Chromatographic separations of aromatic carboxylic acids

The series of L-lysine-derived gelators-modified multifunctional chromatography stationary phase for separation of chiral and achiral compounds

In this study, using chiral L-lysine as the molecular skeleton, three kinds of L-lysine-derived gelators (GBLB, GBLF and GFLF) were synthesized and then bonded to the surface of silica matrix (5 μm) by amide condensation to prepare a series of multifunctional chromatography stationary phases (GBLB-SiO2, GBLF-SiO2, and GFLF-SiO2) were prepared. The L-lysine-derived gelators not only possess chiral recognition ability, but also can spontaneously form oriented and ordered network structures in liquid medium through the interaction of non-covalent bonding forces such as hydrogen bonding, π-π stacking, and van der Waals forces. The comprehensive effect of multiple weak interaction sites enhances the molecular recognition ability and further improves the separation diversity of different types of compounds on stationary phases. The separation and evaluation of chiral compounds showed that benzoin, 1-phenyl-ethanol, 1-phenyl-propanol and 6-hydroxyflavanone could be separated in normal phase mode (NPLC). The separation of different types of non-chiral compounds, such as sulfonamides, nucleosides, nucleobases, polycyclic aromatic hydrocarbons (PAHs), anilines, and aromatic acids, were achieved in hydrophilic interaction/reversed-phase/ion-exchange mode (HILIC/RPLC/IEC), and the separation of polarized compounds could be performed under the condition of ultrapure water as the mobile phase, which has the typical retention characteristics of per aqueous liquid chromatography (PALC). The effects of organic solvent content, temperature, pH value, and buffer salt concentration on the retention and separation performance of the column were investigated. Comparison of the three prepared columns showed that the separation performance (such as aromatic selectivity) could be improved by increasing the types of functional groups on the surface of the stationary phase and the number of aromatic groups. In a word, the prepared stationary phase have multiple retention properties, can simultaneously separate chiral compounds and various types of achiral compounds. This work provides an idea for developing multifunctional liquid chromatography stationary phase materials, and further expands the application of gelators in separation science.

Simultaneous Determination of Trigonelline, Caffeine, Chlorogenic Acid and Their Related Compounds in Instant Coffee Samples by HPLC Using an Acidic Mobile Phase Containing Octanesulfonate

In order to analyze trigonelline, caffeine, chlorogenic acid, and their related compounds simultaneously, an HPLC method using an InertSustain C18 column and a mobile phase containing octanesulfonate as an ion-pairing reagent under an acidic condition was developed. The optimum mobile phase conditions were determined to be 0.1% phosphoric acid, 4 mM octanesulfonate, and 15% methanol at 35°C. Using the proposed method, trigonelline, nicotinic acid, caffeine, theophylline, chlorogenic acid, and caffeic acid in ten instant coffee samples were analyzed. These analytes except for theophylline were detected in all samples. An increase in the caffeine content in instant coffee samples tended to decrease in both trigonelline and chlorogenic acid contents, and the trigonelline content was found to be correlated well with the chlorogenic acid content (R(2) = 0.887).

Preparation, chromatographic evaluation and application of adenosine 5'-monophosphate modified ZrO2/SiO2 stationary phase in hydrophilic interaction chromatography

The zirconia-coated silica (ZrO2/SiO2) material was obtained by coupling layer-by-layer (LbL) self-assembly method and sol-gel technology, to take dual advantages of the suitable porous structure of SiO2 and basic resistance of ZrO2. Adenosine 5'-monophosphate (5'-AMP) was then self-assembled onto ZrO2/SiO2 via Lewis acid-base interaction, generating 5'-AMP-ZrO2/SiO2. The chromatographic properties of 5'-AMP-ZrO2/SiO2 were systemically studied by evaluating the effect of acetonitrile content, pH and buffer concentration in the mobile phase. The results demonstrated that the 5'-AMP-ZrO2/SiO2 possessed hydrophilic interaction chromatographic (HILIC) property comprising hydrophilic, hydrogen-bonding, electrostatic and ion-exchange interactions. For basic analytes, the column efficiency of ZrO2/SiO2 and 5'-AMP-ZrO2/SiO2 was superior to the bare ZrO2, and different selectivity was obtained after the introduction of 5'-AMP. For acidic analytes, good resolution was obtained on 5'-AMP-ZrO2/SiO2 while the analysis failed on the bare ZrO2 column owing to strong adsorption. Hence, the proposed 5'-AMP-ZrO2/SiO2 had great potential in analyzing acidic compounds in HILIC mode. It was an extended application of ZrO2 based SP.

Techniques for analysis of plant phenolic compounds

Phenolic compounds are well-known phytochemicals found in all plants. They consist of simple phenols, benzoic and cinnamic acid, coumarins, tannins, lignins, lignans and flavonoids. Substantial developments in research focused on the extraction, identification and quantification of phenolic compounds as medicinal and/or dietary molecules have occurred over the last 25 years. Organic solvent extraction is the main method used to extract phenolics. Chemical procedures are used to detect the presence of total phenolics, while spectrophotometric and chromatographic techniques are utilized to identify and quantify individual phenolic compounds. This review addresses the application of different methodologies utilized in the analysis of phenolic compounds in plant-based products, including recent technical developments in the quantification of phenolics.

Stable isotopic analysis of pyrogenic organic matter in soils by liquid chromatography-isotope-ratio mass spectrometry of benzene polycarboxylic acids

Pyrogenic organic matter (PyOM), the incomplete combustion product of organic materials, is considered stable in soils and represents a potentially important terrestrial sink for atmospheric carbon dioxide. One well-established method of measuring PyOM in the environment is as benzene polycarboxylic acids (BPCAs), a compound-specific method, which allows both qualitative and quantitative estimation of PyOM. Until now, stable isotope measurement of PyOM carbon involved measurement of the trimethylsilyl (TMS) or methyl (Me) polycarboxylic acid derivatives by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). However, BPCA derivatives can contain as much as 150% derivative carbon, necessitating post-analysis correction for the accurate measurement of δ¹³C values, leading to increased measurement error. Here, we describe a method for δ¹³C isotope ratio measurement and quantification of BPCAs from soil-derived PyOM, based on ion-exchange chromatography (IEC-IRMS). The reproducibility of the δ¹³C measurement of individual BPCAs by IEC-IRMS was better than 0.35‰ (1σ). The δ¹³C-BPCA analysis of PyOM in soils, including at natural and artificially enriched ¹³C-abundance, produced accurate and precise δ¹³C measurements. Analysis of samples that differed in δ¹³C by as much as 900‰ revealed carryover of <1‰ between samples. The weighted sum of individual δ¹³C-BPCA measurements was correlated with previous isotopic measurements of whole PyOM, providing complementary information for bulk isotopic measurements. We discuss potential applications of δ¹³C-BPCA measurements, including the study of turnover rates of PyOM in soils and the partitioning of PyOM sources based on photosynthetic pathways.

Assessment of phenolic acid content and in vitro antiradical characteristics of hawthorn

The infusions and extracts obtained from leaves with flowers, fruit peel, and seed from hawthorn (Crataegus monogyna Jacq., Family Rosaceae) were subjected to evaluation as potential sources of antioxidant phytochemicals on the basis of their total content of phenolics, levels of phenolic acids, and in vitro antiradical activity. Total phenolic content of extracts was determined using the modified Folin-Ciocalteau method. Antioxidant activity was determined for phenolic extracts by a method involving the use of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). Phenolic acids containing extracts and infusions from hawthorn leaves, fruit peel, and seeds were obtained using different polarity solvents and separated by reverse-phase high-performance liquid chromatography, which enabled improved separation by the use of a C(18) column, an acidic mobile phase, and gradient elusion. The highest total phenolic content (343.54 mg of gallic acid equivalents/g) and the highest DPPH radical scavenging activity as the inhibition percentage (60.36%) were obtained in ethyl acetate extract from hawthorn leaves with flower. Also, the highest phenolic acid content was measured in the extracts of hawthorn leaves with flowers: protocathechuic (108-128 mg/100 g), p-hydroxy benzoic (141-468 mg/100 g), caffeic (137-3,580 mg/100 g), chlorogenic (925-4,637 mg/100 g), ferulic (3,363-3,462 mg/100 g), vanillic (214 mg/100 g), and syringic (126 mg/100 g) acids. The results indicate that hawthorn is a promising plant because of its high antioxidant activity.

Simple and practical derivatization procedure for enhanced detection of carboxylic acids in liquid chromatography-electrospray ionization-tandem mass spectrometry

A simple and practical derivatization procedure for increasing the detection responses of carboxylic acids in liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) has been developed. 2-Hydrazinopyridine (HP) and 2-picolylamine (PA) rapidly reacted with biologically and clinically important carboxylic acids [chenodeoxycholic acid, glycochenodeoxycholic acid, prostaglandin E2, 2-(-carboxyethyl)-6-hydroxy-2,7,8-trimethylchroman (gamma-CEHC),alpha-lipoic acid, homovanillic acid (HVA) and 5-hydroxyindole-3-acetic acid] in the presence of 2,2'-dipyridyl disulfide and triphenylphosphine. The resulting HP- and PA-derivatives were highly responsive in ESI-MS operating in the positive-ion mode and gave characteristic product ions during MS/MS, which enabled the sensitive detection using selected reaction monitoring. Among the two reagents, PA was of more practical use; the detection responses of the PA-derivatives were increased by 9-158-fold over the intact carboxylic acids and the limits of detection were in the low femtomole range (1.5-5.6 fmol on column). The PA-derivatization was successfully applied to a biological sample analysis; the derivatization followed by LC-ESI-MS/MS enabled the detection of trace amounts of bile acids, gamma-CEHC and HVA in human saliva with a simple pretreatment, small sample volume and short analysis time.

Antifilarial lead molecules isolated from Trachyspermum ammi

Lymphatic filariasis is caused by infection with the parasitic filarial nematodes Wuchereria bancrofti, Brugia malayi and B. timori, transmitted by mosquitoes. The lack of an adulticidal drug poses a challenge to filariasis elimination, hence it is essential to develop an effective antifilarial drug which could either kill or permanently sterilize the adult worms. In the reported work the in vitro activity of a methanolic extract of fruits of Trachyspermum ammi (Apiaceae) against adult bovine filarial Setaria digitata worms has been investigated. A bioassay-guided fractionation was carried out by subjecting the crude extract to flash chromatography. HPLC analysis was done for the crude extract and active fraction. The crude extract and the active fraction showed significant activity against the adult S. digitata by both a worm motility and MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] reduction assays. The isolated active principle was chemically characterized by IR, (1)H-NMR and MS analysis and identified as a phenolic monoterpene. It was screened for in vivo antifilarial activity against the human filarial worm B. malayi in Mastomys coucha, showing macrofilaricidal activity and female worm sterility in vivo against B. malayi. The findings thus provide a new lead for development of a macrofilaricidal drug from natural products.

Determination of some phenolic acids in Majorana hortensis by capillary electrophoresis with online electrokinetic preconcentration

An online accumulation/mobilization preconcentration technique based on a dynamic pH junction technique and electrokinetic injection was employed for analysis of phenolic acids (sinapic, ferulic, coumarinic, caffeic, syringic, vanillic, and 4-hydroxybenzoic acid) in extracts from Majorana hortensis leaves. Samples were extracted by pressurized solvent extraction with acetone at 150 degrees C and 15 MPa. The capillary electrophoretic method employed 50 mmol.L (-1) sodium borate, pH 9.5, as the sample electrolyte, 50 mmol.L (-1) sodium phosphate, pH 2.5, as the background electrolyte, and 50 mmol.L (-1) sodium phosphate, pH 2.5, with 60 mmol.L (-1) sodium dodecyl sulfate as the mobilization electrolyte. The method allowed 720-fold to 5560-fold preconcentration of the phenolic acids during 30 min of electrokinetic accumulation with detection limits from 0.38 to 4.22 ng.mL (-1).

Analysis of phenolic acids in honeys of different floral origin by solid-phase extraction and high-performance liquid chromatography

The determination of 18 aromatic and arylaliphatic carboxylic acids in honey from different floral origin using solid-phase extraction (SPE) and reversed-phase high performance liquid chromatography (RP-HPLC) is reported. The behaviour of the solutes on SPE cartridges was predicted from preliminary calculations involving the pK(a) constants of the carboxylic groups, the n-octanol:water partition coefficients and the distribution coefficients at different pH values of the conditioning and washing solvents. The proposed SPE isolation and pre-concentration of the acids was achieved on reversed-phase Bond Elut C18 cartridges using an acetonitrile:tetrahydrofuran (1:1, v/v) elution system. RP-HPLC separations were performed on a Spherisorb ODS-2 column using linear gradient elution with a mobile phase composed of 20 mm phosphate buffer (pH 2.92) and methanol, and with UV detection. The reported SPE and RP-HPLC methods were applied to the analysis of 49 authentic honey samples from various floral sources and the results indicate that they may serve with respect to the quantitative control of a number of phenolic acids in plant-derived foods and medicinal plants.

Development of an HPLC-PAD-MS assay for the identification and quantification of major phenolic edelweiss (Leontopodium alpium Cass.) constituents

The analytical assessment of edelweiss (Leontopodium alpinum) herb extracts, used in traditional alpine medicine, has resulted in the development of a HPLC-PAD-MS method that allows baseline separation of almost all constituents. Peak assignment of 14 analytes was achieved by comparison of retention times, UV and mass spectra with those of reference compounds either commercially available (luteolin, apigenin and chlorogenic acid) or isolated from edelweiss plants by column chromatography. Ten of the isolated analytes were identified as the known natural products: quercetin-3-O-beta-D-glucoside, luteolin-7-O-beta-D-glucoside, luteolin-3'-O-beta-D-glucoside, luteolin-4'-O-beta-D-glucoside, apigenin-7-O-beta-D-glucoside, 6-hydroxy-luteolin-7-O-beta-D-glucoside, luteolin-7,4'-di-O-beta-D-glucoside, chrysoeriol-7-O-beta-D-glucoside, leontopodic acid and 3,5-dicaffeolyquinic acid. One analyte, 3,4,5-tri-(E)-caffeoly-D-glucaric acid proved to be a new natural product and was named leontopodic acid B. Structure elucidation was carried out by means of MS and NMR spectroscopy in all cases. The aerial plant parts of L. alpinum (capitula, inflorescence leaves, stems, stem leaves and leaves of the basal rosette) showed variable amounts of the above-mentioned constituents, although qualitative differences were not observable.

Supramolecular systems-based extraction-separation techniques coupled to mass spectrometry

The combination of supramolecular chemistry and MS has not only been fruitful in the field of gas-phase fundamental studies of host-guest complexes and supramolecular assemblies. Mass spectrometric analysis has also benefited from the ability of supramolecular systems to behave as pseudophases in which solutes partition from the bulk solvent phase. Supramolecular systems-based extraction and concentration schemes and separation techniques have been widely used in different fields of analytical chemistry and are ideally suited for coupling with MS. This review describes the present status of the application of supramolecular chemistry in mass spectrometric analysis and includes topics such as the use of coacervative liquid-liquid extraction and hemimicelle/admicelle-based SPE of organic compounds prior to chromatography and electrophoresis. It also discusses the recent advances in enantioselective analysis using CD in electrophoresis- and chromatography-MS. The potential and analytical challenges of these approaches in environmental and bioanalytical chemistry, where one can expect significant developments in the future, are outlined.

Use of β-cyclodextrin bonded phase with s-triazine moiety in the spacer for separation of aromatic carboxylic acid isomers by high-performance liquid chromatography

The separation and retention behavior of five aromatic carboxylic acid isomers was investigated by means of high-performance liquid chromatography (HPLC) using a beta-cyclodextrin bonded phase with s-triazine ring in the spacer. The influence of mobile phase pH on the retention was examined. The presence of s-triazine moiety in the spacer enhances greatly the selectivity of the isomers of aromatic carboxylic acids. Baseline separations of the five aromatic carboxylic acid isomers were achieved. In particular, the isomers of toluic, aminobenzoic, nitrobenzoic and hydroxybenzoic acid were successfully and effectively separated. The chromatographic results indicate that, in addition to inclusion complexation, pi-pi interaction and hydrogen bonding interaction between the bonded phase and analytes play significant roles in the retention of these acid isomers. Different elution orders were observed for these acidic solutes with different substituents. Possible retention mechanisms are discussed.

Analysis of phenolic acids as chloroformate derivatives using solid phase microextraction–gas chromatography

In the presented study, a simple and original procedure of phenolic acids derivatization treated by ethyl and methyl chloroformate performed in an aqueous media consisting of acetonitrile, water, methanol/ethanol and pyridine has been modified and optimized. Seven phenolic acid standards-caffeic, ferulic, gallic, p-coumaric, protocatechuic, syringic and vanillic were derivatized into corresponding methyl/ethyl esters and subsequently determined by the means of gas chromatography connected to the flame-ionisation detector (FID). Some selected validation parameters as linearity, detection and quantitation limits and peak area repeatability were valued. The total time of gas chromatography (GC) analysis was 24 min for methyl chloroformate and 30 min for ethyl chloroformate derivatization. The more suitable methyl chloroformate derivatization was used for further experiments on the possibility of multiple pre-concentration by the direct solid phase microextraction technique (SPME). For this purpose, polyacrylate (PA), polydimethylsiloxane (PDMS), carboxen/polydimethylsiloxane (CAR/PDMS) and polydimethylsiloxane/divinylbenzene (PDMS/DVB) fibres were tested and the extraction conditions concerning time of extraction, temperature and time of desorption were optimized. The most polar PA fibre gave the best results under optimal extraction conditions (50 min extraction time, 25 degrees C extraction temperature and 10 min desorption time). As a result, the total time of SPME-GC analysis was 74 min and an increase in method sensitivity was reached. The limits of quantitation (LOQ) of p-coumaric, ferulic, syringic and vanillic acid esters after SPME pre-concentration were 0.02, 0.17, 0.2 and 0.2 microg mL(-1), respectively, showing approximately 10 times higher sensitivity in comparison with the original GC method.

Study of the selectivity of reversed-phase columns for the separation of polycarboxylic acids and polyphenol compounds

A systematic investigation was undertaken into the relative separation performance of five reversed-phase chromatography columns including some commercially new hybrid packed columns for a series of polycarboxylic acids and polyphenol compounds. Information theory (IT) and factor analysis (FA), together with a basic evaluation of retention information (band shape, retention factor and elution order) were used to compare four columns to a conventional C18 column. The results revealed very little difference in retention behaviour between the Phenomenex Aqua C18 column, the Waters XTerra RP C18 column, and the conventional Phenomenex Luna C18 column. However, there were notable differences in the retention processes between the Phenomenex Synergi polar-RP column, which is an ether-linked phenyl base with polar endcapping, and the Luna C18 column. The most significant differences were observed between the Luna C18 column and a Phenomenex Luna Cyano column. However, the limited degree of retention of the polycarboxylic acids and polyphenol compounds on the Luna Cyano column permits only limited use for the separation of these types of compounds. Overall, the Phenomenex Synergi polar-RP column exhibited the best performance for the separation of the test solutes compared to that of the conventional C18 column, with IT yielding an Informational Similarity of 0.99 and FA a moderate correlation coefficient of 0.70. The Phenomenex Synergi polar-RP column gave the best peak shape and offered substantial selectivity differences thereby providing a good alternative over the conventional C18 column for separating polycarboxylic acids and polyphenols.

Separation, characterization, and quantitation of phenolic acids in a little-known blueberry (Vaccinium arctostaphylos L.) fruit by HPLC-MS

The aim of this study was the qualitative and quantitative determination of free, ester, glycoside, and ester-bound phenolic acids in the blueberry (Vaccinium arctostaphylos L.) fruit. A method for the determination of the profile of phenolic acids of four different phenolic fractions in the fruit was developed using high-performance liquid chromatography-mass spectrometry (HPLC-MS). Thirteen compounds (gallic, protocatechuic, p-hydroxybenzoic, m-hydroxybenzoic, gentisic, chlorogenic, p-coumaric, caffeic, ferulic, syringic, sinapic, salicylic, and trans-cinnamic acids) were identified and quantified in the berry. These experimental results showed that the predominant phenolic acid in the fruit of V. arctostaphylos is caffeic acid in free and insoluble ester-bound forms and p-coumaric acid in soluble ester and glycoside forms. Seven phenolic acids were identified as hydroxybenzoic acid derivatives (HBAs) and four as hydroxycinnamic acid derivatives (HCAs). Total content of HBAs and HCAs in the four phenolic fractions constituted 30.1 and 69.9% of the free, 27.9 and 72.1% of the ester, 24.7 and 75.3% of the glycoside, and 51.7 and 48.3% of the ester-bound forms, respectively. Total phenolics as the sum of individual phenolic acids identified is 698.5 ng/g of fresh weight (fw) for the free, 3399.2 ng/g of fw for the ester, 3522.1 ng/g of fw for the glycoside, and 3671.6 ng/g of fw for the ester-bound phenolic fractions. The present results were compared with reported levels of phenolic acids in the fruits of different Vaccinium species. These data suggest that the fruit can be considered as a potentially good dietary source of phenolic acids.

pH gradient high-performance liquid chromatography: theory and applications

pH gradient high-performance liquid chromatography (HPLC) is a method of reversed-phase high-performance liquid chromatography suitable for ionogenic substances. It consists in programmed increase during the chromatographic process of the eluting strength of eluent with respect to the analytes separated. On the analogy of the conventional organic modifier gradient reversed-phase HPLC, in the pH gradient approach the eluting strength of the mobile phase increases due to its changing pH: increasing in case of acids or decreasing in case of bases. At the same time the content of organic modifier remains constant. A theory of the pH gradient HPLC has been elaborated. The resulting mathematical model is easily manageable. Its ability to predict changes in retention and separation of analytes following the changes in chromatographic conditions is demonstrated. The pH gradient method is uniquely suitable to determine pKa values of analytes. An equation is presented allowing to calculate pKa values basing on appropriate retention data. The effects on pKa are discussed of the concentration of methanol in the mobile phase. The RP HPLC-derived pKa data correlate to the reference pKa values (w(w)pKa) but are not identical. That may be explained by the effects on the chromatographically determined pKa of the specific interactions of analytes with stationary phases. The proposed pH gradient RP HPLC procedure offers a fast and convenient means to get comparable acidity parameters for larger series of compounds, like drug candidates, also when the analytes are available only in minute amounts and/or as complex mixtures.

Gradient elution of organic acids on a β-cyclodextrin column in the polar organic mode and its application to drug discovery

A high performance liquid chromatographic method was developed that separated organic acids using the polar organic mode. The separation was obtained using a beta-cyclodextrin stationary phase with a mobile phase that was composed of acetonitrile/methanol/triethylamine (TEA)/acetic acid. The compounds were eluted under gradient conditions and the elution order depended on the number, type and position of the hydrogen bonding functional groups present in the molecule. Adjusting the acid to base ratio resulted in the biggest change in selectivity. In addition, increasing the methanol concentration decreased the retention times of the analytes, which had little effect on the selectivity. Using a certain set of conditions one could separate a large number of organic acids, which allowed these acids to be detected by UV and mass spectrometry. These conditions were used to evaluate the purity of potential pharmaceutical drug candidates that showed activity towards a kinase target vascular endothieal growth factor (Vegf). Each compound contained a carboxylic acid group that was critical to the activity. The method was able to give purity estimates of these samples, which were difficult to determine by other HPLC methods.

Anion-Exchange Displacement Centrifugal Partition Chromatography

Ion-exchange displacement chromatography has been adapted to centrifugal partition chromatography. The use of an ionic liquid, benzalkonium chloride, as a strong anion-exchanger has proven to be efficient for the preparative separation of phenolic acid regioisomers. Multigram quantities of a mixture of three hydroxycinnamic acid isomers were separated using iodide as a displacer. The displacement process was characterized by a trapezoidal profile of analyte concentration in the eluate with narrow transition zones. By taking advantage of the partition rules involved in support-free liquid-liquid chromatography, a numerical separation model is proposed as a tool for preliminary process validation and further optimization.

Simple reversed-phase ion-pair liquid chromatography assay for the simultaneous determination of mycophenolic acid and its glucuronide metabolite in human plasma and urine

A simple and reproducible reversed-phase ion-pair high-performance liquid chromatographic (HPLC) method using isocratic elution with UV absorbance detection is presented for the simultaneous quantitation of mycophenolic acid (MPA) and MPA-glucuronide (MPAG) in human plasma and urine. The sample preparation procedures involved simple protein precipitation for plasma and 10-fold dilution for urine. Each analytical run was completed within 15min, with MPAG and MPA being eluted at 3.8 and 11.4min, respectively. The optimized method showed good performance in terms of specificity, linearity, detection and quantitation limits, precision and accuracy. This assay was demonstrated to be applicable for clinical pharmacokinetic studies.

Determination of pKa by pH Gradient Reversed-Phase HPLC

pH gradient reversed-phase HPLC consists of a programmed increase during the chromatographic run of the eluting power of the mobile phase with regard to ionizable analytes. On the analogy of the conventional organic modifier gradient RP HPLC, in the pH gradient mode, the eluting strength of the mobile phase increases due to its increasing (with acid analytes) or decreasing (with basic analytes) pH, whereas the content of organic modifier is kept constant. We have shown previously that the pH gradient separations are technically possible using standard chromatographic equipment. Here we demonstrate that the method is uniquely suitable to determine pK(a) values of analytes. A strict theoretical model is proposed to determine pK(a) values based on the retention data from a pH gradient RP HPLC run. The pK(a) data so obtained are discussed in relation to the concentration of methanol in the mobile phase, the type of stationary phase, and the duration of the gradient. The pK(a) values determined by the pH gradient method are related to the respective data obtained conventionally in a series of isocratic experiments. A close similarity of the two types of chromatographically determined pK(a) data is demonstrated. The HPLC-derived pK(a) parameters correlate to the literature pK(a) values determined by titrations in water. The chromatographically derived and the reference pK(a) values are not identical, however. That is probably due to the effects on the chromatographic pK(a) of the specific sites of interactions with analytes on the surfaces of the HPLC stationary phases. Nonetheless, the proposed pH gradient HPLC method may supply in a fast and convenient manner comparable acidity parameters for larger series of drug candidates, including those available in only minute amounts, without need of their purification, and also when the compounds are provided as complex mixtures, like those produced by combinatorial chemistry.

Phenolic acids in foods: an overview of analytical methodology

Phenolic acids are aromatic secondary plant metabolites, widely spread throughout the plant kingdom. Existing analytical methods for phenolic acids originated from interest in their biological roles as secondary metabolites and from their roles in food quality and their organoleptic properties. Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruits and vegetables, against oxidative damage diseases (coronary heart disease, stroke, and cancers). High performance liquid chromatography (HPLC) as well as gas chromatography (GC) are the two separation techniques reviewed. Extraction from plant matrixes and cleavage reactions through hydrolysis (acidic, basic, and enzymatic) are discussed as are the derivatization reagents used in sample preparation for GC. Detection systems discussed include UV-Vis spectroscopy, mass spectrometry, electrochemical, and fluorometric detection. The most common tandem techniques are HPLC/UV and GC/MS, yet LC/MS is becoming more common. The masses and MS fragmentation patterns of phenolic acids are discussed and tabulated as are the UV absorption maxima.

Quantitative bioanalysis of enantiomeric drugs using capillary electrophoresis and electrospray mass spectrometry

A novel assay method for an enantiomeric pair of drugs has been developed using a combination of capillary electrophoresis and electrospray tandem mass spectrometry connected with a homemade interface. Accurate quantification was demonstrated in plasma from 0.25 to 50 microg/ml. A liquid-liquid sample preparation technique allowed improvement in the quantitation limit to 10 ng/ml. Variables for the enantiomeric separation, including chiral selective reagent, organic solvents, buffer and acid concentration as well as injection technique, were optimized. This assay proved adequate for analysis of neat, spiked plasma, and plasma from a pharmacological study of the drug enantiomers.

Influence of mobile phase acid-base equilibria on the chromatographic behaviour of protolytic compounds

A review about the influence of mobile phase acid-base equilibria on the liquid chromatography retention of protolytic analytes with acid-base properties is presented. The general equations that relate retention to mobile phase pH are derived and the different procedures to measure the pH of the mobile phase are explained. These procedures lead to different pH scales and the relationships between these scales are presented. IUPAC rules for nomenclature of the different pH are also presented. Proposed literature buffers for pH standardization in chromatographic mobile phases are reviewed too. Since relationships between analyte retention and mobile phase pH depends also on the pKa value of the analyte, the solute pKa data in water-organic solvent mixtures more commonly used as chromatographic mobile phase are also reviewed. The solvent properties that produce variation of the pKa values with solvent composition are discussed. Chromatographic examples of the results obtained with the different procedures for pH measurement are presented too. Application to the determination of aqueous pKa values from chromatographic retention data is also critically discussed.

Prediction of chromatographic retention, pKa values and optimization of the separation of polyphenolic acids in strawberries

Polyphenolic acids are a complex group of compounds that have attracted enormous attention in the last few years because of their biological properties. In this work, the proportion of organic modifier and the pH of acetonitrile-water mixtures used as mobile phases were optimized in order to separate a series of polyphenolic compounds. The linear solvation energy relationship formalism based on the single solvent polarity parameter, E(T)N was used to predict their chromatographic behavior as a function of the percentage of acetonitrile in the eluent. Moreover, the correlation established between retention and the pH of the aqueous-organic mobile phase was used to optimize the pH of the mobile phase. The optimized mobile phase is composed of acetonitrile and formic acid buffer adjusted to pH 4.25, with 12% (v/v) acetonitrile. Also, the pKa values of polyphenolic acids in acetonitrile-water mixtures were determined using chromatographic data, and in order to validate the optimized conditions, a series of polyphenolic compounds was studied in strawberries.

Semi-micro liquid chromatography of aromatic amino acid metabolites using isocratic elution and column switching

The concentrations of catecholamine-related compounds in body fluids reflect sympathetic nerve functions. Measuring the enzyme activity of these metabolic pathways will improve diagnosis since a variety of symptoms are reported. An isocratic elution system with two column switching valves was developed using three types of semi-micro columns for fast chromatographic analysis of catecholamine related compounds. Columns are a pentyl-bonded phase, 50 x 2.1 mm i.d., a phenylhexyl-bonded phase, 100 x 2.1 mm i.d. and an octadecyl-bonded phase, 100 x 2.1 mm i.d. The separation of 20 standard compounds was achieved within 25 min using reversed-phase ion-pair liquid chromatography with an electrochemical detector. This new system was applied for analysis of catecholamine-related compounds in pig brain, since pigs are a widely used animal model for transgenic manipulation of neural genes, and MHPG (or VMA), DOPAC, DOPA, NE, EP, DA, 5HTP and 5HIAA were quantified.

Lipophilicity and pKa estimates from gradient high-performance liquid chromatography

The linear-solvent strength (LSS) model of gradient elution has been applied to estimate parameters of lipophilicity and acidity of a series of drugs and model chemicals. Apparent pKa values and log kw values for individual analytes were determined in 2-3 gradient runs. The first experiment (or first two experiments) uses a wide-range organic modifier gradient with pH chosen for suppressed ionization of the analyte. The result of this experiment allows an estimate of contents of organic modifier of the mobile phase (%B) providing the required retention coefficient, k, for the non-ionized analyte. The following experiment is carried out with the latter %B and a pH-gradient of the aqueous component of the eluent that is sufficient to overlap the possible pKa-value of the analyte. The initial pH of the buffer used to make the mobile phase is selected to insure that the analyte is in non-ionized form. The resulting retention time allows an estimate of PKa in a solvent of the selected %B. At the same time, estimates of log kw can also be obtained. The log kw parameter obtained from gradient HPLC by the approach proposed correlated well with the corresponding value obtained by standard procedure of extrapolation of retention data determined in a series of isocratic measurements. Correlation between log kw and the reference parameter of lipophilicity, log P, was very good for a series of test analytes and satisfactory for a structurally diverse series of drugs. The approach supported with specific detection procedures can be recommended for fast screening of lipophilicity of individual components of complex mixtures like those produced by combinatorial chemistry. The values of pKa obtained in a study were found to correlate with the literature pKa data determined in water for a set of aniline derivatives studied. In case of a series of drugs the correlation was less than moderate if the general procedure of pKa determination was applied.

Analysis of highly polar compounds of plant origin: combination of hydrophilic interaction chromatography and electrospray ion trap mass spectrometry

The primary goal of metabolomic analysis is the unbiased relative quantification of every metabolite in a biological system. A number of different metabolite-profiling techniques must be combined to make this possible. Here we report the separation and analysis of highly polar compounds in a proof of concept study. Compounds were separated and analyzed using hydrophilic interaction liquid chromatography (HILIC) coupled to electrospray ionization (ESI) mass spectrometry. Two types of HILIC microbore columns (Polyhydroxyethyl A and TSK Gel Amide 80) were compared to normal phase silica HPLC columns. The best separations of standards mixtures and plant samples were achieved using the Amide 80 stationary phase. ESI enabled the detection of both positively and negatively charged metabolites, when coupled to a quadrupole ion trap mass spectrometer using continuous polarity switching. By stepwise mass spectrometric fragmentation of the most intense ions, unknown compounds could be identified and then included into a custom mass spectrometric library. This method was used to detect oligosaccharides, glycosides, amino sugars, amino acids, and sugar nucleotides in phloem exudates from petioles of fully expanded Cucurbita maxima leaves. Quantitative analysis was performed using external standards. The detection limit for stachyose was 0.5 ng per injection (Amide 80). The concentration of stachyose in investigated phloem samples was in the range of 1-7 mM depending on the plant.