Number Of Theoretical Plates Research Articles

New vinylester-based monoliths as a new stationary phase for capillary electrochromatography.

Vinyl ester-based monoliths are proposed as a new group of stationary phase for CEC. The capillary monolithic columns were prepared by using two vinyl ester monomers, vinyl pivalate (VPV), and vinyl decanoate (VDC) by using ethylene dimethacrylate (EDMA) as the cross-linking agent, and 2-acrylamido-2-methylpropane sulfonic acid as the charge-bearing monomer. The monoliths with different pore structures and permeabilities were obtained by varying the type and composition of the porogen mixture containing isoamyl alcohol and 1,4-butanediol. The electrochromatographic separation of alkylbenzenes was successfully performed by using an acetonitrile/aqueous buffer system as the mobile phase in a CEC system. Vinyl ester monoliths with short alkyl chain length (i.e. poly(VPV-co-EDMA) exhibited better separation performance compared with the monolith with long alkyl chain length (i.e. poly(VDC-co-EDMA). In the case of VPV-based monoliths, the theoretical plate numbers higher than 250000 plates/m were achieved by using a porogen mixture containing 33% v/v of isoamyl alcohol. For both VDC and VPV-based monoliths, the column efficiency was almost independent of the superficial velocity in the range of 2-12 cm/min.

Metal–Organic Framework InH(d-C10H14O4)2 for Improved Enantioseparations on a Chiral Cyclodextrin Stationary Phase in GC

The chiral MOF InH(d-C10H14O4)2 has a left-handed helical structure and adopts a diamond network. In this work, three capillary columns (A, B, and C) containing InH(d-C10H14O4)2, peramylated β-cyclodextrin and sodium chloride, and InH(d-C10H14O4)2 and peramylated β-cyclodextrin have been prepared and their respective enantioseparation abilities have been investigated. The polarity of the MOF is moderate, whereas peramylated β-cyclodextrin provides a stationary phase of low polarity. The numbers of theoretical plates (plate m−1) of the three columns have been measured with n-dodecane at 120 °C, which followed an increasing order of A (2,973) < B (3,563) < C (3,665). 1-Phenylethanol, dihydrocarvyl acetate, citronellal, leucine, methyl l-β-hydroxyisobutyrate, limonene, and menthol were separated with different resolutions on columns A, B, and C. The relative standard deviations (RSDs) in the run-to-run and column-to-column retention times were 0.50 and 2.50 %, respectively. The test results indicated that the incorporated InH(d-C10H14O4)2 enhanced the separations of racemates on a peramylated β-cyclodextrin stationary phase, resulting in high column efficiency and good reproducibility in gas chromatography.

Fabrication of a totally renewable off-channel amperometric platform for microchip electrophoresis

In this approach, a novel method to fabricate an integrated amperometric platform used in off-channel electrophoresis has been introduced. A simple screen printed protocol combining a wet etching procedure was used to define the pattern on a glass substrate, and whole electrodes were constructed by filling the conductive carbon ink into the etched cavities. A simple Teflon tape was used to align this platform with the micro-channel, and the variation of reassembling of this device can be down to 2.2% without the assistance of microscope. This device was characterized by dopamine (DA) and catechol (CA), and the width of half peak is around 4s, even a 100μm double T shape injection design and a 550μm working electrode were used in this work. Under the optimum condition, this device possesses a low background with a noise level of 1.4pA (peak to peak). The linear range for DA and CA are 0.1–100μM (R=0.998) and 0.2–200μM (R=0.996) with a theoretical plate number of 1.57×104 and 3.46×104 (plate/m), respectively.

Evaluation of new silica‐based humic acid stationary phase for the separation of tocopherols in cold‐pressed oils by normal‐phase high‐performance liquid chromatography

A new humic acid stationary phase was prepared by immobilizing humic acid onto aminopropyl silica via an amide linkage formation and used, for the first time, for the separation and quantification of the tocopherol compounds in cold-pressed oil samples under normal-phase high-performance liquid chromatography conditions. Parameters affecting the chromatographic separation such as mobile phase composition and flow rate were optimized. By evaluating the calculations of capacity factor, asymmetry factor, resolution, selectivity factor, and theoretical plate number, the best separation was obtained with isocratic elution of n-hexane and isopropyl alcohol (99:1% v/v) at a flow rate of 1.0 mL/min. The effluent was monitored by a fluorescence detector set at excitation and emission wavelengths 295 and 330 nm, respectively. All compounds were separated in 20 min. The method was validated according to international guidelines and found to be linear in a wide concentration range, also the mean recovery of the compounds ranged from 97.9 to 99.2%, with a CV less than 2.7% in all cases. The results showed that the developed stationary phase is suitable for the separation and quantification of the tocopherol compounds in real oil samples.

Simulation of Citronellal Extraction Tower Based on Aspen Plus Software

Aspen Plus is a simulation system of large-scale generalized flowsheet. It is to describe the chemical process by using a digital model, and to obtain the desired results on the computer by changing various effective conditions. The main component with higher value in citronella oil is high purity citronellal. Due to its boiling point, temperature-sensitive and other issues, the difficulty of distillation process increases. This paper conducts process design and simulation on citronella oil high vacuum distillation process by using Aspen Plus software, obtains the process parameters of citronellal extraction tower, explores the relationship between reflux ratio and the number of theoretical plates, and provides data support for the continuous production of citronellal in citronella oil system.

Preparation and Detection of the Resolution and Number of Theoretical Plates for High Purity Steviosides

Macroporous resin was the most common method for the separation and purification of steviosides. But the purity was only 90% influenced by less selectivity of the resin. And Rebaudioside A could not be separated from other steviosides at the same time. Purity of total glycosides could be up to 99% and Rebaudioside A could be enriched efficiently by section desorption after one-dimensional chromatography of macroprous resins. The content of Rebaudioside A increased from 39.46% to 63.70% gradually in each desorption solution of the 4 resin columns. Moreover, the resolution between Rebaudioside A, Rebaudioside C and stevioside, and the number of theoretical plates for them were detected meanwhile. These results could provide theoretical foundation for the separation of high purity steviosides and enrichment of Rebaudioside A.

RP-HPLC Method for Determination of Metaxalone Using Box- Behnken Experimental Design.

The present study demonstrates a chemometric approach to identify the significant factors influencing the reverse phase high performance liquid chromatography (RP-HPLC) determination of metaxalone in bulk and pharmaceutical dosage forms. The combined effect of buffer pH, % organic phase and flow rate, each at three levels was screened for responses such as retention time, tailing factor and number of theoretical plates. The responses were statistically optimized with the aid of response surface methodology using Box–Behnken experimental design (BBD). Phosphate buffer (pH 5): acetonitrile (41.74: 58.26 v/v) as eluent pumped at 0.8 ml/minflow rate was found to be the optimal condition to attain desired responses. The optimized method was further validated with respect to linearity, precision, accuracy and sensitivity according to the current regulatory requirements. Finally, the developed experimental design was validated by statistical analysis using one-way ANOVA showed that the proposed method was simple, sensitive, robust and represents an improvement over the existing methods for routine analysis of the metaxalone.

New peak broadening parameter for the characterization of separation capability in capillary electrophoresis.

The influence of separation conditions on peak broadening is usually estimated by the number of theoretical plates. Using the data available in literature and experimental data, it is shown that in pressure-assisted capillary electrophoresis the plate number is not directly related to the separation capability of conditions used. The experiments at different electrolyte flow velocities demonstrate that a higher plate number (the best separation efficiency) can be obtained with a lower peak resolution. Since ions are separated by electrophoresis due to the difference in electrophoretic mobilities, the peak width in terms of electrophoretic mobility is suggested as a new peak broadening parameter describing the separation capability of the conditions used. The parameter can be calculated using the tailing factor and the temporal peak width at 5% of the peak height. A simple equation for the resolution calculation is derived using the parameter. The advantage of the peak width in terms of mobility over other parameters is shown. The new parameter is recommended to be used not only in pressure-assisted capillary electrophoresis but also in general capillary electrophoresis when in a number of runs the virtual separative migration distance and separation capability of the conditions used change widely.

Preparation and chromatographic performance of a eugenol-bonded silica gel stationary phase for high performance liquid chromatography

A eugenol-bonded silica gel stationary phase (EGSP) for high performance liquid chromatography ( HPLC) has been synthesized by the solid-liquid successive reaction method. The preparation process included two steps: firstly, γ-glycidoxypropyltrimethoxy-silane (KH-560) was covalently attached to the surface of spherical silica gel. Then the bonded silica gel continued to react with eugenol ligand, which was a plant active component, and obtained EGSP. The structure of EGSP was characterized by elemental analysis, thermogravimetric analysis and Fourier transform infrared spectroscopy. Using naphthalene as a probe, the column efficiency was tested under the mobile phase of acetonitrile-water (35:65, v/v) at a flow rate of 0.8 mL/min. The chromatographic properties and the retention mechanism of EGSP were evaluated by using neutral, basic and acidic analytes as solute probes. Meanwhile, the comparative study with C18 column and phenyl column was also carried out under the same chromatographic conditions. The result showed that the eugenol ligand was successfully bonded to the surface of silica gel with a 0.28 mmol/g of bonded amount, and the theoretical plate number of EGSP column was about 24 707 N/m. The EGSP appeared to be a kind of excellent reversed-phase stationary phase with suitable hydrophobicity and various synergistic sites. The eugenol ligand bonded on silica gel could first provide π-π interaction sites for different analytes because of its benzene ring and alkenyl. In addition, the methoxy groups of eugenol were responsible for dipole-dipole and hydrogen-bonding interactions between the ligand and solutes in the effective separation process. Comparing with traditional C18 column and phenyl column, EGSP has an advantage in the fast separation of polar compounds under simple experimental conditions.

Non-Gaussian Theoretical Plate Number in Chromatography

Non-Gaussian Theoretical Plate Number in Chromatography

Preparation, characterization and application of N-methylene phosphonic acid chitosan grafted magnesia–zirconia stationary phase

A hydrophilic stationary phase (SP) was prepared through grafting N-methylene phosphonic acid chitosan on magnesia–zirconia particles (P-CTS-MgO–ZrO2) via Lewis acid–base interaction. The resulting material was characterized by thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscope and nitrogen adsorption analysis. The chromatographic performance of P-CTS-MgO–ZrO2 was systemically evaluated by studying effect of acetonitrile content, pH and buffer concentration in the mobile phase. The results demonstrated that the novel SP provided hydrophilic, electrostatic-repulsion and ion-exchange interactions. Compared to the bare MgO–ZrO2, P-CTS-MgO–ZrO2 exhibited superior peak shape, reasonable resolution and reduced analysis time in separation of basic analytes. Besides, remarkable resolving power of acids, i.e. six non-steroidal anti-inflammatory drugs which failed to be eluted from the bare MgO–ZrO2, was obtained with the theoretical plate number (N/m) of 4653–31313, asymmetry factor <1.21 and the resolution of 1.6–3.4. Finally, P-CTS-MgO–ZrO2 SP was applied to separate monosaccharides, phospholipids and peptides. P-CTS-MgO–ZrO2 was a promising hydrophilic SP for wide applications.

Polymer monolithic methacrylate base modified with tosylated-polyethylene glycol monomethyl ether as a stationary phase for capillary liquid chromatography

A polyethylene glycol (PEG) monolithic column was successfully prepared in situ for the separation of inorganic anions in ion exchange capillary chromatography. By attaching PEG-groups into the methacrylate-based polymer, the number of theoretical plates was improved from 1433 to 3346plates/m (when nitrate was used as the analyte). The retention behavior of iodate, bromate, nitrite, bromide and nitrate was observed under various salt aqueous solutions. The retention was based on cations trapped among PEG chain and the positively charged pyridine that work as the anion exchange sites in the PEG monolith. The relative standard deviations (RSDs, for n=7) of retention time, peak height and peak area were less than 2.27% for all the analyte anions. The PEG monoliths showed satisfactory mechanical stability and did not swell or shrink significantly with swelling propensity value of 0.34 and 0.64 for methanol and THF, respectively. This stationary phase was successfully applied to the determination of these anions in seawater as well as public drinking water samples.

Liquid chromatography of some steroid hormones in aqueous-organic, micellar, and cyclodextrin mobile phases

The retention of hydrocortisone, progesterone, dexamethasone, and prednisolone is studied by HPLC and thin-layer chromatography using aqueous-organic, micellar, and cyclodextrin mobile phases. It is demonstrated that micellar and cyclodextrin mobile phases improve the efficiency and selectivity of the chromatographic separation of the indicated hormones compared to aqueous-organic eluents. By comparing the numbers of theoretical plates, the heights equivalent to a theoretical plate, and resolution it was found that micellar mobile phases are more efficient than cyclodextrin ones and can be used for work with Sorbfil plates.

In situ polymerization of monolith based on poly(Triallyl Isocyanurate-co-trimethylolpropane triacrylate) and its application in high-performance liquid chromatography.

A novel organic monolithic stationary phase was prepared for high-performance liquid chromatography (HPLC) by in situ copolymerization. In which, triallyl isocyanurate (TAIC) and trimethylolpropane triacrylate (TMPTA) in a binary porogenic solvent consisting of polyethylene glycol 200 and 1, 2-propanediol were used. The resultant monoliths with different column properties (e.g., morphology and pressure) were optimized by adjusting the ratio of TMPTA/TAIC and the composition of porogenic solvent. The resulting poly(TAIC-co-TMPTA) monolith showed a relatively homogeneous structure, good permeability and mechanical stability. The chemical group of the monolith was assayed by the infrared spectra method, the morphology of monolithic material was studied by scanning electron microscopy and the pore size distribution was determined by a mercury porosimeter. A series of small molecules were used to evaluate the column performance in terms of hydrophobic mode. At an optimized flow rate of 1.0 mL min(-1), the theoretical plate number of analyte was >15,000 plates m(-1). These applications demonstrated that the monoliths could be successfully used as the stationary phase in conjunction with HPLC to separate small molecules from the mixture.

Using active flow technology columns to achieve near ‘direct’ injection through-put in HPLC –MS

A new column technology referred to as active flow technology was used to undertake the analysis of amino acids at very high through-puts. Using active flow technology chromatography columns functioning in a parallel segmented flow mode the separation efficiency was shown to be up to 65% higher than conventional columns — based on the number of theoretical plates. Since the volumetric flow that elutes from the active flow technology column is reduced in proportion to the segmentation ratio, the volume of mobile phase that enters the mass spectrometer is reduced. This enables separations to be undertaken at higher volumetric flow rates without compromising the flow limitations of the MS. When 2.1mm i.d. active flow technology columns were employed separations with MS detection were completed in less than 6s, time scales that rival those for direct injection, but with the advantage of having a chromatographic separation step prior to the MS.

Exponential Function Shortcut Method for the Calculation of the Number of Theoretical Plates in a Distillation Column

An exponential function shortcut calculation (EFSC) method is proposed for the estimation of the number of theoretical plates, NT, in distillation units. We set up the new concept of the thermal st...

Click N-benzyl iminodiacetic acid: Novel silica-based tridentate zwitterionic stationary phase for hydrophilic interaction liquid chromatography

Iminodiacetic acid (IDA) is dicarboxylic acid amine, which may produce stronger interaction with polar or charged compounds than bidentate α,β-amino acid. In this article, a novel type of tridentate zwitterionic HILIC stationary phase was prepared by covalently bonding N-benzyl IDA on silica gel via copper(I) catalyzed Huisgen azide-alkyne 1,3-dipolar cycloaddition (CuAAC). The structure of this stationary phase and all related intermediates was confirmed by NMR, FT-IR, MS spectrum and elemental analysis. The new stationary phase showed good HILIC characteristics and high column efficiency (the theoretical plate number is up to 44000 plates m−1 in the case of guanosine) in the application of separation of polar compounds, including organic acids, organic bases, as well as highly polar and hydrophilic compounds, such as cephalosporins and carbapenems. Most of them displayed good peak shape and selectivity.

Quercetin-imprinted chromatographic sorbents revisited: Optimization of synthesis and rebinding protocols for application to natural resources

Molecularly imprinted polymers (MIPs) based on quercetin and synthesized by either bulk, precipitation or suspension polymerization were characterized in terms of size and shape by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After a study of rebinding protocols, the optimal materials were evaluated as sorbents for solid-phase extraction (SPE) and high-performance liquid chromatography (HPLC) to confirm the presence of imprinted cavities and to assess their selectivity. Besides quercetin, other structurally related natural compounds, naringenin, daidzein and curcumin, were employed for selectivity tests of MIPs. Although rebinding protocols previously described for such MIPs are typically based on binding, washing and eluting methanol-based solutions, we show that this highly polar solvent leads to weak specific interactions (imprinting factor<1) and poor sorbent properties, most probably because of hydrogen-bonding interferences between the MIP and MeOH. Similar experiments performed in tetrahydrofuran yield to much more improved properties (imprinting factor>2.4). This calls for reviewing most of previously published data on quercetin-MIPs; in proper binding conditions, published MIPs may prove more performing than initially assessed. As expected, tested MIPs exhibited the highest selective rebinding towards quercetin template (imprinting effect, quercetin, 3.41; naringenin, 1.54; daidzein, 1.38; curcumin, 1.67); the differences in selectivity between quercetin analogues were explained by the ligand geometries and H-bonding patterns obtained from quantum-chemical calculations. The evaluation of MIPs under identical analytical conditions allowed investigating the effect of the production method on chromatographic performances. The MIPs in bead materials (for quercetin, peak width, 0.69; number of theoretical plates, 143; symmetry factor, 2.22) provided a significant improvement in chromatographic efficiency over the bulk materials (for quercetin, peak width, 1.25; number of theoretical plates, 115; symmetry factor, 2.92). Using the quercetin-beaded MIP as SPE sorbent, quercetin was selectively extracted from Allium cepa L. extract. The MIP developed in this work therefore appears highly promising for the enrichment and determination of quercetin in natural products.

Evaluation of the Effects of Sample Dilution and Volume in Hydrophilic Interaction Liquid Chromatography

The effects of sample dilution and volume on the peak shapes in hydrophilic interaction liquid chromatography (HILIC) were evaluated using fluorescence-labeled thiols as model compounds and ZIC-HILIC as the HILIC column. The content of acetonitrile, which was selected as the diluent of the aqueous samples in this study, was varied 0−95 % in the injection samples, and the numbers of theoretical plates (NTPs) and retention times were compared using a mobile phase composed of ammonium formate buffer/acetonitrile (25:75, v/v). Although the NTPs and retention times decreased with decreasing acetonitrile content, the peak shapes were acceptable for samples with acetonitrile contents down to 50 % based on asymmetry factor. Furthermore, the sample volume had a serious effect for samples with low acetonitrile contents. Although a high content of acetonitrile in samples is still recommended, the capacity for the aqueous solution in the injection samples under HILIC conditions should vary with the composition of the mobile phase and may be larger than previously thought. These findings should be helpful in deciding the sample composition under HILIC conditions, particularly in bioanalysis, where aqueous solution is often contained in the injection samples and the sample dilution with an organic solvent may decrease the detection sensitivity.

Synthesis of a mixed-model stationary phase derived from glutamine for HPLC separation of structurally different biologically active compounds: HILIC and reversed-phase applications

A novel mixed-mode stationary phase was synthesised starting from N-Boc-glutamine, aniline and spherical silica gel (4µm, 60Å). The prepared stationary phase was characterized by IR and elemental analysis. The new stationary phase bears an embedded amide group into phenyl ring, highly polar a terminal amide group and non-polar groups (phenyl and alkyl groups). At first, this new mixed-mode stationary phase was used for HILIC separation of four nucleotides and five nucleosides. The effects of different separation conditions, such as pH value, mobile phase and temperature, on the separation process were investigated. The optimum separation for nucleotides was achieved using HILIC isocratic elution with aqueous mobile phase and acetonitrile with 20°C column temperature. Under these conditions, the four nucleotides could be separated and detected at 265nm within 14min. Five nucleosides were separated under HILIC isocratic elution with aqueous mobile phase containing pH=3.25 phosphate buffer (10mM) and acetonitrile with 20°C column temperature and detected at 265nm within 14min. Chromatographic parameters as retention factor, selectivity, theoretical plate number and peak asymmetry factor were calculated for the effect of temperature and water content in mobile phase on the separation process. The new column was also tested for nucleotides and nucleosides mixture and six analytes were separated in 10min. The chromatographic behaviours of these polar analytes on the new mixed-model stationary phase were compared with those of HILIC columns under similar conditions. Further, phytohormones and phenolic compounds were separated in order to see influence of the new stationary phase in reverse phase conditions. Eleven plant phytohormones were separated within 13min using RP-HPLC gradient elution with aqueous mobile phase containing pH=2.5 phosphate buffer (10mM) and acetonitrile with 20°C column temperature and detected at 230 or 278nm. The best separation conditions for seven phenolic compounds was also achieved using reversed-phase HPLC gradient elution with aqueous mobile phase containing pH=2.5 phosphate buffer (10mM) and acetonitrile with 20°C column temperature and seven phenolic compounds could be separated and detected at 230nm within 16min.