Publications by authors named "Nimisha Thakur"

6 Publications

  • Page 1 of 1

Evaluation of gas chromatography for the separation of a broad range of isotopic compounds.

Anal Chim Acta 2021 Jun 10;1165:338490. Epub 2021 Apr 10.

Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, TX, 76019, United States. Electronic address:

The separation of deuterated compounds from their protiated counterparts is essential in areas of drug discovery and development, investigating kinetic isotope effects and quantitative methods of non-mass spectrometry-based stable isotope dilution assay (non-MS SIDA). The separations of 47 isotopologue pairs of common compounds and drugs were achieved by gas-liquid chromatography, employing twelve different stationary phases. Polydimethylsiloxane phase, phenyl substituted polydimethylsiloxane phases, wax phases, ionic liquid phases, and chiral stationary phases were selected to encompass a wide polarity range and diverse chemical interactions. The best-performing stationary phases are presented for separation of protic-polar, aprotic-dipolar, nonpolar analytes. Overall, the IL111i, SPB-20, and PAG stationary phases were remarkable in their ability to separate the isotopologues. The isotope effect was also evaluated. It was observed that nonpolar stationary phases often exhibit an inverse isotope effect in which heavier isotopic compounds elute earlier than their lighter counterparts. Conversely, polar stationary phases often show a normal isotope effect, while those of intermediate polarities can show both effects depending on the isotopologues. The location of deuterium atoms, however, affects isotopologue retention times. Deuterium substituted aliphatic groups appear to have a greater inverse isotope effect on retention than aromatic substituents.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.aca.2021.338490DOI Listing
June 2021

A Gas Chromatography-Molecular Rotational Resonance Spectroscopy Based System of Singular Specificity.

Angew Chem Int Ed Engl 2020 01 19;59(1):192-196. Epub 2019 Nov 19.

BrightSpec, Inc., Charlottesville, VA, 22903, USA.

We designed and demonstrated the unique abilities of the first gas chromatography-molecular rotational resonance spectrometer (GC-MRR). While broadly and routinely applicable, its capabilities can exceed those of high-resolution MS and NMR spectroscopy in terms of selectivity, resolution, and compound identification. A series of 24 isotopologues and isotopomers of five organic compounds are separated, identified, and quantified in a single run. Natural isotopic abundances of mixtures of compounds containing chlorine, bromine, and sulfur heteroatoms are easily determined. MRR detection provides the added high specificity for these selective gas-phase separations. GC-MRR is shown to be ideal for compound-specific isotope analysis (CSIA). Different bacterial cultures and groundwater were shown to have contrasting isotopic selectivities for common organic compounds. The ease of such GC-MRR measurements may initiate a new era in biosynthetic/degradation and geochemical isotopic compound studies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.201910507DOI Listing
January 2020

Enhancing the selectivity of polar hydrophilic analytes with a low concentration of barium ions in the mobile phase using geopolymers and silica supports.

Talanta 2020 Jan 7;207:120339. Epub 2019 Sep 7.

Department of Chemistry & Biochemistry, University of Texas at Arlington, Texas, 76019, USA. Electronic address:

Charged analytes such as organic sulfonic acids, sulfates, carboxylates, and phosphates are often analyzed by hydrophilic interaction liquid chromatography (HILIC). In many cases, these analytes do not show any selectivity and elute near the dead time using the conventional acetonitrile-ammonium acetate buffers. In this work, we introduce a powerful selectivity enhancing technique by using a trace amount of Ba ion in the mobile phase as a general approach for HILIC with UV-Vis detection. Silica and a newly developed material called geopolymers are used as stationary phases. Geopolymers are X-ray amorphous aluminosilicate inorganic polymers with cation exchange properties. Barium exchanged geopolymers (Ba-NM-GP) are synthesized from metakaolin based geopolymer. Thorough characterization of Ba-NM-GP is reported using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Brunauer-Emmett-Teller (BET) surface area analyzer and laser diffraction particle size distribution analyzer for the determination of their shape, size, porosity, surface area and particle size distribution respectively. It is demonstrated that in the absence of Ba, baseline separations of sulfonates, carboxylates, and phosphates is not possible, whereas, in the presence of Ba in the mobile phase, these analytes are easily separated. Barium perchlorate is suggested as an additive for it is UV transparent, and it has practically an unlimited solubility in acetonitrile.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.talanta.2019.120339DOI Listing
January 2020

Synthetic aluminosilicate based geopolymers - Second generation geopolymer HPLC stationary phases.

Anal Chim Acta 2019 Nov 9;1081:209-217. Epub 2019 Jul 9.

Department of Chemistry and Biochemistry, University of Arlington, Arlington, TX, 76019, United States. Electronic address:

A survey of existing stationary phases classified by the United States Pharmacopeia reveals that 120 groups of chromatographic supports mostly utilize silica-silane chemistry, polymeric materials along with some niche metal oxides. In this work, the synthesis and characterization of transition-metal free geopolymers as a new class of stationary phases for hydrophilic interaction liquid chromatography and normal phase separations is reported. The geopolymers were synthesized by reaction of synthetic aluminosilicate with potassium silicate (fumed silica dissolved in KOH) in a water-in-oil emulsion. For comparative purposes of peak shapes, a geopolymer from natural metakaolin was also synthesized. The geopolymers were examined by X-ray diffraction, energy dispersive spectroscopy, laser diffraction, and N-adsorption isotherms. This two-step approach gives spherical microparticles with surface area and pore size comparable to silica phases (150 m/g and 120 Å respectively). Both synthetic aluminosilicates based and natural metakaolin based geopolymers occupy a unique "spot" in the HILIC selectivity chart when compared to 35 HILIC phases. An additional promising feature of geopolymers is high pH and temperature stability which are used to tune selectivity for small polar analytes. High pH separations are shown with carboxylic acids. Geopolymers also show mixed mode behavior in retention with ion-exchange properties in purely aqueous mobile phases. The separation of derivatized sugars is demonstrated and compared with porous graphitic carbon (Hypercarb™) as another pH-stable stationary phase.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.aca.2019.07.017DOI Listing
November 2019

Enantiomeric impurities in chiral catalysts, auxiliaries, and synthons used in enantioselective syntheses. Part 5.

Chirality 2019 09 18;31(9):688-699. Epub 2019 Jul 18.

Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas.

The enantiomeric excess of chiral starting materials is one of the important factors determining the enantiopurity of products in asymmetric synthesis. Fifty-one commercially available chiral reagents used as building blocks, catalysts, and auxiliaries in various enantioselective syntheses were assayed for their enantiomeric purity. The test results were classified within five impurities level (ie, <0.01%, 0.01%-0.1%, 0.1%-1%, 1%-10%, >10%). Previously from 1998 to 2013, several reports have been published on the enantiomeric composition of more than 300 chiral reagents. This series of papers is necessitated by the fact that new reagents are forthcoming and that the enantiomeric purity of the same reagent can vary from batch to batch and/or from supplier to supplier. This report presents chiral liquid chromatography (LC) and gas chromatography (GC) methods to separate enantiomers of chiral compounds and evaluate their enantiomeric purities. The accurate and efficient LC analysis was done using newly introduced superficially porous particle (SPP 2.7 μm) based chiral stationary phases (TeicoShell, VancoShell, LarihcShell-P, and NicoShell).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/chir.23086DOI Listing
September 2019

Geopolymers as a New Class of High pH Stable Supports with Different Chromatographic Selectivity.

Anal Chem 2018 07 12;90(13):8139-8146. Epub 2018 Jun 12.

Department of Chemistry and Biochemistry , University of Texas at Arlington , Arlington , Texas 76019 , United States.

Geopolymers belong to an interesting class of X-ray amorphous polycondensed aluminosilicate ceramic solids. The high mechanical strength, chemical stability in basic conditions, and water insolubility make geopolymers a unique solid support in separation science. This work describes a new straightforward synthetic procedure for making spherical porous geopolymer particles with high surface area which are amenable for chromatographic purposes. In-depth physicochemical evaluation of geopolymers is conducted via particle size distribution, porosity measurements, X-ray diffraction, pH titration, and energy-dispersive spectroscopy and compared with silica, titania, and zirconia. Chromatographic selectivity shows that the surface chemistry of geopolymers has strong hydrophilic and electrostatic character, which makes it different from 36 chromatographic columns. Hydrophilic interaction liquid chromatography in columns packed with geopolymer particles shows different selectivity than that in silica columns, with excellent peak shapes. Phosphate or fluoride additives are not required as they are for zirconia or titania phase.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.analchem.8b01426DOI Listing
July 2018