Fingerprinting and identification of isolated plastic polymers with pyrolysis comprehensive two-dimensional gas chromatography and flame ionization detection.

An approach using pyrolysis with comprehensive two-dimensional gas chromatography with flame ionization detection is introduced for identifying common isolated plastic polymers. A quadrupole mass spectrometer is employed as a parallel detector to aid method development and improve polymer identification in complex matrices. Common plastic polymers including polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyamide, poly(methyl methacrylate), styrene-butadiene rubber, and polyethylene terephthalate are accurately identified within a total analysis time of 45 min. A strategy to enhance compatibility of high-resolution capillary gas chromatography using a 150-µm internal diameter column technology and a larger internal volume microfurnace-based pyrolyzer is discussed. This strategy resulted in minimizing the band broadening effect caused by the pyrolyzer's internal volume and overcoming the slow pressure buildup when the sample is inserted into the furnace. Prolonged pressure buildup to reach a final pressure setting can cause a safety shutdown to the pneumatic control system. The developed approach is complementary to spectroscopic techniques by offering mass based, chemical composition analysis of plastics.

Targeted Analysis of Microplastics Using Discrete Frequency Infrared Imaging.

An analytical strategy to improve sample throughput with discrete frequency infrared image-based targeted analysis of microplastics using a laser direct infrared chemical imaging system was successfully developed and implemented. Leveraging a quantum cascade laser as a light source, the system could lock the frequency at predetermined wavelengths and use a discrete frequency infrared imaging technique to identify particles with absorption at desired wavelengths. In this way, targeted analysis can be achieved by selectively characterizing these particles. In the concept demonstration study, the targeted analysis was able to identify 87.7% of spiked polyethylene particles by scanning only 20% of the particles in the sample. The technique substantially improves sample throughput by at least a factor of 4 under conditions used. In the tests performed with real environmental samples, the targeted analysis workflow correctly identified eight types of common microplastics by only investigating around 60% of the particles and less than 30% of the sample area. Results obtained demonstrated that this scanning strategy is a game changer to enhance sample throughput in microplastic analysis. The technique has the potential of being applied to other infrared-based analytical platforms.

Quasi-Stop-Flow Modulation Strategy for Comprehensive Two-Dimensional Gas Chromatography.

An easy-to-implement strategy of differential flow modulation for comprehensive two-dimensional gas chromatography was innovated. With this approach, an independent auxiliary pneumatic control device for flow modulation was not a prerequisite. The strategy involved splitting the carrier gas stream into two separate streams before reaching the inlet embodiment. One stream was employed as a mobile phase for chromatographic separation. The other stream, for flow modulation, was routed to one of the ports of a three-way solenoid valve. The modulation stream flowed onward to a fluidic path and a T-junction that joined the primary and secondary dimension columns. With this arrangement and depending on the configuration of the three-port valve, the analytical platform can be operated in three different modes: bypass stop-flow, vent stop-flow, and quasi-stop flow. Quasi-stop-flow mode was demonstrated to have a significantly better chromatographic performance, as demonstrated in various types of real-life petroleum samples such as gasoline and light cycle oil. In the light cycle oil sample, a respectable separation between compound classes was achieved with peak width at half height of 34 ms or less for alkanes on a second dimension with polyethylene glycol stationary phase. Excellent repeatability was shown with normal alkanes standards of nC8-nC25. Relative standard deviations for retention times are almost zero in 1D, less than 0.2% in 2D, and less than 3.5% for peak areas (n = 9).

Uniformity and Sensitivity Improvements in Comprehensive Two-Dimensional Gas Chromatography Using Flame Ionization Detection with Post-Column Reaction.

A 3D-printed microreactor for post-column reactions was successfully integrated with comprehensive two -dimensional  gas chromatography. A two-stage post-column reaction provided a carbon-independent response, enhanced the flame ionization detection uniformity, and improved the detector sensitivity. These enhancements are critical to overcome challenges in analyses using comprehensive two-dimensional gas chromatography and flame ionization detection, which aim to separate and quantify multiple components. Post-column reaction flame ionization detection eliminated the requirement of multilevel and multicompound calibration, it enabled the determination of target analytes with a single-carbon-containing calibration compound with an accuracy of ±10%, and it improved the sensitivity for compounds that were not efficiently ionized by flame ionization detection. Extra column band-broadening caused by the incorporation of the 3D-printed microreactor was minimized using optimized reactor operating parameters and intercolumn connectivity. Chromatographic fidelity was in the practical domain of comprehensive 2D gas chromatography. Typical peak widths at half-height using the described approach ranged from 165 to 235 ms for probe compounds with retention factors spanning 5 < k < 40.

Metal 3D-printed catalytic jet and flame ionization detection for in situ trace carbon oxides analysis by gas chromatography.

A gas chromatographic approach for the determination and quantification of trace levels of carbon oxides in gas phase matrices for in situ or near-line/at-line analysis has been successfully developed. Catalytic conversion of the target compounds to methane via the methanation process was conducted inside a metal 3D-printed jet that also acted as a hydrogen burner for the flame ionization detector. Modifications made to a field transportable gas chromatograph enabled the leveraging of advantaged microfluidic-enhanced chromatography capability for improved chromatographic performance and serviceability. The compatibility with adsorption chromatography technology was demonstrated with in-house constructed columns. Sustained reliable conversion efficiencies of greater than 99% with respectable peak symmetries were attained at 400°C. Quantification of carbon monoxide and carbon dioxide at a parts-per-million level over a range from 0.2 ppm to 5% v/v for both compounds with a respectable precision of less than 3% relative standard deviation for peak area (n = 10) and a detection limit of 0.1 ppm v/v was achieved. Linearity with correlation coefficients of R2 greater than 0.9995 and measured recoveries of >99% for spike tests were achieved. The 3D-printed steel jet was found to be reliable and resilient against potential contamination from the matrices owing to the in situ backflushing capability.

Two-dimensional liquid chromatography with active solvent modulation for studying monomer incorporation in copolymer dispersants.

A pseudo-comprehensive two-dimensional liquid chromatography approach with size exclusion chromatography in the first dimension and gradient reversed-phase liquid chromatography in the second dimension was successfully developed for the characterization of vinyl acetate/acrylic acid copolymers and vinyl acetate/itaconic acid/acrylic acid terpolymers. Active solvent modulation was exploited to prevent the polymer breakthrough in the second dimension separation caused by the strong solvent used in the first dimension. The conditions of the active solvent modulation valve were optimized to achieve sufficient on-line dilution and to completely prevent polymer breakthrough without adding excessive time to the modulation cycle. Using this approach, copolymers made with different monomer ratios and processes were studied. Heterogeneous composition distribution due to insufficient monomer incorporation was detected in some of the copolymer samples. We demonstrated that with active solvent modulation, the two-dimensional liquid chromatography approach is no longer limited to water-soluble polymers and can be used for a broader range of polymers and copolymers.

Gas Chromatography with In Situ Catalytic Hydrogenolysis and Flame Ionization Detection for the Direct Measurement of Formaldehyde and Acetaldehyde in Challenging Matrices.

A gas chromatographic strategy to advance the direct detection and quantification of volatile aliphatic aldehydes such as formaldehyde and acetaldehyde in gas phase matrices without the need for sample pretreatment or concentration has been successfully developed. The catalytic hydrogenolysis of aldehydes to alkanes is conducted in situ within the 3D-printed steel jet assembly of the flame ionization detector and without any additional hardware required. Reliable conversion efficiencies of greater than 90% with respectable peak symmetries for the analytes were attained at 400 °C. Quantification of formaldehyde and acetaldehyde at parts-per-million levels over a range of 0.5-300 ppm (v/v) for formaldehyde and 0.2-430 ppm (v/v) for acetaldehyde with a respectable precision of less than 5% RSD ( n = 10) was achieved. The total analysis time was less than 10 min. Linearity with a correlation coefficient ( R2) greater than 0.9997 and measured recoveries of >99% for spike tests under the specified conditions were achieved. The 3D-printed steel jet assembly was found to be reliable and resilient to matrices such as air, water, hydrocarbons, and aromatics. An additional benefit realized with this analytical strategy is that the slight restriction induced by the presence of the catalyst in the 3D-printed jet assembly enables backflush via the inlet split vent without the need for additional pressure control or intercolumn-connection devices. The utility of this technique was demonstrated with important aldehyde applications from various segments.

Positive Temperature Coefficient Compensating Heating for Analytical Devices.

Positive temperature coefficient thermistors acting as heating devices are quickly growing in popularity and are being adapted into critical applications in many sectors from medical to space discovery. Positive temperature coefficient heating offers substantial benefits for miniaturized and portable analytical devices in key aspects such as energy efficiency, safety in overheating, size, scalability, and in discovering new thermal management strategies. These heaters can reach 230 °C without additional requirements for regulating electronics. By incorporating positive temperature coefficient technology into a commercial diode array photometric detector, the detector is made suitable for coupling with gas chromatography. The detector cartridge flow cell is heated to a specific target temperature within the range of 70 to 150 °C without impacting the detector's construction material or imparting any negative effect to the surrounding detector system electronics. Applying a temperature of 150 °C to the cell permits analysis of volatile and semivolatile compounds with a boiling point equivalent to that of n-hexadecene (285 °C). Model compounds of alkene homologues from C8 to C16 showed a maximum peak asymmetry of 1.10 with the heated cell design. A high degree of repeatability was observed with RSD of less than 0.01% in retention time and 3% in peak area ( n = 10).

Flow injection gas chromatography with sulfur chemiluminescence detection for the analysis of total sulfur in complex hydrocarbon matrixes.

A fast and reliable analytical technique for the determination of total sulfur levels in complex hydrocarbon matrices is introduced. The method employed flow injection technique using a gas chromatograph as a sample introduction device and a gas phase dual-plasma sulfur chemiluminescence detector for sulfur quantification. Using the technique described, total sulfur measurement in challenging hydrocarbon matrices can be achieved in less than 10 s with sample-to-sample time <2 min. The high degree of selectivity and sensitivity toward sulfur compounds of the detector offers the ability to measure low sulfur levels with a detection limit in the range of 20 ppb w/w S. The equimolar response characteristic of the detector allows the quantitation of unknown sulfur compounds and simplifies the calibration process. Response is linear over a concentration range of five orders of magnitude, with a high degree of repeatability. The detector's lack of response to hydrocarbons enables direct analysis without the need for time-consuming sample preparation and chromatographic separation processes. This flow injection-based sulfur chemiluminescence detection technique is ideal for fast analysis or trace sulfur analysis.

High-throughput gas chromatography for volatile compounds analysis by fast temperature programming and adsorption chromatography.

The synergy of combining fast temperature programming capability and adsorption chromatography using fused silica based porous layer open tubular columns to achieve high throughput chromatography for the separation of volatile compounds is presented. A gas chromatograph with built-in fast temperature programming capability and having a fast cool down rate was used as a platform. When these performance features were combined with the high degree of selectivity and strong retention characteristic of porous layer open tubular column technology, volatile compounds such as light hydrocarbons of up to C7 , primary alcohols, and mercaptans can be well separated and analyzed in a matter of minutes. This analytical approach substantially improves sample throughput by at least a factor of ten times when compared to published methodologies. In addition, the use of porous layer open tubular columns advantageously eliminates the need for costly and time-consuming cryogenic gas chromatography required for the separation of highly volatile compounds by partition chromatography with wall coated open tubular column technology. Relative standard deviations of retention time for model compounds such as alkanes from methane to hexane were found to be less than 0.3% (n = 10) and less than 0.5% for area counts for the compounds tested at two levels of concentration by manual injection, namely, 10 and 1000 ppm v/v (n = 10). Difficult separations were accomplished in one single analysis in less than 2 min such as the characterization of 17 components in cracked gas containing alkanes, alkenes, dienes, branched hydrocarbons, and cyclic hydrocarbons.

Thermal Independent Modulator for Comprehensive Two-Dimensional Gas Chromatography.

We introduce a modulation strategy for comprehensive two-dimensional gas chromatography (GC×GC) with complete thermal independence between the cooling and heating stages and without the need for GC oven heat for remobilization. Based on this approach, a compact thermal independent modulator (TiM) with thermoelectric cooling and micathermic heating has been successfully innovated for use in GC×GC. The device operates externally to a gas chromatograph, does not require liquid cryogen, and has minimal consumables requirements. The augmentation of an additional gas flow stream results in a number of critical chromatographic parameter improvements such as the decoupling of flows of first- and second-dimension columns to attain both efficiency and speed optimized flow in each dimension, the potential for independent retention time locking or scaling in either dimension, the improvement of modulator reinjection efficiency, as well as facilitating back-flushing for the first dimension to enhance system cleanliness and throughput. TiM was found to be useful for chromatographic applications over a volatility range equivalent to nC6 to nC24 under conditions used. Repeatability of retention time for model compounds such as benzene, toluene, ethyl benzene, and xylenes were found to be quite satisfactory with relative standard deviations of less than 0.009% in (1)D and less than 0.008% in (2)D (n = 10). Typical peak widths of 120 ms or less with a relative standard deviation of less than 4.7% were achieved for the aromatic model compounds. In this article, the performance of the modulator is demonstrated and a series of challenging chromatographic applications are presented to illustrate usefulness of the apparatus.

Flow-modulated targeted signal enhancement for volatile organic compounds.

Comprehensive two-dimensional gas chromatography is a technique that is becoming more widespread within the analytical community, especially in the separation of complex mixtures. Modulation in comprehensive two-dimensional gas chromatography can be achieved by manipulating temperature or flow and offers many advantages such as increased separation power, but one underutilized advantage is increased detectability due to the reduction of peak width from the use of a modulator. A flow modulator was used to selectively target analytes for increased detectability with a standard flame ionization detector operated at 100 Hz, without the need for cryogens or advanced modulation software. By the collection of the entire peak volume followed by peak transfer rather than further separation, an increase of 12 times in peak height and detectability was realized for the analytes tested using an internal loop modulator configuration. An external loop flow modulator configuration allowed for more volatile analytes (with k < 5), and demonstrated an analyte detectability enhancement factor of at least 6. The collection loop size can be readily increased with an external loop configuration to accommodate for these naturally broader peaks. This novel flow modulated targeted signal enhancement approach was applied to industrially significant analyses like the analysis of methanol in a hydrocarbon streams. Methanol was detected at 7 ppb with a conventional flame ionization detector and without the need for pre-concentration.

Ultra-trace analysis of furanic compounds in transformer/rectifier oils with water extraction and high-performance liquid chromatography.

A novel approach for the determination of parts-per-billion level of 5-hydroxymethyl-2-furaldehyde, furfuryl alcohol, furfural, 2-furyl methyl ketone, and 5-methylfurfural in transformer or rectifier oils has been successfully innovated and implemented. Various extraction methods including solid-phase extraction, liquid-liquid extraction using methanol, acetonitrile, and water were studied. Water was by far the most efficient solvent for use as an extraction medium. Separation of the analytes was conducted using a 4.6 mm × 250 mm × 3.5 µm Agilent Zorbax column while detection and quantitation were conducted with a variable wavelength UV detector. Detection limits of all furans were at 1 ppb v/v with linear ranges range from 5 to 1000 ppb v/v with correlation coefficients of 0.997 or better. A relative standard deviation of at most 2.4% at 1000 ppb v/v and 7.3% at 5 ppb v/v and a recovery from 43% to 90% depending on the analyte monitored were obtained. The method was purposely designed to be environmental friendly with water as an extraction medium. Also, the method uses 80% water and 20% acetonitrile with a mere 0.2 mL/min of acetonitrile in an acetonitrile/water mixture as mobile phase. The analytical technique has been demonstrated to be highly reliable with low cost of ownership, suitable for deployment in quality control labs or in regions where available analytical resources and solvents are difficult to procure.

Trace-level screening of dichlorophenols in processed dairy milk by headspace gas chromatography.

A headspace gas chromatographic approach based on flame ionization detection has been successfully developed for the determination of parts-per-billion levels of 2,4-dichlorophenol and 2,6-dichlorophenol in processed dairy milk. Under the right environmental conditions, these compounds are produced as products of the reductive dechlorination of pentachlorophenol. Maintaining a highly inert chromatographic system and employing a recently commercialized inert capillary column permits the analysis of 2,4-dichlorophenol and 2,6-dichlorophenol without derivatization. Further, a detection limit improvement of more than a factor of two was achieved by adding sodium sulfate to substantially decrease the solute partition coefficient in the matrix. A detection limit of 1 ng/g and a limit of quantitation of 2 ng/g were attained, and complete analysis can be conducted in < 13 min. Reproducibility of area counts over a range from 20 to 200 ng/g and over a period of 2 days were found to be less than 6% (n = 20). A linear range from 5 to 500 ng/g with a correlation coefficient of at least 0.9992 was obtained for 2,4-dichlorophenol and 2,6-dichlorophenol. Spike recoveries from 10 to 500 ng/g for all the analytes range from 92 to 102%.

Direct Measurement of Trace Elemental Mercury in Hydrocarbon Matrices by Gas Chromatography with Ultraviolet Photometric Detection.

We introduce a technique for the direct measurement of elemental mercury in light hydrocarbons such as natural gas. We determined elemental mercury at the parts-per-trillion level with high precision [<3% RSD (n = 20 manual injection)] using gas chromatography with ultraviolet photometric detection (GC-UV) at 254 nm. Our approach requires a small sample volume (1 mL) and does not rely on any form of sample preconcentration. The GC-UV separation employs an inert divinylbenzene porous layer open tubular column set to separate mercury from other components in the sample matrix. We incorporated a 10-port gas-sampling valve in the GC-UV system, which enables automated sampling, as well as back flushing capability to enhance system cleanliness and sample throughput. Total analysis time is <2 min, and the procedure is linear over a range of 2-83 µg/m(3) [correlation coefficient of R(2) = 0.998] with a measured recovery of >98% over this range.

Selectivity tuning via temperature pulsing using low thermal mass liquid chromatography and monolithic columns.

Low thermal mass LC was applied to the capillary LC separation of a complex insecticide mixture by increasing temperature and decreasing gradients, as well as fast selected temperature pulses to increase resolution of overlapped components. The technology was applied using a new generation of capillary monolithic stationary phases. Considerable peak shifts and selectivity changes were observed for given temperature conditions. The concept of temperature pulsing during an elution profile shows promise for increasing resolution in difficult separations and can provide a relatively simple means to solve coelution problems.

Applications of planar microfluidic devices and gas chromatography for complex problem solving.

The application of planar microfluidic devices in GC for the separation of components of interest otherwise difficult to separate in a single analysis is presented. A variety of configurations were used for parallel chromatography, column effluent splitting, back flushing, selectivity tuning, valve less switching and column isolation, heart cutting, and comprehensive multidimensional chromatography. The synergies of recently commercialized planar microfluidic devices combined with the resolving power of fused-silica capillary columns are demonstrated. Difficult separations were accomplished in one single analysis, such as light hydrocarbons in air with high-moisture content, fixed gases in hydrocarbons, trace sulfur containing compounds in natural gas, and oxygenated compounds in hydrocarbons, among others.

Multidimensional GC using planar microfluidic devices for the characterization of phenolic antioxidants in fuels.

A multidimensional gas chromatographic approach using planar microfluidic devices for Deans switching has been developed and implemented for the characterization of sterically hindered phenolic compounds used as antioxidants in fuels. Detection and quantitation was conducted with MS in selected ion monitoring mode. A complete analysis is conducted in less than 15 min with precision greater than 5.5% at 1 and 25 ppm w/w (ppm(w)). LODs of 50 ppb w/w (ppb(w)) or better in selected ion monitoring mode and a linear range of 100 ppb(w) to 100 ppm(w) with a correlation coefficient greater than 0.998 were attained for all analytes. Unique to this analytical configuration is the use of a mass spectrometer capable of monitoring the column effluent from either dimension by incorporating a high-temperature rotary valve and a three-port planar microfluidic device. High-molecular-weight (C25-C40) fuel contaminants eluting from the first column can be selectively sent to the mass spectrometer for profile characterization in scan mode. These compounds would otherwise be retained substantially by the low-phase-ratio analytical column employed in the second dimension.

Direct measurement of part-per-billion levels of dimethyl sulfoxide in water by gas chromatography with stacked injection and chemiluminescence detection.

Dimethyl sulfoxide (DMSO) is a chemical of industrial significance with many important applications. DMSO is used as an industrial solvent, in drug delivery and healthcare applications, among others. Analysis of DMSO in water typically involves extensive sample preparation, enrichment, and derivatization to improve solute detectability. A novel gas chromatographic procedure has been developed for the direct measurement of trace levels of DMSO in an aqueous matrix, such as potable water. The technology utilizes stacked injection techniques for in-column solute enrichment, a precolumn to enhance solute focusing effects, and sulfur chemiluminescence detection for matrix suppression and sensitivity. A detection limit of 2 parts per billion (ppb) (v/v) of DMSO in water was attained. Relative precision of less than 7% at the concentration of 10 ppb (v/v) of DMSO was demonstrated. A correlation coefficient of 0.9988 was obtained over a range of 2 ppb (v/v) to 100 ppb (v/v). No detectable carry-over was found at the 5 ppb (v/v) level whereas less than 4% carry-over was observed at the 100 ppb (v/v) level. Various sample storage media including glass, polyethylene, and polycarbonate were also studied to minimize solute loss. Recoveries greater than 84% were achieved with all storage media tested. The method was found to be reliable and simple to implement.

Thermal modulation for multidimensional liquid chromatography separations using low-thermal-mass liquid chromatography (LC).

We report on a proof-of-principle experiment with a novel thermal modulation device with potential use in two-dimensional liquid chromatography (LC × LC) systems. It is based on the thermal desorption concept used in two-dimensional gas chromatography (GC × GC) systems. Preconcentration of neutral analytes eluting from the first dimension column is performed in a capillary "trap" column packed with highly retentive porous graphitic carbon particles, placed in an aluminum low-thermal-mass LC heating sleeve. Remobilization of the trapped analytes is achieved by rapidly heating the trap column, by applying temperature ramps up to +1200 °C/min. Compared to the nonmodulated signal, the presented thermal modulator yielded narrow peaks, and a concentration enhancement factor up to 18 was achieved. With a thermally modulated LC separation of an epoxy resin, it is shown that when the thermal modulation is applied periodically, the trapped and concentrated molecules can be released periodically and that the modulating interface can both serve as a preconcentration device and as an injector for the second dimension column of an LC × LC setup. Because of the thermal modulation, a high-molecular-weight epoxy resin could be adequately separated and the different fractions were identified with a GPC analysis, as well as an offline second dimension LC analysis.