Patent Application
20120270330
Detection and quantification of trace chemicals in air are basic and necessary requirements for many applications, including environmental, epidemiological, occupational safety and toxicological studies. In spite of recent advances, the tasks of analyzing multiple chemicals in a complex environment with thousands of interfering chemicals and substances, using a miniaturized and inexpensive device in real time, remains a difficult challenge.
A common strategy in sensor development is to rely on specific binding between a probe and a target molecule, or on molecular recognition. This strategy can lead to highly selective and sensitive detection of the analyte if the binding is strong and specific, but it has two inherent drawbacks. First, the strong binding results in slow recovery of the sensing elements, making it difficult for real-time monitoring of analytes with concentrations that vary with time. Second, this strategy cannot be applied to target analytes, such as aromatic hydrocarbons, that are weakly interactive and do not have a probe molecule to interact with.
A compromise strategy is based on pattern recognition, which uses an array of sensing elements. Another widely used strategy is separation-based techniques, including gas chromatography-mass spectrometry (GC-MS). Conventional GC-MS equipment is bulky, slow and expensive. Portable GC devices using different detectors1, 2, 3, 4, 5, 6, 7, 8, 9, 10 have been developed, but their sensitivity and selectivity are limited, cost is high, and physical size is still bulky for many applications.11 As an effort to miniaturize GC, micromachined columns and preconcentrators have been investigated in combinations with SAW and chemresistor detectors.8, 12, 13 Pn-column micro GC with capillary-based ring resonators have also been developed.14, 15
Thus, there remains a need for efficient devices that can be used for the analysis of air pollutants.
The present invention provides a device that makes it possible to perform real-time detection and analysis of BTEX components in real samples using an inexpensive and miniaturized hybrid specific binding-separation device. The device may be used in occupational health and safety applications as well as for toxicological population studies to determine the presence of organic volatile components in an air sample.
In exemplary embodiments, the present invention provides a device for the detection of chemical moieties in a gas sample, said device comprising (a) a flow control unit; (b) a separation unit; and (c) a detection unit comprising at least one sensor, such as resonating quartz crystal tuning fork, comprising a coating of a sensing material that specifically detects the presence of said chemical moieties in said gas sample; wherein said flow control unit is operably linked to and controls the flow of a mobile gas phase through the separation unit, said separation unit is operably linked to said detection unit and separates the components in said gas sample and provides said separated components to said detection unit; and said resonating tuning fork in said detection unit provides a signal to a high frequency resolution detection circuit that allows a qualitative and/or quantitative detection of said chemical moieties separated by said separation unit.
In specific embodiments, the flow control unit comprises a sample trap loop, a miniaturized valve for sample delivery to the device and a gas pump for introducing said mobile phase to said separation unit.
In specific embodiments the device is a hybrid device that includes a gas chromatograph as a separation unit and a specifically coated quartz crystal resonating tuning fork device as a detection unit. More specifically, the separation unit is a gas chromatography device coupled to a capillary gas chromatography column that is capable of separating volatile organic compounds. Preferably, the gas chromatography column is a two meter gas chromatography column comprised of carbowax and cyanopropylphenyl silicone capillary columns connected in series.
The device is preferably one in which the resonating quartz crystal tuning fork is coated with a highly cross-linked molecular imprinted polymer. The molecular imprinted polymer was prepared by either a polystyrene base cross-linked with divinylbenzene in the presence of the template of interest or just by polymerization of the cross-linking agent, i.e. divinylbenzene in the presence of the template. The template molecules are chosen based on the target analytes. For example, for BTEX detection, aromatic hydrocarbons are the choice target analytes, and xylenes mixture the preferred one target.
In one embodiment, the resonating quartz crystal tuning fork is coated with a highly cross-linked polystyrene with biphenyl imprints to create high density binding sites in the polystyrene for sensitive and selective detection of BTEX.
In other specific embodiments, the resonating quartz crystal tuning fork is coated with a highly cross-linked divynilbenzene matrix imprinted with molecules selected from the group consisting of benzene, toluene, ethylenebenzene, a xylene, a naphthalene and pyrene, or a polyaromatic hydrocarbon.
In other specific embodiments, the template molecule used to imprint the polymer is selected from the group consisting of benzene, toluene, ethylenebenzene, a xylene, a naphthalene and pyrene, or a polyaromatic hydrocarbon.
In specific embodiments, the tuning fork after coating has a resonance frequency of about 32 kHz. In the devices of the invention the tuning fork is connected to a high performance digital counter wherein said counter allows measurement of a resonance frequency of <2 mHz resolution, which resonance frequency allows detection of <4 pg/mm2 substance weight. In specific embodiments, the high frequency resolution detection circuit comprises a chip to allow wireless capability.
It is contemplated that the detection unit comprises a plurality of resonating tuning forks, wherein different tuning forks comprising different coatings to selectively detect chemical moieties in said gas sample.
Specifically contemplated is a device that has at least one resonating quartz crystal tuning fork comprising a coating of a sensing material that allows the detection of one or more of benzene, toluene, ethylbenzene and xylenes in said gas sample. In other examples, the device further comprises a second resonating quartz crystal tuning fork comprising a coating of a sensing material that detects non-BTEX materials in said gas sample. The said coating comprises polycarbosilane derivative, polysiloxane derivative, fluoroalcohol polycarbosilanes, polycarbosilane polysiloxanes, and crown-ether derivatives.
The coating of the forks should be such that it does not impede the resonance of the forks. In exemplary embodiments, the coating comprises between 0.5-3 ug of coating material per tuning fork of a size of 250 μm×430 μm×3 mm.
In specific embodiments, the device is used to detect the presence of chemical moieties in air, and said chemical moieties are aromatic volatile organic compounds associated with traffic pollution.
In some embodiments, it is contemplated the device does not contain a column heating device for the chromatography columns and yet produces rapid separation of the VOCs. In other embodiments, the device further comprises a heating element to heat the chromatography columns.
Further embodiments contemplate devices that further comprise a sampling unit comprising a column packed with a material for the preconcentration of analytes prior to loading of said analytes to said separation unit. For example, such a preconcentration sample unit may comprise a column packed with a material selected from the group consisting of highly cross-linked polystyrene polymers, molecular sieves, carbopack X, carbopack B, carboxen and/or amorphous carbon materials or combinations thereof.
Other examples of the devices within the present invention are those that further comprise a zero filter for providing carrier gas to the separation device. Such a zero filter may be comprised of activated carbon, a molecular sieve, alumina, silica, activated carbon, graphite, polymers or a strong oxidizing material.
It should be understood that the devices of the present invention are to be used in the field and as such preferably are portable. More preferably, the device is a hand-held combined miniaturized gas-chromatograph and a detection apparatus comprising an array of quartz crystal tuning forks coated with molecularly imprinted polymers that specifically detect benzene, toluene, ethylbenzene and xylenes.
Also contemplated are methods of detecting the presence of benzene, toluene, ethylbenzene and xylenes in a gas sample comprising performing a miniaturized gas chromatographic separation of said sample and detecting the presence of BTEX therein using a device of the present invention. The method has a detection a lower detection limit of about 5 parts per million by volume (ppmV) and an upper detection limit of about 250 ppmV. Further the method is able to detect the presence of BTEX in a sample run time of from about 2 minutes to about 10 minutes.
Solid line: SCF-TF.
There is a need for sensitive perform real-time detection and analysis of BTEX components in real air samples that allows on-sight detection of these components in sampled air. In the present invention, there is described an inexpensive and miniaturized hybrid specific binding-separation device for achieving such detection. The invention combines and integrates two strategies, specific binding and chromatographic separation, synergistically into a miniaturized palm-size device that can quickly detect volatile organic compounds (VOCs) in complex environments (
As shown in
The injection unit can contain one or several valves to control the injection from sampling or purging modes and set-up to redirect the sample directly into the separation unit, or to a sample trap loop, or into a predefined volume loop. The injection unit also includes one or two pumps working in pushing or pulling mode that can carry the gas carrier or gas sample through the different units composing the full system.
The sample trap loop of the injection unit consist of an inert tube, with optimized dimensions, containing one (one-stage trap) or several (multi-stage trap) absorbing/adsorbing materials that can be packed in beds separated by inert mesh stoppers.
The separation unit part of the device is a hand-held gas chromatograph that is fitted with capillary columns optimized to allow the gas chromatographic separation of the components of the samples. The coupled capillary column can have different lengths, internal diameters, inner thickness of the polymeric layer and inner materials chosen from the set of alkylpolysiloxanes, fluoroalkylpolysiloxanes, allylpolysiloxanes, cyanoalkyl- or cyanoallyl-polysiloxanes or polyethyleneglycol and its derivatives.
In specific embodiments, the GC is fitted with a tandem arrangement of two different columns coupled in series wherein one of the columns is based on poly(ethylene glycol) and the other is based on cyanopropylphenyl silicone.
The hybrid apparatus of the invention is one in which the separation unit (i.e., GC) is operatively linked to a detection system that is made of a sensing apparatus that contains microelectromechanical resonators linked to a high performance digital counter that detects the presence of the volatile organic compounds in the sample separated in the separation device by use of the signal produced by the microelectromechanical resonators as the VOCs come into contact with them.
The microelectromechanical resonators are quartz crystal resonator based tuning forks. Such quartz crystal resonating tuning forks are well known to those of skill in the art (see e.g., US Patent Publication 20080297008). Such tuning fork resonators work on the principle that where a tuning fork is oscillating at a resonant frequency, compounds present in the gas environment in which the forks are placed will collide with the tines of the tuning fork and cause a mechanical energy loss in the tines. This then creates a change in the resonant oscillation frequency of the fork with the frequency shift depending on the momentum that the tine imparts to the compounds in the gas.
In the present invention, the sensitivity of the tuning forks to BTEX is increased by modifying the tuning forks to give them a coating of an appropriate sensing material. For this purpose, a highly cross-linked polystyrene formed by divinylbenzene (Sigma), a high density of binding sites generated by the biphenyl (BP) (Sigma) template and the porogen solvent (ethylbenzene+o-,p-,m-xylenes)(Mallinckrod) gave the best performance. Synthesis of this polymer was performed according to the methods described by Lieberzeit et al. 2005.27 Based on the chemical nature of the materials, the MIP binding sites bind to the target analytes primarily via multiple Π-Π and van der Waals interactions, which allows for selective but reversible binding. As the VOCs from the air bind to the tuning fork, the resonating frequency is altered and the presence of the VOCs is detected.
While in some embodiments, the device contains only tuning forks coated with materials that will specifically bind to one or more components of BTEX, in other embodiments, the sensitivity of the device can be improved by including tuning forks that are specifically coated with polymers that recognize non-BTEX molecules. In this regard, in one specific example, the tuning forks are coated with a fluoroalcohol polysiloxane polymer (SC-F201, Seacoast Science, Inc.) at a concentration of 20 mg·mL−1 in toluene. This polymer is acidic and hydrophobic which can discriminate non-BTEX components. The use of multiple sensing elements with complementary functions improves the selectivity of the system, which is critical for real sample analysis. It is particularly contemplated that the device may contain 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more tuning forks. Each tuning fork may be coated with a polymer that is derivatized to recognize only one component or it may recognize multiple components.
Molecularly imprinted polymers (MIPs) are synthetic materials with artificially generated recognition sites able to specifically rebind a target molecule in preference to other closely related compounds. In some approaches for preparing MIPs involve the formation of reversible covalent bonds between the template and monomers before polymerization. Then, the template is removed from the polymer by cleavage of the corresponding covalent bonds, which are re-formed when the polymer comes into contact with the analyte (e.g., benzene, toluene etc.) again. The high stability of template-monomer interaction leads to a rather homogenous population of binding sites, minimizing the existence of non-specific sites. An intermediate option is the semi-covalent approach in which the template is also covalently bound to a functional monomer, but the template rebinding is based only on non-covalent interactions and the formation of relatively weak non-covalent interactions (i.e. hydrogen bonding, ionic interactions) between template molecule and selected monomers before polymerization. This approach is by far the most used for the preparation of MIPs, mainly due to its experimental simplicity and to the commercial availability of different monomers able to interact with almost any kind of template. In this approach it is desirable to use a high amount of monomer material so that the excess of free monomers is randomly incorporated to the polymeric matrix leading to the formation of non-selective binding sites. A preferred method for use in the present invention for preparing MIPs is described in Reference 27.
In the present case, the target molecules would be molecules such as aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylenes, naphtalenes, pyrenes and the like. These molecules are mixed with the polystyrene polymers and once polymerization and cross-linking has occurred the target molecules are removed from the polymer leaving behind a “recognition site” for those molecules. The coating materials are typically made of polymers based on polystyrene (PS) or polyurethane (PU). The coating may be presented as a uniform coating on the sensing surface or alternatively may be in the form of a microparticle-coating. In specific examples described below, several molecules were used as templates of MIPs (e.g.: benzene, toluene, biphenyl (BP) and pyrene (Pyr)).
For one particular case, the molecular imprinted polymer for BTEX detection can be synthesized by a typical bulk polymer preparation scheme were the reaction mixture is maintained in a sealed deoxygenated vial at a temperature of 60-80° C. during 24-48 hours. The reaction mixture can be composed of different relationships of template, cross-linking agent, monomer and initiator. Particularly, for this example the template is xylenes, the cross-linking agent is divinylbenzene, the monomer could be styrene or low molecular weight polystyrene and the initiator is azobis-(isobutyronitrile); all of these compound in a relationship of 14:2:1:0.25 in weight or in a relationship of 14:2:0:0.25 to get a more compact structure. After the reaction ends the solid block of polymer is mesh to a dust-like powder and washed several times with few milliliters of xylenes. Finally, a mass of 2 mg of the polymer is suspended in a polystyrene/xylenes solution (2 mg in 1 mL) by sonication during three hours. This suspension is later used to cast or spin-coat the prongs of the tuning forks. Using the devices described herein the inventors were able to analyze samples of air for the presence of pollutants such as ethylbenzene, xylenes, naphtalenes, pyrenes. This work has successfully demonstrated that it is possible to perform real-time detection and analysis of BTEX components in real samples using an inexpensive and miniaturized hybrid specific binding-separation device. The system allows the detection and analysis of low ppmV detection of BTEX in complex samples within 3-4 minutes. The hybrid device is more selective than sensors based on specific binding only; and faster, simpler and more miniaturized than conventional portable GCs. It achieves the improved performance without pre-concentration, programmed heating elements and processes. The system is appropriate for occupational health and safety applications as well as for toxicological population studies.
These devices and the methods of use of the same are described in greater detail in the examples herein below.
Target Analytes:
BTEX is a group of aromatic volatile organic compounds (VOCs) associated with traffic pollution and represents more than 80% of automobile exhaust and other related traffic processes. Many epidemiological studies have associated BTEX to adverse effects in human health mainly on the nervous and hematopoietic systems with some incidence on cancer development.16 However, fundamental uncertainties persist due to the lack of appropriate tools for their detection and analysis in an efficient, fast, inexpensive, easy, and reliable way.
Quartz Crystal Tuning Forks (TFs) Sensors:
These are microelectromechanical resonators that provide intrinsic sensing properties, including low noise, low power consumption and high quality factor (˜10,000 in ambient air) associated to high sensitivity of mass or force (pN) detection (˜4 pg/mm2).17-19 The tuning forks have a resonance frequency of 32.768 kHz (Newark (none Electronics), a small size of 250 μm×430 μm×3 mm and an effective spring constant of 20 kN·m−1. The thermal noise of the tuning fork is small with rms oscillation amplitude of 4×10−4 nm for the prongs at room temperature.17, 20, 21 A high performance digital counter was built which allows us to measure the resonance frequency with ˜2 mHz resolution, or equivalently 4 pg/mm2 of mass detection limit. In addition, a Bluetooth chip connected to a microcontroller gives wireless capability to the detector. This enables transmission of data to other wireless-enabled systems, such as a computer or a cell phone, improving signal processing, storage and further transmission capabilities.
Sensing Materials Modifying the TF Sensors:
To obtain the desired selectivity22-26 for detection of BTEX components in complex environmental matrices, the tuning forks must be modified with appropriate sensing materials.
For this purpose, molecularly imprinted polymers (MIPs) were synthesized and screened to provide the needed selectivity as well as sensitivity, response time and reversibility (
The tuning forks were first coated with a hydrophobic layer by immersing them in toluene containing 10% (v/v) phenyltrimethoxysilane for 24 hrs. After rinsing the tuning forks with toluene and drying out with clean air, they were dipped 20 mM dodecanetiol in isopropanol for 60 minutes. In this way the quartz surface and also the electrodes of the tuning forks became hydrophobic. Subsequently the tuning forks were coated with BP-MIP (MIP-TF) and SC-F201 (SCF-TF) polymer. The masses of the polymer coatings ranged from 0.5-3 μg, and the sensitivities of the tuning forks vary within 20%.
Chromatographic Separation:
Gas samples were prepared by injecting microliter amounts of the liquid analyte into 40 L Tedlar bags filled with synthetic air (breathing quality) in order to get the desired concentration. Additional dilutions of samples were performed with a precision gas diluter or by mixing the contents of Tedlar bag with air, using an inert gas pump and timed valve switching. An inert gas pump provides a constant flow of air through a coupled stainless steel capillary column, made of two 2 meters long columns in series. The inner filling materials of the two columns are carbowax and cyanopropylphenylsilicone (Quadrex Inc.), respectively, to provide optimal separation performance. As best shown in
The detector was made of TFs coated with BP-MIP, and its response to toluene is shown in
Although the detection limits of the MIP-TF sensing element are below 1 ppmV, it should be noted that the chromatographic separation, i.e. the Sep-TF detection, has detection limits of few ppmV levels. The difference is due to the flow rate dependence of the MIP-TF sensing element. Signal response for the MIP-TF sensor depends strongly on the sampling flow rate, reaching a steady state signal for flow rates higher than 100 ml·min−1; however this high flow rate can not be reached with the chromatographic separation mainly because of two aspects: first one, the separation is not efficient at this high flow rate values and second one, such a big flow rates can not be reached with miniaturized gas pumps that are needed for a wearable device. The volume and geometry of the detection chamber that houses the tuning forks could also have an effect on the detection limits. We have examined the dead volume of the chamber, and found no obvious change in the detection limits for dead volumes varying from 50 to 500 μL.
Separation of BTEX
The selectivity of BP-MIP has been systematically tested using common interferents, such as alcohols, ketones and humidity (
Several calibration tests indicate a linear dynamical range from ˜5 ppmV and up to 250 ppmV without hysteresis after the analysis of high concentrations. Beyond this, a stability test with the same sensing platform during seven months shows less than 5% of dispersion for BTEX detection; more than 200 tests with different concentrations were performed during this period of time.
It is worthy to mention that columns based on silica particles packed in ⅛″ internal diameter Teflon tubing of ˜10-15 cm were also effective for separation of BTEX and offered an alternative for separation (
Selective Detection of BTEX with the Hybrid Device.
In order to ensure a reliable analysis of BTEX mixtures without false positives or false negatives for real samples, we have tested the hybrid system using various interferents commonly found in ambient air. The selectivity coefficients, defined as the ratio of the response to each of the analytes to the interferents, range between 3-40 for common interferents, such as polar hydrocarbons, chlorinated hydrocarbons and some aliphatic hydrocarbons (Table 2). These selectivity coefficients were determined with samples having concentrations for the interferent at least three times higher than for the BTEX sample (BTEX sample: 40-50 ppmV; Interferent: 150 ppmV). We found good discrimination capability, due to the hybrid selective bind-separation approach. First, the BP-MIP TFs provide an intrinsic selective detection of benzene, toluene, and ethylbenzene & xylenes. Second, the separation mechanism allows us to separate analytes from the interferents in time domain and provide additional discrimination capability.
As a matter of fact, the signal for benzene at 150 ppmV does not depend on the concentration of acetone when this latter one is changing from 50 to 250 ppmV (
Detection of BTEX in Real Samples
Gasoline vapors, one of the most complicate real samples,29 were used to test the performance of the hybrid device.
Detector Based on Multiple Tuning Fork Sensors
We have added another TF sensing element modified with fluoroalcohol polysiloxane (SCF-TF). In contrast to BP-MIP, SCF-TF is particularly selective and sensitive to polar VOCs, which is used to detect low molecular weight interferents at short elution times.
This application hereby claims the benefit of prior filed co-pending U.S. provisional patent application No. 61/242,256, filed Sep. 14, 2009, of Tao et al., entitled “A HYBRID SEPARATION AND DETECTION DEVICE FOR CHEMICAL DETECTION AND ANALYSIS,” which is incorporated herein by this reference.
This research was funded in part by NIH/NIEHS (U01 ES0106064-01) via the GEI program awarded by the National Institutes of Health (NIH). The United States government has certain rights to this invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2010/046702 | 8/25/2010 | WO | 00 | 5/8/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61242256 | Sep 2009 | US |
