Porous Column Research Articles

Development of a liquid chromatography–tandem mass spectrometry method for plasma-free metanephrines with ion-pairing turbulent flow online extraction

Liquid chromatography-tandem mass spectrometry has become the preferred technology to measure unconjugated metanephrine and normetanephrine in plasma because of its high sensitivity and specificity over immunoassay and gas chromatography-mass spectrometry. In our earlier study, plasma metanephrines were extracted with offline ion-pairing solid-phase extraction and quantified by liquid chromatography-tandem mass spectrometry with porous graphitic carbon column based chromatography. In this study, we aim to automate the sample preparation with turbulent flow online extraction technology and maintain or improve the analytical performance previously achieved from the offline approach. The online extraction was done with a mixed-mode cation exchange turbulent flow chromatography column assisted with ion-pairing reagent and porous graphitic column was used for chromatographic separation. The total online extraction and analytical LC runtime was 12 min. This method was linear from 6.3 to 455.4 pg/mL for metanephrine; 12.6 to 954.5 pg/mL for normetanephrine with an accuracy of 80.6% to 93.5% and 80.9% to 101.7%, respectively. The lower limit of quantitation was 6.3 pg/mL for metanephrine and 12.6 pg/mL for normetanephrine. Inter-assay and intra-assay precision for metanephrine and normetanephrine at low and high concentration levels ranged from 2.0% to 10.5%. In conclusion, we have developed a fast and sensitive automated online turbulent flow extraction method for the quantitative analysis of plasma metanephrines. Ion-pairing reagent was necessary for the success of this method.

A comparison of overload behaviour for some sub 2 μm totally porous and sub 3 μm shell particle columns with ionised solutes

The overloading performance of some 2.7μm shell and sub 2μm totally porous columns, including one pair manufactured from similar materials with similar bonding chemistries, was compared using strongly acidic and basic probe compounds. In general, the capacity of shell particles was not greatly reduced, despite containing a smaller porous volume. Nevertheless, at low pH, both types of column were overloaded by only small concentrations of ionised solute. Considerable improvement could be gained by increasing the buffer concentration, although sensitivity in mass spectrometric detection may be compromised. The capacity of columns of different internal diameter may not be directly compared merely by scaling the injection volumes, as it is possible that the sample is not homogeneously distributed across the column radius, especially in larger diameter columns, where the sample may travel preferentially through a central core of the packing. A totally porous charged surface hybrid phase gave much improved loading properties of the basic probe in low ionic strength mobile phases such as formic acid, often used in mass spectrometry. However, its relative advantage over conventional phases was reduced as the mobile phase ionic strength was increased. Furthermore, acidic compounds may give tailing on this phase. At pH 7, all columns tested showed evidence of interaction with ionised silanols; peak shapes improved as the buffer concentration was increased. Column efficiency first increased and then decreased as solute concentration was increased at constant buffer concentration, which can be attributed to the decreasing proportion of solute molecules retained by the ion exchange process.

Superficially porous particles columns for super fast HPLC separations

Superficially porous silica particles columns (SPSPCs) are manufactured by different companies. The most common have the brand names Halo, Ascentis Express and Kinetex. These columns provide super fast, sharp peaks and moderate sample loading and back pressure. These are available in different chemistries such as C₈, C₁₈, RP Amide and Hilic. Normally, the silica gel particles have 2.7 and 1.7 µm total and inner solid core diameters with 0.5 µm thick outer porous layer, 90 Å pore size and 150 m²/g surface area. They have been used for the separation and identification of low and high molecular weight compounds. The present article describes the state of the art of superficially porous silica particles based columns with special emphasis on their structures, mechanisms of separation, applications and comparison.

Preparation of aligned porous silica monolithic capillary columns and their evaluation for HPLC

Porous monolithic columns have been investigated and used to achieve fast separation with low back pressure and good resolution. Silica monoliths are favoured over polymeric monoliths due to their chemical/mechanical stability and non-swelling property. The monolithic columns with randomly distributed pore structures have been used so far, leading to eddy diffusion and peak broadening. Here, a directional freezing and freeze drying method is used for the first time to produce aligned porous silica monoliths in a silica capillary from commercially available silica colloidal suspension. Both silica and silica–polymer composite monoliths are produced and the full pore characterizations are performed. These monoliths contain parallel microchannels and thus minimize eddy diffusion. The HPLC tests on these monoliths under both normal phase and reversed phase conditions demonstrate the feasibility for use in chromatography. The aligned microchannel structures of the monoliths can achieve efficient separation with significantly low back pressure, compared to conventional porous monoliths. There is great potential to further develop this new type of monoliths for highly efficient HPLC with low operating back pressure.

Application of Fused-Core Particle Column in Two Dimensional Reversed Phase - Reversed Phase LC/MS Analysis of Biological Samples. Impact of Extra-Column Volume

We demonstrate successful applicability of fused–core column in analysis of biological samples (diluted urine and SPE–pre-purified plasma or serum) using 2 dimensional chromatography. Implementation of fused-core material in the second dimension of 2D chromatographic platform allowed improving S/N above 4 fold in comparison to traditional porous C18 column. Sample load onto the first dimension (5 μm) eliminates limitations of sample volume to be injected. The main limitation factor of the fused-core material–low loading capacity was overcome by using 2D chromatography. We emphasize importance of reduction of post-column (extra column) volume in LC/MS applications utilizing fused-core columns.

Preparation and Evaluation of a Porous P(NIPAAm-MAA-EDMA) Monolithic Column for HPLC

A novel porous monolithic column was prepared by in situ free-radical polymerization in a 50 mm × 4.6 mm i.d. stainless steel column at 60 °C, which was in the presence of 2,2′-azobisisobutyronitrile (AIBN) as initiator, with N-isopropylacrylamide (NIPAAm) and methacrylic acid (MAA) as monomers, ethylene dimethacrylate (EDMA) as crosslinker, methanol and dodecanol as a porogenic solvent. The micrographs of monolithic column were characterized by scanning electron microscopy (SEM) and pore size distribution was determined by mercury intrusion porosimetry. The pressure drop across the columns was measured as a function of flow rate using water and methanol. The result showed that the monolithic columns had an excellent permeability and a high mechanical stability. Moreover, the influence of several parameters which were column temperature, mobile phase composition and flow rate was investigated in the separation of mixtures of nifedipine and nisoldipine. In addition, the effects of type and concentration of buffer on the elution of human plasma protein were investigated. The results showed that a better peak shape was obtained at higher temperature and larger flow rate on the monolithic column.

Investigation of Nanoparticle Transport Inside Coarse-Grained Geological Media Using Magnetic Resonance Imaging

Quantifying nanoparticle (NP) transport inside saturated porous geological media is imperative for understanding their fate in a range of natural and engineered water systems. While most studies focus upon finer grained systems representative of soils and aquifers, very few examine coarse-grained systems representative of riverbeds and gravel based sustainable urban drainage systems. In this study, we investigated the potential of magnetic resonance imaging (MRI) to image transport behaviors of nanoparticles (NPs) through a saturated coarse-grained system. MRI successfully imaged the transport of superparamagnetic NPs, inside a porous column composed of quartz gravel using T(2)-weighted images. A calibration protocol was then used to convert T(2)-weighted images into spatially resolved quantitative concentration maps of NPs at different time intervals. Averaged concentration profiles of NPs clearly illustrates that transport of a positively charged amine-functionalized NP within the column was slower compared to that of a negatively charged carboxyl-functionalized NP, due to electrostatic attraction between positively charged NP and negatively charged quartz grains. Concentration profiles of NPs were then compared with those of a convection-dispersion model to estimate coefficients of dispersivity and retardation. For the amine functionalized NPs (which exhibited inhibited transport), a better model fit was obtained when permanent attachment (deposition) was incorporated into the model as opposed to nonpermanent attachment (retardation). This technology can be used to further explore transport processes of NPs inside coarse-grained porous media, either by using the wide range of commercially available (super)paramagnetically tagged NPs or by using custom-made tagged NPs.

A reversed-flow differential flow modulator for comprehensive two-dimensional gas chromatography

A simple and reliable differential flow modulator has been demonstrated which reverses the flow during the flush step. The modulator is constructed with commercially available capillary flow technology tees which simplifies the apparatus and permits wide range of column dimensions to be used because the modulator volume is adjustable. Using a reverse flush arrangement the tailing of the peak at the base (baseline rise between modulations) is reduced 10–20 fold as compared to forward flush modulation. This is most easily observed for peaks overloaded in the first dimension. Excellent reproducibility (<2% RSD) of area measurements has been demonstrated with a complex fragrance sample as well as the capacity to handle significant overloading without loss of resolution in the second dimension. Further demonstrating the flexibility of this modulator, separation of C1–6 alkanes and olefins are demonstrated with two porous layer open tubular columns.

One‐dimensional freezing of nonheaving unsaturated soils: Model formulation and similarity solution

Freezing of unsaturated soils is associated with the formation of a moving freezing zone and liquid water flow toward the zone. An equilibrium thermodynamic formulation of coupled flow and heat transport in variably saturated partially frozen porous media is developed and a self‐similar solution is derived for the case of a semi‐infinite horizontal porous media column with a constant freezing temperature on one boundary. Solutions to the self‐similar equations are derived using a Runge‐Kutta solution procedure. The solution is found to yield two possible modes distinguished by zones composed of different combinations of ice, liquid water, and air. One of the modes contains three zones: a frozen zone (WI) with just ice and liquid water; a transition zone (AWI) with ice, liquid water, and air; and an unsaturated zone (AW) with liquid water and air. The second mode contains only the WI zone and the AW zone. It is found that the WI zone is a quintessential part of the solution. The AWI zone is found to exist when the advancement of the freezing zone is relatively fast, while it is absent when the zone advances slowly. Predictions of ice saturation and liquid water saturation with the self‐similar solution are compared to published experimental data. Pore pressure is calculated as a linear combination of ice pressure and liquid water pressure, and the calculated figures are used to provide a condition for model limitation in the case of incipient ice lens formation. The developed similarity solution provides insight into the mechanics of liquid water movement and pore filling with ice and the conditions for incipient heaving.

Enantioselective recognition of glutamic acid enantiomers based on poly(aniline- co- m-aminophenol) electrode column

A copolymer, poly(aniline- co- m-aminophenol), was synthesized by copolymerization of aniline and m-aminophenol in the presence of l-glutamic acid ( l-Glu) as template molecules. The copolymer was filled into a porous ceramic column as the conductive stationary phase. Enantioselective recognition of Glu enantiomers was achieved on this special electrode column by applying different potentials on copolymers in the ceramic column. l-Glu anions were ejected from the ceramic column after electrochemical reduction due to the reversible redox of the copolymer while complementary cavities were left. Then a positive potential was applied to the molecularly imprinted poly(aniline- co- m-aminophenol) for re-binging Glu enantiomers in the mobile phase. The results showed good enantioselectivity.

Compaction-dissolution waves in an upwelling mantle column

SUMMARY Compaction–dissolution waves in porosity and melt pressure form spontaneously in numerical simulations of melt migration in an upwelling, viscously compacting, porous column in a solubilitygradient.Themeltfractionisassumedtobesmallandthesolidcomprisesolivineand orthopyroxene. The solubility of orthopyroxene in the melt is assumed to increase linearly with height and induces a gradient reaction, assumed to be at local equilibrium. Approximations for the vertical, 1-D, steady-state solutions are derived assuming negligible resistance to compaction. The linear stability of the steady-state solutions is characterized by complex eigenvalues and an oscillatory instability with strong wavenumber selection. This instability leads to the formation of checkerboard compaction–dissolution waves observed in the nonlinear numerical simulations. The phase velocity of these waves is larger than the solid velocity but smaller than the melt velocity. The oscillatory instability is realized over a range of parameters and the variation in wave properties is explored. A power-law bulk-viscosity formulation, ξ = η/φ m , is shown to decrease growth rates linearly in the exponent, m. For small perturbations, the growth rates and phase velocities measured from high-resolution numerical simulations are predicted by the linear theory as well as the dominant wavenumber in the non-linear regime. We present a regime diagram for reaction infiltration instabilities in viscouslycompacting porousmediaandshowthatcompaction–dissolutionwaves arefavoured by increasing solid upwelling and small solubility gradients relative to high-porosity channels. The regime diagram suggests that the formation of compaction–dissolution waves is a feasible new physical mechanism for melt transport beneath mid-ocean ridges.

Imaging and modeling of flow in porous media using clinical nuclear emission tomography systems and computational fluid dynamics.

This paper presents experimental and modeling aspects of applying nuclear emission tomography to study fluid flow in laboratory packed porous media columns of the type frequently used in geophysics, geochemistry and hydrology research. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are used as non-invasive tools to obtain dynamic 3D images of radioactive tracer concentrations. Dynamic sequences obtained using 18F-FDG PET are used to trace flow through a 5 cm diameter × 20 cm tall sand packed column with and without an impermeable obstacle. In addition, a custom-made rotating column setup placed in a clinical two-headed SPECT camera is used to image 99mTc-DTPA tracer propagation in a through-flowing column (10 cm diameter × 30 cm tall) packed with recovered aquifer sediments. A computational fluid dynamics software package FLUENT is used to model the observed flow dynamics. Tracer distributions obtained in the simulations in the smaller column uniformly packed with sand and in the column with an obstacle are remarkably similar to the reconstructed images in the PET experiments. SPECT results demonstrate strongly non-uniform flow patterns for the larger column slurry-packed with sub-surface sediment and slow upward flow. In the numerical simulation of the SPECT study, two symmetric channels with increased permeability are prescribed along the column walls, which result in the emergence of two well-defined preferential flow paths. Methods and results of this work provide new opportunities in hydrologic and biogeochemical research. The primary target application for developed technologies is non-destructive, non-perturbing, quantitative imaging of flow dynamics within laboratory scale porous media systems.

Transient dynamic analysis of a fluid-saturated porous gradient elastic column

The dynamic response of a fluid-saturated porous gradient elastic column to a transient disturbance is determined analytically and numerically. The basic dynamic theory of a fluid-saturated poroelastic medium due to Biot is modified by replacing the classical linear elastic model of the solid skeleton by the simple gradient elastic model of Mindlin with just one elastic constant (internal length scale) in addition to the classical ones. Thus, the new theory, which is presently restricted to the one-dimensional case, can take into account the microstructural effects of the solid skeleton. After the establishment of appropriate boundary and initial conditions, the one-dimensional dynamic column problem is solved analytically with the aid of the Laplace transform with respect to time. The time domain response is finally obtained by a numerical inversion of the transformed solution. The effect of the solid microstructure on the response is assessed and discussed.

Determination of serum vitamins 25-OH-D2 and 25-OH-D3 with liquid chromatography–tandem mass spectrometry using atmospheric pressure chemical ionization or electrospray source and core-shell or sub-2 μm particle columns: A comparative study

Objective Develop a simple LC–MS–MS method for determination of 25-hydroxymetabolites of vitamin D2 and D3 in serum. Design and methods 0.1 mL serum was mixed with 0.3 mL deproteinizing mixture containing two deuterated internal standards. Two LC–MS–MS systems were used: Waters Premier with an ESI source and a Hypersil Gold RP 18 50 × 2.1 mm column, and a Thermo Quantum, with an APCI source and a Kinetex RP 18 core-shell 50 × 2.1 mm column. Certified reference material 972 from NIST was used for method calibration. The samples were additionally analyzed with commercial HPLC procedure. Results Baseline separation of compounds was achieved in 6 min run time. Both systems have limits of detection of 0.5 nmol/L and recovery ranging from 86% to 112%. The measurement of 25-OH-D3 gave comparable results for all procedures; the results of 25-OH-D2 measurement obtained with the HPLC often differed from those obtained with LC–MS–MS methods. Conclusions Established procedure performed well for ESI and APCI sources and totally porous and core-shell LC columns. The use of two internal standards enhanced the accuracy of the measurements.

Preparation and evaluation of a porous monolithic capillary column for microextraction of estrogens from urine and milk samples online coupled to high-performance liquid chromatography

A porous monolithic capillary column based on poly (methacrylic acid-co-ethylene glycol dimethacrylate) (poly (MAA-co-EGDMA)) was prepared using methanol and polyethylene glycol 6000 as mixed porogens. The monolith has the characteristics of good permeability, high extraction efficiency and long lifetime. Improved permeability of the monolith could realize sample loading with high flow rate. A simple and convenient construction that employed valve-switch technique was designed for online coupling of the monolithic capillary column to high performance liquid chromatography. In order to obtain optimum extraction efficiency, the extraction conditions including sample pH, sample volume, extraction and desorption flow rate were investigated. Under the optimum conditions, the enrichment factors were 180–362 for five estrogens, indicating remarkable preconcentration ability of the monolithic capillary column. The dynamic binding capacity (DBC) was estimated to be 3.73mgmL−1 via frontal analysis. Finally the monolithic capillary column was successfully applied to online enrichment of estrogens from urine and milk samples followed by high performance chromatography. Low detection limits (S/N=3) of the proposed method were achieved in the range of 0.04–0.35μgL−1. The recoveries were 95.6–106.1% and 76.5–116.8% for the spiked urine and milk samples respectively, with the RSDs of 1.7–9.9%.

Reequilibration time of superficially porous silica based columns in gradient elution reversed phase liquid chromatography

Between repetitive analyses using gradient elution liquid chromatography the column must be reequilibrated to the initial conditions, extending run times. We studied the reequilibration time of three superficially porous silica columns compared to one fully porous silica column on a chromatograph with a reduced flush-out volume. Post-gradient acetone injections made at the interface of the pure organic-highly aqueous phase show anomalous, pressure-related band focusing, and increased retention compared to injections on either side of the interface. These anomalies are explained by applying the Buckley–Leverett theory of oil displacement in sands to column reequilibration. Reequilibration was shown to occur quickly, with less than three column volumes of conditioning solvent, and depends on the reproducibility as required by the application. Offline LC–GC was used to quantitate the percent acetonitrile eluting from each column post-gradient. After an initial, large expulsion of acetonitrile, a steady small amount (∼0.03%) of acetonitrile is detected long after the column is considered equilibrated. The limiting variable with column equilibration is not the desorption of organic modifier from the stationary phase, but rather the pressure required to force the aqueous phase into the pores.

RP-HPLC LIPOPHILICITY STUDIES FOR SOME (HETERO)ARYLAMIDES DERIVED FROM 2-AMINO 4,6-DIMETHYL PYRIDINE: INTRODUCTION OF AN HYDROGEN-BOND DESCRIPTOR

RP-HPLC measurements (in water + 60% v/v acetonitrile, on a porous polystyrene column) lead to a linear calibration between the retention factor logk and the lipophilicity logP for a series of 9 OECD standards. Introduction of a variable indicator D for hydrogen-bond donors enhances the goodness of fit. From experimental retention factors, estimated values of logP are given for 23 substituted (hetero)arylcarboxamides derived from 2-amino 4,6-dimethyl pyridine, whose anti-inflammatory behavior and anti-cholinesterase activity were previously characterized. From the logP versus logk relationship, a family-dependent indicator variable D is attributed to each compound.

Application of a Tandem Mass Spectrometer and Core–Shell Particle Column for the Determination of 151 Pesticides in Grains

A comparison of ultrahigh performance liquid chromatography (UHPLC) with a 2.6 μm core-shell particle column (Kinetex C(18)) and conventional liquid chromatography (LC) with a 3 μm porous particle column (Atlantis dC(18)), coupled with electrospray ionization tandem mass spectrometry (ESI-MS/MS), for the determination of 151 pesticides in grains is presented in this study. Pesticides were extracted from grain samples using a procedure known as QuEChERS (quick, easy, cheap, effective, rugged, and safe). Quantification, with an analytical range from 5 to 500 μg/kg, was achieved using matrix-matched standard calibration curves with isotopically labeled standards or a chemical analogue as internal standards. The method performance parameters that included overall recovery, intermediate precision, and measurement uncertainty were evaluated using a designed experiment, that is, the nested design. The UHPLC (Kinetex C(18)) was superior to conventional LC (Atlantis dC(18)) as it yielded a shorter analytical run time, increased method sensitivity, and improved method performance. For UHPLC/ESI-MS/MS (Kinetex C(18)), 90% of the pesticides studied had recoveries between 81 and 110%, 88% of the pesticides had intermediate precision ≤20%, and 84% of the pesticides showed measurement uncertainty ≤40%. As compared to UHPLC/ESI-MS/MS (Kinetex dC(18)), the LC/ESI-MS/MS (Atlantis dC(18)) showed a relatively lower sensitivity, less repeatability, and larger measurement uncertainty. UHPLC/ESI-MS/MS with 2.6 μm core-shell particle column and scheduled MRM proved to be a good choice for quantification or determination of pesticides in grains.

A high-order numerical study of reactive dissolution in an upwelling heterogeneous mantle—I. Channelization, channel lithology and channel geometry

SUMMARY High-porosity channels are important pathways for melt migration in the mantle. To better understand the lithology and geometry of high-porosity melt channels, we conduct high-order accurate numerical simulations of reactive dissolution along a solubility gradient in an upwelling and viscously deformable porous column. In contrast to earlier studies, we assume the dissolution reaction to be at equilibrium, consider a finite soluble mineral abundance, and employ a high-order accurate numerical scheme. Using sustained perturbations in porosity and soluble mineral abundance at the inflow boundary, we explore the structure of steady-state high-porosity melt channels and their associated lithologies over a range of parameters, including soluble mineral abundance, solubility gradient, amplitude and lateral variation in inflow melt flux, melt fraction and upwelling rate. In general, high-porosity dunite channels are transient and shallow parts of pathways for melt migration in the mantle. The lower parts of a high-porosity channel are orthopyroxene-bearing dunite, harzburgite and possibly lherzolite. A wide orthopyroxene-free dunite channel may contain two or three high-porosity melt channels. The depth of dunite channel initiation depends on the solubility gradient, soluble mineral abundance, inflow melt flux, melt suction rate and upwelling rate. The amplitude and length scale of lateral variation in porosity and orthopyroxene abundance at the base of the upwelling column are important in determining the size and dimension of dunite channels, the strength of melt focusing and the melt suction rate, the presence of compacting boundary layer, as well as the number of high-porosity melt branches within a dunite channel. The spatial relations among the high-porosity melt channels, dunite and harzburgite channels documented in our numerical simulations may shed new light on a number of field, petrological and geochemical observations related to melt migration in the mantle.

Determination of phospholipids in milk samples by means of hydrophilic interaction liquid chromatography coupled to evaporative light scattering and mass spectrometry detection

The combined use of the state-of-the-art hybrid mass spectrometers together with high efficient liquid chromatography could surely be a useful tool for such a challenging task, as phospholipids (PLs) analysis. In this research, we used hydrophilic interaction liquid chromatography (150 mm × 2.1 mm I.D., 2.7 μm d.p. partially porous column) to achieve the separation of major PLs classes in cow's and donkey's milk samples. Solid-phase extraction (SPE) was performed in order to pre-concentrate minor PLs from non polar lipids (triacylglycerols) and the recovery for the extraction method was assayed on a milk sample, fortified with 5 μg/mL of SM pure standard, and analyzed in triplicate. A value of 89.99% was calculated, with a coefficient of variation (CV%) of 1.93. A 70-min long stepwise gradient of water/acetonitrile afforded baseline separation of PLs classes, at 50 μL/min flow rate. Accurate detection by an ion trap-time of flight (IT-TOF) mass spectrometer (in both positive and negative ionization mode) allowed to fully characterize the distinctive phospholipid profile and fatty acid composition of cow's and donkey's milk, the latter being analyzed for the first time. Evaporative light scattering detection was further employed to attain the quantitative evaluation of major PLs classes identified, by the external calibration method using reference material solutions in the 5–200 μg/mL concentration range. Major difference between the two analyzed samples consisted in the total PLs amount, which in cow's milk was determined as over 20-fold higher than the donkey's.