Retention mechanism for ion-pair chromatography with chaotropic reagents

Quantitative structure-retention relationship for reliable metabolite identification and quantification in metabolomics using ion-pair reversed-phase chromatography coupled with tandem mass spectrometry

Hydrophilic metabolites are essential for all biological systems with multiple functions and their quantitative analysis forms an important part of metabolomics. However, poor retention of these metabolites on reversed-phase (RP) chromatographic column hinders their effective analysis with RPLC-MS methods. Herein, we developed a method for detecting hydrophilic metabolites using the ion-pair reversed-phase liquid-chromatography coupled with mass spectrometry (IPRP-LC-MS/MS) in scheduled multiple-reaction-monitoring (sMRM) mode. We first developed a hexylamine-based IPRP-UHPLC-QTOFMS method and experimentally measured retention time (t_R) for 183 hydrophilic metabolites. We found that t_Rs of these metabolites were dominated by their electrostatic potential depending upon the numbers and types of their ionizable groups. We then systematically investigated the quantitative structure-retention relationship (QSRR) and constructed QSRR models using the measured t_R. Subsequently, we developed a retention time predictive model using the random-forest regression algorithm (r² = 0.93, q² = 0.70, MAE = 1.28 min) for predicting metabolite retention time, which was applied in IPRP-UHPLC-MS/MS method in sMRM mode for quantitative metabolomic analysis. Our method can simultaneously quantify more than 260 metabolites. Moreover, we found that this method was applicable for multiple major biological matrices including biofluids and tissues. This approach offers an efficient method for large-scale quantitative hydrophilic metabolomic profiling even when metabolite standards are unavailable.

Development of Reverse Phase Ultra-fast Liquid Chromatography Using Ion-pairing Reagent for Quantitative Assessment of Ceftriaxone in Rat Serum and Cerebrospinal Fluid

Background:

In case of meningitis, the meninges are inflamed and the blood brain barrier is distorted, therefore, there is no hindrance to drug penetration. The problem arises when the disease is at the verge of cure, the meninges become uninflamed and the permeability of the drug is reduced to such extend that it becomes nearly impossible to maintain the minimum inhibitory concentration of drug at the site of infection. This problem was overcome by formulating ceftriaxone loaded NLCs and administrating it through intraperitoneal route to Wistar Albino rat. For quantitative assessment of drugs in rat serum and cerebrospinal spinal fluid, a new RP-UFLC (reverse-phase ultra-fast liquid chromatography) method has been developed and validated.

Objective:

Development and validation of RP-UFLC (using ion-pairing reagent) method for accurate estimation of ceftriaxone in rat serum and CSF.

Methods:

Method validation is done according to the ICH Guidelines (Q2) for the estimation of ceftriaxone in rat serum and its CSF (cerebrospinal fluid) by the RP-UFLC method. The blood was collected from the rat tail vein and CSF was collected carefully from cisterna magna of the rats by a 23G syringe. The mobile phase was used in a ratio of 70:30%v/v of phosphate buffer with ion-pairing reagent and acetonitrile with a pH of 8.0

Results:

Limit of detection and limit of quantification of ceftriaxone in rat serum was 1.08 μg/ml and 3.84 μg/ml, respectively. Similarly, the limit of detection and limit of quantification of ceftriaxone in CSF of rats was 0.94 μg/ml and 2.84 μg/ml, respectively. In both cases, the R2value was more than 0.99 and showed 99% accuracy.

Conclusion:

The experimental result suggests that the new RP-UFLC method developed and validated can be effectively used to assess ceftriaxone in preclinical studies in rats.

Beta-Alanine and Tris-(hydroxyl methyl) Aminomethane as Peak Modifiers in the Development of RP-HPLC Methods Using Aceclofenac and Haloperidol Hydrochloride as Exemplar Drugs

In the present analytical approach, beta-alanine (ALA) and tris-(hydroxyl methyl) aminomethane (TRIS) were investigated as peak modifiers due to their water solubility and their possible peak modifying a property. These reagents were tested for their efficacy on the elution of aceclofenac (ACF) and haloperidol hydrochloride (HLC) from C18 column (250 mm × 4.6 mm, 5 μ) equipped with a photodiode array detector. The test reagents were investigated at 0.25 ± 0.05% concentration with a varying % aqueous composition on elution efficacy of HLC and ACF. The added ALA/TRIS in the mobile phase significantly (P < 0.05) improvised the symmetrical elution of HLC with 3-fold theoretical plates increase (P < 0.05) and 10-fold reduced capacity factor as compared to the control run. For ACF, the shoulder effect observed for ACF peak was eliminated. The optimized mobile phase was a combination of acetonitrile and water containing 0.25% beta-alanine/TRIS (pH 3.5 with ortho-phosphoric acid) at the ratio of 70:30 and 60:40% v/v, respectively, for ACF and HLC. The method was validated as per ICHQ2 guidelines. The column performance was tested for reproducibility in non-peak modifier applications and revealed a null effect on the column, thus these agents are relatively less toxic to HPLC columns.

Fluorinated carboxylic acids as "ion repelling agents" in reversed-phase chromatography

Fluorinated carboxylic acids have been in use as ion-pairing reagents for over three decades. It has been observed that ion-pairing reagents not only increase the retention of oppositely charged analytes on reversed-phase HPLC columns but also decrease the retention of similarly charged analytes; these latter effects, however, have not been thoroughly investigated for the fluorinated carboxylic acids, and the application of these reagents has been rather restricted to their ion-pairing capacity to separate basic analytes. In the present study, we report a systematic investigation about the effects of three fluorinated carboxylic acids (trifluoroacetic acid (TFA), pentafluoropropionic acid (PFPA), and heptafluorobutyric acid (HFBA)) on the retention and selectivity of the separation of halogenated carboxylic acids and sulfonic acids by reversed-phase chromatography with an inductively coupled plasma mass spectrometry detector (ICPMS). Several eluents were tested and compared at different concentrations (0-100 mM) and pH values, including sulfate, nitrate, phosphate, oxalate, TFA, PFPA, and HFBA. The fluorinated carboxylic acids resulted in a consistent decrease in the retention factors (up to ca. 9-fold with HFBA) in a concentration dependent manner, which plateaued at around 50 mM. Significant improvement of the peak symmetry of the chromatographed acids was also observed. We highlight the advantages of incorporating the fluorinated carboxylic acids in modifying the selectivity and retention of organic acids in reversed phase chromatography in general, and particularly when employing chromatographic detectors with limited compatibility with organic mobile phases such as the ICPMS.

Fluoroalkylamines: Novel, Highly Volatile, Fast-Equilibrating, and Electrospray Ionization-Mass Spectrometry Signal-Enhancing Cationic Ion-Interaction Reagents

A new class of cationic ion-interaction reagents for reversed-phase chromatography is introduced in the present work. Compounds belonging to a homologous series of linear fluoroalkyl chains including trifluoroethylamine (TFEAm), pentafluoropropylamine (PFPAm), heptafluorobutylamine (HFBAm), and nonafluoropentylamine (NFPAm) were tested and compared with ammonia and triethylamine (TEA) for the separation of selected organic acids of general interest such as the herbicides glyphosate, ethephon, and fosamine and arsenic metabolites methylarsonic acid and dimethylarsinic acid as well as other compounds. Depending on the carbon and fluorine atom number, the fluoroalkylamines were shown to be effective cationic ion-interaction reagents, significantly enhancing the retention of organic acids on a C18 reversed-phase column. Contrary to the general behavior of ion-interaction reagents (a broader term than ion-pairing reagent), significant (up to 5-fold) and consistent enhancement in the electrospray ionization mass spectrometry signal (ESI-MS) was observed relative to ammonia and triethylamine. Overall, among the tested series HFBAm was found to offer the best overall properties among the tested series as it provided a good compromise between column equilibration time (ca. 25 column volumes) and retention behavior (up to a 10-fold increase in the retention factor of acids relative to ammonia) while providing the same general advantages found for the fluoroalkylamines such as fast washout times from the ESIMS system (ca. 30 min) and a 3–5-fold signal enhancement. The fluoroalkylamines are a new class of cationic ion-interaction reagents with clear advantages over the currently employed alkylamines and may revive the general interest in ion-interaction chromatography.

Boosting basic-peptide separation through dynamic electrostatic-repulsion reversed-phase (d-ERRP) liquid chromatography

The application of a novel chromatographic approach to therapeutic peptides bearing basic amino acids in their structure allowed unprecedented resolution of their related impurities (including epimeric isobaric ones), resulting in a superior analytical tool for the evaluation of the quality of these drugs in the market.

A simple and effective chromatographic method has allowed unprecedented resolution of basic peptide and their related impurities, including the very challenging epimeric isobaric ones.

Ultra-high performance separation of basic compounds on reversed-phase columns packed with fully/superficially porous silica and hybrid particles by using ultraviolet transparent hydrophobic cationic additives

The use of the tetrabutylammonium additive was investigated in the ultra-high performance reversed-phase liquid chromatographic elution of basic molecules of pharmaceutical interest. When added to the mobile phase at low pH, the hydrophobic tetrabutylammonium cation interacts with the octadecyl chains and with the residual silanols, thus imparting a positive charge to the stationary phase, modulating retention and improving peak shape of protonated basic solutes. Two sources of additive were tested: a mixture of tetrabutylammonium hydroxide/trifluoroacetic acid and tetrabutylammonium hydrogen sulfate. Retention and peak shape of 11 basic pharmaceutical compounds were evaluated on commercially available ultra-fast columns packed with octadecyl stationary phases (Ascentis Express C18 2.0 µm, Acquity BEH C18 1.7 µm, Titan C18 1.9 µm). All columns benefit from the use of additive, especially tetrabutylammonium hydrogen sulfate, providing very symmetric peaks with reasonable retention times. Focusing on the probe compounds amitriptyline and sertraline, efficiency and asymmetry values were investigated at increasing retention factor. The trend is very different to that obtained in reversed-phase conditions and the effect lies in the complex molecular interaction mechanisms based on hydrophobic and ion exchange interactions as well as electrostatic repulsion.

Influence of the mobile phase and molecular structure parameters on the retention behavior of protonated basic solutes in chaotropic chromatography

In this study, we present novel insights into the pH-dependent retention behavior of protonated basic solutes in chaotropic chromatography. To this end, two sets of experiments were performed to distinguish between mobile phase pH and ionic strength effects. In the first set, the ionic strength (I) was varied with the concentration of NaPF₆ and additives that adjusted the mobile phase pH, while in the second set, I was kept constant by adding the appropriate amount of NaCl. In each set, the retention behavior of 13 analytes was qualitatively examined in 21 chromatographic systems, which were defined by the NaPF₆ concentration in their aqueous phases (1-50mM) and the pH of their mobile phases (2, 3 or 4); the acetonitrile content was fixed at 40%. The addition of NaCl significantly reduced the differences among retention factors at studied pH values due to the effect of the Na⁺ ions on PF₆^-adsorption to the stationary phase and the magnitude of the consequential development of the surface potential. A quantitative description of the observed phenomenon was obtained by an extended thermodynamic approach. The contribution of ion-pair formation in the stationary phase to the retention of the solutes was confirmed across models at the studied pH values in the set with varying I. In the systems with a constant I, the shielding effect of the Na⁺ ions on the surface charge lowered the attractive surface potential and diminished the aforementioned interactions and hence the effect of the mobile phase pH on analyte retention. Eventually, we developed a readily interpretable empirical retention model that simultaneously takes into account analyte molecular structures and the most relevant chromatographic factors. Its coefficients have clear physical meaning, and owing to its good predictive capabilities, the model could be successfully used to clarify the contributions of analyte molecular structures and chromatographic factors to the specific processes underlying separation in chaotropic chromatography.

Determination of urazamide in pharmaceutical preparation with room temperature ionic liquid

A high performance liquid chromatographic method was developed and validated for the determination of urazamide in pharmaceutical preparation with novel green aqueous mobile phase modified with room temperature ionic liquids (RTILs). 1-Ethyl-3-methyl-imidazolium tetrafluoroborate ([EMIM][BF4]) was selected as a mobile phase additive to improve retention and avoid baseline disturbances at t₀. Various mobile phase parameters such as cation moiety, chaotropic anion moiety, pH and concentration of RTILs were optimized to determine urazamide at the proper retention time. The assay was validated according to International Conference on Harmonization guidelines. The linearity of the calibration curve was good (r² > 0.999). Intra-day precision varied between 0.50 and 1.23%. Relative standard deviations of inter-day precision ranged between 1.07 and 1.66%. Recoveries in tablets ranged between 99.7 and 101.2% and it was successfully applied to determine urazamide in pharmaceutical preparations.

Implementation of infrared and Raman modalities for glycosaminoglycan characterization in complex systems

Glycosaminoglycans (GAGs) are natural, linear and negatively charged heteropolysaccharides which are incident in every mammalian tissue. They consist of repeating disaccharide units, which are composed of either sulfated or non-sulfated monosaccharides. Depending on tissue types, GAGs exhibit structural heterogeneity such as the position and degree of sulfation or within their disaccharide units composition being heparin, heparan sulfate, chondroitine sulfate, dermatan sulfate, keratan sulfate, and hyaluronic acid. They are covalently linked to a core protein (proteoglycans) or as free chains (hyaluronan). GAGs affect cell properties and functions either by direct interaction with cell receptors or by sequestration of growth factors. These evidences of divert biological roles of GAGs make their characterization at cell and tissue levels of importance. Thus, non-invasive techniques are interesting to investigate, to qualitatively and quantitatively characterize GAGs in vitro in order to use them as diagnostic biomarkers and/or as therapeutic targets in several human diseases including cancer. Infrared and Raman microspectroscopies and imaging are sensitive enough to differentiate and classify GAG types and subtypes in spite of their close molecular structures. Spectroscopic markers characteristic of reference GAG molecules were identified. Beyond these investigations of the standard GAG spectral signature, infrared and Raman spectral signatures of GAG were searched in complex biological systems like cells. The aim of the present review is to describe the implementation of these complementary vibrational spectroscopy techniques, and to discuss their potentials, advantages and disadvantages for GAG analysis. In addition, this review presents new data as we show for the first time GAG infrared and Raman spectral signatures from conditioned media and live cells, respectively.

Diisopropylethylamine/hexafluoroisopropanol-mediated ion-pairing ultra-high-performance liquid chromatography/mass spectrometry for phosphate and carboxylate metabolite analysis: utility for studying cellular metabolism

RATIONALE

Mass spectrometric (MS) analysis of low molecular weight polar metabolites can be challenging because of poor chromatographic resolution of isomers and insufficient ionization efficiency. These metabolites include intermediates in key metabolic pathways, such as glycolysis, the pentose phosphate pathway, and the Krebs cycle. Therefore, sensitive, specific, and comprehensive quantitative analysis of these metabolites in biological fluids or cell culture models can provide insight into multiple disease states where perturbed metabolism plays a role.

METHODS

An ion-pairing reversed-phase ultra-high-performance liquid chromatography (IP-RP-UHPLC)/MS approach to separate and analyze biochemically relevant phosphate- and carboxylic acid-containing metabolites was developed. Diisopropylethylamine (DIPEA) was used as an IP reagent in combination with reversed-phase liquid chromatography (RP-LC) and a triple quadrupole mass spectrometer using selected reaction monitoring (SRM) and negative electrospray ionization (NESI). An additional reagent, hexafluoroisopropanol (HFIP), which has been previously used to improve sensitivity of nucleotide analysis by UHPLC/MS, was used to enhance sensitivity.

RESULTS

HFIP versus acetic acid, when added with the IP base, increased the sensitivity of IP-RP-UHPLC/NESI-MS up to 10-fold for certain analytes including fructose-1,6-bisphosphate, phosphoenolpyruvate, and 6-phosphogluconate. It also improved the retention of the metabolites on a C18 reversed-phase column, and allowed the chromatographic separation of important isomeric metabolites. This methodology was amenable to quantification of key metabolites in cell culture experiments. The applicability of the method was demonstrated by monitoring the metabolic adaptations resulting from rapamycin treatment of DB-1 human melanoma cells.

CONCLUSIONS

A rapid, sensitive, and specific IP-RP-UHPLC/NESI-MS method was used to quantify metabolites from several biochemical pathways. IP with DIPEA and HFIP increased the sensitivity and improved chromatographic separation when used with reversed-phase UHPLC.

Analysis of 3-O-sulfo group-containing heparin tetrasaccharides in heparin by liquid chromatography-mass spectrometry

Complete heparin digestion with heparin lyase 2 affords a mixture of disaccharides and resistant tetrasaccharides with 3-O-sulfo group-containing glucosamine residues at their reducing ends. Quantitative online liquid chromatography-mass spectrometric analysis of these resistant tetrasaccharides is described in this article. The disaccharide and tetrasaccharide compositions of seven porcine intestinal heparins and five low-molecular-weight heparins were analyzed by this method. These resistant tetrasaccharides account for from 5.3 to 7.3wt% of heparin and from 6.2 to 8.3wt% of low-molecular-weight heparin. Because these tetrasaccharides are derived from heparin's antithrombin III-binding sites, we examined whether this method could be applied to estimate the anticoagulant activity of heparin. The content of 3-O-sulfo group-containing tetrasaccharides in a heparin correlated positively (r=0.8294) to heparin's anticoagulant activity.

Application of partially fluorinated carboxylic acids as ion-pairing reagents in LC/ESI-MS

This report describes the application of partially fluorinated carboxylic acids as ion-pairing reagents for basic analytes in high-performance liquid chromatography coupled with electrospray ionization mass spectrometry (LC/ESI-MS) in positive-ion mode. Partially fluoridated carboxylic acids such as difluoroacetic acid, 3,3,3-trifluoropropionic acid and 3,3,3-trifluoromethyl-2-trifluoromethylpropionic acid functioned as volatile paired-ion similarly as trifluoroacetic acid (TFA). These acids provided basic analytes larger retention factor (k) compared to acetic acid or formic acid in LC. The ESI-MS signal strength of analytes with these acids were higher than that of TFA and was analogous to that of acetic acid or formic acid. The performances of partially fluorinated carboxylic acids in LC and ESI-MS for basic analytes were analyzed by multivariate statistical analysis using physicochemical descriptors of acids. Equations obtained in the analysis enabled us the quantitative evaluation of the performance of fluorinated carboxylic acids as ion-pair reagents for basic analytes in LC/ESI-MS.

Chromatography and the hundred year mystery of inorganic ions at aqueous interfaces: adsorption of inorganic ions at the Porous Graphitic Carbon Aqueous Interface follows the Hofmeister series

Many physical phenomena are affected by the structure of water interfaces, yet it remains an active and controversial subject. A great deal of recent theoretical endeavour and computer simulations question the validity of the Onsager Samaras theory of the ion-free interface between an electrolyte solution and an hydrophobic surface. Experimental results play a crucial role in assessing the legitimacy of the theories. Experimental data are scarce, while simulation results suggest an increasing surface affinity of ions with increasing chaotropic character, in dramatic contradiction to the classical view. Chromatography is a powerful separative technique, but we originally used it as a tool to detect the adsorption of chloride electrolytes and sodium electrolytes, strongly expected to shun any dielectric boundary, onto an hydrophobic surface, and to rank ions according to their adsorbophilicities. Frontal analysis gave unequivocal experimental evidence to this unexpected phenomenon and it was used to quantify it. The infinite dilution equilibrium constants for adsorption of kosmotropes and chaotropes onto the interface were obtained and contrasted to the Jones-Dole B viscosity coefficients, that is a common quantifier of the Hofmeister effect. It is clear that (i) the more chaotropic the ion is, the more it contributes to the global adsorbophilicity of the electrolyte; (ii) the influence of the variable anion is more than twofold that of the variable cation, thereby confirming a robust observation in many other physical systems. Standard free energy of adsorption for each electrolyte was calculated and its reliability was commented upon. The central issue in this paper is the effective and ascertained adsorption of electrolytes onto an hydrophobic surface and the fact that the adsorbophilicity of an electrolyte may be inferred from its position in the Hofmeister series.

Disaccharide analysis of glycosaminoglycan mixtures by ultra-high-performance liquid chromatography-mass spectrometry

Glycosaminoglycans are a family of polysaccharides widely distributed in all eukaryotic cells. These polyanionic, linear chain polysaccharides are composed of repeating disaccharide units that are often differentially substituted with sulfo groups. The diversity of glycosaminoglycan structures in cells, tissues and among different organisms reflect their functional an evolutionary importance. Glycosaminoglycan composition and structure also changes in development, aging and in disease progression, making their accurate and reliable analysis a critical, albeit, challenging endeavor. Quantitative disaccharide compositional analysis is one of the primary ways to characterize glycosaminoglycan composition and structure and has a direct relationship with glycosaminoglycan biological functions. In this study, glycosaminoglycan disaccharides, prepared from heparan sulfate/heparin, chondroitin sulfate/dermatan sulfate and neutral hyaluronic acid using multiple polysaccharide lyases, were fluorescently labeled with 2-aminoacridone, fractionated into 17 well-resolved components by reverse-phase ultra-performance liquid chromatography, and analyzed by electrospray ionization mass spectrometry. This analysis was successfully applied to cell, tissue, and biological fluid samples for the picomole level detection of glycosaminoglycan composition and structure.

Ultra-performance ion-pairing liquid chromatography with on-line electrospray ion trap mass spectrometry for heparin disaccharide analysis

A high-resolution method for the separation and analysis of disaccharides prepared from heparin and heparan sulfate (HS) using heparin lyases is described. Ultra-performance liquid chromatography in a reverse-phase ion-pairing mode efficiently separates eight heparin/HS disaccharides. The disaccharides can then be detected and quantified using electrospray ionization mass spectrometry. This method is particularly useful in the analysis of small amounts of biological samples, including cells, tissues, and biological fluids, because it provides high sensitivity without being subject to interference from proteins, peptides, and other sample impurities.

Hyphenated techniques for the analysis of heparin and heparan sulfate

The elucidation of the structure of glycosaminoglycan has proven to be challenging for analytical chemists. Molecules of glycosaminoglycan have a high negative charge and are polydisperse and microheterogeneous, thus requiring the application of multiple analytical techniques and methods. Heparin and heparan sulfate are the most structurally complex of the glycosaminoglycans and are widely distributed in nature. They play critical roles in physiological and pathophysiological processes through their interaction with heparin-binding proteins. Moreover, heparin and low-molecular weight heparin are currently used as pharmaceutical drugs to control blood coagulation. In 2008, the health crisis resulting from the contamination of pharmaceutical heparin led to considerable attention regarding their analysis and structural characterization. Modern analytical techniques, including high-performance liquid chromatography, capillary electrophoresis, mass spectrometry, and nuclear magnetic resonance spectroscopy, played critical roles in this effort. A successful combination of separation and spectral techniques will clearly provide a critical advantage in the future analysis of heparin and heparan sulfate. This review focuses on recent efforts to develop hyphenated techniques for the analysis of heparin and heparan sulfate.

The role of ion-pairing in peak deformations in overloaded reversed-phase chromatography of peptides

The paper reports a study on the role of ion-pairing behind peak deformations, e.g. peak splitting and even peak disappearance, during the elution of a peptide at highly overloaded conditions in reversed-phase chromatography. Deformation of component peaks is not uncommon in chromatography. There are reports which discuss their occurrence, but mostly at analytical scale, while their occurrence is quite common also in the preparative scale, as in the case discussed in this work. This paper first describes the conditions leading to peak splitting and peak disappearance of an industrial peptide, then explains the plausible reasons behind such behaviour, and finally with experimental analysis demonstrates the role of ion-pairing in causing such behaviour.

Improvement of separation efficiencies of anion-exchange chromatography using monolithic silica capillary columns modified with polyacrylates and polymethacrylates containing tertiary amino or quaternary ammonium groups

Anion-exchange (AEX) columns were prepared by on-column polymerization of acrylates and methacrylates containing tertiary amino or quaternary ammonium groups on monolithic silica in a fused silica capillary modified with anchor groups. The columns provided a plate height (H) of less than 10 microm at optimum linear velocity (u) with keeping their high permeability (K=9-12 x 10(-14) m2). Among seven kinds of AEX columns, a monolithic silica column modified with poly(2-hydroxy-3-(4-methylpiperazin-1-yl)propyl methacrylates) (HMPMA) showed larger retentions and better selectivities for nucleotides and inorganic anions than the others. The HMPMA column of 410 mm length produced 42,000-55,000 theoretical plates (N) at a linear velocity of 0.97 mm/s with a backpressure of 3.8 MPa. The same column could be employed for a fast separation of inorganic anions in 1.8 min at a linear velocity of 5.3 mm/s with a backpressure of 20 MPa. In terms of van Deemter plot and separation impedance, the HMPMA column showed higher performance than a conventional particle-packed AEX column. The HMPMA column showed good recovery of a protein, trypsin inhibitor, and it was applied to the separation of proteins and tryptic digest of bovine serum albumin (BSA) in a gradient elution, to provide better separation compared to a conventional particle-packed AEX column.