Simultaneous quantification of five proteins and seven additives in dairy products with a heart‐cutting two‐dimensional liquid chromatography method

A heart‐cutting two‐dimensional high‐performance liquid chromatography method was developed to simultaneously quantify five major proteins and seven food additives (maltol, ethyl maltol, vanillin, ethyl vanillin, benzoic acid, sorbic acid, and saccharin sodium) in milk and milk powders. In this two‐dimensional system, a Venusil XBP‐C4 column was selected in the first dimension for protein separation, and a Hypersil ODS‐2 C18 column was employed in the second dimension for additive separation; a two‐position, six‐port switching valve was used to transfer the targets (additives) from the first dimension to the second dimension. Method validation consisted of selectivity, response function, linearity, precision, sensitivity, and recovery. In addition, a conventional one‐dimensional high‐performance liquid chromatography method was also tested for comparison. The two‐dimensional method resulted in significantly improved recovery of the food additives compared to the conventional method (90.6–105.4% and 65.5–86.5%, respectively). Furthermore, this novel method has a simple one‐step sample preparation procedure, which shortens the analysis time, resulting in more efficient analysis and less solvent usage.     

Comprehensive two‐dimensional monolithic liquid chromatography of polar compounds

Two‐dimensional liquid chromatography largely increases the number of separated compounds in a single run, theoretically up to the product of the peaks separated in each dimension on the columns with different selectivities. On‐line coupling of a reversed‐phase column with an aqueous normal‐phase (hydrophilic interaction liquid chromatography) column yields orthogonal systems with high peak capacities. Fast on‐line two‐dimensional liquid chromatography needs a capillary or micro‐bore column providing low‐volume effluent fractions transferred to a short efficient second‐dimension column for separation at a high mobile phase flow rate. We prepared polymethacrylate zwitterionic monolithic micro‐columns in fused silica capillaries with structurally different dimethacrylate cross‐linkers. The columns provide dual retention mechanism (hydrophilic interaction and reversed‐phase). Setting the mobile phase composition allows adjusting the separation selectivity for various polar substance classes. Coupling on‐line an organic polymer monolithic capillary column in the first dimension with a short silica‐based monolithic column in the second dimension provides two‐dimensional liquid chromatography systems with high peak capacities. The silica monolithic C₁₈ columns provide higher separation efficiency than the particle‐packed columns at the flow rates as high as 5 mL/min used in the second dimension. Decreasing the diameter of the silica monolithic columns allows using a higher flow rate at the maximum operation pressure and lower fraction volumes transferred from the first, hydrophilic interaction dimension, into the second, reversed‐phase mode, avoiding the mobile phase compatibility issues, improving the resolution, increasing the peak capacity, and the peak production rate.     

Off‐line comprehensive two‐dimensional reversed‐phase countercurrent chromatography with high‐performance liquid chromatography: Orthogonality in separation of Polygonum cuspidatum Sieb. et Zucc

Off‐line comprehensive two‐dimensional reversed‐phase countercurrent chromatography with high‐performance liquid chromatography was investigated in separation of crude ethanol extract from traditional Chinese medicinal herb Polygonum cuspidatum Sieb. et Zucc. Two‐dimensional contour plots for countercurrent chromatography with high‐performance liquid chromatography was obtained after comprehensive separation was completed. Total peak capacity was evaluated and approximately 810 peaks were obtained through a comprehensive two‐dimensional separation. A highly orthogonality of 52.23% and a large separation space occupancy of 88.86% were achieved. Meanwhile, it was found that several components could be well separated by countercurrent chromatography while they could not be separated by high‐performance liquid chromatography, and vice versa, which further indicated the orthogonality of the two separation methods. The off‐line comprehensive two‐dimensional countercurrent chromatography with high‐performance liquid chromatography provided a promising and powerful method for separation of complex natural products.     

Two‐dimensional solid‐phase extraction strategy for the selective enrichment of aminoglycosides in milk

An orthogonal two‐dimensional solid‐phase extraction strategy was established for the selective enrichment of three aminoglycosides including spectinomycin, streptomycin, and dihydrostreptomycin in milk. A reversed‐phase liquid chromatography material (C₁₈) and a weak cation‐exchange material (TGA) were integrated in a single solid‐phase extraction cartridge. The feasibility of two‐dimensional clean‐up procedure that experienced two‐step adsorption, two‐step rinsing, and two‐step elution was systematically investigated. Based on the orthogonality of reversed‐phase and weak cation‐exchange procedures, the two‐dimensional solid‐phase extraction strategy could minimize the interference from the hydrophobic matrix existing in traditional reversed‐phase solid‐phase extraction. In addition, high ionic strength in the extracts could be effectively removed before the second dimension of weak cation‐exchange solid‐phase extraction. Combined with liquid chromatography and tandem mass spectrometry, the optimized procedure was validated according to the European Union Commission directive 2002/657/EC. A good performance was achieved in terms of linearity, recovery, precision, decision limit, and detection capability in milk. Finally, the optimized two‐dimensional clean‐up procedure incorporated with liquid chromatography and tandem mass spectrometry was successfully applied to the rapid monitoring of aminoglycoside residues in milk.     

Design of a microfluidic device for comprehensive spatial two‐dimensional liquid chromatography

This study discusses the design aspects for the construction of a microfluidic device for comprehensive spatial two‐dimensional liquid chromatography. In spatial two‐dimensional liquid chromatography each peak is characterized by its coordinates in the plane. After completing the first‐dimension separation all fractions are analyzed in parallel second‐dimension separations. Hence, spatial two‐dimensional liquid chromatography potentially provides much higher peak‐production rates than a coupled column multi‐dimensional liquid chromatography approach in which the second‐dimension analyses are performed sequentially. A chip for spatial two‐dimensional liquid chromatography has been manufactured from cyclic olefin copolymer and features a first‐dimension separation channel and 21 parallel second‐dimension separation channels oriented perpendicularly to the former. Compartmentalization of first‐ and second‐dimension developments by physical barriers allowed for a preferential flow path with a minimal dispersion into the second‐dimension separation channels. To generate a homogenous flow across all the parallel second‐dimension channels, a radially interconnected flow distributor containing two zones of diamond‐shaped pillars was integrated on‐chip. A methacrylate ester based monolithic stationary phase with optimized macroporous structure was created in situ in the confines of the microfluidic chip. In addition, the use of a photomask was explored to localize monolith formation in the parallel second‐dimension channels. Finally, to connect the spatial chip to the liquid chromatography instrument, connector ports were integrated allowing the use of Viper fittings. As an alternative, a chip holder with adjustable clasp locks was designed that allows the clamping force to be adjusted.     

Purification of flavonoids from licorice using an off‐line preparative two‐dimensional normal‐phase liquid chromatography/reversed‐phase liquid chromatography method

An orthogonal (71.9%) off‐line preparative two‐dimensional normal‐phase liquid chromatography/reversed‐phase liquid chromatography method coupled with effective sample pretreatment was developed for separation and purification of flavonoids from licorice. Most of the nonflavonoids were firstly removed using a self‐made Click TE‐Cys (60 μm) solid‐phase extraction. In the first dimension, an industrial grade preparative chromatography was employed to purify the crude flavonoids. Click TE‐Cys (10 μm) was selected as the stationary phase that provided an excellent separation with high reproducibility. Ethyl acetate/ethanol was selected as the mobile phase owing to their excellent solubility for flavonoids. Flavonoids co‐eluted in the first dimension were selected for further purification using reversed‐phase liquid chromatography. Multiple compounds could be isolated from one normal‐phase fraction and some compounds with bad resolution in one‐dimensional liquid chromatography could be prepared in this two‐dimensional system owing to the orthogonal separation. Moreover, this two‐dimensional liquid chromatography method was beneficial for the preparation of relatively trace flavonoid compounds, which were enriched in the first dimension and further purified in the second dimension. Totally, 24 flavonoid compounds with high purity were obtained. The results demonstrated that the off‐line two‐dimensional liquid chromatography method was effective for the preparative separation and purification of flavonoids from licorice.     

Online high‐pH reversed‐phase liquid chromatography × low‐pH reversed‐phase liquid chromatography tandem electrospray ionization mass spectrometry combined with pulse elution gradient in the first dimension for the analysis of alkaloids in Macleaya cordata (willd.) R. Br

An online high‐pH reversed‐phase liquid chromatography× low‐pH reversed‐phase liquid chromatography tandem electrospray ionization mass spectrometry combined with pulse elution gradient in the first dimension was constructed to separate and identify alkaloids from Macleaya cordata (willd.) R. Br. The modulation was performed by using a dual second dimensional columns interface combined with a make‐up dilution pump, which is responsible for dilution and neutralization of the first dimensional effluent, and the dual second dimensional columns integrated the trapping and the separation function to reduce the second dimension system dead volume. Taking advantage of the dissociable characteristics of alkaloids, mobile phases with different pH values were applied in the first dimension (pH 9.0) and the second dimension (pH 2.6) to improve the orthogonality of two‐dimension separation. Besides, the pulse elution gradient in first dimension and second dimensional gradient were carefully optimized and much better separation was achieved compared to the separation with the traditional two‐dimensional liquid chromatography approach. Finally, mass measurement was performed for alkaloids in M. cordata (willd.) R. Br. by coupling proposed two‐dimensional liquid chromatography system with triple quadrupole mass spectrometry, and 39 alkaloids were successfully identified by comparing the obtained result with the former reported results.     

Two‐dimensional liquid chromatography analysis of all‐trans‐, 9‐cis‐, and 13‐cis‐astaxanthin in raw extracts from Phaffia rhodozyma

An effective two‐dimensional liquid chromatography method has been established for the analysis of all‐trans‐astaxanthin and its geometric isomers from Phaffia rhodozyma employing a C18 column at the first dimension and a C30 column in the second dimension, connected by a 10‐port valve using the photo‐diode array detector. The regression equation of astaxanthin calibration curve was established, and the precision and accuracy values were found to be in the range of 0.32–1.14% and 98.21–106.13%, respectively. By using two‐dimensional liquid chromatography, it was found that day light, ultrasonic treatment, and heat treatment have significant influence on the content of all‐trans‐astaxanthin in the extract from P. rhodozyma due to the transformation of all‐trans‐astaxanthin to cis‐astaxanthin. The day light and ultrasonic treatments more likely transform all‐trans‐astaxanthin to 9‐cis‐astaxanthin, and the thermal treatment transforms all‐trans‐astaxanthin to 13‐cis‐astaxanthin. These results indicate that the two‐dimensional liquid chromatography method can facilitate monitoring astaxanthin isomerization in the raw extract from P. rhodozyma. In addition, the study will provide a general reference for monitoring other medicals and bioactive chemicals with geometric isomers.     

Enantioselective and simultaneous determination of lactate and 3‐hydroxybutyrate in human plasma and urine using a narrow‐bore online two‐dimensional high‐performance liquid chromatography system

For the enantioselective and simultaneous analysis of lactate and 3‐hydroxybutyrate, a validated online two‐dimensional high‐performance liquid chromatography system using 4‐nitro‐7‐piperazino‐2,1,3‐benzoxadiazole as a fluorescent derivatization reagent has been developed. For the reversed‐phase separation in the first dimension, a Capcell Pak C18 ACR column (1.5 × 250 mm, particle size 3 μm) was used, and the target fractions were isolated by their hydrophobicity. In the second dimension, a polysaccharide‐coated enantioselective column, Chiralpak AD‐H (2.0 × 250 mm, 5 μm), was used. The system was validated by the calibration curve, intraday precision, interday precision, and accuracy using standards and real human samples, and satisfactory results were obtained. The present method was applied to human plasma and urine, and in the plasma, trace amounts of d‐ lactate (8.4 μM) and l‐ 3‐hydroxybutyrate (1.0 μM), besides high levels of l‐ lactate (860.9 μM) and d‐ 3‐hydroxybutyrate (59.4 μM), were successfully determined. In urine, trace levels of d‐ lactate (3.7 μM), d‐ 3‐hydroxybutyrate (2.3 μM), and l‐ 3‐hydroxybutyrate (3.3 μM) in addition to a relatively large amount of l‐ lactate (15.4 μM) were observed. The present online two‐dimensional high‐performance liquid chromatography system is useful for the simultaneous determination of all the lactate and 3‐hydroxybutyrate enantiomers in human physiological fluids, and further clinical applications are ongoing.     

Peak purity assessment in a triple‐active fixed‐dose combination drug product related substances method using a commercial two‐dimensional liquid chromatography system

Pharmaceutical formulations containing multiple active components challenge the development of analytical methods, especially as the individual active ingredients diverge in their physicochemical properties. Establishing specificity, especially peak purity, is one of the major evaluation criteria when developing a related substances method for drug substances or products. Fixed‐dose combination products may not be amenable to common strategies for assessing peak purity, such as performing orthogonal separations, due to the complexity of the separation and/or diversity of the active ingredients. An alternate approach to evaluating peak purity is demonstrated for a triple‐active component fixed‐dose combination product under development. A commercially available automated two‐dimensional liquid chromatography system was used to perform a selective comprehensive multidimensional separation of an active ingredient peak. The first dimension performed the drug product impurity/degradant profiling method; the second dimension assayed these fractions using the drug substance profiling method, which was pseudo‐orthogonal to the first dimension. A total of 14 targeted fractions were sampled across the first dimension main peak, with 11 containing detectable analytes and the remaining fractions bracketing the main peak. This degree of sampling allowed profiling of a coeluting degradant present at a 0.2% w/w level throughout the main peak.     

Rapid screening and identification of antioxidants in the leaves of Malus hupehensis using off‐line two‐dimensional HPLC–UV–MS/MS coupled with a 1,1′‐diphenyl‐2‐picrylhydrazyl assay

The leaves of Malus hupehensis have a strong antioxidant activity and are commonly consumed as a healthy tea. However, detailed information about its antioxidants is incomplete. Herein, we developed an effective strategy based on combining off‐line two‐dimensional high‐performance liquid chromatography with ultraviolet and tandem mass spectrometry detection with a 1,1′‐diphenyl‐2‐picrylhydrazyl assay to rapidly screen and identify the antioxidants from the leaves of M. hupehensis. In the orthogonal two‐dimensional liquid chromatography system, a Venusil HILIC column was used for the first dimension, while a Universil XB‐C₁₈ column was installed in the second dimension. As a result, 32 antioxidants, including ten dihydrochalcones, two flavanones, nine flavonols, four flavones, and seven phenolic acids were tentatively identified, out of which 23 compounds, as far as we know, were isolated and characterized from the leaves of M. hupehensis for the first time. To the best of our knowledge, this is the first systematic investigation of the antioxidants from the leaves of M. hupehensis. The results indicated that the proposed method is an efficient technique to rapidly investigate antioxidants, especially for coeluted and minor compounds in a complex system.     

Separation and identification of phenolic compounds in Bidens pilosa L. by ultra high performance liquid chromatography with quadrupole time‐of‐flight mass spectrometry

A validated method based on ultra‐performance liquid chromatography coupled with quadrupole time‐of‐flight mass spectrometry was established to separate and identify phenolic compounds in Bidens pilosa L. Mass spectrometry experiments were performed both in positive and negative ion modes. A total of 35 compounds were detected, and 26 phenolic compounds were unequivocally identified or tentatively assigned based on retention time, maximum UV absorption, molecular formula, and fragments. The ultra high performance liquid chromatography method was validated and showed good linearity (R² ≧ 0.9996) over the test range. The limits of detection and quantification were above 0.072 and 0.162 μg/mL, respectively. The relative standard deviations of intraday and interday precision were below 0.3 and 1.6%, respectively.     

Separation of α‐amylase inhibitors from Abelmoschus esculentus (L).Moench by on‐line two‐dimensional high‐speed counter‐current chromatography target‐guided by ultrafiltration‐HPLC

In this study, an on‐line two‐dimensional high‐speed counter‐current chromatography system based on a six‐port valve was developed. Target‐guided by ultrafiltration with high‐performance liquid chromatography, the one‐step isolation of three potential α‐amylase inhibitors from Abelmoschus esculentus (L).Moench was achieved by employing the developed orthogonal system and extrusion elution mode. The purities of three potential α‐amylase inhibitors were all over 95% as determined by high‐performance liquid chromatography. Furthermore, UV, mass spectrometry and ¹H NMR spectroscopy were applied to the structural identification of the isolated three target compounds, their structures were assigned as quercetin‐3‐O‐sophoroside (i), 5,7,3′,4′‐tetrahydroxy flavonol‐3‐O‐[β‐d ‐rhamnopyranosil‐(1→2)]‐β‐d ‐glucopyranoside (ii ) and isoquercitrin (iii), respectively. The Results demonstrated that the proposed method was highly efficient to screen and isolate enzyme inhibitors from complex natural products extracts, and on‐line two‐dimensional high‐speed counter‐current chromatography can effectively increase the peak resolution of target compounds.     

Qualitative and quantitative analysis of the major constituents in Jin‐Mu‐Gan‐Mao tablet by high‐performance liquid chromatography with diode‐array detection and quadrupole time‐of‐flight tandem mass spectrometry

Jin‐Mu‐Gan‐Mao tablet is a well‐known traditional Chinese medicinal preparation, which has been used to treat the common cold in China. In this study, a systematic method was established for the qualitative and quantitative analysis of the major constituents in Jin‐Mu‐Gan‐Mao tablet. First, a method of high‐performance liquid chromatography with diode‐array detection and quadrupole time‐of‐flight mass spectrometry was developed for identification of the multi‐constituents. Thirty‐one compounds including ten phenolic acids, 18 flavonoids, and three iridoid glycosides were clearly identified by comparison with the reference standards, and 11 compounds were deduced by comparison with the literature data. Second, a new quantitative analysis method of Jin‐Mu‐Gan‐Mao tablet was established by high‐performance liquid chromatography with diode‐array detection. Twelve compounds, either with high contents or strong bioactivities, were chosen as marker components. This analytical method was validated through intra‐ and interday precision, repeatability, and stability, with respective relative standard deviations less than 1.74, 2.54, 2.44, and 2.48%. The limits of detection and quantification were less than 0.327 and 0.881 μg/mL, respectively. The overall recoveries ranged from 97.04–102.76% (relative standard deviation ≤ 2.91%). Then this validated method was applied to determine ten batches of Jin‐Mu‐Gan‐Mao tablet. The results indicated that these new approaches can be applicable for the qualitative and quantitative analysis of Jin‐Mu‐Gan‐Mao tablet.     

Simple high‐performance liquid chromatography method for formaldehyde determination in human tissue through derivatization with 2,4‐dinitrophenylhydrazine

A simple high‐performance liquid chromatography method has been developed for the determination of formaldehyde in human tissue. FA Formaldehyde was derivatized with 2,4‐dinitrophenylhydrazine. It was extracted from human tissue with ethyl acetate by liquid–liquid extraction and analyzed by high‐performance liquid chromatography. The calibration curve was linear in the concentration range of 5.0–200 μg/mL. Intra‐ and interday precision values for formaldehyde in tissue were <6.9%, and accuracy (relative error) was better than 6.5%. The extraction recoveries of formaldehyde from human tissue were between 88 and 98%. The limits of detection and quantification of formaldehyde were 1.5 and 5.0 μg/mL, respectively. Also, this assay was applied to liver samples taken from a biopsy material.     

Ultra‐performance convergence chromatography method for the determination of four chromones and quality control of Saposhnikovia divaricata (Turcz.) Schischk.

Ultra‐performance convergence chromatography is an environmentally friendly analytical technique that employs dramatically reduced amounts of organic solvents compared to conventional chromatographic methods. In this study, a rapid, sensitive, and environmentally friendly method based on ultra‐performance convergence chromatography was developed for the quantification of four major chromones present in the roots of Saposhnikovia divaricata (Turcz.) Schischk. Using this method, the analysis time was significantly shortened compared to conventional high‐performance liquid chromatography techniques. In addition, the influence of cosolvent type, cosolvent ratio, column temperature, system pressure, and flow rate on the peak resolution was investigated. The proposed method was validated in terms of its limits of detection, limits of quantitation, linearity, precision, and accuracy. More specifically, the limits of detection of the four chromones ranged from 0.006 to 0.033 μg/mL, while the limits of quantitation ranged from 0.019 to 0.101 μg/mL. Our method also exhibited a good regression (r² > 0.999), excellent precision (RSD < 0.60%), and acceptable recoveries (99.48–102.89%). Finally, the quantities of these four chromones present in 20 commercial samples from Korea and China were successfully evaluated using the developed method, indicating that the proposed method is suitable for the rapid and accurate quality control of Saposhnikovia divaricata.     

Characterization of poly(ethylene glycol)‐b‐ poly(ε‐caprolactone) by two‐dimensional liquid chromatography

Block copolymers of ethylene oxide and ε‐caprolactone were synthesized by microwave‐assisted polymerization of ε‐caprolactone with polyethylene glycol monomethyl ethers as initiator. The samples thus obtained were characterized by two‐dimensional liquid chromatography with liquid chromatography at critical conditions as the first and liquid exclusion adsorption chromatography as the second dimension. A full baseline separation of all oligomers could be achieved in both dimensions.     

Study of the chromatographic parameters of ultra‐high‐performance supercritical fluid chromatography and method development using a design of experiments approach for the quantification of pesticides in lettuce

We examine the potential of ultra‐high‐performance supercritical fluid chromatography for multiresidue quantification of ten pesticides commonly applied to lettuce and compares it to ultra‐high‐performance liquid chromatography. Initially, a thorough study of the stationary and mobile phase composition and injection solvent was carried out. In a second step, a chemometric approach based on design of experiments was used to simultaneously study the influence of temperature, pressure, and percentage of ethanol on the retention, resolution and symmetry of the peaks. Using this approach, it was possible to obtain the Design Space, a robust region where complete separation of the analytes was achieved, with acceptable peak shape. Both methods were validated according to the figures of merit: selectivity, linearity, quantification limit, accuracy (in terms of recovery), and precision (repeatability and intermediate precision) and used to quantify the pesticides in lettuce samples. Comparing both techniques, it was concluded that the limits of quantification, accuracy, and precision were similar. However, in supercritical fluid chromatography, a reduced volume of organic solvent was used, the method was faster and generated lower amounts of residues.     

Comparison of two sample clean‐up methodologies for the determination of polycyclic aromatic hydrocarbons in edible oils

An off‐line high‐performance normal‐phase liquid chromatography procedure with a silica column followed by reversed‐phase high‐performance liquid chromatography (HPLC) with fluorescence detection for the determination of polycyclic aromatic hydrocarbons (PAHs) in edible oils is reported. The method was validated using certified reference materials and compared with a standardized method widely used in the food industry, consisting in low pressure column chromatography with alumina as stationary phase followed by reversed phase HPLC determination. The limits of detection were lower than 1 ng/g and good selectivity was achieved for both methods. There were no significant differences in accuracies and precisions obtained for each approach. The advantages and disadvantages of the two methods are discussed.     

Switchable‐hydrophilicity solvent liquid‐liquid microextraction versus dispersive liquid‐liquid microextraction prior to HPLC‐UV for the determination and isolation of piperine from Piper nigrum L

Switchable‐hydrophilicity solvent liquid‐liquid microextraction and dispersive liquid‐liquid microextraction were compared for the extraction of piperine from Piper nigrum L. prior to its analysis by using high‐performance liquid chromatography with UV detection. Under optimum conditions, limits of detection and quantitation were found as 0.2–0.6 and 0.7–2.0 μg/mg with the two methods, respectively. Calibration graphs showed good linearity with coefficients of determination (R²) higher than 0.9962 and percentage relative standard deviations lower than 6.8%. Both methods were efficiently used for the extraction of piperine from black and white pepper samples from different origins and percentage relative recoveries ranged between 90.0 and 106.0%. The results showed that switchable‐hydrophilicity solvent liquid‐liquid microextraction is a better alternative to dispersive liquid‐liquid microextraction for the routine analysis of piperine in food samples. A novel scaled‐up dispersive liquid‐liquid microextraction method was also proposed for the isolation of piperine providing a yield of 102.9 ± 4.9% and purity higher than 98.0% as revealed by NMR spectroscopy.