Chemical impedance detectors for fluid analyzers

A chemical impedance detector having several electrodes situated on or across a dielectric layer of a substrate. The electrodes may be across or covered with a thin film polymer. Each electrode may have a set of finger-like electrodes. Each set of finger-like electrodes may be intermeshed, but not in contact, with another set of finger-like electrodes. The thin-film polymer may have a low dielectric constant and a high porous surface area. The chemical impedance detector may be incorporated in a micro fluid analyzer system.

BACKGROUND

The present invention pertains to detectors and particularly to detectors for fluid analyzers. More particularly, the invention pertains to chemical impedance detectors.

U.S. patent application Ser. No. 11/383,723, filed May 16, 2006, entitled “An Optical Micro-Spectrometer,” by U. Bonne et al., is hereby incorporated by reference. U.S. patent application Ser. No. 11/383,663, filed May 16, 2006, entitled “A Thermal Pump,” by U. Bonne et al., is hereby incorporated by reference. U.S. patent application Ser. No. 11/383,650, filed May 16, 2006, entitled “Stationary Phase for a Micro Fluid Analyzer,” by N. Iwamoto et al., is hereby incorporated by reference. U.S. patent application Ser. No. 11,383,738, filed May 16, 2006, entitled “A Three-Wafer Channel Structure for a Fluid Analyzer,” by U. Bonne et al., is hereby incorporated by reference. U.S. Provisional Application No. 60/681,776, filed May 17, 2005, is hereby incorporated by reference. U.S. Provisional Application No. 60/743,486, filed Mar. 15, 2006, is hereby incorporated by reference. U.S. patent application Ser. No. 10/909,071, filed Jul. 30, 2004, is hereby incorporated by reference. U.S. Pat. No. 6,393,894, issued May 28, 2002, is hereby incorporated by reference. U.S. Pat. No. 6,837,118, issued Jan. 4, 2005, is hereby incorporated by reference. U.S. Pat. No. 7,000,452, issued Feb. 21, 2006, is hereby incorporated by reference. These applications and patents may disclose aspects of structures and processes related to fluid analyzers, including the PHASED (phased heater array structure for enhanced detection) micro gas analyzer (MGA).

SUMMARY

The present invention may be an implementation of chemical impedance (resistive or capacitive) detector of gases or liquids having a low-k porous and permeable, high surface area thin film polymer on interdigitated electrodes having dimensions compatible with micro-channels of micro analyzers. The detector may be integrated into the structure of a micro analyzer.

DESCRIPTION

Related art CIDs (chemical impedance detectors), i.e., polymer gas chromatography (GC) detectors, appear to be bulky, slow (about 1 second), of low sensitivity and not integrated into the structure of the micro analyzers. Such detectors, whether based on changes in resistivity, dielectric constant or strain of the polymer, should achieve a gas-to-solid solution equilibrium, which is either too slow (for high-speed GC applications), due to the slow rate of diffusion into the polymer lattice, or too insensitive, due to low film thickness required if the response time is to be lowered.

FIGS. 1aand1bshow an illustrative example of an interdigitated capacitive polymer detector10, viz., a chemical impedance detector (CID).FIG. 1areveals a top view andFIG. 1bshows a cross-section view of the detector10at line30ofFIG. 1a. A layer12of dielectric may be situated on a substrate11. For an illustrative example, the substrate11may be silicon and layer12may be SiO2. Substrate11and layer12may be of some other appropriate material. On the SiO2layer12may be finger-like electrodes13and14intermeshed with each other but not in contact with each other. On the electrodes13and14and the SiO2layer12portions between the electrodes may be a layer15of a thin film polymer. The CID10may have a larger or even a smaller structure other than the one shown in the Figures.

The CID10may solve related-art shortcomings by using low-k-porous, high surface area (for low stray capacitance and high sensitivity) thin-film (for ms-level speed)15, on interdigitated pairs of low-height and low-width electrodes13and14(to maximize signal/stray capacitance ratio), compatible with micro-channel dimensions (for short purge times) of a fluid analyzer, such as a phased heater array structure for enhanced detection (PHASED) micro gas analyzer (MGA).

FIG. 2shows a layout of two CIDs10on a chip21. An insert22which is an enlarged area of a CID10reveals an interdigitization of the finger-like portions of electrodes13and14. A gas to be detected may enter an inlet23, and then pass through a three element flow sensor24and a thermo conductivity detector25. From the detector25the gas may go through the CID10for detection and a determination of a magnitude of an amount of a particular analyte. From there, the gas may exit from CID10to the gas outlet26. Both CIDs10may connected and operated in a differential AC-coupled mode where one is exposed to the analyte peaks and the other is not, or to one of a different composition or concentration.

FIG. 3reveals another capacitive impedance detector structure49consisting of an array of holes47into a plane-parallel capacitor48, with spacings comparable to the thin dielectric, so maximize porosity, sensing signal and analyte access, and minimize response time.

FIG. 4shows a layout of an illustrative example of a thermal conductivity detector (TCD)25which may be used in conjunction with the CID10. A sample gas flow27may pass over a detector element28. The structure of detector25may include a wall33, membrane supports29and air gaps31on both sides, provided that both sides are sealed relative to the sample analyte being analyzed; otherwise, there would not be air gaps31present in the structure of detector25. Lead-outs32may be connected to the detector element28. Dimension34of the detector may be about 100 microns.

Polymer film-based sensors, in general, upon exposure to trace gases, may either change film resistivity, dielectric constant, strain and/or weight. Also, metal oxide films may change resistivity and serve as detectors. The porous, spin-coatable materials may be used for GC pre-concentration and separation. Pre concentration or a pre concentrator may be referred to as concentration or a concentrator, respectively, even though it may be actually like pre concentration or a pre concentrator.

Also, polymer films may be used for gas detection in gas chromatography in the form of SAW detectors (surface acoustic wave, sensitive to changes in film mass). Useful detector results may be obtained with MPN (dodecanethiol monolayer protected gold nanoparticle) films, which change in electrical conductivity when exposed to different gases. These films may have excellent results when used as GC separator films in capillary columns.

A table inFIG. 5shows examples of porous materials with measured specific surface area used for GC pre-concentration in packed columns but typically not necessarily as solvent-castable films. This table may provide characteristics of selected adsorbents, such as maximum temperature (° C.), specific surface area (m2/g), pore size (nm), and the respective monomer.

A table inFIG. 6lists non-porous polymers that are solvent-castable. This table provides information relating to detector polymers for solvent casting and stable to 250° C. in inert atmospheres. Films of rubbery (Tg<operating temperature) polymers 100-200 nm in thickness appear to be consistent with fast response times (˜milliseconds). A sample gas velocity of 1 m/s may pass a 1-mm long in only about 1 ms. The table ofFIG. 6lists a chemical name, acronym, character and application.

A table inFIG. 7lists performance in terms of a ppm-level LOD (limit of detection), in relation to the IDLH (immediate danger to life and health) or ppm-level of IDLH, for an analyte detector of industrial solvent vapors, CWA (chemical warfare agent) simulants and ERCs (explosives-related compounds), along with the polymer used for detection. The table may reveal a LEL (lower explosive limit) and an IDLH concentration for selected VOCs (volatile organic compounds), with the lowest detected vapor concentrations and a calculated theoretical LOD in ppm (v/v), signal-to-noise ratio (S:N, measured response in volts/peak-to-peak noise in volts), the polymer used to achieve the lowest detected concentrations, and the oscillation amplitude (ΔVosc).

In order to enable detector performance prediction and comparison between alternate materials, one may define some polymer properties related to their ability to absorb gases: 1) Directly measurable via gas-weight absorption into polymers are partition function values, K, (which are geometry-independent thermodynamic equilibrium values for film materials before and after exposure to analytes); and 2) Also measurable via GC elution or retention times are partition or capacity factors, k′. A comparison between these two is possible by remembering that K=k′·β, where β=volumetric ratio of the gas/film volume of a GC separation column. The values of k′=(tr−to)/todepend on the ratio β and thus on the column ID and film thickness. For example, under otherwise equal conditions, larger ID columns may cause the analyte to reside a greater fraction of the time in the gas phase, and thus be swept through the column within a time closer to that of the unretained analytes, to,thus resulting in smaller k′ values than would be the case with smaller IDs. A similar argument may be made for film thickness. One may herein compare K-values derived from weight gain measurements with GC-derived k′ values obtained for a porous film material, in order to estimate likely detector performance.

Another source of K and k′ data is provided by theoretical predictions, as can be made via molecule-surface energy interaction models, which can then result in predictions of capacitance detector signal and a film's sensitivity to exposure to given analytes. Such models generate molecular internal energy changes, ΔE, corresponding to the ad- or absorption of a gas molecule on the surface (liquid or gas). The relation between ΔE and K follows known laws of thermodynamics, which relates changes in ΔE to changes on enthalpy, ΔH; entropy, ΔS; Gibbs' Free Energy, ΔG; and the equilibrium constant, K: ΔH=ΔE+TΔS; and ΔG=RT·ln(K)=ΔH+TΔS.

The present CID10two capacitors, for which the capacitance is given by C=N.(Ca+Cd), where Ca=2.∈O.D(k1)/D′(k1) capacitance in air, Cd=2.∈O.∈r.D(y1)/D′(y1) capacitance in the substrate, ∈O=0.0886 pF/cm=dielectric constant of vacuum, ∈r=relative dielectric constant of the substrate material, D(y), and D′(y) represent the complete elliptic integral of the first kind and its complement, y=W/(W+2s), W the metal width and s the spacing, respectively.

The ratio D(y)/D′(y) is simplified to

To achieve a response time near 1 millisecond, the plate height and the film thickness (to enable rapid analyte in- and out-diffusion) should be t2˜h˜0.1 μm, which also requires W to be near that dimension. However, fabrication technology may just allow values of W˜1 μm.

FIG. 8lays out a model of porous capacitance information for parameters like those of a micro analyzer, such as a PHASED MGA110(FIG. 16). The above calculations may be compiled into a table inFIG. 8, where the data input cells are highlighted with dashed-line boxes and the CID outputs (signal and background) and results in dotted-line boxes, for a simple alkane such as hexane. Larger molecular weight and more dense analytes may lead to larger ΔC signals. Note, however, that these may be generally maximum ΔC estimates, based on assumption that the analyte concentration on the polymer15would be one monomolecular layer, while ignoring gas-phase analyte concentrations and equilibrium partitions.

One may evaluate non-porous-film type CIDs. An ability to dissolve gaseous analytes into the structure of a polymer may be given by the solubility mass fraction, μ, which is closely related to the partition function or equilibrium where μ=m(vap-in-film)/m(film)=Δm(film)/V(film)/ρ(film), or, if viewed from the gas side, the partition function, K=Δm(film)/V(film)/ρ(vap)=μ·ρ(film)/ρ(vap), both of which may be listed in the table ofFIG. 8. The solubility of gases in polymers may be discussed from a theoretical vantage point, and the experimental results of toluene and DMMP uptake in several polymers may be noted with partition equilibria or coefficients, K=Δm/V(film)/ρ(vap), at room temperature, ranging from 1000 to 100,000 as shown in a graphical plot inFIG. 9, and with some numerical examples at room temperature given in a table inFIG. 10, for vapor-(non-porous) polymer pairs. In the table, PDMS=polydimethylsiloxane, PECH=polyepichlorohydrin, Latex˜polybutylene, ABACD=abietic acid, SEB=styrene/ethylene-butylene (powder), DMMP=dimethyl methylphosphonate, DEEP=diethyl ethylphosphonate, and DB-5=PDMS+5% phenyl substitution, a common GC film. As to DB-5, K=β·k′, with k′ may be from retention times (corrected for a temperature change from the experimental value of 100° C. to 25° C., seeFIG. 11) for a column with 100 μm ID and 0.4 μm film thickness, β=61.7.FIG. 11shows retention time and k′=(tr−to)/to(plotted as ln(k′)) versus 1/T (temperature in 1/K), for DEEP and DMMP.

As anticipated roughly by Raoult's law, the values for the more volatile compounds (toluene relative to DMMP) may be much lower, and those for smaller, more volatile molecules like hexane, may be lower still. The GC-film DB-5 appears to adsorb less gas than the other polymers noted, despite chemical similarities between PDMS and DB-5 and apparently widely diverging values. A significant point is that the estimated partition equilibrium constants for various analytes on the present porous sensor film with values of t4, ρLaand ρgayield saturation k-values which may be worthy of note, as indicated in a table ofFIG. 12. That table reveals partition coefficients derived for porous NG-E films of an 800 m2/g specific surface area.

For the table ofFIG. 12, the “quasi K-values at boiling point” may be computed for an assumed monomolecular (liquid analyte) film inside all pores, which may then be converted to an equivalent analyte gaseous density fraction outside the pores but within the overall confines of the porous film. With the temperature dependence of k′, which may also hold for K=k′·β, then the K-values may be converted from boiling point to 22° C. The so-obtained equivalent K-values (22° C.) should then be comparable to K-values for non-porous films in the table ofFIG. 10. A comparison of these tables appear to show that the alkanes of increasing molecular weight might increase as expected, and that the analyte K-values in the porous film may be generally higher than those in non-porous films.

Large partition coefficients may enable analytes to concentrate in the film at K-times greater concentration than the one in the gas phase, thus reaching monomolecular film coverage before reaching a 100% gas-phase concentration. Therefore, the low-vapor pressure analytes of K˜100,000 may be detected at 1 ppm gas phase concentrations with a capacitance sensitivity, δC˜0.008 pF, rather than estimates of 0.08·10−6pF, corresponding to K˜1.

The present device may involve a monolithic micro GC pre-concentrator121, separator123(FIGS. 15 and 16), TCD (thermal conductivity detector)115,118, flow sensor122, p and T sensors, and a “thin-film” gas detector, e.g., CID10. The present device may incorporate a monolithic micro GC system with a “thin-film” gas detector, in which this thin film may be the same active material in the three micro GC operations—pre-concentration, separation and CID, after a spin-coat application on one wafer, which forms one of the channel walls of, for example, concentrator121and/or separator123. The micro GC may be a PHASED MGA110. The noted “thin-film” material may consist of a porous film such as NGE (nanoglass-E), NGE+TA (toughening agent) or GX-3P as the with TA materials, available from Honeywell International Inc., rather than non-porous material films.

An advantage of the CID10may include greater sensitivity and speed combination than conventional CIDs, as provided by the large-surface area of the open-pore film, which enables analyte to more rapidly adsorb or dissolve into and desorb from the film15in this CID10, than with a film of equal mass without pores. Another advantage may include the capability of one and the same GC stationary film to serve for pre-concentration and separation (of devices121and123, respectively), as well as for gas detection, i.e., to observe the elution time and the peak area, to identify and quantify each analyte, respectively. Further, an application of the porous film via spin-coat on just one wafer to form one of the channel walls, may keep fabrication costs lower than fabrication involving coating of all channel walls.

Separation performance of GC columns with partial stationary phase coverage may be noted. Because k′ depends on β, i.e., the ratio of gas/stationary phase volumes, stationary film temperature, and the stationary film thickness, then analyte molecules near the uncoated, wall may tend to elute without any retention (k′=0), whereas analyte molecules near the coated part of the column may be retained and correspond to k′>0, each of which is expected to result in significant deterioration of achievable separation efficiency. Countering this effect may be a rapid and short time diffusion between the walls of the (square) column. It may be calculated that an increase in plate height and associated loss in resolution in rectangular channels with only one or two coated walls coated, relative to columns with the four walls coated, shows that (the table ofFIG. 13) for the square channels with one coated wall, the increased plate height is a factor of 1.50 relative to one with all walls coated, and a factor of 1.92 relative to a cylindrical column. These calculations appear to omit temperature effects. The table appears to concern separation impedances for pressure-driven systems. The retention factor k′ may equal=4. Codes in first column may be as follows: “OT”=open cylindrical; “BT”=coating at bottom and top; “AL”=coating at all walls; “BO”=coating at bottom wall only; numbers by the codes refer to width-to-height ratio, φ; “NE”=edge effects neglected; um=mean gas velocity; νm=reduced gas velocity; h=H/dc=reduced plate height; H=plate height, dc=column ID; and Φ=permeability factor.

Temperature effects may be noted. Because the PHASED MGA columns may operate with temperature control of just one wall, temperature ramping, as needed to achieve high-speed analyses, can cause temperature gradients between the heated “wall” and the other uncoated ones. Fortunately, one may take advantage of the temperature-dependence of k′ by applying a much thinner “coating” to the unheated walls, e.g., in the form of a thin “deactivation” film, which should exhibit smaller k′-values. During ramping, the lower temperatures of the cooler walls may qualitatively increase k′-values to approach the hot-wall values. If one selects the coating thicknesses carefully and remembering (FIG. 14) that for every 17° C. temperature drop, k′ may approximately double or triple its value.FIG. 14, likeFIG. 11, shows the retention time and k′=(tr−to)/to(plotted as ln(k′)) versus 1/T, for a number of analytes.

Performance of a GC separation column may be expressed by a performance index, π:
π=N2/(toΔp),
where N=number of theoretical plates, to=elution time for unretained peak, and Δp=pressure drop; and in terms of its reduced plate height, h=H/d, which may have a theoretical minimum of h=1, and d=capillary ID.

Other polymers, such as Nafion™, a proton-conducting, perfluoro-polymer, may be considered as material for a polymer film for the CID10.

The invention may incorporate a channel or channels for a flow of a sample along a membrane that supports heaters and a stationary phase for sample analysis. The channel or channels may be an integral part of a micro fluid analyzer. The analyzer may have a pre-concentrator (PC) (viz., concentrator) and chromatographic separator (CS) that incorporates the channel or channels.

FIG. 15shows a pre concentrator121and a separator123for a fluid analyzer110. A CID10may be situated at an inlet of a pre concentrator121and an outlet of a separator123. The CIDs10may be connected in a differential mode. A sample111may be moved with a thermal pump46in pre concentrator121. The pump could instead be in the separator123. The thermal pump46may have three heaters receiving sequenced energizing signals to provide heat pulses by the heaters in a fashion to move the sample fluid111through the pre concentrator121and separator123. Other kinds of thermal pumps may be used, or an ordinary pump116(like that inFIG. 16) may be used. A controller119may provide signals to the pump46, and to heaters in the pre concentrator121and separator123via contact pads41. Controller119may process signals from the CIDs10, and other detectors and flow sensor(s) (FIG. 16). Controller119may have, for instance, a pre amplifier, an analog-to-digital converter (and vice versa), a timer, and a microprocessor. The microprocessor may manage and process signals from the CIDs10, flow sensor(s), TCDs, and so on, and provide signals for power to the heaters, including the pump, and to detectors (e.g., TCDs) and sensors as needed.

FIG. 16is a system view of a fluid analyzer which may be a phased heater array structure for enhanced detection (PHASED) micro gas analyzer (MGA)110. It reveals certain details of the micro gas apparatus110which may encompass the specially designed channel described herein. The PHASED MGA110, and variants of it, may be used for various chromatography applications.

Sample stream111may enter input port112to the first leg of a differential thermal-conductivity detector (TCD) (or other device)115. A pump116may effect a flow of fluid111through the apparatus110via tube117. There may be additional or fewer pumps, and various tube or plumbing arrangements or configurations for system110inFIG. 16. Fluid111may be moved through a TDC115, concentrator121, flow sensor122, separator123and TDC118. Controller119may manage the fluid flow, and the activities of concentrator121and separator123. Controller119may be connected to TDC115, concentrator121, flow sensor122, separator123, TDC118, and pump116. Data from detectors115and118, and sensor122may be sent to controller119, which in turn may process the data. The term “fluid” may refer to a gas or a liquid, or both.

FIG. 17is a schematic diagram of part of the sensor apparatus110representing a heater portion of concentrator121and/or separator123inFIG. 16. This part of sensor apparatus110may include a substrate or holder124and controller119. Controller119may or may not be incorporated into substrate124. Substrate124may have a number of thin film heater elements125,126,127, and128positioned thereon. While only four heater elements are shown, any number of heater elements may be provided, for instance, between two and one thousand, but typically in the 20-100 range. Heater elements125,126,127, and128may be fabricated of any suitable electrical conductor, stable metal, alloy film, or other material. Heater elements125,126,127, and128may be provided on a thin, low-thermal mass, low-in-plane thermal conduction, membrane, substrate or support member124, as shown inFIGS. 17 and 18.

InFIG. 18, substrate130may have a well-defined single-channel phased heater mechanism and channel structure131having a channel132for receiving the sample fluid stream111. The channel may be fabricated by selectively etching a silicon channel wafer substrate130near the support member124. The channel may include an entry port133and an exhaust port134.

The sensor apparatus110may also include a number of interactive elements inside channel132so that they are exposed to the streaming sample fluid111. Each of the interactive elements may be positioned adjacent, i.e., for closest possible thermal contact, to a corresponding heater element. For example, inFIG. 18, interactive elements35,36,37, and38may be provided on a surface of support member124in channel132, and be adjacent to heater elements125,126,127, and128, respectively. There may be detectors115and118at the ends of channel132.

There may be other channels having interactive film elements which are not shown in the present illustrative example. The interactive elements may films be formed from any number of substances commonly used in liquid or gas chromatography. Furthermore, the interactive substances may be modified by suitable dopants to achieve varying degrees of polarity and/or hydrophobicity, to achieve optimal adsorption and/or separation of targeted analytes.

The micro gas analyzer110may have interactive elements35,36, . . . ,37and38fabricated with various approaches, such that there is a pre-arranged pattern of concentrator and separator elements are coated with different adsorber materials A, B, C, . . . (known in gas chromatography (GC) as stationary phases). Not only may the ratio of concentrator121/separator123elements be chosen, but also which elements are coated with A, B, C, . . . , and so forth, may be determined (and with selected desorption temperatures) to contribute to the concentration and separation process. A choice of element temperature ramping rates may be chosen for the A's which are different for the B, C, . . . , elements. Versatility may be added to this system in a way that after separating the gases from the group of “A” elements, another set of gases may be separated from the group of “B” elements, and so forth.

Controller119may be electrically connected to each of the heater elements125,126,127,128, and detectors115and118as shown inFIG. 17. Controller119may energize heater elements125,126,127and128in a time phased sequence (see bottom ofFIG. 19) such that each of the corresponding interactive elements35,36,37, and38become heated and desorb selected constituents into a streaming sample fluid111at about the time when an upstream concentration pulse, produced by one or more upstream interactive elements, reaches the interactive element. Any number of interactive elements may be used to achieve the desired concentration of constituent gases in the concentration pulse. The resulting concentration pulse may be sensed by detector118for analysis by controller119.

FIG. 19is a graph showing illustrative relative heater temperatures, along with corresponding concentration pulses produced at each heater element. As indicated herein, controller119may energize heater elements125,126,127and128in a time phased sequence with voltage signals50. Time phased heater relative temperatures for heater elements125,126,127, and128may be shown by temperature profiles or lines51,52,53, and54, respectively.

In the example shown, controller119(FIG. 17) may first energize heater element125to increase its temperature as shown at line51ofFIG. 19. Since the first heater element125is thermally coupled to first interactive element35(FIG. 18), the first interactive element desorbs selected constituents into the streaming sample fluid111to produce a first concentration pulse61(FIG. 19), while no other heater elements are not yet pulsed. The streaming sample fluid111carries the first concentration pulse61downstream toward second heater element126, as shown by arrow62.

Controller119may next energize second heater element126to increase its temperature as shown at line52, starting at or before the energy pulse on element125has been stopped. Since second heater element126is thermally coupled to second interactive element36, the second interactive element also desorbs selected constituents into streaming sample fluid111to produce a second concentration pulse. Controller119may energize second heater element126in such a manner that the second concentration pulse substantially overlaps first concentration pulse61to produce a higher concentration pulse63, as shown inFIG. 19. The streaming sample fluid111may carry the larger concentration pulse63downstream toward third heater element127, as shown by arrow64.

Controller119may then energize third heater element127to increase its temperature as shown at line53inFIG. 19. Since third heater element127is thermally coupled to the third interactive element37, the third interactive element37may desorb selected constituents into the streaming sample fluid to produce a third concentration pulse. Controller119may energize the third heater element127such that the third concentration pulse substantially overlaps the larger concentration pulse63, provided by the first and second heater elements125and126, to produce an even larger concentration pulse65. The streaming sample fluid111may carry this larger concentration pulse65downstream toward an “Nth” heater element128, as shown by arrow66.

Controller119may then energize “N-th” heater element128to increase its temperature as shown at line54. Since “N-th” heater element128is thermally coupled to an “N-th” interactive element38, “N-th” interactive element38may desorb selected constituents into streaming sample fluid111to produce an “N-th” concentration pulse. Controller119may energize “N-th” heater element128in such a manner that the “N-th” concentration pulse substantially overlaps the large concentration pulse65as provided by the previous N−1 interactive elements, to produce a larger concentration pulse67. The streaming sample fluid111may carry the resultant “N-th” concentration pulse67to either a separator123and/or a detector118.

Although the invention has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.