Patent Application
20090114090
Existing air sampling technologies have major limitations. Summa canisters have been the gold standard for air sampling technologies. A Summa canister is a stainless steel vessel which has had the internal surfaces specially passivated using a “Summa” process. This process combines an electropolishing step with chemical deactivation to produce a surface that is chemically inert. A Summa surface has the appearance of a mirror, bright and shiny. In some cases, an additional adsorption layer is added to the surface of the summa canister. When combined with subsequent gas chromatography-mass spectrometry (GC-MS) analysis, they yield the most accurate data of all commercially available technologies. The cost, size, weight, and labor intensive handling of Summa canisters, however, make them unattractive for chemical reconnaissance. In terms of cost and size, solid-absorbent sampling tubes and solid phase micro-extraction (SPME) sampling technologies would be more suitable, but these technologies show large discrepancies when compared to Summa canister data. These discrepancies are due in large part to the comparatively narrow adsorption spectrum and selective gas adsorption of the solid absorbents and the limited absorption capabilities of the polymeric coatings on the SPME fiber.
In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.
An ionic liquid electrospray with a cooled counter electrode is used to collect polar and polarizable molecules. Various embodiments are described that utilize the electrospray in combination with solid absorbents in a cartridge or canister form that can also be used with a system to remove water from a gas such as air, and move the air through the cartridge. Multiple cartridges may be used in a sampling system, and in some embodiments, each cartridge may be sealed after used for sampling air, and sent to be analyzed in a laboratory. In some embodiments, in combination with superb non-polar compound capturing capability of existing solid absorbent, charged nanodroplet enhanced sampling may be used to capture organics, acids, halogens, noble gases, and organometallics with a dynamic range from 10 ppt to 100 ppm with sufficient quantity for subsequent GC-MS analysis. The cartridge may be a highly miniaturized sampling capsule (<3 ml in volume). The sampling capsules utilize the ionic liquid electrospray to pre-concentrate, dissolve, and preserve polar and polarizable gases. The sampling capsules may also utilize layered solid absorbents to capture low polarizability non-polar gases and a void space to contain noble gases as well as other gases that do not interact with the ionic liquid or solid phase materials.
In one embodiment, canister 100 is an open tubular structure with flanges at both ends. As manufactured, the ends may be covered by metallized polymer films adhesively bonded to the flanges. The film extends over one side of the end of the flange and is bonded to a small diameter shaft. The seal is opened by turning the shaft, breaking the adhesive seal and tightly rolling up the polymer film onto the shaft. The two ends of the capsule can be opened synchronously providing openings for drawing air samples through the capsule.
Resealing the capsule after exposure is equally simple, assuming that suitable sealing films and adhesives can be found. Ideally the original sealing film can be reused by unrolling the rolled-up film back onto the flanged surfaces of the capsule. Axial pressure would be applied to the two ends to insure the integrity of the adhesive seal. The upstream end of a canister would be closed first, establishing a vacuum (a few psi) in the canister that would aid the sealing process. If tight seals cannot be achieved in this way, fresh polymer films with fresh adhesives can be used with minor complication of the robotic actuator system.
In one embodiment, an inner side of wall 105 of the canister 100 may be passivated using a “Summa” process, such as the same one used in Summa® canister construction, before loading other components into the canister. This process combines an electropolishing step with chemical deactivation to produce a surface that is chemically inert. The canister may be pre-heated right before sampling to desorb any “left-over” gases adsorbed during storage.
In addition to the sampling canister treatment, tubing in the gas path may be passivated to avoid adsorption of analytes. Swagelock manufactures and sells deactivated stainless steel tubing for the analytical production of trace gas standards.
In one embodiment, a gas stream is directed through the canister. Gas that interacts with the nano-droplets in the spray plume is captured by dissolving into the ionic liquid forming a nanodroplet-analyte combination, sometimes simply referred to as an analyte. Nano droplets of the ionic fluid are represented by round particles. A gas 240 to be sampled contains chemical represented by circles 230 in the inset. The chemicals pass through the spray plume 220 and are coated with the nanoparticles. The nanoparticles are then electrostatically attracted to the counter electrode 120, which is cooled. The cooled electrode causes the ionic liquid to transition into a glassy state, effectively trapping the analyte.
In one embodiment, the counter electrode 120 is thermoelectrically cooled to approximately 250° K. Different ionic fluids may provide desired results at different temperatures of the counter electrode 120. Other methods of cooling the electrode 120 may be used, such as the use of a refrigerant or other mechanism. The cooled electrode causes the ionic liquid spray to transition into a glassy state, effectively trapping the analytes. In one embodiment, the electrode may be designed to ensure that the pores will not clog as the analyte accumulates. The small amounts of analytes that aren't captured by the ionic liquid, such as light hydrocarbons, remain in the air stream, and are drawn through a solid phase adsorbing material, such as nanostructured activated carbon. Solid phase adsorbents are well known for their capability in capturing light hydrocarbon gases, ensuring high sampling efficiency.
As a final step, the capsule is resealed trapping a small volume (˜1 cc—depending on the size of the canister) of air which will enable sampling of gases that are above the parts-per-billion level. Pertinent gases, such as krypton and xenon, which have background levels above the 10 ppb level will be trapped in this “dead volume” as well as being weakly adsorbed by the ionic liquid.
Electrosprayed ionic liquids will sample all polar and polarizable gas molecules in an air stream passed through sprayed nano-droplets. The nano-droplets provide a greatly increased surface area of the ionic liquid, increasing the number of molecules attaching to the spray. The use of electrosprayed ionic liquids provides the ability to pre-concentrate, dissolve, and preserve all polar gases. It may be used in combination with layered solid absorbents to capture low polarizability non-polar gases and a void space in the canister to contain noble gases.
In one embodiment, solid phase adsorptive material 125 may be a combination of loosely packed micro- or nano-powders whose surface structures and chemistries collectively bind many classes of molecules firmly but reversibly. Mixtures of Zeolites and other adsorbents, with various pore sizes, may be used to trap a wide range of analytes. For example, a hydrophobic adsorbent like a nanoporous carbon or high silica Zeolite may be used along or may be paired with a more hydrophilic adsorbent to maximize the variety of adsorbents to be collected. In one embodiment, Zeolite, activated carbon, and silica gels, may be used with pores of approximately 8-15 Angstroms. Such a pore size will work for a wide range of molecules, since the great majority of analytes will enter the pores.
The canister 100 is designed to capture lighter hydrocarbons, halogens and other less polar gases that are not effectively captured by electrospray ionic liquids. The solid adsorbent 125 may serve a scavenging role, intercepting traces of more polar analytes which may, for whatever reason, have bypassed the electrospray system. The solid adsorbent is adjacent to the cooled copper foam, so adsorption of analytes will be enhanced. Light hydrocarbons have significant vapor pressure, even at the lower temperatures expected, and it is expected that their background concentrations will be significantly higher than 10 ppt.
Adsorbents in various embodiments may include hydrophobic inorganic materials like silicalite, nanoporous carbons, activated carbons, and Zeolite. These materials combine high micropore surface area (>800 m2/g for pores 10-15 Å in diameter), with efficient mesopore channel networks for acceptable uptake rates. Commercially available solid phase adsorbents comprising a support coated with an organic coating including long alkyl sidechains may also be of interest. Though lower in surface area, these materials are particularly effective for adsorption of hydrocarbons. If required multiple adsorbents can be combined to provide optimum sorption.
The format of the solid adsorbents may be selected to ensuring efficient sampling of the passing air stream. Low pressure drop is desired, but bypass should be avoided. In one embodiment, a structured format, either as coatings on a corrugated support or in a monolith may be utilized based on anticipated flow characteristics of the gas stream through canister 100.
The use of the ionic electrospray in place of additional solid phase adsorbents helps overcome a problem referred to as competitive displacement. The majority of gases have very similar solid phase absorption mechanisms. This similarity is useful in enabling exhaustive sampling of all possible atmospheric compounds. Understanding this mechanism sheds light on the difficulties solid phase absorbers face in meeting the first two challenges. Generally, gas adsorption starts with physisorption. For instance, Zeolite and other high surface area adsorbents function by adsorbing analytes first as a physisorption process, and subsequently as a pore-filling condensation process as the physisorption sites get filled.
When only trace amounts of gas are present, the gases form a physisorbed monolayer on the Zeolite surface. During this phase, interactions between the analyte and the adsorbent surface will dominate. As the total gas concentration increases, the pores of the absorber begin filling at which time the adsorbed gas will form an adsorbed liquid-like phase. The driving force will now be the interactions between the analyte and this incipient phase. The efficiency of adsorption depends on the concentration of the analyte in the air. At low concentrations on a virgin adsorbent, the adsorption affinity will be high but the capacity will be low (being restricted to a monolayer). As the quantity of adsorbate on the adsorbent increases, the average strength of each interaction will decrease. When various analytes are present in the sampled air, each analyte competes for adsorption sites. A large concentration of analyte will dominate the strong monolayer interactions leaving the low concentration analyte to have weaker interaction with the insipid layer. The problem is exacerbated if the high concentration analyte-absorbent interaction is energetically preferred. This solid absorbent phenomena of high concentration analytes prevents other analytes from adsorbing and is referred to as competitive displacement.
Ionic liquid materials offer many advantages for gas capture, including wide range of solubilities and thermal and electrical stability. Many different ionic liquids materials may be used. In one embodiment, 1-butyl-3-methylimidazolium tetrafluoroborate or [bmim][BF4] is used.
Electrospraying any liquid produces an incredible high surface area mist. The ultra-fine charged droplet mist results from applying a high voltage (several kVs/cm) between a small (100 s of micrometers) electrospray nozzle and a counter electrode. Typical droplet generation rates are on the order of 1010 of these droplets are generated per second at 17.5 μl/min spray rate. The droplets in the mist are highly charged, often carrying in excess of 2,000 Coulomb per kilogram. The incredible charge density separates individual droplets in the mist plume. Typical droplet size ranges are from 100 s of nm to ionic emission (sub-nm). The flow rate may have an effect on resulting droplet size and therefore plume surface area. Depending on the selected ionic liquid mixture and resulting flow rate, we can expect droplet between 3.16 to 89.4 nm and surface areas between 678 and 24 m2/g. High surface area and small droplet size are beneficial for effective gas adsorption and dissolution. From the above numbers and our canister size, the typical diffusion time of gases from to the surface of a droplet is approximately 27 μsec. Subsequent dissolution of the surface-adsorbed gases into the droplet take approximately 0.1 μsec. The mean resident time of a droplet inside the canister is ˜5 msec, or ˜200-times longer than the time needed for gas adsorption and dissolution. Stated simply, the electrospray process solves the resistance to mass transfer seen in bulk ionic liquids.
The sweeping volume of the nanodroplets is orders of magnitude higher than the gas volume. Sweeping volume is defined as the total volume sampled by the charged droplet before they reach the counter electrode. Sweeping volume is defined by:
V
s
=N×A
d
×V
gd
where Vs is the nanodroplet sweeping volume in L/s, N is the number density of nanodroplets per second, A is the droplet cross sectional area, and vgd is the relative velocity between the gas stream and the nanodroplets in m/s. For a 17.5 μl/min electrospray, around 10 billion 89 nm nanodroplets are generated per second. Assuming gas/droplet relative velocity of 2 cm/s, the nanodroplets sweeping volume is 300× greater than the gas volume they sample (gas volumetric flow is around 15 ml/s in one embodiment). Such a large sweeping volume ensures complete sampling of entire gas volume by the droplets. The sweeping effect exits for all spray-based air scrubbing technology regardless if the droplets are charged or not. The increase in sweeping volume becomes larger when the droplet sizes are relatively large (>1 μm).
Strong mutually induced dipole-dipole interaction occurs between charged droplet and polar gas molecules. The electrosprayed nanodroplets obtain very high charge-to-mass ratios, often exceeding 2,000 coulomb/kg. They essentially behave like “quasi” mobile ions. When the droplets encounter polar or polarizable gas molecules, both the charged nanodroplets and the polarized gas molecules attract each other. This attraction increases the gas concentration in the micro-environment around the charged droplet. This effect becomes more prominent as the droplet size approaches that of an individual gas molecule, and especially when the gas concentration is at sub-ppm level.
In one embodiment, the counter electrode exhibits good thermal and electrical conductivity. The foam should also allow the gas sample to pass through it with the lowest possible pneumatic pressure drop at complete NAC capturing efficiency. Copper foam, with appropriate pore size, represents an ideal candidate. Copper has very good thermal and electrical properties with a customizable pore size. Larger pore size will decrease the pneumatic pressure drop, while smaller pore sizes favor NAC capturing. In one embodiment, a counter electrode formed of copper foam provides less than an 8 psi pneumatic pressure drop, and an analyte foam residence time greater than the analyte pore wall collision time. In one embodiment, a 1-cm cylindrical copper foam with 1 cm2 cross sectional area is used. The following Table 1 summarizes the trade offs of the pore size for optimal copper foam configuration. As can be seen, copper foam with pore size ranging from 100 to 200 μm offers low pressure drop, good droplet capturing efficiency, and rapid cooling/gelation of the captured droplets (˜29 ns).
Electrospray of ionic liquid into the canister module seamlessly integrates effective trace compounds adsorption offered by highly charged nanodroplets; rapid dissolution of the absorbed compounds into ionic liquid droplets; and effective preservation of captured trace compounds through solidification/gelation of ionic liquid. It may provide complete and exhaustive polar and polarizable analyte capturing, and easy analyte preservation.
In one embodiment, approximately 10 g of Zeolite 3 Å may be used to adsorb all of the water that could be present in 5 L of 100% RH air sample. This amount of mass would be prohibitively large if it were included in each canister. The air stream drying module 635 in system 600 is designed to be reusable. Energy is provided for regenerating 10 g of Zeolite 3A between each sampling cycle in one embodiment. Taking typical value of Zeolite 3A specific heat of 1.07 kJ/kg. ° C. and ˜250° C. regeneration temperature, ˜2.5 kJ energy is used for one cycle of regeneration, which takes about 30 seconds in one embodiment. Primary lithium battery has a typical power density ˜0.6 Watt.hr/g, it is estimated that ˜1.2 g of battery is needed per sample.
Pneumatic pressure drop through the system is minimized, since the entire flow path, including the water removal module, the copper foam counter electrode, and the solid absorbent layer are highly porous. Air pumping at this low pressure can be readily achieved by either motion of a vehicle on which the system 600 is loaded, or a low power diaphragm gas pump. For example, a vehicle cruise at ˜10 mph will provide 20 L/min air through a 1 cm2 port.
However, larger air flow calls for higher power consumption. It is, therefore, highly desirable to have a system that offers high gas capturing efficiency. Less air sample that is used, means a lighter system. For example, a 19 L/min diaphragm pump weighs 17 lb (KNF Model UN813.5), compares to a 1.6 l/min pump that weighs merely 44 g (KNF Model NMPo15). In some embodiments, less than 5 L of actual air sample is needed, thus greatly reduces the weight of the overall system.
Cooling of the capturing counter electrode is provided for preservation of gases captured in the ionic liquid. The side effect is that it cools the air as well, which poses a power challenge for the overall system. In this system, a counter flow heat exchanger “recycles” the spent air to cool the hot side of a thermoelectrical cooler 620, hence reducing the power consumption of the overall system to <3 Watts in one embodiment.
Electrosprayed ionic liquids are used to capture chemicals in the atmosphere or gas sample. Electrosprayed ionic liquids offer several advantages for chemical capture compared to standard materials, such as Zeolites and activated carbons. Electrospray produces many small charged droplets. The droplets remain close to one another. The droplets effectively reduce the sampling time by increasing the surface area of the liquid and reducing mass transfer distances. A charge exists on the surface of the droplets and may electrostatically attract polar gas molecules. The use of ionic liquids for air sampling may also provide high capture capacity. Materials such as Zeolites and activated carbon have a finite number of adsorption sites, which limits the amount of molecules that can be captured. Adsorbing materials can become saturated in the presence of large background chemical signatures, and can potentially miss the presence of smaller amounts of chemicals because there are no adsorption sites left.
Once the chemicals are absorbed, the ionic liquids provide the ability to hold on to the adsorbed chemicals before future analysis. The adsorbed materials are prevented from evaporating by the relatively high freezing point and viscosity intrinsic to ionic liquids. Once solidified, any material not at the air-ionic liquid interface cannot evaporate. Ionic liquids have warmer freezing temperatures than traditional organic solvents. Furthermore, if the ionic liquid remain unfrozen, it remains a viscous liquid delaying the adsorbed material from reaching the gas-ionic liquid interface and thus delaying evaporation.
In one embodiment, a sampler system samples chemical analytes in air. A pump is used to draw in air to the sampler at a rate of up to 5 L/minute. The air that is sampled contains trace amounts of chemical analytes for capture. The air is first streamed over a water absorbing material, such as Zeolite 3A. Once the water content is removed, the air is directed into one of many interchangeable capture chambers. An individual capture chamber contains many components including: An electrospray nozzle with a reservoir of ionic liquid; a porous metal foam counter electrode with an exposed heat coupler; and a separate solid phase absorbing material. A new capture chamber may be used each time a different sample is collected. The air flow into a specific capture chamber is controlled by a series of valves.
Air streaming through the capture chamber flown through a plume of ionic liquid material that is generated by the electrospray nozzle. The chemical analytes in the air sample interact with the ionic liquid by first adsorbing on the droplet surface, then completely dissolving. The analyte-droplet combination is electrostatically driven towards a grounded porous metal foam that is thermoelectrically cooled. The analyte-droplet combination solidifies as they impact the cooled copper foam. The high surface area of the copper foam allows continuous capture of the ionic liquid droplets without clogging, permitting continuous air flow through the system.
Analytes that do not interact with the ionic liquids will not be captured on the metal foam, and will remain in the airflow. The remaining analytes will flow through a solid phase adsorbing material, such as activated carbon or Zeolite. Chemical analytes will interact with the solid phase material and adsorb to the surface. The air flowing out of the capture chamber is directed over a heat-sink attached to the thermoelectric cooler. The exhaust air is slightly chilled compared to ambient after interacting with the copper foam, and dissipates heat from the “hot side” of the cooler.
Less energy may be used to cool the copper foam because of the enhanced heat dissipation. The air-flow control valves operate to ensure that a small volume of air, typically on the order of 1 mL, remains in the capture chamber after the sampling cycle is over.
The Abstract is provided to comply with 37 C.F.R. § 1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.
