Functionalized metal oxides as a stationary phase and a surface template for micro gas chromatography separation columns

Abstract

The present invention provides a detector and method for detecting substances in complex mixtures. The detector includes a microfabricated preconcentrator, a separation column with an on-chip thermal conductivity detector, a controller for controlling flow and thermal management and a user interface. The thermal conductivity detector includes a first resistor located at an inlet of the separation column and a second resistor located at an outlet of the separation column.

Claims

1. A detector for measuring water organic compounds (WOCs) in an aqueous sample, comprising: a trap having a first cavity and purge extractor; said purge extractor defining a second cavity for receiving an extraction of WOCs from an aqueous sample, said purge extractor having spaced apart inlets and a first outlet and second outlet, one of said inlets configured to allow the passage of an aqueous sample to be analyzed into said second cavity and said other inlet configured to allow the passage of inert gas into said second cavity to purge WOCs from said second cavity; said inlets spaced apart to promote the uniform distribution of said inert gas and said aqueous sample; said first outlet and said second outlet configured to allow waste water to drain from said second cavity and to prevent water from entering said trap; said trap comprised of a distribution network, said distribution network is comprised of a plurality of posts, said posts configured to have an affinity for a compound to be detected; said posts having a metal oxide deposited thereon by atomic layer deposition to create a stationary phase medium; said metal oxide is aluminum oxide, hafnium oxide or zirconium oxide; and said first cavity is coated with SiO.sub.2 nanoparticles (SNPs).

2. The detector of claim 1 wherein when in a vertical position water is prevented from entering into said trap by locating said second outlet above said first outlet.

3. The detector of claim 1 wherein said metal oxide is aluminum oxide, hafnium oxide or zirconium oxide and is comprised of a plurality of layers.

4. The detector of claim 3 wherein said metal oxide is aluminum oxide, hafnium oxide or zirconium oxide and is comprised of a plurality of layers each of said layers is about 1-2 angstroms in thickness.

5. The detector of claim 4 wherein said posts are round, elongated or are in a zigzag pattern.

6. The detector of claim 5 wherein said posts are further coated with a Tenax TA poly (2,6-diphenylene oxide) film.

7. The detector of claim 1 wherein said first cavity is coated with SiO.sub.2 nanoparticles (SNPs).

8. The detector of claim 1 wherein said trap is connected in series with a column, said column defining a third cavity in which a plurality of posts are located, said posts configured to have an affinity for a compound to be detected.

9. The detector of claim 8 further including a heater connected to said column.

10. The detector of claim 8 wherein said post of said column are coated with aluminum oxide, hafnium oxide or zirconium oxide.

11. A detector for measuring water organic compounds (WOCs) in an aqueous sample, comprising: a purge extractor; a trap defining a first cavity; a thermal conductivity detector defining a second cavity; a column defining a third cavity; said purge extractor defining a fourth cavity for receiving an extraction of WOCs from an aqueous sample, said purge extractor having spaced apart inlets and a first outlet and second outlet, one of said inlets configured to allow the passage of an aqueous sample to be analyzed into said fourth cavity and said other inlet configured to allow the passage of inert gas into said fourth cavity to purge WOCs from said fourth cavity; said inlets spaced apart to promote the uniform distribution of said inert gas and said aqueous sample; said first outlet and said second outlet configured to allow waste water to drain from said fourth cavity and to prevent water from entering said trap; said trap comprised of a distribution network; said thermal conductivity detector includes a first resistor located at an inlet of said separation column and a second resistor located at an outlet of said separation column; and said column in which a plurality of posts are located, said posts configured to have an affinity for a compound to be detected.

12. The detector of claim 11 wherein when in a vertical position water is prevented from entering into said trap by locating said second outlet above said first outlet.

13. The detector of claim 12 wherein said distribution network is comprised of a plurality of posts, said posts configured to have an affinity for a compound to be detected.

14. The detector of claim 13 wherein said posts having a metal oxide deposited thereon by atomic layer deposition to create a stationary phase medium on said metal oxide.

15. The detector of claim 14 wherein said metal oxide is aluminum oxide, hafnium oxide or zirconium oxide and is comprised of a plurality of layers.

16. The detector of claim 15 wherein said metal oxide is aluminum oxide, hafnium oxide or zirconium oxide and is comprised of a plurality of layers each of said layers is about 1-2 angstroms in thickness.

17. The detector of claim 16 wherein said posts of said column are coated with aluminum oxide, hafnium oxide or zirconium oxide.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) 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.

(2) FIG. 1: Schematic representation of ALD-based stationary phase coating for an embodiment of the present invention. (I) anisotropically etched semipacked column, (II) and (III) atomic layer deposition of 10 nm alumina film and (II) 95 cycles with each cycle having four steps (a) TMA exposure (b) purge (c) H.sub.2O exposure (d) purge, (IV) anodic bonding of ALD-coated wafer with glass substrate, (V) silane deactivation for 24 hours.

(3) FIGS. 2A and 2B: Depict a 1 m-long semipacked column with integrated 20 m circular microposts and post spacing of 40 m for an embodiment of the present invention.

(4) FIG. 3: Effect of silane functionalization on a ALD-coated semipacked column at 50 C., 250:1 split ratio, 0.1 L injection volume and 10 psi column pressure (compound identification in the order of elution n-hexane (solvent), n-nonane and n-decane).

(5) FIG. 4: Separation performance of an ALD-coated/silane functionalized semipacked MEMS column at 50 C., 7.5 psi inlet pressure with split ratio of 75:1. Compound identification in order of elution 1. dichloromethane (solvent) 2. n-hexane (k=0.23) 3. benzene (k=0.38) 4. toluene (k=1.01) 5. Tetrachloroethylene (k=1.55) 6. chlorobenzene (k=1.90) 7. Ethylbenzene (k=2.34) 8. p-xylene (k=2.75) 9. n-nonane (k=4.00).

(6) FIG. 5: Diagram of an embodiment of the present invention showing the topology for the extraction and analysis of water organic compounds. A back-side heater is utilized for thermal desorption of analytes from the TPC for chromatographic analysis.

(7) FIGS. 6A, 6B and 6C: An image of an embodiment of the present invention showing a detector fabricated (A) PE, (B) TPC and (C) GC chip with embedded resistors utilized as the thermal conductivity detector for aqueous analysis.

(8) FIG. 7: Set of chromatograms indicating increase in TCD response with increase in purge time and concentration of WOCs.

(9) FIG. 8: Graph showing the TCD response variation with increasing purge time for a sample containing four WOCs at 1 ppm concentration.

(10) FIGS. 9A, 9B, 9C and 9D: (A) Micro-devices (B) images showing micro-posts in PC (C, D) polydimethylsiloxane coating on the interior wall of the column channel.

(11) FIGS. 10A. 10B and 10C: (A) GC system block diagram of one embodiment of the present invention, (B) Operation cycles and timing, (C) Operation temperature, control Signal and Time.

(12) FIG. 11: FM response for separation of test compounds with integrated PC and SC. Injection performed using flow manipulation technique with PC heated to 200 C. and desorption flow rate set to 1 mL/min.

(13) FIGS. 12A and 12B: Chromatogram of (A) gasoline vapor and (B) standards using Zebra GC

(14) FIG. 13: Chromatogram of gasoline vapor sampled at ambient pressure and temperature using sorbent tubes containing 200 mg of Tenax TA. Gasoline vapor was analyzed by thermal desorption coupled to GC-FID using conventional column containing (5% phenyl-, 95% dimethyl-polysiloxane). Desorption temperature and time were 300 C. and 25 min respectively. Toluene peak at 1.4 min (16 ppmv) and p-xylene (14 ppmv) peak at 2.4 min. Benzene and ethylbenzene not detected. Temperature programming: 35 C., hold for 10 min, 5 C./min to 150 C., final hold time 1 min.

(15) FIG. 14: A PTGC chip of an embodiment of the present invention.

(16) FIG. 15: Schematic of the interface of the PTGC chip with a Texas Instruments MSP430 series controller for automatic operation of the system, for on-site water monitoring. Valve A and B are inert control valves (LFYA1226032H) while the rest are (LHLA0531111H) latching low power valves, from The Lee Co. The pump (270 EC-LC-L) from Schwarzer Precision consumes 0.6 W. A 0.2 pm filter (WU-81054-42) from Cole Parmer precedes the pump. A 14 oz., 48.8 Wh, 11.1V Li-Ion battery powers the system.

DETAILED DESCRIPTION OF THE INVENTION

(17) 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.

(18) As shown in FIG. 1, in one preferred embodiment, the present invention provides a method for fabricating semipacked or packed columns that may be about 1 m-long, 190 m wide with 180 m deep channels, with 20 m integrated circular micropillars, with 40 m post spacing. In other embodiments, the posts may have other cross-sectional geometries and be arranged in regular patterns and irregular patterns. In a preferred embodiment, the posts may be rectangular and be arranged in a zigzag pattern.

(19) The process starts with a standard RCA cleaning of 4 inch, 500 m thick n-type single side polished silicon wafers (University Wafers). After performing wafer priming using hexamethyldisilizane (HMDS), AZ9260 photoresist (AZ Electronic Materials) is spin coated at 2000 rpm to acquire an 8 m thick resist layer. This is followed by soft-baking the resist-coated wafer at 110 C. for 1 minute. The wafer is then exposed for 50 seconds using a mask aligner (Karl Suss) to transfer features from a chrome mask on to the soft-baked resist-coated wafer. Following exposure, the features are developed in AZ400K developer (AZ Electronic Materials) and DI water (3:1). Afterwards, the developed wafer is hard-baked for 3 minutes at 110 C. SF6 and C4F8 are used to anisotropically etch the wafer (via a standard Bosch process) using an Alactel deep reactive ion etcher (DRIE).

(20) Following etching, the photoresist is first removed using acetone and the wafer is then placed in a piranha etch solution to remove any organic residue left during either the etching or photoresist removal steps (FIG. 1, step-I). Representations of a 1 m-long and 180 m-deep microfluidic channel are shown in FIG. 2 after anisotropic etching and photoresist removal. As shown, one or more microposts 101-113 are created as described above.

(21) The ALD coating process for aluminum oxide using trimethylaluminum (TMA) and water as precursors is well established. The successive reactions during aluminum oxide growth on the silicon surface from TMA and water is described by two sequential reactions as given below.
SiOH+Al(CH.sub.3).sub.3.fwdarw.SiOAl(CH.sub.3).sub.2+CH.sub.4(A)
SiOAl(CH.sub.3).sub.2+2H.sub.2O.fwdarw.SiOAlO*(OH)+2CH.sub.4(3)

(22) The asterisk in reaction (B) represents an oxygen shared between adjacent aluminum atoms on the surface. During the first reaction (A), the surface silicon hydroxyl species react with TMA forming a dimethyl-terminated aluminum species and methane as a byproduct. The excess TMA will not react further with the surface, resulting in a self-limiting process. In the second reaction (B), two water molecules react with the dimethyl-terminated aluminum species, forming an oxide bridge between adjacent aluminum species and a terminal hydroxide on each aluminum atom as well. The terminal hydroxide then allows for the production of additional aluminum oxide monolayers by the same two step cycles.

(23) Thin alumina film deposition on the etched silicon wafer was performed at 250 C. using a thermal ALD Cambridge NanoTech system (Savannah S100). TMA (Sigma-Aldrich) and water precursors (deionized water used from clean room) were used to deposit a 10 nm aluminum oxide film. Each ALD deposition cycle consists of a 15 ms sequential precursor gas exposure of either TMA or H.sub.2O (20 sccm flow) followed by a 4 second purge (FIG. 1, step-II) to provide a film thickness of 1.07 per cycle. The wafer was coated for approximately 16 minutes (95 cycles) to get the desired film thickness of 10 nm. The surface of the ALD-coated silicon was characterized using an atomic force microscope (Bruker Nanoscope V) with a silicon tip in a tapping mode. A 1 m.sup.2 area was scanned at a rate of 0.98 Hz and recorded using 512 scan lines. The 3-D surface plot of a 10 nm aluminum oxide film gave a root mean square roughness of 0.264 nm, clearly demonstrating that the aluminum oxide film generates a highly smooth and uniform surface.

(24) The thin-film deposition rate of the ALD system may be improved further by employing a separated ALD process. It has been shown that growth rates as high as 1.2 nm/sec may be achieved for aluminum oxide thin films using this technique. Therefore, this modified ALD process could be integrated into MEMS fabrication of columns for high throughput stationary phase deposition.

(25) After performing the ALD coating, the etched silicon wafer was anodically bonded with a 700 m thick and 4 inch wide double side polished Borofloat wafer (Coresix Precision Glass) at 1250V and 400 C. for 45 minutes (FIG. 1, step-III). The wafer was then diced into individual devices for functionalization and chromatographic testing. In order to provide an interface between the GC injection port and detector, a 220 m outer diameter by 100 m internal diameter fused silica capillary tubing (Polymicro Technologies) was attached to the column inlet and outlet ports using Epoxy 907 (Miller Stephenson). The surface of the ALD oxide film and the glass cover was finally functionalized by filling each column with 10 mM chlorodimethyloctadecylsilane (CDOS, Sigma Aldrich), in toluene solution for 24 hours at room temperature (1, step-IV).

(26) Before performing chromatographic tests, each device was first purged for 30 minutes with nitrogen. This was followed by column temperature conditioning in the GC oven for approximately one hour (35 C. ramped at 2 C./min to 150 C.) at a constant inlet pressure of 10 psi.

(27) A bench top 7890 series Agilent GC system equipped with a flame ionization detector (FID), an electronic pressure controller and an autosampler (7359A) unit was used for injection and detection purposes. Both the injector and detector temperatures were maintained at 280 C. Ultrapure nitrogen (>99.99%) was purchased from Air Gas Ltd, USA and used as the carrier gas. Methane (99% grade, Matheson Trigas, Ohio) was used for calculating the gas velocities to generate Golay plots. All chromatographic test mixtures were prepared using standard HPLC grade chemicals (Sigma-Aldrich).

(28) Ideally a chromatographic peak should be symmetrical, or Gaussian, in shape. Asymmetric peaks, usually in the form of peak tailing, influence the column performance. A tailing factor (T.sub.f) can be used to quantify the peak tailing and is given by:

(29) $T_f=\frac{a+b}{2a}$

(30) where both a and b, measured at 5% of the peak height, represent the front and back half-widths respectively. A T.sub.f=1 represents a perfectly symmetric peak (a=b), whereas peak tailing (a<b) will yield values of T.sub.f>1.

(31) Alumina as an adsorbent material has been successfully used for conventional gas-solid chromatographic separations. Although the ALD-based alumina coating (without any alkylsilane based treatment) of one embodiment of the present invention clearly demonstrates separation capabilities, significant asymmetric peak shapes are observed as shown (FIG. 3) and tailing factors (T.sub.f) of 5.95 and 7.15 were calculated for n-nonane and n-decane peaks (7000 ppm diluted in n-hexane) respectively. This may be the result of unknown changes in the alumina structure from the ALD process that affects the equilibration of the solutes with the stationary phase. FIG. 3 also shows the separation profile for the same hydrocarbon mix on the atomic layer deposited alumina column after silane functionalization. The chromatographic performance of the alumina adsorbent is significantly enhanced using alkylsilane functionalization performed after anodic bonding of the cover glass wafer (tailing factor of 1).

(32) Organochloroalkylsilanes may also be used to functionalize the medium to enhance retention times and peak symmetry.

(33) The primary performance criterion for GC columns is expressed by the number of theoretical plates (N) for a certain compound and is calculated from chromatogram using the following formula.

(34) $N=5.54*{\left(\frac{t_r}{w_{1/2}}\right)}^2$

(35) where t.sub.r is the retention time and w.sub.1/2 is the peak width at half height. Another related parameter that takes into account the column length (L) for the comparison of different columns is the height-equivalent-to-a-theoretical-plate (HETP), where

(36) $\mathrm{HETP}=\frac{L}{N}$

(37) The ALD-coated/silane functionalized semipacked MEMS columns provided up to 4200 plates per meter using n-decane (diluted in dichloromethane) at 50 C. at an optimum inlet pressure of 7.5 psi (linear velocity of 8.15 cm/sec). Since a meter long microfabricated column coated with gas-solid stationary phases (sputtered silica or carbon nanotubes) typically yields around 4000-5000 plates/meter, accordingly, in one embodiment, the present invention provides a method for creating micro GC columns that is comparable to other non-conventional coatings.

(38) The separation capability of the embodiment was tested using a mixture containing eight compounds: n-hexane, benzene, toluene, tetrachloroethylene, chlorobenzene, ethylbenzene, p-xylene and n-nonane diluted in dichloromethane to a concentration of 15000 ppm. An autosampler was used to inject 0.3 L of the mixture with a split ratio of 75:1. The column pressure and oven temperature were maintained at 7.5 psi and 50 C. respectively. The resulting chromatographic separation (FIG. 4) provided good resolution and retention of the compounds, with the entire separation achieved in less than a minute at isothermal conditions. Capacity factors (k), the ratio of the time spent in the stationary phase relative to the mobile phase, ranged from 0.23 for hexane to 4.0 for n-nonane, with the separation driven primarily by differences in boiling points as reflected by the order of elution.

(39) In one application, the present invention may be used to separate mixtures with either straight chain alkanes or aromatic hydrocarbons. The functionalized alumina thin film of the present invention also shows similar separation capabilities as afforded by conventional alumina particles.

(40) The long-term and thermal stability of the present invention was also evaluated. Chlorobenzene and n-nonane (diluted in dichloromethane) were utilized as probes with multiple injections made using an autosampler unit. The deviations in the plate numbers (N) and the capacity factors (k) of each compound were calculated from the 1.sup.st, 25.sup.th, 50.sup.th, and 75.sup.th injections as presented in Table 3.

(41) TABLE-US-00003 TABLE 3 Long-term stability of ALD based stationary phase with multiple injections (GC testing at 50 C., 7.5 psi) Chlorobenzene n-Nonane Plate Number Capacity Factor Plate Number Capacity Injection (N) (k) (N) Factor (k) 1.sub.st 3368 1.87 4380 3.77 25.sup.th 3508 1.85 4359 3.82 50.sup.th 3319 1.85 3962 3.89 75.sup.th 3886 1.85 3948 3.90 % RSD 7.3 0.64 5.7 1.6

(42) These results demonstrate that after multiple injections the silane-functionalized alumina coating remains stable, with less than 8% deviation in plate numbers and less than 3% deviations in k values. Similarly, using the same compounds, the thermal stability of the ALD based stationary phase was also evaluated. The column was subjected to thermal cycles of 100 C., 150 C. and 200 C. for 8 hours each under a constant inlet nitrogen pressure of 2.5 psi. After each thermal cycle, the column testing was carried out at 50 C. and 7.5 psi. The deviations in the N and k values are shown in Table 4.

(43) TABLE-US-00004 TABLE 4 Temperature stability test of ALD based stationary phase (GC testing at 50 C., 7.5 psi) Column Chlorobenzene n-Nonane heating for 8 Plate Number Capacity Plate Number Capacity hours @ (N) Factor (k) (N) Factor (k) Before Heating 3990 1.80 4343 3.92 100 C. 3930 1.82 4251 3.94 150 C. 3808 1.80 4308 3.91 200 C. 3128 1.65 4020 3.59 % RSD 10.7 4.3 3.4 4.3

(44) The results show that the alumina based stationary phase remains stable up to 150 C. with small variations in retention times; however, there is some degeneration at 200 C., and column performance deteriorates considerably if heated beyond temperatures of 200 C.

(45) The effect of variations in microfabrication processes on chromatographic efficiency was also considered. Three separation columns were fabricated on different wafers using identical fabrication techniques as discussed above. A 15% variation (RSD) was observed in the plate number values (using n-decane) for the tested columns. This high chip-to-chip variation could be attributed to variations in the end connections.

(46) In yet other embodiments, the present invention concerns a device and method using atomic layer deposited alumina with alkylsilane functionalization for use in gas chromatographic separations. The use of ALD ensures highly conformal film deposition inside complex column designs and affords good selectivity, separations and retention of different compounds. Compared to very recently reported methods for GC columns that utilize sputtering systems for alumina deposition, the present invention achieves very symmetric peaks and separations. Moreover, the present invention is not limited by the column depth; thus improving column flow rate and the sample capacity by increasing the cross-sectional area of the microfluidic channel. In yet other embodiments, the present invention, provides a method for tuning the selectivity of alumina films by using silanes with different functional groups. Since ALD is used for very thin film depositions, in range of 5-15 nm, there will not be a significant change in the separation performance.

(47) FIGS. 5 and 6 show a preferred embodiment of the present invention that includes detector 700 and a micro purge extractor (PE) 704 for extraction of WOCs from an aqueous sample. PE 704 contains two spaced apart inlets, inlet 708 for the aqueous sample to be analyzed and inlet 710 for a pure inert gas to purge the WOCs from cavity 705.

(48) Spacing the inlets apart promotes uniform distribution. In other embodiments, distribution network 712 may be provided to uniformly spread the sample inside the chip. Similarly, network 714 for the purging gas is used to enhance the interaction between two phases (air and water) inside the chip and to facilitate the removal of WOCs from the streaming water. Other networks may also be used to improve gas flow and distribution at other locations as well. The chip also contains two outlets. Outlet 720 is used for waste water and outlet 722 directs purged WOCs to trap TPC 730 which may be distributed by network 732. Purge outlet 722 directs WOCs to TPC 730. It may be located at the top corner of the PE chip. Device 700 is operated in two phases namely; (1) the extraction phase and (2) the analysis phase.

(49) During the extraction phase, two microfabricated chips (PE 704 and TPC 730) may be connected in a tandem, combined on a single chip or configured to use valve 740 while PE chip 704 is maintained vertically to prevent water from entering into TPC 730 chip 730 via the air or purging gas outlet 710. As shown, this locates outlet 722 above outlet 720, which permits water to drain from outlet 720 while preventing it from entering outlet 722.

(50) With a source of analyte such as a vial connected to sample inlet 708, the aqueous solution is introduced into the PE chip 704 using purified nitrogen. High purity nitrogen gas is supplied through the air inlet 710 of the PE chip trapping WOCs on the adsorbent surface on TPC chip 730, which may include square shaped columns 750 or the other micro pillar configurations described above.

(51) The analyzed mass is calculated from the sample concentration and the volume of water collected during the purged time. During the analysis phase, PE 704 is taken offline and TPC 730 is connected in series with GC column 745, which may be constructed as was described above, with the embedded TCD 755 detector using a six port switching valve 760. Helium is used as a carrier gas while the outlet of the column is connected to the FID of a commercial Agilent HP7890 GC system for verification purposes. The sensor 762 on the backside of TPC 730 is used to monitor the temperature profile of the chip when heated by a heater or heating element 763. A voltage applied to heater 763 on the backside of TPC 730 heats it up from room temperature to 150 C. The desorbed WOCs are separated by GC 745. A 40 mA current is sourced into a Wheatstone bridge with two resistors of TCD 755 in each of its arms. The differential voltage measured across the two resistors enables the detection of WOCs, which is fed into a Keithley 2700 and recorded on a LabVIEW program.

(52) The efficiency of the coated column was evaluated with the TCD switched to the ON condition by applying an 8.3 V DC voltage. This voltage corresponds to a temperature of 95 C. This was measured with helium flowing at the operating pressure of 12 psi. The metric commonly used for column performance is height equivalent to a theoretical plate (HETP) as described above.

(53) The plate number was calculated over a range of column pressures with the constant split injection ratio of 150:1 using chlorobenzene diluted to 2% (v/v) in hexane.

(54) The separation and identification of the four WOCs using only the column and its TCD (without the PE and TPC) was performed by installing the chip inside the GC oven with its inlet and outlet connected to the injector and the GC FID, respectively. A 0.1 ml volume of the sample containing the WOCs diluted to 2% (v/v) in hexane was injected into the GC column for separation and identification of the four WOCs by the chip.

(55) Similarly, for TCD response calibration, five samples (0.5%, 10%, 20%, 30%, and 40% (v/v) in hexane) for each WOC were prepared and tested. A 0.1 ml of each sample was injected three times in succession using the GC autosampler module with the split ratio maintained at 150:1. By using the density, the mass for each WOC was calculated taking the split injection ratio into account.

(56) The fabrication of the TPC was performed on a standard 4 inch wafer using MEMS processing technology. First, photolithography was performed to pattern micro-posts/fluidic ports. The wafer was then subjected to deep reactive ion etching (DRIE, Alcatel) to achieve a depth of 250 m. After stripping the photoresist off the front-side, a 500 nm thick oxide layer was deposited on the backside and the wafer diced into individual chips. The chip was then filled with Tenax TA solution (10 mg ml.sup.1 in dichloromethane) and allowed to evaporate to deposit a thin film (200 nm) of the polymer adsorbent on the cavity surfaces. The chip was then capped with a Borofloat wafer by anodic bonding. Following bonding, the chips were loaded onto the platen of an e-beam evaporator (PVD-250, Kurt Lesker) with the backside facing the crucible. The chips were masked by a stainless steel shadow mask patterned with the features defining the heater and the sensor. Following this, 40 nm/100 nm/25 nm of Cr/Ni/Au was deposited to get nominal resistances of 15 ohm and 250 ohm for the heater and the sensor, respectively. Finally, the devices were unloaded; the shadow masks removed off and fused capillary tubes epoxied into the inlet/outlet ports. The fabrication process of the PE chip followed that of the TPC but without the adsorbent coating and backside oxide/metal deposition.

(57) For the fabrication of the GC column with embedded TCD, a two-step anisotropic etching of silicon was performed for hosting the feedthroughs and the microfluidic channel by spin coating the wafer with S1813. A shallow depth of 2-3 m was achieved which prevented a contact between the metal inter-connects on the Borofloat wafer and the walls of the separation column in silicon upon bonding. A 12 m thick AZ9260 photoresist was patterned with a mask for subsequent deep etching of the channels resulting in 250 m deep channels for the separation. Then, TCD resistors were fabricated on a glass substrate by utilizing a lift-off process of a 40 nm/100 nm/25 nm Cr/Ni/Au stack in the e-beam evaporator. After aligned anodic bonding of the diced detector on glass and the diced separation column on silicon, capillary tubes were epoxied into the inlet/outlet ports. The chip was static coated with polydimethylsiloxane by filling it with a solution of 10 mg ml.sup.1 OV-1 in pentane, followed by carefully sealing one end with wax and pulling a vacuum at the open end. This procedure left a thin layer of OV-1 coating (250 m) on the walls of the column channel.

(58) To avoid changing the concentration of WOCs, a 24 ml cylindrical vial was filled completely with deionized (DI) water leaving no headspace. Both 1 ppm and 500 ppb solutions (v/v) were prepared in two steps. First, 1000 ppm (v/v) solution was made by adding 24 l of each WOC to 24 ml of DI water. Second, the solution was further diluted 1:24 and 1:12 with DI water to achieve concentrations of 1 ppm and 500 ppb, respectively. The solution was analyzed immediately to avoid compromising the sample integrity. Before processing any sample, all parts of the equipment in contact with the sample were demonstrated to be interference free. This was accomplished through a blank run.

(59) Before evaluating the performance of the integrated purge and trap GC system, the heating and sensing elements of the microfabricated preconcentrator, separation column, and the detector were calibrated and the separation performance of the column was evaluated.

(60) A 12 V DC voltage was applied to the heater and the sensor resistance was measured until the resistance representing the desired temperature value was reached. The sensor resistance varied with the applied voltage due to ohmic heating. The final temperature of 150 C. was attained within 7 seconds representing a ramp rate of 20 C. s.sup.1. This condition remained constant during the desorption process of the WOCs trapped on the Tenax TA polymer coating of the TPC.

(61) The maximum plate number (optimum condition) observed for the 2 m long column was about 6200 at 12 psi (flow rate 0.62 ml ml.sup.1.). The column was operating at this optimum flow condition for further investigations.

(62) The separation and identification of the four WOCs was performed by the method described previously. FID was used to verify the chromatogram generated by the TCD. WOCs were successfully separated and detected by the chip within 1.5 min. Next, a calibration curve showing the output of the TCD as a function of the injected WOC concentration was obtained by the method described previously. The injected mass varied from about 3 ng to 23 ng for toluene, 5.4 ng to 43 ng for PCE, 3.7 ng to 29.3 ng for chlorobenzene and 3 ng to 23 ng for ethylbenzene.

(63) Following calibration and performance evaluation of each GC unit, the PE was put in place. The ability of the complete system comprising PE and TPC chips, separation column, and the thermal conductivity gas detector (TCD) to continuously monitor WOCs in the aqueous sample was realized experimentally by the method explained earlier. The aqueous solution was introduced into the PE chip using purified nitrogen at 10 psi. High purity nitrogen gas was supplied through the air inlet of the PE chip trapping WOCs on the adsorbent surface with a flow rate maintained at 0.4 ml min.sup.1 (5 psi) through the TPC chip. The extraction period was varied for three discrete periods of 7, 14 and 21 min. The set of chromatograms in FIG. 7 was generated using 500 ppb and 1 ppm aqueous samples for two different extraction periods. The initial negative dip is due to the sample mixture passing under the reference detector. At this stage, the signal detector experiences the carrier gas and hence is constant. This results in a negative voltage output as explained before. As the sample mixture moves through the column, it is separated over time. When the individual components pass under the sample detector, the reference detector experiences the carrier gas and hence results in positive peaks corresponding to each eluted compound. The second peak is due to trace moisture extracted from the purge chip and is not seen on the FID signal which is insensitive to the trace water content. The increase in peak heights for all WOCs with the increase in extraction time was observed which validates the design. It is also evident that rapid chromatographic separation and detection of all four WOCs within 1.5 min is achieved at room temperature. The method's precision was evaluated by three repetitive analyses for each test. After each analysis, the TCP was heated to 150 C. (conditioning step) to prevent carry over from the previous runs, following which a blank run was performed to conform the same. The change in the detector response (area under the peak) with the purge time was then monitored for a sample containing four WOCs at 1 ppm concentration. The experiment was repeated thrice for three different purging times and the average value was plotted for each WOC.

(64) FIG. 8 shows that the peak area increases with purging time. The increase in the peak area was attributed to the increase in the quantity of nitrogen (inert gas used) that bubbled through the aqueous sample, and consequently, more quantity of WOC moved from the liquid to the vapor phase. Additionally, in streaming mode, a fresher sample entered the PE chip replacing the old one, thereby increasing the amount of WOC purged over time. The results in FIG. 8 indicate that ethylbenzene and chlorobenzene are purged easily from the aqueous sample as compared to PCE and toluene. This is due to their relatively high partition coefficient (K.sub.ow) value, which is defined as the ratio of concentration of a compound in a hydrophobic solvent (usually octanol) to its concentration in water at equilibrium. In other words, it is a measure of hydrophobicity and depends on size, polarity and hydrogen bond strength of a compound. Hydrophilic compounds (with low partition coefficient) are held by a very strong dipole-dipole interaction and hydrogen bond in water and thus could not be easily purged from the sample. Increasing the temperature of the sample should increase the purged amount by supplying enough thermal energy to the molecule to break the dipole-dipole interaction. The sample analyzed during the purge time was collected to determine the percent recovery of each compound. Assuming that the trap is able to capture the entire purged amount, percent recovery is defined as the ratio of the amount that is collected for chromatographic analysis relative to the amount that was originally present in the aqueous sample. Table 5 summarizes the percent recovery for each compound at 1 ppm concentration.

(65) TABLE-US-00005 TABLE 5 List of water organic compounds with their originating sources and potential health risks Potential health Amount Contaminants effect Contamination sources recoverd (ng) Recovery log(K.sub.ow) MCL Toluene Nervous system, Petroleum factories 5.7 23% 2.75 1 mg l.sup.1 liver problems PCE Liver problems, Discharge from factories 5.4 18% 2.57 5 g l.sup.1 increased risk and dry cleaners of cancer Chlorobenzene Liver and kidney Discharge from chemical 9 25% 2.86 0.1 mg l.sup.1 problems and agricultural chemical factories Ethylbenzene Liver and kidney Petroleum refineries 18.7 38% 3.14 0.7 mg l.sup.1 problems

(66) The percent recoveries are lower than those reported in the literature. This can be attributed to the fact that commercial purge and trap systems use high purging gas flow rates (normally 40 ml min.sup.1) and also use traps consisting of a short length micro-bore tubing packed with the granular form of the adsorbent material. Such traps at the cost of high pressure drops and high power consumptions can provide higher adsorption capacity. It is notable that low recoveries have also been reported previously by Sandia National Laboratories in their bench-top (WASP) system described earlier due to the flow limitations in their setup. In addition, embodiments of the present invention have achieved a detection limit of 500 ppb, which is comparatively higher than the commercial purge and trap systems. Part of this is attributed to small sample volumes (in ml) analyzed by the PE chip when compared to the commercial purge and trap systems.

(67) The present invention, in yet other embodiments, provides a micro-scale version of a purging device for the extraction of WOCs from an aqueous sample. The potential application of the chip for on-site monitoring of the aqueous sample when equipped with all necessary GC components as described herein. In other embodiments, to enhance the recovery of analytes may be accomplished by modifying the design of the PE chip and integrating temperature programming ability on this chip.

(68) As shown in FIGS. 9A-9D and FIGS. 10A-10B, in another embodiment, the present invention provides a detector that has two major units: 1) sampling and preconcentration, and 2) separation and detection. PC 1500 chip and SC-TCD 1502 chip have integrated thin film heaters 1505 and 1506 and sensors 1510 and 1512 which are used for temperature control during desorption and separation. The microfabricated components may be integrated with off-the-shelf flow controllers to implement GC flow cyclesloading, injection, analysis, and cleaning. An embedded platform, based on an 8-bit microcontroller (ATmega640, Atmel Corporation), is responsible for fluidic and thermal control. It also implements a user interface, signal processing, and data acquisition circuitry. The system is highly portable, battery powered, and easy to operate. It can be paired with a laptop for device control and data visualization through a Labview application.

(69) PC 1500 is a 13 mm13 mm silicon-glass chip and consists of an array of high aspect ratio micro-posts inside its 1 mm square cavity. Micro-posts 1520-1524 are realized by bulk micromachining of a 4 inch silicon wafer utilizing a deep reactive ion etching process to achieve a depth of 240 m. Micro-posts 1520-1524 may also be configured as described above.

(70) A 1-m thick plasma enhanced chemical vapor deposition (PECVD) oxide layer that acts as an insulator is deposited on the backside. The wafer is then diced into individual chips. The micro-posts are then coated with a thin film (200 nm) layer of adsorbent 1530 which may be Tenax TA followed by capping with a Borofloat wafer via anodic bonding. A 40 nm/230 nm of Cr/Ni stack is deposited which serves as a heater and temperature sensor on the backside of the chip using an e-beam evaporator (PVD-250, Kurt Lesker). The nominal resistance of the heater and sensor is around 15 ohm and 250 ohm, respectively. Finally, fused capillary tubes are inserted and epoxied to the inlet/outlet ports.

(71) For the SC-TCD, a two-step anisotropic etching of silicon is performed. First, a shallow depth of 2-3 pm is achieved which prevents the contact between the metal interconnects on the Borofloat wafer and the walls of the separation column in silicon upon bonding. Second, a 2 m long, 70 m wide and 240 in deep channel is etched into the silicon wafer. TCD resistors 550 are fabricated on a glass substrate using a lift-off process for a 40 nn/100 nm/25 mu Cr/Ni/Au stack deposited employing the e-beam evaporator. The glass and silicon substrates are then aligned and bonded together. The heaters and temperature sensors are fabricated on the backside of the chip using stainless steel shadow mask. Afterwards, the capillary tubes are epoxied into the inlet/outlet ports. The chip is finally coated with a thin layer (250 nm) of OV-1 on the walls of the column channel.

(72) An SEM image of the Tenax TA and OV-1 coating is shown in FIG. 9. The optical image of all fabricated chips is shown as well.

(73) The microfabricated components may be integrated with a (Parker Hannifm Co), multi-way valves (The Lee Co.), and a portable helium cylinder. System control is through an integrated electronic module managed by an 8-bit micro-controller. Latching valves are selected to optimize power consumption and controlled by applying a 100 ms 5 V DC pulse through an H-Bridge. The pump flow rate is adjusted by varying the pulse-width modulation (PWM) duty cycle, which is an important parameter during sample collection. The on-chip temperature sensors 1510 and 1512 are connected in a 3-wire resistance temperature detector (RTD) configuration by using two well-matched current sources with a high precision 24-bit ADC. The reference voltage for the ADC is also generated using these matched current sources through a precision resistor and applied to the differential reference pins of the ADC. This scheme ensures that the span of the analog input voltage remains ratio-metric to the reference voltage and any error in the former due to temperature drift of the excitation current is compensated by the variation of the latter.

(74) On-chip heaters 1505 and 1506 are controlled through PWM channels and a digital proportional control system is implemented as part of the embedded firmware, which generates different profiles for temperature reference signal based on the user input (initial temperature, step, ramp, final temperature).

(75) The TCD is connected in a Wheatstone bridge, driven by 7.5 V DC, with low noise thin film resistors (PF1260 series, Riedon Inc). The differential signal is conditioned and filtered prior to feeding into an ultra-low noise 24 bit ADC (AD7793, Analog Devices). The signal is further filtered digitally, using an on-chip low pass modified Sin c3 filter that also provides 60 Hz rejection. The TCD, along with the entire system, is operated at a data rate of 10 Hz, which provides substantial resolution for the peaks.

(76) Microfabricated components along with the flow controllers, integrated electronic module, and user interface circuitry are assembled in a 30 (l)15 (w)10 (h) box, schematically shown in FIG. 10-a. The box also houses a lithium ion battery (2200 mAh) pack and a small helium cylinder (95 uff, 2700 psi) to make the GC highly portable (1.8 kg). The system can be operated in a manual or automatic mode using the LCD/Keypad based human-machine interface, which has a menu driven system. Once the mode is selected, the screen shows the state of the system in terms of valve positions, temperature readings, pump duty cycle, and sensor value. Sensor data can be visualized and recorded in the Labview application, which receives data packets through USB or Bluetooth interface.

(77) FIG. 10-b shows the timing diagram for the following automated stages: loading, injection, analysis, and cleaning. In the loading stage, the pump applies negative pressure at the PC outlet to load it with VOCs present in the air sample. Once a sufficient sample is loaded in the PC, the valves are switched to flow helium through the bypass path into the SC-TCD. In order to ensure a sharp injection plug, the PC is heated first at a rate of 25 C./s to 200 C. without flow, and then the valve is switched to inject analytes into the SC-TCD. The valve is switched back to the bypass path after injection and this stage typically lasts 10-12 s. Once the analytes are injected, they are separated and simultaneously identified in the SC-TCD. This separation for the analytes of interest (BTEX) takes 1-2 min; the SC can be operated at higher temperatures to reduce the analysis time or to resolve higher boiling compounds. Once the analysis phase is complete, the valves are switched back to flow helium at a rate of 3 mL/min through the PC. The PC is heated several times if necessary, to minimize residual analytes from the previous run. Typically, one temperature cycle (10-12 seconds) is sufficient to desorb the remaining analytes because of the high desorption efficiency of our silicon-based PC.

(78) The integrated electronic module primarily consists of temperature controllers, TCD interface, flow controllers, user interface, and data acquisition circuitry. Each module was tested and optimized separately before integration. For accurate temperature measurement, on-chip sensors were calibrated by placing the MEMS devices in a conventional GC (7890, Agilent, Palo Alto, Calif.) oven to characterize sensor resistance that responded linearly with respect to temperature with correlation coefficient of (le)>0.99. The calibration was completed by updating the firmware with calibration slope and offset, which were computed from the resistance vs. temperature data. The temperature profile required for the PC is very different from that required for the SC; the former requires heating at a high ramp rate (20-100 C./s) to quickly desorb the analytes and generate a sharp injection plug, whereas the SC requires heating at much lower ramp rates (0.2-1 C./s) during the analysis phase to accelerate elution of high-molecular weight analytes. The PC temperature reference was generated through the firmware, and a step input was given to heat the PC to 200 C., which was sufficient to completely desorb 99% of the analytes of interest. The heating ramp rate depends on the thermal mass, power dissipated, and heat losses.

(79) A maximum heating ramp rate of 25 C./s was achieved for the PC, by applying an 18 V DC across the heater resistance (15 LI). Further increases in the voltage resulted in deterioration of the thin film heater due to high current density. The SC was temperature programmed for ramp rates of 20 C./min and 30 C./min. The power consumptions were determined to be 0.5 W and 1.2 W for isothermal operation of the SC at 45 C. and 65 C., respectively.

(80) The sensitivity of the TCD detector was improved by increasing the signal-to-noise (S/N) ratio. The signal was amplified (Gain 32), and the noise was reduced by filtering the signal and packaging the detector in a small aluminum box to mitigate the effects of ambient fluctuations. Once the measurement circuitry was tuned, noise measurements were made under normal operating conditions with the carrier gas flowing and the TCD turned ON. The average peak-to-peak detector noise was 8.08 V and measured for baseline signal captured for 10 s. The power consumption for the TCD and pump operation was 280 mW and 165 mW, respectively. The full measurement cycle of the GC consumes an average power of 2.75 W meaning that the battery can last up to 8 hours (110 full cycles). It is notable that in one embodiment, each helium refill (95 mL, 2700 psi) will last around 10,000 full cycles, meaning that the size of the helium cylinder and subsequently the GC can be considerably reduced.

(81) For optimum operation, the PC was characterized in terms of four parameters: adsorption capacity, breakthrough volume, desorption peak width, and desorption efficiency. While evaluating the maximum adsorption capacity of the PC, the effect of flow rate on the adsorption process was minimized by keeping it to a low value of 1 mL/min analytes were injected into the PC from headspace in sealed 1 mL vials using a conventional GC autosampler module. The split ratio and the injection volume were changed to vary the amount of analyte introduced into the PC. Analytes not retained by the adsorbent bed appeared as a breakthrough peak, which was allowed to return to the baseline prior to heating the PC. The maximum adsorption capacity was defined as the mass retained in the PC when the injection led to 10% immediate breakthrough. The PC can adsorb 30-400 ng of analytes depending on their affinity to Tenax TA. The masses retained were 30 ng, 130 ng, 240 ng, and 350 ng for benzene, toluene, chlorobenzene, and ethylbenzene, respectively. These results indicate that the PC can retain a sufficient amount of compound well above the detection limit of TCD (1 ng) and that it has higher affinity to high boilers.

(82) For breakthrough volume (BV) identification, about 4 ng of each analyte was loaded separately on the PC at a flow rate of 1 ml/min and then 5, 10, 15, 25, and 30 mL of the carrier gas was passed through the PC at the same flow rate. The PC was subsequently heated and the volume of carrier gas, which resulted in a 10% reduction in the total mass retained, was noted.

(83) Another important parameter is the width of the desorption peak that can directly influence the chromatographic resolution achieved by the separation column. The initial desorption peak width attained on the PC at a ramp rate of 25 C./s and flow rate of 1 mL/min, was 4 seconds. This was reduced by first heating the PC without the carrier gas flowing and next, passing the carrier gas through when the chip temperature reaches 200 C. This flow-manipulation technique resulted in a reduction of the peak width at half height (PWHH) from 4 to 8 seconds.

(84) A minimum PWITH of 350 ms was achieved when the desorption flow rate was increased to 2.5 mL/min. It is notable that 99% desorption efficiency for the analyte of interest was achieved by heating the PC to 200 C. The remaining amount was removed by subsequent heating of the PC prior to another run to minimize carry over from the previous adsorption run.

(85) The efficiency of the coated column was evaluated with the TCD switched ON by applying a 7.5 V DC to the Wheatstone bridge. This voltage corresponds to a temperature of 80 C. for the TCD and was measured with helium flowing. The heated TCD elevated the temperature of the column to 32 C. The metric commonly used for the column performance is the height-equivalent-to-a-theoretical-plate (HETP).

(86) The plate number was calculated over a range of column pressures. The maximum plate number (optimum condition) observed was 6200 for 2-m long column at 12 psi (flow rate of 0.7 mL/min).

(87) Further, the embodiment was tested for the separation and identification of six VOCs using the column and its TCD. The SC along with the interface circuitry was installed inside a conventional GC and connected to the injection port and FD with fused-silica capillaries. A mixture of 6 compounds (headspace), containing benzene, toluene, tetrachloroethylene, chlorobenzene, ethylbenzene, and p-xylene, was injected by autosampler through the heated injection port (1 l, 50:1 split ratio). The peaks were found to be well resolved and the separation required less than 2 minutes. Next, a calibration curve showing the output (peak area) of the TCD as a function of the VOC injected mass was obtained. For that purpose, a headspace sample for each VOC was prepared and tested. The split ratio was varied from 120:1 to 50:1 based on the vapor pressure of the VOC. Injected volumes were varied from 0.5 L to 4 L, to achieve the mass injected in the range of 1 to 5 ng.

(88) Once individual chips were tested, the PC was connected upstream of the SC-TCD to test the hybrid integration. The integration was expected to both improve and compromise the system performance of different aspects of the GC. The compact design reduced the transfer lines, thereby reducing the formation of a cold spot that decreases efficiency. On the other hand, the optimal flow rate for operating the PC and MSC was different; therefore, there was a trade-off in establishing the flow rate for the integrated system. The inlet port of the PC was connected to the GC injector (280 C., split ratio 50:1) and it was loaded with a mixture containing six compounds (headspace, 1 L). For initial testing, the outlet port of SC was connected to the FID detector of a conventional GC. The flow rate was set to 1 ml/min, for which the PWHH was measured to be 0.8 seconds from the PC and the SC exhibited well resolved peaks for the analytes of interest. As shown in FIG. 11, the six compounds were separated and identified in less than 2 min.

(89) The TCD is sensitive to flow perturbation during the switching of carrier gas into the PC; therefore, the integrated TCD presented some challenges because of interference stemming from the injection of the analytes by the PC. The problem was solved by adopting an innovative system architecture. As shown in FIG. 10-a, an alternate flow path for the PC was provided to maintain a steady flow in the SC-TCD during the injection cycle. The fluidic resistance of the alternate flow path was matched closely with that of the PC path to minimize flow perturbations while switching between the two paths. The new architecture decreased the TCD stabilization time by one order of magnitude (from 1-2 min to 10 seconds) and ensured continuous flow of the carrier gas as shown in FIGS. 12A-12B.

(90) The components of the GC were assembled as schematically shown in FIG. 10-a. In addition to the integration of PC and SC-TCD mentioned above, a Y-connector may be added between them to connect the small pump through a valve. The connector isolates the loading path of the PC from the SC-TCD, which reduced contamination in the SC-TCD. This also permits loading at higher flow rates to the PC since the high fluidic resistance in the SC was avoided.

(91) The GC was tested by loading the system with a mixture of five compounds (headspace), containing toluene, tetrachloroethylene, chlorobenzene, ethylbenzene, and p-xylene, injected by the GC autosampler through the heated injection port (2 L, 40:1 split ratio) at 1 mL/min. As shown in FIG. 12-b, calibration standards were generated with this method. The injected mass ranged from 1-3 ng, which is equivalent to 10 mL loading of 100 ppbv gas mixture, approximating a 10-min loading using a pump operated at 1 mL/min The test was performed three times and retention times were highly repeatable with an RSD less that 1.3% for all analytes. The peak areas and peak heights had average RSDs less than 4.7% and 8%, respectively.

(92) The fully-assembled GC was evaluated in a simulated environment using gasoline as the source of exposure. The test atmosphere was generated by placing 50 mL of gasoline in a 100 mL beaker that was placed inside a large glass chamber (4 L). Air was circulated inside the chamber but outside the gasoline beaker at 500 ml/min to simulate a car refueling scenario in which gasoline vapors displaced from the tank disperse in the atmosphere, where they may be inhaled. The top of the chamber was kept open to the atmosphere and the chamber was allowed to be filled with vapors for 10 minutes. The vapors were sampled through the GC, by keeping the system inlet close to the top of the chamber. Vapors were sampled under ambient temperature and pressure for two different sampling times, 5 and 15 minutes. To compare results with a conventional sampling system, gasoline vapors were also sampled using sorbent tubes packed with Tenax TA at a sampling flow rate of 69 ml/min for 3 hours. Sorbent tubes were desorbed using a thermal desorption system coupled to a GC-FID (TD-GC-FID, Perkin-Elmer ATD 400). Toluene and p-xylene were identified at a concentration of 16 and 14 ppmv, respectively (FIG. 13). In both systems (GC and TD-GC-F1D), benzene was not identified because it co-elutes with the other low-boiling point components in gasoline. FIG. 12-a illustrates that the GC detected five peaks, three of which were identified as toluene, ethylbenzene, and p-xylene based on retention times. Also, because Tenax TA has low affinity to benzene, the GC system retained lesser mass of it compared to the other analytes. The sample volume collected and analyzed in the GC was 3 orders of magnitude lower than those collected on sorbent tubes. These results illustrate that the present invention is capable of detecting and separating compounds with a much shorter sampling time and lower sample volume compared to conventional systems to complete one full cycle of analysis.

(93) In other embodiments, the present invention provides a GC system suitable for environmental monitoring applications. The system leverages micromachined components to achieve low power consumption (2.75 W) and fast analysis time (4.4 min). A Limit of Detection (LOD) of 1 ng was achieved, which enables monitoring of HAPs at sub-100 ppbv concentrations. In yet other embodiments, a deep-etched PC, enabling high sample volume, and utilizing semi-packed/multi-capillary columns for increased separation efficiency may be provided.

(94) FIGS. 14-15 show another embodiment of the present invention. FIG. 14 shows various column patterns of designs that may be used with Trap as well as with the other components described above such as PC, GC, PE, TPC, TCD and SC. Specifically, traps and concentrators are concentration amplifiers used to improve the detection limit of an analytical system. Preconcentration may be achieved by collecting analytes on an adsorbent material or medium over a period of time. The collected samples are then released via rapid thermal desorption in the form of a highly concentrated plug for subsequent chemical analysis. In one embodiment, micro pillars may be round 1610 or elongated and in a zigzag pattern 1611 and/or in other configurations as described above to increase the effective surface area of the device and/or to ensure the enhancement of gas molecules interaction with the adsorbent surface. The pillars may also be coated with Tenax TA thin film using an inkjet printing technique. In addition to its high adsorption capacity and thermal stability (up to 450 C.), one interesting feature of Tenax TA is its low affinity to water making it a suitable adsorbent for the extraction of VOCs from aqueous samples.

(95) In yet another embodiment, the present invention enhances the adsorption capability of the Tenax TA poly (2,6-diphenylene oxide) through its deposition on a nano-structured template. Modified Tenax TA-coated SiO.sub.2 nanoparticles (SNPs) are incorporated as an adsorbent bed in silicon based micro-thermal preconcentrator with an array of microposts as described above embedded inside the cavity and sealed with a cover. The interior surface of the chip is first modified by depositing SNP using a layer-by-layer self-assembly technique followed by coating with Tenax TA. The adsorption capacity of the SNP-Tenax TA medium is enhanced by as much as a factor of three compared to thin films of Tenax TA. The increased adsorption ability is attributed to the higher surface area provided by the underlying porous SNP coating and the pores between SNPs affecting the morphology of deposited Tenax TA film by bringing nano-scale features into the polymer.

(96) In addition, SNPs may solely be used as the adsorbent. The medium may be created using a layer by layer coating technique. In addition, surface functionalization may be undertaken using silanes as described.

(97) In one preferred embodiment, the present invention has achieved more than 10,000 concentration factors for the disclosed MEMS-based pTraps. Separation columns 1700 may be used to separate a mixture of gases into its constituents. The column is basically a long channel whose walls are coated with a stationary phase, as described above, that is responsible for the separation of the various gaseous species. As shown, column 1700 may be serpentine in configuration to reduce the overall footprint of the detector. Other channel configurations may be used. In one alternate embodiment, the present invention provides a linearly-variable column (LVC) and the other a step-gradient column (SGC). In some preferred embodiments, the width of a 1 m long 250 m-deep LVC is gradually reduced from 120 m to 20 m at 1 m/cm. While that of a 1 m-long SGC is modulated in five steps (120 m, 95 m, 70 m, 45 m and 20 m) each with a length of 20 cm.

(98) Microfabricated columns typically are comprised of high-aspect-ratio rectangular channels etched in silicon and laid out in circular or square-spiral configuration. In other embodiments, MEMS columns including semi-packed (having embedded micro posts 1701-1705) and multicapillary (having parallel channels) and demonstrated columns with 12,000-20,000 number of theoretical plates per meter, the highest reported separation efficiencies in GCs. In other embodiments, serpentine microfabricated semi-packed columns (SPC) with three circular micro pillars (20 pm-wide, 20 pm-post spacing) embedded along a 1 m-long and 150 pm-wide microfluidic channel may be used.

(99) As shown in FIG. 14, unlike conventional TCDs, in one embodiment, the present invention provides a monolithically integrated detector that includes Purge section 1900, Trap section 1600, TCD section 1800 and separation column 1700. TCD 1800 includes a first resistor 1802 located at the inlet 1812 of separation column 1700. TCD 1800 also includes a second resistor 1803 located at the outlet 1813 of separation column 1700. This configuration reduces reagent usage by eliminating the need for a reference channel with a carrier gas.

(100) In use, the overall size of the PTGC chip will be about 25 mm90 mm. The chip will be connected to auxiliary micro/mini systems including a pump, valves, and a small cylinder of compressed helium gas as schematically shown in FIG. 15. The chip operation has 4 stages. In the purge and trap stage, water from sample source flows into the Purge 1900 through the water inlet. Helium through the gas inlet purges the WOCs, through the gas outlet, to be adsorbed in Trap 1600. An elevated temperature in the Purge enables easier extraction of WOCs. In the next stage, temperature of the Trap is raised to remove the low boilers out through Aux-2. In the third stage, the temperature of the Trap is raised further to release the target into the separation column 1700, resulting in their separation and detection by detector 1800. In the final stage, the Trap and SPC are raised to a higher temperature to condition and prepare for the next run. It should be noted that after the initial purge and trap stage, the Purge is cleaned with DI water and methanol to prevent microbial growth. The valves, pump and chip heating circuitry require signal coordination to lead through the stages with minimal computation workload. This, combined with the fact that the unit must be portable and compact, warrants the use of a low power processor with multiple sleep modes. By far the most popular in this class is the Texas Instruments MSP 430 series processors available with various cache, memory size and on-chip peripheral specifications. A low 100 nA sleep current and a wake-up time of few microseconds makes it well suited for on-site deployment.

(101) In other embodiments, the present invention provides a method of fabricating a separation column for use with a gas chromatograph. An oxide, such as a metal oxide or even an aluminum oxide, is deposited by atomic layer deposition to create a stationary phase medium on the separation column. The oxide may be deposited as a plurality of layers with each layer deposited in a cycle comprising (a) exposure to trimethylaluminum (b) purge (c) exposure to water (d) and purge. In a preferred embodiment, each cycle deposits a layer of about 1-2 angstroms. The oxide, including the aluminum oxide, may be functionalized by exposure to one or more silanes. In other embodiments the silane is an alkylsilane or is chlorodimethyloctadecylsilane.

(102) In yet other embodiments, the present invention provides a detector having a micro-purge extractor in communication with a micro-scale gas chromatography column for the extraction and analysis of water organic compounds from an aqueous sample. The micro-purge extractor has a cavity in communication with a sample inlet port, a purge gas inlet port, a waste outlet port and a purged water organic compound outlet port, The sample inlet port is adapted to receive an aqueous sample. The purge gas inlet port is spaced apart from the sample inlet port and adapted to receive inert gas, which is used to purge water organic compounds from the cavity of the micro-purge extractor. The waste outlet is opposingly located from the purge outlet port, and the waste outlet is adapted for draining water from the chip. The purge outlet in is communication with a micro-thermal preconcentrator, which is adapted to absorb and desorb water organic compounds. Also included is at least one resistive heating element that when activated, causes the water organic compounds to be desorbed. Lastly, the micro-scale gas chromatography column is adapted to separate the water organic compounds and a micro-thermal conductivity detector is used to identify the water organic compounds. The separation column may have an oxide, metal oxide or aluminum oxide stationary phase medium, which may be functionalized by exposure to one or more silanes. The silane may be an alkylsilane or a chlorodimethyloctadecylsilane. The purge outlet port and the sample inlet port are located on a top side of the micro-purge extractor and the purge gas inlet is located on a side of the micro-purge extractor, and the waste outlet port is located on a bottom side of said micro-purge extractor.

(103) In yet other embodiments, the present invention provided a detector for detecting hazardous air pollutants at parts-per-billion concentrations in complex mixtures. The detector includes a microfabricated preconcentrator, a separation column with an on-chip thermal conductivity detector, a controller for controlling flow and thermal management and a user interface. The thermal conductivity detector includes a first resistor located at an inlet of the separation column and a second resistor located at an outlet of the separation column. The separation column includes a medium comprised of silica a nanoparticle layer with a Tenax TA coating. In other embodiments, the separation column may include at least one channel that linearly decreases in width and/or at least one channel that decreases in width in a stepwise fashion.

(104) 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. For example, while the terms adsorb, absorb, adsorption and absorption are used herein, the intention of the application is to use all term interchangeably with the broadest meaning applied to all without any one term having a narrower meaning than the other.