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This paper compares static (i.e., temporally unchanging) thermal gradient gas chromatography (GC) to isothermal GC using a stochastic transport model to simulate peak characteristics for the separation of C12-C14 hydrocarbons resulting from variations in injection bandwidth. All comparisons are made using chromatographic conditions that give approximately equal analyte retention times so that the resolution and number of theoretical plates can be clearly compared between simulations. Simulations show that resolution can be significantly improved using a linear thermal gradient along the entire column length. This is mainly achieved by partially compensating for loss in resolution from the increase in mobile phase velocity, which approximates an ideal, basic separation. The slope of the linear thermal gradient required to maximize resolution is a function of the retention parameters, which are specific to each analyte pair; a single static, thermal gradient will not affect all analytes equally. A static, non-linear thermal gradient that creates constant analyte velocities at all column locations provides the largest observed gains in resolution. From the simulations performed in this study, optimized linear thermal gradient conditions are shown to improve the resolution by as much as 8.8% over comparative isothermal conditions, even with a perfect injection (i.e., zero initial bandwidth).

This article presents a method of simulating molecular transport in capillary gas chromatography (GC) applicable to isothermal, temperature-programmed, and thermal gradient conditions. The approach accounts for parameter differences that can occur across an analyte band including pressure, mobile phase velocity, temperature, and retention factor. The model was validated experimentally using a GC column comprised of microchannels in a stainless-steel plate capable of isothermal, temperature-programmed, and thermal gradient GC separations. The parameters governing retention and dispersion in the transport model were fitted with 12 experimental isothermal separations. The transport model was validated with experimental data for three analytes using four temperature-programmed and three thermal gradient GC separations. The simulated peaks (elution time and dispersion) give reasonable predictions of observed separations. The magnitudes of the maximum error between simulated peak elution time and experiment were 2.6 and 4.2% for temperature-programmed and thermal gradient GC, respectively. The magnitudes of the maximum error between the simulated peak width and experiment were 15.4 and 5.8% for temperature-programmed and thermal gradient GC, respectively. These relatively low errors give confidence that the model reflects the behavior of the transport processes and provides meaningful predictions for GC separations. This transport model allows for an evaluation of analyte separation characteristics of the analyte band at any position along the length of the GC column in addition to peak characteristics at the column exit. The transport model enables investigation of column conditions that influence separation behavior and opens exploration of optimal column design and heating conditions.

A wide variety of analytical techniques have been employed for monitoring chemical reactions, with online instrumentation providing additional benefits compared to offline analysis. A challenge in the past for online monitoring has been placement of the monitoring instrumentation as close as possible to the reaction vessel to maximize sampling temporal resolution and preserve sample composition integrity. Furthermore, the ability to sample very small volumes from bench-scale reactions allows the use of small reaction vessels and conservation of expensive reagents. In this study, a compact capillary LC instrument was used for online monitoring of as small as 1 mL total volume of a chemical reaction mixture, with automated sampling of nL-scale volumes directly from the reaction vessel used for analysis. Analyses to demonstrate short term (~2 h) and long term (~ 50 h) reactions were conducted using tandem on-capillary ultraviolet absorbance followed by in-line MS detection or ultraviolet absorbance detection alone, respectively. For both short term and long term reactions (10 and 250 injections, respectively), sampling approaches using syringe pumps minimized the overall sample loss to ~0.2% of the total reaction volume.

This paper compares dynamic (i.e., temporally changing) thermal gradient gas chromatography (GC) to temperature-programmed GC using a previously published stochastic transport model to simulate peak characteristics for the separation of C12-C40 hydrocarbons. All comparisons are made using chromatographic conditions that give approximately equal analyte retention times (tR). As shown previously, a static thermal gradient does not improve resolution (Rs) equally for all analytes, which highlights the need for a dynamic thermal gradient. An optimal dynamic thermal gradient should result in constant analyte velocities at any instant in time for those analytes that are actively being separated (i.e., analytes that have low retention factors). The average separation temperature for each analyte is used to determine the thermal gradient profile at different times in the temperature ramp. Because many of the analytes require a similar thermal gradient profile when actively being separated, the thermal gradient profile in this study was held fixed; however, the temperature of the entire thermal gradient was raised over time. From the simulations performed in this study, optimized dynamic thermal gradient conditions are shown to improve Rs by up to 13% over comparative temperature-programmed conditions, even with a perfect injection (i.e., zero injection bandwidth). In the dynamic thermal gradient simulations, all analytes showed improvements in Rs along with slightly shorter tR values compared to simulations for traditional temperature-programmed conditions.

This paper reports the first results of a robust, high-performance, stainless-steel microchip gas-chromatography (GC) column that is capable of analyzing complex real-world mixtures as well as operating at very high temperatures. Using a serpentine design, a 10 m column with an approximately semicircular cross-section with a 52 μm hydraulic diameter (Dh) was produced in a 17 × 6.3 × 0.1 cm rectangular steel chip. The channels were produced using a multilayer-chemical-etch and diffusion-bonding process, and metal nuts were brazed onto the inlet and outlet ports allowing for column interfacing with ferrules and fused silica capillary tubing. After deactivating the metal surface, channels were statically coated with a ≈0.1 μm layer of 5% phenyl–1% vinyl–methylpolysiloxane (SE-54) stationary phase and cross-linked with dicumyl peroxide. By using n-tridecane (n-C13) as a test analyte with a retention factor (k) of 5, a total of 44 500 plates (≈4500 plates per meter) was obtained isothermally at 120 °C. The col...

Methyl sulfate (MeSO4-) salts were explored as thermochemolysis-methylation (TCM) reagents for gas chromatographic (GC) analysis of dipicolinic acid (DPA) as its dimethyl ester (Me2DPA) from bacterial endospores. The reaction was carried out under non-pyrolytic conditions by inserting a small coiled wire filament coated with the sample and reagents directly inside a GC injection port at 290 °C. Above 10 : 1 methyl donor/DPA ratios, alkali metal salts of MeSO4- effected 80-90% conversion of DPA to Me2DPA, which was 10-20 times more active than the same amount of tetramethylammonium hydroxide (TMA-OH) at this temperature. A quaternary salt mixture consisting of 1 : 3 : 1 : 3 TMA+/Na+/OH-/MeSO4- methylated spore DPA with an average conversion of 86% (mean conversion by TMA-OH under the same conditions was 4%). Therefore, the sensitivity for detection of bacterial endospores was increased over 20-fold compared to that observed with the more commonly employed TMA-OH methylating reagent. The limit of detection by this method was 9 × 104 total spores. Mechanisms describing the observed behavior are proposed and discussed. This is the first use of MeSO4- as a TCM reagent for GC.

A newly developed portable capillary liquid chromatograph was investigated for the separation of various pharmaceutical and illicit drug compounds. The system consists of two high-pressure syringe pumps capable of delivering capillary-scale flow rates at pressures up to 10 000 psi. Capillary liquid chromatography columns packed with sub-2 μm particles are housed in cartridges that can be inserted into the system and easily connected through high-pressure fluidic contact points by simply applying a specific, predetermined torque rather than using standard fittings and less precise sealing protocols. Several over-the-counter analgesic drug separations are demonstrated, along with a simple online measurement of tablet dissolution. Twenty illicit drug compounds were also separated across six targeted drug panels. The results described in this study demonstrate the capability of this compact liquid chromatography instrument to address several important drug-related applications while simplifying system operation, and greatly reducing solvent usage and waste generation essential for onsite analysis.

A simple device based on a tiny, coiled wire filament (CWF), similar in operation to solid phase microextraction (SPME), was reported almost a decade ago for sampling, concentrating, and minimizing contamination for Injection of liquid samples in gas chromatography (GC). However, the pliability of the platinum wire from which the CWF was fabricated was a hindrance when using this technique. In this work, we utilized a much stiffer stainless steel wire to form the CWF, and explored a number of ways to use it for sampling and sample introduction in GC and GC–MS. The coil radius and number of turns in the CWF (i.e., length of the coil) determined the sampling volume. The CWF was attached to a retractable plunger (part of a hand-held syringe-type holder) such that it could be retracted inside a 19 G needle for insertion into a standard GC injection port. Sampling was easily performed by either dipping the CWF in a liquid sample (i.e., sample dissolved in a solvent), which was drawn up into the wire coil by capillary action, or by applying a specific volume of liquid sample onto the CWF using a micro-syringe. Analytes trapped in/on the CWF could be introduced into the GC injection port before or after solvent evaporation. For semi-volatile compounds, the sample solvent was evaporated before injection, while for volatile compounds, some or all of the sample solvent was retained in the coil during injection. Repeatable volumes from 0.05 to 0.7 μL were sampled using CWFs with 5–70 turns, respectively, when sampling by dipping. Advantages of using a CWF compared to a conventional syringe include: (1) the chromatographic system is protected from contamination caused by accumulation of sample residues, (2) high quantitative repeatability is obtained for small volume injections (0.05–0.2 μL), (3) large sample injections can be performed for trace analysis by evaporating the solvent before injection, and (4) carryover and discrimination of semi-volatile compounds are minimized. These advantages enable easy and rapid (10 min total analysis time) trace (0.1–5 ppb) detection of a variety of different types of compounds, including polycyclic aromatic hydrocarbons, biphenyl congeners, organochlorine pesticides, pyrethroid pesticides, phthalate esters, and n -alkanes from C10 to C40 in water and waste water.

The design of a miniaturized LED-based UV-absorption detector was significantly improved for on-column nanoflow LC. The detector measures approximately 27mm×24mm×10mm and weighs only 30g. Detection limits down to the nanomolar range and linearity across 3 orders of magnitude were obtained using sodium anthraquinone-2-sulfonate as a test analyte. Using two miniaturized detectors, a dual-detector system was assembled containing 255nm and 275nm LEDs with only 216nL volume between the detectors A 100μm slit was used for on-column detection with a 150μm i.d. packed capillary column. Chromatographic separation of a phenol mixture was demonstrated using the dual-detector system, with each detector producing a unique chromatogram. Less than 6% variation in the ratios of absorbances measured at the two wavelengths for specific analytes was obtained across 3 orders of magnitude concentration, which demonstrates the potential of using absorption ratio measurements for target analyte detection. The dual-detector system was used for simple, but accurate, mobile phase flow rate measurement at the exit of the column. With a flow rate range from 200 to 2000nL/min, less than 3% variation was observed.

Organic monolithic columns based on single crosslinking of trimethylolpropane trimethacrylate (TRIM) monomer were prepared in a single step by living/controlled free-radical polymerization. Full optimization of the preparation, such as using different percentages of TRIM and different amounts of radical promoter as well as various porogen solvents were explored. The resulting monolithic columns were characterized by scanning electronic microscopy and nitrogen sorption for structure morphology studies and surface area measurements, respectively. Using capillary liquid chromatography, 150 μm i.d. columns were applied to separate a mixture of small hydrophobic molecules. The results indicated that column performance is highly sensitive to the type and the amount of porogen solvents used in the polymerization mixture composition. Good resolution factors and methylene selectivity were obtained, indicating the promising potential of this material for capillary liquid chromatography separations. This article is protected by copyright. All rights reserved

Rapid determination of trace level (parts-per-trillion) volatile organic compounds cannot always be achieved with conventional analytical techniques. In this study, a device was developed to sample a large volume of air in a short time period. The basic design involves packing sorbent layers concentrically around an empty permeable tube. Single digit parts-per-trillion detection limits were reached in less than 25min with this sampling system using gas chromatography-mass spectrometry for analysis. The concentric packed trap can sample at high flow rates (>10L/min) because it has a large sampling surface cross-section and short combined sorbent bed. Additionally, the large sorbent amount (>1g) provides large breakthrough volume (>100L) required to achieve low detection limits. The trapped analytes were thermally desorbed and transferred into a needle trap device for final analysis. This high flow sampling system was explored for detection of low ppt benzene, toluene, ethylbenzene and xylenes (BTEX) in air.

Retention indices for 10 sets of alkyl-substituted polycyclic aromatic sulfur heterocycles (PASHs) isomers (total of 80 PASHs) were determined using gas chromatography with three different stationary phases: a 50% phenyl phase, a 50% liquid crystalline dimethylpolysiloxane (LC-DMPS) phase, and an ionic liquid (IL) phase. Correlations between the retention behavior on the three stationary phases and PASH geometry [length-to-breadth (L/B) and thickness (T)] were investigated for the following PASHs: 4 methyl-substituted dibenzothiophenes (DBTs), 3 ethyl-substituted DBTs, 15 dimethyl-substituted DBTs, 8 trimethyl-substituted DBTs, 15 methyl-substituted naphthothiophenes, 30 methyl-substituted benzonaphthothiophenes, and 5 methyl-substituted tetrapheno[1,12-bcd]thiophene. Correlation coefficients for retention on the 50% phenyl phase vs L/B ranged from r = −0.28 (MeBbN23Ts) to r = 0.92 (EtDBTs). Correlation coefficients for retention on the IL phase vs L/B ranged from r = 0.13 (MeN12Ts) to r = 0.83 (EtDBTs). Correlation coefficients for retention on the 50% LC-DMPS phase vs L/B ranged from r = 0.22 (MeDBTs) to r = 0.84 (TriMeDBTs).

Retention indices for 48 polycyclic aromatic sulfur heterocycles (PASHs) were determined using gas chromatography with three different stationary phases: a 50% phenyl phase, a 50% liquid crystalline dimethylpolysiloxane (LC-DMPS) phase, and an ionic liquid (IL) phase. Correlations between the retention behavior on the three stationary phases and PASH geometry (L/B and T, i.e., length-to-breadth ratio and thickness, respectively) were investigated for the following four isomer sets: (1) 4 three-ring molecular mass (MM) 184 Da PASHs, (2) 13 four-ring MM 234 Da PASHs, (3) 10 five-ring MM 258 Da PASHs, and (4) 20 five-ring MM 284 Da PASHs. Correlation coefficients for retention on the 50% LC-DMPS vs L/B ranged from r = 0.50 (MM 284 Da) to r = 0.77 (MM 234 Da). Correlation coefficients for retention on the IL phase vs L/B ranged from r = 0.31 (MM 234 Da) to r = 0.54 (MM 284 Da). Correlation coefficients for retention on the 50% phenyl vs L/B ranged from r = 0.14 (MM 258 Da) to r = 0.59 (MM 284 Da). Several correlation trends are discussed in detail for the retention behavior of PASH on the three stationary phases.

A compact ultrahigh-pressure nanoflow liquid chromatograph (LC) was developed with the purpose in mind of creating a portable system that could be easily moved to various testing locations or placed in close proximity to other instruments for optimal coupling, such as with mass spectrometry (MS). The system utilized innovative nanoflow pumps integrated with a very low volume stop-flow injector and mixing tee. The system weighed only 5.9 kg (13 lbs) or 4.5 kg (10 lbs) without a controller and could hold up to 1100 bar (16000 psi) of pressure. The total volume pump capacity was 60 μL. In this study, the sample injection volume was determined by either a 60 nL internal sample groove machined in a high-pressure valve rotor or by a 1 μL external sample loop, although other sample grooves or loops could be selected. The gradient dwell volume was approximately 640 nL, which allowed significant reduction in sample analysis time. Gradient performance was evaluated by determining the gradient step accuracy. A low RSD (0.6%, n = 4) was obtained for day-to-day experiments. Linear gradient reproducibility was evaluated by separating a three-component polycyclic aromatic hydrocarbon mixture on a commercial 150 μm inner diameter capillary column packed with 1.7 μm particles. Good retention-time reproducibility (RSD0.17%) demonstrated that the pumping system could successfully generate ultrahigh pressures for use in capillary LC. The system was successfully coupled to an LTQ Orbitrap MS in a simple and efficient way; LC-MS of a trypsin-digested bovine serum albumin (BSA) sample provided narrow peaks, short dwell time, and good peptide coverage.

Retention indices for 70 polycyclic aromatic sulfur heterocycles (PASHs) were determined using reversed-phase liquid chromatography (LC) on a monomeric and a polymeric C18 stationary phase. Molecular shape parameters [length, breadth, thickness (T), and length-to-breadth ratio (L/B)] were calculated for all the compounds studied. Correlations between the retention on the polymeric C18 phase and PASH geometry (L/B and T) were investigated for six specific PASH isomer groups with molecular mass (MM) 184 Da, 234 Da, 258 Da, 284 Da, 334 Da, and 384 Da. Similar to previous studies for polycyclic aromatic hydrocarbons (PAHs), PASH elution order on the polymeric C18 phase was generally found to follow increasing L/B values. Correlation coefficients for retention vs L/B ranged from r = 0.45 (MM 184 Da) to r = 0.89 (MM 284 Da). In the case of smaller PASHs (MM ≤ 234 Da), the location of the sulfur atom in the bay-region of the structure resulted in later than expected elution of these isomers based on L/B. In the case of the larger PASHs (MM ≥ 284 Da), nonplanarity had a significant influence on earlier than predicted elution based on L/B values.

Almost four decades of investigations have opened up many avenues to explore the production and utilization of planar (i.e., microchip) gas chromatographic columns. However, there remain many practical constraints that limit their widespread commercialization and use. The main challenges arise from non-ideal column geometries, dead volume issues and inadequate interfacing technologies, which all affect both column performance and range of applications. This review reflects back over the years on the extensive developments in the field, with the goal to stimulate future creative approaches and increased efforts to accelerate microchip gas chromatography development toward reaching its full potential.

This paper examines the separation effects of a moving thermal gradient on a chromatographic column in gas chromatography. This movement of the gradient has a focusing effect on the analyte bands, limiting band broadening in the column. Here we examine the relationship between the slope of this gradient, the velocity of the gradient and the resulting band width. Additionally we examine how transport of analytes along the column at their analyte specific constant temperatures, determined by the gradient slope and velocity, affects resolution. This examination is based primarily on a theoretical model of partitioning and transport of analyte under low concentration conditions. Preliminary predictions indicate that analytes reach near constant temperatures, relative positions and resolutions in less than 100 cm of column transport. Use of longer columns produces very little improvement in resolution for any fixed slope. Properties of the thermal gradient determine a fixed solute band width for each analyte. These widths are nearly reached within the first 40–70 cm, after which little broadening or narrowing of the bands occur. The focusing effect of the thermal gradient corrects for broad injections, reduces effects of irregular stationary phase coatings and can be used with short columns for fast analysis. Thermal gradient gas chromatographic instrumentation was constructed and used to illustrate some characteristics predicted from the theoretical results.

Fabrication technologies for microelectromechanical systems (MEMS) allow miniaturization of conventional benchtop gas chromatography (GC) to portable, palm-sized microfabricated GC (μGC) devices, which are suitable for on-site chemical analysis and remote sensing. The separation performance of μGC systems, however, has not been on par with conventional GC. Column efficiency, peak symmetry and resolution are often compromised by column defects and non-ideal injections. The relatively low performance of μGC devices has impeded their further commercialization and broader application. In this work, the separation performance of μGC columns was improved by incorporating thermal gradient gas chromatography (TGGC). The analysis time was ∼20% shorter for TGGC separations compared to conventional temperature-programmed GC (TPGC) when a wide sample band was introduced into the column. Up to 50% reduction in peak tailing was observed for polar analytes, which improved their resolution. The signal-to-noise ratios (S/N) of late-eluting peaks were increased by 3–4 fold. The unique focusing effect of TGGC overcomes many of the previous shortcomings inherent in μGC analyses.

Miniaturization of gas chromatography (GC) instrumentation is of interest because it addresses current and future issues relating to compactness, portability and field application. While incremental advancements continue to be reported in GC with columns fabricated in microchips (referred to in this paper as "microchip columns"), the current performance is far from acceptable. This lower performance compared to conventional GC is due to factors such as pooling of the stationary phase in corners of non-cylindrical channels, adsorption of sensitive compounds on incompletely deactivated surfaces, shorter column lengths and less than optimum interfacing to injector and detector. In this work, a GC system utilizing microchip columns was developed that solves the latter challenge, i.e., microchip interfacing to injector and detector. A microchip compression clamp was constructed to heat the microchip (i.e., primary heater), and seal the injector and detector fused silica interface tubing to the inlet and outlet ports of the microchip channels with minimum extra-column dead volume. This clamp allowed occasional operation up to 375°C and routine operation up to 300°C. The compression clamp was constructed of a low expansion alloy, Kovar™, to minimize leaking due to thermal expansion mismatch at the interface during repeated thermal cycling, and it was tested over several months for more than one hundred injections without forming leaks. A 5.9m long microcolumn with rectangular cross section of 158μm×80μm, which approximately matches a 100μm i.d. cylindrical fused silica column, was fabricated in a silicon wafer using deep reactive ion etching (DRIE) and high temperature fusion bonding; finally, the channel was coated statically with a 1% vinyl, 5% phenyl, 94% methylpolysiloxane stationary phase. High temperature separations of C10-C40 n-alkanes and a commercial diesel sample were demonstrated using the system under both temperature programmed GC (TPGC) and thermal gradient GC (TGGC) conditions. TGGC analysis of a complex essential oil sample was also demonstrated. Addition of a secondary heater and polyimide insulation proved to be helpful in achieving the desired elution temperature without having to raise the primary heater temperature above 300°C for high boiling point compounds.

Highly cross-linked monolithic networks (i.e., polyethylene glycol diacrylate, PEGDA) synthesized from monomers containing varying ethylene oxide chain lengths were fabricated inside fused silica capillary columns for use in liquid chromatography (LC) of small molecules. Tergitol was used as a surfactant porogen in combination with other typical organic liquid porogens. Column performance was correlated with quantitative descriptors of the physical/chemical properties of the monomers and porogens using a statistical model. Solubility and viscosity values of the components were identified as important predictors of monolith morphology and efficiency. The chromatographic retention mechanism was determined to be principally reversed-phase (RP) with additional hydrogen bonding between the polar groups of the analytes and the ethylene oxide groups embedded in the monolith structure. The fabricated monolithic columns were evaluated under RPLC conditions using phenols, hydroxy benzoic acids, and alkyl parabens as test compounds. Isocratic elution of hydroxy benzoic acids at a linear velocity of 0.04 cm/s using a PEGDA-700 monolith gave chromatographic peaks with little tailing (i.e., tailing factor1.28). The chromatographic efficiency measured for a non-retained compound (uracil) using this column was 186,000 plates/m when corrected for injector dead volume. High resolution gradient separations of selected pharmaceutical compounds and phenylurea herbicides were achieved in less than 18 min. Optimized monoliths synthesized from all four crosslinking monomers exhibited high permeability and demonstrated little swelling or shrinking in different polarity solvents. Column preparation was highly reproducible, with relative standard deviation (RSD) values less than 2.1%, based on retention times of the phenol standards (3 different columns).

Monoliths are considered to be a low pressure alternative to particle packed columns for liquid chromatography (LC). However, the chromatographic performance of organic monoliths, in particular, has still not reached the level of particle packed columns. Since chromatographic performance can be attributed to morphological features of the monoliths, in-situ characterization of the monolith structure in three dimensions would provide valuable information that could be used to help improve performance. In this work, serial sectioning and imaging were performed with a dual-beam scanning electron microscope for reconstruction and quantitative characterization of poly(ethylene glycol) diacrylate (PEGDA) monoliths inside a capillary column. Chord lengths, homogeneity factors, porosities and tortuosities were calculated from three-dimensional (3D) reconstructions of two PEGDA monoliths. Chromatographic efficiency was better for the monolith with smaller mean chord length (i.e., 5.23 μm), porosity (i.e., 0.49) and tortuosity (i.e., 1.50) compared to values of 5.90 μm, 0.59 and 2.34, respectively, for the other monolithic column. Computational prediction of tortuosity (2.32) was found to be in agreement with the experimentally measured value (2.34) for the same column. The monoliths were found to have significant radial heterogeneity since the homogeneity factor decreased from 5.39 to 4.89 (from center to edge) along the column radius.

Reports of novel micro/nanostructures designed to separate biomacromolecules and bioparticles are increasing in number, and these studies have greatly advanced our understanding of nanoscale fluidics and nanoparticle behavior in confined channels. This review is aimed at summarizing previous developments in micro/nanofabricated systems for nanoparticle separations. These are discussed in three groups based on architecture, namely, micro/nanopillar array structures, nanoplane gap structures and artificial nanoporous membranes.

Capillary columns containing butyl or lauryl methacrylate monoliths were prepared using two different free-radical polymerization methods: conventional free-radical polymerization and controlled/living free-radical polymerization, both initiated thermally, and these methods were compared for the first time. Both monolith morphology and chromatographic efficiency were compared for the synthesized stationary phases using scanning electronic microscopy (SEM) and capillary liquid chromatography, respectively. Columns prepared using controlled method gave better chromatographic performance for both monomers tested. The lauryl-based monolith showed 7-fold improvement in chromatographic efficiency with a plate count of 42,000 plates/m (corrected for dead volume) for a non-retained compound. Columns fabricated using controlled polymerization appeared more homogenous radially with fused small globular morphologies, evaluated by SEM, and lower column permeability. The columns were compared with respect to resolving power of a series of alkylbenzenes under isocratic and gradient elution conditions.

Three crosslinking monomers, i.e., 1,6-hexanediol dimethacrylate (HDDMA), cyclohexanediol dimethacrylate (CHDDMA) and 1,4-phenylene diacrylate (PHDA), were used to synthesize highly cross-linked monolithic capillary columns for reversed-phase liquid chromatography (RPLC) of small molecules. Selection of porogen type and concentration was investigated in detail. Isocratic elution of alkylbenzenes at a flow rate of 300nL/min was performed using HDDMA and CHDDMA monolithic columns. Gradient elution of alkylbenzenes using all three monolithic columns showed good separations. Optimized monoliths synthesized from all three crosslinking monomers possessed high permeabilities. Poly(HDDMA) monoliths demonstrated column efficiencies up to 86,000 plates/m. Column preparation of poly(HDDMA) monolithic columns was highly reproducible; the relative standard deviation (RSD) values (n=3) for run-to-run and column-to-column were less than 0.26% and 0.70%, respectively, based on retention times of alkylbenzenes.

RPLC is the most common mode of LC. It is widely used for separations of both small and large molecules. Monolithic columns, which are currently under intensive study by many groups, have the potential of becoming attractive alternatives to particle-packed columns. They are generally easier and faster to fabricate, and they demonstrate a lower pressure drop, less nonspecific adsorption, and richer chemistry (in the case of organic polymer monoliths) for providing broad selectivity. Silica monoliths, as is also true for columns packed with silica particles, are best applied to small-molecule separations. Organic polymer monoliths, on the other hand, have shown advantages for large-molecule separations. Recently, improvements in organic monoliths have led to efficiencies for small molecules that are approaching and even surpassing 100 000 plates/m. While this performance is still far short of what is currently available using modern small particles and silica monoliths in RPLC, steady progress is being made. This review describes recent developments in the synthesis and performance of organic polymer RPLC monoliths, and identifies areas where additional work is needed to significantly improve their performance for both small- and large-molecule separations.

The use of negative axial thermal gradients in gas chromatography (TGGC) has intrigued chromatographers since the early 1950s because of the dramatic narrowing of analyte bands and concomitant raised expectations for improving resolving power. However, technical difficulties experienced in construction of TGGC instrumentation and control of the temperature along the column have made its implementation and, hence, detailed study difficult. In this work, we describe a TGGC system capable of rapidly producing and varying thermal gradient profiles by simultaneous use of resistive heating and convective cooling. Heating and cooling rates as high as 1200 and 2500 °C/min, respectively, allowed the creation of dynamic temperature gradients. The separation characteristics of TGGC with dynamically changing temperature gradients are demonstrated. A gradient velocity of 2.22 cm/s provided repetitive separations every 45 s, and injection band widths of 45 s duration were transformed into approximately 1-s peak widths. Peak tailing for basic compounds was nearly eliminated. Dynamic TGGC allows unique control over separations, oftentimes improving resolution and detection signal-to-noise. Thermally controlled elution in TGGC holds great promise for performing smart separations in which the separation time window is most efficiently utilized, and optimized separations can be quickly achieved. Rapid adjustment of relative compound elution can be used to greatly reduce GC method development time.

An automated sample preparation system was developed and tested for the rapid detection of Bacillus anthracis endospores by gas chromatography-mass spectrometry (GC-MS) for eventual use in the field. This reactor is capable of automatically processing suspected bio-threat agents to release and derivatize unique chemical biomarkers by thermochemolysis (TCM). The system automatically controls the movement of sample vials from one position to another, crimping of septum caps onto the vials, precise delivery of reagents, and TCM reaction times and temperatures. The specific operations of introduction of sample vials, solid phase microextraction (SPME) sampling, injection into the GC-MS system, and ejection of used vials from the system were performed manually in this study, although they can be integrated into the automated system. Manual SPME sampling is performed by following visual and audible signal prompts for inserting the fiber into and retracting it from the sampling port. A rotating carousel design allows for simultaneous sample collection, reaction, biomarker extraction and analysis of sequential samples. Dipicolinic acid methyl ester (DPAME), 3-methyl-2-butenoic acid methyl ester (a fragment of anthrose) and two methylated sugars were used to compare the performance of the autoreactor with manual TCM. Statistical algorithms were used to construct reliable bacterial endospore signatures, and 24 out of 25 (96%) endospore-forming Bacillus species were correctly identified in a statistically designed test.

In this paper, a new approach to prepare monolithic molecularly imprinted polymer (MIP) fibers for solid-phase microextraction is proposed with the help of microwave irradiation. Imprinting polymerization was carried out within silica capillaries in 4.5 min, using dimethyl phthalate (DMP) as a template molecular, α-methacrylic acid as a functional monomer and ethylene dimethacrylate as a crosslinker, acetonitrile as the porogenic solvent. The synthesis was optimized by varying the ratio of template/monomer and different volume of porogen. The resulted MIP fibers were obtained after silica being etched away with a controlled length of 1 cm, and subsequently characterized by SEM. In order to increase the selective extraction of DMP, factors affecting the extraction including extraction time, salt concentration, desorption time, and desorption solvents were investigated for solid-phase microextraction procedures in detail. The selectivity coefficients, defined as the extraction amount ratio of MIP to its nonimprinting fiber, were 5.6, 2.6, and 1.4 for DMP and its counterpart including dibutyl phthalate and di-n-octylo-phthalate, respectively. The resulted fibers were also applied to detect DMP, dibutyl phthalate, and di-n-octylo-phthalate in bottled beverage samples coupled to HPLC and resulted in relative recoveries of up to 73.8-98.5%, respectively.

Retention indices for 79 alkyl-substituted polycyclic aromatic sulfur heterocycles (PASHs) were determined by using reversed-phase liquid chromatography (LC) on a monomeric and polymeric octadecylsilane (C18) stationary phase. Molecular shape parameters [length, breadth, thickness (T), and length-to-breadth ratio (L/B)] were calculated for all the compounds studied. Based on separations of isomeric methylated polycyclic aromatic hydrocarbons on polymeric C18 phases, alkyl-substituted PASHs are expected to elute based on increasing L/B ratios. However, the correlation coefficients had a wide range of values from r = 0.43 to r = 0.93. Several structural features besides L/B ratios were identified to play an important role in the separation mechanism of PASHs on polymeric C18 phases. First, the location of the sulfur atom in a bay-like-region results in alkylated-PASHs being more retentive than non-bay-like-region alkylated-PASHs, and they elute later than expected based on L/B value. Second, the placement of the alkyl group in the k region of the structure resulted in a later elution than predicted by L/B. Third, highly nonplanar methyl-PASHs (i.e., 1-Me and 11-MeBbN12T) elute prior to the parent PASH (BbN12T).

A novel porous poly(ethylene glycol) methacrylate-based monolithic column for normal phase liquid chromatography was prepared by thermally initiated polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) and ethylene dimethacrylate (EDMA) in the presence of selected porogens. The monolith was macroscopically homogeneous, had low flow resistance, and did not swell or shrink significantly in solvents of different polarities. Inverse size-exclusion data indicate that the monolith had a total porosity of 79.2%, including an external porosity of 69.3% and an internal porosity of 9.9%. Due to its mild polarity (hydrophilicity), the PEG-functionalized monolith could perform traditional normal phase chromatography using non-polar solvents The van Deemter plot demonstrated that the column efficiency of 33,600–34,320 theoretical plates/m could be achieved at a linear flow velocity of 0.9–1.5 mm/s. The dual retention capability (both weak hydrophilic and hydrophobic interactions) investigated in this paper explains well why the PEG-functionalized monolith could operate in various chromatographic modes.

Seven crosslinking monomers, i.e., 1,3-butanediol dimethacrylate (1,3-BDDMA), 1,4-butanediol dimethacrylate (1,4-BDDMA), neopentyl glycol dimethacrylate (NPGDMA), 1,5-pentanediol dimethacrylate (1,5-PDDMA), 1,6-hexanediol dimethacrylate (1,6-HDDMA), 1,10-decanediol dimethacrylate (1,10-DDDMA), and 1,12-dodecanediol dimethacrylate (1,12-DoDDMA), were used to synthesize highly cross-linked monolithic capillary columns for reversed-phase liquid chromatography (RPLC) of small molecules. Dodecanol and methanol were chosen as “good” and “poor” porogenic solvents, respectively, for these monoliths, and were investigated in detail to provide insight into the selection of porogen concentration using 1,12-DoDDMA. Isocratic elution of alkylbenzenes at a flow rate of 300 nL/min was conducted for all of the monoliths. Gradient elution of alkylbenzenes and alkylparabens provided high resolution separations. Optimized monoliths synthesized from all seven crosslinking monomers showed high permeability. Several of the monoliths demonstrated column efficiencies in excess of 50,000 plates/m. Monoliths with longer alkyl-bridging chains showed very little shrinking or swelling in solvents of different polarities. Column preparation was highly reproducible; the relative standard deviation (RSD) values ( n = 3) for run-to-run and column-to-column were less than 0.25% and 1.20%, respectively, based on retention times of alkylbenzenes.

Monolithic stationary phases show promise for LC as a result of their good permeability, ease of preparation and broad selectivity. Inorganic silica monoliths have been extensively studied and applied for separation of small molecules. The presence of a large number of through pores and small skeletal structure allows the chromatographic efficiencies of silica monoliths to be comparable to columns packed with 5 μm silica particles, at much lower back pressure. In comparison, organic polymeric monoliths have been mostly used for separation of bio-molecules; however, recently, applications are expanding to small molecules as well. Organic monoliths with high surface areas and fused morphology rather than conventional globular morphology have shown good performance for small molecule separations. Factors such as domain size, through-pore size and mesopore size of the monolithic structures have been found to govern the efficiency of monolithic columns. The structure and performance of monolithic columns are reviewed in comparison to particle packed columns. Studying and characterizing the bed structures of organic monolithic columns can provide great insights into their performance, and aid in structure-directed synthesis of new and improved monoliths.

Polyethylene glycol diacrylate monoliths prepared using different amounts of monomer, porogen ratio, and capillary dimensions were characterized using capillary flow porometry (CFP) and scanning electron microscopy (SEM). Our results reveal good agreement between SEM and CFP measurements for through-pore size distribution. The CFP measurements for monoliths prepared by the same procedure in capillaries with different diameters (i.e., 75, 150, and 250 μm) clearly confirmed a change in through-pore size distribution with capillary diameter, thus, certifying the need for in-column measurement techniques over bulk measurements (e.g., mercury intrusion porosimetry). The mean through-pore size varied from 3.52 to 1.50 μm with a change in capillary diameter from 75 to 250 μm. Consistent mean through-pore size distribution for capillary columns with the same internal diameter but with different lengths (1.5, 2, and 3 cm) confirms the high interconnectivity of the pores and independence of CFP measurements with respect to capillary length. CFP and SEM measurements not only allow pore structure analysis but also prediction of relative column performance. Monoliths with narrow through-pore size distribution (0.8-1.2 μm), small mean through-pore size, and thin skeletal size (0.55 μm) gave the best performance in terms of efficiency for polyethylene glycol diacrylate monoliths.

Hydrophilic C18 monolithic polymer sorbents were synthesized for use in solid phase extraction (SPE) and in capillary liquid chromatography (LC). The approach involved incorporating both hydrophobic and hydrophilic monomers into a monolithic material, by copolymerization of stearyl methacrylate (SMA), poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) and ethylene dimethacrylate (EDMA) in the presence of selected porogens, to produce translucent mesoporous monolithic materials in bulk (SPE) or white macroporous monoliths inside fused silica capillary columns (capillary LC). A capillary column containing one of the hydrophilic C18 monoliths (i.e. poly(SMA-co-PEGMEMA-co-EDMA) with 15% (w/w) PEGMEMA) demonstrated nearly 35% reduction in retention of polycyclic aromatic compounds and greater than 40% increase in retention of phenols compared to a hydrophobic C18 monolithic column. In addition, the hydrophilic monolith demonstrated significantly improved resolution of phenols. Similar monolithic materials prepared in bulk were ground and sieved to obtain 45-65 μm particles with desired rigidity for SPE. To achieve optimum extraction performance for phenols, several parameters, including sample pH and volume, and eluent type and volume, were investigated. Under optimized experimental conditions, the method demonstrated good sensitivity (1.6 ng/mL LOD) and linearity (R(2)>0.97 for 10-200 ng/mL). Again, incorporation of 15% (w/w) PEGMEMA in the monolith increased the extraction efficiency of phenols in water from approximately 20% to 67-92% compared to a hydrophobic C18 monolithic material. Increased wettability of the sorbent by the aqueous sample matrix and the presence of hydrogen-bonding interactions are responsible for the improved retention of polar compounds.

Highly cross-linked networks resulting from single crosslinking monomers were found to enhance the concentrations of mesopores in, and the surface areas of, polymeric monoliths. Four crosslinking monomers, i.e., bisphenol A dimethacrylate (BADMA), bisphenol A ethoxylate diacrylate (BAEDA, EO/phenol = 2 or 4) and pentaerythritol diacrylate monostearate (PDAM), were used to synthesize monolithic capillary columns for reversed phase liquid chromatography (RPLC) of small molecules. Tetrahydrofuran (THF) and decanol were chosen as good and poor porogenic solvents for BAEDA-2 and BAEDA-4 monoliths. For the formation of the BADMA monolith, THF was replaced with dimethylformamide (DMF) to improve the column reproducibility. Appropriate combinations of THF, isopropyl alcohol and an additional triblock poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) or PPO–PEO–PPO porogen were found to be effective in forming rigid PDAM monoliths with the desired porosities. Selection of porogens for the BADMA and PDAM monoliths was investigated in further detail to provide more insight into porogen selection. Isocratic elution of alkyl benzenes at a flow rate of 0.3 μL/min was conducted for BADMA and PDAM monoliths. The peaks showed little tailing on both monoliths without addition of acid to the mobile phase. The column efficiency measured for pentylbenzene using the BADMA monolithic column was 60,208 plates/m (k = 7.9). Gradient elution of alkyl benzenes and alkyl parabens was achieved with high resolution. Optimized monoliths synthesized from all four crosslinking monomers showed high permeability, and demonstrated little swelling or shrinking in different polarity solvents. Column preparation was highly reproducible; relative standard deviation (RSD) values were less than 1.2% and 7.5% based on retention times and peak areas, respectively, of alkyl benzenes.

Methyl sulfate (MeSO4−) salts were explored as thermochemolysis–methylation (TCM) reagents for gas chromatographic (GC) analysis of dipicolinic acid (DPA) as its dimethyl ester (Me2DPA) from bacterial endospores. The reaction was carried out under non-pyrolytic conditions by inserting a small coiled wire filament coated with the sample and reagents directly inside a GC injection port at 290 °C. Above 10 : 1 methyl donor/DPA ratios, alkali metal salts of MeSO4− effected 80–90% conversion of DPA to Me2DPA, which was 10–20 times more active than the same amount of tetramethylammonium hydroxide (TMA-OH) at this temperature. A quaternary salt mixture consisting of 1 : 3 : 1 : 3 TMA+/Na+/OH−/MeSO4− methylated spore DPA with an average conversion of 86% (mean conversion by TMA-OH under the same conditions was 4%). Therefore, the sensitivity for detection of bacterial endospores was increased over 20-fold compared to that observed with the more commonly employed TMA-OH methylating reagent. The limit of detection by this method was 9 × 104 total spores. Mechanisms describing the observed behavior are proposed and discussed. This is the first use of MeSO4− as a TCM reagent for GC.

Biocompatible poly(ethylene glycol methyl ether acrylate-co-polyethylene glycol diacrylate) monoliths were prepared for size exclusion chromatography (SEC) of proteins in the capillary format using Brij 58P in a mixture of hexanes and dodecanol as porogens. The monolithic columns provided size separation of four proteins in 20 mM sodium phosphate buffer (pH 7.0) containing 0.15 M NaCl, and there was a linear relationship between the retention times and the logarithmic values of the molecular weights. Compared to SEC monoliths previously synthesized using a triblock copolymer of polyethylene oxide and polypropylene oxide, an increase in mesoporosity was confirmed by inverse size exclusion chromatography. As a result, improved protein separation in the high molecular weight range and reduced column back-pressure were observed.

A simple capillary flow porometer (CFP) was assembled for through-pore structure characterization of monolithic capillary liquid chromatography columns in their original chromatographic forms. Determination of differential pressures and flow rates through dry and wet short capillary segments provided necessary information to determine the mean diameters and size distributions of the through-pores. The mean through-pore diameters of three capillary columns packed with 3, 5, and 7 μm spherical silica particles were determined to be 0.5, 1.0 and 1.4 μm, with distributions ranging from 0.1 to 0.7, 0.3 to 1.1 and 0.4 to 2.6 μm, respectively. Similarly, the mean through-pore diameters and size distributions of silica monoliths fabricated via phase separation by polymerization of tetramethoxysilane (TMOS) in the presence of poly(ethylene glycol) (PEG) verified that a greater number of through-pores with small diameters were prepared in columns with higher PEG content in the prepolymer mixture. The CFP system was also used to study the effects of column inner diameter and length on through-pore properties of polymeric monolithic columns. Typical monoliths based on butyl methacrylate (BMA) and poly(ethylene glycol) diacrylate (PEGDA) in capillary columns with different inner diameters (i.e., 50-250 μm) and lengths (i.e., 1.5-3.0 cm) were characterized. The results indicate that varying the inner diameter and/or the length of the column had little effect on the through-pore properties. Therefore, the through-pores are highly interconnected and their determination by CFP is independent of capillary length.

Fine particulate matter is believed to be more toxic than coarse particles and to exacerbate health problems such as respiratory and cardiopulmonary diseases. Specific organic compounds within atmospheric fine particulate material can be used to differentiate specific inputs from various emissions and thus is helpful in identifying the major urban air pollution sources that contribute to these health problems. Particular marker compounds that carry signature information about different emission sources (i.e., gasoline or diesel motor vehicles, wood smoke, meat cooking, vegetative detritus, and cigarette smoke) are reviewed. Aerosol organic types (e.g., from mass spectrometry data, which can also help in elucidation of carbonaceous material sources) are also discussed. Apportionment of the primary source contributions and atmospheric processes contributing to fine particulate matter and fine particulate organic material concentrations are outlined. This review provides an overview of the latest developments in chemical characterization approaches for identification and quantification of compounds in complex organic mixtures associated with fine atmospheric particles and their use in chemical mass balance (CMB) and positive matrix factorization (PMF) source apportionment models.