) chromatography is principally used to separate samples on the basis of the differences in the non-polar parts of their molecules. Fast gradient elution at a high flow-rate allowed rapid and detailed profiling of urine and cell culture samples in metabonomic fingerprinting studies [93]. Silica monolithic C18 columns were used for separations of isoflavones in in natural samples [94,16] The analytes could be divided into three groups according to their hydrophobicity and were resolved by a ternary gradient elution in less than 15 min [95]. Anthocyanins and phenolic acids were separated on a monolithic RP-C18 HPLC column using UV [96,97] or sensitive chemiluminiscent detection [98]. A monolithic RP-C18 HPLC column was employed for rapid screening of compounds assumed to possess either nutritional value, or to be involved in plant desease resistance [99], and for fast resolution of ascorbic acid (vitamin C) from lipophilic compounds [100]. Preserving additives in various soft drinks were separated on a C18 monolithic column using a buffered (pH 2) aqueous acetonitrile mobile phase, with the addition of 10 mM octylamine to improve the peak shape [101]. Reversed-phase separations of low-polarity compounds need highly organic mobile phases. For example, rapid separation of fat-soluble vitamins A and E in human plasma was achieved on a 100 mm long Chromolith RP-18 column using a gradient from 90 to 100% methanol in water, after separation from proteins on an in-line connected Random Access Material (RAM) column [102]. Lycopen in food products was determined after supercritical fluid extraction with CO2 , using a single silica C18 monolithic column for analyte trapping and subsequent separation; the fast method eliminated sample degradation during the analysis [103]. A Chromolith RP-18 column with non-aqueous acetonitrile-2-propanol mobile phase allowed fast analysis of triacylglycerols, at a cost of lower resolution, in comparison to the separation on conventional particle-packed columns [104]. Short (50 mm) monolithic silica C18 columns were used for fast determination of caffeine and related products [105]. Recently

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introduced narrow-bore (2 mm i.d.) Chromolith RP18 column allowed rapid resolution of theobromine, theophylline and caffeine in less than 5 min, with large decrease in mobile phase consumption [106]. Commercial silica C18 monolithic columns were also employed for analysis of veterinary and human drugs, such as residues of nitrofuran veterinary drugs [107], of cephalosporin antibiotics [108] and in the LC–ESI-MS/MS analysis of five nut allergens in breakfast cereals and biscuits [17]. After pre-column derivatisation, aflatoxin contaminants in food were separated in 3 min on a Chromolith Performance RP18 column, coupled to a triple quadrupole mass spectrometer for identification [109]. A Chromolith RP 18 column was used for arsenic speciation in urine and food samples. Arsenobetaine, dimethyl arsenide and monomethyl arsonate could be separated within 3 min, using ionpair chromatography with 2.5 mM tetrabutylammonium bromide in 10 mM phosphate buffer (pH 5.6) and 1% methanol additive. The column was directly coupled with atomic absorption spectrometry detection [110]. Even though the main application areas of the silica monoliths are in the separation of low-molecular weight compounds, they can be also used for separations of biopolymers such as monitoring of protein profiles. Reversed-phase LC on a Chromolith Performance RP-18e column was used for establishing the genetic identity of food products [111]. Recently, reversed-phase gradient separation of peptides from tryptic digests on 4 serially connected, 0.1 mm i.d., monolithic silica gel capillary columns of total length 1 m, with impressing peak capacity of 1000, even though at extremely long separation times (700 min) was reported [112]. In addition to chemically bonded C18 silica monolithic columns, hybrid phenyl-silica monolithic capillary columns prepared by condensation and polymerization of benzylmethacrylate and vinyl trimethoxysilane in fused silica capillaries were used for separation of tryptic digests in buffered acetonitrile mobile phases [113]. Miniaturized commercial Silica C18 monolithic columns (10 mm × 4.6 mm i.d.) were incorporated into a commercial sequential injection analysis (SIA) system to enhance the possibilities of fast simultaneous analysis of several compounds in a single run to simultaneous injection chromatographic (SIC) analysis. The SIC methods were applied to simultaneous determination of fenoxycarb and permethryn pesticide residues in veterinary pharmaceutical foams and sprays [114], or to the simultaneous determination of food preservatives [115], antioxidants, and sweetener additives in food and cosmetics [116]. Some RP chromatography applications of silica-based monoliths in food analysis are reviewed in Table 1. 3.2. Organic polymer monoliths Monoliths based on hydrophobic organic polymers of a nature are very popular for the separation of proteins. Depending on the nature of the functional monomer, the monolithic columns show various degrees of non-polar interactions in RP applications [14]. Polystyrene-co-divinylbenzene (PS-DVB) monolithic columns usually provide better separation of bio polymers than silica-based monoliths, due to small portion of micro- and mesopores in the skeleton. PS-DVB monolithic columns are available commercially in conventional format and can be used for fast separations of proteins [117]. Tetrahydrofuran – decanol porogen mixtures provide PS-DVB monolithic capillary columns with lower hydrophobicity than C18 or C8 silica-bonded stationary phases, similar to C4 -silica particles, conventionally used for the separations of hydrophobic proteins [118]. The porous structure of PS-DVB monoliths for chromatography of biopolymers can be optimized by adjusting the concentration ratios of the monomers

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Table 1 Reversed-phase chromatography monolithic applications in food analysis. Analyte

Matrix

Monolithic column

Mobile phase

Flavonoids Anthocyanins Phenolic acids Phenolic preservatives

Soy extracts Red cabbage Fruit ripening process Health care products

Silica C18 Silica C18 Silica C18 Silica C18

Aqueous organic gradient

Ascorbic acid, phenolics, chaconine, solanine Ascorbic acid Sorbate and benzoate additives

Potato

Silica C18

Berry fruit extracts Soft drinks

Silica C18 Silica C18

Vitamins A and E

Human plasma

Chromolith RP-18

Lycopen

Tomato and food products

Silica C18

Triacylglycerols Caffeine Theobromine, theophylline, caffeine Nitrofuran veterinary drug Cephalosporin antibiotics Nut allergens Aflatoxins B-1, B-2, G(1), G(2)

Fats and oils Beverages, coffee Soft drinks, herbal products and pharmaceuticals Farm water, animal feeds Drug residues Breakfast cereals, biscuits Food contaminants

Chromolith RP-18 Silica C18 Chromolith RP18

Arsenobetaine, dimethyl arsenide, methyl arsonate

Urine and food

Chromolith RP18

Protein profiles Peptides

Food products Tryptic digests

Chromolith RP18 4 silica C18 capillaries

Peptides

Tryptic digests

Phenyl-silica capillaries

Fenoxycarb, permethryn

Veterinary pharmaceuticals

Silica C18

Preservatives, antioxidants, sweetener additives Antenna proteins Concanavalin A, lectins Photosynthetic membrane proteins Trypticin digests Tryptic digests

Food and cosmetics

Ribonuclease A lysozyme albumin myoglobin s-lactoglobulin human insulin bovine insulin porcine insulin Peptidic fragments Insulin, cytochrome C, albumin, lactoglobulin Ribonuclease A lysozyme albumin bovine insulin cytochrome C Caffeine

Note

Chemiluminiscent detection

Reference [94,16,95] [96] [97] [98] [99]

Aqueous acetonitrile, pH 2 Gradient 90–100% methanol in water

10 mM octylamine added to improve the peak shape After in-line separation from proteins on a RAM column On-line connected with supercritical fluid extraction

Acetonitrile-2-propanol

Silica C18 Silica C18 Silica C18 Chromolith RP18

2.5 mM tetrabutylammonium bromide in 10 mM phosphate buffer (pH 5.6) + 1% methanol

[100] [101]

[102]

[103]

[104] [105] [106]

LC–ESI-MS/MS Pre-column derivatisation, triple quad MS identification AAS detection for arsene speciation

Genetic identity study Buffered aqueous acetonitrile, gradient Buffered aqueous acetonitrile, gradient

[107] [108] [17] [109]

[110]

[111] [112] [113]

Sequential injection chromatographic analysis

[114]

[115,116]

Spinach, pea leaves Lentil, wheat germ Photosynthetic membrane proteins Rhizobia bacteria Various proteins

PS-DVB PS-DVB PS-DVB

Gradient ACN-water Gradient ACN-water Gradient ACN-water

[120,121] [123] [124]

PS-DVB disk PS-DVB capillary

[125] [126]

Bovine pancreas chicken egg white bovine serum horse muscle bovine milk yeast bovine pancreas porcine pancreas Tryptic digests Bovine serum and other

Poly(norbornene)

Gradient ACN-water Ion-pair RP chromatography Gradient ACN-water

Gradient ACN-water Gradient ACN-water

[132] [58]

Gradient ACN-water

[133]

Bovine pancreas chicken egg white human serum bovine pancreas bovine milk Tea, coffee and cocoa extracts

Poly(methacrylate) Poly(methacrylate) capillaries Poly(methacrylate) capillaries

Poly(methacrylate) capillaries

[127]

[134]

ACN: acetonitrile; PS-DVB: polystyrene-co-divinylbenzene monolithic columns.

to porogen solvents and the polymerization temperature. The retention of oligonucleotides increases on monolithic columns prepared with higher concentrations of monomers in the polymerization mixture, due to a larger surface area of the pores [52]. Courtois et al. [119] proposed using poly(ethylene glycol) (PEG) in the porogen solvent mixture to obtain more polar monoliths containing large pores with a protein-friendly surface, which

do not cause unfolding or denaturation of proteins. With increasing molecular weight of PEG in the polymerization mixture, the pore size increase and the surface area of the monoliths decrease. On PS-DVB monolithic columns, separation of proteins with similar molecular masses and amino acid sequences could be accomplished in approximately half the separation time in comparison to a 5 ␮m C4 -silica porous particle column [120,121].

P. Jandera / J. Chromatogr. A 1313 (2013) 37–53

Separations of peptide fragments from tryptic digests and membrane proteins on PS-DVB monolithic stationary phases in a single run in approximately 10 min, six time faster than on a C8 -silica bonded particle stationary phase were reported [122–124]. In proteome analysis, peptides in tryptic digests of various proteins were separated by ion-pair-reversed-phase chromatography on commercial poly(styrenedivinylbenzene)-capillary columns [125,126]. Monolithic capillary columns were prepared via ring-opening metathesis polymerization (ROMP) using norborn-2-ene (NBE) and 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene (DMN-H6) as monomers, were employed for HPLC–UV and HPLC–MS analysis of proteins and peptides [127]. Poly(methacrylate) capillary monolithic stationary phases for separation of biopolymers, differing in hydrophobicities were prepared from various functional monomers, such as glycidyl methacrylate with a highly reactive epoxy ring [38], isobornylmethacrylate [128], or C6- [129], C8- [130], C16- [131] methacrylate esters. Lauryl methacrylate-based monoliths provided fast gradient separation of peptidic fragments containing less than 20 amino acids in tryptic digests [132,58]. 0.1 mm i.d. monolithic capillary columns prepared by co-polymerization of ethyl methacrylate or of less polar lauryl methacrylate with trimethylolpropane triacrylate cross-linker were employed for separation of proteins in less than 10 min [133]. In comparison to the broad application area of organic polymer monoliths for separation of biopolymers, applications of organic polymer monoliths for small molecules in food analysis have been rather limited in the past; however the recent progress in preparation of monoliths with more favorable pore morphology, considerably improving the separation efficiency of organic monolithic polymer media, as discussed in Section 2, has opened new perspectives in this field. Polymethacrylate monolithic capillary columns were probably most frequently used for separation of lowmolecular compounds. For example, a hexyl methacrylate capillary monolithic column was used for fast and sensitive determination of caffeine in food (tea, coffee and cocoa extracts) [134]. Columns prepared with tetraoxyethylene dimethacrylate cross-linker show high efficiencies of approximately 70,000 theoretical plates/m, high permeability, and could be used for fast efficient separations of both low-molecular compounds [65]. Some examples of RP chromatography applications of organic polymer monoliths in food analysis are listed in Table 1.

4. Normal-phase and HILIC chromatography on monolithic columns in food analysis Strongly polar or ionic samples are usually too weakly retained in RP systems to allow successful separation. This problem can be often solved by employing polar columns in aqueousorganic mobile phases with high concentrations of acetonitrile or another water-miscible organic solvent. This aqueous-organic normal-phase LC separation mode (ANP) is also called Hydrophilic Interaction Liquid Chromatography (HILIC) [135]. Properties of stationary phases suitable for HILIC applications were recently reviewed [136,137]. In some applications, hydrophilic monolithic columns were successfully used [138,139]. Plain silica monoliths such as commercial unmodified Chromolith can be used in some HILIC applications. For example, highly efficient nano-LC separation of neutral xanthines was achieved on silica capillaries [140]. However, modification of polar bonded silica-based stationary phases with polar or ionexchange functional groups, such as coating with polyacrylamide, can significantly improve HILIC separations of peptides [141,142]. Comparable resolution, but in shorter analysis time with respect to

43

conventional TSKgel Amide-80 column, was achieved for pyridylamine derivatives of mono-, di- and trisaccharides from extracts of plants such as corn and soybean [143,144]. HILIC and ion-exchange separations were reported on silica monolithic columns with electrostatically attached latex particles [145]. Metal oxide monoliths can be also used for separations of polar low-molecular compounds. Zirconia monolithic columns were prepared by coating a silica-based monolith in a fused silica capillary with zirconium butoxide, followed by hydrolysis with 0.01 M acetic acid [146]. The zirconia monolith retained xanthine alkaloids considerably more strongly than silica, due to ligand-exchange process on the metal ion surface, so that dimethylxanthine isomers could be easily resolved [147]. Titania monolithic columns, providing improved pH and thermal stability with respect to silica-based monoliths, were recently synthesized through a sol–gel process and employed for HILIC separation of benzoic acid and vanillin in food samples such as plum, peach and apricot preserves or milk jelly, using isocratic elution with buffered acetonitrile-rich aqueous-organic mobile phases [148]. Monolithic polymer-based columns for HILIC separation mode can be prepared by incorporating polar (hydrophilic) moieties in the organic polymer matrix. Because of high hydrophobicity of the styrene backbone, the attempts at preparing styrene-based monoliths suitable for HILIC separations have not been very successful so far. Poly(methacrylate) monoliths are more frequently employed in the HILIC mode. Compared to poly(acrylamide) monoliths, broader range of commercially available hydrophilic methacrylate monomers with various functionalities are available. The alkyl methacrylate functional monomer can be substituted by a more polar neutral, ionic or zwitterionic acrylate or methacrylate to prepare poly(methacrylate) monoliths suitable for normal-phase, HILIC or ion-exchange separations [149]. A monolithic poly(N-(hydroxymethyl)methacrylamide)-co-EDMA capillary column was employed for HILIC separations of nucleic acid bases, nucleosides and oligonucleotides [150]. Similar monolithic capillary column for separation of various ionized solutes was prepared by co-polymerization of 2-hydroxymethacrylate (HEMA), ethylene dimethacrylate (EDMA) and methacrylamide [151]. Capillary monolithic columns were prepared by in situ reaction of tris(2,3-epoxypropyl) isocyanurate (TEPIC) with 4[(4-aminocyclohexyl)methyl] cyclohexylamine (BACM) in the presence of PEG. Moderately efficient isocratic separations of nucleic bases and nucleosides were possible on a TEPIC-BACM column under HILIC conditions in 90% acetonitrile in water [152]. Co-polymerization of glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA) with subsequent hydrolysis of the oxirane group yielded a polymeric monolithic Diol phase suitable for HILIC separations [153]. Five major opium alkaloids (narcotine, papaverine, thebaine, codeine and morphine) were separated on a hydrolyzed poly (GMA-co-EDMA-co-SPMA) monolithic column [154]. Monolithic columns containing neutral hydrophilic groups prepared by copolymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) with ethylene dimethacrylate (EDMA) showed dual retention mechanism and could be used both for RP and for normal-phase separation of small molecules in organic mobile phases, but provided too low polarity to be useful for HILIC separations [155]. Recently, monolithic stationary phases with hydrophilic functionalities, incorporating gold nanoparticles, attached through layered architecture on hypercrosslinked styrene – divinylbenzene matrices, were employed in hydrophilic interaction chromatography of nucleosides and peptides [156]. Some HILIC chromatography applications of monoliths in food analysis are reviewed in Table 2.

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Table 2 Applications of HILIC, ion-exchange and zwitterionic chromatography on monolithic columns in food analysis. Analyte Xanthine alkaloids Peptides Mono-, di- and trisaccharides Dimethylxanthine isomers Benzoic acid, vanillin Bases, nucleosides, oligonucleotides Nucleic bases, nucleosides Narcotine, papaverine, thebaine, codeine, morphine S-lactoglobulin, albumin, thyroglobulin Pectin methylesterase DNA

Matrix

Monolithic column

Mobile phase

Corn, soybean

Silica capillaries Polyacrylamide on silica Polyacrylamide on silica

ACN-water ACN-water ACN-water

Zirconia monolith Titania monolith

ACN-water BufferedACN-water

[147] [148]

Poly(methacrylate-comethacrylamide) TEPIC-BACM Hydrolyzed GMA-co-EDMA diol phase

BufferedACN-water

[149]

ACN-water BufferedACN-water

[152] [154]

Anion-exchange DEAE CIM disk SO3 -CIM + CM-CIM disks DEAE-CIM, QA-CIM disks

BufferedACN-water

[160]

BufferedACN-water BufferedACN-water

[161] [162]

Poly(acrylate) cation exchangers Polymethacrylate anion exchanger Cetylpyridinium on silica Cationic latex particles on silica Zwitterionic silica monolith Zwitterionic silica monolith Zwitterionic BVPE Zwitterionic polymethacrylate Zwitterionic polymethacrylate Zwitterionic polyacrylate

Salt gradient

[163–165]

Aqueous buffer

[166]

Aqueous buffer Aqueous buffer

[167] [168]

BufferedACN-water

[169]

BufferedACN-water

[170]

BufferedACN-water BufferedACN-water

[172] [173,174]

BufferedACN-water

[175]

BufferedACN-water

[176]

BufferedACN-water

]185–187]

BufferedACN-water

[186]

Plum, peach, apricot preserves, milk jelly

Bovine milk, bovine serum Tomato Genetically modified plants

Peptides, proteins Bromate Inorganic anions Inorganic anions Bases, nucleosides, 2-deoxynucleosides Glycopeptides Nucleic bases Proteins Neutral, basic, acidic Amides, phenols, benzoic acids Polyphenolic acids, flavone glycosides Peptides, amides, phenols

Drinking water

Zwitterionic polymethacrylate Cation exchange polymethacrylate - PETA

Note

Pyridylamine derivatives

Reference [140] [141,142] [143,144]

ACN: acetonitrile; TEPIC-BACM: tris(2,3-epoxypropyl) isocyanurate with 4-[(4-aminocyclohexyl)methyl] cyclohexylamine-co-polymer; GMA: glycidyl methacrylate, EDMA: ethylene dimethacrylate, CIM: continuous interaction medium; DEAE: diethylamino ethyl; BVPE: 1,2-bis(p-vinylphenyl ethane; PETA: pentaerythritol triacrylate.

5. Ion-exchange separations on monolithic columns in food analysis Organic polymer ion-exchange monolithic columns are available in conventional (rod), capillary, or disk format, and are applied mainly to separations of biopolymers (proteins, peptides, nucleic acids fragments), or of small anions (Table 2). Extremely short monolithic columns – disks based on PS-DVB, or poly(methacrylate) matrices are useful for fast isolation and purification of proteins, viruses, nucleic acids, etc., from biological matrices, and are often employed for this purpose in food analysis. Short length and high porosity of these materials enable high flow rates to be used at a small pressure drop. High separation efficiency of these materials for biopolymers is attributed to the fast mass transfer due to convection; that is why these materials are called “convective interaction media” (CIM). Diffusivity enhanced by the convective flow is approximately four orders of magnitude larger than the “free” diffusivity of biopolymers in the pores of standard particle-packed columns [157]. Biopolymers strongly interact with the monolith surface through multiple binding sites. The strength of the attachment to the disk strongly differs between the macromolecules and their separation is based on selective elution during a steep mobile phase gradient. Therefore the column performance is only slightly influenced by the column length and very short columns can be used to achieve efficient separations [158]. If

necessary, several monolithic discs can be stacked to form thicker separation media (3–5 mm) of the desired length for more efficient separations. The materials are available also in tubes of different sizes, even in large “production” format. Ion-exchange CIM disks, either with weak carboxymethyl (CM-CIM) or strong sulfonic acid cation-exchange groups (SO3 -CIM), and weak diethylaminoethyl (DEAE-CIM) or strong quaternary amine (QA-CIM) anion-exchange functional groups are used for separation of biopolymers. For example, stacked anion-exchange discs were employed for separation of nucleic acid fragments using linear salt gradients [159]. S-lactoglobulin from bovine milk, bovine serum albumin, thyroglobulin and plasmid DNA were separated on a diethylamine modified poly(methacrylate) anion-exchange monolithic disk [160]. Pectin methylesterase isoenzymes were extracted from tomato fruits and isolated on an SO3 -CIM disk, before subsequent purification in a second, semi-preparative step on two stacked CMCIM disks. The method was reported faster and more efficient in comparison to the earlier published isolation and purification procedures from plant materials [161]. In another application, DNA from maize and corn processed food was isolated on DEAE-CIM and QA-CIM anion-exchange disks, to detect genetically modified plants [162]. Poly(acrylate) and poly(methacrylate) ion-exchange monolithic capillary columns also can be used for separation of ionic compounds. For example, a hydrophilic poly(2-carboxyethyl

P. Jandera / J. Chromatogr. A 1313 (2013) 37–53

acrylate-co-poly(ethylene glycol) diacrylate) weak cationexchange monolithic capillary column enabled separation of 11 peptides with a 10-min salt gradient, or of 39 proteins with a 20-min salt gradient [163]. Monolithic capillary cation-exchange columns with moderately strong phosphoric acid and polyethylene glycol (PEG) functional groups are less hydrophobic in comparison to strong cation-exchange monoliths prepared using PEGDA as cross-linker, so that peptides and proteins could be separated without addition of acetonitrile to the mobile phase [164,165]. Trace levels of bromate in food and drinking water were determined by anion-exchange chromatography on a capillary poly(methacrylate) monolithic column modified with quaternary ammonium groups [166]. Inorganic anions in food samples were separated on a short (25 mm × 4.4 mm i.d.) silica monolithic column coated with cetylpyridinium chloride cationic surfactant [167], or on silica monoliths coated with polycationic latex nanoparticles [168]. Some ion-exchange chromatography applications of monoliths in food analysis are reviewed in Table 2.

6. Zwitterionic and mixed-mode separations on monolithic columns in food analysis Zwitterionic stationary phases with functionalities containing both cationic and anionic groups provide mixed-mode separation mechanism combining advantages of high hydrophilic selectivity and weak electrostatic (ion-exchange or ion-repulsion) interactions with charged analytes. Monolithic stationary phases with sulfobetaine groups, comprise cationic quaternary amines and sulfonic acid anions. The surface of a silica monolith can be modified via a free radical polymerization of a zwitterionic sulfobetaine monomer; or capillary organicsilica hybrid monolithic zwitterionic columns can be prepared using a single-step approach by in situ polymerization of a mixture containing inorganic and organic precursors, such as N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine (MEDSA), or 3-methoxysilylpropyl methacrylate (␥MAPS). By acidic hydrolysis of tetramethoxysilans in the presence of polyethylene glycol and urea, a silica monolithic capillary column was prepared and subsequently modified with MEDSA, and was employed for fast and efficient separations of nucleic bases, nucleosides and 2-deoxynucleosides under HILIC conditions [169]. Silica-based monolithic zwitterionic columns were used for separations of glycopeptides by their glycan composition, which could not be achieved in reversed-phase HPLC [170]. Porous zwitterionic monolithic columns were used also for efficient HILIC separations of various low-molecular weight neutral, basic and acidic compounds [171]. Zwitterionic monolithic columns on the basis of non-polar organic-polymer matrices are only rarely used. An exemption is a 0.2 mm i.d. capillary column, prepared by co-polymerization of MEDSA with 1,2-bis(p-vinylphenyl ethane) (BVPE), which was employed for separation of nucleic bases [172]. More frequently, zwitterionic groups are incorporated into poly(methacrylate) structures. Poly(methacrylate) monolithic columns prepared by photoinitiated co-polymerization of MEDSA and ethylene dimethacrylate (EDMA) cross-linker, and initially intended for cation-exchange separation of proteins [173,174]. Later, Jiang et al. [175] prepared a hydrophilic monolithic column from the same monomers, by thermally initiated co-polymerization inside a 100-␮m i.d. fused-silica capillary, suitable for HILIC separations of neutral, basic, and acidic polar analytes in aqueous-organic mobile phases with 60% or more acetonitrile, and reported very good separation selectivity in comparison to particle-packed zwitterionic columns. By photo-initiated co-polymerization of MEDSA and poly(ethylene glycol) diacrylate in a binary porogen system

45

comprising isopropanol and decanol in 75-␮m-i.d. fused silica capillaries, monolithic columns were prepared showing good separations of amides, phenols, and benzoic acids by hydrophilic interaction chromatography in mobile phase with high concentrations of acetonitrile [176]. The porosity, permeability, selectivity and retention characteristics of monolithic sulfobetaine columns depend on the concentration of MEDSA and the composition of the watercontaining porogen solvents in the polymerization mixture [177]. Generally, the retention of low-molecular samples on zwitterionic columns is little affected by temperature [178]. Probably, entropic effects play important role in the separation, possibly due to different hydrogen-bonding sample solvation in the occluded stationary liquid phase and the bulk mobile phase. Even though the quality of isocratic and gradient separations of phenolic acids and flavonoid compounds in beverages on the MEDSA-EDMA monolithic column significantly improved at 60–80 ◦ C, in comparison to the separation at 30 ◦ C, the separation efficiency was still rather low [179]. A recent study revealed that the efficiency of monolithic columns for polar low-molecular compounds in the HILIC mode can be significantly improved by co-polymerization of MEDSA with polar cross-linking molecules, dioxyethylene dimethacrylate (DiEDMA), or bisphenol A glycerolate dimethacrylate (BIGDMA). After improved pre-treatment of the fused-silica capillary walls before in situ polymerization, poly(methacrylate) monolithic columns, 0.32 and even 0.56 mm i.d., 15–66 mm long, stable up to 100 ◦ C, could be reproducibly prepared. The columns showed increased proportion of pores below 50 nm and efficiencies of at least 60,000–70,000 theoretical plates/m for low-molecular compounds in the HILIC mode [180] and were used for separations of strongly polar polyphenolic acids and flavone glycosides [181]. Various interactions may contribute to the retention on HILIC stationary phases containing anionic, cationic or zwitterionic groups, depending on the type of stationary phase, sample, and on the composition of the aqueous-organic mobile phases [136,182,183]. Hydrogen bonding is probably the most important effect at very low concentrations of water [184]. Electrostatic interactions affect mainly the retention of ionic or ionizable polar analytes. Mixed-mode hydrophilic interaction/ion-exchange chromatography (HILIC/IEX) columns can be applied to the separation of peptides with low differences in hydrophilicity/hydrophobicity (by only a single carbon atom); or of peptides differing only in the sequences of amino acids. The greater the charge on the peptide molecules, the better is the separation achievable by HILIC/IEX. Furthermore, HILIC/IEX separation of peptides on columns with cation-exchange functionalities was superior in comparison to the more commonly applied RP mode [185]. A capillary column prepared by co-polymerization of 2-acrylamido-2-methyl1-propanesulfonic acid with pentaerythritol triacrylate (PETA), was suitable for separation of charged peptides, amides and phenols by mixed-mode HILIC-cation exchange mechanism [186]. Solvophobic interactions may more or less contribute to the retention on polar columns by reversed-phase mechanism. Many polar columns, including monolithic ones, show combined HILC/RP retention mechanism for strongly or moderately polar compounds. The RP behavior predominates in water-rich mobile phases, and the retention decreases at increasing concentrations of the organic solvent. When a minimum retention is achieved, the retention again increases at higher concentrations of the organic solvent, as the polar interactions become stronger than the solvophobic ones. Hence U-shape plots of the retention factors (k) versus the concentration of the organic solvent in the mobile phase are observed, with a minimum corresponding to the transition from the RP to the NP (HILIC) mechanism [187]. Dual HILIC/RP retention mechanism

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P. Jandera / J. Chromatogr. A 1313 (2013) 37–53

7. Bioaffinity monolithic materials and sorbents with imprinted molecules in food analysis 7.1. Materials and techniques

Fig. 3. The effects of the mobile phase on the retention factor, k, of salicylic acid on a monolithic zwitterionic DiEDMA-MEDSA column (179 × 0.36 mm i.d.). Polymerization mixture: N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine (MEDSA) + dioxyethylene dimethacrylate cross-linker. ϕ, H2 O – volume fraction of water in aqueous acetonitrile + 10 mM ammonium acetate.

and U-shape plots were observed for various polar compounds on monolithic sulfobetaine polymethacrylate capillary columns prepared by polymerization of MEDSA with various cross-linkers [177,180], see the example in Fig. 3. The HILIC/RP dual retention mechanism for phenolic acids and flavonoid compounds was typical for all poly(methacrylate) zwitterionic columns: HILIC in acetonitrile-rich mobile phases and RP in mobile phases with higher concentrations of water. Some columns show broad pore distribution and can be used for size-exclusion chromatography of non-polar polymers in tetrahydrofuran. As the separation selectivities in the HILIC and in the RP modes on the polar columns are more or less complementary, additional information on the sample can be obtained in alternating experiments with gradients of increasing (RP mode) and decreasing (HILIC mode) concentrations of acetonitrile over a broad mobile phase composition range, as show Figs. 4a and 4b for phenolic compounds on a zwitterionic MEDSA-BIGDMA column [181]. Some chromatographic applications of zwitterionic and mixed-mode monoliths in food analysis are reviewed in Table 2.

Fig. 4. Separation of phenolic acids on on a monolithic zwitterionic DiEDMA-MEDSA column (179 × 0.36 mm i.d.). (a) HILIC mode, 85% acetonitrile and 15% 10 mM NH4 Ac, flow rate 2.5 ␮L/min. (b) RP mode, 40% ACN/60% 10 mM NH4 Ac, flow rate 6.3 ␮L/min.

Bioaffinity monolithic materials include covalently immobilized antibodies for selective isolation, purification, pre-concentration and analysis of biological agents, usually pollutants or toxins, from clinical, environmental, or food samples. Hydrophilic monoliths, either in the form of rod columns, or of disks, are suitable materials for attaching the desired antibody molecules, because of suppressed non-specific binding activity. Molecularly imprinted polymer materials (MIPs) mimic the function of bioaffinity adsorbents by introducing selective molecular features of samples to the stationary phase [188]. MIPs can be simply prepared on the basis of organic polymer monolithic skeleton. The target analyte, or a structurally similar template is included as a component of the polymerization mixture, thus introducing memory to a three-dimensional polymer network in the final polymer material in the form of shape-selective sites (cavities), which can recognize the molecules of target analytes in complicated samples and extract them from a matrix. MIPs have found applications in food analysis, mainly for sample pre-treatment using solid-phase extraction (SPE). MI-SPE is principally based on the preferential partitioning of sample constituents between the solid stationary and the liquid mobile phase, controlled mainly by differences in the shape of molecules and can be applied either off-line, or connected in line with a HPLC column. Off-line applications of MI-SPE columns consist first in conditioning with an appropriate solvent/buffer; then the sample is loaded. In “molecular recognition” step, the loaded column is washed with a solvent that will promote specific interactions between the stationary phase and the imprinted analyte, while disrupting non-specific binding. An additional washing step is usually added, followed by elution of the selectively bound analyte(s) with a strong eluting system [189] (Fig. 5). MIP monoliths can be synthesized inside capillaries or as SPE extraction fibers, to be employed prior to injection into a liquid or gas chromatograph. Such approach has been successfully used in food analysis, e.g., for the determination of triazine herbicides in cereals and vegetables [190]. A major problem encountered in practical MI-SPE applications are non-specific effects, causing sorption of both target and non-target analytes on the MIP surface. The problem can be often solved by careful selection of washing and extracting solvents [191]. The in-line MI-SPE systems approaches offer possibilities for automation of the whole pre-treatment/enrichment and analytical process [192]. A MI-SPE pre-column packed with a few milligrams of imprinted polymer is connected to an LC analytical column directly via a switching valve (Fig. 6). Initially, the MIP column is loaded with the sample of interest and all necessary intermediate washing steps are performed using a secondary pump. The analytes retained on the MIP column are desorbed and transferred into a holding loop, using the LC mobile phase as the eluting solvent for the MIP-SPE. After valve switching, the sample is injected into the analytical column for separation. The on-line technique can be applied also in immunoaffinity chromatography. For example, ochratoxin A is natural contaminant, which can occur in a large variety of commodities (cereals, beans, nuts, spices, dried fruits, coffee, beer, wine) and in the meat and intestines of animals fed with contaminated feed. Antiochratoxin antibodies immobilized through epoxy groups in a poly(methacrylate) monolithic capillary column, which was used for specific pre-concentration of ochratoxin-A natural pollutant [193].

P. Jandera / J. Chromatogr. A 1313 (2013) 37–53

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Fig. 5. Schematic diagram of the imprinting process.

7.2. Applications of MI-SPE in food analysis The applications of MI-SPE in food analysis involve the extraction and isolation of food components, additives, contaminants, minerals, trace elements, pesticide and pharmaceutical residues. Non-selective adsorption based on RP-behavior may cause problems with MIP applications for aqueous samples. Nevertheless, caffeine-imprinted polymers were used for extraction and isolation of caffeine and theophylline from coffee and cola soft drinks [194] or from green tea [195]. Sequential selective MI-SPE was applied to the analysis of waste water from the production of olive oil for isolation of caffeic acid, followed by p-hydroxybenzoic acid, each on an individual imprinted polymer cartridge column [196].

Fig. 6. Principle of the on-line coupling to MIP-SPE.

MIPs based on 4-vinylpyridine (4-VPy) as the functional monomer were applied to the extraction of quercetin and other related flavonols, such as naringin, from white and red wine, orange juice and tea, with significant enrichment of the templated analytes [197]. MIPs prepared using a 2,6-bis(acrylamido)pyridine (BAAP) monomer, with a hydrophilic pentaerythritol triacrylate (PETA) cross-linker and tetraacetate ester of riboflavin as the template, for recognition and selective removal of riboflavin (vitamin B2) from highly aqueous samples, such as commercial multivitamin tablets, milk and beer, with recoveries up to 95% [198]. The commercial SupelMIPTM Riboflavin material was applied in an online SPE-LC determination of the vitamin in milk and energy drinks [199]; ␣tocopherol (vitamin E) was extracted from vegetable leaves [200]. MIPs with cholesterol templates have been used in the analysis of cholesterol in cheese products, providing 80% recovery of cholesterol [201]. Benzo[a]pyrene, which is considered a dangerous carcinogenic and endocrine disrupting agent, is common contaminant found in foodstuffs whose production entails high temperature treatment, such as roasted coffee beans. As this polyaromatic hydrocarbon does not possess reactive functional groups, molecular imprinting is rather difficult. The MIPs prepared using 4-vinylpyridine-co-divinylbenzene with aromatic hydrocarbon skeleton matching the polycyclic aromatic hydrocarbon structure of the benzo[a]pyrene, provided up to 72.5% recovery of the template when applied to the extraction of PAHs from instant coffee samples [202]. Bisphenol A is often used as polymer additive in the synthesis of common polycarbonate food and plastic beverage containers, from which this endocrine disruptor can penetrate into foods. 4Vinylpyridine/ethylene dimethacrylate co-polymers were found to be suitable monolithic matrices for imprinting bisphenol A, allowing extraction recoveries of up to 76% from shrimp samples [203]. MI-SPE columns could extract this contaminant from spiked milk samples, after dilution with water and methanol, with recoveries of 80% [204]. These materials could also selectively extract phenols and phenoxyacids from spiked honey samples [205].

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Phthalates, commonly used plasticizers, are environmental pollutants often contained in food products, distributed in plastic boxes or vessels. Using dibutyl phthalate as the template, diethyl, dibutyl and dioctyl phthalates could be detected in real samples of soybean milk and extracted using a dibutyl phthalate imprinted MIP [206]. Melamine, originated from melamine-formaldehyde plastic materials, or intentionally added to milk in order to artificially increase the nitrogen content, can induce renal failure and even death in children and infants. Using melamine-imprinted MIPs, recoveries higher than 90% from milk samples could be achieved [207]. Pesticides are frequently found contaminants in food. Among them, triazine herbicides are probably the most frequently analyzed using the MI-SPE techniques. Atrazine is very frequently used template, due to its stability and relatively low toxicity. Using a triazine MIP in a clean-up step prior to analysis by either HPLC or ELISA, the limit of detection of 0.005 ppm atrazine was achieved

in beef liver organic extracts [208]. Other triazine MIP templates were also reported. An MI-SPE column with simazine template, coupled in-line with a C18 HPLC column, provided enrichment factors up to 100 for triazines in water samples containing humic acid, but significantly less in urine or apple extract samples, due to non-specific sorption of matrix components [209]. An ametryn imprinted polymer has a high degree of “class-selectivity” for fourteen triazines and metabolites, but the solvents had to be carefully selected to reduce non-specific interactions [210]. A MIP column with propazine template, was used for the analysis of corn and potato [211]. A monolithic column with imprinted fungicide thiabendazole was employed in capillary electrochromatography of various fruit samples, without additional need for sample cleaning pre-treatment [212]. A fenuron-imprinted (poly)methacrylate monolithic sorbent [213], or a poly(trifluoromethacylic acid) MIP imprinted with isoproturon [214], were used for the analysis of phenylurea herbicides in wheat, carrot, barley and potato.

Table 3 Applications of monolithic molecularly imprinted polymer sorbents (MIP) for SPE in food analysis. Analyte

Matrix

MIP

Caffeine theophylline Caffeine, theophylline Quercetin, naringin

Coffee, cola Green tea Wine, orange juice, tea

Riboflavin

Multivitamin tablets, milk, beer

Riboflavin ␣-Tocopherol Cholesterol Benzo[a]pyrene

Milk, energy drinks Vegetable leaves Cheese Instant coffee

Bisphenol A

Shrimp Milk

Caffeine-imprinted polyamide Caffeine-imprinted polyamide Quercetin-imprinted poly(vinylpyridine) BAAP - PETA imprinted with tetraacetate ester of riboflavin Supel MIPTM Riboflavin vitamin E MIP Cholesterol MIP Vinylpyridine-codivinylbenzene Bisphenol A-imprinted 4-vinylpyridine/ethylene dimethacrylate co-polymer

Phenols, phenoxyacids Diethyl, dibutyl, dioctyl phthalates Melamine Triazines Triazines Triazines Triazines Thiabendazole Phenylurea herbicides

Honey Soybean milk Milk Beef liver Water, urine, apple extracts Class-selective applications Corn, potato Fruit Wheat, carrot, barley, potato

Phenylurea herbicides Dichlorvos Carbaryl Domoic acid Ochratoxin A

Cucumber and lettuce Apple homogenate Shellfish, mussel Wine

Zearalenone Ampicillin Chloramphenicol Tetracycline Ofloxacin, ciprofloxacin, eurofloxacin

Swine feed, cereals Honey Milk Milk, honey Baby food, egg, swine Honey

Cimaterol, ractopamine, clenproperol, clenbuterol, brombuterol, mabuterol, mapenterol, isoxsuprine Steroid endocrine disruptors

Bovine muscle, rabbit, duck, turkey, fish Milk powder

Sulfamethazine, fenbendazole

Milk, yoghurt, beef, pork, chicken Milk, beef liver

Dibutyl phthalate MIP Melamine MIP Atrazine MIP Simazine MIP Ametryn MIP Propazine MIP Thiabendazole MIP Fenuron-imprinted (poly)methacrylate Isoproturon-imprinted (poly)trifluoroacrylate Dichlorvos-imprinted MIP Carbaryl-imprinted MIP o-Phthalic acid MIP (S)-2-(1-hydroxy-2naphthamido)-3phenylpropanoic acid MIP Zearalenone MIP Ampicillin MIP Chloramphenicol MIP Tetracycline MIP Ciprofloxacin MIP Ofloxacin-poly(methacrylic) MIP Supel MIPTM ␤-agonists feed additives

Note

Reference

Flavones

[194] [195] [197]

Vitamin B2

[198]

Online SPE-LC Vitamin E PAH contaminant

[199] [200] [201] [202]

Contaminant

[203][204]

Contaminants Contaminants Contaminant Herbicides Herbicides Herbicides Herbicides Fungicide, CEC

[205] [206] [207] [208] [209] [210] [211] [212] [213] [214]

Organophosphorous pesticide Carbamate insecticide Natural toxin Natural toxin

[215] [216] [217] [218]

Mycotoxin Antibiotics Antibiotics Antibiotics Fluoroquinolone residues

[219] [220] [221] [222] [223] [224]

Banned ␤-agonists

[225]

17␤-Estradiolpoly(methacrylic) MIP

[226]

[227] Sulfamethazine MIP, fenbendazole MIP

Antibacterial drug, antihelmintic drug

[228]

SPE: solid-phase extraction; PAH: polyaromatic hydrocarbons; BAAP-PETA: poly(2,6-bis[acrylamido]pyridine-co pentaerythritol triacrylate); CEC: capillary electrochromatography.

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MIPs with a dichlorvos and carbaryl templates, packed into precolumns coupled directly to an HPLC analytical column, were employed to detect and identify the organophosphorous pesticide residues [215] and carbamate insecticides [216] in fruit and vegetable. Naturally occurring toxins in food such as the domoic acid was isolated from the contaminated shellfish samples or mussel extracts by MI-SPE monolithic polymers with imprinted o-phthalic acid as a toxin analog template with matching structure and pKa [217]. MIPs for selective SPE extraction of ochratoxin A, were prepared by polymerization using poly(acrylamide) functional monomers, and (S)-2-(1-hydroxy-2-naphthamido)-3phenylpropanoic acid as the template analog. Ochratoxin A in wine samples was determined using first a C18 column sample pre-treatment, followed by MI-SPE of the toxin, with the recoveries higher than 90% [218]. Zearalenone, an estrogenic mycotoxin produced by several Fusarium species in maize, sorghum, wheat, barley, oats and other cereal grains, was selectively extracted on a MIP with imprinted template, with reported recoveries between 85 and 95% from swine feed and cereal samples [219]. Pharmaceuticals, employed in animal agriculture for treating animal bacterial diseases as well as to promote animal growth when used as feed additives, are often administered at too high doses, causing contamination of the environment and of food products of animal origin and eventually inducing pathogen resistance to clinical drugs (antibiotics). MI-SPE has found applications in the analysis of pharmaceuticals mainly in meat, fish and milk products. Ampicillin imprinted microspheres prepared by suspension polymerization were employed to determine the concentrations of the antibiotic in honey with recoveries higher than 89%, however the MIP materials provide low selectivity factors over other antibiotics, amoxicillin, oxacillin and penicillin V [220]. A simple and fast selective method using MI-SPE was reported for the extraction of chloramphenicol from milk, prior to the final analysis by LC–MS, with recoveries up to 87% [221]. A monolithic column imprinted with the tetracycline antibiotic prepared by in situ polymerization was coupled in-line to a C18 LC analytical column and applied for the determination of antibiotics in milk and honey, with recoveries up to 82–90% [222]. A ciprofloxacin MIP column was applied for the SPE extraction of nine quinolones and fluoroquinolones from baby food, without co-extracted interfering substances, unlike the SPE on an anion-exchange column [223]. Recently, an ofloxacin-imprinted poly(methacrylic acid-EDMA) monolithic column was prepared by in situ polymerization in a stainless-steel column and applied for isolation of fluoroquinolone residues (ofloxacin, ciprofloxacin and eurofloxacin) in honey samples [224]. MI-SPE on a commercial SupelMIPTM ␤-agonists material, followed by LC–MSn analysis, was used for the determination of cimaterol, ractopamine, clenproperol, clenbuterol, brombuterol, mabuterol, mapenterol and isoxsuprine, banned beta-agonists feed additives for growth promotion in cattle and other farm animals, in digested bovine muscle, in rabbit, duck, turkey and fish samples [225]. A MIP prepared with 17␤-estradiol template, trifluoromethacylic acid monomer and trimethylolpropane trimethacrylate cross-linker, were applied for the recovery of related steroids in milk powder samples [226] This endocrine disruptor could be detected off-line at low levels (0.116–0.461 nmol kg−1 ) in milk, yoghurt, beef, pork and chicken samples with recoveries ranging from 64.20 to 74.18% [227]. A coupled MI-SPE–square wave voltammetry provided recoveries as high as 95% in the analysis of antibacterial sulfamethazine in milk and later of anthelmintic drug fenbendazole in beef liver [228]. Some applications of MIP sorbents in food analysis are overviewed in Table 3.

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Fig. 7. A standard comprehensive LC × LC setup with a ten-port valve modulator interface and two collecting loops, operating in alternating cycles, A and B.

8. Two-dimensional applications of monolithic columns in food analysis Two-dimensional (2D) HPLC separations can be used to significantly increase the number of compounds separated in a single run in comparison to single dimension separations and to classify the sample on the basis of structural similarities. Combining different LC separation mechanisms, such as reversed-phase (RP), ion-exchange (IEX) and normal-phase (NP) or HILIC chromatography is aimed to accomplish non-correlated retention with maximum peak capacity. 2D liquid chromatography is practiced either in on-line or in off-line setup. In the “heart-cut” setup, only a few selected fractions from the first-dimension column are separated on a second-dimension column in alternating cycles controlled via a switching valve interface (Fig. 7). In comprehensive 2D LC × LC chromatographic techniques, the whole effluent from the first-dimension column is collected in fractions, which are inline transferred to the second-dimension column. Comprehensive LC × LC has experienced rapid development during the past years, because it allows automatic performance avoiding tedious and time-consuming off-line fraction transfer between the separation systems. The chemistry of the stationary phase and the composition of the mobile phase in each dimension should match for real-time 2D operation. For in-line comprehensive LC × LC practice, it is essential that the separation in the second dimension should be accomplished while the next fraction from the first dimension is collected, usually in less than 1 min [229]. Such fast efficient separations require short efficient columns, fast gradients and high flow-rates of the mobile phase [230,231]. This can be accomplished on columns packed with sub-2 ␮m particles, but at the cost of very high operation pressures. Good second-dimension performance can be achieved also at 40 MPa or less, with standard HPLC instrumentation, when using core–shell particles [232]. Monolithic silica-based columns

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are also very useful in the second-dimension, due to their high permeability and efficiency [233]. In-line two-dimensional combination of two reversed-phase systems, including one or two monolithic columns, is most straightforward. A monolithic silica-bonded C18 column in the second dimension, in combination with cyano- and amino-bonded columns in the first dimensions, were used already 10 years ago for separations of pharmaceuticals and of aromatic amines [234]. The number of applications of short monolithic columns in the second dimension has rapidly increased. For example, using a 2D set-up with a monolithic silica C18 column in the second dimension, coupled to a cation-exchange column in the first dimension, more than 1000 proteins were identified in arabidopsis leaves [235]. In another application, natural compounds in the extracts of umbelliferae herbs were separated using a gradient of acetonitrile in water on a bonded cyanopropyl column in the first dimension and a gradient of methanol in water on a monolithic C18 column in the second dimension [236]. Comprehensive 2-D HPLC systems for the separation of phenolic antioxidants have been developed on the basis of different selectivities of a PEG-silica column in the first dimension and a packed or monolithic C18 or a ZR-CARBON column, respectively, in the second dimension [237]. Combinations of PEG-silica, phenyl-silica and C18 silica-bonded columns in the first dimension and a monolithic Chromolith C18 column in the second dimension provided low selectivity correlations, improved system orthogonality and bandwidths suppression in comprehensive LC × LC separation of phenolic and flavone natural antioxidants, with more regular band distribution over the whole 2D retention plane and increased peak capacity [238]. For the LC × LC separation of phenolic acids in wine and juices, an RPLC separation in the first dimension was combined with ion-pair chromatography in the second dimension, where a monolithic column provided better results in comparison to short columns, packed with either 2.5 or 1.7 ␮m particles, in terms of the peak capacities [239]. Two C18 monolithic silica columns were combined in a 2D setup; the selectivity differences between the two reversed-phase systems were obtained by employing different aqueous-organic mobile phases in the first and in the second dimensions, either using different concentrations of methanol on each column [233], or 22% tetrahydrofuran in the first and 45% methanol in the second dimension [240]. Acidic compounds were preferentially adsorbed in aqueous tetrahydrofuran, while neutral ones in methanolic mobile phases. Because of the opposite effects of sample polarity on the retention, combinations of normal-phase (NP) and reversed-phase (RP) systems in 2D systems principally provide a highly orthogonal system with complementary selectivities in the two dimensions, which can be very useful for achieving high numbers of separated compounds (high peak capacity) when analyzing complex samples containing mixtures of uncharged compounds of comparable dimensions, differing in polarity and (or) hydrophobicity. However, the problem is in poor compatibility of the organic mobile phase in the NP system and the aqueous-organic mobile phase in RP mode. A microbore silica column operated at a flow rate of 20 ␮L/min in the NP mode in the first dimension and a monolithic type C18 column in the second dimension at 4 mL/min in gradient mode permits the transfer of a small fraction volume in the secondary column, making the transfer of incompatible solvents from the first to the second dimension possible, unfortunately at a cost of some loss of sensitivity in two-dimensional separations. Such 2D systems were used in the analysis of the oxygen heterocyclic fraction of cold-pressed lemon oil, containing coumarins and psoralens, which contain hydroxyl, methoxyl, isopentenyl, isopentenyloxyl, geranyloxyl groups and terpenoid groups, such as epoxides or vicinal diol groups [241].

Fig. 8. 2-D comprehensive LC × LC separation of 28 phenolic acids and flavones using simultaneous gradient elution in the HILICxRP setup. First dimension: a BIGDMA-MEDSA monolithic column, 210 × 0.53 mm i.d.; (60 ◦ C); second dimension: Kinetex XB-C18, 30 × 3 mm i.d., core–shell column (50 ◦ C). The profiles of acetonitrile gradients in D1: dashed lines; in D2: full lines Interface: 10 ␮L sampling loops, switching time 1.5 min. Polymerization mixture: N,N-dimethylN-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine (MEDSA) + bisphenol A glycerolate dimethacrylate (BIGDMA).

Similar system compatibility problems arise from the differences in the elution strengths of aqueous-organic mobile phases in the HILIC and RP modes in comprehensive in-line HILIC × RP 2D systems. This problem can be solved by semi-offline approach, trapping fractions from the first, HILIC dimension (possibly after diluting with a solvent possessing low elution strength) and removing the solvent before the injection into the second dimension [242]. An in-line solution consists in using very small transferred fraction volumes (e.g., 2 ␮L) to suppress the unwanted sample solvent effects on band broadening or deformation in the second dimension, as shown above. This approach was employed using a novel monolithic zwitterionic bisphenol A glycerolate dimethacrylate – N,N-dimethyl-N-methacryloxyethylN-(3-sulfopropyl)ammonium betaine (BIGDMA-MEDSA) microcolumn of 0.53 mm i.d. in the first dimension, which could be operated either in the HILIC or in the RP mode, coupled in-line with a short non-polar core–shell or silica-monolithic column in the second (RP) dimension. The BIGDMA-MEDSA microcolumn provided significantly improved separation in comparison to a commercial ZIC-HILIC zwitterionic microcolumn [181]. Figs. 8 and 9 show comprehensive two-dimensional LC × LC separations of phenolic and flavonoid compounds on a 0.53 mm i.d. monolithic BIGDMAMEDSA zwitterionic (poly)methacrylate monolithic column in the first dimension, in combination with a Kinetex XB-C18, 30 × 3 mm i.d. core–shell column in the second dimension, in the NP × RP and in the RP × RP setups. The setup can be used with repeated sample analysis, employing alternating gradients of decreasing (HILIC mode), and increasing (RP mode) concentration of acetonitrile in the first-dimension, to obtain useful complementary information on the sample [181].

P. Jandera / J. Chromatogr. A 1313 (2013) 37–53

Fig. 9. 2-D comprehensive LC × LC separation of 28 phenolic acids and flavones using simultaneous gradient elution in the RPxRP setup. Conditions as in Fig. 8, except for reversed gradient direction (mobile phase A: 0.01 mol/L in water (pH = 3.1); mobile phase B: 0.01 mol/L CH3 COONH4 in acetonitrile).

In proteome analysis, a multidimensional shotgun approach allows separations of hundreds of unique proteins in a single run. Strong cation-exchange (SCX) chromatography combined with reversed-phase LC and coupled to MS or to MS/MS is widely employed for peptide analysis. Here again, C18 silica monolithic columns can be used in the fast second dimension. The use of capillary monolithic silica column for this purpose was also reported [243]. 9. Conclusions, future perspectives Monolithic columns present some significant advantages over particle-based materials, which explains the continuing efforts in the development and improvement of monolithic technology and ever increasing interest in their practical applications. These columns are free from a densely packed bed of particles and do not contain frits, which may cause problems when plugged by solid impurities. Much higher external porosity compared to conventional particle-packed columns results in higher permeability and lower column backpressure, which allows high flow rates and fast separations be achieved at resolution comparable to that obtained with conventional particle-packed columns. In food analysis, silica- and organic-polymer based monolithic columns are employed for isolation, enrichment, separation and determination of food components, additives, contaminants, minerals, trace elements, pesticides and pharmaceutical residues. Silica-based monoliths are highly competitive with packed columns intended for fast analysis, containing sub-2 ␮m or porous shell particles, in terms of speed and efficiency of separation. The basic types of monolithic stationary phase chemically bonded on monolithic silica, are commercially available for reversed-phase and HILIC applications, in various lengths to suit either very fast separation times, or to more demanding resolution of complex samples. Silica monolithic columns are valuable tools in in-line

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two-dimensional liquid chromatography, where high speed and efficiency of separation in the second dimension is of primary importance. Besides the analytical column format, monolithic silica capillaries with i.d. 0.1 mm are also available. Recently, also 0.2 mm i.d. columns have been introduced on the market to fill the gap in the microbore format. Organic polymer monoliths are the ideal support media for immobilization of antigens, enzymes and other bioaffinity agents and for molecular imprinting of target molecule templates, for selective sample isolation, enrichment and pre-treatment by solidphase extraction (MI-SPE) and obviously will keep this position in future. A disadvantage of the MIPs consists in the necessity of the synthesis and of the optimization of the isolation method for a specific sample, or for a relatively narrow group of sample compounds. Nevertheless, the applications of MIPs in food analysis are continuously extending. Recently, in-line connection of MIP cartridges with standard analytical columns is becoming increasingly popular, as it combines the sample pre-treatment and isolation in a single step. A traditional application field of organic polymer monoliths is in pre-treatment, isolation, purification and fast separation of proteins and other biopolymers. As their chromatographic resolution on these media in gradient elution is largely independent of the column length, very thin CIM disks are very popular for this purpose. Besides the CIM disks, monolithic organic polymer capillaries, 0.1 mm i.d. or less, were introduced for capillary electrochromatography and are increasingly used for capillary LC, due to recent significant improvements in pore morphology of organic polymer monoliths, either by optimization of in situ polymerization procedure, or by post-polymerization modifications, which significantly increased the separation efficiency for small molecules. We can expect further important future progress in this direction. Especially attractive feature of organic polymer monoliths is large variety of functional elements that can be incorporated into their structure. Especially attractive seem polar, mixed-mode ion-exchange/HILIC and zwitterionic monolithic stationary phases, enabling adapting the separation conditions to non-polar, polar or ionic samples, which are difficult to separate in other chromatographic systems. We can expect future development of monolithic media, offering new possibilities of increasing selectivity in various application areas, including the food analysis. Acknowledgment The present work was funded by the Grant Agency (Science Foundation) of the Czech Republic under the project 206/12/0398. References ˇ cek, R. Chromeˇcek, Collect. Czechosl. Chem. Commun. 32 [1] M. Kubín, P. Spaˇ (1967) 3881. [2] S. Hjerten, j.l. Liao, R. Zhang, J. Chromatogr. 473 (1989) 273. ˇ [3] T.B. Tennikova, B.G. Belenkii, F. Svec, J. Liq. Chromatogr. 13 (1990) 63. ˇ [4] F. Svec, J.M.J. Fréchet, Anal. Chem. 64 (1992) 820. ˇ [5] C. Viklund, F. Svec, J.M.J. Fréchet, K. Irgum, Chem. Mater. 8 (1996) 744. [6] K. Nakanishi, N. Soga, J. Am. Ceram. Soc. 74 (1991) 2518. [7] H. Minakuchi, K. Nakanishi, N. Soga, N. Ishizuka, N. Tanaka, Anal. Chem. 68 (1996) 3498. [8] P. Aggarawal, H.D. Tolley, M.L. Lee, J. Chromatogr. 1219 (2012) 1. [9] J. Wei, Z.T. Jiang, R. Li, J. Tian, Anal. Lett. 45 (2012) 1724. [10] K. Cabrera, J. Sep. Sci. 27 (2004) 986, 843. [11] V.F. Samanidou, E.G. Karageorgu, J. Sep. Sci. 34 (2011) 2013. [12] J.M. Saz, M.L. Marina, in: P.G. Wang (Ed.), Monolithic Chromatography and its Modern Applications, LLM Publications, St. Albans, Hertfordshire, UK, 2010. [13] O. Nunez, H. Gallart-Ayala, C.P.B. Martins, P. Lucci, J. Chromatogr. A 1228 (2012) 298. [14] G. Guiochon, J. Chromatogr. A 1168 (2007) 101. [15] H. Minakuchi, K. Nakanishi, N. Soga, N. Ishizuka, N. Tanaka, J. Chromatogr. A 762 (1997) 135. [16] N. Manchon, M. D’Arrigo, A. Garcia-Lafuente, E. Guillamon, A. Villares, J.A. Martinez, A. Ramos, M.A. Rostagno, Anal. Bioanal. Chem. 400 (2011) 1251.

52

P. Jandera / J. Chromatogr. A 1313 (2013) 37–53

[17] C. Bignardi, L. Elviri, A. Penna, M. Careri, A. Mangia, J. Chromatogr. A 1217 (2010) 7579. ˇ [18] J. Urban, F. Svec, J.M.J. Fréchet, J. Chromatogr. A 1217 (2010) 8212. [19] C.G. Huber, P.J. Oefner, G.K. Bonn, Chromatographia 37 (1993) 653. ˇ [20] F. Svec, J. Chromatogr. A 1228 (2012) 250. ˇ ´ [21] M. Janˇco, D. Sykora, F. Svec, J.M.J. Fréchet, J. Schweer, R. Holm, J. Polym. Sci. Pol. Chem. 38 (2000) 2767. [22] I. Nischang, I. Teasdale, O. Bruggemann, J. Chromatogr. A 1217 (2010) 7514. [23] I. Nischang, I. Teasdale, O. Bruggemann, Anal. Bioanal. Chem. 400 (2011) 2289. ˇ [24] F. Svec, J. Sep. Sci. 32 (2009) 3. [25] A. Premstaller, H. Oberacher, C.G. Huber, Anal. Chem. 72 (2000) 4386. ˇ [26] S. Xie, R.W. Allington, F. Svec, J.M.J. Fréchet, J. Chromatogr. A 865 (1999) 169. ´ Anal. Chem. 69 (1999) 169. [27] A. Palm, M.V. Novotny, ˇ [28] E.C. Peters, M. Petro, F. Svec, J.M.J. Fréchet, Anal. Chem. 69 (1997) 3646. [29] H. Oberacher, C.G. Huber, Trends Anal. Chem. 21 (2002) 166. [30] D. Moravcová, P. Jandera, J. Urban, J. Planeta, J. Sep. Sci. 26 (2003) 1005. [31] P. Holdˇsvendová, P. Coufal, J. Suchánková, E. Tesaˇrová, Z. Bosáková, J. Sep. Sci. 26 (2003) 1623. ˇ [32] F. Svec, J. Chromatogr. A 1217 (2010) 902. [33] P. Jandera, J. Urban, J. Sep. Sci. 31 (2008) 2521. ˇ [34] F. Svec, C.G. Huber, Anal. Chem. 78 (2006) 2100. ˇ [35] F. Svec, J.M.J. Fréchet, Chem. Mater. 7 (1995) 707. ˇ [36] F. Svec, J. Sep. Sci. 27 (2004) 747. ˇ [37] Q. Wang, F. Svec, J.M.J. Fréchet, J. Chromatogr. A 669 (1994) 230. [38] M.R. Buchmeiser, Polymer 48 (2006) 2187. [39] K. Kanamori, K. Nakanishi, T. Hanada, Adv. Mater. 18 (2006) 2407. [40] J. Hasegawa, K. Kanamori, K. Nakanishi, T. Hanada, S. Yamago, Macromolecules 42 (2009) 1270. [41] H. Aoki, T. Kubo, T. Ikegami, N. Tanaka, K. Hosoya, D. Tokuda, N. Ishizuka, J. Chromatogr. A 1119 (2006) 66. [42] T. Kubo, N. Kimura, K. Hosoya, K. Kaya, J. Polym. Sci. Polym. Chem. 45 (2007) 3811. [43] Y.Y. Li, H.D. Tolley, M.L. Lee, Anal. Chem. 81 (2009) 4406. [44] E.S. Sinitsyna, Y.N. Sergeeva, E.G. Vlakh, N.N. Saprikina, T.B. Tennikova, React. Funct. Polym. 69 (2009) 385. [45] E.S. Sinitsyna, E.G. Vlakh, M.Y. Rober, T.B. Tennikova, Polymer 52 (2011) 2132. [46] J. Hasegawa, K. Kanamori, K. Nakanishi, T. Hanada, S. Yamago, Macromol. Rapid Commun. 30 (2009) 986. [47] Z.W. Wu, K.J. Frederic, M. Talarico, D. De Keel, Can. J. Chem. Eng. 87 (2009) 579. [48] Y.Y. Li, H.D. Tolley, M.L. Lee, J. Chromatogr. A 1218 (2011) 1399. [49] Y.Y. Li, H.D. Tolley, M.L. Lee, J. Chromatogr. A 1217 (2010) 4934. ˇ [50] Q.C. Wang, F. Svec, J.M.J. Fréchet, Anal. Chem. 65 (1993) 2243. [51] L. Trojer, S.H. Lubbad, C.P. Bisjak, W. Wieder, G.K. Bonn, J. Chromatogr. A 1146 (2007) 216. [52] W. Wieder, S.H. Lubbad, L. Trojer, C.P. Bisjak, G.K. Bonn, J. Chromatogr. A 1191 (2008) 253. [53] B. Mayr, G. Holz, K. Eder, M.R. Buchmeiser, C.G. Huber, Anal. Chem. 74 (2002) 6080. [54] F.M. Sinner, M.R. Buchmeiser, Angew. Chem. Int. Ed. 39 (2000) 1433. ˇ [55] F. Svec, J.M.J. Fréchet, Ind. Eng. Chem. Res. 38 (1999) 34. ˇ [56] M. Merhar, A. Podgornik, M. Barut, M. T¸igon, A. Strancar, J. Sep. Sci. 26 (2003) 322. [57] B.L. Waguespack, S.A. Hodges, M.E. Bush, L.J. Sondergeld, M.M. Bushey, J. Chromatogr. A 1078 (2005) 171. [58] J. Urban, P. Jandera, P. Langmaier, J. Sep. Sci. 34 (2011) 2054. [59] M. Jesus Lerma-Garcia, E.F. Simo-Alfonso, G. Ramis-Ramos, J.M. HerreroMartinez, Electrophoresis 29 (2008) 4603. [60] S.H. Lubbad, M.R. Buchmeiser, J. Chromatogr. A 1217 (2010) 3223. ˇ ˇ [61] A. Svobodová, T. Kˇrít¸ek, J. Sirc, P. Sálek, E. Tesaˇrová, P. Coufal, K. Stulík, J. Chromatogr. A 1218 (2011) 1544. [62] Y.-J. Cheng, S.-H. Huang, B. Singco, H.-Y. Huang, J. Chromatogr. A 1218 (2011) 7640. ˇ [63] A. Nordborg, F. Svec, J.M.J. Fréchet, K. Irgum, J. Sep. Sci. 28 (2005) 2401. [64] Z.D. Xu, L.M. Yang, Q.Q. Wang, J. Chromatogr. A 1216 (2009) 3098. ˇ ríková, J. Urban, J. Chromatogr. A 1274 (2013) ˇ [65] P. Jandera, M. Stanková, V. Skeˇ 97. ˇ [66] F.F. Svec, J.M.J. Fréchet, Biotechnol. Bioeng. 48 (1995) 476. [67] Y. Ueki, T. Umemura, J.X. Li, T. Odake, K. Tsunoda, Anal. Chem. 76 (2004) 7007. [68] C. Legido-Quigley, N.D. Marlin, V. Melin, A. Manz, N.W. Smith, Electrophoresis 24 (2003) 917. [69] O. Kornyˇsova, A. Maruˇska, P.K. Owens, M. Ericsson, J. Chromatogr. A 1071 (2005) 171. [70] F. Plieva, X. Huiting, I. Galaev, B. Bergenstahl, B. Mattiasson, J. Mater. Chem. 16 (2006) 4065. [71] R. Dario Arrua, T.J. Causon, E.F. Hilder, Analyst 137 (2012) 5179. [72] R. Darrio Arrua, M. Talebi, T.J. Causon, E.F. Hilder, Anal. Chim. Acta 738 (2012) 1. [73] J. Urban, P. Jandera, Anal. Bioanal. Chem. 405 (2013) 2123. ˇ [74] F. Svec, J.M.J. Frechet, Macromolecules 28 (1995) 7580. [75] M. Szumski, B. Buszewski, J. Sep. Sci 32 (2009) 2574. [76] T. Hirano, S. Kitagawa, H. Ohtani, Anal. Sci. 25 (2009) 1107. [77] I. Nischang, O. Bruggemann, J. Chromatogr. A 1217 (2010) 5389. [78] A. Greiderer, L. Trojer, C.W. Huck, G.K. Bonn, J. Chromatogr. A 1216 (2009) 7747. [79] L. Trojer, C.P. Bisjak, W. Wieder, G.K. Bonn, J. Chromatogr. A 1216 (2009) 6303.

ˇ [80] S. Xie, F. Svec, J.M.J. Fréchet, J. Chromatogr. A 775 (1997) 65. [81] D. Moravcová, P. Jandera, J. Urban, J. Planeta, J. Sep. Sci. 27 (2004) 789. ˇ ˇ [82] P. Coufal, M. Cihák, J. Suchánková, E. Tesaˇrová, Z. Bosáková, K. Stulík, J. Chromatogr. A 946 (2002) 99. ˇ cík, J. [83] J. Grafnetter, P. Coufal, E. Tesaˇrová, J. Suchánková, Z. Bosáková, J. Sevˇ Chromatogr. A 1049 (2004) 43. [84] S. Eeltink, J.M. Herrero-Martinez, G.P. Rozing, P.J. Schoenmakers, W.T. Kok, Anal. Chem. 77 (2005) 7342. [85] M. Jesus Lerma-Garcia, E.F. Simo-Alfonso, G. Ramis-Ramos, J.M. HerreroMartinez, Electrophoresis 28 (2007) 4128. [86] Q. Zhang, Y. Yin, Chem. Commun. 49 (2013) 231. ˇ [87] T. Rohr, E.F. Hilder, J.J. Donovan, F. Svec, J.M.J. Fréchet, Macromolecules 36 (2003) 1084. [88] V.A. Davankov, S.V. Rogozhin, M.P. Tsyurupa, U.S. Patent 3,729,457 (1971). ˇ J.M.J. Frechet, Anal. Chem. 82 (2010) 1621. [89] J. Urban, F. Svec, [90] Y. Li, Y. Chen, R. Xiang, D. Ciuparu, L.D. Pfefferle, C. Horváth, J.A. Wilkins, Anal. Chem. 77 (2005) 1398. ˇ [91] S.D. Chambers, F. Svec, J.M.J. Fréchet, J. Chromatogr. A 1218 (2011) 2546. ˇ [92] S.D. Chambers, T.W. Holcombe, F. Svec, J.M.J. Fréchet, Anal. Chem. 83 (2011) 9478. [93] H. Pham-Tuan, L. Kaskavelis, C.A. Daykin, H.G. Janssen, J. Chromatogr. B 789 (2003) 283. [94] S. Apers, T. Naessens, K. Van Den Steen, F. Cuyckens, M. Claeys, L. Pieters, A. Vlietinck, J. Chromatogr. A 1038 (2004) 107. [95] C. Repolles, J.M. Herrero-Martinez, C. Rafols, J. Chromatogr. A 1131 (2006) 51. [96] P. Arapitsas, C. Turner, Talanta 74 (2008) 1218. [97] M. Biesaga, U. Ochnik, K. Pyrzynska, J. Sep. Sci. 30 (2007) 2929. [98] J. Ballesta-Claver, M.C. Valencia, L.F. Rostagno, M. Palma, C.G. Barroso, Anal. Chim. Acta 582 (2007) 243. [99] R. Shakya, D.A. Navarre, J. Agric. Food Chem. 54 (2006) 5253. [100] P.G. Walker, S.L. Gordon, R.M. Brennan, R.D. Hancock, Phytochem. Anal. 17 (2006) 284. [101] N.O. Can, G. Arli, Y. Lafci, J. Sep. Sci. 34 (2011) 2214. ˇ ´ M. Ot’apka, R. Sladkovsky, ´ P. Solich, Curr. Anal. Chem. y, [102] P. Sadílek, D. Satínsk 5 (2009) 311. [103] J. Pól, T. Hyotylainen, O. Ranta-Aho, M.L. Riekkola, J. Chromatogr. A 1052 (2004) 25. [104] M. Lísa, M. Holˇcapek, H. Sovová, J. Chromatogr. A 1216 (2009) 8371. [105] P.D. Tzanavaras, D.G. Themelis, Anal. Chim. Acta (2007) 89. [106] P.D. Tzanavaras, C.K. Zacharis, D.G. Themelis, Talanta 81 (2010) 1494. [107] P. Vinas, N. Campillo, L. Carrasco, M. Hernandez-Cordoba, Chromatographia 65 (2007) 85. [108] E.G. Karageorgou, V.F. Samanidou, J. Sep. Sci. 33 (2010) 2862. [109] W.S. Khayoon, B. Saad, T.P. Lee, B. Salleh, Food Chem. 133 (2012) 489. [110] G.F. Pearson, G.M. Greenway, E.I. Brima, P.I. Haris, J. Anal. Atom. Spectrom. 22 (2007) 361. [111] J.M. Rodriguez-Nodales, M. del Alamo, M.C. Garcia, A. Cifuentes, M.L. Marina, J. Agric. Food Chem. 57 (2009) 3014. [112] H. Eghbali, K. Sandra, F. Desmet, F. Lynen, K. Nakanishi, P. Sandra, G. Desmet, J. Chromatogr. A 1218 (2011) 3360. [113] Z.B. Zhang, H. Lin, J.J. Ou, H.Q. Quin, R.A. Wu, J. Dong, H. Zhou, J. Chromatogr. A 1228 (2012) 263. ˇ ´ R. Sladkovsky, ´ M. Pospíˇsilová, P. Solich, Talanta 77 [114] P. Chocholouˇs, D. Satínsk y, (2008) 566. [115] W. Jangbai, W. Wongwilai, K. Grudpan, S. Lapanantnoppakhun, Food Anal. Methods (2012) 631. [116] J.F. Garcia-Jimenez, M.C. Garcia-Jimenez, L.F. Valencia, Capitan-Vallvey, Anal. Chim. Acta 594 (2007) 226. [117] T.J. Causon, A. Nordborg, R.A. Shellie, E.F. Hilder, J. Chromatogr. A 1217 (2010) 3519. [118] A. Premstaller, H. Oberacher, W. Walcher, A.M. Timperio, L. Zolla, J.P. Chervet, N. Cavusoglu, A. van Dorsselaer, C.G. Huber, Anal. Chem. 73 (2001) 2390. [119] J. Courtois, E. Byström, K. Irgum, Polymer 47 (2006) 2603. [120] D. Corradini, C.G. Huber, A.M. Timperio, L. Zolla, J. Chromatogr. A 886 (2000) 111. [121] W. Walcher, H. Oberacher, S. Troiani, G. Holzl, P. Oefner, L. Zolla, C.G. Huber, J. Chromatogr. B 782 (2002) 111. [122] W. Walcher, A.M. Timperio, L. Zolla, C.G. Huber, Anal. Chem. 75 (2003) 2390. [123] E.O. Hochleitner, R. Bakry, C.W. Huck, F. Flores, W.M. Stoeggl, G. Stecher, G.K. Bonn, Int. J. Mass Spectrom. 223 (2002) 111. [124] C.G. Huber, W. Walcher, A.M. Timperio, S. Troiani, A. Proceddu, L. Zolla, Proteomics 4 (2004) 3909. [125] J.A. Rodrigues, F.J. Lopez-Baena, F.J. Ollero, J.M. Vinardell, M.D. Espuny, R.A. Bellogin, J.E. Ruiz-Sainz, J.R. Thomas, D. Sumpton, J. Ault, J. Thomas-Oates, J. Proteome Res. 6 (2007) 1029. [126] H. Toll, H. Oberacher, R. Swart, J. Chromatogr. A 1079 (2005) 274. [127] F.M. Sinner, C. Gatschelhofer, A. Mautner, C. Magnes, I.R. Buchmeiser, T.R. Pieber, J. Chromatogr. A 1191 (2008) 274. [128] B. Buszewski, M. Szumski, Chromatographia 60 (2004) S261. [129] T. Umemura, Y. Ueki, K. Tsunoda, A. Katakai, M. Tamada, H. Haraguchi, Anal. Bioanal. Chem. 386 (2006) 566. [130] X. Huang, Q. Wang, H. Yan, Y. Huang, B. Huang, J. Chromatogr. A 1062 (2005) 183. [131] Z. Jiang, N.W. Smith, P.D. Ferguson, M.R. Tailor, J. Biochem. Biophys. Methods 70 (2007) 39. ˇ [132] S. Eeltink, L. Geiser, F. Svec, J.M.J. Fréchet, J. Sep. Sci. 30 (2007) 2814.

P. Jandera / J. Chromatogr. A 1313 (2013) 37–53 [133] B. Schlemmer, R. Bandari, L. Rosenkranz, M.R. Buchmeiser, J. Chromatogr. A 1216 (2009) 2664. [134] Z.A. Al-Othman, A. Aqel, M.K.E. Alharbi, A.Y. Badjah-Hadj-Ahmed, A.A. AlWarthan, Abdulrahman, Food Chem. 132 (2012) 2217. [135] A.J. Alpert, J. Chromatogr. A 449 (1990) 177. [136] P. Jandera, Anal. Chim. Acta 692 (2011) 1. [137] B. Buszewski, S. Noga, Anal. Biochem. 402 (2012) 231. [138] Z. Jiang, N.W. Smith, Z. Liu, J. Chromatogr. A 1218 (2011) 2350. [139] D.N. Gunasena, Z. El Rassi, Electrophoresis 33 (2012) 251. [140] G. Puy, R. Roux, C. Demesnay, J.-L. Rocca, J. Iapichella, A. Galameau, D. Bunuel, J. Chromatogr. A 1160 (2007) 150. [141] K. Horie, T. Ikegami, K. Hosoya, N. Saad, O. Fiehn, N. Tanaka, J. Chromatogr. A 1164 (2007) 198. [142] T. Ikegami, K. Tomomatsu, H. Takubo, K. Horie, N. Tanaka, J. Chromatogr. A 1184 (2008) 474. [143] T. Ikegami, H. Fujita, K. Horie, K. Hosoya, N. Tanaka, Anal. Bioanal. Chem. 386 (2006) 578. [144] T. Ikegami, K. Horie, N. Saad, K. Hosoya, O. Fiehn, N. Tanaka, Anal. Bioanal. Chem. 391 (2008) 2533. [145] K.M. Glenn, C.A. Lucy, P.R. Haddad, J. Chromatogr. A 1155 (2007) 8. [146] J. Randon, S. Huguet, A. Piram, G. Puy, C. Demesnay, J.-L. Rocca, J. Chromatogr. A 1109 (2006) 19. [147] J.A. Blackwell, P.W. Carr, Anal. Chem. 64 (1992) 853. [148] J. Wei, Z.-T. Jiang, R. Li, J. Tan, Anal. Lett. 45 (2012) 1724. [149] E.G. Vlakh, T.B. Tennikova, J. Sep. Sci. 30 (2007) 2801. [150] P. Holdˇsvendová, J. Suchánková, M. Bunˇcek, V. Baˇckovská, J. Biochem. Biophys. Methods 70 (2007) 23. [151] X.C. Wang, H.X. Lu, X.C. Lin, Y.H. Xie, J. Chromatogr. A 1190 (2008) 365. [152] K. Hosoya, N. Hira, K. Yamamoto, M. Nishimura, N. Tanaka, Anal. Chem. 78 (2006) 5729. ˇ J.M.J. Frechet, Anal. Chem. 75 (2003) [153] M.C. Xu, D.S. Peterson, T. Rohr, F. Svec, 1011. [154] X. Lin, J. Wang, L. Li, X. Wang, H. Lu, Z. Xie, J. Sep. Sci. 30 (2007) 3011. [155] Y. Li, M.L. Lee, J. Jin, J. Chen, Talanta 99 (2012) 91. [156] Y. Lv, Z. Lin, F. Svec, Anal. Chem. 84 (2012) 8457. ˇ [157] F. Svec, T.B. Tennikova, J. Chromatogr. A 646 (1993) 279. [158] R.R.M. Moore, R.R. Walters, J. Chromatogr. 317 (1984) 119. [159] S. Yamamoto, T. Okada, M. Abe, N. Yoshimoto, J. Chromatogr. A 1218 (2011) 2460. [160] V. Frankovic, A. Podgornik, N.L. Krajnc, F. Smrekar, P. Krajnc, A. Strancar, J. Chromatogr. A 1207 (2008) 84. [161] I. Vovk, B. Simonovska, M. Bencina, J. Chromatogr. A 1065 (2005) 121. [162] S. Jerman, A. Podgornik, K. Cankar, N. Cadez, M. Skrt, J. Zel, P. Raspor, J. Chromatogr. A 1065 (2005) 107. [163] X. Chen, H.D. Tolley, M.L. Lee, J. Sep. Sci. 34 (2011) 2063. [164] X. Chen, H.D. Tolley, M.L. Lee, J. Chromatogr. A 1217 (2010) 3844. [165] X. Chen, H.D. Tolley, M.L. Lee, J. Chromatogr. A 1218 (2011) 4322. [166] N. Wang, S. He, Y. Zhu, Eur. Food Res. Technol. 235 (2012) 685. [167] J. Li, Y. Zhu, Y.Y. Guo, J. Chromatogr. A 1118 (2006) 46. [168] M.E.A. Ibrahim, C.A. Lucy, Talanta 100 (2012) 313. [169] D. Moravcová, J. Planeta, V. Kahle, M. Roth, J. Chromatogr. A 1270 (2012) 178. [170] J. Wohlgemuth, M. Karas, W. Jiang, R. Hendriks, S. Andrecht, J. Sep. Sci. 33 (2010) 880. [171] H. Lin, J. Ou, Z. Zhang, J. Dong, M. Wu, H. Zou, Anal. Chem. 84 (2012) 2721. [172] H.C. Foo, J. Heaton, N.W. Smith, S. Stanley, Talanta 100 (2012) 344. [173] C. Viklund, K. Irgum, Macromolecules 33 (2000) 25391223. [174] C. Vicklund, A. Sjorgen, K. Irgum, I. Nes, Anal. Chem. 73 (2001) 444. [175] Z. Jiang, N.W. Smith, P.D. Ferguson, M.R. Taylor, Anal. Chem. 79 (2007) 1243. [176] X. Chen, H.D. Tolley, M.L. Lee, J. Sep. Sci. 34 (2011) 2088. ˇ ríková, P. Jandera, R. Kubíˇcková, M. Pospíˇsilová, J. Sep. Sci. 32 [177] J. Urban, V. Skeˇ (2009) 2530. [178] Y. Guo, S. Gaiki, J. Chromatogr. A 1074 (2005) 71. ˇ ríková, P. Jandera, J. Chromatogr. A 1217 (2010) 7981. [179] V. Skeˇ ˇ ríková, J. Chromatogr. A 1289 (2013) ˇ [180] M. Stanková, P. Jandera, J. Urban, V. Skeˇ 47. ˇ [181] P. Jandera, M. Stanková, T. Hájek, J. Sep. Sci. 36 (2013) 2430. [182] Z. Hao, B. Xiao, N. Weng, J. Sep. Sci. 31 (2008) 1449. [183] J.Y. Wu, W. Bicker, W. Lindner, J. Sep. Sci. 31 (2008) 1492. [184] A. Berthod, S.S.C. Chang, J.P.S. Kullman, D.W. Armstrong, Talanta 47 (1998) 1001. [185] C.T. Mant, R.S. Hodges, J. Sep. Sci. 31 (2008) 1573. [186] J. Lin, S. Lin, J. Lin, X. Lin, Z. Xie, J. Chromatogr. A 1218 (2011) 4671. [187] J.J. Pesek, M.T.M.T. Matyska, J. Sep. Sci. 30 (2007) 637. [188] B. Sellergren, Molecularly Imprinted Polymers. Man Made Mimics of Antibodies and Their Applications in Analytical Chemistry, Elsevier Science B.V., Amsterdam, 2001. [189] F. Lanza, B. Sellergren, Chromatographia 53 (2001) 599. [190] D. Djozan, M. Mahkam, B. Ebrahimi, J. Chromatogr. A 1216 (2009) 2211. [191] M. Lasáková, P. Jandera, J. Sep. Sci. 32 (2009) 799. [192] G. Theodoridis, C.K. Zacharis, P.D. Tzanavaras, D.G. Themelis, A. Economou, J. Chromatogr. A 1030 (2004) 69. [193] K. Faure, N. Delaunay, G. Alloncle, S. Cotte, J.L. Rocca, J. Chromatogr. A 1149 (2007) 145.

[194] [195] [196] [197] [198] [199] [200] [201] [202] [203] [204] [205] [206] [207] [208] [209] [210] [211] [212] [213] [214] [215] [216] [217] [218] [219] [220] [221] [222] [223] [224] [225] [226] [227] [228] [229] [230] [231] [232] [233] [234] [235] [236] [237] [238] [239] [240]

[241] [242] [243]

53

G. Theodoridis, P. Manesiotis, J. Chromatogr. A 948 (2002) 163. H.W. Sun, F.X. Qiao, G.Y. Liu, J. Chromatogr. A 1134 (2006) 194. C. Michailof, P. Manesiotis, C. Panayiotou, J. Chromatogr. A 1182 (2008), 25.43. ˇ ríková, A. Tegou, N. Giantsiou, P. Jandera, G. Theodoridis, M. Lasáková, V. Skeˇ J. Sep. Sci. 29 (2006) 2310. P. Manesiotis, A. Hall, J. Courtois, K. Irgum, B. Sellergren, Angew. Chem. Int. Ed. 44 (2005) 3902. H. Oliveira, M.A. Segundo, J. Lima, M. Miro, V. Cerda, Anal. Bioanal. Chem. 397 (2010) 77. F. Puoci, G. Cirillo, M. Curcio, F. Iemma, U.G. Spizzirri, N. Picci, Anal. Chim. Acta 593 (2) (2007) 164. F. Puoci, M. Curcio, G. Cirillo, F. Iemma, U.G. Spizzirri, N. Picci, Food Chem. 106 (2) (2008) 836. J.P. Lai, R. Niessner, D. Knopp, Anal. Chim. Acta 522 (2004) 137. J.H. Zhang, M. Jiang, L.J. Zou, D. Shi, S.R. Mei, Y.X. Zhu, Y. Shi, K. Dai, B. Lu, Anal. Bioanal. Chem. 385 (2006) 780. D.K. Alexiadou, N.C. Maragou, N.S. Thomaidis, G. Theodoridis, M.A. Koupparis, J. Sep. Sci. 31 (2008) 2272. E. Herrero-Hernandez, R. Carabias-Martinez, E. Rodriguez-Gonzalo, Anal. Chim. Acta 1268 (650) (2009) 195. J. He, R.H. Lv, J. Zhu, K. Lu, Anal. Chim. Acta 661 (2010) 215. H.H. Yang, W.H.X.C. Guo, F.R. Chen, H.Q. Zhao, L.M. Lin, X.R. Wang, Talanta 80 (2009) 821. M.T. Muldoon, L.H. Stanker, Anal. Chem. 69 (1997) 803. B. Bjarnason, L. Chimuka, O. Ramström, Anal. Chem. 71 (1999) 2152. F. Chapuis, V. Pichon, F. Lanza, B. Sellergren, M.C. Hennion, J. Chromatogr. B 804 (2004) 93. C. Cacho, E. Turiel, A. Martin-Esteban, D. Ayala, C. Perez-Conde, J. Chromatogr. A 1114 (2006) 255. E. Turiel, J.L. Tadeo, P.A.G. Cormack, A. Martin-Esteban, Analyst 130 (2005) 1601. F.G. Tamayo, F.J.L. Casillas, A. Martin-Esteban, Anal. Chim. Acta 482 (2003) 165. F.G. Tamayo, F.J.L. Casillas, A. Martin-Esteban, J. Chromatogr. A 1069 (2005) 173. Z.X. Xu, G.Z. Fang, S. Wang, Food Chem. 119 (2010) 845. J. Hantash, A. Bartlett, P. Oldfield, G. Denes, R. O’Rielly, D. Roudiere, S. Menduni, J. Chromatogr. A 1125 (2006) 104. T. Kubo, M. Nomachi, K. Nemoto, T. Sano, K. Hosoya, N. Tanaka, K. Kaya, Anal. Chim. Acta 577 (2006) 1. N.M. Maier, G. Buttinger, S. Welhartizki, E. Gavioli, W. Lindner, J. Chromatogr. B 804 (2004) 103. J.L. Urraca, M.D. Marazuela, M.C. Moreno-Bondi, Anal. Bioanal. Chem. 385 (2006) 1155. X.Z. Shi, S.Q. Song, G.R. Qu, S.L. Zheng, A.B. Wu, D.B. Zhang, Anal. Lett. 43 (2010) 757. R. Mohamed, J. Richoz-Payot, E. Gremaud, P. Mottier, E. Yilmaz, J.C. Tabet, P.A. Guy, Anal. Chem. 79 (2007) 9557. X.L. Sun, X.W. He, Y.K. Zhang, L.X. Chen, Talanta 79 (2009) 926. H.Y. Yan, F.X. Qiao, K.H. Row, Anal. Chem. 79 (2007) 8242. Y.K. Lv, C.-L. Jia, J.-Q. Zhang, P. Li, H.-W. Sun, J. Liq. Chromatogr. Relat. Technol. 36 (2013) 249. P.R. Kootstra, C.J.P.F. Kuijpers, K.L. Wubs, D. van Doorn, S.S. Sterk, L.A. van Ginkel, R.W. Stephany, Anal. Chim. Acta 529 (2005) 75. Q.J. Zhu, L.P. Wang, S.F. Wu, J. Wasswa, X.H. Gu, J. Tang, Food Chem. 113 (2009) 608. Y. Shi, D.-D. Peng, C.-H. Shi, X. Zhang, Y.-T. Xie, B. Lu, Food Chem. 126 (2011) 1916. A. Guzman-Vazquez de Prada, O.A. Loaiza, B. Serra, D. Morales, P. MartinezRuiz, A.J. Reviejo, J.M. Pingarron, Anal. Bioanal. Chem. 388 (2007) 227. P.J. Schoenmakers, P. Mariott, J. Beens, LCGC Eur. 16 (2003) 335. D.R. Stoll, P.W. Carr, J. Am. Chem. Soc. 127 (2005) 5034. P. Jandera, J. Chromatogr. A 1255 (2012) 112. T. Hájek, P. Jandera, J. Sep. Sci. 35 (2012) 1712. T. Ikegami, T. Hara, H. Kimura, H. Kobayashi, K. Hosoya, K. Cabrera, N. Tanaka, J. Chromatogr. A 1106 (2006) 112. C.J. Venkatramani, Y. Zelenchok, Anal. Chem. 75 (2003) 3484. V.V. Tolstikov, A. Lommen, K. Nakanishi, N. Tanaka, O. Fiehn, Anal. Chem. 75 (2003) 6737. L. Hu, X. Chen, L. Kong, X. Su, M. Ye, H. Zou, J. Chromatogr. A 1092 (2005) 191. F. Cacciola, P. Jandera, E. Blahová, L. Mondello, J. Sep. Sci. 29 (2006) 2500. ˇ F. Cacciola, P. Jandera, Z. Hajdú, P. Cesla, L. Mondello, J. Chromatogr. A 1149 (2007) 73. M. Kivilompolo, T. Hyotylainen, J. Sep. Sci. 31 (2008) 3466. N. Tanaka, H. Kimura, D. Tokuda, K. Hosoya, T. Ikegami, N. Ishizuka, H. Minakuchi, K. Nakanishi, Y. Shinatani, M. Furuno, K. Cabrera, Anal. Chem. 76 (2004) 1273. P. Dugo, O. Favoino, R. Luppino, G. Dugo, L. Mondello, Anal. Chem. (2004) 2525. S.R. Wilson, M. Jankowski, M. Pepaj, A. Mihailova, F. Boix, G.V. Truyols, E. Lundanes, T. Greibrokk, Chromatographia 66 (2007) 469. H. Kimura, T. Tanigawa, H. Morisaka, T. Ikegami, K. Hosoya, N. Ishizuka, H. Minakuchi, K. Nakanishi, M. Ueda, K. Cabrera, N. Tanaka, J. Sep. Sci. 60 (2004) 897.