Mehdi Ashraf-Khorassani

Design, Modeling, and Fabrication of MEMS-Based Multicapillary Gas Chromatographic Columns

This paper describes different approaches to achieve high-performance microfabricated silicon-glass separation columns for microgas chromatography systems. The capillary width effect on the separation performance has been studied by characterization of 250-, 125-, 50-, and 25-¿m -wide single-capillary columns (SCCs) fabricated on a 10 × 8 mm2 die. The highest plate number (12 500/m), reported to date for MEMS-based silicon-glass columns, has been achieved by 25-¿m-wide columns coated by a thin layer of polydimethylsiloxane stationary phase using static coating technique. To address the low sample capacity of these narrow columns, this paper presents the first generation of MEMS-based ¿multicapillary¿ columns (MCCs) consisting of a bundle of narrow-width rectangular capillaries working in parallel. The theoretical model for the height-equivalent-to-a-theoretical-plate (HETP) of rectangular MCCs has been developed, which relates the HETP to the discrepancies of the widths and depths of the capillaries in the bundle. Two-, four-, and eight-capillary MCCs have been designed and fabricated to justify the separation ability of these columns. These MCCs capable of multicomponent gas separation provide a sample capacity as large as 200 ng compared to 5.5 ng for 25-¿m-wide SCCs.

Supercritical fluid extraction of metal ions and metal chelates from different environments

Abstract Metal ions and metal chelates have been extracted from both solid and aqueous matrices using both pure and modified supercritical fluid CO 2 . It has been demonstrated that fluorinated chelates have higher solubility in the supercritical fluid which can cause enhanced extraction efficiency. Stability of the neutral metal chelate under supercritical conditions plays an important role in the extraction of metal ions from water. Also, it has been demonstrated that the matrix can have a major effect on extraction of both metal ions and metal chelates. Addition of modifier to both matrix and supercritical fluid can increase extraction efficiency of analytes.

Formation of dimers of inclusion cryptand/paraquat complexes driven by dipole–dipole and face-to-face π-stacking interactions

Dimers of inclusion complexes were formed from a new cryptand and viologens (paraquats) driven by dipole-dipole and face-to-face pi-stacking interactions as shown by mass spectrometric characterization and X-ray analysis.

Packed column supercritical fluid chromatography-mass spectroscopy : A review

Abstract The literature is reviewed with regard to packed column supercritical fluid chromatography (SFC) coupled directly with mass spectrometry (MS) using either analytical scale or microbore columns that have typically been used for liquid chromatography analysis. Efforts with direct fluid introduction, moving belt interface, thermospray, particle beam, and atmospheric pressure chemical ionization have been discussed. No longer should SFC be a last resort technique but it should be exercised to its full advantage, especially when coupled to MS. With continual advances in commercially available instrumentation, sample assay by SFC-MS should become routine and rugged.

Subcritical fluid chromatography of water soluble nucleobases on various polar stationary phases facilitated with alcohol‐modified CO2 and water as the polar additive

The separation of four water soluble nucleobases (thymine, uracil, adenine, and cytosine) via supercritical fluid chromatography with a CO(2)-based mobile phase containing an alcohol modifier and additive is described. Methanol, ethanol, 1-propanol, and 1-butanol were examined in conjunction with water as a neutral additive. Packed column stationary phases included silica bonded diol, cyanopropyl, and 2-ethyl pyridine. Thymine and uracil eluted with good peak shapes without additive, while adenine and cytosine yielded late eluting, severely tailing peaks. The addition of up to 5% water to each of the five alcohols gave rise to much sharper peaks that eluted under gradient conditions in less than 10 min with no baseline noise. Results with water under identical chromatographic conditions were compared with formic acid and ammonium acetate as additives. Water proved to be much superior to formic acid, and it was comparable to ammonium acetate. The role of water was speculated to not only enhance the solvating power of the binary mobile phase for water soluble analytes, but the common elution pattern exhibited by each of the three stationary phases suggested that water had altered the surface chemistry of the packed phase.

Solubility of metal chelates and their extraction from an aqueous environment via supercritical CO2

Solubility of nickel(II), copper(II), and chromium(III) hexafluoroacetylacetone and chromium(III) acetylacetone chelates was measured in supercritical CO(2) at two different pressures (200 and 400 atm) and 60 degrees C. Solubility of fluorinated acetylacetone chelates was at least an order of magnitude higher than the non-fluorinated complexes. These pre-formed metal chelates as well metal diethyldithiocarbamate (DDC) and metal bis(trifluoroethyl)dithiocarbamate (FDDC) have also been extracted from aqueous environment using pure supercritical CO(2). It was demonstrated that metal HFA chelates while exhibiting higher solubility in supercritical CO(2) compared with metal FDDC chelates, exhibited lower extraction efficiency using the same extraction conditions. This behavior of metal HFA chelates is related to their stability in an aqueous environment. Direct extraction of Ni(+2) and Cu(+2) from an aqueous matrix was also achieved via in-situ chelation using diethyldithiocarbamate and bis(trifluoroethyl)dithiocarbamate as the ligands. Bis(trifluoroethyl)dithiocarbamate proved to be a more effective ligand for direct extraction of metal ions from aqueous environment using supercritical CO(2).

Microwave-Induced Plasma as an Elemental Detector for Packed-Column Supercritical Fluid Chromatography

The direct introduction of supercritical fluid chromatography effluent from a packed DeltaBond 300 phenyl column with the MIP as an elemental detector, using a high efficiency TM010 cavity, has been employed for SFC separations. A toroidal plasma is sustained in the cavity by using an ICP type torch and an argon auxiliary flow (2 L/min) over all pressures (1500–5000 psi). Methanol modifier concentrations (0–5%) have also been studied. Studies of the effects of CO2 on excitation temperature, electron number density, and analyte emission signals are presented. In addition, the background broad-band emission signal with 100% CO2 and a methanol modifier up to 5% are presented.

Supercritical fluid chromatographic resolution of water soluble isomeric carboxyl/amine terminated peptides facilitated via mobile phase water and ion pair formation

Both analytical scale and preparative scale packed column supercritical fluid chromatography (SFC) have found widespread applicability for chiral separations of multiple polar pharmaceutical candidates. However, SFC is rapidly becoming an achiral technique. More specifically, ion pair SFC is finding greater utility for separation of ionic analytes such as amine salts and organic sulfonates. The key to this success is, in part, the incorporation of additives such as trifluoroacetic acid and ammonium acetate into the mobile phase in association with a wide variety of both bonded silica stationary phases and high purity bare silica. Ion pairing SFC coupled with evaporative light scattering detection and mass spectrometric detection is presented here for the separation of water soluble, uncapped, isomeric peptide pairs that differ in amino acid arrangement. The separation is best achieved on either diol-bonded silica or bare silica with 1-5% (w/w) water as a significant ingredient in the mobile phase. Nitrogenous stationary phases such as 2-ethylpyridine, which had been very successful for the separation of capped peptides failed to yield the desired separation regardless of the mobile phase composition. A HILIC type retention mechanism is postulated for the separation of both isomeric uncapped peptide pairs.

Screening strategies for achiral supercritical fluid chromatography employing hydrophilic interaction liquid chromatography-like parameters

A thorough evaluation of 5 μm bare silica from two major vendors for achiral supercritical fluid chromatography of polar analytes has been carried out. Columns were the same dimension, and a virgin column was reserved for each modifier-mixture combination. Three mixtures were prepared and chromatographically separated via a gradient of methanol-modified CO(2) that incorporated 5% (w/w) water as a neutral additive. Mixture (A) invoked both trifluoroacetic acid and water as additives. Mixture (B) utilized isopropyl amine and water; while mixture (C) employed either ammonium acetate and water as additives or only water. Regardless of the mixture components and mobile phase composition, duplicate separations with superior selectivity and excellent peak resolution were observed on five analysis days over a 15-day period. Subsequent removal of water (i.e. primary additive) from each of the mobile phases led to lower selectivity for early eluting components but excellent peak resolution prevailed for later eluting peaks during a later 5-day testing period with only the secondary additive. The re-introduction of 5% water into the mobile phase (after allowing the bare silica columns that were used with no water to sit for 30 days) slowly yielded the original separation after approximately five injections. A hydrophilic interaction liquid chromatography (HILIC)-like mechanism for SFC whereby analyte partitions between water absorbed on the silica and water in the mobile phase is proposed. The general utility of this experimental approach with bare silica was subsequently demonstrated by single injection of ten drug-like compounds with each of the four mobile phases that previously were utilized with the three model compound mixtures. In each case, sharp peaks were observed for each drug-like compound regardless of the additive although retention times varied with the additive employed.

Supercritical Fluid Chromatography of polyphenolic compounds in grape seed extract

SummarySupercritical Fluid Chromatography with a packed column on a mixture of eight polyphenols has been optimized. Carbon dioxide which was modified with methanol which contained less than 1% (w/w) citric acid as a secondary additive served as the mobile phase. Two tandem diol columns were used sequentially. The optimized method was applied to a supercritical fluid grape seed extract. Various components in the extract could be identified by retention time and ultraviolet spectral comparison with our synthetic mixture of polyphenols.