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Quantifying the greenness of chromatographic separations using CO2 in the mobile phase.

Life cycle analysis allows comparison of RP-HPLC to SFC.

The understanding of the desorption mechanism in surface-assisted laser desorption/ionization (SALDI) remains incomplete because there are numerous types of SALDI materials with a broad range of physical and chemical properties, many of which impact the ultimate analytical performance in terms of signal generation. In this study, the chemical thermometer molecule, benzylpyridinium chloride, is applied to investigate the desorption process of SALDI using electrospun nanofibrous polymer and polymer composite substrates. The ion desorption efficiency was inversely related to the ion internal energy, which could not be fully explained by a thermal desorption mechanism. A competing non-thermal desorption (i.e., phase transition/explosion) was proposed to be involved in this SALDI process. The influence of the orientation and dimension of the nanofiber structure revealed that a cross-linked nanofiber network with a small diameter favored the nanofiber-assisted LDI to provide efficient ion desorption. Graphical abstract.

The first discussions on the use of inorganic gases above their critical point as mobile phase in chromatography systems stems back to 19571 and it was Klesper et al who reported on a preliminary study in which chlorofluorocarbons were used above their critical point to separate metal phophorins.2 Supercritical fluid chromatography (SFC) with carbon dioxide as the mobile phase emerged in the late 1960s. In the past decade, a new generation of SFC instruments are commercially available and also the number of applications of SFC have grown considerably. Here, Susan Olesik, Caroline West, Davy Guillarme, Debby Mangelings, and Lucie Nováková share their thoughts on the technology and discuss the challenges and potential of SFC.

Recent advances in the analysis of proteins have increased the demand for more efficient techniques to separate intact proteins. Enhanced-fluidity liquid chromatography (EFLC) involves the addition of liquefied CO2 to conventional liquid mobile phases. The addition of liquefied CO2 increases diffusivity and decreases viscosity, which inherently leads to a more efficient separation. Herein, EFLC is applied to hydrophobic interaction chromatography (HIC) stationary phases for the first time to study the impact of liquefied CO2 to the chromatographic behavior of proteins. The effects of liquefied CO2 on chromatographic properties, charge state distributions (CSDs), and ionization efficiencies were evaluated. EFLC offered improved chromatographic performance compared to conventional liquid chromatography (LC) methods including a shorter analysis time, better peak shapes, and higher plate numbers. The addition of liquefied CO2 to the mobile phase provided an electrospray ionization (ESI)-friendly and "supercharging" reagent without sacrificing chromatographic performance, which can be used to improve peptide and protein identification in large-scale application.

Gold nanoparticles (AuNPs) are of great interest in many fields, especially in biomedical applications. Thiol terminated polyethylene glycol (PEG) is the most widely used polymer to increase the biocompatibility of nanoparticle therapeutics. Herein, a rapid method for separation and characterization of PEGylated AuNPs on an ultrathin layer chromatographic (UTLC) plate using electrospun polyacrylonitrile (PAN) nanofibers as the stationary phase is described. AuNPs with sizes ranging from 10 to 80 and 30 nm AuNPs coated with various molecular weight of PEG (2, 5, 10, and 20 kDa) were all successfully separated by UTLC using optimized conditions. The fabrication of electrospun UTLC is simple, fast, and inexpensive. The UTLC, with much thinner sorbent layer (10× thinner than traditional TLC) and small fiber size (∼300 nm), requires minimal mobile phase solvent and provides faster separation and higher resolution compared to other separation methods for AuNPs. AuNPs with different sizes and different PEG molecular weights were well separated within 5 min with lowest plate height2 μm and resolution value1.5. As an example of this method, the size transformation of AuNPs in serum protein was determined quantitatively.

The value of exploring selectivity and solvent strength ternary gradients in enhanced fluidity liquid chromatography (EFLC) is demonstrated for the separation of inulin-type fructans from chicory. Commercial binary pump systems for supercritical fluid chromatography only allow for the implementation of ternary solvent strength gradients which can be restrictive for the separation of polar polymeric analytes. In this work, a custom system was designed to extend the capability of EFLC to allow tuning of selectivity or solvent strength in ternary gradients. Gradient profiles were evaluated using the Berridge function (RF1), normalized resolution product (NRP), and gradient peak capacity (Pc). Selectivity gradients provided the separation of more analytes over time. The RF1 function showed favor to selectivity gradients with comparable Pc to that of solvent strength gradients. NRP did not strongly correlate with Pc or RF1 score. EFLC with the hydrophilic interaction chromatography, HILIC, separation mode was successfully employed to separate up to 47 fructan analytes in less than 25 min using a selectivity gradient.

Enhanced-fluidity liquid chromatography (EFLC) methods using methanol/H2O/CO2 and hydrophilic interaction liquid chromatography (HILIC) were explored for the separation of proteins and peptides. EFLC is a separation mode that uses a mobile phase made of conventional solvents combined with liquid carbon dioxide (CO2) in subcritical conditions. The addition of liquid CO2 enhances diffusivity and decreases viscosity while maintaining mixture polarity, which typically results in reduced time of analysis. TFA additive and elevated temperature were leveraged as key factors in the separation of a 13-analyte intact protein mixture in under 5min. Under these conditions EFLC showed modest improvement in terms of peak asymmetry and analysis time over the competing ACN/H2O separation. Protein analytes detected by electrospray ionization - quadrupole time of flight, were shown to be unaffected by the addition of CO2 in the mobile phase. Herein, the feasibility of separating hydrophilic proteins up to 80kDa (with transferrin) is demonstrated for CO2-containing mobile phases.

A "green" hydrophilic interaction liquid chromatography (HILIC) technique for separating the components of mixtures with a broad range of polarities is illustrated using enhanced-fluidity liquid mobile phases. Enhanced-fluidity liquid chromatography (EFLC) involves the addition of liquid CO2 to conventional liquid mobile phases. Decreased mobile phase viscosity and increased analyte diffusivity results when a liquefied gas is dissolved in common liquid mobile phases. The impact of CO2 addition to a methanol:water (MeOH:H2O) mobile phase was studied to optimize HILIC gradient conditions. For the first time a fast separation of 16 ribonucleic acid (RNA) nucleosides/nucleotides was achieved (16min) with greater than 1.3 resolution for all analyte pairs. By using a gradient, the analysis time was reduced by over 100% compared to similar separations conducted under isocratic conditions. The optimal separation using MeOH:H2O:CO2 mobile phases was compared to MeOH:H2O and acetonitrile:water (ACN:H2O) mobile phases. Based on chromatographic performance parameters (efficiency, resolution and speed of analysis) and an assessment of the environmental impact of the mobile phase mixtures, MeOH:H2O:CO2 mixtures are preferred over ACN:H2O or MeOH:H2O mobile phases for the separation of mixtures of RNA nucleosides and nucleotides.

A matrix-free laser desorption/ionization mass spectrometry (LDI-MS) approach was developed using electrospun composite nanofibers with controllable size, morphology and composition. Composite polyvinylpyrrolidone (PVP) nanofibers with added nanoparticles to enhance the absorption of laser energy absorber were studied. Fractal dimensional analysis was conducted to evaluate the electrospinning process for the first time, which provided useful information on the repeatability of nanofiber production. The performance of the nanofiber-assisted laser desorption/ionization method was investigated through characterization of small drug molecules and synthetic oligomers. Homogeneous sample distribution was achieved by eliminating the “sweet spot”, resulting in good reproducibility. Mass spectra features clean background, that is especially beneficial for interpretation of small molecules. High sensitivity for small drug molecules and synthetic oligomers was obtained with limits of detection limits down to the low attomolar range. The combination of composite nanofibers with LDI-MS is a versatile and sensitive approach for detection and characterization of analytes with broad range of molecular weight.

The potential of enhanced-fluidity liquid chromatography, a subcritical chromatography technique, in mixed-mode hydrophilic interaction/strong cation-exchange separations is explored, using amino acids as analytes. The enhanced-fluidity liquid mobile phases were prepared by adding liquefied CO2 to methanol/water mixtures, which increases the diffusivity and decreases the viscosity of the mixture. The addition of CO2 to methanol/water mixtures resulted in increased retention of the more polar amino acids. The “optimized” chromatographic performance (achieving baseline resolution of all amino acids in the shortest amount of time) of these methanol/water/CO2 mixtures was compared to traditional acetonitrile/water and methanol/water liquid chromatography mobile phases. Methanol/water/CO2 mixtures offered higher efficiencies and resolution of the ten amino acids relative to the methanol/water mobile phase, and decreased the required isocratic separation time by a factor of two relative to the acetonitrile/water mobile phase. Large differences in selectivity were also observed between the enhanced-fluidity and traditional liquid mobile phases. A retention mechanism study was completed, that revealed the enhanced-fluidity mobile phase separation was governed by a mixed-mode retention mechanism of hydrophilic interaction/strong cation-exchange. On the other hand, separations with acetonitrile/water and methanol/water mobile phases were strongly governed by only one retention mechanism, either hydrophilic interaction or strong cation exchange, respectively.

A self-polymerizable octatetrayne, 1,8-dialdehydebenzyl-1,3,5,7-octatetrayne, is synthesized and covalently attached to an amino-functionalized surface of silica particles. The silica particles with a monolayer coverage of octatetrayne were then thermally processed to various final temperatures of 200, 400 and 700°C. The amino-functionalization, covalent attachment of octatetrayne and thermal process of silica particles were monitored by scanning electron microscopy (SEM), infrared (IR) spectroscopy and thermogravimetric analysis (TGA). The thermally processed particles were then packed into a capillary column and evaluated as a stationary phase for HPLC. After chromatographic evaluation, the optimized temperature for thermal processing was determined to be 400°C, which provides the best modified silica particles SiO2-OCT-T400 with an ultra-thin glassy carbon film coating. The linear solvation energy relationship model indicated that the primary contributors in retention are dispersion and H-bond basicity. The application of SiO2-OCT-T400 as a stationary phase was further demonstrated by successful separation of nonpolar hydrocarbons mixture and a nucleosides mixture.

Polyacrylonitrile/Nafion®/carbon nanotube (PAN/Nafion®/CNT) composite nanofibers were prepared using electrospinning. These electrospun nanofibers were studied as possible substrates for surface-assisted laser desorption/ionization (SALDI) and matrix-enhanced surface-assisted laser desorption/ionization time-of-flight mass spectrometry (ME-SALDI/TOF-MS) for the first time in this paper. Electrospinning provides this novel substrate with a uniform morphology and a narrow size distribution, where CNTs were evenly and firmly immobilized on polymeric nanofibers. The results show that PAN/Nafion®/CNT nanofibrous mats are good substrates for the analysis of both small drug molecules and high molecular weight polymers with high sensitivity. Markedly improved reproducibility was observed relative to MALDI. Due to the composite formation between the polymers and the CNTs, no contamination of the carbon nanotubes to the mass spectrometer was observed. Furthermore, electrospun nanofibers used as SALDI substrates greatly extended the duration of ion signals of target analytes compared to the MALDI matrix. The proposed SALDI approach was successfully used to quantify small drug molecules with no interference in the low mass range. The results show that verapamil could be detected with a surface concentration of 220 femtomoles, indicating the high detection sensitivity of this method. Analysis of peptides and proteins with the electrospun composite substrate using matrix assisted-SALDI was improved and a low limit of detection of approximately 6 femtomoles was obtained for IgG. Both SALDI and ME-SALDI analyses displayed high reproducibility with %RSD ≤ 9% for small drug molecules and %RSD ≤ 14% for synthetic polymers and proteins.

The application of carbon nanotube or nanorod/polyacrylonitrile (PAN) composite electrospun nanofibrous stationary phase for ultrathin layer chromatography (UTLC) is described herein. Multi-walled carbon nanotubes (MWCNTs) and edge-plane carbon (EPC) nanorods were prepared and electrospun with the PAN polymer solution to form composite nanofibers for use as a UTLC stationary phase. The analysis of laser dyes demonstrated the feasibility of utilizing carbon nanoparticle-filled electrospun nanofibers as a UTLC stationary phase. The contribution of MWCNT or EPC in changing selectivity of the stationary phase was studied by comparing the chromatographic behavior among MWCNT–PAN plates, EPC–PAN plates and pure PAN plates. Carbon nanoparticles in the stationary phase were able to establish strong π–π interactions with aromatic analytes. The separation of five polycyclic aromatic hydrocarbons (PAHs) demonstrated enhanced chromatographic performance of MWCNT-filled stationary phase by displaying substantially improved resolution and separation efficiency. Band broadening of the spots for MWCNT or EPC-filled UTLC stationary phases was also investigated and compared with that for pure PAN stationary phases. A 50% improvement in band dispersion was noted using the MWCNT based composite nanofibrous UTLC plates.

Electrospun polymeric nanofibers (polyacrylonitrile, poly(vinyl alcohol), and SU-8 photoresist) and carbon nanofibers pyrolyzed to final temperatures of 600, 800, and 900 °C were used as substrates for surface-assisted laser desorption/ionization (SALDI) and matrix-enhanced surface-assisted laser desorption/ionization (ME-SALDI) analyses. Sample preparation of polymeric analytes using the electrospun target plate for SALDI analysis is simple and fast. Signal enhancements for poly(ethylene glycol) were noted with nanofibrous carbon substrates compared to those obtained with commercially available stainless steel plates when no organic matrix is used. Minimal fragmentation was observed. Poly(ethylene glycol) with a molecular weight as high as 900 000 Da was successfully detected using the carbon nanofibrous substrate processed to 800 °C, which is the highest molecular weight that has been studied by SALDI. Small molecules were detected using nanofibrous carbon substrate processed to 800 °C. For example, spectra of glucose, arginine, and crystal violet were obtained with no observed interferences in the low molecular weight range. The SALDI results show enhanced shot-to-shot reproducibility compared to matrix-assisted laser desorption/ionization (MALDI). High-quality polystyrene spectra were obtained for the first time using SALDI nanofibrous polyacrylonitrile substrates. Significantly enhanced signal-to-noise ratios were obtained using ME-SALDI compared to conventional MALDI or SALDI for the studied analytes. A detection limit of 400 amol was achieved for angiotensin I using the nanofibrous carbon ME-SALDI substrate.

An ion-exchange method to separate charged biomolecules on ultrathin layer chromatographic (UTLC) plates using electrospun Nafion-Polyacrylonitrile (PAN) nanofibers as the stationary phase is described. Sulfonate groups on Nafion provide the ion-exchange sites. The addition of PAN (a higher molecular weight polymer than Nafion) was used to facilitate the nanofiber formation process using electrospinning. Electrospinning parameters and separation conditions were optimized using fractional factorial design and response surface methodology. Nafion-PAN nanofibers containing 45% (w/w) Nafion with 0.407 mmol/g of SO3H group and 16.0 mmol/g of fluorine as an ion exchange stationary phase for UTLC were evaluated using the separations of amino acids and proteins, followed by visualizations using ninhydrin and fluorescamine, respectively. The electrospun Nafion-PAN plates showed high chemical stability under various mobile phase conditions. Mobile phase velocity decreased with the addition of Nafion into the electrospinning solutions. The sources of band broadening of analyte spots were investigated. The separation of amino acids showed high selectivity and separation efficiency. The separation of four proteins demonstrated the feasibility of Nafion-PAN UTLC for separating large biomolecules.

Enhanced fluidity liquid chromatography using the hydrophilic interaction retention mechanism (EFLC-HILIC) is studied as an alternative separation mode for analyzing oligosaccharides and other sugars. These carbohydrates, which are important for the study of foods and biological systems, are difficult to comprehensively profile and either require a non-green, expensive solvent (i.e. acetonitrile) or derivatization of the analytes at the expense of time, sample loss, and loss of quantitative information. These difficulties arise from the diverse isomerism, mutarotation, and lack of a useable chromophore/fluorophore for spectroscopic detection. Enhanced fluidity liquid chromatography is an alternative separation method that involves the use of conventional polar solvents, such as methanol/water mixtures, as the primary mobile phase component and liquid carbon dioxide (CO2) as the modifier in subcritical conditions. The addition of liquid CO2 enhances diffusivity and decreases viscosity while maintaining mixture polarity, which typically results in reduced time of analysis and higher efficiency. This work illustrates an optimized EFLC-HILIC separation of a test mixture of oligosaccharides and simple sugars with a resolution greater than 1.3 and an analysis time decrease of over 35% compared to a conventional HPLC HILIC-mode analysis using acetonitrile/water mobile phases.

In this work, the use of patterned proteins and peptides for the deposition of gold nanoparticles on several substrates with different surface chemistries is presented. The patterned biomolecule on the surface acts as a catalyst to precipitate gold nanoparticles from a precursor solution of HAuCl4 onto the substrate. The peptide patterning on the surfaces was accomplished by physical adsorption or covalent attachment. It was shown that by using covalent attachment with a linker molecule, the influence of the surface properties from the different substrates on the biomolecule adsorption and subsequent nanoparticle deposition could be avoided. By adjusting the reaction conditions such as pH or HAuCl4 concentration, the sizes and morphologies of deposited gold nanoparticle agglomerates could be controlled. Two biomolecules were used for this experiment, 3XFLAG peptide and bovine serum albumin (BSA). A micro-transfer molding technique was used to pattern the peptides on the substrates, in which a pre-patterned poly(dimethylsiloxane) (PDMS) mold was used to deposit a lift-off pattern of polypropylmethacrylate (PPMA) on the various substrates. The proteins were either physically adsorbed or covalently attached to the substrates, and an aqueous HAuCl4 solution was applied on the substrates with the protein micropatterns, causing the precipitation of gold nanoparticles onto the patterns. SEM, AFM, and Electron Beam Induced Current (EBIC) were used for characterization.

The application of enhanced fluidity liquid (EFL) mobile phases to improving isocratic chromatographic separation of nucleosides in hydrophilic interaction liquid chromatography (HILIC) mode is described. The EFL mobile phase was created by adding carbon dioxide to a methanol/buffer solution. Previous work has shown that EFL mobile phases typically increase the efficiency and the speed of the separation. Herein, an increase in resolution with the addition of carbon dioxide is also observed. This increase in resolution was achieved through increased selectivity and retention with minimal change in separation efficiency. The addition of CO₂ to the mobile phase effectively decreases its polarity, thereby promoting retention in HILIC. Conventional organic solvents of similar nonpolar nature cannot be used to achieve similar results because they are not miscible with methanol and water. The separation of nucleosides with methanol/aqueous buffer/CO₂ mobile phases was also compared to that using acetonitrile/buffer mobile phases. A marked decrease in the necessary separation time was noted for methanol/aqueous buffer/CO₂ mobile phases compared to acetonitrile/buffer mobile phases. There was also an unusual reversal in the elution order of uridine and adenosine when CO₂ was included in the mobile phase.

A method of producing solid-phase microextraction (SPME) fibers based on electrospinning polymers into nanofibrous mats is demonstrated. Using this method the polymer mat is attached to a stainless steel wire without the need of a binder. While applicable to any polymer that can be electrospun, a polymeric negative photoresist, SU-8 2100, is used for this initial study. SPME devices composed of carbon nanofibers are also illustrated by pyrolyzing SU-8 to produce amorphous carbon. Nonpolar compounds, benzene, toluene, ethylbenzene, and o-xylene (BTEX) and polar compounds, phenol, 4-chlorophenol and 4-nitrophenol are extracted under headspace SPME conditions. Extraction efficiencies are compared to commercial polydimethylsiloxane (PDMS), polydimethylsiloxane/divinylbenzene (PDMS/DVB), and polyacrylate (PA) fibers. For both the nonpolar and polar compounds, the carbon nanofiber based phases demonstrated enhanced or comparable (o-xylene only) extraction efficiencies. Distribution constants, K, for benzene on the electrospun fibers are of greater or similar magnitude to those of the compared commercial fibers and increase with carbonization temperature. Finally, the measured detection limits for all the organic compounds are similar to those measured with other SPME gas chromatography-flame ionization detector (GC-FID) methods with a large linear dynamic range (3 orders of magnitude) for quantification.

The optimization of electrospinning SU-8 2100 negative photoresist was performed to create carbon micro/nanofibers and beads that can be patterned after electrospinning by using UV radiation. The fiber diameters had a range of 300 nm to 1 μm, based upon the selected electrospinning parameters. Low concentrations of the SU-8 2100 resulted in beads while specific higher concentrations produced well-defined fibers. Fibers and beads were converted to carbon through pyrolysis and retained their three dimensional structure. By utilizing the photosensitive properties of the SU-8 negative photoresist, the electrospun fibers were patterned by UV photolithography. The fibers and beads were characterized by SEM and Raman microscopy, respectively. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

A technique for creating devices for ultrathin layer chromatography (UTLC) using an electrospinning method is described. The devices use a nanofibrous stationary phase with fiber diameters that are 400 nm. Separations of mixtures of laser dyes and mixtures of steroidal compounds were performed to illustrate the capabilities of these new UTLC media. The complete analyses were found to require very little development time and require less solvent than typical TLC methods. The efficiency of the separations was substantially improved compared to that determined using commercial phases. The retention properties and efficiency of the technique are discussed as are the effects of mat thickness and mobile phase composition on the chromatographic properties of the devices.

The retention of solutes on two fluorinated low temperature glassy carbon (F-LTGC) stationary phases under reversed-phase liquid chromatographic conditions was studied by employing the solvation parameter model. The two fluorinated glassy carbon stationary phases were produced by slowly heating zirconia particles that were encapsulated with oligo[1,3-dibutadiyne-1,3-(tetrafluoro)phenylene] precursor polymer to two different final temperatures (200 and 400 degrees C). The resulting carbon particles had different amounts of fluorine after thermal processing. The solvation parameter models indicated that different intermolecular interactions are important in describing retention on the two stationary phases. The interactions that are important for describing retention on the 200 degrees C processed F-LTGC stationary phase are hydrogen bond basicity> or =dispersion>hydrogen bond acidity>dipolarity/polarizability. The interactions that describe the retention on the 400 degrees C processed F-LTGC are hydrogen bond basicity>dispersion>excess molar refraction> or =hydrogen bond acidity. The solvation parameter model for the 200 degrees C processed F-LTGC showed similar trends in the relative importance of intermolecular interactions as previously found for octadecyl-polysiloxane stationary phases, while the 400 degrees C processed F-LTGC had similar intermolecular interactions with solutes as found with porous glassy carbon in that pi-pi interactions with the carbon surface contribute more so to the retention.

Carbon stationary phases have been widely used in HPLC due to their unique selectivity and high stability. Amorphous carbon as a stationary phase has at least two sites of interaction with analytes: basal-plane and edge-plane carbon sites. The polarity and adsorptivity of the two sites are different. In this work, the edge-plane carbon stationary phase is prepared by surface-directed liquid crystal assembly. Specific precursor polymers form discotic liquid crystal phases during the pyrolysis process. By using silica as the substrate to align the discotic liquid crystal, edge-plane carbon surfaces were formed. Similar efficiencies as observed for Hypercarb were observed in chromatograms. The column efficiency was studied as a function of linear flow rate. A minimum reduced plate height of 6 was observed in these studies. To evaluate the performance of the homogeneous edge-plane carbon stationary phase, linear solvation energy relationships were used to compare these ordered carbon surfaces to commercially available carbon stationary phases, including Hypercarb. Reversed-phase separations of nucleosides, nucleotides, and amino acids and derivatives were demonstrated using the ordered carbon surfaces, respectively. The column batch-to-batch reproducibility was also evaluated. The retention times for the analytes were reproducible within 1-6% depending on the analyte.

A self-polymerizable liquid octatetrayne, 1,8-dibutyl-1,3,5,7-octatetrayne, was heated to 80 °C and produced cross-linked poly(1,8-dibutyl-1,3,5,7-octatetrayne) microbeads via suspension polymerization with diameters adjustable from hundreds of micrometers to a few micrometers. Carbon microbeads that may be useful for catalyst supports or in high-performance carbon materials were obtained by the pyrolysis of poly(1,8-dibutyl- 1,3,5,7-octatetrayne) microbeads at 800 °C under an inert atmosphere.

The amphiphilic and self-polymerizable monomer 1,8-dihydroxymethyl-1,3,5,7-octatetrayne (DHMOTY) was prepared and dispersed in water by itself or with the help of phenyl trimethylamine hydroxide. Accordingly, cross-linked poly(1,8-dihydroxymethyl-1,3,5,7-octatetrayne) (polyDHMOTY) nanospheres were produced due to spontaneous polymerization of octatetrayne structures. XPS shows the presence of a large number of hydroxyl groups on the surface of polyDHMOTY nanospheres. Carbon nanospheres were generated by pyrolysis of cross-linked polyDHMOTY nanospheres at high temperatures in inert atmosphere in the yield of 60%.

The dispersion polymerization of styrene in carbon dioxide with a series of copolymers of poly(propylene glycol) methacrylate (PPGMA) and 2-(perfluorooctyl)ethyl methacrylate (FOEMA) as the polymerization dispersants was examined. It was demonstrated that PPGMA and FOEMA copolymers and polymers containing 52-100% FOEMA could be used as effective dispersants for the polymerization, and the composition of the copolymeric dispersant had a dramatic effect on both the polymerization yield and the morphology of the resulting polystyrene. The effects of the concentrations of the copolymeric dispersants, the concentrations of the monomer, and the reaction pressure were also investigated.

Liquid chromatography at the critical condition (LCCC) is a chromatographic technique that allows for the isolation of one area of the polymer matrix so that other areas of the polymer may be probed with size-exclusion or adsorptive chromatographic modes. This technique has been successfully applied to the analysis of functionality distributions in functionalized oligomers and to polymer distributions within copolymers. Herein, the critical conditions of two polar polymers, poly(acrylic acid) and polystyrene sulfonate, are determined. These conditions were identified by varying buffer concentration, organic modifier within the mobile phase, or both. At the critical condition of poly(acrylic acid), the retention characteristics of a copolymer of acrylic acid and vinyl pyrrolidinone were determined. This extension to water-based mobile phase conditions will substantially broaden the possible applications of LCCC.

Structural information on humic acids is difficult to obtain because of the heterogeneity of the acids. Herein liquid chromatography at the critical condition, LCCC, is used to provide a sorting mechanism for the diverse types of molecules contained in humic acids. The critical condition of polymers that are believed to model some subunit of the humic acid is determined. Humic acids from three different terrestrial sources (soil, compost, and peat) are then separated under these chromatographic conditions. The portion of the humic acid that has structure similar to that of the model polymer elutes at the retention volume of the critical condition of the model. Next, fractions are collected and further characterized. This detailed characterization includes high-efficiency size-exclusion chromatography and electrospray mass spectrometry. The size-exclusion chromatograms of the fractions were found to be markedly different from that of the original humic acid sample. This is strong evidence that the LCCC separation mechanism is different from size fractionation. The mass spectra of the humic acid fractions were also markedly different from those of the bulk humic acids previously reported. The mass spectra of specific fractions collected had repeating clusters of m/z values, which is more evidence that the critical condition separation is a powerful sort function.

A series of random copolymers, poly(perfluorooctylethylene methacrylate)−poly(propylene glycol) methacrylate, were synthesized as dispersants for dispersion polymerizations of methyl methacrylate in CO2. The results show that the composition of copolymeric dispersants has a dramatic effect on the polymerization yield and on the morphology of the resulting poly(methyl methacrylate) particles. The effect of varying the dispersant concentration, monomer concentration, and the initial medium pressure on the resultant polymerization was also investigated.

With increasing interest in the detection of disease-related volatile organic compounds (VOCs) found in human breath, breath analysis could prove to be a very useful diagnostic tool, especially for the early detection of lung cancer. Solid-phase microextraction (SPME) is a technique well suited for breath analysis and has been applied to studying VOCs in the nanomolar concentration range. However, many compounds of interest in human breath are excreted at picomolar concentrations and may be unsuitable for analysis using conventional SPME sorbent phases. To extend the concentration range of conventional SPME, a novel 4-cm-long, low-temperature glassy carbon (LTGC) macrofiber was developed. The LTGC SPME macrofibers were used to extract five lung cancer-related VOCs (2-methylheptane, styrene, propylbenzene, decane, undecane) at conditions simulating human breath, and they were analyzed via gas chromatography/mass spectrometry. Results show that detection limits are lower using the SPME macrofibers compared to a conventional SPME fiber, in the low- to sub-picomolar range for the compounds of interest, which should be adequate for the analysis of these compounds in human breath. Also, the LTGC SPME macrofibers demonstrate significantly greater extraction efficiencies, sensitivity, and peak identification accuracy compared to that of commercial PDMS/DVB fibers without excessive chromatographic peak broadening. The use of SPME macrofibers broadens the potential range of application of SPME where the rapid extraction of very low levels of volatile compounds is required.

High fluidity solvents, such as supercritical fluids, have several advantages over traditional solvents as polymerization media, such as offering a more environmentally-friendly reaction media, providing increased reaction rates, and simplifying the separation and purification of polymers. In this study, a traditional glass-ionomer polymer, poly(acrylic acid-co-itaconic acid) (PAA/IA) was synthesized by using mixtures of CO2 and methanol as the reaction solvent and was characterized by 1H-NMR, FT-IR, GPC, and viscometry. The mechanical and working properties of the glass-ionomer cements, prepared by mixing aqueous solutions of the polymers with Fuji II glass powder, were evaluated for compressive strength (CS), diametral tensile strength (DTS) and flexural strength (FS), as well as setting time and working time. The results showed that the polymerization reaction in CO2/methanol mixtures was faster and had higher conversion than the polymerization reaction in water. The glass-ionomer formulations...

The improvement in the analysis of telechelic polymer matrixes continues to be a pursuit for many scientists of varying disciplines. This quest for a new technique has led to the continued development of liquid chromatography at the critical condition (LCCC) or liquid chromatography at the critical adsorption point (LC-CAP). LCCC allows for the isolation of one area of the polymer matrix so that other areas of the polymer can be probed with size-exclusion or adsorptive chromatographic modes. Although this technique has been successfully applied to the analysis of telechelic polymers, the practice of LCCC can be difficult. These difficulties include finding and maintaining a solvent system appropriate for the practice of LCCC as well as deterioration of peak shape once the system is operating at the LCCC mode. Because of the specificity of the mobile phase required for the practice of LCCC, the work is routinely practiced by premixing solvents. Previous work with enhanced-fluidity liquid mobile phases demonstrated that these mobile phases removed many of the aforementioned challenges associated with working at the LCCC mode. These mobile phases utilize both pressure and temperature variation in order to maintain the specific solvent strength necessary for the LCCC work. This work studies the coupling and optimization of enhanced-fluidity, EF, liquid mobile phases for LCCC. Several EF-LCCC systems, differing in mobile phase composition, temperature, and pressure, were routinely established, resulting in the effective practice of critical chromatography. The practice of LCCC with on-line mobile phase preparation is demonstrated using commercially available instrumentation. Finally, EF-LCCC is used to analyze triblock and diblock copolymers.

Electrospun polymer nanofiber stationary phases were examined for their application to planar electrochromatography (PEC). Separations were performed on polyacrylonitrile nanofiber ultra-thin-layer chromatography (UTLC) plates in 1-2 min using a ternary mobile phase. The influences of buffer concentration and pH, ratio of organic modifier, and development time on analyte migration distances were studied. Band broadening in this system was studied as a function of distance. The plate height initially decreased and then plateaued with a minimum plate height value as low as 11 μm. Nanofiber alignment considerably increased analyte migration rate, but larger spot sizes were noted when nearly complete fiber alignment was used. The easily tunable stationary phase thickness can be tailored to a given separation, where thinner layers promote faster separations and thicker layers are ideal for more complex mixtures. Compared to UTLC, PEC offers unique selectivity and decreased analysis time (4 times faster over 15 mm). Results for a two-dimensional separation using UTLC and PEC are also reported. These rapid separations required 11 min using a 40 × 40 mm plate and exhibited a significant increase in separation number (70-77).

Small diameter nanofibers of silica and silica/polymer are produced by electrospinning silica/polyvinylpyrrolidone (SiO2/PVP) mixtures composed of silica nanoparticles dispersed in polyvinylpyrrolidone solutions. By controlling various parameters, 380 ± 100 nm diameter composite nanofibers were obtained with a high silica concentration (57.14%). When the polymer concentration was low, “beads-on-a-string” morphology resulted. Nanofiber morphology was affected by applied voltage and relative humidity. Tip-to-collector distance did not affect the nanofiber diameter or morphology, but it did affect the area and thickness of the mat. Heat treatment of the composite nanofibers at 200°C crosslinked the polymer yielding solvent-resistant composite nanofibers, while heating at 465°C calcined and selectively removed the polymer from the composite. Crosslinking did not change the nanofiber diameter, while calcined nanofibers decreased in diameter (300 ± 90 nm) and increased in surface area to volume ratio. Nanofibers were characterized by scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40966.

The separations of five groups of ionogenic disubstituted benzene isomers were achieved by taking advantage of the pH variation caused by proportions of CO 2 in the methanol–water–CO 2 mobile phases without the addition of an aqueous buffer. The retention behavior of acidic, basic, and zwitterionic isomers was predominantly affected by the dissociation of these analytes within methanol–water–CO 2 mobile phases of different pH. A comparison with previously published reversed-phase high-performance liquid chromatography (HPLC) of these isomers revealed that resolution improvement for anisic acid, toluic acid and aminobenzoic acid isomers was achieved with methanol–water–CO 2 enhanced-fluidity liquids. Significant reductions in the retention factors were also obtained. Meanwhile, the detrimental effect of the presence of an aqueous buffer on the stationary phase was avoided which made fast repetitive analyses possible.

The polymerization products of C18 fatty acids are complex mixtures of high dimeric, trimeric and higher molecular weight liquid acids. The current separation method for these mixtures involves a lengthy normal-phase HPLC gradient and the use of primary standards that are prepared via a semi-preparative HPLC. In this study, enhanced-fluidity liquid mobile phases, methanol/CO 2 and methanol/CHF 3 , were studied as a possible alternative methodology for the separation of dimer and trimer acids. This study demonstrated that enhanced-fluidity liquid chromatography provides several advantages over normal-phase HPLC with conventional solvents. These advantages included faster speed of analysis, lower consumption of organic solvents, and much improved sample throughput. The best separation of dimer acids was achieved on a silica column using a steep methanol concentration gradient. Methanol/CO 2 and methanol/CHF 3 mixtures showed comparable elution strength using this steep gradient on the silica stationary phase. However, these polar compounds showed a normal-phase type retention on five reversed-phase columns which indicated an unusual retention mechanism.

Low-temperature glassy carbon (LTGC) films were investigated as a sorbent coating for solid-phase microextraction because of its uniquely selective adsorptive characteristics. The selectivity of these coatings is primarily controlled by shape characteristics of the solute molecule and the final processing temperature used to form the LTGC, demonstrating unique adsorptive characteristics compared to commercial phases. The LTGC films were prepared by first coating porous silica particles with a diethylnyl oligomer precursor and then heat curing at temperatures between 300 and 1000 degrees C to form the LTGC. Then, using a sol-gel process, the LTGC-coated silica particles were immobilized onto stainless steel fibers and subsequently used for headspace and liquid extractions followed by GC-FID analysis. The selectivity of the LTGC is demonstrated by the extraction of a variety of aromatic hydrocarbons as well as the taste and odor contaminants geosmin, 2-methylisoborneol, and 2,4,6-trichloroanisole commonly found in water supplies. The data show that the LTGC coating has the highest affinity for molecules with the greatest cross-sectional surface area and polarizability and that this selective mechanism increases as a function of LTGC processing temperature.

Comprehensive phase diagrams of methanol/CHF3 and methanol/H2O/CHF3 mixtures over the temperature range of 25–100°C and pressure range of atmospheric to 340 atm are reported. Fluoroform is expected to be useful as an alternative to CO2 for enhancing the fluidity of liquid mixtures due to its high polarity and low viscosity. Therefore, these mixtures will be studied as mobile phases for enhanced-fluidity liquid chromatography and extraction. The phase behavior of methanol/CHF3 and methanol/H2O/CHF3 was compared to that of methanol/CO2 and methanol/H2O/CHF3. Fluoroform is markedly more miscible with methanol or methanol/H2O than is CO2. Data for methanol/CHF3 binary mixtures were also modeled by the Peng–Robinson equation of state. The correlation results showed that the PR equation of state with two temperature-independent binary parameters was capable of representing the experimental data over the entire temperature range with an average relative deviation within 6%.

Microscale chromatography has found numerous applications in liquid chromatography. The combination of enhanced-fluidity liquid mobile phases with packed-capillary LC is evaluated for polymer characterization using size-exclusion chromatography (SEC) and liquid chromatography at the critical condition (LCCC) phase. Separations of polystyrene polymers and copolymers are completed using liquid chromatography at the critical condition. The critical conditions of polystyrene polymers were approached by changing the concentration of CO(2) in the mixture combined with temperature and pressure variation. Because the solvent strength of enhanced-fluidity liquid mixtures is affected by temperature and pressure variation, the solvent strength could be fine-tuned to accurately find the critical condition. Long packed capillaries could be used in this application because the enhanced-fluidity mobile phases have low viscosities. High efficiencies resulted. The performance of packed-capillary and analytical-scale analytical columns containing the same packing material was compared for a challenging separation at the critical condition.

Supercritical and liquid mixtures of ethanol/H2O/CO2 are increasingly used to extract solutes from solid or semi-solid matrixes when nontoxic solvents and fast extraction kinetics are desired. Accordingly, to better understand the mass transport capabilities of these mixtures, the diffusion coefficients of benzene, anthracene, m-cresol, and p-nitrophenol in enhanced-fluidity liquid mixtures of ethanol/H2O/CO2 were studied. The effect of mixture composition and temperature variation on the measured diffusion coefficients was studied. In a mixture containing 0.61/0.39 mole ratio ethanol/H2O, the diffusion coefficients of the four solutes increased comparably either by adding 30 mol % CO2 or by changing the temperature of the mixture from 25 to 60 degrees C. The experimental data were compared to that predicted by the Stokes-Einstein and the Wilke-Chang relations. Often, the experimental diffusion coefficients were greater than those predicted by these mass transport relations. However, the Eyring relationship was useful in describing the variation of diffusion coefficients as a function of temperature change for all of the ethanol/H2O/CO2 mixtures tested.

Lowering the viscosity of an HPLC mobile phase using liquid CO2/organic solvent mixtures has proven to be an effective means of increasing chromatographic efficiency, shortening analysis time, and ...

Capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS) and electrospray (ES) or ion spray (IS) mass spectrometry (MS) are recently introduced techniques for elemental speciation. Both techniques have the potential for rapid elemental speciation with low detection limits. Examples of the use of CE-ICP-MS for elemental speciation of positive, neutral and negative species are discussed. Issues in interfacing CE and ICP-MS are considered briefly. The potential advantages and disadvantages of laminar flow in CE-ICP-MS are examined. Potential difficulties in CE-ICP-MS including loss of sample, chemical matrix effects and changes in speciation during separation are discussed. The interpretation of ES or IS-MS spectra and analysis of complex mixtures are considered. Calibration and chemical matrix effects are assessed. Potential pitfalls of interpreting bare metal ion spectra as elemental analysis are discussed. The need for fundamental understanding of the processes that control ES and IS-MS signals is examined. High conductivity samples currently present difficulties for CE-ICP-MS or ES and IS-MS.

A comparison among modified and unmodified supercritical fluid extraction (SFE), enhanced-fluidity liquid extraction, and accelerated solvent extraction (ASE) techniques was made for the extraction of polynuclear aromatic hydrocarbons (PAHs) from an aged, spiked lignite coal fly ash. All of the attempted extraction conditions allowed the extraction of the PAHs to some degree, but no single extraction technique proved to be superior for all of the PAHs used. Three groups of PAHs with similar extraction efficiencies were identified. The group with the lowest molecular weights was best recovered using a 90% CO2-10% methanol mixture at 70 degrees C and 238 atm. The group of medium-molecular-weight PAHs was recovered equally well using any of three extraction conditions: SFE (100% CO2, 90 degrees C, and 238 atm), enhanced-fluidity liquid mixture (60% CO2-40% methanol, 70 degrees C, and 238 atm), and a methanol ASE mixture. The group of high-molecular-weight PAHs seemed to be equally well recovered with all of the attempted extraction conditions, but the enhanced-fluidity conditions (60% CO2-40% methanol, 70 degrees C, and 238 atm) had extraction recoveries (> 85%) with the lowest standard deviations (approximately 5%).

A comparison among supercritical fluid extraction (SFE), modified SFE, enhanced-fluidity extraction, and accelerated solvent extraction (ASE) techniques was made for the extraction of polynuclear aromatic hydrocarbons (PAHs) from an aged, spiked bituminous coal fly ash. Non-ASE extraction techniques used in this study could not recover PAHs with molecular weights greater than that of pyrene. ASE techniques using methylene chloride (with and without a static step) and toluene were able to recover most of the PAHs studied. None of the ASE techniques could quantitatively extract the low-molecular-weight PAHs from the bituminous fly ash. The medium-molecular-weight PAHs were best recovered with toluene ASE. The high-molecular-weight PAHs were best recovered with the toluene ASE technique (> 80%), but the overall precision of these measurements was low. Methylene chloride ASE with a static step recovered the high-molecular-weight PAHs with the next highest efficiency (approximately 55%) and had standard deviations less than 10% (longer extraction times [> 30 min] with the methylene chloride would increase the recoveries of these analytes.) A comparison of the results from this study with those of a previous study using lignite coal fly ash illustrates the difficulty in developing and adapting analyte-specific extraction methods for analytes that are adsorbed on different matrices.

Nanofibrous silica-based stationary phases for electrospun ultra-thin layer chromatography (E-UTLC) are described. Nanofibers were produced by electrospinning a solution of silica nanoparticles dispersed in polyvinylpyrrolidone solutions to create composite silica/polymer nanofibers. Stationary phases were created from as-spun nanofibers, or the nanofibers were heated either to crosslink the polyvinylpyrrolidone or to calcine and selectively remove the polymer. As-spun, crosslinked, and calcined nanofibers with similar mat thicknesses (23–25 μm) were evaluated as stationary phases for E-UTLC separations of laser dyes and amino acids and compared to commercial silica TLC plates. As-spun nanofiber plates offered fast mobile phase velocities, but like other polymer-based nanofibers, separations were only compatible with techniques using nonsolvents of the polymer. Crosslinked nanofibers were not as limited in terms of chemical stability, but separations produced tailed spot shapes. No limitations in terms of mobile phases, analyte solvents, and visualization techniques were observed for calcined nanofibers. Highly efficient separations of amino acids were performed in 15 mm on calcined nanofiber plates, with observed plate heights as low as 8.6 μm, and plate numbers as large as 1400. Additional alignment of the nanofibers provided shorter analysis times but also larger spot widths. The extension of stationary phases to silica-based nanofibers vastly expands the range of mobile phases, analyte solvents, and visualization techniques which can be used for E-UTLC separations.

The use of enhanced-fluidity liquid mixtures of tetrahydrofuran (THF)-CO2 as the mobile phases in the separation of styrene and polystyrene standards by size-exclusion chromatography (SEC) was compared to high temperature SEC. The phase diagram of a THF-CO2 binary system was determined. Adding CO2 to THF or increasing the mobile phase temperature produce similar results in the observed chromatography. The pressure drop across the column decreases, the optimal linear velocity shifts to a higher value, the chromatographic efficiency increases and the analysis time shortens for silica-based or polymer-based columns. The combination of both approaches provided the greatest improvement in chromatographic performance for CO2 proportions of 0–30% and temperatures of 24–100°C. Molecular weight calibration curves indicated that the SEC mechanism was maintained for these conditions. However, non-size-exclusion interactions were detected when the proportion of CO2 in the mobile phase was 50% mole fraction or more. Under those conditions, temperature elevation no longer enhanced the performance of the size-exclusion separation. Instead, increased adsorption of the polymer was observed due to significant loss of mobile phase solvent strength. However, by increasing the pressure of the mobile phase, the solvent strength could be restored.