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Mobility based apparatus and methods using dispersion characteristics, sample fragmentation, and/or pressure control to improve analysis of a sampleRelated Patent Categories: Radiant Energy, Ionic Separation Or Analysis, MethodsMobility based apparatus and methods using dispersion characteristics, sample fragmentation, and/or pressure control to improve analysis of a sample description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070252082, Mobility based apparatus and methods using dispersion characteristics, sample fragmentation, and/or pressure control to improve analysis of a sample. Brief Patent Description - Full Patent Description - Patent Application Claims
REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. patent application Ser. No. 10/998,344, filed on Nov. 24, 2004, which claims the benefit of and priority to: U.S. Provisional Application No. 60/524,830, filed on Nov. 25, 2003, entitled "System for Identification of Ion Species in an Electric Field"; U.S. Provisional Application No. 60/549,004, filed on Mar. 1, 2004, entitled "Chemical Agent Detector"; and U.S. Provisional Application No. 60/549,952, filed on Mar. 4, 2004, entitled "Tunable Chemical Agent Detector"; U.S. Provisional Application No. 60/556,349, filed on Mar. 25, 2004, entitled "Tunable DMS Recirculation System"; and U.S. Provisional Application No. 60/556,198, filed on Apr. 28, 2004, entitled "Reduced Pressure DMS." The entire teachings of the above referenced applications are incorporated herein by reference. [0002] This application also incorporates by reference the entire contents of the following co-pending U.S. patent applications: U.S. Ser. No. 10/187,464, filed on 28 Jun. 2002; U.S. Ser. No. 10/215,251, filed on 7 Aug. 2002; U.S. Ser. No. 10/462,206, filed on 13 Jun. 2003; U.S. Ser. No. 10/684,332, filed on 10 Oct. 2003; U.S. Ser. No. 10/734,499, filed on 12 Dec. 2003; U.S. Ser. No. 10/738,967, filed on 17 Dec. 2003; U.S. Ser. No. 10/797,466, filed on 10 Mar. 2004; U.S. Ser. No. 10/821,812, filed on 8 Apr. 2004; U.S. Ser. No. 10/824,674, filed on 14 Apr. 2004; U.S. Ser. No. 10/836,432, filed on 30 Apr. 2004; U.S. Ser. No. 10/840,829, filed on 7 May 2004; U.S. Ser. No. 10/866,645, filed on 10 Jun. 2004; U.S. Ser. No. 10/887,016, filed on 8 Jul. 2004; U.S. Ser. No. 10/894,861, filed on 19 Jul. 2004; U.S. Ser. No. 10/903,497, filed on 30 Jul. 2004; U.S. Ser. No. 10/916,249, filed on 10 Aug. 2004; U.S. Ser. No. 10/932, 986, filed on 2 Sep. 2004; U.S. Ser. No. 10/943,523, filed on 17 Sep. 2004; and U.S. Ser. No. 10/981,001, filed on 4 Nov. 2004. FIELD OF THE INVENTION [0003] The invention relates generally to mobility-based systems, methods and devices for analyzing samples. More particularly, in various embodiments, the invention relates to improving sample collection, filtration, detection, measurement, identification and/or analysis (collectively "analysis") using, for example: dispersion characteristics (2-, 3-, and n-dimensional); sample fragmentation; and/or variations in flow channel/filter field conditions. Such conditions may include, without limitation: pressure; temperature; humidity; field strength, duty cycle, and/or frequency; and/or compensation voltage. BACKGROUND [0004] There are a number of different circumstances in which it is desirable to perform analysis to identify compounds in a sample. Such samples may be taken directly from the environment or they may be provided by front end specialized devices to separate or prepare compounds before analysis. There exists, a demand for low cost, compact, low-power, accurate, easy to use, and reliable devices capable of detecting compounds in a sample. [0005] One class of known analyzers are mass spectrometers (MS). Mass spectrometers are generally recognized as being the most accurate type of analyzers for compound identification. However, mass spectrometers are quite expensive, easily exceeding a cost of $100,000 or more and are physically large enough to become difficult to deploy everywhere the public might be exposed to dangerous chemicals. Mass spectrometers also suffer from other shortcomings such as the need to operate at relatively low pressures, resulting in complex support systems. They also need a highly trained operator to tend to and interpret the results. Accordingly, mass spectrometers are generally difficult to use outside of laboratories. [0006] A class of chemical analysis instruments more suitable for field operation is known as Field Asymmetric Ion Mobility Spectrometers (FAIMS) or Differential Mobility Spectrometers (DMS), and also known as Radio Frequency Ion Mobility Spectrometers (RFIMS) among other names. Hereinafter, FAIMS, DMS, and RFIMS, are referred to collectively as DMS. This type of spectrometer subjects an ionized fluid (e.g., gas, liquid or vaper) sample to a varying high-low asymmetric electric field and filters ions based on their field mobility. [0007] The sample flows through a filter field which allows selected ion species to pass through, according to a compensation voltage (Vcomp) applied to filter electrodes, and specifically those ions that exhibit particular mobility responses to the filter field. An ion detector then collects ion intensity/abundancy data for the detected ions. The intensity data exhibits attributes, such as "peaks" at particular compensation voltages. [0008] A typical DMS device includes a pair of electrodes in a drift tube. An asymmetric RF field is applied to the electrodes across the ion flow path. The asymmetric RF field, as shown in FIG. 1, alternates between a high or "peak" field strength and a low field strength. The field varies over a particular time period (T), frequency (f) and duty cycle (d). The field strength E varies with an applied field voltage (Vrf) and the size of the gap between the electrodes. Ions pass through the gap between the electrodes when their net transverse displacement per period of the asymmetric field is zero. In contrast, ions that undergo a net displacement eventually undergo collisional neutralization on one of the electrodes. In a given RF field, a displaced ion can be restored to the center of the gap (i.e. compensated, with no net displacement for that ion) by superimposing a low strength direct current (dc) electric field (e.g., by applying Vcomp across the filter electrodes) on the RF. Ions with differing displacement (owing to characteristic dependence of mobility in the particular field) pass through the gap at differing characteristic compensation voltages. By applying a substantially constant Vcomp, the system can be made to function as a continuous ion filter. Alternatively, scanning Vcomp obtains a spectral measurement for a sample. A recorded image of the spectral scan of the sample is sometimes referred to as a "mobility scan" or as an "ionogram." [0009] Examples of mobility scans based on the output from a DMS device are shown in FIGS. 2A and 2B. The compounds for which scans are depicted are acetone and an isomer of xylene (o-xylene). The scan of FIG. 2A resulted from a single compound, acetone, being independently applied to the DMS analyzer. The illustrated plot is typical of the observed response of the DMS device, with an intensity of detected ions dependent on Vcomp. For example, the acetone sample exhibits a peak intensity response at a Vcomp of approximately -2 Vdc. [0010] FIG. 2B illustrates the results when analyzing a mixture of acetone and o-xylene. The combined response shows two peaks in approximately the same region as for the independent case. The compounds in the mixture can be detected by comparing the response against a library, for example, of stored known responses for independently analyzed compounds, or libraries of mixtures. Thus, the scans for independently analyzed compounds, such as the scan of FIG. 2A for acetone, can be stored in a computer system, and when compound responses such as that in FIG. 2B are observed, the relative locations of the peaks can be compared against the stored responses in the library to determine the constitution of the compound. [0011] A specific RF field voltage and field compensation voltage Vcomp permits only ion species having a particular ion mobility characteristic to pass through the filter to the detector. By noting the RF level and compensation voltage and the corresponding detected signal, various ion species can be identified, as well as their relative concentrations (as seen in the peak characteristics). [0012] Consider a plot of ion mobility dependence on Vrf, as shown in FIG. 3. This figure shows ion intensity/abundancy versus RF field strength for three examples of ions, with field dependent mobility (expressed as the coefficient of high field mobility, a) shown for species at greater, equal to and less than zero. The velocity of an ion can be measured in an electric field (E) low enough so that velocity (v) is proportional to the electrical field as v=KE, through a coefficient (K) called the coefficient of mobility. K can be shown to be related to the ion species and gas molecular interaction properties. This coefficient of mobility is considered to be a unique parameter that enables the identification of different ion species and is determined by, ion properties such as charge, size, and mass as well as the collision frequency and energy obtained by ions between collisions. [0013] When the ratio of E/N, where N is gas density, is small, K is constant in value, but at increasing E/N values, the coefficient of mobility begins to vary. The effect of the electric field can be expressed approximately as K(E)=K(0)[1+.alpha.(E)], where K(0) is a low voltage coefficient of mobility, and .alpha. is a specific parameter showing the electric field dependence of mobility for a specific ion. [0014] Thus, as shown in FIG. 3, at relatively low electric field strengths, for example, of less than approximately 8,000 V/cm, multiple ions may have the same mobility. However, as the electric field strengths increase, the different species diverge in their response such that their mobility varies as a function of the applied electric field. This shows that ion mobility is independent of applied RF field voltage at relatively low RF field strengths, but is field-dependent at higher RF field strengths. [0015] FIGS. 2A and 2B demonstrate that species can have a unique behavior in high fields according to mobility characteristics. The ions passing through the filter are detected downstream. The detection signal intensity can be plotted as a characteristic detection peak for a given RF field voltage and field compensation voltage Vcomp. Peak intensity, location, and shape are typically used for species identification. [0016] However, a problem occurs in that the peaks, as seen in the typical DMS spectra, are generally broad in width. Therefore, compounds exhibiting intensity peaks at similar compensation voltages may be difficult to separate from each another. Consequently, there may be particular conditions under which two different chemicals generate indistinguishable scans for a particular Vcomp and a particular RF field voltage, or for other combinations of filter field/flow channel parameters. In such a case, it is may not be possible to differentiate between the two different compounds. Another problem may occur when two or more chemical species have the same or almost the same ion mobility characteristic for a particular set of field/flow channel parameters. This is most likely to happen in the low electric field regime (referred to herein as Ion Mobility Spectrometry or IMS), where many existing ion mobility spectrometer systems operate. Therefore, if two or more chemical species have the same or almost the same mobility characteristic, then their spectroscopic peaks will overlap, and identification and quantification of individual species will be difficult or impossible. [0017] FIG. 4 is a graph of Vcomp versus Vrf according to an illustrative embodiment of the invention, but also highlighting the above described prior art drawback. More particularly, FIG. 4 depicts a graph of Vcomp versus Vrf for four compounds: lutidine; cyclohexane; benzene; and dimethyl-methl-phosphonate (DMMP). Each curve shows the location of detected ion intensity peaks, such as those circled at 100, at the various (Vrf, Vcomp) locations, which in total provide the peak characteristics for each particular compound. As shown, there is a region 100 in which the intensity peaks and mobility curves for DMMP and cyclohexane overlap with each other. As can be seen, operating in a Vrf region of from approximately 2,500 Vpeak to approximately 2,650 Vpeak, at a Vcomp of about -6 Vdc to about -8 Vdc, one would find it virtually impossible to discriminate between the two compounds based on a single Vcomp scan at a single Vrf. Specifically, in a conventional spectral scan approach that plots intensity/abundance versus Vcomp over a range of Vcomp for a single Vrf would plot the overlapping peaks as a single peak. [0018] Accordingly, there is a need for an improved ion mobility-based compound identification approach that addresses peak overlap issues and provides improved compound analysis features. SUMMARY OF THE INVENTION [0019] The invention addresses the deficiencies of the prior art by providing, in various embodiments, improved mobility-based systems, devices and methods for analyzing constituents in a sample. More particularly, in various embodiments, the invention provides improved sample collection, filtration, detection, measurement, identification and/or analysis (collectively "analysis") using, for example: dispersion characteristics; sample fragmentation; and/or sample processing variations, such as and without limitation, variations in flow channel/filter field conditions. Such conditions may include, any spectral changes, including, without limitation changes in: pressure; temperature; humidity; field strength, duty cycle, and/or frequency; field voltage amplitude, frequency and/or duty cycle; detector bias voltage magnitude and/or polarity; and/or filter field compensation voltage magnitude and/or polarity. [0020] In one practice, the invention employs one or more of the above to provide a library of spectral signatures for a plurality of known species, and identifies unknown species by comparing at least a portion of a spectral signature for the unknown species to at least a portion of one or more of the spectral signatures stored in the library. The spectral signature is a compilation of spectral information for a particular species. The spectral information may include, without limitation, spectral peak amplitude; spectral peak width; spectral peak slope; spectral peak spacing; spectral peak quantity; relative shifts in spectral peaks due, for example, to changes in processing conditions; spectral discontinuities; Vrf versus Vcomp characteristics or any other characteristics of any of the above described conditions plotted against any one or more other above described conditions. 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