Patent Application
20180164260
Not applicable.
Not applicable.
Over the past several decades, the progress in microfabrication technology has revolutionized the fields of computing and signal processing as well as the automobile industry. The miniaturization of well-established analytical instruments is another paradigm in which the use of this technology has produced significant advancements. One such example is the gas chromatography (GC) system, which is used in various scientific, medical, and industrial settings to separate and analyze volatile organic compounds (VOCs). The monitoring of VOCs is of interest in various applications, including homeland security, space and fossil fuel exploration, worker exposure assessment and biomedical diagnostics. Miniaturized GCs (μGCs) are being intensely developed to enable the rapid diagnosis of VOCs in remote locations with low cost and low consumption. A typical μGC consists of three components: an injector/pre-concentrator for sample introduction, a micromachined column for VOC separation and a single detector or array of detectors located at the column exit to identify the separated compounds.
The majority of standalone μGC systems reported to date address the manual assembly of separately fabricated μGC components using commercially available off-chip fluidic interconnects. This approach is commonly known as the hybrid integration method. The hybrid integration method has certain benefits, such as the optimization of components and the absence of thermal crosstalk between individual μGC components.
Nevertheless, the implementation of μGC systems in a hybrid format is time consuming, cumbersome, expensive and prone to error. In addition, the hybrid format degrades the overall performance of μGCs due to the presence of cold spots between transfer lines. The condensation of compounds in these cold spots can result in extensive peak broadening. In particular, high boilers experience peak broadening in a more profound manner, which can negatively impact their analysis.
In contrast, chip-scale GC systems, in which all three components are located on a single chip, may suffer from thermal crosstalk between the individual μGC components. Thermal crosstalk can occur during two chromatographic events: 1) the desorption process of the pre-concentrator and 2) the temperature programming of the column. Thermal crosstalk could potentially change the retention time of a compound and also produce undesirable drift or noise in the detector signal.
These issues can be avoided in chip-scale GC platforms by developing an optimal back-end detector technology. Recently, there have been advancements in detector technology, and a wide variety of chemical detectors have been developed.
The micro helium discharge photoionization detector (μDPID) has demonstrated a minimum detection limit of approximately 10 pg, which is the same as that of a flame ionization detector (FID), which is mainly used in bench-top GC systems. The high sensitivity of μDPIDs reduces the system's reliance on the front-end pre-concentration step, which is typically performed to improve the detection limit of the detector. In addition, the μDPID can be easily integrated with a separation column. These two features of μDPIDs make them a promising candidate for developing chip-scale GC systems, thereby addressing the above-mentioned issues and improving the overall cost and performance of this technology.
The first μGC, which was developed in 1979, included a monolithic integrated sample injection loop, a 1.5 m long separation column and a separately fabricated thermal conductivity detector (TCD). The hybrid μGC system required the TCD to be integrated after fabrication. In addition, TCDs generally have poor sensitivity (e.g., detection limit of 10 parts per million) and also produce an unstable baseline when operated under flow and temperature programming conditions (the TCD temperature must be controlled to ±0.1° C. or better for baseline stability).
Sandia National Laboratories launched μChemLab™ for homeland security applications in 1998. Their research is unique in that it was focused on the development of a truly monolithic integrated μGC system. Their system included a preconcentrator, a separation column (86 cm long, 100 μm wide and 400 μm deep) and a magnetically actuated pivot plate resonator (PPR) fabricated on a silicon-on-insulator (SOI) wafer. Although the fabrication and subsequent coating mechanism for the integrated chip were demonstrated, no chromatogram was published using an on-chip PPR detector.
In addition, the heart of a GC system is the separation column which is crucial for overall performance of a chromatographic analysis. There have been intensive studies in the development of miniaturized separation columns, and in this regard micromachined silicon-glass chips have been a focus of several research studies. These columns can consume low power (less than 100 mW) and enable rapid temperature programming (up to 60° C./s). Additionally, it is possible to make micro columns of desired shapes and geometrical patterns which are otherwise unobtainable in traditional GC columns. These micromachined silicon columns have been successfully integrated with other microfabricated components to develop portable gas chromatography systems in both research and commercial laboratories.
The stationary phase coated inside microfabricated channels plays a central role in the separation process, and there have been considerable efforts to develop new stationary phase materials for micro columns (μCs). Several reports have been recently published on the use of monolayer-protected gold, carbon nanotubes, polymers, metal-organic frameworks, atomic layer-deposited alumina, sputtered oxides or graphite, and silica nanoparticles as stationary phases in μCs. Despite these encouraging studies, there is still a need to overcome a number of limitations and critical points. First, a considerable band broadening and/or tailing has been observed leading to a loss in separation power. Second, some of these microfabricated columns show limited chemical selectivity—although they show promising separation performance—thereby confining their applications. It is therefore desirable to develop and evaluate new stationary phases that can overcome the aforementioned limitations.
In one embodiment, the present invention provides a miniaturized gas chromatography (μGC) systems that can rapidly perform an analysis of volatile organic compounds (VOCs) in an extremely compact and low-power enabled platform.
In another embodiment, the present invention provides a compact chip-scale GC platform that has dimensions of 1.5 cm×3 cm. This lab-on-a-chip GC has three essential elements—a sample injector, separation column and detector—all on the same platform.
In another embodiment, the present invention provides a compact chip-scale GC platform that requires a loading time of a few seconds to introduce a VOC mixture into the chip, with the subsequent analysis performed in less than a minute for compounds with a wide range of boiling points (110-216° C.). This embodiment of the present invention is particularly suitable for producing low-cost and efficient μGC systems for the rapid analysis of compounds in real-time situations.
In another embodiment, the present invention provides a microfabrication method for the single chip integration of the key components of a μGC system in a two-step planar fabrication process. The 1.5 cm×3 cm microfluidic platform includes a sample injection unit, a micromachined semi-packed separation column (μSC) and a micro helium discharge photoionization detector (μDPID). The sample injection unit consists of a T-shaped channel operated with an equally simple setup involving a single 3-way fluidic valve, a micropump for sample loading and a carrier gas supply for subsequent analysis of the VOCs. This preferred embodiment only requires a loading time of only a few seconds and produces sharp and repeatable sample pulses (full width at half maximum of approximately 200 ms) at a carrier gas flow rate that is compatible with efficient chromatographic separation.
In another embodiment, the present invention provides a compact chip-scale GC platform having a wide variety of VOCs with boiling points in the range of 110-216° C. that can be analyzed in less than 1 minute by optimizing the flow and temperature programming conditions. Moreover, the analysis of four VOCs at the concentration level of 1 part per million in an aqueous sample (which corresponds to a headspace concentration in the lower parts-per-billion regime) was performed with a sampling time of only 6 s.
In another embodiment, the present invention provides a compact chip-scale GC platform having a linear dynamic range over three orders of magnitude. The embodiments of the present invention may be used to monitor hazardous VOCs in real time in industrial workplaces and residential settings
In another embodiment, the present invention provides a high-speed and efficient gas chromatographic separation of a mixture of organic compounds using semi-packed columns (SPCs) coated with room temperature ionic liquids (RTILs). A 1 m long, 240 μm deep, 190 μm wide column with embedded circular micropillars of 20 μm in diameter and 40 μm post spacing may be fabricated using microelectromechanical systems (MEMS) technology.
In another embodiment, two conventional RTILs were deposited inside the channels of the SPCs, and these columns were tested for gas chromatographic separation of a mixture of 12 compounds spanning a wide boiling point range (80 to 214° C.). The separation was achieved in 45 seconds with a separation efficiency of approximately 3000 plates/m.
In yet other embodiments, the present invention may use a dynamic coating method to deposit one or more RTILs in the channels of microfabricated SPCs. These columns may be used to separate a number of different chemical mixtures comprising both polar and non-polar compounds. These RTIL-coated SPCs displayed sharp and symmetrical peaks, offered high separation efficiency, and increased the separation speed. The number of theoretical plates obtained using helium as a carrier gas was as high as 2300 plates per meter.
In yet another embodiment, a [BPyr][NTf2]-coated column may also be used in the separation of BTEX and naphthalene.
The embodiments of the present invention using RTILs may be used as excellent stationary phases for SPCs, thereby dramatically expanding the range of complex mixtures that could be analyzed using a micro gas chromatograph.
In yet another embodiment, coating of the silicon surface with atomic layer deposited alumina or atomic layer deposited hafnium oxide prior to the deposition of an RTIL produced significant improvement in separation performance. The separation efficiency as measured by the plate numbers (or number of theoretical plates) was found to be approximately 8,000 plates per meter (“plates/m”). This number is two times plate number obtained by directly immobilizing the RTIL on silicon surface. This shows that the surface on which an RTIL is immobilized plays a major role in separation performance. Other surfaces that can be used include gold, platinum, other inert metals, metal carbides, and metal nitrides.
In other embodiments, the present invention provides a miniaturized gas chromatography system integrated on single chip comprising a sample injection unit having a T-shaped configuration; a separation column having an inlet, an exit and an interior surface and; at least one detector located at the separation column exit.
In other embodiments, the present invention provides a miniaturized gas chromatography system that reduces the need for heated interconnect lines between individual components of μQC reducing the footprint, cost and power budget for the operation of μQC technology.
In other embodiments, the present invention provides a miniaturized gas chromatography system that reduces band-broadening of compounds having high molecular weight thus improving the performance of μQC technology.
In other embodiments, the present invention provides a miniaturized gas chromatography system including a plurality of separation columns each having an interior surface, the interior surfaces coated with one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with an alumina surface, the alumina surface coated with a one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with a hafnium oxide surface, the hafnium oxide surface coated with a one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the at least one detector, i.e. micro discharge photoionization (μDPID), has a response that remains linear to an injected mass of the test compounds.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the at least one detector, i.e. micro discharge photoionization (μDPID), has a response that remains linear to an injected mass of the test compounds over three orders of magnitude.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the at least one detector, i.e. micro discharge photoionization (μDPID), has a minimum detection limit of 10 pg comparable to commercialized flame ionization detector (FID).
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the detection limit of 10 pg can be improved further by the optimizing gap between the excitation electrodes, the collector volume, packaging of the detector and density of micro-plasma.
In other embodiments, the present invention provides a miniaturized gas chromatography system having the ability to operate under temperature programming conditions to reduce analysis time.
In other embodiments, the present invention provides a miniaturized gas chromatography system that can be integrated with other types of detectors such as micro thermal conductivity detector (μTCD).
In other embodiments, the present invention provides a miniaturized gas chromatography system having a separation efficiency of approximately 3000 plates/m.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein separation is achieved in 45 seconds with a separation efficiency of approximately 3000 plates/m.
In other embodiments, the present invention provides a miniaturized gas chromatography system having a separation efficiency of approximately 8000 plates/m.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with a gold surface, the gold surface coated with a one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with a platinum surface, the platinum surface coated with a one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with an inert metal surface, the inert metal surface coated with a one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system including a pump in communication with a branch of the T-shaped sample injection unit, the pump creates a negative pressure to load a gaseous sample present above the headspace of a liquid into a fluidic channel formed by two branches of the T-shaped sample injection unit and away from the separation column. This preconcentrates the sample.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the miniaturized gas chromatography system has a fast sample introduction (full width at half maximum of approximately 200 ms at room temperature) and repeatable injection of samples into the separation column.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein system of claim 1 can be improved further by heating the T-shaped injector making it comparable to the commercially available GC injectors.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the T-shaped sample injection unit of can be further used to inject liquid samples. Thus, there is potential for this injector technology to be utilized for other analytical tools/applications beside μGC.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the T-shaped sample injection unit of can be replaced with micropillars coated with variety of commercially available adsorbents to trap analyte of interest (selective, nonselective adsorption) and release them on demand through thermal desorption process.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the pump after creating the negative pressure creates a positive pressure which injects a sample into the separation column.
In other embodiments, the present invention provides a miniaturized gas chromatography system further including a valve which cooperates with the pump to create negative pressure and positive pressure.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with a material that increases the uniformity of the one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with an oxide that increases the uniformity of the one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface coated with an oxide surface, the oxide surface coated with one or more room temperature ionic liquids.
In other embodiments, the present invention has a separation efficiency of approximately 8000 plates/m and where the separation of 21 compounds is achieved in approximately 3 minutes.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface is coated with a metal carbide surface and the metal carbide surface is coated with a one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system wherein the interior surface of the separation column has a silicon surface, the silicon surface is coated with a metal nitride surface and the metal nitride surface is coated with a one or more room temperature ionic liquid films.
In other embodiments, the present invention provides a miniaturized gas chromatography system integrated on single chip comprising a sample injection unit; a separation column having an inlet, an exit and an interior surface; at least one detector located at the separation column exit; and the sample injection unit including micropillars coated with adsorbent to trap analytes of interest and release them on demand through a thermal desorption process.
In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.
Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.
The integration of the μGC components on a common substrate reduces the footprint of this technology, enhances the overall performance and allows for a lower fabrication cost. Furthermore, each component can perform its role effectively and rapidly; for example, sample injection unit 110, which may be made of a plurality of branches that form a T-shape 111 as shown in
The circuit shown in
All VOCs listed in Table 1 were purchased from Sigma-Aldrich (St. Louis, Mo.) with >99% purity.
Silicon wafers (n-type, 4 in. diameter, 500 μm thickness, single side polished) and Borofloat wafers (4 in diameter, 700 μm thickness, double side polished) used to fabricate the embodiments of the present invention were purchased from University Wafers (Boston, Mass.) and Coresix Precision Glass (Williamsburg, Va.), respectively. Ultra-high-purity helium (UHP 300) was purchased from Airgas (Christiansburg, Va.). Fused silica capillary tubes (100 μm I.D. and 200 μm O.D.) were purchased from Polymicro Technologies (Phoenix, Ariz.). The miniaturized pump (P/N SP 270 EC-LC-L) and 3-way latching solenoid valve (LHLA0521111H) were purchased from Schwarzer Precision (Germany) and Lee Company (Westbrook, Conn.), respectively.
For a preferred embodiment of the present invention, a method of chip fabrication follows the following processing steps. As shown in
The top and cross-sectional images of the μSC using scanning electron microscopy (SEM) are shown in
The performance of the sample injection unit was characterized in terms of the peak parameters (full width at half maximum [FWHM], peak height and area). Five compounds—heptane, toluene, chlorobenzene, ethylbenzene and p-xylene—were used. Each compound was separately injected at flow rates set to five discrete values of 0.3, 0.45, 0.72, 1 and 1.4 mL min−1. These flow rates correspond to inlet pressures of 10, 15, 20, 25 and 30 psi. The loading time of 500 ms was used for all VOCs. The outlet of the injector was connected to the FID installed in the conventional GC system (HP 7890). Triplicate runs were performed for each value, and the average values of the FWHM and peak height were plotted, as shown in
The results illustrate that for all VOCs, the FWHM depends on the injector flow rate condition. All compounds experienced a sharp decrease in FWHM between 0.3 and 0.72 mL min−1 followed by a more gradual decrease. A similar trend has been observed previously and when a 20 cm-long uncoated capillary tube was connected between a conventional GC injector and the FID. The inverse proportionality of the FWHM with the flow rate can be attributed to different factors. First, increasing the carrier gas flow rate will increase the sample injection rate from the sample loop to the detector, which reduces the extra-column band broadening. Second, the solutes are swept faster, and the longitudinal diffusion in the mobile phase is decreased as a result. Molecules diffuse in the carrier gas from the region of high concentration to that of lower concentration over time.
Increasing the flow rate decreases the time that molecules spend in the injector and connecting tubes, and therefore, the diffusion of molecules in the carrier gas decreases, which results in lower FWHM values. Furthermore, the on-chip injector tested herein operates at room temperature, which can contribute to the differences in the FWHM of high-and low-volatility compounds. In summary, the overall decreases in the FWHM for heptane, toluene, chlorobenzene, ethylbenzene and p-xylene over the entire flow rate range were 40%, 59%, 59%, 50% and 64%, respectively.
Because the FID is sensitive to the mass flow rate, the decrease in the FWHM was compensated for by a corresponding increase in the peak height to maintain uniform peak area.
Furthermore, the repeatability of the sample injection unit was investigated by monitoring the change in the peak parameters (FWHM, peak height and area) over multiple headspace injections. For this purpose, toluene was chosen as a test compound, and multiple injections were performed in succession approximately every 30 s. The flow rate in this experiment was set to 1.4 mL min−1. The sample injection unit produced highly repeatable results, with less than 5% variation in the standard error values for peak parameters over six injections. The FWHMs for these injections were approximately 200 ms.
The μDPID of an embodiment of the present invention using T-shape unit 111 has a minimum detection limit of approximately 10 pg, a response time of approximately 200 ms and highly stable excitation electrodes over a long period of time. To evaluate the linear range of a μDPID for this embodiment of the present invention, the inlets of the detector were connected to injectors A and B of the conventional GC system. The pressures of injectors A and B were set to 15 and 10 psi, respectively. A DC voltage of 550 V was applied (using PS-310, Stanford Research Systems) to create the discharge, and the bias voltage was set to 30 V. This value of the bias voltage was selected based on previous work, which showed the enhanced sensitivity of the detector at 30 V.
Four test compounds—toluene, tetrachloroethylene, chlorobenzene and ethylbenzene—were used. The different headspace volumes of the test compounds sealed in a vial were sampled using an autosampler (7359A) module to ensure repeatable injections. Assuming ideal gas law behavior, the mass of the compound injected from a saturated vapor above the pure liquid could be calculated from the injection volume and split ratio used for the injection. The injected mass was in the range of 10 pg to 10 ng. The response of the detector was measured in terms of the peak area for each injection. The peak area was selected as an indicator for the quantitative data analysis (instead of peak height) based on our previous results, which indicated that the μDPID is a mass flow rate-sensitive detector (MSD). For any MSD, variations in the flow rate could give rise to changes in the peak height or width; however, negligible changes are observed in the peak area. Therefore, the quantification of a compound in terms of peak area is more accurate. Each data point was repeated three times, and the average peak area was plotted against the injected mass of each test compound (
These results illustrate that the response of the detector using the T-shape unit 111 remains linear to the injected mass of the test compounds over three orders of magnitude. The R-squared values and their respective slopes obtained by linear regression analysis (forced zero Y-intercept) have been indicated for each curve. Moreover, as evident from
The sensitivity of the detector is defined as the signal output per unit mass of the compound in the carrier gas. For MSDs, the sensitivity S, is defined as
where A is the integrated peak area, and W is the mass of the compound. Therefore, the slopes of the curves in
The chip-scale GC platform of the present invention was tested following the characterization of the sample injection unit and μDPID. Twelve VOCs—toluene, tetrachloroethylene, chlorobenzene, ethylbenzene, p-xylene, n-nonane, bromobenzene, n-decane, 1,2-dichlorobenzene, n-undecane, 1,2,4-trichlorobenzene and n-dodecane—were selected to evaluate the performance of the embodiment. The following experiments were performed to demonstrate the sampling, separation and detection of compounds with boiling points in the range of 110-216° C. as quickly as possible through the optimization of the flow and temperature programming conditions.
The chip was configured as discussed above with the carrier gas and auxiliary helium supplied by the conventional GC system. The pressures on these supplies were set to 22 and 10 psi, respectively, corresponding to a flow rate of 0.77 mL min−1 at the chip outlet port. The chip temperature was maintained at 40° C. During sampling, the pump was turned on to load the sample into the fluidic connection between the chip and e valve. A sampling time of 3 seconds was considered sufficient for testing the compounds. The chromatogram in
where d is the distance between the peak maxima for the two compounds, A and B. The resolution between p-xylene and C9/bromobenzene was 2.16, and that between C9/bromobenzene and C10 was 4.60.
To reduce the analysis time and increase the peak width of the high-boiling-point compounds, a series of experiments was performed under flow programming conditions. One of the best chromatographic results is shown in
Temperature is one of the most important variables in GCs and an effective way of optimizing the analysis time. The superposition of temperature (Tinitial=40° C., ramp=30° C. min−1, Tfinal=65° C.) and flow programming (Pinitial=22 psi, ramp=35 psi min−1, Pfinal=35 psi) has been demonstrated in the chromatogram shown in
A separate sample containing high-boiling-point compounds, including n-nonane, bromobenzene, n-decane, 1,2-dichlorobenzene, n-undecane, 1,2,4-trichlorobenzene and n-dodecane, was also analyzed. The carrier gas pressure was increased to 35 psi, whereas the auxiliary helium pressure was maintained at 10 psi respective, which corresponds to a flow rate of 0.9 mL min−1 at the chip outlet port. The chip temperature was increased to 70° C. The loading time was increased to 6 because of the low vapor pressure of these compounds. The resulting chromatogram is shown in
The chip performance was also evaluated for the analysis of VOCs in aqueous media. Four VOCs—toluene, chlorobenzene, ethylbenzene and p-xylene—were diluted to a concentration of 1 ppm (1 mg L−1) in deionized (DI) water. A small aliquot (500 μL) of the prepared sample was transferred into a 1.8 mL vial. The vial was subsequently sealed to avoid compromising the sample integrity. A sampling time of 6 seconds was considered sufficient for extracting VOCs from the headspace of the sample. The concentration in the headspace was calculated using Henry's Law. The headspace concentrations for toluene, chlorobenzene, ethylbenzene and p-xylene were 270, 152, 323 and 314 ppb, respectively. The representative chromatogram is shown in
The embodiments of the present invention are further enhanced by the use of semi-packed columns (SPCs) which provide higher separation efficiencies and sample capacities compared to the open-channel counterparts. The columns of the present invention exhibit the properties of both packed columns—high sample capacity—and open tubular columns—high speed separation, high separation efficiency, and low pressure drop.
In contrast to open channel columns, the SPCs exhibit smaller loss of efficiency with the increase in flow rate, thereby making them suitable for high speed separations. Among a number of different stationary phase materials, one class of compounds that may be used with the embodiments of the present invention in microfluidic columns is ionic liquids (ILs).
ILs constitute a group of organic salts which are liquid below 100° C.; and the ILs that are liquid at room temperature are commonly known as room temperature ionic liquids (RTILs). ILs are polar, chemically inert, nonflammable, thermally stable, easy to synthesize, possess low vapor pressure, and their selectivity can be easily tuned by altering the constituent cation or anion; and hence they have been widely used as stationary phases in conventional gas chromatography. Currently, more than 300 ILs are commercially available and more than a trillion ILs have been estimated. Remarkably, RTILs can separate both polar and non-polar analytes. They have abilities to undergo multiple solvent-solute interactions which include: nonbonding and π-electron interactions, dipole-type interactions, hydrogen bonding (basicity and acidity) interactions, and cohesion and dispersion interactions. ILs show significant hydrogen bond acidity, a feature that is absent in commonly used conventional stationary phases, such as poly(siloxane) and poly(ethylene glycol). Unlike conventional stationary phases which provide limited selectivity variations, ILs offer excellent opportunities for fine-tuning the selectivity of the stationary phase.
As shown in
The performance of RTIL-coated SPC can be further enhanced by depositing a thin layer alumina or hafnium oxide or other appropriate materials. The performance increase is shown in
The RTILs [BPyr][NTf2] and [P66614][NTf2] were obtained from Ionic Liquids Technology, Inc. Silicon wafers were obtained from University Wafers. Borofloat wafers were purchased from Coresix Precision Glass, Inc. Fused silica capillary tubes of 100 μm internal diameter and 200 μm outer diameter were obtained from Polymicro Technologies. A two-part epoxy system was obtained from J-B Weld. Acetone was obtained from Spectrum Chemicals. Benzene, n-hexane, n-heptane, n-octane, n-nonane, benzene, toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, isobutylbenzene, n-butylbenzene, styrene, benzyl chloride, 2-chlorotoluene, 2,5-dichlorotuluene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene, naphthalene, 2-nitrotoluene, 3-nitrotoluene, 4-nitrotoluene, and a standard mixture of C7-C30 saturated alkanes (in hexane) were obtained from Sigma Aldrich. A mixture containing 8 fatty acid methyl esters (FAME #20 mix) was obtained from Restek Corporation. Gasoline (octane rating of 87), diesel, and kerosene were obtained from local stores. Air and ultrapure helium were purchased from Airgas, Inc. Methane was obtained from Air Liquide, Inc. Ultrapure hydrogen for flame ionization detector (FID) was produced by using Parker Domnick Hunter hydrogen generator All of these chemicals were used as received without further purification.
The separation columns were produced employing microelectromechanical system (MEMS) processes including photolithography, etching, and silicon-glass anodic bonding. The fabricated columns were 1-meter long, 240 μm deep and 190 μm wide with circular pillars of 20 μm diameter and 40 μm pillar spacing.
A silicon wafer was cleaned using standard RCA cleaning, and then it was primed with hexamethyldisilazane (HMDS) which acts as an adhesion promotor for a photoresist. This is followed by the deposition of 8 μm thick AZ9260 photoresist by spin coating at 2000 rpm for 1 minute. The photoresist-coated wafer was then soft-baked at 110° C. for 1 minute. The pattern from a chrome mask was transferred to the soft-baked wafer by using ultra violet light source and a mask aligner. The wafer was then hard-baked at 110° C. for 3 minutes. Following this, the wafer was anisotropically etched using Alcatel deep reactive ion etcher (DRIE) via a standard Bosch process with SF6 as etching and C4F8 passivation reactants. The photoresist was removed by treating the wafer with acetone and subsequently with piranha solution. A 10-nm layer of aluminum oxide was deposited at 300° C. via atomic layer deposition using trimethylaluminum and water as precursors. Similarly, hafnium oxide was deposited using tetrakis(dimethylamino)hafnium and water as precursors. The etched silicon wafer was anodically bonded with a 700 μm thick Borofloat wafer (Coresix Precision Glass) at 1250 V and 400° C. for 45 minutes, and finally the wafer was diced into individual devices.
The outlet and inlet of the column were then connected to fused silica capillary tubing (internal diameter: 100 μm and outer diameter: 200 μm) using J-B Weld twin tube epoxy in order to connect the micro column to GC injection port and detector. The total length of the two capillary tubes was 27 to 30 cm.
A freshly prepared solution of an RTIL in acetone at a concentration of 4 mg/mL to 16 mg/mL (for dynamic) and 2 mg/mL (for static) was used for deposition of RTILs into the channels of the SPCs. Both static and dynamic coating methods were used. Static coating did not produce high yields for SPCs since air bubbles were formed during the coating procedure preventing the deposition of the RTILs in some of these columns. Therefore, the columns coated with the dynamic technique for our chromatographic evaluation of RTIL-functionalized SPCs. The columns coated with 8 mg/mL of RTIL solution showed optimal performance and these were evaluated in more detail.
The separations were performed using an Agilent 7890A GC system equipped with an automatic sampler (7693A) and two FIDs. Helium was used as a carrier gas. Before installation, the columns were flushed with nitrogen for 30 minutes. Following the installation, each column was conditioned from 30 to 200° C. at a ramp rate of 2° C./min followed by holding at 200° C. for 15 minutes, while the inlet pressure was held at 10 psi during the column conditioning. The inlet temperature was kept at 280° C. and the detector temperatures was kept at 300° C. during the measurements.
[P66614][NTf2] and [BPyr][NTf2] were selected as the model RTILs since they are expected to show altered selectivity due to the presence of distinctly different cations. Among these two RTILs, [P66614][NTf2] has been previously used as a GC stationary phase and it has been shown to be stable up to 380° C. RTIL [BPyr][NTf2], however, has not been used as a stationary phase in GCs.
A thin film of [BPyr][NTf2] was coated inside a fused silica capillary tubing. Then the coated capillary was heated in the GC oven. The temperature was ramped from 30 to 320° C. at a rate of 10° C./min. FID signal during this time did not show any significant rise in its baseline indicating that the RTIL is stable at least up to 320° C., which is sufficient for the separation of a wide range of analytes.
The coated SPCs were imaged using an optical microscope.
The separation efficiency of each column was evaluated by determining the height-equivalent-to-a-theoretical plate (HETP) or plate numbers (N) as a function of the average carrier gas linear velocity. The retention time of methane was taken as the hold-up time. HETP or N were determined at 100° C. isothermal conditions using naphthalene as a probe. The following equations were used for calculations.
where, ū is average linear velocity, L is the length of a column, tM is the retention time of methane, tR is the retention time of the compound of interest, N is the number of theoretical plates (or plate numbers), wh is the peak width at half height of the compound of interest, and H is HETP.
The separation performance of the columns was characterized by separating a number of different mixtures. The first test mixture was a 15-component mixture comprising hydrocarbons, aromatic halides, and nitroaromatic compounds. These compounds include heptane, benzene, toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, styrene, benzyl chloride, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene, naphthalene, 2-nitrotoluene, 3-nitrotoluene, and 4-nitrotoluene. The boiling points of these compounds range from 80.1 to 238° C. These toxic chemicals are widely distributed in environmental and occupational settings, and there has been substantial scientific interest in monitoring these chemicals.
Another group of analytes was a standard mixture containing 8 fatty acid methyl esters (FAMEs) obtained from Restek Corporation. Analysis of FAMEs is very important for food and biodiesel characterization. The different FAMEs present in our test mixture were methyl caprylate (C8:00), methyl caprate (C10:00), methyl laurate (C12:00), methyl myristate (C14:00), methyl palmitate (C16:00), methyl stearate (C18:00), methyl oleate (C18:01), and methyl linoleate (C18:02). The mixture was dissolved in hexane (50 mg of total FAMEs was dissolved in 1 mL hexane).
The third mixture was a sample of gasoline. Automotive gasoline is a complex mixture of mostly low-boiling hydrocarbons, but it also contains hazardous chemicals such as BTEX (benzene, toluene, ethylbenzene, and xylenes) and naphthalene. The separation and determination of BTEX, naphthalene, and other hazardous chemical entities in gasoline and gasoline-contaminated environmental samples is crucial for efficient health risk assessment and management of occupational and environmental exposure to these chemicals.
Overall, these improved columns provide a series of advantages over the currently available micro columns. The first advantage is that these columns provide very sharp and symmetrical peaks. A careful examination of chromatograms of the earlier works or the commercially available micro columns shows that there is a significant peak broadening, fronting, or tailing. These effects arise due to either pooling of stationary phase at the corners of the rectangular columns or limited sample capacity of the columns. Given the fact that the present invention achieved symmetrical peaks for a number of polar and non-polar compounds, it is presumed that there are less pooling effects in the case of RTILs.
In addition, the embodiments of the present invention were able to immobilize the RTILs without having to pretreat the columns with sodium chloride although this pretreatment may enhance the separation performance. It is presumed that the presence of pillars may partially prevent the pooling of the RTILs inside the column. A highly polar compound, acetone, was passed through a column coated with [BPyr][NTf2], and as evident from
The second advantage of this research is that it provides a route to create micro columns having different selectivity for separation of a wide range of chemical mixtures. Changing or modifying the constituent ions of an IL will modify the solvation properties of an IL, thereby altering the relative retention times. The separation performance of these columns can also be enhanced by modifying the surface with metal oxides before coating with an RTIL.
Although researchers are successful in the development of micro columns for high speed and highly efficient separation by using stationary phases, such as sputtered silica or graphite and atomic layer deposited alumina, these stationary columns do not offer tunable selectivity. The third advantage offered by the embodiments of the present invention is the speed of separation. For example, some embodiments were able to baseline separate BTEX in less than 50 seconds and baseline separate naphthalene from gasoline in less than two and a half minutes. This is a substantial improvement in speed as compared to the current state-of-the-art micro GC technology. As discussed above, SPCs provide high efficiency and flatter Golay plots. It is therefore possible to work at higher flow rates to increase the separation speed without a significant loss in separation efficiency.
In yet other embodiments, surface modification of the channels of SPCs prior to coating with RTILs may be performed. The surface of the silicon channels, including the pillars, may be modified by depositing a thin film of hafnium oxide or aluminum oxide via atomic layer deposition (ALD). A thin film of an RTIL was subsequently deposited on the surface of the oxide layer, and the performance of these columns was evaluated by separating a number of complex chemical mixtures, including a 21-component mixture of hazardous chemical pollutants, a sample of kerosene, and a sample of diesel. A vastly improved separation performance, including an enhancement in separation efficiency was observed due to the presence of an oxide film underneath the RTIL layer. The number of theoretical plates, as measured by using naphthalene as a probe at 100° C., for the column with an RTIL immobilized on aluminum oxide was found to be as high as 8,000 plates/m, which is more than 2 times the number of theoretical plates obtained by depositing the same RTIL on silicon surface. This shows that by using RTILs in SPCs to create chip-based separation columns, devices can be created that rival the commercial capillary columns.
In yet other embodiments, other surface coatings that may be applied as an intermediate layer between the silicon surface and the one or more room temperature ionic liquid films include oxides and other materials, such as metal, metal carbides, metal nitrides, known to those of skill in the art that will increase the uniformity of the one or more room temperature ionic liquid films.
The RTILs may be deposited inside the channels of the separation columns using a dynamic coating procedure at room temperature by employing a freshly prepared solution of an RTIL in acetone. The concentration of RTIL in the solution was varied from 0.4 to 1.6% (w/v). The entire column was first filled with a solution of RTIL, and the solution was removed by using nitrogen gas at a pressure of 10 psi. After removing the bulk of the solution, the column was placed under vacuum to evaporate the residual solvent in quiescent conditions.
The maximum plate numbers (Nmax) for silicon surface coated with [BPyr][NTf2] was found to be 3822±195 per meter at an optimum average flow velocity of 31 cm·s−1. By coating [BPyr][NTf2] on hafnium oxide surface, the Nmax was found to increase by 18%. Interestingly, a column prepared by coating [BPyr][NTf2] on alumina surface produced Nmax of 8,000 plates m−1, which is more than double the maximum plates numbers obtained by immobilizing the same RTIL on silicon surface. Another RTIL, [P66614][NTf2], immobilized on alumina surface produced Nmax of 7,158 plates m−1. This indicates that highly efficient separation columns can be designed by suitably modifying the silicon surface prior to RTIL deposition.
In other embodiments, the present invention provides another way to preconcentrate the sample. Instead of using the T-shape unit, this embodiment uses micropillars coated with adsorbents to trap analytes of interest and release them on demand through thermal desorption process. This arrangement may be integrated with the T-shaped sample injection unit.
While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure.
This application claims the benefit of U.S. Provisional Application No. 62/431,476 filed Dec. 8, 2017 and U.S. Provisional Application No. 62/467,387 filed Mar. 6, 2017 both of which are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62431476 | Dec 2016 | US | |
| 62467387 | Mar 2017 | US |
